I . . ..
Acknowledgements All members of theTask Group contributed significantlyto the publication but special mention must be made of the joint editors, Ceorge SomeM'lle OBE, a past Chairman of CBDC, and Bob Lark at Cardiff University. Both expended much time and energy in transformingthe information and findings into the finished article and CBDC would like to express its gratitude to these two and all the other membersof the Group.
Publishedfor and on behalf of the Concrete Bridge Development Group by The Concrete Society Riverside House, 4 Meadows BusinessPark,Station Approach, Blackwater,Camberley,Surrey CUI7 9AB Tel: +44 (0)1276 607140 Fax: +44 (0)1276 607141 www.concrete.org.uk CCIP-024 PublishedJune 2007 ISBN 1-904482-35-X 0 Concrete Bridge Development Group Order reference: CBDG/TG9 CClP publications are produced by The Concrete Society on behalf of the Cement and Concrete Industry Publications Forum -an industry initiativeto publish technicalguidance in support of concrete design and construction. CClP publications are available from the Concrete Bookshop at www.concretebookshop.com Tel: +44 (0)7004 607777 All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in any form or by any means, without the prior permission of the copyright owner. Enquiries should be addressed to the Concrete Bridge Development Group.
Although the Concrete Bridge DevelopmentGroup (limited by guarantee)does its best to ensure that any advice, recommendations or information it may give either in this publication or elsewhere is accurate, no liability or responsibility of any kind (including liability for negligence) howsoever and from whatsoever cause arising, is accepted in this respect by the Group, its servants or agents.
Printed by Cromwell Press, Trowbridge, UK
I ~
Guidance on t h e Assessment of Concrete Bridges Contents Foreword
X
Abbreviations and acronyms
xii
Notation
xiv
1.
The assessment process
1
1.1
The need for assessment
1
1.1.1 Asset management
1
1.1.2 Effectiveness of current programme
1
1.1.3 Network management
2
1.1.4 Sub-standard bridges
2
1.1.5 Assessment documentation
2
Management of the assessment process
2
1.2.1 Experience
2
1.2.2 The brief
3
1.2.3 Existing information
3
1.2
~
1.3
1.2.4 Access
4
1.2.5 Site work
4
1.2.6 Level of assessment and analysis
5
1.2.7 Management of resuks
5
Staged assessments
5
1.3.1 BA79: The management of sub-standard highway structures
6
1.3.2 BA79: Advice on assessment
6
~
1.4
1.3.3 Levels of assessment
7
1.3.4 Conclusions
8
Assessment standards
8
1.4.1 Highways Agency documentation
8
1.4.2 DeDartures from standards
10
~~
1.4.3 Other bridge owners
10
1.5.1 Whole life assessment 1.5.2 Asset management
11
1.5.3 Consequences for assessment
13
11
~
1.5.4 A continuous assessment Droeramme
2.
1.6 Assessment of the serviceability limit state 1.6.1 Serviceability assessment
13 14
1.7 Historical aspects of assessment 1.8 European developments
15 17
1.8.1 The BRlME proiect 1.8.2 Structural assessment
17 17
1.8.3 Deterioration
18
1.8.4 Chloride ingress
18
1.9 References
18
Technical approval and documentation
19
2.1 2.2
General Drocedures
19 19
2.3 2.4
Obiectives Key stages
20 21
2.5
‘Departures’ and ‘Aspects not covered by Standards’
25
2.6
Reference
26
General principles
Inspection for assessment
3. ~
27
~
3.1
Process and eeneral Drocedures
27
3.1.1 Background and introduction
27
3.1.2 Reauirements for insDection
27
3.1.3 Future requirements for inspection
28 29
3.1.4 The inspection process 3.1.5 Risk assessment and safety issues 3.2
Condition assessment
3.2.1 BD2l approach 3.2.2 CSS bridge condition indicator
~~
3.3 3.4 3.5 3.6
ii
13
Network Rail’s Structures Condition Markine Index (SCMI) Other work Preferred approach References
30 30 30 30 31 32 33 33
4.
In-situ and laboratory testing
34
4.1
Background and Introduction
34
4.2
Current testing practice
34
4.3
In-situ sampling and testing
35
4.4
Laboratory testing
35
4.5
Concrete parameters for assessment
35
4.6
Reinforcement parameters for assessment
37
4.7
In-situ stress measurement - concrete
37
4.8
In-situ stress measurement - steel reinforcement
38
4.8.1
38
4.9
5.
4.8.2 Cut bar method
38
Geophysical Techniques
39
4.10 References
39
Loading
40
5.1
General principles
40
5.1.1 Scope of live loading
40
5.1.2
41
5.2
Background t o assessment live loading
Bridge specific Loading
42
5.2.1 Bridge specific probabilistic loading model
42
5.3
Highway surfacing effects
43
5.4
Road traffic and abnormal loads
44
5.4.1 Purposes of highway structure assessment
46
5.4.2 Design and assessment
46
5.4.3 BD86 - Assessment of highway structures for the effects of Special Types General Order (STGO) and Special Order (SO) Vehicles 5.4.4 BD86 Load Models - SV Vehicles
46
5.4.5 BD86 - HB Conversion Charts
47
5.4.6 Assessment of Abnormal Load Movements using BD86
47
References
48
5.5 6.
Blind hole drilling
47
Analysis for assessment
49
6.1
Assessment principles
49
6.1.1 General principles
49
6.1.2 Analytical procedure
49
6.1.3 Lower bound and upper bound methods
50
6.1.4 Ductility
50
6.1 .S
50
Differences from design
6.2
6.3
6.1.6 Structural condition
51
6.1.7 Global and local analvsis
52
Simple methods
52
6.2.1 Strip method
52
6.2.2
52
BA16 method
6.2.3 Upper bound limit and check
52
Elastic methods
53
6.3.1 Elastic grillage
53
6.3.2
54
Elastic finite elements
~~
6.4
6.5
6.3.3 Westergaard and Charts
56
Plastic equilibrium methods
57
6.4.1 Plastic redistribution
58
6.4.2 Hillerborg strip method
58
Yield-line analysis
59
6.5.1 Principles
59
6.5.2 Upper bound solutions
59
6.5.3 Slab decks
60
6.5.4 Beam and slab decks
62
6.5.5
~
7.
Box culverts and retaining walls
62
6.5.6 Limitations of yield-line methods
63
6.5.7 Computer analysis
64
6.5.8 Shear
65
6.6
Strut and tie action
66
6.7
Serviceability limit state
66
6.7.1 Inspections
66
6.7.2 Plastic methods of analysis
67
6.7.3 Special inspections
67
6.8
Soil structure interaction
67
6.9
Conclusions
68
~~~
6.10 References
68
Hidden strengths
69
7.1
Reinforcement anchorage and bond
69
7.1.1 First principles
69
7.1.2 Reinforced concrete beams
69
7.1.3 Sub-standard cover and deteriorated concrete
71
7.1.4 Bond a t laps
72
7.2
7.3
7.1.5 Haunched sections
72
7.1.6 Bent-up reinforcement
72
7.1.7 Inclined links
74
7.1.8 Bearing clamping of reinforcement
75
Shear
76
7.2.1 Shear in reinforced concrete slabs
76
7.2.2 Effect of varying section size
77
7.2.3 Shear enhancement a t supports
77
7.2.4 Shear in mestressed flawed beams
79
7.2.5 Shear in webs of post-tensioned prestressed beams
79
Deck surfacing
79
7.3.1 Properties of surfacing materials
79
~
7.4
7.5
7.3.2 Discussion
80
Moment field analysis of reinforced concrete slabs
80
7.4.1 Moment fields
80
7.4.2 Wood-Armer equations
81
7.4.3 Moment field approach
82
7.4.4 Torsionless grillages
82
ComDressive membrane action
82
~
7.6
~
~~
~
7.5.1 Local strength of deck slabs and restraint
83
7.5.2 Global behaviour of beam and slab decks
84
7.5.3 External restraint
84
7.5.4 Non- Iinear nu mer ical a nalvs is
85
Piers
85
7.6.1 Assessment of piers
85
~~
8.
7.6.2 Slender piers
86
7.6.3 Global restraints
87
7.6.4 Buckling of multiple piers
88
7.7
Redundancy of elements
89
7.8
Parapet edge stiffening
89
7.9
Width of sumorts
90
7.10 Foundations
90
7.1 1 References
91
Specific structural forms
92
8.1
Reinforced concrete slabs
92
8.2
Reinforced concrete beams
93
V
9.
8.3
Half joints
94
8.4
Hinge joints
94
8.5
Box culverts
95
8.6
Precast pre-tensioned beams
96
8.7
Reinforced concrete arches
97
8.7.1 Elastic analysis
97
8.7.2 Mechanism analvsis
98
8.8
Post-tensioned structures
99
8.9
References
99
Soecific material and assessment factors
100
9.1
Introduction and general principles
100
9.2
Alkali-Silica Reaction (ASR)
101
9.2.1 Background
101
9.2.2 Structural assessment: current practice and reference documents 9.2.3 Kev Doints in structural assessment
102 103
9.2.4 Structural sensitivity: elements most a t risk
103
9.2.5 Indicative modifications to design models for strength assessment 9.2.6 Assessment of Highway Structures - BD44/95 and BA44/96; BA52/94; BD21/01
104
Aggressive chemical attack
105
9.3.1 Background
105
9.3.2 ‘Conventional’ form of sulfate attack
107
9 . 3 . F Thaumasite form of sulfate attack (TSA)
108
9.4
Freeze-thaw action
109
9.5
High Alumina Cement (HAC)
110
9.6
Supersulfated Cement (SSC)
111
9.7
Chlorides
112
9.8
Carbonation
113
Steel corrosion
115
9.9.1 Introduction and background
115
9.9.2 Effects of corrosion
115
9.9.3 Corrosion rate
116
9.9.4 Prestressed concrete
116
9.3
105
~~
9.9 ~
vi
9.10 Fatigue
117
9.1 1 Sub-standard reinforcement detailing
118
9.12 Deteriorated reinforced concrete structures
120
9.12.1 Background and introduction
120
9.12.2 Effects of deterioration, and principles for assessment
121
~
~
9.12.3 Sources of detailed information
122
9.12.4 Procedures in accordance with Highways Agency documentation
122
9.13 References
123
10. Load testing
126
10.1 General principles
126
10.1.1 Background
126
10.1.2 Reasons for presence of reserves of strength
126
10.1.3 Types of load test
127
10.1.4 The aims of load testing
127
10.1.5 Documents in the Design Manual for Roads and Bridges
128
10.1.6 Safety
128
~~
~
10.2 Supplementary load testing
129
10.2.1 Status
129
10.2.2 Methodology
129
10.2.3 Further guidance
129
10.3 Proof load testing
129
10.3.1 Background
129
10.3.2 Methodology
130
10.3.3 Application
130
10.3.4 Current status
131
10.3.5 Safety
131
10.4 Full-scale or model-scale testing to failure
131
10.4.1 Background
131
10.4.2 Broad principles
132
~
10.4.3 Extent of testing
133
10.4.4 Application
134
10.5 Hybrid and other forms of load testing
134
10.5.1 Examples
134
10.5.2 Load testing for monitoring
134
10.5.3 Reliability and updating
135
10.6 Dvnamic load testing
135
10.6.1 Structures where dynamic response is critical
135
10.6.2 Dynamic tests for health monitoring
136
vii
10.6.3 Applications for assessment
136
10.6.4 Case studies and research in the literature
137 138
~~
10.7 References 11. Reliability and risk-based techniques
139
1 1 .I Background
139
I 1.2 The appropriate application of reliability and risk-based assessments 1 1.3 Overview of reliability-based assessments 11.3.1 General
140 141
11.3.2 Uncertainties in bridge assessment 11.3.3 Reliability Index
141 141 142
11.5 Bridge specific risk-based assessments
143 144 146
11.6 References
147
1 1.3.4 Interpretation of Reliability Analysis Results 11.4 AcceDtance criteria ~~
12. Bridge management and assessment
12.1 General principles
148
148
12.1.I Managing sub-standard bridges
148
12.1.2 The BA79 Advice Note
148
12.1.3 Managing the results of assessments 12.1.4 Immediate risk structures
149 149
12.1.5 Interim measures during assessment 12.1.6 Interim measures on completion of assessment 12.1.7 Assessment and maintenance
150
12.2 Monitoring 12.2.1 Appropriate monitoring
150 151 151 152
12.2.2 Classes and frequency of monitoring 12.2.3 Application of monitoring
153 153
12.2.4 Permanent monitoring 12.2.5 Monitoring specifications
154 154
~~
12.2.6 Methods and took for monitoring 12.3 Cracking and crack widths 12.3.1 Crack widths
156
12.3.2 Types of crack and diagnosis
157 158 158
12.4 Deterioration rates 12.4.1 The causes of deterioration 12.4.2 Timescales and rates of deterioration
viii
155 156
158
12.4.3 Structural assessment
161
12.4.4 Structural amraisal
162
~
12.5 Legal matters
163
12.5.1 Historical background
163
12.5.2 Current situation
164
12.6 The assessment of bridges for abnormal loads
167
12.6.1 Abnormal load categories
167
12.6.2 Management of abnormal indivisible load movements
167
12.7 References
Appendix Relevant historical references to design and materials specifications and standards used in concrete bridge construction
170 171
ix
Foreword This publication contains guidance on a variety of topics related t o the assessment of concrete bridges. It was prepared by the Assessment Task Group (ATG) of the Concrete Bridge Development
Group and is a development of the report of the ATG published in 1997. Each Chapter is based on an early draft prepared by a member of ATG, which was subsequently reviewed and extended by other members. The whole publication was then edited by Professor Ceorge Somerville and the Convenor of the group, Dr R. J. Lark (Cardiff University). At the time of publication, the Assessment Task Group consisted of the following members: Bob Lark (Convenor, 2001 t o date) Graham Cole (Convenor, 1997 t o 2001) David Bone Mike Chubb Robin Church Steve Denton lan Frostick Paul Jackson Martin Lynch George Somerville
Cardiff University Surrey County Council Royal Haskoning Atkins Highways & Transportation Essex County Council
Parsons Brinckerhoff Network Rail Gifford and Partners Highways Agency Consultant
The input of Brian Barton of the Highways Agency and David Cullington, formerly of the Transport Research Laboratory, is also acknowledged. The current assessment programme effectively started in 1984 with the publication of Departmental Standard BD2l. Substantial progress has been made since then and it might appear that there was little need for guidance on assessment. However, substantial numbers of sub-standard structures still exist, many of which may benefit from further assessment. There is a growing realisation that a continuous assessment process forms an essential part of the sustainable maintenance of the national bridge stock. The purpose of this Guide is t o both record the experiences of past years and t o give examples of best practice for the future. It should be used with due care and attention, as they may not be applicable t o a specific project. The intention is that they should be used as supporting documentation in conjunction with the appropriate standards and, therefore, have been written based on the assumption that the reader will have some experience and a basic knowledge of bridge assessment. The Concrete Bridge Development Group (CBDG) and the members of the ATG assume no responsibility for the adequacy of the advice given, nor for the legal, contractual or financial consequences of its use. The intention of this Guide is t o address the assessment of all types of concrete bridge, irrespective of their use or ownership. It is recognised nevertheless that a significant
X
proportion of such structures are reinforced concrete highway structures, and that many of the standard procedures that are currently used are founded in the requirements of the Highways Agency and/or, when road over rail, Railway Group Standards. As such, much of the guidance that is given here is also based on these requirements, but wherever possible reference is also made t o new research and alternative points of view, which it is believed will both aid and advance the assessment process. References and discussion of the advice and requirements contained in other publications and Standards, particularly the Highways Agency’s Design Manual for Roads
andBridges, will inevitably become out of date, as these documents are themselves updated from time to time In drafting this guidance, the task group has endeavoured t o focus on the principles that underpin best practices in the assessment of concrete bridges, and therefore t o provide guidance which, whilst set in the context of current requirements, should hopefully remain useful even i f these requirements change Comments and feedback on this Guide would be welcomed. They should be sent t o the Secretary, Concrete Bridge Development Group, at the address given inside the front cover. R. J . Lark July 2006
Abbreviations and acronyms AIL AIP
ALL AS R
AW AWR BCA BCI BRIME
BSALL BSLL C&U CBDG CEB
CDM COBRAS CONTECVET
css DAF DfT DMRB
fib FIP FTA HA HAC HACC HCV HRM HSE IAN N DT NLFE NSCLTB
Assessment Live Load Alkali-Silica Reaction Authorised Weight Authorised Weight Regulations British Cement Association Bridge Condition Indicator Bridge Management in Europe research project Bridge Specific Assessment Live Loading The Basic Static Live Load effect Construction and Use Concrete Bridge Development Group Comite Euro-International du Beton (Euro-International Concrete Committee) Construction, Design and Management Concrete Bridge Assessment Package EU project on the Service Life of Concrete Structures (BE 4062) CSS (Formerly the County Surveyors Society) A Dynamic Amplification Factor The Department for Transport Design Manual for Roads and Bridges Federation internationale du beton (The International Federation for Structural Concrete) - Formed by the merger of CEB & FIP Federation lnternationale de la Precontrainte (International Federation for Prestressing) Freeze-Thaw action The Highways Agency High Alumina Cement High Alumina Cement Concrete Heavy Goods Vehicle High Speed Road Monitor The Health and Safety Executive Interim Advice Note Non-destructive testing Non-linear finite element analysis National Steering Committee for the Load Testing of Bridges
Pfa REHABCON
National Vocational Qualification Pulverised fuel ash EU project on Strategies for the Maintenance and Rehabilitation of
RH SCC SCMI
Concrete Structures Relative Humidity Stress corrosion cracking Structure Condition Marking Index
NVQ
xii
Abnormal Indivisible Load Approval in Principle
sls
so SRPC
ssc
Sewiceability limit state. Special Order Sulfate-Resisting Portland Cement
UDL U Is VSE
Supersulfated cement Special Types General Order Technical Approval Technical Approval Authority Technical Appraisal Form Technical Approval Schedule Transport Research Laboratory Thaumasite form of sulfate attack Uniformly distributed load Ultimate limit state Vehicle Standards and Engineering
wcs
Worst Credible Strength
STG 0 TA TAA TA F TAS TRL TSA
xiii
Notation Ash ASV
B C C
D
6 E F", fCl
fC" fsh
ft fY fsv
H
h I0
I,,,,. 1, K
I,1, 4 M, m M* P
Q R Rtt
Area o f bent-up bars Cross-sectional area of vertical stirrup legs at each stirrup location Force in bent-up reinforcement Live load capacity factor/Compression force Diameter of loaded area Width of support Deflection Elastic modulus The total ultimate anchorage force in the longitudinal reinforcement Concrete strength at transfer Concrete cube strength Stress in bent-up bars Concrete tensile strength Yield stress Yield strength of reinforcement Depth of section Slab thickness The performance level of a structure at the time of construction The critical performance level of a structure The performance level of a structure at the time of an assessment Load reduction factor Linear measures/span Values of moment Resistance moment Value o f point load Applied force Reaction force A random multiplier t o model the uncertainty in a load effect due t o the static weight of vehicles Radius Stirrup spacing along a beam Slab span Live load effect (e.g. bending moment or shear force) Tensile force Wall thickness Shear force The fully anchored shear capacity Shear capacity Concentrated point load Uniformly distributed load Linear measure Effective depth Angular measures Factor t o allow for enhancement of bond due t o transverse pressure
xiv
P Yi
Ym
A P
P
r
Factor used in the calculation of bond stress influenced by bearing clamping action Loading partial safety factor Material partial safety factor Joint displacement Coefficient of friction Transverse bearing clamping pressure Reduction factor
1. The assessment process 1.1 The need for assessment
The highway bridge assessment programme in the United Kingdom effectively began with the publication of Departmental Standard BD21 in 1984. (Frequent reference will be made throughout this Guidance document to relevant Standards (BDs) and Advice Notes (BAs) produced by the Highways Agency. A full list of these is given in Section 1.4,) This Standard replaced the previous document, BE1/73, and introduced the limit state concept based on the Bridge Code, BS 5400. The requirement to permit the passage of vehicles of up to 40 tonnes and with 11.5 tonne axles was introduced by directive 85/3/EEC. The United Kingdom and Ireland obtained derogation from the directive until 31 December 1998. Despite having 14 years to complete the assessment programme, work was not completed by the due date and risk management measures had to be introduced. Current estimates are that the initial programme will take a further 1 to 10 years to complete, depending on the bridge owner. Nevertheless,the main area for future work, under current vehicle regulations, is with re-assessment.
1.1.1 Asset tTlanagement
The emphasis today is very much on asset management. This means moving away from reactive maintenance (simply spending roll forward budgets), and towards targeted schemes which increase the overall value of the bridge stock (the asset), but at lowest whole life cost. Key activities in asset management are the inspection and assessment process. It is necessary to determine the general condition and structural safety of each item in the overall asset, i.e. each bridge. Having established the current state, it is also necessary to estimate deterioration rates, so that some idea of the residual life of the structure can be determined. This work should be done to a consistent standard, so that investment decisions can be made with some degree of confidence across the network. Use of this Guide will assist in the effective management of an important national asset.
1 . I .2Effectiveness O f Current programme
The assessment programme has been carried out by a very large number of organisations to varying standards. The actual capacity of structures which were given a 40 tonne rating is not known. This approach was satisfactory a t the time but does not lend itself to determining the residual life of a structure and, ultimately, the value of the asset. Many bridges that just passed assessment in 1984 may now have become sub-standard.
Previous page is blank
1
A t the beginning of the assessment programme it was anticipated that the majority of the problems would occur with shear capacity in bridges built before 1973 when design rules changed. Although this did occur there were a significant number of theoretical
failures in flexure. The assessed levels of loading were also found, in many cases, to be well below 32 tonnes, which was the previous loading standard. A significant amount of experience has been acquired during the assessment
programme, much of which has been highlighted in a review of the programme undertaken by Parsons Brinckerhoff on behalf of the Highways Agency’ ’. This Guide draws on this experience and summarises good practice.
1. I .3 Network management
1. I .4 Sub-standard bridges
1.1.5 Assessment documentation
2
The Highways Agency, in particular, is looking a t route management strategies and the management of abnormal load vehicles. For this, it will be necessary to know the assessed capacity and condition of a bridge to a consistent standard.
Bridges that fail assessment are described as sub-standard. In a simplistic approach, these ‘failures’ then become the strengthening programme for future years. There are thousands of these bridges across the country. It may be that a careful application of further assessment techniques could remove these structures from the programme saving millions of pounds in both direct and indirect costs. Use of this Guide will assist in the maintenance of a safe highway network.
There is information on assessments contained in Highways Agency publications, text books, conference proceedings, learned journals, research papers, etc. This Guide provides easy reference to much of this source material.
1.2 Management of the assessment process
This Section sets out some basic concepts that need to be taken into account in the management of the assessment process and serves as an introduction to later, more detailed, guidance. It is based on the conclusions of the work of the first Concrete Bridge Development Group (CBDG) Assessment Task Group, which was completed in 199712.
I .2.1Experience
Bridge assessment requires the use of experienced engineers and inspectors who can interpret defects present in a bridge and who understand the background to Codes of Practice. The Highways Agency and the CSS (formerly the County Surveyors Society) are considering the implementation of an accreditation scheme for bridge inspectors. Other bridge owners already run NVQ schemes. The client should ensure that the staff proposed for the project have sufficient experience.
I
1.2.2The brief
The initial stages of the assessment programme were primarily used t o identify those bridges which, when using conservative assumptions, could be shown t o be satisfactory. More sophisticated, and expensive, techniques could then be concentrated on the 30% or
so of the bridge stock that failed this initial assessment. This concept of staged assessment has now been incorporated in HA Advice Note BA79. More recently, the need t o carry out assessments t o a consistent minimum standard, say Level 3 of BA79, has been proposed t o assist with asset management. Therefore, the requirements for a brief have changed since the assessment programme began with the first publication of BD2l in 1984. In inviting tenders for assessment, the bridge owner is required t o produce contract
documents that include the scope of the work and available existing information. A key objective should be t o allow the assessing engineer t o concentrate on the investigation and analysis of the bridge concerned by removing, as far as possible, the uncertainties of the process. Typical examples of uncertainties are:
0 Existing information 0 Statutory undertakers
0 Access 0 Possessions 0 Testing 0 Analysis 0 HB, C&U and abnormal load assessments 0 Health and Safety. For a lump sum bid, the assessment engineer requires a clear brief, free of requirements t o make guesses at the tender stage on the extent of work required. The high level of risk, which exists during the desk study and site stage, tends t o reduce money available for assessment calculations with the inevitable consequences. The approach which should give best value is t o use a mixture of lump sum and time charge work. The lump sum should be restricted t o work that can be defined clearly. Time charge work could be set against a pre-agreed budget that should not be exceeded without the client’s permission. However, this requires a professionally qualified client who can enter into technical discussion with the assessment engineer. It does allow more of a team approach and the clients are much more able t o ensure that the end product meets their requirements.
1.2.3 Existing information
The existence of good records of design, alterations and previous assessments, as well as drawings, considerably assists the assessment engineer. The procurement of drawings is sometimes left t o the assessment engineer. This may be an easy task or may involve considerable time and effort. This is a risk item in a lump sum bid. The client is generally in the best position t o obtain this information. The assessment engineer is often requested t o contact the statutory authorities t o determine the approximate location of existing services. The client should obtain this
3
information. If it is necessary to determine precisely the position of services by constructing trial pits, then this should be incorporated in a comprehensive site testing and investigation programme.
1.2.4 Access
Frequently, assessment engineers are requested to make their own arrangements for access across private land. This is a risk item since the difficulty of making such arrangements cannot generally be determined at tender stage. Local Authorities have powers under the Highways Act 1980 to obtain access for the purpose of inspecting, surveying and maintaining highway structures. They are also responsible for making compensatory payment if damage is caused to private property. Details of adjacent land owners and access arrangements should be provided by the client leaving the assessment engineer only to obtain access. The need to arrange for road closures can be assessed and undertaken by the assessment engineer, but adequate time should be allowed for the process when setting a contract programme. A further complication is introduced when dealing with railway tracks. Obtaining track possession can be a protracted and time-consuming business, which is a risk item for the
assessment engineer. Track possessions should be booked and paid for by the client, with the assessment engineer being responsible for undertaking the work within the possession and for the cost of any overrun. This cost must be stated clearly in the tender documents. The possessions must also be a reasonable period within which to undertake the work required. The costs associated with cancelled possessions should be recoverable. Similar considerations can apply to lane closures on motorways. Health and Safety is becoming ever more important and the Construction, Design and Management (CDM) regulations impose certain obligations on the client. Bridge inspection work often falls outside the regulations. However, the client should ensure that adequate Health and Safety measures are proposed by the assessment engineer. Providing adequate Health and Safety is expensive and in the past, attempting to minimise the tender price, assessment engineers have sometimes taken unacceptable risks. The client should carry out risk analysis of each bridge in a commission so that access arrangements and methods of working are common to all tenderers.
1.2.5 Site work
Having carried out a desk study, collected existing information, obtained the necessary approvals and established a safe method of working, the inspection and possible testing of the structure can be carried out. It is important to realise that an inspection for assessment differs from other types of
inspection.Given limited possession times, it is essential that effort is concentrated on areas that will be critical to the assessment calculations.It may be useful to carry out a preliminary analysis of the structure prior to site works, so that these critical areas can be identified.
4
The extent of testing required is often left t o the discretion of the assessment engineer. This is an unacceptable practice in a lump sum bid as it leaves the assessment engineer with the dilemma of how much t o include and the client with lump sum bids which may vary widely depending upon the assumptions made by the assessment engineer. The requirements for testing can generally be best determined after or during the inspection and should be paid for based upon a schedule of rates t o be expanded after discussion between the client and assessment engineer on the testing required t o achieve the clients objectives. Alternatively, the client must define precisely what testing is t o be undertaken in the contract documents.
1.2.6 Level O f aSSeSSment and analysis
The expectations of the client on the degree of sophistication of analysis must be defined clearly. The client should not, as was frequently the practice, leave the assessment engineer with the requirement t o include for more sophisticated analysis if the bridge fails under simple methods. This approach requires the assessment engineer t o determine at tender stage whether a particular bridge is going t o fail a simple assessment and, therefore, t o include for further work. In view of the introduction of BA79, and the growing interest in asset management, the client should state the minimum level of assessment. Therefore a clearly defined approach t o analysis is required with payments for the work undertaken. Non linear computer work, special modelling and reliability analysis in accordance with a Level 4 or 5 assessment in BA79 should be paid for o n a time basis, or by an agreed lump sum when the extent of the work can be defined clearly. Generally, an analysis of the assessment of a bridge t o carry abnormal load vehicles should be carried out if a 40 tonne Assessment Live Loading is obtained. There may be bridges on certain minor roads where this process cannot be justified. The assessment live loading model has changed from HB loading t o Special Types General Order (STGO) loading (see Section 5.4).
1.2.7 Management
Of
reSU[tS
1.3 Staged aSSeSSmentS
Once an assessment has been completed t o an appropriate level of sophistication, which has been agreed with the client and Technical Approval Authority (TAA), see Chapter 2, then the results need to be managed. If the result is less than 40 tonnes then the bridge is defined as sub-standard and should be managed in accordance with BA79. If the bridge has achieved 40 tonne Assessment Live Load (ALL) t o an agreed minimum level of analysis then the result can be used t o assist in determining a long-term maintenance regime for the structure. Finally, it is necessary t o determine a date by when the bridge is re-assessed.
The traditional approach t o assessments has been t o use the simplest method of analysis in order t o achieve a 40 tonne Assessment Live Loading. Indeed, this technique is still referred t o in paragraph 6.2 of the 2001 version of BD2l as follows:
5
0 me assessment process
the choice of the appropriate method (of analysis) will depend upon the structural form of the bridge and the required degree of accuracy. The simple methods, although conservative, are quick to use and should be tried initially where appropriate, before ‘I.
..
progressing to more accurate but more complex methods.
’I
A staged approach t o assessment has been defined in BA79
1.3.1 BA79: The management of substandard highway structures
This Highways Agency (HA) Advice Note was published t o provide guidance on the implementation of interim measures for structures that were assessed to be substandard, or provisionally sub-standard, in order t o maintain the safety of the highway network. It provided further advice on assessment methods and, for the first time, set out formal
definitions of levels of assessment in its Appendix B. It was also intended that the Advice Note would provide guidance applicable t o “future
cyclical assessments within maintenance management programmes”. The Advice Note was drafted by a working group consisting of representatives from a large number of bridge owners, consulting engineers, safety experts and the Transport Research Laboratory. I t s use is applicable t o virtually all highway bridges and, for the first time, gives the bridge owner a sound method for dealing with the problem of managing bridge and other highway structures that have failed assessments.
1.3.2 BA79: Advice O n assessment
The details given in paragraph 2.1 of the BA79 are important and are repeated here in full. “The process of assessment and subsequent action is of crucial importance for ensuring that all highway structures remain in a safe and serviceable state. The assessment rules and criteria need to be applied rigorously and in a consistent manner. Ifassessments are unduly conservative, structures will be unnecessarilystrengthened, using up scarce resources and causing traffic disruptions. A t the same time, if the rules are lax, or applied unevenly, some structures where the margins of safety are unacceptable may be left without appropriate measuresbeing implemented. I’
The Advice Note provides a flow chart t o illustrate the assessment process and related measures. It also recognises the importance of recording the various deliberations associated with the management of sub-standard structures and provides a typical proforma. This Advice Note has more references t o the role and requirements of the Technical Appraisal Authority than probably any other HA Standard or Advice Note. These topics are discussed further in Chapter 12.
6
Paragraph B1.3 of Appendix B of the Advice Note states: “Structural failure is not acceptable t o the public hence the order of probability of failure inherent in the assessment criteria is very small. When a structure is assessed to be substandard, it does n o t mean therefore that it will necessarilyfail or collapse. However, if such structures were left in large numbers without remedial action, there may be an unacceptable risk that a collapse in service would occur. The assessments are based on probabilities and therefore it is impossible to know beforehand which bridges would actually fail in practice. ”
1.3.3Levels O f aSSeSSment
It is possible t o carry o u t assessments at five distinct levels, as defined by BA79. Existing assessment Standards and Advice Notes are only applicable t o Levels 1, 2 and 3. Any assessment carried out t o Levels 4 and 5 will require the detailed approval of the TAA concerned. Each level of assessment has been summarised below. Reference should be made t o Appendix B of BA79 for full details.
Level 1 - Simple The simplest level of assessment, using simple analytical methods, which will give a conservative estimate of load capacity.
Level 2 - Refined This level of assessment involves the use of more refined analysis and better structural idealisation. More refined analysis may include grillage analyses and finite element analyses. Non-linear and plastic methods of analysis may also be used. This level also includes the determination of characteristic strength for materials based on existing available data, but not specific site testing.
Level 3 - Bridge Specific A bridge specific assessment live loading may be derived rather than the use of values from BD21 or BD50 (see Section 1.4). This approach is unlikely t o be justified for short span bridges. This level allows the use of specific material testing t o determine Worst Credible Strengths.
Level 4 - Modified Criteria This level involves the use of partial safety factors that would be specific t o a particular bridge. It should be noted that the derivation of structure specific partial safety factors involves applying the fundamental principles of reliability theory. The Advice Note offers the following warning: “The background knowledge and engineeringjudgement requiredfor this level of assessment is of a high order and hence, the TAA needs to be closely involved.” Draft rules have been prepared but not issued by the HA although advice can be sought from the HA if required.
7
Level 5 - Reliability This level involves reliability analyses that require probability data for all the variables defined in the loading and resistance equations. The Advice Note states: “This type of assessment requires specialist knowledge and expertise and is only likely to be worthwhile and possible in exceptional cases.”
1.3.4COnChSlOnS
For bridge owners with a small number of structures on mainly local roads it would be sensible to continue to use the simplest level of assessment that is appropriate. However, for larger owners with a more complex stock of bridges on primary routes or motorways, there are good economic reasons for developing whole life costing maintenance strategies. In these circumstances it is necessary to carry out assessments to a consistent standard, say Level 3, rather than just determine ‘pass’ or ‘fail’.
1.4 Assessment standards
The Highways Agency provides the principal source of Standards applicable to the design and assessment of highway bridges in the United Kingdom. These documents are contained within the Design Manual for Roads andBridges. For completeness, the documents current at the time of writing are listed here; the Reader should check that he/she is using the current versions and any standards issued subsequently. The requirements of other bridge owners are also considered.
1.4.1 Highways Agency documentation
The documents listed below form part of the Design ManualforRoadsandBridges (DMRB) in Volumes 1 and 3. They are logged on website:-
http://www.standardsforhighways.co.uk/dmrb/index.htm, which should be consulted for revisions and updates. Throughout they are referenced as Highways Agency publications but in this context this normally includes the other overseeing departments - The Scottish Executive, The Welsh Assembly and The Department of the Environment for Northern Ireland. For details refer to the publications themselves.
Volume 1 Section 1 Approval Procedures BD2/05 Technical Approval of Highway Structures on Motorways and Other Trunk Roads Part 1 : General Procedures
Volume 3 Section 1 Inspection Part 2 BD54/93 Post-tensioned Concrete Bridges. Prioritisation of Special Inspections Part 3 BA50/93 Post-tensioned Concrete Bridges. Planning, Organisation and Methods for Carrying Out Special Inspections BD63/07 Inspection of Highway Structures Part 4
a
Section 2 Maintenance Part 1
BD62/07
As Built, Operational and Maintenance Records for Highway
Structures Section 3 Repair
Part 1
BA30/94
Strengthening of Concrete Highway Structures Using Externally Bonded Plates
Part 2
BA43/94 BD27/86 BA35/90
Part 3
BA83/02
Strengthening, Repair and Monitoring of Post-tensioned Concrete Bridge Decks Materials for the Repair of Concrete Highway Structures The Inspection and Repair of Concrete Highway Structures Cathodic Protection for use in Reinforced Concrete in Highway Structures
Section 4 Assessment Part 1
BD46/92
Technical Requirements for the Assessment and Strengthening Programme for Highway Structures. Stage 2 - Modern Short Span Bridges
Part 2
BD50/92
Technical Requirements for the Assessment and Strengthening Programme for Highway Structures. Stage 3 - Long Span Bridges
Part 3
BD21/01
Part 4
BA16/97
The Assessment of Highway Bridges and Structures The Assessment of Highway Bridges and Structures (Incorporating Amendment No. 1 dated November 1997 and Amendment No. 2 dated November 2001)
I
Part 5
BA38/93
Assessment of the Fatigue Life of Corroded or Damaged Reinforcing Bars
Part 6 Part 7
BA39/93
Assessment of Reinforced Concrete Half-joints The Assessment and Strengthening of Highway Bridge Supports
Part 8
BD48/93 BA54/94
Part 9
BA55/06
Part 10
BA52/94
Part 1
BD56/96
The Assessment of Steel Highway Bridges and Structures
Part 2 Part 3
BA56/96 BA51/95
The Assessment of Steel Highway Bridges and Structures The Assessment of Concrete Structures Affected by Steel Corrosion
Part 4
BD44/95
The Assessment of Concrete Highway Bridges and Structures
Part 5 Part 16
BA44/96 BD61/96
The Assessment of Concrete Highway Bridges and Structures The Assessment of Composite Concrete Highway Bridges and
Part 17
BA61/96
Structures The Assessment of Composite Concrete Highway Bridges and Structures
BD34/90
Technical Requirements for the Assessment and Strengthening
BA34/90
Programme for Highway Structures. Stage 1 - Older Short Span Bridges and Retaining Structures Technical Requirements for the Assessment and Strengthening Programme for Highway Structures. Stage 1 - Older Short Span Bridges and Retaining Structures
Load Testing for Bridge Assessment The Assessment of Bridge Substructures and Foundations, Retaining Walls and Buried Structures The Assessment of Concrete Structures Affected by Alkali Silica Reaction
9
1 Uhe assessment process
Part 18 BA79/06 Part 19
Part 20
1.4.2 Departures from Standards
The Management of Sub-Standard Highway Structures (Incorporating Amendment No. 1 dated August 2001) BD86/04 The Assessment of Highway Bridges & Structures For the Effects of Special Types General Orders (STGO) and Special Order (SO) Ve hicles BD81/02 Use of Compressive Membrane Action in Bridge Decks
The relevant Highway Authority’s procedure for agreeing structural ‘Departures from Standards’ and ‘Aspects not covered by Standards’ is an important part of the Technical Approval procedure but will vary between different Authorities. It is usual for the procedure to require the assessor/designer to recommend a Departure and, with the agreement of the checker, submit this with a justification to the Technical Approval Authority (TAA). The TAA is required to consider the proposal and should seek advice from an appropriate specialist, who has a particular expertise in the subject of the Departure. Usually, final approval and endorsement requires a positive recommendation from both the TAA and the relevant specialist. See Chapter 2 for further information on the Technical Approval process.
1.4.3Other bridge Owner8
This Guide is written primarily for highway bridges. They are equally applicable to motorway bridges and bridges carrying local roads. Therefore, there will be a large number of bridge owners involved besides the Highways Agency.
~
Another major national bridge owner, Network Rail, is responsible for a significant number of road over rail bridges although these are mostly of metal or masonry construction. Network Rail issues Railway Group Standards that are similar to, and make extensive reference to, Highways Agency Standards. Network Rail also issues a number of Current Information sheets, which effectively provide information on seeking Departures from Standards. There are a large number of autonomous local authorities, some of which have specialist bridge teams. The Bridges Group of the Engineering Committee of the CSS (formerly the County Surveyors Society) provides a network support group for local authority bridge owners. CSS Bridges Group also co-ordinates local authority input into Highways Agency Technical Project Boards. The Department for Transport (DfT) is embarking currently on a programme of providing national advice to local authority engineers - including those responsible for the bridge stock. This is particularly important as the HA Standards have generally been focussed towards the high-speed motorway and trunk road network. Normally, local authorities would follow Highways Agency Standards and Advice Notes, particularly with regard to assessment and design Standards. However, few Standards are written specifically with local roads in mind and fully justified Departures from Standards may be necessary, particularly for geometric problems.
10
1.5 Assessment and asset management
It has been acknowledged by the Highways Agency (HA) that the assessment of the bridge stock is likely t o become a ‘continuous’ process. This is so that bridge maintenance strategies can become safety related and based on whole life performance techniques rather than solely based on condition. It is interesting t o note that Network Rail has a domestic assessment programme based o n an 1 8 year cycle. This Section is based substantially on a paper entitled Whole life Performance BasedAssessment Rules BackgroundandPrinciples’ It provides useful background although some of the details have changed since publication of the paper.
’.
1.5.1 Whole life assessment
The procedures proposed by the Highways Agency are intended t o ensure that present levels of reliability o f existing bridge stock should be maintained at levels considered t o be socially and economically acceptable, without the economic consequences of undue conservatism. To provide marginally excessive reserves of safety in a design adds little t o first cost, but the provision of unnecessary margins in assessing existing bridges for increased loading or deterioration can be disproportionately expensive. For ease of use in conjunction with current practice it was proposed t o relate reliability t o the load capacity factor, K,on HA loading as defined in BD21, which a structure in service may be assessed t o achieve. That factor takes into account any deterioration in structural resistance. ‘Profiles’ of diminution of ‘available’ K factors with time may be derived from surveys of representative bridges and empirical predictions of deterioration. The procedure proposed by Flint and Das’ was developed into a draft Highways Agency Advice Note BA81 entitled Whole life assessment of highway bridges andstructures. This was trialled by a number of HA Maintaining Agents during 2000. The requirement was t o determine the performance level of the structure at the time of assessment, I,, and compare this t o the Critical Performance level, I,, ,. The present assessment process (BD21) is such that I , has t o be greater than I,, , t o achieve an assessment ‘pass’. If I , is less than I,, , then the structure has t o be managed in accordance with the advice given in BA79 (see Figure 1.I). The next stage in the development of any asset management plan is t o attempt t o determine when structures, where I , is greater than I,,,,, will reach a Critical Performance level. In order t o achieve this goal i t is necessary t o have an understanding of the deterioration rate. Unfortunately, published deterioration models need t o be treated with care and considerable judgement is required in their use.
1.5.2 Asset management
If I , and I,,,, can be reliably determined, together with a reasonable approximation of future deterioration, then it is possible t o evaluate various maintenance options using whole life costing techniques (see Figure 1.2). If the maintenance options for individual structures are combined in a comprehensive bridge management system then it is possible t o determine a strategy t o maximise the overall value of the asset.
11
No. of years from construction t Performance level a t time of construction I, Performance level of the structure being assessed a t time t I, lCri Critical performance level Whole life
- -
- ...
performance profile shown dashed \
\
- - - - - - - - - - > - - -\ \
0
t
No. of years from construction
Structure (a)
Option 1 2
Maintenancestrategy strengthen the structure now weight restrict structure/implement interim measures and monitor
Structure (b) Option 1 2 3
Maintenancestrategy strengthen the structure now undertake minor repairs to abate deteriorationhonitor the structure do minimum work now and replace at reduced life t,
0
t,
t2
No. of years from construction
The simplistic annual review of maintenance budgets is now outdated. Bridge owners need to be able to demonstrate the effects of various levels of funding on the value of the asset. The CSS produced a useful paper’ on recommended levels of maintenance spending and this approach is probably satisfactory for owners of small numbers of bridges. However, a substantial amount of work is now under way to link the production
12
of performance indicators, asset management plans and asset valuation models, and although the discussion of these topics is beyond the scope of this Guide, i t is clear that there will be a substantial role for effective bridge assessment techniques t o play in the overall process.
1.5.3COnSeqUenCeS for assessment
1.5.4A
COntinUOUS
a s s e s s m e n t programme
1.6 Assessment of the serviceability limit state
If comprehensive asset management plans are t o be developed based on whole life costing techniques and asset management plans, then it will be necessary t o accurately determine I,, I,,,, and deterioration rates. Use of this Guide should assist in this process.
Currently, the Highways Agency is considering proposals t o re-assess a proportion of its bridge stock every year t o ascertain their adequacy t o support imposed loads. Such reviews would be undertaken when significant events occur that could increase the imposed loads above those previously assessed and/or reduce the load-bearing capacity of structures. Typically, a re-assessment every 12 years might be anticipated. It will be for other users t o determine whether t o follow a similar approach.
An alternative approach t o the notion of a continuous assessment programme is the development of procedures for assessment at the serviceability limit state (sls). Although currently some checks are required at the sls, particularly when plastic methods of analysis are used (see Section 6.7),the UK approach t o assessment has safety and the ultimate limit state (uls) capacity of a structure as its focus. Proposed by Lark and Mawson’ 5! the aim of assessment at the sls would be t o create a more positive link between assessment procedures and the sls criteria that drive maintenance decisions, and t o provide a framework for recording and incorporating the information that has allowed engineers in the past t o assess a structure on the basis of engineering judgement. Although this concept is closer t o what is currently continental practice, there is evidence that the latter is moving more towards the contemporary UK approach of ultimate checks and so the demand for a serviceability approach has yet t o be proven. Nevertheless, it is also recognised in the UK that many of the structures that have been assessed as sub-standard have been so classified, n o t because of deterioration but because of the enhanced requirements of the technical standards. The response of Collins16 of the National Assembly of Wales has been therefore t o call for the development of asset management policies based on the whole life cost of maintaining the function of highway structures rather than just their strength, an approach that is more compatible with the concept of assessment at the sls. However, if such an approach is t o be used in the UK, further work is required t o support its general implementation and therefore this Section is provided for information rather than direct guidance, as is the case in Chapters 2 t o 12.
13
1.6.1 Serviceability assessment
The justification for proposing an alternative approach t o assessment based on serviceability limit state criteria is that certification of a structure today is no guarantee of its performance tomorrow. To address this, what has been suggested is a procedure in which the everyday response of a structure is monitored and then projected forward, and a ‘change management’ procedure is adopted whereby it is the variation of the reliability of the in-service structural and environmental response of the structure with time that is of interest. It has been shown that the reliability of a structure with respect t o an environmental
serviceability limit state, such as the onset of corrosion, can be expressed as a reliability index, the time-dependent variation of which can then be derived by assuming a deterioration model. This, typically, is a function of the chloride levels in the concrete’ The latter is a measurable parameter. Therefore by monitoring chloride levels, the likelihood of corrosion being a problem can be anticipated and, by comparing the development of this likelihood with time, with that predicted on the basis of the design parameters, a rational decision on the need for preventative action can be made.
’.
Likewise, but not yet explored t o the same extent, it is also possible to calculate the reliability of a bridge with respect t o structural serviceability limit states. The variation of this reliability with either the applied loads or the material resistance can be determined therefore, and if these in turn are related t o time, (for example due t o changes in loading regimes and the perceived likelihood of extreme loading events, or t o changes in material resistance due to the effects of creep, shrinkage and/or deterioration, etc.), then the variation of this reliability with either these changes themselves or with time can also be examined. As with the environmental example given above, the significant benefit of adopting a sls approach is that the analytical procedure and limit state models adopted in this procedure can be verified by monitoring the response of the structure. Then, if found t o be inappropriate, they can be modified t o reflect the actual behaviour of the structure and, when satisfactory, can be updated in response t o the past performance of the structure. In other words, the actual service of the structure can be used as a ‘proof’ load and by monitoring the structure’s behaviour the uncertainties associated with its assessment can be continually refined and reduced. The challenge of this approach will be t o identify appropriate criteria that can express capacity a t the sls. This is not something that will be easy to do with confidence, because existing definitions of sls behaviour are generally stiffness related whereas uls behaviour is strength related and currently there are no uls criteria for effects such as those due t o shear. Nevertheless, what is being proposed is a procedure which seeks to make use of an understanding of the reliability of bridge structures applied in such a way that it can support bridge managers’ engineering judgement of the response of their stock, and can be enhanced by both traditional and future bridge monitoring techniques. For example, strain, deflection and/or the nature of cracking patterns may be used currently to characterise the response of a structure to both loads and movements from which its structural reliability can be identified. Likewise chloride levels, moisture content, porosity and degree of corrosion, etc. are measures of environmental reliability. In the future therefore, it can easily be envisaged that defect, vibrational and other NDT monitoring techniques may also provide suitable methods for gauging the ‘in-service’ reliability of a structure.
14
i
Whichever ‘measure’ is adopted, it is the variation of the resulting reliability from that anticipated a t the time of design which is significant; with improvements representing an enhanced understanding of the structure’s behaviour and reductions typically being due to changes in loading and deterioration, although they can also be due to changes in understanding of the performance of the structure. What matters is that these variations in reliability are managed. Thus, if the in-service reliability of a structure is reduced because of deterioration or an increase in loading, the question that must be answered is can it be restored through a knowledge of the actual behaviour of the bridge or by tighter control of the traffic using it, or are remedial works required. Unlike assessment a t the uls, the adequacy of the response to this question can be monitored and the bridge manager can be assured that a margin of safety against total collapse still exists. The suggestion is that this provides an opportunity to manage the risk posed by the structure and, in each period, to address the changes which have occurred in both the response and loading of the structure, whether due to traffic, environmental or deterioration effects; to assess the consequences of these changes and, on the basis of this assessment, to implement the necessary monitoring, controls or remedial action. Further research is required to identify exactly how the above will be achieved and the data that is needed to implement it.
1.7 Historical aspects O f assessment
This Section is a summary of a paper entitled A review ofbridge assessments through time, presented by Barry Mawson, Gwent Consultancy, to the South Wales branch of the Institution of Highways and Transportation on 10 February 2000, and has been reproduced by kind permission of the author. To review developments in bridge assessment it is necessary to consider the history o vehicular loading and the link with design standards. The French appear to have been the first to produce a design loading code for highway bridges in about 1850. In 1894 the Dutch produced a similar loading standard consisting of two alternative carts, a lightweight and a heavyweight cart, both drawn by a single heavy horse. Nineteenth century bridge designers in the UK were still allowed some freedom. A uniformly distributed load of Icwtlft’ together with the local effect of a 10 tonne wheel load was commonly applied. A typical British bridge design towards the end of the 19th century would accommodate a single 30 tonne vehicle on four wheels. In 1922 the Ministry of Transport produced its first standard load train for highway
bridges. It consisted of an engine pulling three heavy trailers. In 1931 the Ministry of Transport devised a standard loading for highway bridges, referred to as the equivalent loading curve. For any particular loaded length, this curve represented an equivalent uniformly distributed load, which, when combined with a specific point load located within the loaded length, would produce the same bending moment and shear force as the original loading train. This simplified quick analysis of smaller and standard bridges in the design process.
15
The assessment process
Axle weights and gross vehicle weights continued to increase and by 1973 a 10 tonne
axle and 32 tonne maximum gross vehicle weight was permitted. The Ministry of Transport published a loading standard BE1/77, which was in parallel with the revised loading standard B S I 53. The loading consisted of two types, HA loading which was similar to the earlier equivalent loading curve, and an additional HB loading model to represent abnormal vehicles. Overloading of vehicles tended to increase, traffic densities increased, and in May 1983 gross vehicle weights were permitted under ‘Construction and Use Regulations’ to increase from 32.5 tonnes to 38 tonnes provided they were on 5 or 6 axles. Then EC Directive 85/3 required a minimum standard to be allowed in all EC countries, i.e. 40 tonnes on 5 axles, and that axle weights should be permitted to a level of 11.5 tonnes. A derogation allowed the UK to delay implementation until 1 January 1999. In 1984 the Department of Transport embarked on a 15 year programme for rehabilitation of highway structures. The assessment programme was organised in three
stages. Stage 1 covered older short span bridges which were not designed for, or known to be designed for, present day loading requirements, i.e. pre-1922. Stage 2 of the programme covered modern short span bridges designed prior to changes in design rules introduced in 1973, notably for shear in concrete. Stage 3 included long span bridges where increase in long length loading was important, due to the effects of a larger HCV proportion in traffic queues. The history of bridge assessment goes back well into the 19th century for railway bridges. The London North Western Railway commenced a programme of assessment in the 1880s. This was followed by a more general assessment of railway bridges in the 1920s and led to the Bridge Stress Committee during the period 1923 to 1928, which specifically covered the dynamic effects on railway bridges. The need to assess road bridges was first properly considered in the 1960s with the move towards the 32 tonne vehicle. This was implemented with the Assessment Standard BE4 and was aimed a t the pre-1922 bridge stock. The further increase in lorry weight to 38t was again covered in 1984 by the Department of Transport Standard BD21/84. This was amended with the introduction of the potential for the 40t lorry in 1987 and consolidated into the standard in 1993 (BD21/93 - now BD21/01). A considerable number of highway bridges were in the ownership of the railways and their duty was limited to a design capacity of ‘the traffic of the day’. This was defined clearly in the Statutory Instrument 1705 in 1972 which set the railway’s loading obligation as that defined by assessment standard BE4. No allowance was made for the reference in BE4 to the fact that it covered bridges:
0 Not designed to HA loading and so considered as sub-standard and having a limited life. 0 That their replacement should not be unduly delayed. 0 That structures should be critically examined a t such intervals as their conditions require but a t least once every three years.
16
The standard loading curves used in assessment had been developed from the design standard and allowed for the likelihood of impact, bunching and potential overload of vehicles. Research showed that this was unduly conservative for assessment and the introduction of assessment standard BD21/97 gave reductions of up to 35% of load effect. Following the new vehicular standards of January 1999, modifications required to the assessment standard were introduced to cover the weight limits of 18 tonnes to 26 tonnes, which allow a 11.5 tonne axle. Further changes may be necessary if the general use of 44 tonne, or heavier, vehicles is permitted.
1.8 European develop ment s
It is beneficial to study the process of bridge assessments overseas, particularly in Europe. A European research project, known as Bridge Management in Europe (BRIME), has been
undertaken to develop an outline framework for a bridge management system for the European road network. This Section summarises five papers, which were presented at the Fourth International Conference on Bridge Management a t the University of Surrey in April 2000. The original papers are detailed in the References below.
1.8.1 The BRIME project
The project was co-ordinated by the Transport Research Laboratory in the UK in collaboration with five European partners. The project was divided into eight work packages as follows: 1. Classifying the condition of a structure. 2. Assessing the load carrying capacity of existing bridges, including the use of risk-
based methods. 3. Modelling of deteriorated structures and affect of deterioration on load carrying capacity. 4. Modelling of deterioration rates. 5. Deciding whether a sub-standard or deteriorated structure should be repaired, strengthened or replaced. 6. Prioritising bridges in terms of their need or repair, rehabilitation or improvement. 7. Reviewing systems for bridge management and development of a framework for a bridge management system. 8. Project co-ordination. The deliverables from the BRIME project may be viewed on the TRL website, a t http://www.trl.co.uk/brime.This Section summarises the contents of work packages 1 to 5 1 8 ' 0 ' 1 2
1.8.2 Structural assessment
In most of the partner countries, assessment is only carried out on specific structures when there is a need to carry an exceptional load or where the bridge has been subjected to deterioration, mechanical damage, repair or change of use. The UK is
17
unusual in that it has carried out a comprehensive programme of assessment where bridges have been built t o outmoded design standards and have not been assessed t o current standards. The rules used in bridge assessment are provided mainly by design standards with additional standards relating t o testing methods including load testing. The UK is unusual in that it has a comprehensive set of assessment standards. Generally, limit state methods are used, although Germany still uses allowable stress techniques.
1.8.3 Deterioration
The current practice is for all of the partner countries t o take account of deterioration in some way. Except for the UK, taking account of deterioration is, in general, carried out on an adhoc basis and depends on the knowledge and experience of the assessment engineer. The UK documentation tends t o be general in nature and contains little quantitative guidance. At present there are few procedures available for taking deterioration into account in a structural assessment.
1.8.4 Chloride ingress
In order t o estimate the remaining service life of a bridge, it is necessary t o predict deterioration rates once the existing condition of the bridge has been assessed. The objective of this work package was t o consolidate and improve existing knowledge of chloride penetration in concrete, which is the principle mechanism of deterioration. The work package was not complete a t the time the paper was written. However the initial conclusion was that the technique was not suitable for general use and that only experienced engineers or corrosion experts should be allowed t o act upon the results generated by a chloride ingress model. The authors stated that although chloride ingress models could be used for determining future maintenance, they should not be used for assessing structural capacity or deterioration.
I .9References
1.1
PARSONSBRINCKERHOFF, TechnicaiAuditoftbeappiication ofBA79 and A rewewofbridgeassessmentfailureson the Motorway and TrunkRoadnetwork, Final Reports HBR80616 - Highways Agency Contract 2/419, Parsons Brinckerhoff, December 2003
1.2 CONCRETE BRIDGE DEVELOPMENT GROUP, ibeassessment ofconcrete bridges, 1, CBDG, Camberley, 1997 1.3 FLINT, AR and DAS, PC. Whole life performance based assessment rules - Background and principles, SafetyofBndges Conference, Thomas Telford, London, 1996.
1.4 CSS, FundingforBridge Maintenance, CSS Bridges Croup, February 2000. 1.5 LARK, RJ and MAWSON, BR, Assessment at the serviceability limit state. BridgeManagement 4, (Ryall, MJ, Parke, CAR and Harding, JE, eds), Thomas Telford, 2000, pp 426-433.
1.6 COLLINS, TJA, "Highway Function" approach t o bridge maintenance and a more flexible approach t o design loading and dimensional requirements, Surveyors'Annuai Bridge Conference, March 2002
1.7 CEHLEN, C and SCHIESSL, P, Probability-based durability design for the Western Scheldt Tunnel, Structura,'Concrete, Vol P1, No 2, 2000. References 1 8 to 1 12 below were all in BndgeManagementl (Ryall, MJ, Parke, CAR and Harding, JE, eds), published by Thomas Telford Ltd in 2000.
1.8 WOODWARD, RJ, VASSIE, PR and CODART, MB, Bridge Management in Europe (BRIME) overwew of project and review of bridge management systems.
1.9 BLANKROLL, A eta/, BRIME -Chloride ingress and bridge management 1.10 KASCHNER, R, HAARDT, P, CREMONA, C, CULLINCTON, D and DALY, AF, Bridge Management in Europe (BRIME) structural assessment
1.11 DALY, AF, Bridge Management in Europe (BRIME). modelling of deteriorated structures 1.12 BERC, L eta/, Bridge Management in Europe (BRIME) condition assessment of bridge structures
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2. Technical approval and documentation 2.1 h l e r a l procedures
This Chapter is intended t o introduce the Technical Approval procedure for the assessment, design of strengthening and refurbishment of highway structures t o engineers with little previous experience of the subject. It includes: 0 General principles of the procedure
0 An explanation of its objectives 0 A description of the key stages of the procedure 0 Flow charts and figures.
The Chapter is not intended t o give the full details of every aspect of the procedure. Nor can i t cover in detail how i t is implemented by different Highway Authorities, and the many different types of contract and procurement methods currently in use. However, i t should give assessing and checking engineers basic information about the procedure and, most importantly, explain the need t o involve the Technical Approval Authority (TAA) right from the start of the assessment process. The details included refer specifically t o the Highways Agency procedures. Local Authority and other bridge owners (e.g. British Waterways and Network Rail) have similar Technical Approval procedures using the principles given in this Section. One key difference is that normally the TAA function is not separated from the in-house engineers.
2.2 General principles
The Technical Approval procedure is a form of Quality Assurance for Highway Authorities and the owners of highway structures used by the general public. The procedure was first introduced into the UK following a number of bridge failures in the early 1970s. These failures were blamed in part on the use of inappropriate design Standards and also because designers of the time were adapting the Standards for use on structures that were outside the limits for which they could be used safely. The procedure was introduced following an official inquiry into these failures and has been modified and adapted over time. The procedure is a mandatory requirement for all Department for Transport (DfT) funded highway structural design work of any significance* '. This includes designs for new bridges and other highway structures, bridge assessments and structural modifications, refurbishments and repairs; the standard procedure for the assessment and strengthening of highway structures and its relationship with the design approval procedure is illustrated in Figure 2.1. The procedure is also applied t o any temporary structural works on the highway that could affect the safety of the public. Some form of Technical Approval or Appraisal Procedure is a requirement of all other Highway Authorities and rail structure owners in the United Kingdom. The detailed requirements of the procedure in use by the Highways Agency are set out in the standard BD2/02 (see Section 1.4). Technical Approval of highway structures is an ongoing process, which starts at the concept or feasibility stage. It continues through design, construction and on through the
19
Design standards
Assessment standards
Departures from standard to be used in assessment agreed with TAA
E
i
Assessment report Non-compliance with standard revealed in assessment
Departures to be retained in the structure agreed with Highway Authority
1
I Strengthening/ refurbishment AIP
Implement agreed action refer to BA 79/98 Management of Substandard Structures
I
a
I
I
Departuresfrom Standard (ULS & SLS) proposed for strengthening / refurbishment works agreed with Highways Authority
New works to current design standards plus ULS departures in existing elements agreed with the Highway Authority
I
1
Strengthening / refurbishment design
1
-.. .. +-.
nt
Figure 2.1 Standardsfor assessment and strengthening of highway structures.
-
life of the structure, including steady-state assessment and any strengthening, repairs or modifications. The process is only concluded finally with the demolition of the structure. The various documents, which are a product of the process throughout the structure’s life, form a very important part of the structure records and should be carefully retained.
2.3 Objectives
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The original, and still the primary, objective of the procedure is to ensure the safety of highway structures by ensuring adequacy of structural design, assessment, construction and of steady state maintenance. Recent CDM legislation and court findings have made it even more important for Highway Authorities and other structure owners to maintain a robust procedure for ensuring structural safety.
The procedure for assessment has had to be developed and extended to incorporate resulting strengthening, modification and refurbishment of the structures. Highway Authorities and bridge owners have introduced additional objectives with the aim of ensuring good value for money, minimising environmental impact, improving sustainability and aesthetics. The recent introduction of new forms of contract for the maintenance and procurement of highway works has led to the need to further modify how the procedures are applied.
2.4 Key Stages
The detailed application of the Technical Approval (TA) procedures for the assessment of highway structures depends on the particular requirements of the Highway Authority or structure owner. However, this does not alter the general principles of the procedure, which can be adapted to apply, in most instances, to all forms of structure. Technical Approval or TA should start a t the concept planning or feasibility stage of any assessment, strengthening, modification or refurbishment work involving highway structures. The earlier the assessment engineer involves the Technical Approval Authority (TAA), usually the better. The procedure requires that the TAA have a responsibility to ensure that only competent and experienced engineers carry out the assessment of highway structures and the check of the structural work. The precise nature of the check and the relationship between assessor and checker is dependent on the category of the structure. As soon as an assessor is designated or appointed, it is usual for discussions to take place between them and the TAA on the various assumptions to be made and options available for inspection and testing for the assessment. The assessor will also agree the category of the structure with the TAA, based on the complexity, size, value or risk of the project. BD2 defines four categories of structure: 0, 1, 2 or 3
0 Category 0 structures are typically buried structures of less than 3 m clear span and having more than 1 m cover, multi-cell buried structures where the cumulative span is less than 5 m and having more than 1 m cover or single span simply supported structures with a span less than 5 m. 0 Category 1 structures are those with a single simply supported span of less than 20 m and having less than 25" skew or buried concrete box structures with a clear span of less than 8 m. 0 Category 2 structures are those that are not otherwise defined as Category 0, 1 or 3. 0 Category 3 structures are typically complex structures, which require sophisticated analysis or with features such as high structural redundancy, unconventional, novel or esoteric design aspects, a span exceeding 50 m, skew exceeding 45", moveable bridges, bridges with suspension systems, post-tensioned concrete structures with internal grouted tendons.
21
2
However, BD2 does note that the category boundaries are not rigid. If in doubt, the assessor is required t o make the decision in consultation with the TAA having regard t o the wider issues of the value of the structure and the potential consequences of failure. In the case of Category 3 structures, an independent consultant is required t o carry out a check of the assessment. For Category 2 structures, this check may be carried out by a checking team, which may be from the same organisation but must be independent of the assessment team, while for Category 0 and 1 structures, although an independent check is still required, it may simply be carried out by another engineer from the original assessment team. The procedure for obtaining Technical Approval for an assessment is given in figure 2.2 and can be described as follows. The assessor completes an Approval in Principle document, which is generally called an AIP, or Technical Appraisal Form (TAF) depending on the procurement route being adopted, the latter typically being more used on design and build or related projects. This sets out the technical proposals for the assessment or design of strengthening and includes all the structure geometry, ground conditions and nature of the existing foundations. It also includes assumptions on material properties or proposed testing, methods of analysis and assessment loading criteria, etc. Any non-compliances or ‘Departures from Standard’ or ‘Aspects not covered by Standards’, which are being proposed at this stage should be identified and recorded on the AIP. Advice should be sought from the Highways Agency when considering Departures from their Standards or Codes of Practice. Usually, it will be necessary for the assessor t o discuss the proposed ‘Departures’ with the Highway Authority designated representative and/or TAA, who normally are required t o accept any such proposals. A description of the usual procedure for agreeing ‘Departures’ and ‘Aspects not covered by Standards’ is given below. A list of relevant approved Standards that are t o be used in the assessment is attached in the form of the Technical Approval Schedule (TAS). It is important to note that the AIP or TAF is a ‘living’ document subject t o revision and amendment during the assessment or design process. These are important record documents, which need t o be retained for future reference. The signed AIP is submitted t o the TAA for acceptance and to the checker for comment and agreement. The significance of a personal signature by the individual design or assessment team leader is to ensure ‘ownership’ and responsibility for the accuracy of the information in the AIP. Acceptance by the TAA will be dependent on their confidence in the assumptions, the method of analysis and material properties t o be adopted in the assessment or design. Usually, the TAA will be required t o seek approval for any ‘Departures’ from the Highway Authority’s designated representative if distinct from the TAA a t this stage. The detailed calculations required for the assessment or the design of the structure can then start, followed shortly by the check. At any stage the assessor/designer, with the agreement of the checker, may consider that further ‘Departures from Standards’ could safely be applied t o the particular structure t o improve the assessment. The introduction of any material changes or the introduction of further ‘Departures’ would require an
22
-----
Figure 2.2 Technical approval procedure for assessment.
4-
Assessment AIP submitted toTAA
I0
I
Proposed departures from standards* agreed by Highway Authority’s designated representative? Initial or higher level of assessment agreed?
I
1 I
L L TAA sign AIP/Addendum
Addendum t o AIP identify departures and/or higher level of assessment
Carry out assessment and check
tyes
Amendment t o AIP identified? Assessment required at next level? no Acceptable non-compliances with standards revealed by assessment?
Assessment and check certificates
Assessment and check certificates with reference t o assessment report (where non-compliances with standards revealed by the assessment are recorded)
+
+
Accepted criteria for departures from standard* entered in the AIP with an addendum endorsed by the TAA and the Highway Authority’s designated representative (departures and endorsement could be recorded on certificates i f required by the Highway Authority)
+ I
Assessment and check certificates accepted by TAA
TAA return a facsimile of the assessment and check certificates and retain the originals
and aspects not covered by standards
addendum to the AIP, recording all the changes to the original. This applies equally to any significant changes that might occur during the construction of strengthening works, etc. The approval procedure for strengthening and improvement works is similar to that above and is detailed in Figure 2.3. It is quite common for it to become necessary for a strengthening design to be modified during construction. This can result from a number of causes; the most common being unforeseen conditions necessitating a change to the
23
Non-compliance with standards* revealed by assessment of the existing structure?
1
.
Engineer considers each non-compliance with standards* and proposes whether it can be retained in the structure, or upgrading (to current design standards) t o be carried out
no
I
I
I
1
4-
I
Non-compliance with standards* revealed by the assessment t o be retained in the structure agreed by the Highway Authority’s designated representative?
I
I
Strengthening / improvement AIP
I Proposed departures from standards* t o be used in the design agreed by the Highway Authority
TAA sign AIP
Carry out the design and check
Design and check certificates
Accepted criteria for departures from standards* entered on the assessment and check certificates and endorsed by the Highway Authority’s designated representative
I Assessment and check certificates accepted by the TAA
I TAA return a facsimile of the assessment and check certificates and retain the originals for their records
24
proposed strengthening details. It is most important that any changes affecting the structure are subjected t o the same rigorous examination of the TA procedure as the original proposals. The final part of this stage of the TA procedure is the submission by the assessor/ designer and checker of the design and check certificates and their acceptance by the TAA. In the case of an assessment the assessor would normally submit a detailed Assessment Report, which will include a comprehensive extract of the Inspection Report and any test results. The Report will contain full details of the assessment findings and the assessed capacity of the structure. The assessment certificate and assessment check certificate should state the assessed capacity of the structure. As with the AIP and reports, these are important documents and need t o be carefully retained with the structure records.
2.5 'Departures' and 'Aspects not covered by Standards'
It has been suggested that the application of the TA procedure will inevitably stifle innovation. This can happen with the rigid application or misinterpretation of assessment Standards. In contrast, careful application of the procedures by experienced bridge engineers has been demonstrated t o encourage innovation by the use of 'Departures' or 'Aspects n o t covered by Standards'. The acceptance of non-compliances t o Standards, which can be retained safely, has been shown t o avoid unnecessary restrictions and expensive strengthening. It should be noted that most Highway Authority bridge engineers, who are designated as or by the TAA, are involved with the approval of the assessment of many structures. They are very well placed therefore t o promulgate best practice and safely consider 'Departures from Standards'. The full effect of any 'Departure from Standard' or 'Aspect not covered by Standards' is considered and formally notified to, and has t o be accepted by, the TAA. This includes any 'Departures', which can be justified by the use of higher levels of assessment. The procedure for the management of standards is detailed in Figure 2.1. The full effect of any 'Departure from Standard' or 'Aspect not covered by Standards' has t o be considered by the TAA including any 'Departures' arising from the adoption of different levels of assessment. The Highways Agency's Advice Note BA79/98 sets out the philosophical approach t o assessment and the principle of different levels of assessment. It identifies where 'Departures' are required for the higher, more complex levels of assessment. The assessment process can be carried out in 5 distinct levels with Level 1 being the simplest t o Level 5, the most sophisticated:
0 Level 1 assessments are carried out using simple analysis methods, in accordance with the assessment Standard BD2l and the accompanying Advice Note BA1 6. 0 Level 2 assessments involve more refined analysis and the use of characteristic strengths of materials. 0 Level 3 introduces the concept of Bridge Specific Assessment Live Loading and makes use of material test results t o determine worst credible strength values. 0 Level 4 assessments include a reduction in partial safety factors if these can be justified particularly where dead load or superimposed dead loads are high. 0 Level 5 assessments involve complex reliability analysis for particular structures.
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The relevant Highway Authority’s procedure for agreeing structural ‘Departures from Standard’ and ‘Aspects not covered by Standards’ is an important part o f the TA procedure but will vary between different authorities. It is usual for the procedure t o require the assessor/designer t o recommend a departure and, with the agreement of the checker, submit this with a justification t o the TAA. The TAA is required t o consider the proposal and should seek advice from an appropriate specialist, who has a particular expertise in the subject of the departure. Final approval and endorsement usually requires a positive recommendation from both the TAA and the relevant specialist. In the case of most Highway Authorities, the designated representative usually only approves ‘Departures’ and ‘Aspects not covered by Standards’. It is the responsibility of the TAA t o resolve any differences of opinion between the assessor and checker and this includes the use o f specific ‘Departures’ or ‘Aspects not covered by Standards’. This is particularly important in relation t o any ‘Departures from Standard’, which may be accepted by the Highway Authority, as all parties are required t o agree.
2.6 Reference
26
2.1
HIGHWAYS AGENCY, Highways structures Technical Approval, Easy Guide, Advice and Technical Appraisals Group, May 2002
3. Inspection for assessment 3.1 Process and general procedures
3.1 . I Background and introduction
Inspection is the first step generally in the assessment process, with the double objectives of establishing the condition and dimensions of the structural components and of verifying the form of construction. The extent of the work will depend on what is already known from archive and maintenance records, and therefore physical inspection is integrated with desktop studies. It may also be necessary to carry out in-situ and laboratory testing to establishkonfirm assessment parameters, and this is covered in Chapter 4. Even a t this early stage of the process, it must be remembered that the purpose of assessment is t o evaluate current and future structural capacity. Generally, this is done via analytical methods (Chapter 6), while taking advantage of any hidden strengths (Chapter 7). A major objective of inspection and testing is therefore t o determine input parameters for this analysis, particularly with regard t o geometrical and mechanical properties; where these properties are affected by different forms of deterioration, then the guidance given in Chapters 3 and 4 is supplemented by that in Chapter 9. For concrete bridges, the process and general procedures involved are those given in relevant Standards and Advice Notes (see Section 1.4), with BD21, BA50, BD and BA44, BD and BA63 being particularly relevant. Chapter 3 follows these procedures, although it should also be recognised that the situation is undergoing continuous change, as represented by the references at the end of this Chapter. This Chapter therefore deals with inspection for assessment, the management of the inspection process and the application of inspection results t o both one-off and continuous assessments. Section 3.2 covers condition assessment and specific test methods and techniques are discussed in more detail in Chapter 4. The concept of inspection, or in an ongoing context, condition monitoring, is to make best use of past, present and future performance-based data and observations to verify the current integrity, and more ambitiously t o predict the future performance, of a structure.
3.1.2 Requirements for inspection
BD21 states that inspection for assessment: . . is necessary to verfy the form of construction, the dimensions of the structure and the nature and condition of the structural components. Inspection should cover not only the condition of individual components but also the condition of the structure as an entity and especially noting any signs of distress andits cause.” ‘I.
The standard also gives specific requirements for Inspection for Loading and for Inspection for Resistance.
27
BD2l refers t o Volume 3, Section 1 of the Design Manua[forRoadsandBridges, which essentially means BD63 and BA63 for concrete bridges. BD63 defines the following four main categories of inspection: 1. Superficial involving a cursory check for obvious deficiencies. 2 . General involving a visual inspection of representative parts of the structure t o
ascertain its condition and note any items requiring special attention. 3. Principal involving a close examination of all parts of the bridge at six-yearly, or
exceptionally up to ten-yearly, intervals. Originally, Principal Inspections solely comprised visual examinations, but more recently limited testing has also been included. 4. Special Inspections involving a close inspection or testing of a particular area or defect. They are carried out for specific reasons such as: following up on a defect identified during an earlier inspection; checking for any effects caused by particular events (e.g. flooding, vehicle impact, etc.); and before and after the passage of abnormal loads. BD2l notes that General Inspections are unlikely t o be adequate for assessment purposes and also states that “all constituent parts ofthe superstructure shall be inspected to determine their respective strengths”. As a consequence, inspections for assessment are normally carried out in conjunction with Principal Inspections, but may also involve
Special Inspections. With regard t o substructures, foundations, retaining walls and wing walls, BD2l requires that all accessible parts be examined and any defects noted. This should also take into account any evidence of ground movements around foundations and behind abutments and wing walls. Underwater inspection of any submerged parts of structures should also be carried out. Special Inspections are also required for post-tensioned concrete bridges. Unusually these involve more invasive inspection techniques, the main aim of which is t o identify corrosion or the propensity for corrosion of the prestressing tendons. Details for the planning, organisation and methods for carrying out such inspections are given in BASOl93.
3.1.3 Future requirements for inspection
The Highways Agency proposed the introduction of new procedures for bridge inspections, aimed at those aspects of a structure that are most relevant t o its load carrying capacity and durability. The objective of these proposals was to make more effective use of testing, monitoring and the growing range of improved non-destructive testing techniques, and t o improve the quality and consistency of both the inspections themselves and the way in which they were reported. This was t o be done by combining regular visual inspections of the whole structure with a programme of more detailed investigations concentrating on known or suspected areas of deterioration. For each structure, a unique schedule was t o be developed that was specifically designed t o provide the information needed t o assess the structure adequately. However, these proposals have not received universal support and discussions are currently ongoing as t o how t o ensure that this information is acquired.
28
To provide a starting point for assessment, all structures should be subject t o a detailed inspection, the purpose of which should be t o establish precisely the condition of each structure a t the time of its assessment and t o provide a reference against which any future deterioration of the structure can be monitored. Such inspections should record the location of all imperfections and defects on scale drawings complemented with photographs and text. It is anticipated that regular inspections should then be carried out a t programmed intervals, concentrating on the condition of particular parts of the structure where deterioration has been found before or where it is suspected.
3.1.4 The inSpeCtiOtl process
The inspection process starts with the Client’s Brief and should include some or all of the following activities:
0 Review archive and maintenance records for pertinent information (e.g.as-built drawings, soils data, previous inspection reports, Health and Safety File, etc.). 0 A preliminary inspection t o determine access requirements and strategy, particularly in respect of critical elements. 0 Organise traffic management, access equipment, inspection, testing and diving teams, as appropriate. U Carry out element and segmental inspections, recording main dimensions, carriageway and soffit levels (if possible), confirm structural section sizes and condition. 0 Record width, length, location, orientation and pattern of all cracking; size, depth and location of spalled areas; size and location of areas of suspected delamination; size and location of areas of scaling; size, location and description of deposits and staining, especially rust staining; loss of section of any exposed reinforcement; and all other defects and damage. 0 Excavate trial holes t o expose buried members where doubt exists with regard t o detail, dimensions, condition, etc. 0 Carry out in-situ testing and sampling as required (see Section 4.3), implement condition monitoring if appropriate. 0 Record all information relating t o the inspection on sketches, drawings and in photographs, identify areas of specific concern and implement Special and Particular Inspections. 0 Receive, assess and report laboratory and ongoing monitoring results. 0 Establish schedules for implementing and reporting condition monitoring, Particular Inspections and ongoing assessments. Summarise all of the findings from the inspection for assessment in a report which will form the basis for the assessment itself. U Consider a post-assessment inspection, concentrating on those areas where cracking/defects would occur under the serviceability limit state (see Section 6.7).
29
~~~
3 Onopeaion for assessment
3.1.5 Risk assessment and safety issues
The main Health and Safety Issues to be considered in risk assessments for bridge inspections are: U Working a t height 0 Working in confined spaces
0 Working adjacent to traffic 0 Working with and adjacent to 'live' services 0 Working from a boat 0 Working on a railway U Working a t night 0 Working near water (leptospirosis) 0 Use of mechanical access equipment 0 Use of small tools 0 Fire 0 Presence of asbestos3' - (movement joints, waterproofing, pipework, permanent formwork, etc.) U Lead paint. This list is not exhaustive and risk assessments should be carried out for each and every bridge site. For additional advice see HSE best practice In particular where destructive testing is being used the specific and special risks associated with such procedures must be considered.
3.2 Condition assessment
3.2.1 BD21 approach
The actual condition of a concrete structure a t the time of the inspection for assessment could have a major influence on the assessed capacity of the structure and therefore must be taken into account in that assessment.
BD2l states that the assessment resistance shall be determined from the calculated
resistance multiplied by an overall condition factor. It further states that the condition factor, determined on the basis of engineering judgement, shall represent an estimate of any deficiency in the integrity of the structure and may relate to a member, a part of a structure or the structure as a whole. This approach is considered inappropriate for concrete bridges.
3.2.2 CSS bridge condition indicator
30
CSS Bridges Group concluded that to assist the maintenance management of a stock of bridges it is essential to have a 'Condition Indicator' which can be used to determine whether the overall condition is deteriorating over time or not. Their view is that such trends will indicate whether adequate funding is being provided for structures' maintenance work. It was also recommended that all authorities should use a single system in order to ensure consistency and credibility. Therefore, CSS commissioned the development of two guidance documents: A review carried out by the
1. A guidance note on the inspection of highway bridges33. 2. A guidance note on the evaluation of a Bridge Condition Indicator based on element condition data collected during these inspection^^^.
The CSS inspection system is based on a fixed list of 38 elements that occur on highway bridges and a pro forma containing these elements and other data collection requirements has been produced. If an element on the list is present on the bridge then the inspector must record its condition. The condition is recorded as a combination of the Severity and Extent of the defects on the element. The Severity scale ranges from 1 (no defect) t o 5 (failed) and the Extent scale from A (no extent) t o E (extensive). The inspection document provides sound guidance on the classification of defects, with tables and photographs of defects t o assist the inspector. The severity/extent ratings provided by the inspections are used t o evaluate the Bridge Condition Indicator (BCI) score. The BCI score is built up as follows: 1. The element severity/extent alphanumeric ratings are translated to an Element
Condition Score which has a scale of 1 (best condition) to 5 (worst condition). 2. The Element Condition Score is translated to an Element Condition Index, still on the 1 t o 5 scale, which takes into account the importance of the element. (N.B.All 38 elements on the CSS pro forma are classified as being of very high, high, medium or low importance.) 3. The weighted average of all the Element Condition Indices on a bridge gives the Bridge Condition Score, where 5 is the worst possible score and 1 is the best possible score for a bridge. 4. The Bridge Condition Score is translated t o the BCI, the BCI is on a 0 (worst condition) t o 100 (best condition) scale for ease of external reporting. 5. The weighted average of the BCls for a stock gives the Bridge Stock Condition Index,
retaining the 0 t o 100 scale. (N.B. The individual BCls are weighted by deck area.) 6. Guidance is provided on how t o interpret and present the BCI data. The BCI does not represent the true functionality/safety requirements of a bridge, i.e. t o safely carry the required service loads, and as such should not be used in isolation. The BCI is just one component of a full set of Asset Management tools currently being developed by the CSS and Highways Agency in partnership for highway bridges, which includes Reliability and Availability Indicators and Asset Valuation procedures. The BCI will, in the long term, be used in the Asset Management of highway structures for setting targets and the prioritisation of maintenance work. However, in the interim it is being used as a high level management tool t o monitor the trend of bridge condition over time.
3.3 Network Rail's Structures Condition Marking Index (SCMI)
Network Rail's Structures Condition Marking Index (SCM1)34was developed by external consultants for Network Rail as a means of providing a more objective measure of recording the condition of its structures. The scoring takes place during each detailed examination, which occurs every six years. The scoring system and associated algorithm are currently in use for bridges but have also been developed for other structural assets, such as tunnels and retaining walls.
31
The system requires on the first visit the key components of the structure to be identified and the defects associated with that component to be scored based upon their extent and severity. At the following detailed examinations only the scoring of defects is required. The extent and severity scores are material specific. Each structure (or each span in multi-span structures) is scored out of 100, with 100 being perfect condition. The average score for the whole population scored is published by the company in its annual Business Plan and is a requirement of the Office of the Rail Regulator. Extensive trials were undertaken with bridge engineers and examiners to validate the output of the system to ensure that the score accurately reflects the condition of the structure. It does not give a direct indication of serviceability as no account is taken of the assessed loading capacity. The system was not developed to manage directly either safety or work prioritisation, although clearly condition influences decisions in both these areas. The scores are to provide Network Rail with information on the condition of its bridge stock or other asset type primarily nationally, regionally, by route and material type. This information can then used to assist in determining national policies on the management of structures and provide one of a number of measures input into tools used to manage work prioritisation, future funding and route upgrade policies.
3.4 Other work
The BRIME project35identified that in Europe generally current methods of condition assessment use two different approaches for both the assessment of individual elements and of the structure as a whole. The first uses a cumulative condition rating which is derived from the condition of individual elements, while the second uses the condition rating of the bridge element in the worst condition as the condition rating of the structure itself. The condition assessment is based on bridge inspection and similar procedures, although with different intervals between inspections, being used in different countries. An alternative procedure is to use damage classification systems36.More recently, this approach has been developed as part of the CONTECVET project37and the findings have been implemented by the British Cement Association as part of a structures management package for the Hong Kong Buildings Department. The BRIME project also investigated the use of damage categorisation models as a basis for an expert system to assess damage and deterioration, although they concluded that: research is needed to improve the methods developedfor categorising damagedareas, , , this requires the development of a larger database and the addition of new parameters. ‘I.
. . further
’I
32
3.5 Preferred approach
Where an inspection for assessment of a concrete bridge highlights areas of deterioration such as spalling, loss of reinforcing bar cross section, etc. then that deterioration must be measured and recorded to enable it to be taken into account in the assessment of individual elements. Procedures for doing this have been investigated by WS at kin^^^ in work for the Highways Agency and by Webster3’ as part of the CONTECVET project3’. Recommendations as to the most appropriate methods for bridge assessment are given in Chapter 7. Similar advice to that given above appears in a technical audit of BA79 undertaken by Parsons Brinckerhoff310for the Highways Agency, and the latter are currently considering these and other recommendations as draft amendments to BD44. It is intended that these amendments will permit various forms of deterioration to be taken into account in the condition assessment of individual elements. Chapter 9 gives information on specific material factors that may need to be taken into account.
3.6 References
3.1
HIGHWAYS AGENCY, Draft for Comment, Interim Advice Note on Asbestos Management Application to the Strategic Road Network, (2004)
3.2 HEALTH & SAFETY EXECUTIVE, Bestpracticeguides 3.3 CSS, Bridge Condition Indicators Volume 1 CommissionReport, Volume 2 Guidance Note on Bridge Inspection Reporting (Addendum No 1, May 2004), Volume 3 Guidance Note on EvaluationofBridge ConditionIndicators (Addendum No 1, May 2004) 3.4
NETWORK RAIL Structures Condition Marking Index, Company code of Practice RTICEICI041
3.5 WOODWARD, RJ eta/, Bridge Management in Europe (BRIME), Deliverable D14 Final Report, Contract N o ’ RO-97SC.2220, BRIME. March 2001.
3.6 SODERQUVIST,The Finnish practice and experience regarding bridge inspection and management, The managementof Highwaystructures,Highways Agency, 22-23 June 1998.
3.7 BRITISH CEMENT ASSOCIATION et a / , A vabdatedusers manualforassessing therexidualservicelife ofconcretestructures, EC Project Ref. IN 309012, CONTECVET, BCA, Camberley, 2001
3.8 WS ATKINS, Deterioratedstructures- The assessmentofdeteriorated reinforced concrete bridge structures, Highways Agency Final Report, November 2000.
3.9 WEBSTER. M. The assessment of corrosion-damagedconcretestructures, PhD Thesis. Birmingham University.July 2000. 3.10 PARSONS BRINCKERHOFF, TechnicalAudit ofthe applicationofBA79 and A review ofbridge assessmentfai/ures on the Motorway and Trunk Roadnetwork,Final Reports HBR80616 - Highways Agency Contract 2/41 9, Parsons Brinckerhoff, December 2003
33
a 4. In-situ and laboratory testing 4.1 Background and introduction
Testing is closely associated with the Inspection process, in terms of establishing/confirming assessment parameters (Section 3,1), and in particular for condition assessment, where some form of deterioration is involved (Section 3.2 and Chapter 9). It is important t o have clear objectives in selecting test methods and in developing a test programme t o ensure that these are met, with an acceptable level of confidence.
Precision of tests and sampling regimes are crucial in this regard, particularly for the key stage of data interpretation.
1
Testing may be required for any of the following reasons: 1. To establish/confirm representative mechanical properties for the concrete, reinforcement or prestressing tendons, for use in subsequent structural analysis. 2. To locate the position of reinforcement or prestressing tendons, and t o establish the
extent of any section loss or concrete spalling. 3. To identify possible deterioration mechanisms, and t o establish which one is dominant.
4.To quantify relevant material properties and characteristics, relevant to current and possible future deterioration, under the measured/assessed local micro-climate and general environmental conditions. In all of this, the purpose is t o augment and enhance the overall Inspection for Assessment process, and t o produce data on the general condition of the structure and/or t o serve as input for subsequent structural analysis. The above objectives require the availability of a wide range of test methods, which may either be non-destructive or intrusive, carried out on site or in the laboratory. This is an evolving field, but details of current relevant test methods are given at the end of this Chapter, together with some guidance on choice, and application in practice. This Chapter can only give a brief review, and this is done in the context of practice and recommendations in relevant Highways Agency documents, most notably in BD21, BD and BA44, BD and BA63, and the Bridge Inspection Guide.
4.2 Current testing practice
34
The emphasis here is on practice in determining assessment parameters for subsequent structural analysis in accordance with Highways Agency guidance documents, i.e. on objectives 1 and 2 in Section 4.1
L
4.3 In-SitU sampling and testing
The extent of in-situ sampling and testing on any particular structure varies from structure to structure, depending on the existence or otherwise of as-built and maintenance records, the form of the structure and its general condition. Sampling and intrusive investigations should be carried out in areas which will not impair the capacity or the durability of the structure, and such sites should be made good using appropriate materials. As a minimum, in-situ sampling and testing should include a cover survey, verified by
local breakout if possible. This is often carried out in conjunction with half cell potential measurement, sampling for chloride ingress profiles, and depth of carbonation measurement, all of which help to give an indication of the condition of the structure and the potential for future problems. Occasionally, resistivity testing is also undertaken to provide information regarding the likelihood of ongoing corrosion. Further in-situ operations can include: exposure of an area of reinforcement to determine size, type, spacing and condition; the recovery of cores; and the recovery of steel reinforcement samples. In areas of concrete where delamination is suspected, e.g. deck or beam soffits, a tapping or sounding survey with a light hammer is useful in detecting potential spalling and loss of cover. Information regarding in-situ stress measurement of concrete and steel is given later in this Chapter.
’
Durability of concrete structures - Investigation, repair, protection4 contains information on planning, test and inspection techniques and on the interpretation of condition surveys. Further information is contained in CBDG Technical Guide 2 - Guide to testing and monitoring the durability of concrete
4.4 Laboratory testing
Laboratory tests may include the determination of chloride and cement content plus the determination of estimated in-situ cube strength. Steel tensile testing may also be carried out. Testing to identify possible deterioration mechanisms and related material properties (objectives 1 and 4 in Section 4.1) involves significant laboratory testing.
4.5 ConCrete parameters for assessment
The worst credible strength (WCS) for concrete in an element can be determined from a minimum of three estimated in-situ cube strength results using a formula given in BA44, where estimated in-situ cube strengths are determined from concrete core tests. It is important to recognise that the BA44 formula was derived on the basis of the variation in the estimate of concrete strengths from core test results at a particular
‘location’, within which the variation of concrete strength is small, and not across an entire structure. The use of this formula to estimate concrete strengths for an entire structure or even element must be approached therefore with caution since test results at one location may well be unrepresentative of the material properties a t another.
35
4 In-situ and laboratory testing
Non-destructivetechniques such as Schmidt or Rebound hammer may be used to demonstrate that core locations are representative of the properties of wider regions of a structure or to identify the need to take additional cores. Although Schmidt or Rebound hammer tests can be calibrated against a core test they are no real substitute for core tests in their own right when feasible. It is advisable that the value obtained from the BA44 formula is compared with values obtained from other methods and an appropriate value selected by engineering judgement. Table 4.1 illustrates four different approaches that might be considered, with example values based upon an actual set of 5 core test results with estimated in-situ cube strengths of 84.5, 86.0,97.0, 83.5 and 83.0N/mm2.The mean value of this set of test results is 86.8N/mm2and the standard deviation 5.82N/mm2.This is greater than the standard deviation assumed in deriving the BA44 formula and illustrates that in this case it may be prudent to use a worst credible strength somewhat lower than the value determined using the BA44 formula. In fact, it actually shows that the cores do not all come from a ‘location’ within which the variation of concrete strength is small. When larger numbers of cores are taken, it is common for some to be below the WCS. Strictly, the BA44 formula gives an indication of the worst likely mean strength, and is only valid if either the variation in concrete strength is small or the behaviour is sensitive to mean concrete strength, rather than the strength in critical local areas. It is emphasised that the second approach below, based on using the BA44 formula in conjunction with the lowest recorded core strength, is included only as an aid to establishing a sensible concrete strength for elements when the number of cores taken is limited. It might be appropriate when it was considered that only a particular core was likely to be representative of the concrete in a critical area. In practice, it is often necessary to use cores such as these, which do not come from one ‘location’, to obtain a single WCS for the whole structure or element. Strictly, the most correct statistical approach is to use the T distribution which allows for uncertainty in determining the standard deviation as well as the mean. However, when the number of cores is small, it becomes necessary to use more judgemental approaches, considering such aspects as the lowest core result or, when results are similar, the possibility that this may be coincidence. Table 4.1 Examplesof four different approaches.
I
Method
I
Description
I
Estimated WCS (N/mm*
4.6 Reinforcement parameters for assessment
The reinforcement parameters required for assessment are type, size, condition, spacing, location and yield strength. AS previously noted, type, size and condition are often best determined by breaking open a small area of concrete for examination, whilst spacing can be determined using a cover meter. When it comes to yield strength, BA44 states that a minimum of three samples should be taken for tensile testing. These samples should be of similar type and diameter and should be taken from low stress areas. Steel samples taken from concrete cores are not recommended for tensile testing. Determining the worst credible strength from a set of three samples requires engineering judgement. BA44 states that the formula for concrete strengths may also be used for reinforcing steel. However, the characteristic strength of the same three samples can be lower than the value calculated by the BA44 formula, depending on the spread of results, and the formula therefore does not provide the worst credible strength. Statistical analysis of a large number of samples tested by the TRL indicates that the worst credible strength tends towards the mean strength with a sufficiently large population. Indeed, testing reinforcing bars can show a sizeable variation and may also show significantly greater yield strengths than the characteristic values specified during design. It is noteworthy, however, that variations in reinforcement yield strengths within a structure will essentially be averaged because the number of bars that collectively contribute to the flexural strength of a section or yield-line. In such cases therefore, the use of the WCS, which is actually an indication of the mean strength, is more correct than in cases where behaviour is sensitive to the local strength of individual bars. In dealing with variations in reinforcement strength, some comfort can also be drawn from the fact that, typically, assessments are based on reinforcement yield strengths, whereas before flexural col[apse occurs some strain hardening of the reinforcement will generally occur, particularly for significantly under reinforced sections and mild steel bars.
~
4.7 In-SitU Stress measurement - concrete
There are two basic techniques that have been widely used: slot-stress and hole drilling. Both rely on measuring the strain relief around a hole which is cut into the concrete. The semi-circular slot, typically about 150 to 300 mm diameter, is cut with a diamond saw and the hole, generally about 75 to 100 mm diameter, with a diamond tipped corer. Both techniques are semi-destructive and care has to be taken to ensure that the
In the case of slot-stress, the strain can be restored by jacking the slot apart. This jack load can be related to the initial stress state. It is also possible to determine the initial stress state by using results of finite element analysis backed up by laboratory testing from the strain releases across and on either side of the For hole drilling, then the relaxation in strain as the hole is progressively drilled can be
related to the initial strain and, hence, stress state by semi-empirical methods. These are established and correlated by both finite element analysis and laboratory testing. As part
37
4
of this work, a circular jack was also developed with can be used to determine the inplane elastic modulus of c o n ~ r e t e ~ ~ , ~ ~ . Care should be taken to allow for the variation in concrete physical properties as well as the internal stress state. The strains and stresses in a cross-section can be affected by locked in stresses due to differential shrinkage and creep resulting from construction and subsequent load history as well as transient temperature fluctuations. The technique has been successfully applied to a number of prestressed concrete bridges. The field accuracy of the technique is plus or minus 2N/mm2.
4.8 In-situ stress measurement - steel reinforcement 4.8.1 Blind hole drilling
A similar principle 3 the slc stress technique can be used for measuring steel resses and is sometimes referred to as blind hole drilling. This entails drilling a small hole in a reinforcing bar or prestressingtendon and measuring the strain relief around the hole. This can be calibrated to give the steel stress and hence force.
One of the problems with this technique is that reinforcing bars have built in residual stress profiles which are caused by the rolling process. Prestressing wires also have drawing residual stresses. This stress profile has to be known, hente the need to calibrate against an identical or a t least similar bar -which would need to be taken from the structure. Additional complications can arise from the stress concentrations caused by the deformations on deformed bars although these can be ground off before strain gauging and drilling. The accuracy is unlikely to be less than plus or minus 20N/mmZ which is often of little use for reinforced structures. Tests carried out in the mid-1980s by WS Atkins to explore this technique were not pursued for this reason.
4.8.2 Cut bar method
The application is to expose a bar in the structure, strain gauge it,&$ then to cut it (with minimum heat). The strain relaxation then gives the in-situ stress in the bar. The bar is then reinstated with a full penetration butt weld which would re-tension the bar due to weld shrinkage. This technique was referred to in a paper presented in 199446in respect of a box culvert. The significance for culverts is that the actual soil pressures are somewhat indeterminate and the readings allow the capacity available for carrying live load to be estimated. The drawbacks are that dead load stress levels can be low; the stress measured after exposing the bar over sufficient length is the average between points where it is properly anchored; actual stresses are dependent on the amount of tension contained by the concrete. However, if the reinforcement stresses are predicted to be very high but there are no signs of cracking to confirm this then using the technique could be justified.
38
I
4.9 Geophysical techniques
4.10 References
The most commonly used geophysical technique in the investigation of concrete bridges is surface impulse radar. This technique can be used t o determine hidden detail in abutments or other elements and can also be used t o detect the presence of reinforcement adjacent to remote faces. However, specialist interpretation is required, quite often coupled with trial holes or cores for calibration.
4.1 MAYS,cc (ed.),Durabi/ityofConcreteStructures-Investigation, Repair, Protection. E&FN Spon, London, 1992 4.2
CONCRETE BRIDGE DEVELOPMENT CROUP, Guide to testing andmonitoring the durabiOtyofconcretestructures. Technical Guide 2, The Concrete Society, Camberley, 2002
4.3
FORDER, S. Ca/ibrationofsaw cutting techniquefor insit ustress determination MEng Project Dissertation, University of Surrey, 1992
4.4
MEHRKAR-ASL, 5, Direct measurement ofstresses in concretestructures,PhD Thesis, University of Surrey, 1988.
4.5
MEHRKAR-ASL, S. Concrete stress-relief coring theory and application, ProceedingsofFP Symposiumon Post-tensioned Concrete Structures, London, 25-27 September 1996, pp 569-576.
4.6
CHALKLEY, C, Proof load testing - a recent proof load test, and its results, Proceedingsofa Surveyorconferenceon Hidden Strength-Load Testingfor Bridge Assessment, London, February, 1994.
39
D Loading
5. Loading 5.1 General PrinCipkS
Loading rules are given in BD2l for short and medium span bridges and in BD50 for long span bridges. These documents are fundamental to the assessment loads for all types of bridge and reference should be made to them for the rules and explanatory notes. The main purpose of this Chapter is to explain the principles and point to references that provide further background details. There have been several changes to loading rules in BD21 since it was first introduced, one being to account for the increase in gross vehicle and axle loads. The most recent changes allow the use of bridge specific live loading for short and medium span bridges, where good surface condition and low traffic flow lead to less onerous requirements (See Section 5.2). Assessment loading is limited generally to the application of dead, superimposed dead and Type HA live loads. (Dead loads are the weight of the structural (i.e. load bearing) elements whereas superimposed dead loads are the weight of the non-structural elements, such as surfacing, parapets, services and miscellaneous street furniture.) Type HA live loading consists of a uniformly distributed load (UDL) together with a knife-edge load (KEL), which can be determined as described in BD21. For assessment purposes, the values specified by this Standard have to be factored to give the Assessment Live Loading (ALL). Although BD2l only covers permanent and Type HA live loads, most bridges that can carry the 40t ALL have also been checked for Type HB loading and, therefore, should now also be checked for STGO loading (See Section 5.4).
5.1.1 Scope O f h e loading
The Type HA (design) loading given in BD2l allows for the effects of 40 tonne vehicles and includes a contingency margin for unforeseeable changes in traffic patterns. For assessment, reduction factors are applied to this Type HA loading to give the various ALL levels with no contingency provision. The 40 tonne ALL covers the effects of normal vehicles (Authorised Weight Regulation) of up to 40 tonne gross vehicle weight [including 41 tonne 6 axle lorries, 44 tonne 6 axle bimodal articulated lorries and draw bar trailer combinations and 44 tonne 6 axle general haulage lorries. (Although the gross weights of these vehicles exceed 40 tonnes, the load is distributed over six axles, rather than the five axles of the 40 tonne vehicle, as well as a longer wheelbase. Thus, the load effects generated by these vehicles are lower than those caused by the 40 tonne vehicle.)] and 11.5 tonne axle weight. For cases where structures are found to be incapable of carrying the full 40 tonne ALL,
loading criteria are given which correspond to specified limits on gross vehicle weights Special loading criteria are also given for fire engines. The Type HA UDL and KEL given in BD2l are generally only suitable for modelling longitudinal load effects. Alternative loads are given for the effects of vehicles on trough
40
,
decks, short span masonry arches, decks with main members that span transversely including skew slabs with significant transverse action, and buried concrete box structures with cover greater than 0.6 m. A recent change requires longitudinal elements to be checked using specific vehicles where there is a low capacity for transverse distribution.
5.1.2 Background to assessment live loading
I
The Type HA ALL that is given in B D 2 l for short spans (2-50 m length) is derived from an ultimate or extreme loading as opposed to a working load. This ultimate load was derived from first principles and is based on the assumption that the worst credible load that can reasonably be expected to occur in the lifetime of the bridge will be equivalent to some multiple of Type HA loading. It has been shown that this extreme load has a return period of 200,000 years or a 0.06% chance of occurring in 120 years. The values of nominal HA loading that are given in BD2l were determined therefore by dividing the extreme loading by 1.5. Four elements have been used to generate the extreme loads. They are: 1. Loading from Authorised Weight (AW) vehicles 2. Impact 3. Overloading 4. Lateral bunching.
Details of these are given in Annex C of BD21 The loading has been derived for a single lane only. It has been assumed that if two adjacent lanes are loaded there is a reasonable chance that they will be equally loaded The various factors that have been used in determining the loading are span dependent. (The exception is impact for the single vehicle case.) The derived loading has taken account of the possibility of convoys of eight or more HCVs, but, on more lightly trafficked routes, the probability of having a bridge completely filled with heavy vehicles is small. The loading is conservative therefore for lightly trafficked medium span structures. The impact factor that has been used was derived from measurements taken on motorway overbridges, which were of modern construction and where the road surface and bridge joints should have been in good condition. The overload factors were derived from a sample survey of approximately 3500 vehicles and thus can be assumed to be typical of what may occur at any time, or in any place in the country. The loading that is derived using these procedures is considered therefore to be fairly universal in its application and to reflect situations that can occur a t any bridge site.
41
D Loading
5.2 Bridge Specific loading
For short and medium spans, bridge specific live loading is derived by the application of a load reduction factor (K) t o account for different traffic flows and road surfaces. Bridge Specific Assessment Live Loading (BSALL) for long span bridges is based on traffic surveys and is described in BD50. Specific guidelines for Trunk Roads are given in BD2l and appropriate K factors can be determined from graphs [the K diagrams) for loaded lengths between 2 and 50 m and six combinations of traffic flow and surface condition. If a bridge is found t o be inadequate at the 40 tonne load level, the permissible weight restriction level is the highest capacity for which the K value in the appropriate K diagram
is less than the live load capacity factor C, derived using the available live load capacity. For bridge specific reliability-based assessments a different loading model is required. For the Highways Agency this has been developed in a project described by Coopers’, the result of which is a probabilistic loading model which accounts for traffic flow, road surface characteristics and the dynamic response of the bridge under consideration.
5.2.1 Bridge Specific probabilistic loading mode[
For a site-specific reliability-based assessment, the load effect [bending moment or shear force) 5,, a t any section can be taken t o be a random variable expressed as: S,, = BSLL x R,, x DAF
Where BSLL = the Basic Static Live Load effect [defined below) = a random multiplier t o model the uncertainty in the load effect due t o the static weight of vehicles DAF = a Dynamic Amplification Factor t o account for the dynamic amplification of the static load effect due t o the impact of axles and the dynamic response of the bridge.
R,,
The Basic Static Live Load effect at the critical section is determined by loading the relevant influence surface with the following: 0 A uniformly distributed load (UDL) of 27 kN/m run across a width equal t o the
smaller of the notional lane width or 3 m,placed centrally within the lane. 0 Two axle loads, each 300 kN, with a 1.2 m spacing between axles, placed anywhere
within the lane t o maximise the load effect at the section considered. This follows the geometrical configuration of the load model in Eurocode 1, Part 3 because it was felt that the tandem axle configuration provides geometrically realistic force distributions on the structure. Each axle consists of two wheels a t a 2 m track width placed centrally within the lane. The same loading is used for both Lane 1 and Lane 2 but a reduced UDL of 7 kN/m run and two axle loads of 100 kN each are used for the remaining lanes. The carriageway is divided into notional lanes according t o BD21.
42
The random multiplier R,, is assumed to have a Cumbel distribution, the parameters of which are given separately for bending and shear in Cooper” and they depend on the span and traffic flow rate. The maximum value of this extreme type distribution also depends on the number of repetitions of a load event within the given reference time. Bridge natural frequencies are considered to be closely correlated to the span length. The Dynamic Amplification Factor (DAF) is assumed therefore to have a Normal distribution, the parameters for which are based on span, the number of loaded lanes and the pavement condition.
5.3 Highway Surfacing effects
Significant research has taken place to enable Bridge Specific Loadings to be determined (see Section 5.2). Application of this work first appeared in the 1997 version of BD21. Not only were the notional lane widths modified from previous calculation methods but the number of HCV movements and the road surface condition were also taken into account. Three traffic flow categories (High, Medium and Low) and two road surface categories (Good and Poor) were defined. This gave six possible combinations of effects, e.g. Hp (High-Poor) and Mg (Medium-Good), and six separate graphs of Reduction Factor, K, against Loaded Length, I ,were produced. The category Hp gave the worst loading condition whereas the category Lg gave the least onerous loading condition. The assessment engineer was left with the task of determining whether the road surfacing was ‘Good’ or ‘Poor’ quality. The three traffic flow categories are defined as follows in BD21: 1. High (H) 2. Medium (M) 3. Low (L)
Flow > 70 70 > Flow > 7 7 > Flow
Flow is equal to the total annual two-way HCV traffic over the bridge divided by 8760 where HCV is defined as goods vehicles that are over 3.5 tonnes maximum permissible gross vehicle weight. The 1997 version of BD2l required road surface categories to be determined using equipment described in TRL Report L R I 125”such as the High Speed Road Monitor (HRM). The acceptance criteria quoted approximated to the mid threshold value for roads with 50 mph speed limit or greater as given in Highway Standard HD2gS3.This technique was reasonable for application to high speed roads which were regularly checked using equipment such as HRM, but proved to be problematical for local roads where the use of such specialised vehicles was rare and the criteria quoted were inappropriate for roads with lower speeds and greater rates of change in gradient. The 2001 version of BD2l simplified the process by allowing the subjective determination of road surface quality. For instance, it is stated that “Motoways and trunk roads may generally be consideredas ‘good’surface category if they are maintainedand repairedbefore they deteriorate to ‘poor’surface”. The ‘poor’ classification is applied if, when driving a vehicle over the bridge in free flow traffic condition, any of the following applies:
43
5 Loading
0 Subsidence - vehicle bounces 0 Sub-base deterioration - vehicle pitches locally U Surface deterioration -visual deterioration of surfacing or steps in expansion joints
Additionally, where practicable, the assessment should be confirmed by observation of HGVs crossing the structure (a full description of the criteria is contained in BD21). If a quantitative assessment of the road surface condition is carried out then it is now permissible t o use the criteria in HD29 appropriate t o the speed of the road carried by the bridge. ‘Good’ roads are those in categories 0 and 1 as given in HD29 whereas ‘Poor’ roads are those in categories 2 and 3. Measurements are required t o be taken between points 20 m beyond the bridge. This increase over the previous figure of 5 m is because road profile unevenness at bridge abutments is a major cause of significant dynamic loading on bridge structuress4. The results of these changes mean that for minor roads the maintenance of a ‘Good’ category may require maintenance at more frequent intervals than would be demanded by consideration of the pavement alone. It may also be necessary t o enter into agreements between the bridge owner and the highway maintenance authority t o ensure that adequate standards of maintenance are achieved. Specialised inspection requirements should be included on bridge files.
5.4 Road traffic and abnormal loads
There are two main classifications of road traffic in the UK. These are: 1. Normal vehicles: The large majority of vehicles using the highway network are regarded as ‘normal’ traffic, which covers cars, light goods vehicles, rigid and articulated vehicles and heavy goods vehicles up to a gross weight of 44 tonnes. These vehicles comply with the Road Vehicles Construction and Use (C&U) Regulations 1998 and Authorised Weight (AW) Regulations 1998. 2. Abnormal vehicles: These are vehicles, either empty or laden, which do not comply with C&U Regulations. This non compliance with the Regulations could be because: 0 The load carried exceeded dimension limits
0 The vehicle and load exceeded weight limits 0 The vehicle had been designed for carrying outsized loads and/or for particular purposes. These vehicles include mobile cranes, construction plant and low loaders carrying exceptional industrial loads (e.g. electrical transformers, machine presses, etc.). These vehicles and payloads are commonly referred t o as an Abnormal Indivisible Load (AIL) There are two types of abnormal vehicles:
1 , Special Types General Order (STGO) vehicles: This group includes vehicles that do not comply with the AW Regulations but comply with The Motor Vehicles (Authorisation of Special Types) General Order (STGO Regulations). Under Article 18 of the STGO Regulations, the maximum gross vehicle weight and maximum axle weight are 150 tonnes and 16.5 tonnes respectively.
44
Loading 5
2. Special Order (SO) vehicles: This group includes vehicles that do not comply with the AW Regulations or STCO Regulations and is covered by Section 44 of the 1988 Road Traffic Act. SO vehicles have maximum axle weights of greater than 16.5 tonnes or gross vehicle weights in excess of 150 tonnes. Both types of Order work in conjunction with the Road Vehicles (Construction and Use) Regulations. The movement of an AIL is covered by the provisions of Section 44 of the Road Traffic Act 1988 and is required to be notified to the appropriate authorities (police, Highway Authorities and bridge owners). Indivisible load movements that take place under the STCO do so under a number of conditions. Some of the C&U Regulations apply and, depending on the size and weight of the load, some special conditions. The STCO divides vehicles into three categories depending on their weight. The chief criteria are summarised in Table 5.1. If the Category 1 or 2 axle limit is exceeded, then the load must be moved under the higher category 2 or 3. All vehicles must carry a 'Category Sign'. In addition to the above, the STCO also makes special provision for 'Engineering Plant', which may be self-propelled or towed. For example: a mobile crane or bulldozer, which cannot meet some of the C&U Regulations because of the task it is designed to do. Any vehicle moving in excess of 150 tonne gross vehicle weight, 16.5 tonne axle weight, 5.0 m wide, or 27.4 m long (excluding the length of the tractor unit) requires the permission of the Highways Agency. Two types of AIL require signed authorisation, as shown in Table 5.2.
Table 5.1
'
Categoriesof Special Types General Order vehicles.
'9
Category
Maximum gross weight of combined vehicle
Maximum axle weight
Minimum number of axles
Speed limits (mph)
Other conditions
I
45
- -
5.4.1 Purposes O f highway structure assessment
5.4.2 Design and assessment
Dimensions and weights
5 m < width < 6.1 m and < 150,000 kg and <27.4 m long
> 150,000 kg or >6.1 m wide or >27.4 m long
Required authorisation
STGO VSE - Form VR1
Special order under Section 44 of the RTA 1988 from the AIL Section of the VSE
The prime purpose of highway structure assessment is to verify that the structure is capable of carrying the intended road traffic safely. The secondary purpose is to determine the carrying capacity or the critical pinch point of the route from assessment of the structures on it, if all structures on that route were assessed.
Since 1973 (BE5) it has been a requirement to design ‘other public roads’, ‘Principal roads’ and ‘Trunk Roads and motorways’ for 30 (120 tonne), 37.5 (150 tonne) and 45 (180 tonne) units of HB loading respectively. These design load requirements are now embodied in the current UK highway loading Standard BD37.This Standard defines Type HA and Type HB load models, which are intended to allow for the loading effects of ‘normal’ and ‘abnormal’ vehicles respectively. UK highway bridge assessment loadings for normal traffic are given in BD2l. Until 2001 there was no assessment load model for abnormal vehicles, so the Type HB load model in BD37 was also used for assessment. The HB model was derived in the late 1940s. An assessment load model for AILS was developed because the HB model was found to be unduly conservative when used in assessment of short span structures. Furthermore, the HB model could not be readily correlated to the current fleet of abnormal vehicles. Consequently, assessments in HB units do not fully meet the prime purpose of assessment.
5.4.3 BD86 - Assessment of highway structures for the effects Of Types General Order (STCO) and Special Order (SO) Vehicles
For short span structures with loaded lengths of less than 50 m, load assessment models for abnormal vehicles have been developed based on the load effects from actual STCO vehicle weights and configurations and traffic data. These models can be used to assess the load effects from STCO vehicles more accurately than the HB load model in BD37. In addition to meeting the purposes of AIL assessment, the BD8655load models enable the following augmentations: Attainment of higher load capacity ratings, particularly for structures with loaded lengths of less than 10 m.
-’
I
I 0 Flexibility t o modify various factors such as dynamic amplification factor, overload factor, etc. t o suit a specific structure. Consistent levels of safety for structures of different spans and for different STCO and SO vehicle movements.
5.4.4 BD86 Load Models sv Vehicles
Five load models (SV vehicles) were derived t o represent the load effects from the axle arrangements permitted under the STCO Regulations, which produce the most severe load effects. The five SV vehicles are: 1. SV80 covering Category 2 STCO vehicles 3.0 m wide up t o 80 tonnes. 2. SVIOO covering Category 2 STCO vehicles 3.0 m wide up t o 100 tonnes. 3. SV150 covering Category 3 STCO vehicles 3.0 m wide up t o 150 tonnes. 4. SV-Train consisting of the SV150 STCO vehicle drawn by a 3.0 m wide tractor unit (locomotive) . 5. SV TT covering military Tank Transporters 3.7 m wide up t o 105 tonnes.
The SV80, SVIOO and SV150 vehicles have axle spacings of 1.2 m, with a variable length at the centre between two bogies of 1.2 m, 5.0 m or 9.0 m. The 1.2 m spacing reflects the findings of the database studies and the STCO Regulations. The 5.0 m and 9.0 m spacings are only used above specified loaded lengths for load effects with two or more peaks in the influence line (e.g. for continuous span structures). Inevitably this may require a large number of load cases t o be considered and therefore general guidance has been given in BD86 t o assist in reducing the number. The most significant difference between the SV load models and HB model is that the latter tends t o be over conservative for spans less than about 10 m where the two adjacent heavy axles tend t o dominate. This is because STCO vehicles have a greater number of lighter axles, which tend t o spread the load more than HB loading.
5.4.5 BD86 - HB Conversion Charts
A number of simple conversion charts based on influence lines have been derived t o facilitate the conversion of SV rating t o HB units. These enable the methods contained
in BD86 for the assessment and management of STCO movements t o be used for most bridges that have previously been assessed for Type HB loading. Examples of the use of the conversion charts are given in the Standard, together with some limitations on applicability.
5.4.6 Assessment O f Abnormal Load Movements using BD86
.
Guidance is given in BD86 to allow abnormal load movements t o be managed where a structure has already been assessed for SV vehicles, or where HB conversion charts may be used. The first stage of the assessment for the particular notified STCO vehicle is a simple screening assessment. The screening compares the vehicle type, gross weight,
axle weight and axle spacing of the notified STCO vehicle with limits appropriate t o the
I
SV vehicles. The STCO vehicle may pass if its gross weight is less than the gross weight of the appropriate SV vehicle multiplied by the corresponding Reserve Factors for the load effects being considered. The original SV assessment would have given the Vehicle Ratings and Reserve Factors for the structure. Prior t o a rigorous SV assessment, the HB t o SV conversion charts may be used t o obtain Vehicle Ratings and Reserve Factors. Note that the Vehicle Rating for a structure is defined as the most onerous SV vehicle that can safely pass over the structure (i.e. the vehicle with the smallest Reserve Factor greater than 1.O). Reserve Factor is defined as the factor on the assessment SV load required t o reach the first failure. When a STCO vehicle does not satisfy the axle weight and spacing characteristics of the SV vehicles, a detailed assessment is carried out. The load effects caused by the STCO vehicle are compared with those of the SV vehicle, multiplied by the Reserve Factors, using one or more influence lines most appropriate for the bridge. If load effects due t o the STCO vehicle still exceed those of the factored SV vehicle, further refinements such as reductions in dynamic amplification factor and/or overload factor and associated loading, can be made where justifiable. These reductions depend on the extent to which the movement can be regulated with regard t o keeping the structure clear of associated normal traffic, restricting the speed of the STCO vehicle over structures and certification by the haulier of the load carried.
5.5 References
5.1
COOPER,DI, Development of short span bridge-specific assessment live loadlng In /nternationa/symposiumonsafetyof bridges (Das, PC, ed.) Institution of Civil Engineers and the Highways Agency, London, 4-5 July 1996, Thomas Telford, 1997.
5.2
TRANSPORT RESEARCH LABORATORY, Measurementandassessment ofunevenness on malorroads. Report LR1125, TRL Ltd, Crowthorne, 1984.
5.3 HIGHWAYS AGENCY, HD29 Structural Assessment Methods. Design ManuallorRoadsandBridges, Vo1.7, Section 3, The Highways Agency, London. 2001
48
5.4
JORDAN, P, Unpublished TRL Report PR/CE/18/98, Comment on Departmental Standard BD21/97. RoadSurface Categories, 1998
5.5
HIGHWAYS AGENCY, BD86. The Assessment of Highway Bridges for the Effects of Special Types General Order (STGO) and Special Order (SO) Vehicles, Design ManualforRoads andBridges, The Highways Agency, London, 2003.
6. Analysis for assessment 6.1 Assessment principles 6.1.I General PrinCipkS
Reserves of strength can exist but remain unrecognised if full advantage is not taken of the range of analytical methods available for the assessment of concrete bridges. As a result, in some instances, assessments can be excessively conservative and adequate structures may be condemned as unsafe. Analytical methods appropriate for bridge assessment vary greatly in their complexity and in their cost of application. Therefore, it is appropriate generally to begin an assessment with a simple method and then to extend the analysis if a shortfall in capacity is identified. Details of specific analytical approaches are set out in this Chapter. Usually, it is the strength and safety of a structure that is the principal concern when an assessment is undertaken. The guidance provided is focussed therefore on assessment a t the ultimate limit state.
6.1.2 Analytical procedure
Typically, the assessment of concrete bridges is undertaken using the following simple procedure: U Establish load effects, using an analytical method chosen by the assessing engineer. 0 Compare load effects with capacities, determined in accordance with BD44 and
’,
BA446 ‘.
Improvements in assessment ratings may be achieved therefore through judicious selection of analytical methods or by recognising unnecessary conservatism in the capacities determined. However, this simple approach to assessment does not recognise one of the principal reasons why concrete bridges are able to carry far greater loads than might be expected; their capacity for redistribution. For a concrete bridge to collapse it is necessary for a failure mechanism to form. Therefore, the maximum load a structure can carry safely is not necessarily governed by the load that causes the first ‘region’ within a structure to reach its capacity (i.e. to yield), as often assumed in design. If a structure is sufficiently ductile and provided alternative load paths exist, additional loading can be carried safely by the structure through redistribution until sufficient ‘regions’ are yielding for collapse to occur. Generally, concrete bridges have both adequate ductility and redundancy to enable significant redistribution so that there is a sizeable difference between the load a t ‘first yield’ and the ultimate collapse load. If the benefits of redistribution are to be realised in assessment, the simple approach of separating load effect calculations and capacity calculations cannot be retained generally. Instead, a form of analysis that accounts for both load effects and capacities in combination must be adopted, such as plastic methods or non-linear numerical methods, usually non-linear finite element analysis (NLFE). Methods that can account for
49
6 Analysis for assessment
redistribution require careful application, in particular, in understandingthe implications of the ductility of concrete structures. However, if distress occurs at the serviceability limit state, a structure may be considered to be ‘unsafe’ and usage may be restricted as a result.
6.1.3 Lower bound and Upper bound methods
The methods used for the analysis of concrete structures a t the ultimate limit state are underpinned generally by Plastic Theory (see, e.g., Clark6.3).When an elastic analysis or a strip method is used, the resulting distribution of stresses is in equilibrium with the applied loads and therefore, provided the stress nowhere exceeds the capacity of the structure, a lower bound assessment of the strength of the structure is obtained. Numerical methods, such as grillage and finite element analysis, can be used to find distributions of stresses that satisfy equilibrium approximately. The results of applying such methods can be considered generally as lower bound assessments, although strictly this is not the case since equilibrium is not satisfied exactly. Methods, such as yield-line analysis, identify a failure mechanism but do not check stresses everywhere. They are upper bound methods therefore. They can be particularly useful in bridge assessment since they account directly for a structure’s capacity for redistribution. The degree of complexity of different methods of analysis and uses are shown in Table 6.1.
6.1.4 Ductility
‘2
Plastic methods implicitly assume that structures are ductile. Generally, concrete structures are sufficiently ductile for this assumption to be valid and some guidance on limits is given in standards. However, establishing precise bounds on the-ductility requirements for plastic analysis to be valid is not ~traightforward~.~,~.~. Non-linear numerical methods, such as non-linear finite element analysis, can be used to account for the effects of limited ductility. However, such methods can be highly complex (and therefore costly) to apply, and particular care and expertise is required to ensure that the results are reliable.
.
6.1.5 Differences from design
Table6.1
Superficially, assessment is similar to undertaking analysis and checking sections in design. There are, however, important differences which should alter the approach fundamentally. If this is not appreciated, it can result in a significant waste of resources
- 1 Increasing complexity I Analysis approach
I Example
on unnecessarily sophisticated analysis. More seriously, it can result in structures being strengthened or even condemned when, in fact, they are satisfactory. Even quite minor repairs, strengthening or modifications to existing bridges are very expensive. This contrasts sharply with the situation in design. A structure that does not yet exist, is very easy to change. Therefore, the designer chooses a convenient analytical approach and then adjusts the structure to comply with the results of the analysis. In assessment, in contrast, the approach should be to alter the analysis to suit the structure. There are a wide variety of analyses normally which will give different but safe assessments. The further the structure is from being statically determinate, the greater the range of possible solutions. The analytical approaches used in design are often conservative. Alternative approaches are available which give more realistic results. These approaches are often, although not always, more expensive. The high cost of strengthening existing structures makes it more likely to be worth using them in assessment. However, whilst the designer aims for a consistent margin of safety, and hence uses consistent analytical approaches, the margin of safety in existing bridges may vary wildly. Because of this, there are also cases where a very simple conservative analytical approach is quite adequate to show that a structure is satisfactory. Another difference between design and assessment relates to the relative importance of the different limit states. In design, serviceability is often critical. In assessment, the first and most important responsibility is always to ensure safety, to ensure that the ultimate strength is adequate. BD2l says that pre-1965 bridges will not normally be checked for serviceability.The serviceability checks in BS 5400 Part 4, particularly the stress check, impose a severe limitation on the benefit that can be gained from using non-elastic analyses. Without this limitation, the scope for using plastic and other inelastic methods is much greater. The effect of these differences is that a wider range of analytical approaches are used in assessment than in design. Faced with a beam and slab bridge to analyse for design, most engineers would use a linear elastic analysis, probably using a grillage model and section properties based on gross-concrete sections. They would tend to use the same form of analysis for all such bridges. In assessment, it may be appropriate to use a simple static analysis, a grillage based on a variety of section properties, a threedimensional elastic finite element model, a plastic analysis, a non-linear finite element analysis, a load test or a combination of these approaches.
6.1.6 Structural condition
One of the most important differences between design and assessment lies in the fact that assessments are undertaken on actual structures that typically have been in service for some years. As a result their condition may not be as it was intended when designed either as a result of construction errors or subsequent deterioration. Assessments should be undertaken on the basis of the actual properties of the structure, as built and as matured. Further guidance on such issues is included in Chapter 9.
51
6.1.7 Global and local analysis
In assessment, as in design, it is usual (although not universal) to consider global and local analysis separately. In a beam and slab type deck, the global analysis gives the moments and forces in the beams and diaphragms. If there are no intermediate diaphragms, it will also give significant transverse moments in the slab. These moments, which are known as global transverse moments, have to be considered in the slab design. The distinction between global and local analysis does not arise in slab type bridges. The overall or global analysis of these can be treated in a similar fashion to global analysis in beam and slab bridges. However, particularly for smaller bridges, it will often be easier to use analytical approaches more often used for deck slab analysis.
6.2 Simple methods 6.2.1 Strip method
The simplest way to determine forces and moments in the beams of a beam and slab structure is to use a static load distribution ignoring the capacity of the deck to distribute loads. The approach can also be used to determine the moments in a section of a slab deck, although when axle loads (rather than uniform lane loads) are considered, it will be necessary to assume some degree of spread of the individual wheel loads. Because it is so simple, this method is useful as an independent check when more sophisticated methods are used. A static load distribution provides a set of forces which are in equilibrium. It is therefore
a lower bound (safe) method according to plastic theory. Hence, if a structure has adequate strength according to such an analysis, it is safe. If only a strength assessment is required, there is no reason to go to a more sophisticated analysis. The analysis also gives conservative figures for serviceability assessment of longitudinal members. However, it cannot provide any information about transverse moments since it ignores them. Hence, if serviceability assessment of transverse members is required, a more sophisticated analysis is needed.
6.2.2 BA16 method
6.2.3 Upper bound limit and check
52
BA16 provides a simple assessment method using empirical charts. This approach is more restricted in application than a static distribution and cannot be used for HB loading, but it can be less conservative. There is also a similar approach for transverse members but this is still more restricted in application.
The opposite extreme from a static load distribution is to assume the distribution is perfect and compare the total longitudinal moment and vertical shear across the width of a bridge with the total available capacity. This is an unsafe method (it is related to yieldline analysis) but it does give useful information. If the bridge fails this test, no amount of improved load distribution will make it work. It provides an upper bound to strength and also another useful check on other analyses. The distribution analysis should always give results which fall between those obtained using a static and perfect load distribution.
If both the section and the loading are fairly uniform across the width of the bridge, there may be little difference between a static and ideal load distribution, in which case there is little advantage in doing more advanced distribution analysis.
6.3Elastic methods 6.3.1 Elastic grillage
If the simple methods do not show the bridge to be adequate, the usual next step is to use the type of elastic methods used in design. Before embarking on such an analysis, it is worth reviewing its limitations. The increase in strength comes only from improving the distribution of load between the beams. This distribution comes as a result of the transverse moments. The distribution analysis cannot increase the total moment capacity of the bridge. This means that if the bridge has no transverse moment capacity, or if all the beams are fully loaded in the simple analysis, a distribution analysis cannot increase the capacity.
Section properties When elastic analysis is applied to reinforced concrete structures, BD44 (like 855400: Part 4) allows considerable freedom in the choice of section properties. It allows them to be calculated from either cracked or uncracked sections. Either approach leads to satisfactory, but sometimes significantly different, results. In design, uncracked properties are used almost invariably. The main reason for this is that cracked section properties can only be calculated after the reinforcement has been designed, whilst the primary object of the analysis is to determine the required amount of reinforcement. Reinforcement can be designed directly using uncracked concrete properties whilst, if cracked properties are used, the process becomes iterative. In assessment, this disadvantage of using cracked properties does not arise.
Cracked v uncracked properties In a slab with isotropic reinforcement, it makes no difference to the predicted moments whether cracked or uncracked properties are used as the longitudinal and transverse stiffnesses are equally affected. However, most bridges are far from being isotropic. In the longitudinal direction they may be heavily reinforced or they may have prestressed concrete or steel beams. In any of these cases, the choice of approach will have little or no effect on stiffness. Transversely, however, they may be very lightly reinforced and the uncracked stiffness can be as much as ten times greater than the cracked stiffness. This is often the case in older structures which were designed using a static load distribution and so have very little transverse steel. When such structures are assessed using a conventional grillage based on gross-concrete properties, the transverse steel often appears overstressed. However, a more realistic assessment using uncracked transverse stiffnesses will show the transverse steel to be less highly stressed a t the expense of giving a less favourable distribution between beams.
53
Varying section properties Reducing the transverse stiffness can result in an analysis that suggests that the longitudinal members are inadequate.Sometimes analysis using cracked properties gives overstressed longitudinal members, whilst analysis using uncracked properties suggests that the transverse steel is inadequate. In such cases, an analysis with intermediate properties could show that both are satisfactory. Using intermediate properties is also a useful technique when the top and bottom steel is different, and it is not obvious whether the section will be in sagging or hogging. It will be safe to use section properties calculated from the greater steel area. The strength is then checked using the appropriate area.
Torsionless gri llages Another property that frequently has to be varied in analysis for assessment is the torsional stiffness. The beams in many older structures were not designed for torsion and will appear to have inadequate torsional strength. However, a grillage analysis using reduced or zero torsional stiffness will still produce a safe assessment. This is a useful technique for assessing skew slabs, particularly where the transverse reinforcement is either skewed or very light. A torsionless skew grillage can be used with the elements running in the reinforcement directions. The moments from this can be used directly to check the reinforcement without the need for post-processing, The approach is safe but it is not valid for serviceability assessment. In particular, it will not predict the top cracking which can arise in the obtuse corner of simply supported slabs.
Slab reinforcement Whilst beams can be checked directly from grillage results, more interpretation is required in slabs, particularly skew slabs. The reinforcement in these is normally designed, and often assessed, using Wood-Armer equations, usually via post processors on the grillage or finite element program. The solution is derived by determining the minimum total steel to resist the moments a t the point. This approach has limitations in assessment but is almost always conservative as is explained in more detail in Section 7.5.
6.3.2 Elastic finite e[efTlentS
Elastic finite element analysis is used in various ways in assessment and the main ones will now be considered.
Plate models of slabs Plate finite element analysis is sometimes used for geometrically complicated slab decks
because, with modern programs, it is easier to assemble the models. Finite elements give a better representation of the behaviour of elastic plates than grillage models. However, as a reinforced concrete slab cracks, the moments tend to approach the distribution given
by a grillage using cracked section properties. If isotropic plate elements from simpler programs are used this loses the ability to take advantage of cracked properties or of reduced stiffnesses to allow redistribution. In extreme cases of highly skewed slabs with light transverse reinforcement placed parallel with the abutments, rather than perpendicular to the main steel, the cracked elastic stiffness in the stiffest and least stiff direction can differ by a factor of 100. It is common for a conventional assessment based
54
on isotropic elastic finite element analysis to give such a structure a significantly lower assessed capacity than a static load distribution. This problem can be avoided by using more sophisticated programs which allow orthotropic properties.
When plate analysis is used with either point loads or point supports, some care is needed in interpreting the results. The peak moment intensity under a point force in an elastic thin plate finite element analysis is very ‘mesh sensitive’; it tends towards infinity as the element mesh is made finer. Because the real slab has a finite thickness, and the load has a finite size, this peak does not arise in practice. The problem can be relieved by modellingthe size of the load or support. However, for the ultimate limit state it is considered appropriate to spread the peaks over a reasonable width; perhaps three times the slab depth. Another consequence of mesh sensitivity is that if the elements are too big in relation to slab thickness, it is possible to get unsafe results with the analysis failing to pick up moment peaks which can be important.
Beam and slab decks In a finite element model of a beam and slab deck, the beams and the parapet upstands can be modelled using offset beam elements with the slab modelled with plates. This is more realistic than a grillage analysis in the sense that it gives a more realistic representation of the interaction between string course, slab and beams. It also enables the real behaviour of intermediate diaphragms (or, more commonly, steel cross girders) which are separate from the slab to be modelled. However, a finer than normal element mesh is required to achieve this. If this type of analysis is done with a fine enough element mesh, the results can be used directly to check the slab reinforcement, avoiding the requirement for a separate local analysis. This approach used to be very expensive but, as computer power becomes cheaper, it becomes more economic to use it in a wider range of circumstances. It is preferable to use a program which enables the plate elements to be made orthotropic, even if the initial analysis is undertaken using isotropic properties. If, as often happens, initial analysis gives excessive stresses in the transverse steel, the analysis can then be adjusted by changing the element properties. This is much easier than transferring to a completely new analysis.
Three-dimensional models Full three-dimensional finite element analysis of box girder and cellular structures is a very powerful and, with modern hardware and software, reasonably economic way of obtaining realistic serviceability assessments. However, its use in strength assessment is more restricted. It does have application on major structures where, for example, it can be useful in determining the implications and causes of observed stresses and behaviour.The difficulty in strength assessment is that code approaches for determining bending and shear strength work on whole sections. Thus, having undertaken a sophisticated analysis which gives elastic stresses throughout the section, moments and shears for comparison with code predicted capacities can only be obtained by integrating the stresses over the section. The results are very similar usually to those obtained from a simple beam analysis.
55
8 Analysis for assessment
6.3.3 Westergaard and Charts
Global analysis As noted above, it is possible and now sometimes economic to analyse a whole bridge
with a finite element model with a fine enough mesh to use the results directly to check the slab reinforcement. However, it is more usual to use less detailed analyses and these call for a separate analysis to determine the local moments in the deck slab due to wheel loads. These local moments have to be combined with the global transverse moments from the global computer model: the moments induced in the deck slab by its action in distributing load between beams. Only coexistent values (values occurring in the same piece of slab under the same load case) need to be combined.
Nodal loading In order to avoid the combination of global and local moments being over-conservative, it is necessary to ensure that the results in the global model do not include any component due to local moments. This is ensured if there are no nodes between the beams and the loads are applied to the nodes rather than using the option available in many programs to apply them to the members.
Slabs The methods used to determine local moments can also be used to analyse slab structures and are usually quicker to use than a grillage for short span structures where only a few wheels have to be considered. Conversely, on major bridges with wide spans between beams or webs, it may be easier to analyse the deck slab using finite elements or a grillage.
Westergaard The simplest approach is that due to Westergaard66.This considers a one-way spanning simply supported slab allowing for its finite thickness. It gives the maximum moment per unit width under a single wheel as: M, = 0.2107 [0.4825
+ Io~(s/c,)]
My= M, - 0.0676P
Where P = value of point load s =slabspan C, = 2[,/(0.4c2+ h2)] c = diameter of loaded area h =slab thickness Strictly, 5 should include the spread through the surfacing allowed by 8037 but not the spread through the slab as this is allowed for in the analysis. In reality, most slabs are built into beams and are continuous. An approximate
correction for this is to reduce M,and Mygiven by the above equations by 0.07P and 0.1065P respectively.
When the slab span is longer than a critical value, normally 1.7 times load spacing, the effect of more than one wheel has to be considered. Westergaard derived solutions for this and they are also given by R o ~ and e ~elsewhere. ~
56
Pigeaud Charts Older bridges often had intermediate diaphragms so that the slab was two-way spanning. A simple elastic way of analysing these for concentrated loads using charts has e ~by ~Reynolds been derived by Pigeaud and the necessary charts are given by R o ~ and and Steedmad8.They can be used for one-way spanning slabs as well but other methods are usually easier.
Influence surfaces The other common way to undertake elastic analyses of slabs is using published influence surfaces. These are the two-dimensional equivalent of influence lines. They plot the bending moment intensity (for example) at a particular point in a slab due to point loads applied a t all positions in the slab. To use them the wheel load area, including the spread allowed by the code, is plotted on a copy of the chart and a numerical integration of the influence factors over this area undertaken. For simply supported, one-way spanning slabs, the approach gives very similar answers to Westergaard. However, for continuous slabs it has the advantage of giving a more realistic account of the effect of continuity, giving reduced bottom steel requirements, and of giving a value to check the top steel against. The best known influence surfaces are due to Pucher6’. There are others, however, including some which enable slabs with haunches to be considered, as well as skew slabs.
6.4 P h S t i C equilibrium methods
The lower bound theorem of plastic limit analysis states that if a distribution of stresses can be found in equilibrium with an applied loading, and which nowhere exceeds the capacity of the structure, then that load can be safely carried by the structure. If a structure is statically indeterminate (i.e. it has some redundancy), then different equilibrium distributions of stresses can be found and any one of these would be acceptable as the basis of a plastic lower-bound analysis. This theorem underpins many of the approaches used in the design of structures, particularly elastic methods, see, e.g., Clark63.It is important to recognise, however, that this theorem is strictly only applicable to ductile structures and cases where displacements are small. Furthermore, it is essential that equilibrium is satisfied everywhere throughout the structure. The ductility of reinforced concrete sections in flexure can be assessed from their rotation capacity. Considerable research has been focused on establishing the rotation capacity of reinforced concrete sections, a good review of which is given in CfB/NP Bulletin d’lnformation No. 2436’0.The rotation capacity of concrete sections is governed either by concrete crushing or by reinforcement fracture. Criteria are included in BD44 for both of these cases, although the expressions given do not appear to be consistent with the latest research in this field, as described in CfB/NP Bulletin d’lnformation No. 243. Particular care should be taken when considering structures which are either heavily reinforced or which are lightly reinforced with reinforcement that itself has limited ductility. Structures that are moderately reinforced typically have a high degree of ductility, making them suitable for plastic analysis.
57
If lower-bound plastic methods of analysis are to be used, it is also necessary to consider carefully the shear capacity requirement for the structure. Typically, this may be done by ensuring that the shear capacity of the structure is sufficient for both an elastic analysis and for the lower-bound equilibrium stress distribution.
6.4.1 Plastic redistribution
If an elastic analysis is undertaken for a structure with a particular applied loading, the resulting distribution of stresses will be in equilibrium with the applied loading, and therefore provide a plastic lower-bound stress distribution. further lower-bound stress distributions may then be found be adding any self-equilibrating stress distribution to the elastic result. (A self-equilibrating stress distribution is a non-zero distribution of stresses throughout a structure, in equilibrium with itself and with zero external applied loads, except for support reactions. Parasitic moments that develop in continuous prestressed structures are an example of a self-equilibrating stress distribution.) This approach is commonly used in the design of continuous beams in buildings, where moments are redistributed from mid-span to supports, or visa-versa, by adding a moment distribution consisting of straight lines between supports (i.e. a self-equilibrating moment distribution). Such moment distributions can be derived from considering the effect of a small rotation occurring through the formation of a plastic hinge. A similar approach can be used in the analysis of reinforced concrete bridges. This is
most simply undertaken by considering the effect of a small rotation in a yield line across the supports if there is a hogging deficiency a t this location or a t mid-span if there is a shortfall in sagging capacity. The degree of redistribution permitted by BD44 can be checked by comparing the rotation required in the yield line with the specified limits. More sophisticated self-equilibrating stress distributions can be developed.
6.4.2 Hillerborg Strip method
The Hillerborg strip method6”,612is an alternative lower-bound plastic method in which a distribution of stresses in a structure, in equilibrium with an applied loading, is found by dividing the structure into longitudinal and transverse strips, and assigning a proportion of the loading applied to the region where a longitudinal and transverse strip intersect, to each of the two strips. The sum of the loads applied to the longitudinal and transverse strips in the intersecting region must be equal to the applied loading. The method is very convenient for designing two-way spanning slabs in buildings. The approach is less well suited however to the assessment of one-way spanning structures (such as most bridge decks) with complex loading patterns. Yield lines are usually a more appropriate approach as is outlined in the following section
58
6.5 Yield-line analysis
6.5.1 P r i n C i p k S
Yield-line analysis is a long established method of using the plasticity of reinforced concrete slabs in order to obtain greater capacity than that obtained by elastic analyses. It was pioneered by K W Johansenin his doctorate thesis in 19436’3. Geometrically compatible plates of a bridge deck are deflected under load to simulate a failure mechanism. Each plate is bounded by straight lines and the boundaries form plastic hinges with the reinforcement yielding such that a mechanism forms. The work done in deflecting the load is equated to the work done in yielding the reinforcement along the plate boundaries. See Figure 6.1.
6.5.2 Upper bound SO[UtiOnS
This method provides an upper bound solution so it is necessary to examine all conceivable combinations and permutations of plate configurations to determine the failure mode
Figure 6.1 General principles of yield-line analysis for a simply supported slab considering one simple mechanism.
7 I (I Plate A
Plate B
L
I Sagging yield line
Plan Slab simply supported on t w o sides subject t o uniformly distributed load
Unit displacement
Section A-A External work done = Load on Plate X displacement of centroid
Section A-A Internal work done =Yield moment of reinforcement crossing the yield line
X (OA
+ OB)
59
Awalysis for assessment
which gives the lowest ratio of applied work to internal work. Therefore, higher partial safety factors are used usually for yield-line analysis in order to allow for this uncertainty. An upper bound to the load capacity is derived from equating external and internal work done.
6.5.3 Slab decks
Yield-line analysis is a simple and effective analytical technique for slabs. Care is needed in the analysis as any given mechanism gives an upper bound for strength. Published and affinity theorems can be helpful in finding the worst mechanism and the corresponding strength, particularly for short spans where fan mechanisms around individual wheels have to be considered in order to identify the worst case. In the simple example shown in Figure 6.2, the circular fan is a significantly worse mechanism than the easier to analyse square. Work equations for deflection of 6 (uniform slab sagging capacity M/unit width, hogging capacity m/unit width):
Circle P ~ = I -d4+m(?) ~Mr6
r
:.
P = 2~ (M
+ m)
Square
:. P=8 ( M + m ) ~ T ( M m)<8(M + m )
+
i.e. the circular yield-line pattern gives a lower capacity than the square pattern. The affinity theorem? l 4 allow standard solutions for right isotropic slabs to be applied to skewed and orthotropic slabs. This is particularly helpful as the worst case solutions for orthotropic slabs include elliptical fans. For some slab bridges, it is relatively easy, if laborious, to write the work equation which gives upper bounds to the failure load (see Clark63)for yield-line mechanisms such as Figure 6.2 Yield-line mechanisms for a point load on a slab.
60
the ones shown in Figure 6.3 and for a skew bridge in Figure 6.4. However, care must always be taken to ensure that the most critical mechanism has been found, and thorough account must be taken of the influence of reinforcement curtailment, variations in section properties and support conditions. Computer methods can be extremely helpful for rapidly analysing more complex structures to identify critical yieldline mechanisms (See Section 6.5.6). Figure 6.3 Typical yield-line mechanisms for slabs.
(uniform steel giving moment capacity M per unit width)
/ / / / / / / / / / / I
-Sagging yield line Hogging yield line
._____
Figure 6.4
Simple support
Typical yield-line pattern for a slab bridge (from Clark63).
Footway
-
Footway
Hogging yield line Sagging yield line
61
8 Analysis for assessment
Yield lines v. Elastic analyses Yield-line analysis is generally only used when elastic analysis has failed to show adequate strength. However, for simple slabs subjected to wheel loads, yield-line analysis is often quicker than elastic analysis so it may be appropriate to start with yield-line analysis. The method can also be applied to continuous and trapezoidally-shapedslabs.
6.5.4 Beam and slab decks
Principles Yield-line analysis can also be used for beam and slab decks. However, this requires more judgement as methods of analysis are less established. Recent work on developing a computer program to analyse beam and slab decks has been carried out by Middleton6”. The work equation is written and solved in the same way as for slabs. As with all yield-line analysis, it is necessary to check shear strength separately. Ductility problems are also much more likely as beams can often be over-reinforced.
Torsional hinges and transverse hinges in slabs Some engineers have included work done by torsional hinges in the beams and also moment hinges in slabs acting in conjunction with beam hinges. However, when Jackson616investigated the approach both theoretically and by comparison with tests, he concluded that it was better not to do this. The deformations required to develop full torsional resistance are unrealistically large whilst the transverse section of the slab acts as the top flange of the beam. It cannot simultaneously be fully stressed in compression under global moments and acting as a yield line. Ignoring work done against these hinges makes the analysis only marginally more conservative but considerably easier.
6.5.5 BOX Culverts and retaining walls
Yield lines can be particularly helpful in the assessment of box culverts and portal structures in order to take full account of their transverse and longitudinal capacity Transversely, the full support and mid-span section capacity can be used although redistribution methods described in Section 6.4.2 can also be used. Care must be taken to consider plastic hinges that can occur where corner bars are curtailed either in the top slab or part way down the walls, see Figure 6.5
Figure 6.5 Yield lines for the transverse analysis of box structures.
Uniform reinforcement
62
Effect of curtailment in top slab
Effect of curtailment in wall reinforcement (1)
Effect of curtailment in wall reinforcement (2)
Figure 6.6 Yield lines for the longitudinal analysis of box
1
structures.
Capacity of this yield line may be affected by transverse hinge location as shown in Figure 6.5
Longitudinally, more conventional slab yield lines can be used to increase HB or BD21 vehicle assessment capacities, assuming the walls provide moment restraint. The slab capacity a t the wall joint can also be limited by curtailment of the corner bars, see Figure 6.6. Yield lines can also be used to analyse retaining walls and wing walls as is described by Clark63.
6.5.6 Limitations O f yield-line methods
There are a number of important points to be considered when undertaking yield-line analyses.
Ductility It is essential that reinforced concrete slab sections have sufficient ductility to justify
using the method. However, as discussed by Denton 6 5 , it is difficult to quantitative precisely such ductility requirements since they can be dependent upon the structure and the loading arrangement. Certainly, the sections should be under-reinforced such that reinforcement yields substantially before concrete crushing occurs. Codes generally allow the application of yield-line analysis when the neutral axis depth is less than of the section depth. Such a limit will be safe, and may be quite conservative in some cases. Jackson616has proposed significantly less onerous rules such that if the strain in the outermost layer of reinforcement under ultimate capacity, as defined in BD44/90, exceeds 0.0024 + 1.2fy/€,, the ductility is adequate. Similar requirements are given in the draft BD on the use of yield-line analysis. This will not be a problem unless the slab is heavily reinforced or concrete is of very low strength. It is often worth taking cores for structures which may have low strength concrete in order to justify higher strengths and ductility. Problems with ductility may also occur in very lightly reinforced slabs with brittle reinforcement.
Edge beams It may be necessary to be conservative when estimating any benefit from edge upstands
as the composite section may be non-ductile. Torsional resistance of edge or parapet beams is also best neglected as is discussed in Section 6.5.4.
63
6 Anallyis for assessment
Anchorage and curtailment Bars must be fully anchored on each side of the yield line as the reinforcement must be able to reach its full yield strength. Therefore, curtailment of the reinforcement or inadequate laps can influence significantly the failure mechanism.
Concrete deterioration and corrosion Delamination of the cover concrete adversely affects the bond strength of reinforcement and extends the effective curtailment lengths. It can also reduce the effectiveness of laps such that full yield cannot occur. Moreover, pitting corrosion of reinforcement can cause premature yielding on a reduced section, which greatly reduces the overall ductility. In general, plastic and yield-line analyses are not recommended for deteriorated concrete structures.
Lift off at supports When analysing heavily skewed slabs, it may be necessary to consider mechanisms in which the slab lifts off the supports. Mechanisms that rely on corners being held down are potentially unsafe.
Slab voids For voided slabs, it is necessary to check that excessive local bending moments are not developed in the top and bottom flanges through transverse shear.
Prestressed slabs The ductility of prestressed slabs can be different to reinforced concrete and serviceability is more important. Therefore, the method is not recommended generally for these types of structures.
Local and global effects Further to the points discussed in Section 6.5.4, when yield lines are used for beam and slab bridges, due to redistribution, global flange forces or global transverse moments in the deck slab can be ignored when assessing local strength except in slabs which are highly stressed under both global and local effects. In such cases, the brittleness of local failure modes can prevent redistribution. When the slab is used to enhance the plastic moment capacity of the beam, the longitudinal moment capacity in the slab should be ignored in the work done against yield lines in the slab.
6.5.7 Computer analysis
Yield-line analysis can require a large number of calculations to check all the possible failure configurations. Computer programs and spreadsheets have been written to speed up the analysis process, for example, as described by Denton617. Work by Middleton a t Cambridge University in conjunction with the Highways Agency6’’ has led to the development of the computer program COBRAS6’*for bridge deck yield-line analysis, which is now commercially available. The program is easy to use and undertakes a large number of yield-line analysis patterns with the minimum of data.
64
COBRAS is understood to be the most sophisticated commercially-availableyield-line analysis program and enables a wide range of yield-line configurations to be selected and analysed. The program varies each yield-line configuration in turn on the bridge deck in small increments, so finding the critical failure geometry for each configuration. The optimum solutions of each of the various configurations are then compared with each other and the overall optimum solution obtained for the particular loading case. In this way, the assessed capacity of the bridge deck is determined.
6.5.8 Shear
6.6 Strut and tie action
Conventional yield-line analysis only assesses the ultimate bending capacity. It does not deal with shear capacity. Normally, shear capacity is checked both by punching shear and by the shears output from a standard elastic analysis, for example grillage or finite element analyses. Research has concentrated on plastic methods for the assessment of shear, but the work is quite
Strut and tie action has been used for the analysis of pile caps and anchorages. The principles are set out in Reference 6.20 and an example of use is given in Reference 6.21. It can be used also for the analysis of diaphragms and deep beams. The principle is based on establishing compressive struts of concrete and tensile ties of reinforcement. The size of the compressive struts is dependent on the allowable compressive stress for the grade of concrete and the force carried. The method usually requires iteration to establish the appropriate geometry of the strut and reinforcement in the ties (see Figure 6.7). Plane frame or space frame analyses can be carried out to solve two and threedimensional strut and tie problems. These are linear-elastic analyses of a non-linear problem and serviceability needs to be considered as discussed in Section 6.7.
Example - Column head A reinforced concrete column of diameter D supported a steel box crossbeam carrying a motorway deck through a steel rocker bearing. Below the top solid section of depth H of the column, the column was hollow with reinforced concrete walls with a thickness of t.
Hoop reinforcement was provided in the solid section as anti-bursting reinforcement. From strut and tie action, a calculation of triangle of forces provided the transverse tension Tand, hence, the load in the hoop reinforcement, TI2 as shown on the plan. It can be shown that the force in the hoop reinforcement is given by: T=P x
(D- t ) / H
In this case the reinforcement was partly corroded and the upper part of the column was strapped with steel rings to compensate for the loss of capacity.
65
Analysis for assessment
Figure 6.7 TI2 Hoop force
Example of strut and tie action.
Hoop reinforcement
Radial tie and 1 strut forces
PLan
J , Bearing load P
Compression struts
1" O
0
0
Tie provided by hoop steel
+ 4
--t
D
-
Strut and tie action on the top of a hollow section
6.7 Serviceability limit state 6.7.1 Inspections
BD21 assumes that for older structures an inspection gives the best indication of serviceability and therefore quantitative analysis is n o t required. For newer structures, serviceability assessments are recommended because there is insufficient service experience to ensure that problems will be apparent from inspection. The same arises for structures where the loading is being increased significantly, such as due t o road
widening or increased railway loads, although this is n o t mentioned in the BD. BD21 and BD44 do not give serviceability criteria. The usual approach is t o report against the criteria used in design. However, remedial action is not always required for 'failures'. Experience shows that structures that are predicted by elastic analysis t o suffer serious yielding and cracking often show no signs of distress. Where it is unclear if remedial action is needed, a quantitative elastic assessment can be used t o identify areas for closer inspection and future monitoring.
66
6.7.2 Plastic methods of a na ly s is
When elastic methods of analysis with representative section-stiffnesses are used t o assess the ultimate limit state loading effects, this generally does not result in excessive stresses and deformations at the serviceability limit state. However, the same is not necessarily the case for yield-line, strut and tie action and other plastic methods of analysis which assume that yielding of reinforcement can occur as the ultimate loading levels are approached. Consequently, if plastic methods of analysis are used it is prudent t o consider separately the serviceability performance of the structure, either using elastic methods or special inspection.
6.7.3 Special inspections
If structures fail the serviceability limit state assessment, it is recommended that a special inspection of the parts of the structure that are expected t o show signs of distress is carried out. It is best that such inspections are carried out after assessment as the pre-assessment inspections often do not identify the problems or do not record the information in sufficient detail for its significance t o be appreciated. In practice, inspections have shown that slabs failing their ultimate capacity by elastic analysis but passing by yield line, may not be severely cracked, and their serviceability condition may be deemed t o be acceptable based on inspection. In such cases, continued monitoring t o ensure durability is advised.
6.8 Soil structure interaction
Soil structure interaction can be modelled by finite element analysis and finite difference programs such as FLAC to realistically assess the soil forces which act on the structure. Typical examples are reinforced concrete arches and buried box and portal structures. Particular care is required, however, with such an approach due t o the variable properties of the soil, and the risk that the relieving effect of the soil on the structure may not be as great as predicted. Advice from geotechnical engineers is recommended and a range of Sod properties used t o take account of the uncertainties. The distribution of bearing pressures under spread foundations depends on the type of soil and the duration of loading. Cohesive soils which are 'soft' because of long-term settlement give uniformly varying bearing pressures under bases. Cohesionless soils are 'hard' and slight deformation of the structure, such as the hog of the base slab of boxes, can lead t o parabolic bearing pressure distributions with peak values at the hard spots, which in the case of the box would be under the walls. This effect can be analysed with simple plane frame analysis of the box with springs representing the soil. Again, a range of soil stiffnesses should be used. Embedded retaining walls such as contiguous and secant piles walls or diaphragm walls should be analysed with suitable soil structure interaction programs such as FREW and WALLA P.
67
8 Analysis for assessment
6.9COndUSiOnS
Many detailed points have been made in the previous sections. However the important principles to note are: 1. In contrast to design, the details of a structure being assessed are fixed. It is necessary
therefore to choose an analysis which is appropriate to the structure. The structure cannot be altered to suit the analytical results; the analysis has to be adjusted to suit the structure. 2. When an analysis suggests that something is inadequate, its significance should always be questioned. It may be a real problem but, frequently, there are other analytical methods available that can prove that it isn’t. Even if this analysis is very expensive, it is still likely to be much cheaper than resorting to strengthening the structure. 3. There is no reason to use an analysis that is any more expensive than the minimum required to prove the structure adequate. If a more sophisticated and expensive analysis is used (compared with a static load distribution) then there should be logical reasons for the choice. 4. A simple, non-linear or plastic analysis may not fully satisfy serviceability requirements. Serviceability checks and inspections can be used to assess the potential risks of durability problems arising during the remaining life of the structure.
6.10References
6.1
HIGHWAYS AGENCY, BD 44 The Assessment Concreteof Highway Bridges andStructures, The Highways Agency, London. 1995
6.2
HIGHWAYS AGENCY, BA 44. The use ofBD44 forthe Assessment ofconcrete Highway BridgesandStructures. The Highways Agency, London. 1996.
6.3
CLARK, LA, Concrete bridge design to 55 5400, Construction Press, London, 1983.
6.4
MORLEY, C and DENTON. SR, Modified plasticity theory for reinforced concrete slab structure of limited ductility, StructuralEngineering, Mechanics and Computation,Vol. 2, 2001
6.5
DENTON. SR, Theanalysis ofreinforcedconcreteslabs, and the implicationsoflimiredductility, PhD thesis, Cambridge University, 2001
6.6
WESTERGAARD. HM, Computation of stresses in bridge slabs due to wheel loads, PublicRoads. Vol. 2. No 1. March 1930, pp 1-23.
6.7
ROWE, RE, Concrete Bridge Design. Applied Science Publishers ttd, London, 1962.
6.8
REYNOLDS, CE and STEEDMAN, JC, Reinforcedconcrete designers handbook, Viewpoint Publications. (Various editions)
6.9
PUCHER, A, Influencesurfacesforelasticplates, Springer Verlag, Vienna and New York, 1977.
6.10 CE8 Bulletin 243, Strategiesfor testing andassessment ofconcretestructures, CEB/FIP. Lausanne. Switzerland, May, 1998. 6.11 HILLERBORG, A, The Advanced Strip Method - A simple design tool, MagazineofConcreteResearch, Vol. 34, No. 121. December 1982 6.12 HILLERBORG, A, Strip MethodDesignHandbook,E and F Spon, 1996 6.13 JOHANSEN,KW. YieldLine Formulaefor Slabs, Cement and Concrete Association, Camberley, 1972. 6.14 JONES, LL, WOOD, RH, Y,eld-LineAnalysisofSlabs, Thames and Hudson, Chatto and Windus, 1967. 6.15 MIDDLETON, CR, Case Studies from the UK using Yield-Line Analysis for Concrete Bridge Assessment, Austroads Bridge Conference 1997, Sydney, Australia Vol 1, 1997. 6.16 JACKSON, PA, The global and local behaviour of bridge deck slabs, TheStructuralEngineer, Vol 68. No. 6. March 1990, pp 112-116 6.17 DENTON, SR. Hidden Strengths, Highways, November 1995. 6.18 COBRAS Concrete Bridge Assessment Package User Manual, University of Cambridge Department of Engineering 6.19 NEILSEN, MP, LimitAnalysisandConcretePlasticity, Prentice Hall, USA, 1984 6.20 LEONHARDT, F, Reducing the shear reinforcement in reinforced beams and slabs, Magazine ofConcrete Research, Vol 17, No. 53, 1965. 6.21 At-SHAWAF, S and JACOBS, P, The Basingstoke Canal Aqueduct, Proceedingsof Institution of Civil Engineers. Vol. 126, No. 1, February 1998, pp 19-30,
68
7. Hidden strengths The large number of structures which have failed assessment to current standards yet do not display any signs of distress, suggests that there are reserves of strength which are not taken into account in the normal assessment process. This Chapter explores some factors that affect the strength of structural elements and may be of use in justifying increased capacities.
7.1 Reinforcement anchorage and bond 7.1. I First PrinCipkS
Structures can often fail assessment because they do not comply with detailing rules for curtailment and laps. In reinforced concrete beams, the apparent shear strength can be very low when the ‘additional longitudinal reinforcement’ is small, i.e. when the main bending reinforcement is highly stressed. In these circumstances, it is worth considering the anchorage and bond of reinforcement from first principles. The following method is based on satisfying bending and shear equilibrium in reinforced concrete elements.
7.1.2 Reinforced Concrete beams
Sometimes called Regan’s method because of his publication7’, it generally applies to reinforced concrete beams where links are provided to carry shear. It is also called ‘variable angle truss analogy’ and is included in EN 199272.It can be illustrated simply in a cantilever situation as shown in Figure 7.1 (a) but it can also be applied to simply supported and continuous beams. A shear failure crack is assumed to form at an angle 8 to the neutral axis of the beam such
that the shear force Vis just carried by the capacity of the links crossing the shear plane. For vertical equilibrium: V = A,,f,,(Zcot
8 /5)
the cross-sectional area of all the (vertical) stirrup legs a t each stirrup location 5 = the stirrup spacing along the beam fsv = the yield strength of the stirrup reinforcement divided by ’ym Z =the effective depth.
Where A,,=
Therefore: cot 8 = (V/ASvfSv)(S/Z)
(Equation 7.1)
Cot 8 is generally taken as not less than 1, or greater than 3 (i.e. 45 degrees > 8 > 17.5 degrees). However, EN 1992 restricts cot 8 to the range 1 to 2.5.
69 ~
4 4 Example of Regan's method
/-rpi W
1
Force
Applied force
2 cot H
Force in bar due t o shear
Force in bar due E t o moment Z
-
A
Distance along cantilever
b J Force to be resisted by main reinforcement to maintain equilibrium (applied force)
Reinforcement capacity
Reinforcement capacity
W
\
I
1 Aggregate of bond strength along bar
A
Bond from bend
Distance along cantilever
4 Force which can be developed in reinforcement from anchorage and bond (capacity)
Figure - 7.1 Example of Regan's method (variable angle truss analogy) for reinforced concrete beams appliedtoa cantilever.
The load on the cantilever tiD W is iust SuDDorted bv a force in the main reinforcement f a t the crack. By considering the equilibrium of the cracked cantilever ABDE by taking moments about the centre of concrete compression C above E, and here neglecting dead loads, then: I
wy + Zcot
vz
8) = FZ + -cot 2
,
8
The bending moment M a t the point in the reinforcement where F is measured is equal to Wl. Also W = V.
70
Therefore: M + VZ cot 6'= FZ + (VZ/2) cot 0 F = (M/Z)
+ Vcot 6'-
(V/2) Cot 6'
And this reduces to: F = (M/Z)
+ (V/2) Cot 0
(Equation 7.2)
The force in the reinforcement is a function of both the moment and the shear force, which is the basis of the additional longitudinal reinforcement requirements and rules for curtailment of reinforcement. as The maximum force in the reinforcement at supports however cannot exceed MJ,Z the angle of the shear failure plane is limited by the physical presence of the support, i.e. cot 0 must be zero. The maximum bending moment in the span is also never exceeded either as a t that point the shear force Vis zero. See Figure 7.l(b).
Having calculated the force in the reinforcement, the adequacy of the anchorage t o the curtailment point can be determined by summing the bond strength along the bar from the point at which the force is measured t o the end of the bar. See Figure 7.1(c). The assessment Standard gives values for the bond strength and further guidance for deteriorated structures is given in Section 7.1.3. Transverse pressures applied t o the reinforcement (e.g. from bearings) can also increase anchorage strength. This is discussed in Section 7.1.8. Hooks and bends can be taken into account in providing extra bond. Providing the maximum shear stresses are checked in accordance with BD44, there should be no problems with compressive shear failure of shallow shear planes with low angles of 0. The benefit of this method is that it calculates the force in the reinforcement at any point by a more exact method than is given in BD44. The method has been used previously and 'Departures from Standard' have been accepted, drawing upon Clause 5.8.7 of BD44, which allows the use of rigorous analysis for calculating curtailment lengths and anchorages of bars.
7.1.3 Sub-standard Cover and deteriorated concrete
Fundamental t o Regan's method is the development of the force in the reinforcement, which is dependent on its bond and anchorage. For undamaged structures the bond strength given in the assessment Standard BD44 can be used to calculate the developed force. Should the cover be less than the normal cover required for fully developing bond (i.e. less than the bar diameter) then reduced values should be used. In the case of damaged or deteriorated structures, the cover can be delaminated and this reduces the bond substantially. Typically, the bond in deformed bars with delamination t o half-barrel depth can be 50% of code values provided they are well constrained by links. However, the bond of plain round bars can reduce t o 10% of the code value. Figure 7.2 illustrates the effect of reduced bond. Guidance on this is given in a revised draft of BD44, which takes account of deteriorated structures. See also Chapter 9.
71
Deficient capacity Delaminated length
1
.. <
Reinforcement capacity for full bond
Force capacity of bar limited by 0.87 X yield strength
- .,-Reduced bond 7 Full bond
Applied force
J
__t
.. -
reinforcement
Distance along cantilever Applied force in main reinforcement compared with capacity
Figure 7.2 Delaminated bond of reinforcement.
7.1.4 Bond a t laps
If the bond strength is reduced significantly there may be insufficient anchorage for reinforcement and laps may be deficient. Regan’s method will identify lap deficiencies as shown in Figure 7.3.
If anchorage or lap lengths are less than those required for the bars to yield then ductility will be compromised and the distribution of load in indeterminate structures needs to be carefully considered. See also Chapter 6 on general analysis and yield-line analysis for comments on ductility.
7.1.5 Haunched Sections
7.1.6 Bent-up reinforcement
Regan’s method of calculating the force in the longitudinal reinforcement resulting from the combination of bending and shear can be used for varying depth sections such as haunched beams by calculating the appropriate value of cot 8 for the section considered, as shown in Figure 7.4.
Where there are bent-up bars towards the beam end, these can be incorporated in the Regan analysis to carry additional loading as shown in Figure 7.5. The angle 8 of the shear failure plane is derived such as to provide sufficient reinforcement crossing the shear failure plane to carry the applied shear. This reinforcement consists of the shear links crossing the failure plane together with the vertical component of the capacity of the bent-up bars. Using the same principles and terms as Section 7.1.2, Cot @canbe found as follows:
(Equation 7.3)
72
..
..
..
...
..
-
.
. ... . -
..
.- ...
'
. . .. 1 . _.:~~
Figure 7.3 Deficient lap lengths identified by Regan's method.
A
A
,
-
1,
1L
-
L
//
LH bars
1,
' Plan of bar laps
Combined capacity
Resistance force
Capacity of RH bars
Capacity of LH bars
d
Distance
Capacity of bars with full bond
f
Combined capacity Resistance force
t
Reduction in capacity
of RH bari
of i H bars d
Distance
Reduced capacity due to reduced bond caused by delamination
Figure 7.4
V
Shear of haunched beams.
t
V =Q-Csin@ 0 = Found by trial and error based on number of links t o carryV
Where B a
= force in the bent-up reinforcement = angle of the inclined bars t o the vertical
= area of bent-up bars = stress in bent-up bars =yield stress divided by ym fsb (VIZ) = reduction factor t o account for the inclined bars not developing their yield strength where they cross the shear plane
A,,
73
Figure 7.5 Regan’s method applied to bent-up reinforcement. 2
cot 0
Vertical links Bent up reinforcement
R
U” 1 2
cot 0
Reduction from inclined bars
Force
A
d
Distance Force to be resisted by main reinforcement
The anchorage of the bent-up bars must be such as t o enablef,, t o be developed. Several different locations of the shear failure plane must be analysed t o provide the critical capacity of the continuing main bars. Where the bottom of the crack reaches the face of the support, further locations of the crack should be considered in the form of a fan with increasing angles of 8. Taking moments about the intersection of the crack and the compression force C,f is calculated from:
The force F must not exceed the capacity of the continuing main reinforcement acting at 0.87 of its yield stress. The anchorage bond of the main reinforcement continuing to the end of the beam must be sufficient t o develop the force f in the bars at their intersection with the crack.
7.1.7Inclined links
74
The above examples only consider vertical links but the principles equally apply t o beams with inclined links. Both the horizontal and vertical components of the link forces are used in checking equilibrium in much the same way as bent-up reinforcement described above.
7.1.8 Bearing Clamping O f reinforcement
Transverse clamping pressure on reinforcing bars can increase the bond strength and can compensate for inadequate anchorage, lap or curtailment lengths which might arise as a result of under-design, increased loading, or delamination or spalling of cover. Transverse pressure may be provided on the soffit of beams or slabs by supporting bearings, piers or abutments. On the top surface of beams, transverse pressure may be provided by the loading through deck beam bearings, see Figure 7.6. The code rules for the anchorage of bars at simply supported ends can be significantly less severe than simple first principles calculations would suggest, because they allow empirically for bearing clamping effects. The increase in bond strength due to transverse clamping pressure depends on a number of different factors including: 0 Depth of cover (the effect is greater as the cover reduces) 0 Whether the concrete is delaminated under the bearing 0 Concrete strength
0 Type of reinforcement - plain round or deformed 0 Bar diameter and the amount of transverse pressure 0 Containment of the bars by links and side cover 0 Type of bearing - rigid or flexible.
References 7.3 to 7.6 give results of work carried out on this subject and which were used to derive rules for the draft revised BD44 dealing with deteriorated structures.
Figure 7.6 Bearing clamping forces and application
The effect on bond of transverse pressure where the concrete under a bearing is delaminated is more significant than for non-delaminated cases but the unclamped delaminated bond value is very low. For clamped delaminated cover over Type 2 deformed bars in tension, the bond strength derived from clamped pull-out tests a t the University of W e ~ t m i n s t e can r ~ ~be found as follows, provided the side cover is not susceptible to failure:
to Regan's method.
(Example based on Figure 7 1)
Bearing clamping force
..
7 Reinforcement capacity
Force capacity of bar limited by 0.87 X yield strength without clamping
Applied force . .
1
-
--..- A 1
Bond from bend
A
Distance along cantilever Force which can be developed in reinforcement from anchorage and bond (capacity)
75
Bond stress over the clamped length of bar is:
Where p = 0.25@ /,/feu ) but not less than 0.7 p =transverse pressure in N/mm2 f,, = cube strength in N/mm2 and p
/,/feu > 0.05
In a delaminated anchorage length where some bars are not in stirrup corners, either the delaminated bond stress for bars not in stirrup corners should be used for all bars including those clamped, or the bond stress should be taken as zero for all bars or part length of bars not in stirrup corners and not clamped. Care must be taken, when allowing for clamping increasing bond strength in beams, that
failure of side cover does not weaken the assumed bond strength. Vertical pressure from the clamping of the bearing can promote failure of the side cover and then reduce rather than increase the bond strength of the sidebars.
7.2 Shear
7.2.1 Shear in reinforced concrete slabs
The rules for the assessment of shear cover two extreme cases: 0 Punching shear a t pier supports or a t points of application of concentrated loads
U Average shear across sections
They do not give specific guidance for the following problem areas: U Concentrated loads close to free edges 0 Concentrated loads close to supported edges
0 Concentrated loads on wide one-way spanning slabs (such as slab bridge decks) 0 Shear a t obtuse corners of slab bridges 0 Shear on sections inclined to the directions of the reinforcement. The following, however, does aid tackling these problems: 0 Unlike BS5400 Part 4, BD44 allows enhancement of shear stresses on short shear spans. 0 It is recommended generally that the distribution of shear is determined from elastic
grillage or finite element analyses. However, analyses tend to give very high local values and the amount of averaging is not defined. 0 In most cases, it is accepted that it is reasonable to distribute the shears over a width of up to 2 to 3 times the section depth. However, all forms of distribution or spreading of the load must satisfy equilibrium. 0 Shear capacities on planes that are not normal to the directions of reinforcement should be calculated using the effective reinforcement crossing the plane using cos4 0 terms. This is based on stiffness requirements rather than strength as proposed by Clark
76
Regan has carried out testing of reinforced concrete slabs covering concentrated loads on free edges and half-width loading and has demonstrated reasonable correlation with the test results using lower bound plastic analysis with checks on the corresponding shears. Hillerborg’s method 78t0710demonstrates this approach. This gives confidence that simple elastic analyses will give safe results. A reasonable approach t o the problem areas identified above, therefore, is t o carry out grillage analyses specifically orientated t o suit potential shear failure planes with member widths of 2 t o 3 times the section depth. This will give the shears appropriate t o average over the section and ensure that equilibrium is being satisfied.
7.2.2 Effect O f Varying section Size
If the longitudinal section of an element varies then the relative angle between the tension and compression forces resisting bending will decrease or increase the shear carried by a section. The effect is most pronounced on haunched members as is shown in Figure 7.7, where the shear carried by the section is reduced by the compression force in the bottom of the section. Similarly, simply supported fish-bellied sections reduce the shear carried because of the tensile force in the bottom flange. However, haunches at simple supports and reverse haunches at continuous supports can increase the shear carried, and this needs t o be taken into account.
7.2.3 Shear enhancement a t supports
Enhancement of shear at supports requires anchorage development lengths of 20 diameters according to BD44. However, tests have been carried out at Queens University Belfast, TRL, Birmingham University and Bath University, which indicate that intermediate values can be u ~ e d ~ ~ - ~Further ” , ~ ’ *details . of this work are included in Section 9.1 1. Regan’s method in combination with bearing clamping as described in Section 7.1.8, can also be used t o justify increased shear capacities a t supports providing links are present.
Figure 7.7 Shear carried by variable section deck.
Continuous bridge deck (also applies t o simply supported)
Inclined neutral axis
Tension force in top flange
+-\t I ,;*? / , I--.---
/--
.--.--/4
straight haunch
Compression force in bottom flange
77
7 Hidden strengths
As an alternative to the method in 7.1.2, a method has been developed by Shave eta1713
that enables the shear capacity t o be calculated in cases where the longitudinal reinforcement has a short anchorage length. The method is based on a lower bound approach, and has been simplified in a form that would be compatible with the general approach of 8044. However, for bridges with short anchorage lengths, the method is much more accurate and less conservative than the existing rules in BD44. For members with shear reinforcement, the theory is based on a variable angle truss approach. The angle of the truss 0 may be chosen such that the following inequalities are satisfied: V < AJYY (z/s) cot0
(ensures that the stirrup stresses do not exceed yield)
V < (bzvf,,) / (cote+ tan0)
V < 2[Fub- (M/z)]/cot0
(ensures that concrete compression does not exceed crushing limit)
(ensures that anchorage failure does not occur)
These inequalities are consistent with the approach in EN 1992, and may be optimised t o determine the best lower bound that does not exceed the relevant limits. The second inequality requires the concrete effectiveness factor v t o be calculated. Various proposals for effectiveness factors have been made72, 14, ”, The approach in Shave et alis based on the formula for effectiveness factor in Reference 7.14. So long as the beam is not over-reinforced in shear, the solution for shear capacity
varies with the square root of the anchorage force f u b . For members loaded a t short shear spans and members without shear reinforcement, a strut and tie approach is used. The variation of shear capacity with anchorage force is more complicated, but may still be approximated by a square root function up t o the fully anchored capacity. These models give results that are consistent with upper bound analyses, and have been verified by a comprehensive programme of experimental testing. BD44 suggests that a t short anchorage lengths, the shear reinforcement is deemed t o be ineffective, and that the enhancement of shear capacity at short shear spans is not allowed. However, the research by Shave et al indicates that this is not the case. Even a t short anchorage lengths, the stirrups contribute t o the shear capacity and shear enhancement occurs a t short shear spans.
The shear capacity is reduced by a reduction factor that accounts for the anchorage length. This reduction factor is written as:
78
- -
where a is factor to allow for enhancement of bond due to lateral pressure, and fubmax is the required anchorage force to give the fully anchored shear capacity This is defined as Fu,
max
= 65svcbd
using the notation of BD44/95 The shear capacity is then found by multiplying the standard fully anchored shear capacity (including shear reinforcement and shear enhancement at short shear spans) by the reduction factor Finally, there are some maximum and minimum limits placed on the shear capacity, which are the same as the existing limits in BD44/95 This method gives predicted shear capacities that have been verified by test data, and is much more accurate than BD44 at short anchorage lengths
7.2.4 Shear in prestressed flanged beams
The standard equations for shear strength in BS 5400 Part 4 and BD44 are based on rectangular sections where the shear stress is 50% higher than the average stress Generally, this is conservative for I section beams which are uncracked in flexure and rigorous checks can be carried out a t the top and bottom web to flange connections as well as intermediate points including the neutral axis, using the appropriate section properties to calculate the principal tensile stress The code rules only check a t the neutral axis Due to the effect of flexural tensile stress, the rigorous check can give a lower strength when the bending moment is significant In such cases, the method does not give any advantage but the code rules can be used since they have been justified by comparison with test results When using this rigorous method on post-tensioned beams, then the reduction of web width for the presence of ducts can take account of their actual position rather than a general reduction See also below for grouted ducts
7.2.5 Shear in webs O f post-tensioned prestressed beams
The reduction in effective width of webs due to the presence of ducts is specified in both BS5400 Part 4 and BD44 as 0 67 times the diameter of the duct Tests have been carried out to suggest that this is conservative providing that the ducts are adequately grouted and reductions to say 50% of the diameter can provide significant benefit
7.3 Deck Surfacing
The use of asphalt surfacing to provide additional stiffness and strength to bridge decks has so far only been applied to extend the fatigue life of orthotropic steel decks A paper on testing mastic asphalt to find a suitable mix for that purpose cites the use of an indirect tensile strength test, an indirect tensile creep test and an indirect tensile fatigue test7 Whether surfacing can be justified in increasing strength is considered below
7.3.1 Properties O f Surfacing materials
Asphalt surfacing IS known widely as flexible surfacing Its success IS based on its ability to flow and to seal up cracks, which form due to temperature and other weather effects and due to the action of wheel loads Clearly, as such, it is not a rigid material - it
79
Hidden strengths
moves under load. It does not appear feasible therefore to derive an E-value for asphall material since creep under load is such a significant factor. Temperature effects are highly significant, asphalt being much softer in high summer temperatures and much more rigid in cold winter temperatures. Therefore, it is a very variable material. Asphalt materials vary widely. While the cube strength of concrete will largely determii its contribution to the strength of a bridge deck, and its E-value will not vary significantly for this purpose even as the strength varies, this cannot be said for asphalt The tests for asphalt surfacing to determine its suitability are generally compaction related rather than specifically strength related. Therefore, if asphalt were to be used t c contribute directly to the strength of bridge decks, new tests would most likely have to be devised to ensure that adequate strength and stiffness were being provided. The bond between the asphalt surfacing and the bridge deck cannot be guaranteed. There are many cases of surfacing peeling away from the deck and therefore there would be a question of reliability if the surfacing were to contribute to the integrity of the bridge deck. The content and laying conditions (temperature and humidity) of bott the waterproofing membrane and the surfacing base course will affect the bond both between the deck and the membrane and between the membrane and the base course
7.3.2 Discussion
It is unlikely that normal depth surfacing on bridge decks will be beneficial to the ultimate strength and capacity. There may be serviceability benefits as for orthotropic decks but these can be judged by performance on most bridges.
7.4 Moment field analysis of reinforced concrete slabs 7.4.1 Moment fields
If an element of a slab is subjected to a combination of bending and twisting moments (i.c a moment field), the bending moment applied about any axis within the plane of the slab can be determined from equilibrium considerations. The applied moment field is given by:
M, = M,cos' a + M, sin' a Where M ,and M, are the principal moment^^'^,^^^ and a is the direction relative to th principal moment M,.This applied bending moment field is illustrated in Figure 7.8, using a polar co-ordinate plot. Similarly, the moment capacity of the slab will vary depending upon the axis about which it is calculated. Assuming that the reinforcement orientated in different directior acts independently, which is reasonable provided the slab is lightly reinforced and the reinforcement is not heavily skewed, the resistance moments vary by a similar relationship to the applied moments, as follows:
M*$ = M*,COS' $
Figure 7.8 Applied moment field.
Where M*, is the resistance in the direction of the reinforcement and the direction being considered.
4 is the angle t o
Again, assuming the reinforcement in different directions acts independently, the contribution from all the reinforcement directions can be combined t o give the Resistance Field by superposition. This approach is illustrated in Figure 7.9.
7.4.2 Wood-Armer equations
When assessing slabs using grillage or finite element methods, i t is necessary t o demonstrate that the bending moment capacity about any axis in the plane of the slab exceeds the moment applied about that axis. In the design of reinforced concrete slabs this requirement is satisfied generally through the use of the Wood-Armer equations (Figure 7.10)7'9,720. The equations ensure that the capacity of a slab is not exceeded in flexure by an imposed loading, whilst minimising the total amount of reinforcement required. However, the use of these equations for assessment leads generally t o a
Figure 7.9 Resistance moment fields.
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Figure 7.10 Wood-Armer resistance field.
Wood's resistance
conservative estimate of structural capacity in all cases where there is spare capacity in one of the reinforcement directions at the critical point. Ignoring this can lead t o highly conservative assessment ratings.
7.4.3 Moment field approach
7.4.4 Torsionless grillages
Alternative approaches are available, based on the same fundamental principles as the Wood-Armer equations, but which are directly applicable t o assessment; see Denton717, '*. These directly compare the applied moment field with the resistance field of the reinforcement provided in the structure and avoid the conservatism of the Wood-Armer equations, see Figure 7.1 1. The LEAP5 post processor RASP also follows the same principles but a simple computer program or spreadsheet which checks the applied moment field against the resistance field in all directions can also be used.
As a further alternative, a torsionless grillage can be run based on the actual reinforcement directions and the stiffnesses varied iteratively if necessary, in order t o satisfy both strength and equilibrium as discussed in Section 6.4.2.
7.5 COmpreSSiVe membrane action
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Tests on slabs restrained around the edges have recorded strengths, which are far greater than predicted by conventional flexural analysis. The strength is also far less sensitive t o reinforcement area so that the ratio of actual strength t o conventional prediction, which typically is around four with conventional amounts of reinforcement, but can be ten or more with very light reinforcement. The reason has been identified as the presence of inclined compressive struts in the concrete providing an arch or dome effect. The phenomenon is called Compressive Membrane Action. Although there are a
..
...
-
--..
.__7r__1__-__
Figure 7.11 Resistancefield which demonstrates satisfactory resistance.
Wood’s moments
moment field
(but ‘failure’ to comply with Wood-Armer resistance requirements)
I
moment field
I
Imoment field I
number of references that explain the principles, the methods are under used because of the lack of usable methodologies and guidance. However, the Highways Agency has produced guidance on the ~ubject’~’. Key aspects will be considered here.
7.5.1 Local strength O f deck slabs and restraint
BD81 is primarily concerned with the deck slabs of beam and slab type bridges. It gives a simple way of assessing the local strength of the slab based on work by Kirkpatrick, Rankin and Long722, An even simpler alternative is to use charts in the Ontario Highway Bridge design code723.Using either method, for most practical cases, the strength will be very high so that its precise value is not important. The major practical restriction on these essentially empirical methods is that they depend on nominal rules to ensure that the restraint required to develop the compressive force is adequate. BD81 imposes restrictions on span, span to depth ratio and edge details as well as requiring support diaphragms. These rules are necessarily conservative.J a ~ k s o n ‘ for ~ ~ ,example, has shown that the effect can still work without support diaphragms. However, there is no simple way of assessing the strength of deck slabs that do not comply with the rules. The only way is to use some form of non-linear numerical analysis. Non-linear finite element analysis has proved able to model the behaviour well in some cases. The predictive capabilities of non-linear numerical methods is less well established than their use in analysing experimental findings. Their use in assessment therefore requires specialist expertise. However, at the present state of the art, it is by no means foolproof and great care is required. Key relevant aspects are considered in 7.5.4.
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7.5.2 Global behaviour O f beam and slab decks
Compressive membrane action has the effect that deck slabs, even very lightly reinforced ones, behave as though they were heavily reinforced. They have both the strength and the lack of ductility of heavily reinforced slabs. Because of this, the ‘global transverse moments’ induced in the slab by its action in distributing load between beams cannot necessarily redistribute away from the areas where there are high local moments. It has been found7” that this can greatly reduce the local strength of deck slabs. This will not be a problem normally in bridges that have intermediate diaphragms or cross frames as these take the moments and minimise the effect on the slab. If the main beams are very stiff and have large reserves of strength, the effect is not likely t o be significant either. BD81 allows the effect t o be ignored if the main beams are prestressed and comply with the serviceability requirements of BD24. For all other cases, it is necessary t o consider the influence of global effects on the slab and the only way of doing this is again with non-linear numerical analysis. In assessment, it is important t o realise that this includes cases where the prestressed beams have adequate ultimate strength but, for example, are found t o be Class 3 under service loads. Although it has been suggested that compressive membrane action could also contribute t o a slab’s ability to resist global transverse moments, this has not been proven and there is evidence that the effect may not be developed a t all. It is recommended therefore that the slab should be checked in the normal way t o ensure that the reinforcement is adequate t o resist these moments. BD81 suggests that the moment could be obtained from a conventional uncracked grillage analysis. However, it says that in the analysis t o determine the moment in the beams, the stiffness of the transverse slab members should be halved t o allow for cracking. This would require two separate analyses. A conservative alternative t o avoid this is t o undertake only the analysis with the halved transverse stiffness but t o check for double the transverse moments this gives. Although this is conservative, it will often still show slabs t o be adequate.
7.5.3 External restraint
BD81, and the majority of the extensive research on compressive membrane action in bridges, concentrates on structures where the restraint is internal, coming from the structure surrounding the critical areas. Moreover, it is possible t o get significant advantages from external restraint from abutments. This can increase significantly the strength of concrete slabs and sometimes beam and slab structures. This effect, which is sometimes known as flat arch action, has been investigated by Das7” and by Jackson726. It has been found that the actual enhancement is often substantial. However, in assessment, it is necessary t o use a conservative estimate of the stiffness of the available restraint. This will give a far lesser enhancement but it can still be significant. Primarily, it is likely t o be of benefit in older reinforced concrete bridges where there was often no provision for expansion. More modern structures, at least prior t o the return t o integral bridges, are provided normally with expansion joints and are less likely t o benefit. An exception, however, is culverts and portal type structures. Even where the restraint stiffness is relatively low, there may be a permanent compression in the deck due t o its action in propping the abutments.
a4
As well as enhancing the global longitudinal capacity, restraint can greatly improve the
distribution properties by increasing the transverse bending capacity. This has proved particularly beneficial in filler beam727 and other bridge types where the transverse steel can be very light or even non-existent. Again, in assessment, it is necessary to use conservative estimates of the restraint which may only come from friction. However, the best predictions of test results have been obtained with what are essentially upper bounds to likely
--
7.5.4 Non-linear numerical analysis
For all but relatively simple cases, non-linear numerical analysis is the only way to obtain realistic analyses allowing for compressive membrane action. It is not possible to consider all the difficulties of non-linear analysis here and only aspects related specifically to the problem considered are discussed. When considering the behaviour of restrained slabs, the biggest variable is nearly always the extent of restraint. Both the strength and stiffness of the restraint are critical to the strength of the restrained slab. Therefore, there is little point in undertaking a relatively sophisticated analysis of a slab model using assumed restraint. Where the restraint is internal, it is much better to model enough of the bridge to model the source of the restraint directly. It will also be necessary to model a t least a span of the bridge when global effects are likely to be important. A potential problem is that the tensile strength of concrete could be a major factor in
providing the restraint. This, however, may be unreliable due to cracking caused by anything from early thermal effects to previous applications of different load cases. Therefore, it is safer generally to use only nominal tensile strength to ensure this is not a significant factor. If the slab is reasonably thin compared with its span, certainly if it is outside the limits
prescribed in BD81 for use of simple methods, the non-linear analysis should include large displacement effects. This is because the deflection reduces the effective lever arm of the restraint force.
7.6 Piers 7.6.1 Assessment
Of
piers
Normally, piers can be analysed by very simple hand calculation methods. However, when they are assessed to modern codes, they frequently appear inadequate. This is often due to increases in the design transverse load, particularly the vehicle collision loads for which piers adjacent to carriageways now have to be checked but also some other loads, such as bearing friction. For very weak piers, the assessment should take account of the loads effects due to the instantaneous removal of one or more supports. Instantaneous load effects can be up to twice that of a gradually applied load and these effects should be redistributed to the remaining supports, as appropriate for the particular superstructure. Equivalent gradually
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applied loads should then be calculated from these redistributed and instantaneous support reactions and the ULS load capacity of the superstructure checked under the application of this equivalent load, with yfLtaken as 1.00.
7.6.2 Slender piers
Another reason for assessment failures, which applies primarily t o slender piers, is the allowance for slenderness moments. These can also cause problems t o foundation assessments as well as pier stems. Past practice was t o allow for buckling by using a reduction factor on axial load, whilst modern practice is t o use added moments. The 'effective heights' used are also different, particularly for cantilever piers729.For slender piers, conventional assessment methods are almost invariably conservative. There are two reasons for this. The first is the way the added moment used t o allow for buckling was derived730.This was based on a non-linear analysis, which aims t o predict the deflection of the pier at failure. The added moment is simply this deflection multiplied by the axial load. The analysis, which was calibrated against tests, gives reasonable predictions of the deflection atfailure of a pier which is loaded t o failure under displacement control. However, the peak load is reached a t a much smaller deflection. This is illustrated in Figure 7.12, which shows a load-deflection plot for an actual pier that was assessed using a non-linear analysis. To determine the actual capacity of a slender pier subject t o axial load and applied moment and/or transverse force, it is possible t o perform a non-linear analysis of the pier. The non-linear analysis must model both the geometric (large displacement) nonlinearity and the material non-linearity of the reinforced concrete section. Commercial
software is available t o model both of these. An alternative to using a package which includes material non-linearity is t o perform an iterative analysis in which the displaced shape is updated at each iteration t o take account of large displacements and the section properties are revised as elements become plastic. The iterations are continued until the displacements converge. This is tedious however and time-consuming. The Figure 7.12 Non-linear analysis of a slender pier.
Safe Safe
_,_
Applied deflections Unsafe
Applied loading
Transverse force F max
Concrete cracks
Transverse deflection A
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Concrete crushes
'
column should either be modelled with an initial bow or the load should be given an initial eccentricity t o account for the actual pier construction tolerances. The eccentricity could be obtained from the assessment standard or it could be obtained by precise survey of the pier. The example shown in Figure 7.1 2 illustrates the benefit of a non-linear analysis. A simple cantilevered pier is subject t o both axial load, P, and a transverse load, f , at the top. The load P is applied as a constant load and the top of the pier is deflected under deflection control in small steps. Initially, the behaviour is linear as the concrete is approximately linear in compression. As the force F increases, the concrete cracks in tension and its tangent modulus in compression progressively reduces. Initially, the moment resistance of the section increases faster than the increase in moment resulting from the growing eccentricity of the load. Consequently, the load f increases with deflection. However, as the pier becomes more flexible, there comes a point at which the rate of increase of moment resistance equals the rate of increase in the moment resulting from the growing eccentricity. This represents the peak in load F on the above graph. A real pier with this combination of applied axial and transverse load would fail at this
peak load point. The modelled pier loaded under displacement control, however, would exhibit a drop in transverse load f until final failure occurred when the concrete reached its crushing strain. This final failure point is likely t o be close t o the failure load predicted by BD44. Depending on the pier geometry and reinforcement, the real failure load at the peak of the graph can be considerably higher than that predicted by BD44. A partial factor of safety appropriate t o upper bound methods should be used with this method of analysis. An example of the use of this method gave a peak load 70% greater than that given by
a conventional assessment. Because of this, the analysis saved many thousands of pounds in strengthening works and associated disruption cost. This provides another illustration of how analytical techniques, which are considered too expensive t o use in design, can be highly economic in assessment.
7.6.3 Cloba restraints
When piers appear t o be 'unsafe' due t o lateral loads, a careful consideration of the real articulation of the bridge is often revealing. For example, consider a simple two span bridge, which is fixed a t the central pier but free a t both ends. Initially, most engineers would assume that any longitudinal load, that is load parallel t o the span, which was applied t o the pier or the deck would be resisted by flexure in the pier. However, bearing friction on the end supports would be reversed and considerably reduce the force applied t o the piers. See Figure 7.1 3. If this force is exceeded, there has t o be a very significant displacement of the pier top and the deck For the pier t o fail. This would almost certainly exceed the movement available a t the expansion joint and a more realistic analysis would assume the load went via the curtain wall into the soil. The pier could then be checked for this amount of
a7
Figure 7.13 Longitudinal force - braking, traction, wind -f
Pier buckling restraints.
-F I
Conservative approach
R
R -F
-f Realistic approach
I
I
eILR
(friction)
t
-f
-f
-
2pR
Joint closes
4bA I
7
R I
Pier checked for A deflection (ignore resistance developed)
Ultimate case
Abutment force oppose by soil resistance
displacement. See Figure 7.13. If the design load was applied to the pier rather than the deck, the connection between the pier and the deck may fail first. However, it may be cheaper and easier t o strengthen this connection rather than t o strengthen the pier itself
7.6.4 Buckling O f multiple piers
When a structure has more than one fixed pier then the buckling of the combined system needs t o be considered. There is a simple relationship for the elastic bucking of multi-leg systems: instead of the buckling of a single column being governed by: = Euler buckling load) P / PEulei (where PEuler
multiple pier buckling is dependent on:
There is a need therefore t o check the global behaviour of the combined system and the local buckling of individual piers. The local buckling capacity of individual columns is then governed by fixed ended buckling rather than sway buckling. However, this will occur a t about four times the sway buckling load and is very unlikely t o be a problem.
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For global buckling of the pier system, the non-linear behaviour has t o be taken into account. This approach is exactly the same as that described in Section 7.6.2 except that the model is more complex as it should take account of all the piers and the combination of eccentricities that may occur, see Figure 7.14. Figure 7.14 Stability of multi-pier supports.
S
S
F
F
F
S
S
I
F - Fixed S - Sliding U
U
U
U
U
Friction forces may reverse due to temperature System stability
Buckling restricted by all fixed piers and friction on sliding bearings
An added complication is that friction on the sliding bearings of multi-column bridges can both resist and assist buckling. Thermal expansion and contraction combined with braking and traction loads need t o be taken into account in the most adverse direction by considering worst-case sequences of loading and effects. This can be carried out by hand methods rather than non-linear analysis t o obtain the loads resisted by the individual piers and then these applied using the standard (conservative) rules. If these are not sufficient then it will be necessary t o carry out a non-linear analysis.
7.7 Redundancy Of e [ements
Structures have redundancy occasionally which is not allowed for in straightforward analysis. For example if a bridge deck beam is damaged, the adjacent beams may be able t o carry the load shed by the damaged beam distributed by the surrounding transverse beams. It can be advantageous t o consider alternative load bearing mechanisms or secondary load paths in bridge systems. See also Section 6.1. I . Piers can be vulnerable t o severe damage if subject t o heavy vehicle impact. However, the removal of a single column or pier may not result in collapse of the deck. Reassurance may be obtained by modelling the deck without the support of the vulnerable column or pier. The structure may be shown t o support its own dead weight in these circumstances, indicating that catastrophic collapse may not be inevitable.
7.8 Parapet edge Stiffening
Parapets often act t o stiffen the edges of decks and can be included in the analysis provided there is sufficient longitudinal shear connection t o ensure that they act compositely with the structure.
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7.9 Width O f SUppOf?.S
The finite width of bridge piers is a useful means of limitingpeak moments in bridge beams and slabs supported monolithically by columns and piers respectively (see Figure 7.15).
Figure 7.15 Effect of finite width of supports.
M, =
WD For circular bearing area 3r
M, =
WD For rectangle bearing area 8
ri”
I
Dispersion through concrete does not satisfy equilibrium - ignore -
dispersion through bearing plate W
Reduction in peak support moment due to finite width of support
7.10 Foundations
The forms of failure that occur in these are likely to be visible. Because of this, normally, they are not analysed in routine assessments unless either the structure is showing visible signs of distress or there are reasons for believing that the load on it has increased significantly since design. An example of the latter that has occurred was a retaining wall; it appeared that a multi-storey building had been built close behind it without anybody having investigated the consequences to the wall. Substructure design is even less of an exact science than superstructure design. Until quite recently, many earth retaining structures were designed only for active pressure which, under current rules, would be designed for at-rest pressure. This has the effect that a simple check to modern standards is likely to fail many structures. However, the observed condition is often sufficient to prove that at-rest pressure is not experienced. It will often be helpful to use lower pressures in assessments and, usually, geotechnical specialists can justify this for specific structures. When calculations suggest that abutments are inadequate, it is always worth reviewing the way they would behave if they moved. Normally smaller bridges decks, particularly slab bridges, will work quite satisfactorily when they are propping the abutments, even if they were not designed to work that way.
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7.11 References
7.1
RECAN, PE, Shear, Current Practice Sheet No 105, Concrete, November 1985, pp 25-26.
7.2
BRITISH STANDARDS INSTITUTION, BS EN1992-1,Designofconcretestructures, Part 1 CeneraiRuiesandRulesfor Buildings, BSI, London, 2004.
7.3
BATAYNEH. MK, The effects of lateral compressionon bondbetweendeformedreinforcing bars andconcrete, Doctoral Thesis at Oxford Brookes University. November 1993.
7.4
UNTRAUER, RE and HENRY, RL. Influence of normal pressure on bond strength,ACi/ournal, May 1965, pp 577-585
7.5
NAVARATNARAJAH,V and SPEARE, PRS, An experimental study of the effects of lateral pressure on the transfer bond of reinforcing bars with variable cover, Proceedings institution ofcivil Engineers, Part 2, Vol 81, December 1986, pp 697-175.
7.6
RECAN, PE, Jests ofdelaminatedanchoragessubiect to transverse pressure, University of Westminster, March 1999.
7.7
CLARK. LA, BALDWIN. MI and CUO, M, Assessment of concrete bridges with inadequately anchored reinforcement, Bridge Management3,(Harding. JE. Parke. CAR and Ryall. MJ,eds), E&FN Spon, London. 1996. pp 225-232.
7.8
HILLERBORC,A, Strip MethodofDesign, Viewpoint Publication, 1975
7.9
HILLERBORC. A, The Advanced Strip Method - A simple design tool, MagazineofConcreteResearch, Vol 34, No. 121, December 1982.
7.10 HILLERBORC, A, StripMethodDesignHandbook. E and F Spon 1996 7.11 CLELAND, D, Tests on supports ofsiab bridge decks with inadequateanchorage,Report for the Roads Service, N Ireland, Queens University Belfast 7.12 CULLINCTON, DW, DALY, AF and HILL, ME, Assessment of reinforced concrete bridges. Collapse test on Thurloxton Underpass, Bridge Management 3, (Harding, JE, Parke, CAR and Ryall, MJ, eds), London, E&FN Spon, 1996. pp 667--674. 7.13 SHAVE, JD, IBELL, TJ and DENTON, SR, A new assessment model for shear in reinforced concrete bridges, Bridge Management 5 (Parke, CAR and Disney, P, eds), London, Thomas Telford, 2005 7.14 IBELL, TJ, MORLEY, CT and MIDDLETON, CR, A plasticity approach to the assessment of shear in concrete beam and slab bridges, The StructuralEngineer, Vol 75 I No. 19, 1997, pp 331-338. 7.15 NIELSEN, MP. limitAnalysisandConcretePlasticity, 2nd ed , CRC Press, 1999. 7.16 .MOHAMMED. LN and PAUL, HR, Design of SMA Mixture for an Orthotropic Bridge Deck Resurfacing,ProceedingsRoads 96 Conference. New Zealand, 1996. 7.17 DENTON, SR, The assessment of reinforced concrete bridge decks, Bridge Modification 2. Proceedings oflnternational Conference, (Pritchard, B, ed.), Thomas Telford Ltd, London, 1997, pp 40-54 7.18 DENTON, SR and BURCOYNE. C, The assessment of reinforced concrete slabs, TheStructuralEngineer.Vol 74, No 9. May 1997, pp 147-152 7.19 WOOD, RH, The reinforcement of slabs in accordance with predetermined field of moments, Concrete, Vol. 2, No. 2, February 1968, pp 69-76 7.20 ARMER, CST, Discussion of the above, Concrete, VoI 2, No. 8, August 1968. pp 319-320 7.21 HIGHWAYS AGENCY, BD 81/02, Use ofcompressivemembraneaction in bridgedecks, May 2002, (Corrected August 2002) Design Manual for Roads and Bridges. Vol. 3, Section 4, Part 20. 7.22 KIRKPATRICK, J, RANKIN, GIB and LONG, AE, Strength evaluation of M-beam bridge deck slabs, The Structural Engineer, Vol 626, No 3, 1984, pp 60-68. 7.23 ONTARIO MINISTRY OF TRANSPORTATION AND COMMUNICATIONS, Ontario HighwayBridge Design Code, Downsview. Ontario, Canada, 1983 (and later editions). p 175. 7.24 JACKSON,PA, The global and local behaviour of bridge deck slabs, JheStructuralEngineer, Vol 68, No. 6, March 1990, ppll2-116 7.25 DAS, PC, Load carrying characteristics of flat arches and their implications for the design assessment and strengthening of bridges, Proceedings offifth internationalconference on structuralfaults and repair, Engineering Technics Press, Edinburgh, 1993, pp 315-320 7.26 JACKSON, PA. Flat arch action, Arch Bridges. Proceedingofthe first internationalconference on arch bridges, Thomas Telford, London, September 1995, pp 407-415 7.27 LOW, AMcC and RICKETTS, NJ,Theassessmentoffillerjoist bridges without transverse reinforcement, Research Report Special 383, Crowthorne, Transport Research Laboratory, 1993. 7.28 JACKSON. PA The analysis and assessment of bridges with minimal transverse reinforcement, Bridge Management 3 (Harding, JE, Parke, CAR and Ryall, MJ,eds), University of Surrey, April 1996, E&FN Spon, pp 779-785. 7.29 JACKSON, PA, Slender concrete bridge piers and the effective height provisions ofBS 5400: Part 4: 1984, Publication 42 561, Cement and Concrete Association (now British Cement Association), Camberley, June 1985 7.30 CRANSTON, WB, Analysis anddesign ofreinforcedconcrete columns, Publication 41.020, Cement and Concrete Association, (now British Cement Association), Camberley, 1972.
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63
8. Specific structural forms Chapter 8 considers particular difficulties in assessing the loading capacity of a number of common structural forms. These are: 0 Reinforced concrete slabs, commonly used as both bridge decks in themselves and
supported by steel or reinforced, prestressed/post-tensioned beams. They are frequently assessed as having inadequate capacity under local wheel effects. 0 Reinforced concrete beams and slab bridges decks suffer from problems of inadequate shear and anchorage bond at the ends of the beams. Care needs t o be taken that what appears t o be a reinforced concrete beam is not a concrete encased steel beam such as the 'Preflex' type manufactured by Boulton & Paul in the 1960s. 0 Half Joints, used particularly in bridge construction t o provide a span greater then the span provided by standard prestressed concrete beams. The detailing of the half joint reinforcement is important in the assessment of these joints. Hinge joints have similar problems t o those of half joints. 0 Box culverts are a common form of construction for culverts and subways. They are often assessed as inadequate t o carry the imposed loads despite no visible signs of distress. Such culverts may be in situ or more often of precast concrete construction. They are sensitive particularly t o analysis of the application of earth pressures and the support t o the base slab. 0 Precast pre-tensioned beams have and continue t o be used t o provide bridge decks for spans of up t o about 30 m. They have been a very successful form of construction but early designs suffer in particular from an absence of shear reinforcement. 0 Arches are more commonly constructed in masonry, however arches are also constructed in both in-situ and precast reinforced concrete and in mass concrete. The mass concrete and lightly reinforced concrete arches rely upon maintaining compression throughout the section as with a masonry arch. More heavily reinforced sections are designed t o carry load in both tension and compression and the method of analysis should take notice of this. They include a wide variety of structural forms. U Post-tensioned structures have particular features that separate them from other structural forms. Each of the above is considered, particular problems are described with reference t o other sections of this Guide and additional information is given on how t o consider the issues.
8.1 Reinforced ConCrete slabs
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Reinforced concrete slabs are used frequently as bridge decks themselves for small span structures and as deck slabs typically supported on steel or reinforced, prestressed/posttensioned concrete beams. The large number of structures that conventional assessments fail t o show t o have adequate load capacity but which, in practice, show no signs of distress indicates that there are hidden reserves of strength that these methods do not account for.
Guidance on appropriate methods of analysis is given in Chapter 6 and areas o f hidden strength are given in Chapter 7. Advice on in-situ and laboratory testing t o provide improved understanding of the material properties is given Chapter 4 and advice o n load testing is given in Chapter 10. Further advice on in-situ and laboratory testing is given in the Concrete Bridge Development Group publication on testing and monitoring the durability of concrete structures”. The key problems areas are typically: 1. Limited reinforcement and inadequate anchorage lengths
2. Cantilever slabs failing in flexure under accidental wheel loading 3. Inadequate transverse reinforcement 4. Inadequate anchorage for shear enhancement at the ends of decks
5. Local wheel effects on footways and verges. Possible solutions: 1. Alternative approaches t o the problem o f inadequate anchorage length are discussed in Section 7.1. 2. The assessed capacity o f cantilever slabs can be increased by the use of Yield- line Analysis, see Section 6.5. 3. The apparent problem of inadequate transverse reinforcement is often as a result of the use of uncracked linear elastic grillage and finite element analysis. Possible solutions are discussed in Sections 6.4 and 6.5. 4. Inadequate anchorage for shear is discussed in Sections 7.1 and 7.2. 5. Analysis of local wheel effects on footways and verges are often undertaken using Westergaard or Pucher Charts. These methods are discussed in Section 6.3.3. Improvements can be achieved by the use of yield-line or finite element analyses.
8.2 Reinforced ConCrete beams
Advice on the analysis for decks including reinforced concrete beams is given in Chapter 6 and on the hidden strengths that may be present in Chapter 7. Key problem areas are inadequate anchorage and bond and shear at the ends of beams, and failure of early beams in both flexure and shear discussed in Sections 7.1 and 7.2. In some beams there is a deficiency in the amount of longitudinal reinforcement, which results in the shear links being ineffective. Some early bridge beams used the Hennibique system of reinforcing82.In this system, 50% o f the bars were bent up at about the 1/3rd point of the span t o enhance the shear capacity at the end of the beam. The design was not based upon current reinforced concrete theory. Later designs have also used the concept of bent-up bars. Advice on the calculation of stress in bent-up bars is given in Section 7.1.6.
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8.3 Half joints
Half joints were introduced into bridge decks as a means of simplifying design and construction operations. This form of joint is vulnerable to deterioration in the event of deck expansion joint failure, where chloride-rich seepage through the joint can cause corrosion of the reinforcement and concrete deterioration. Loss of reinforcement section through corrosion, or associated concrete spalling can induce higher stresses and reduce significantly the safety margins expected of serviceable structures. Half joints are a particular concern because they are not easily accessible for inspection or maintenance and they are mostly located over or under live traffic lanes. They are particularly difficult to repair. Bridges owned by the Highways Agency (HA) incorporating half joints are well distributed throughout England. Many have been subject already to visual inspection, and will have been prioritised for maintenance by the Agents working on behalf of the HA on the basis of their external condition. Some may already have been repaired and/or strengthened. An Interim Management Strategy for all structures of this type has been set out by the Highways Agency in IAN53/0483. This document sets out a risk-based strategy which is necessary to ensure that all structures of this type, which are vulnerable particularly to deterioration and difficult to inspect, are recorded, specially inspected, and remedial works planned, and to allow the future maintenance funding requirements to be identified. Assessments should be based on assumed levels of reinforcement corrosion because the actual level of deterioration is very difficult to establish. This sensitivity analysis is then used in the risk assessments.
8.4 Hinge joints
Hinge joints have similar problems to half joints and can occur in decks and columns. In particular, deck hinge joints are vulnerable to deterioration as they often are restrained from moving by the adjacent supports and shrinkage causes a crack to open up a t the joint. De-icing salts can penetrate directly to the reinforcement and cause pitting corrosion. An Interim Management Strategy for hinge joint structures has been issued by the Highways Agency (IAN51/0384)which has a very similar risk-based approach to the IAN for half joints. A draft BA is in preparation to supersede the IAN but currently is not ready for issue. As for the half joints, assessments should consider different levels of reinforcement
corrosion to determine the sensitivity to deterioration. Fatigue of the reinforcement is a concern especially when the hinge bars crossing the joint are corroding. It is possible to carry out intrusive investigations by water jetting short sections oftthe joint but the variability to deterioration is still difficult to estimate. Hinge joints in piers are less vulnerable but are often in a severe environment in splash zones adjacent to carriageways. The principles for assessment are similar to that given in IAN51/03 which is to use strut and tie models.
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8.5 BOX CU1VertS
The inspection of box culverts reveals structures that are generally in a good condition with little signs of distress. However, the initial assessment of such structures often suggests that they have inadequate load capacity. Assessment failure is generally due t o bendinghhear failure of the top slab and t o inadequate detailing. The consideration of the following may provide an increased assessment capacity: 0 Shear enhancement at the corners due t o the presence of the chamfers. 0 The calculated shear strength may be enhanced using methods given in Section 7.2. 0 The lateral distribution of load through the roof slab may be more realistically
modelled using yield-line theory. This has been proven t o provide significant increases in capacity. Details on the application of yield-line theory are given in Section 6.5.4. 0 The use of membrane action within the roof slab can often provide significant increases in strength; guidance is given in Section 7.5. 0 For culverts with no fill above, the deck surfacing may provide additional stiffness t o the deck slab. Whilst this may not provide an increase in capacity at the ultimate limit state, it may benefit the serviceability limit state; although in practice it is found that it is difficult t o develop this benefit. Further discussion of this is given in Section 7.3. 0 The fill material may arch over the culvert thereby relieving some of the live loading. Further advice is given in Section 6.8. It should be noted that Clause 4.2 of BD31/0Ia5 includes for an additional 15% superimposed dead loading t o be included t o allow for negative arching effects of the fill material. BA55/00 Section 386 provides further advice on the assessment of buried box type structures particularly in relation t o the application of earth pressure. Further advice on the problems of inadequate detailing particularly in relation t o anchorage and bond, which are the most common problems, is given in Section 7.1. Advice on highway loading effects that may influence the assessed capacity is given in Chapter 5. The spring constants used t o model earth pressures in computer frame models can have a significant effect on the moments and shears derived. These or the stiffness of the bottom slab should be reviewed and the sensitivity of the model t o changes in spring constants considered. Some assessment engineers have measured the deflection of the culvert roof slab under known load and then back calculated the spring constants t o produce the same deflection in the model. Due t o the difficulties of estimating the soil pressures the measurement of the stress in the bars using blind hole or stress relaxation technique^^','^ could provide information on the real stresses present in the structure, although both methods should be treated with caution.
95
8.6 PreCaSt pre-tensioned beams
Precast pre-tensioned beams by the nature of the design principles and the process of manufacture have proved generally t o be very durable. The difficulties associated with the assessment of such structures are often an absence of information regarding the prestressing present and calculated shear failures. Often prestressing details are not included in the bridge records and then it is necessary t o establish the strand patterns. This can be achieved by breaking out the ballast wall of the end spans and exposing the beam-ends. The strands can then be seen on the beamend face usually covered with bitumen paint, which can be removed by wire brushing or water jetting. The diameter and pattern can then be measured and presence of debonding identified. The extent of the latter cannot be established easily without resorting t o radiography techniques but, generally, the beam capacity is not particularly sensitive t o the debonded length. The shear behaviour of precast pre-tensioned concrete beams is complex and is still not fully understood. Shear strength in BS 5400 and BD44 is defined using two parameters, V,, and V,,, V,, relates t o failure when the principal tensile stress in the web exceeds the ultimate tensile strength of the concrete, whilst V,, involves combined bending and shear. The values given in both BS 5400 and BD44 for V,, are empirically determined. The frequently used form of construction, using closely spaced beams, means that evidence of shear problems are not visible on site with the exception of the external beams which carry reduced live loading. Early precast pre-tensioned beams often contained no shear reinforcement and this was adopted in BS 5400 Part 4 1978 which allowed the use of beams with no reinforcement under specific circumstances, in particular that the applied shear was less than 50% of the shear resistance. In some early pre-tensioned beams, particularly Tee beams, the spacing of the shear links is too great for them t o be considered effective. Tests were undertaken by the Transport and Road Research Laboratory on a number of precast pre-tensioned beams with no link r e i n f o r ~ e m e n t ~These ~ , ~ ~indicated . that the beams possess reserves of strength due t o the conservative assumptions in the method of calculation and the high strength of the concrete. The tests also indicated that cracking due t o overload would have been visible on the bottom flanges before failure as bending cracks appeared first. These would only be visible however upon close inspection. As noted above, the web cracks would often not be visible due to the close proximity of the beams or concrete infill to provide composite construction. The shear resistance as determined by the tests was at least twice that calculated. Failure of an individual beam is unlikely due t o the ability of the deck t o redistribute load transversely. This may be further enhanced by the use of transverse stressing of the beams, a practice particularly associated with railway overbridges constructed as part of the electrification programme in the 1960s, although in many of these decks inadequate waterproofing means that the transverse stressing has now failed. Some precast pre-tensioned decks may also be constructed as shear key decks where interlocking reinforcement protruding from adjacent beams provides a shear key between the beams when the concrete is placed. Careful consideration is required in the
96
analysis of such decks t o model the shear keys and in the determination of the adequacy of the shear keys. There are no specific guidelines for the assessment of such decks and, therefore, reference is required back t o the design guidance in particular BE238’0. Books on bridge deck analysis by Hambley”’ and others give advice on the analysis of such decks. Further tests for shear in under-reinforced, precast pre-tensioned concrete beams were undertaken in 2002 at the Transport Research Laboratory (TRL) for Railtrack. This included the assessment of infill decks comprising precast pre-tensioned beams placed side by side with infill concrete placed between them and over them t o form a solid slab. The existing code requirements in BA44 are based upon previous tests undertaken at TRL. These allow the assessed shear capacity of infill decks t o be based upon the arithmetic sum o f the shear capacity of both the beam and the infill concrete as calculated separately. Generally, the determination of shear strength is based upon the minimum percentage steel area (0.15%) whether the section is reinforced or not. Information Sheet No 2g812issued by Railtrack for use on Bridgeguard 3 assessments allows shear enhancement t o be considered when calculating the shear capacity of the infill concrete even when no reinforcement is present. This document also provides a justification for the use of short shear span enhancement in pretensioned beams.
8.7 Reinforced Concrete arches
Reinforced concrete arches include a wide variety of structural types. Principally, they are three hinged, t w o hinged or fixed ended. They may have open or solid spandrels. The arch may support a bridge deck above via columns or transverse walls. The bridge may comprise a series o f parallel ribs supporting a bridge deck above. Some reinforced concrete arch bridges are constructed as bow string girders with the deck resisting the horizontal thrust from the arch. Where the deck or carriageway is supported on fill above the arch they are termed closed or solid spandrel, other types are classified as open spandrel. BA55/00 advises that reinforced concrete closed spandrel arch bridges should be assessed using the same requirements of other types of concrete bridges, but that the restraining effects of the surrounding fill should be taken into account in the analysis as appropriate.
The analysis of arches may be undertaken using either elastic or mechanism methods Computer programs have been developed for both.
8.7.1 Elastic analySlS
Hand analysis using the work of Castigliano can be undertaken t o calculate the areas of tension and therefore cracking. The arch thickness and inertia is then reduced t o take account of the area in tension, and the calculation repeated until balance is obtained between the loading and the thickness of the arch not in tension. The above does not allow for the effects of the surrounding fill t o be considered and ignores the presence of any reinforcement.
97
An alternative approach is t o use a structural frame program with the restraining effects of the fill applied as spring constants. As with reinforced box culverts, the derivation of the applied spring constants requires careful consideration as, significantly, these may affect the derived moments and shears. It is also found that unless the stiffness of the arch is reduced (e.g. by using cracked properties or including hinges) this approach often gives a lower strength than treating the arch as un-reinforced. Soil interaction can be modelled using programs such as FLAC813 if the arch is thin and flexible.
The simplest arches for analysis are three hinged arches as they are statically determinate. Formulae are available in many structural textbooks to calculate the moments and shears a t any section. Formulae are available for calculation of two hinged arches which, like the three hinged arch, only transmit thrusts t o the abutmentsBi4. Typically, many smaller arches forming culverts are two hinged arches as the connection t o the abutments cannot be considered t o provide full fixity. The abutments of such structures may also often be inadequate t o resist rotation under load. The effects of temperature, creep and shrinkage need t o be considered with statically indeterminate arches. O f these, temperature is the most easily considered as the effects of creep and shrinkage may be affected by the means of construction. Reynolds and Steedman814suggest that shrinkage can be considered as equivalent t o a fall in temperature of 8.3”C. BD91 gives some advice on when temperature and shrinkage need t o be considered in masonry bridges. This advice should be conservative for RC arches. Formulae are available for the determination of stresses in fixed arches of any profile, although these are difficult t o apply and generally use is made of frame analysis programs. A particular type of fixed arch is the parabolic arch where the thickness of the arch increases as a parabolic function from the crown t o the springing, when only the bending moment and thrust a t the crown and springings need t o be considered and formulae are available t o determine these.
8.7.2 Mechanism analysis
98
Mechanism methods for the analysis of arches have been proposed particularly by Heyman8” for masonry arches and can be applied t o reinforced concrete arches in which the material behaves plastically. The mechanism method is based upon plastic theory, which is discussed in Section 6.4, and the question of ductility of reinforced concrete is discussed in Section 6.1. For reinforced concrete arches, failure will occur on the formation of four hinges. For this t o occur the section must be sufficiently ductile t o allow considerable rotation such that one section does not fail prematurely.
The analysis of lightly or non-reinforced concrete arches may also be undertaken using computer programs designed for the assessment of masonry arches such as RINC816and ARCHIE817. Both analyses give lower bound results.
8.8 Post-tensioned structures
8.9 References
Because, primarily, the design of post-tensioned structures is undertaken at the sls, again, the assessment of such structures is associated generally with checking those sections that behave as reinforced concrete slabs and beams, as discussed in Sections 8.1 and 8.2. However, care must be taken t o ensure that these sections have a satisfactory ductility and are appropriately detailed. In particular, tendon anchorages should be checked as reinforced concrete sections subject t o bursting forces, although unless there is evidence o f distress such sections are unlikely t o be problematic.
8.1
CONCRETE BRIDGE DEVELOPMENT GROUP, Guide to the testing andmonitoring ofdurabiiiryofconcrete bridges, CBDC, Camberley, 2000.
8.2
MULLER, G and RANKIN, G, The Hennibique System. a Renaissance, ProceedinqsoftheInstitutionofCinlEngineers, Part 1.1991, pp 179-187
8.3
HIGHWAYS AGENCY, IAN 53-04, ConcreteHa/~-~ointDeckStructures, February 2004.
8.4
HIGHWAYS AGENCY, IAN 51-03, HingeDeckStructures,July 2003
8.5
HIGHWAYS AGENCY, BD 31/01, The Design of Buried Concrete Box and Portal Frame Structures, DesignManualfor RoadsandBndges,Vol. 2, Section 2, 1987
8.6
HIGHWAYS AGENCY, BA 55/00, The assessment of bridge substructures and foundations, retaining walls and buried structures, Design Manua/forRoadsandBridges, Vol. 3, Section 4, 2000
8.7
ANDERSON, M, Dead load stress measurement, ConcreteEngineering Internationa/,July/August 2000.
8.8
CULLINGTON, D, Assessment of pre-stressed bridge beams, First InternationalConference on Bridge management, inspection, maintenance, assessmentand repair, Surrey University, 1930, pp 447-457
8.9
CULLINGTON, DW and RAGGETT, SJ Shear Strength ofsorne 30year o/dprestressedbeamswithout [inks, Research Report 327, Transport Research Laboratory, Crowthorne, 1991
8.10 HIGHWAYS AGENCY, BE 23/71, Technical Memorandum (Bridges) Shear key decks, DesignManua/forRoadsandBridges, Vol. 1, Section 3 8.1 1 HAMBLEY, EC, Bridge DeckBehaviour, E&F Spon, Various Editions. 8.12 RAILTRACK,Bridgeguard3,Information Sheet No 29, October 2000. 8.13 ITSACA CONSULTING GROUP INC. FLAC-Fast LangranianAnalysisofContinua, 1998 8.14 REYNOLDS, CE and STEEDMAN, JC, ReinforcedConcrete Designers Handbook (Various Editions), Viewpoint Publications.
8.15 HEYMAN, J, TheMasonryArch, Ellis Norwood, 1980. 8.16 UNIVERSITY OF SHEFFIELD, RING. 8.17 OBVlS LID UK, ARCHIE, 1999-2001.
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8 Specific material and assessment factors
9. Specific material and assessment factors 9.1 Introduction and general principles
This Chapter considers specific factors, which may influence the strength, stiffness, stability and serviceability of concrete bridges, and which may require consideration in structural assessment.The sub-sections that follow may be categorised broadly as follows: (A)
Factors affecting concrete strength and stiffness 9.2 9.3 9.4
Alkali-Silica reaction Aggressive chemical attack Freeze-Thaw action.
(B)
Special material issues 9.5 High Alumina Cement 9.6 Supersulfate Cement
(C)
Corrosion issues 9.7 Chlorides 9.8 Carbonation 9.9 Steel corrosion
(D) Structural issues 9.10 Fatigue 9.1 1 Sub-standard reinforcement detailing 9.12 Deteriorated reinforced concrete structures The basic purpose of assessment is to evaluate current and future structural capacity. This is usually done via analytical methods (Chapter 6), while taking account of any hidden strengths (Chapter 7). The process and procedures are as outlined in Chapters 1 and 2. The accuracy of outputs from analysis is highly dependent on the quality of the input assessment parameters. Generally, these are established during the Inspection and Testing phases (respectivelyChapters 3 and 4). Geometrical and mechanical properties are particularly important. In this Chapter 9, guidance is given, to augment that in Chapters 3 and 4, on how the factors itemised above might affect the assessment parameters. By broad category, these are: (A)
Factors affecting concrete strength and stiffness. These three items are deterioration mechanisms, which directly attack the concrete. Stiffness can be reduced, which can influence the distribution of action effects in the structure as a whole. Strength may also be reduced, either by loss of section or by a reduction in mechanical properties; this would reduce resistance to imposed loads a t critical sections. In determining the magnitude of these reductions, it will be necessary to establish the un-deteriorated strength of the concrete in the structure, which will have increased with time from that assumed a t the design stage.
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(B)
Special material issues Some elements may be found, which are made with special cements, such as High Alumina Cement or Supersulfate Cement. Sections 9.5 and 9.6 give guidance on how the general performance of concrete may be modified when these special concretes are present.
(C)
Corrosion issues The most common form of deterioration in concrete bridges is corrosion of reinforcement, usually due t o chlorides from de-icing salts, but also possibly from carbonation of the concrete. This can lead t o loss of rebar cross-section, due either t o general corrosion or local pitting; the resulting loss of strength of section (in bending, shear, torsion, bond, etc.) will depend on the location of the corrosion and on the quality of the reinforcement detailing. Corrosion may also lead t o a loss of ductility in the reinforcement. Usually, there are visual signs of corrosion at the surface of the concrete, in the form of staining, cracking, or, in extreme cases, delamination; this latter factor can result in a loss of concrete section, and a reduction in bond or anchorage.
(D) Structural issues
A treatment for fatigue is proposed in Section 9.10. Reinforcement detailing may also be sub-standard, and this can increase the sensitivity of structural elements to the effects of deterioration, particularly corrosion; this is dealt with in Section 9.1 1. Finally, current approaches t o evaluating the structural effects of deterioration are brought together in Section 9.12. Considerable engineering judgement is required in evaluating the structural effects of deterioration in individual cases. In general, damage classification and condition assessment methods, while useful in giving a qualitative measure of the overall state, are no substitute for quantitative methods, having either a deterministic or probabilistic basis. Real strength is not proportional generally t o the apparent level of deterioration; great care is necessary therefore in establishing values for the assessment parameters, which reflect both the level of deterioration and the sensitivity of the structural form. Particular care may be necessary where a structure has been repaired in the past, since the effectiveness of the repair may be questionable; frequently, conservative assumptions are necessary.
9.2 Alkali-Silica Reaction (ASRI 9.2.1 Background
Alkali-Silica reaction is a reaction between the hydroxyl ions in the pore solution and certain forms of silica occasionally present in significant quantities in the aggregate, in the presence of moisture in the concrete. The product of the reaction is a gelatinous silica hydrate, containing silica, sodium, potassium, calcium and water, The gel can hydroscopically absorb pore solution, and swell; this swelling can induce internal stresses within the concrete, which
101
9 Specific material and assessment ffaaors
may be sufficient to cause internal cracking, expansion and visually severe cracking. The reaction is finite, and its duration depends on the levels of alkalis present, on the size, nature and reactivity of the aggregate, and on sufficient moisture. The causes of ASR are now well recognised and understood, and recommendations have existed since 1983 for minimising the risks due to it, in new construction; References 9.1 and 9.2 are typical. It is unlikely, therefore, that structures built in recent years will suffer from ASR. The correct diagnosis of ASR as the dominant deterioration mechanism is of great importance, since, if not done, there is a risk of an erroneous assessment, and the specification of inappropriate remedial measures. Gels can form from other deleterious reactions, and alkali-silica gel itself can be formed without causing expansion. The level of current and likely future expansion is crucial for subsequently assessing possible structural effects, and therefore is an essential element in the diagnosis procedure. Consensus guidance on diagnosis is availableg3,and for greater detail, see Reference 9.4. Much work has now been done on the assessment of the possible structural effects of ASR. While published recommendations may vary in detail, the principles are broadly the same. The key to all of them is the estimation of expansion during the diagnosis stage. Depending on the magnitude of that, methods are presented for predicting the effect on geometrical and mechanical properties, prior to estimating the influence on relevant action effects, such as bending, shear, axial compression, torsion, bond and anchorage. Mostly, these procedures are well documented. The approach in this Guide is to give an elaborate reference system in the sub-section which follows, while subsequently highlighting important issues in current procedures or the assessment of concrete bridges in the UK. Structural sensitivity, and the vulnerability of the resistances to particular structural actions, are important issues in this regard.
9.2.2 StrUCtUra( aSSeSSment: current practice and reference documents
The actual references are listed a t the end of this Chapter. They are grouped here under a series of different headings, together with brief explanations.
a) General consensus guidance documents - structural assessment ISE Reportg5.This was the first authoritative document in the UK, with a strong
dependence on estimating expansion, and emphasising the importance of detailing sensitivity. It should be read in the wider context of the Institution’s more general report on structural appraisalg6. BA52/9497.This Advice Note refers specifically to Highway Structures, and comes from Section 4, Volume 3 of the Design ManualforRoadsandBridges. It should be read in the context of other relevant BDs and BAs (see Section 1.4), but particularly BD21/01, and BD44/95 and 8A44/96. CSA-A86A-00g8andCONTECVET Manualg9.These two documents introduce an , ~ ~ are . based on modifying design international slant, whiie updating UK p r a c t i ~ e ~ ’Both equations for assessment purposes.
102
Clarkglo.This report is included in this category, since it identified the types of element most at risk, and made recommendationsfor modifying design strength models in BS 5400. It was the basis for much that appeared in later guidance documents listed above.
b) Research reports on the effect of ASR on various structural actions. The eight references placed in this c a t e g ~ r y give ~ ” test ~ ~ data ~ ~ on ~ the structural performance of concrete elements affected by alkali-silica reaction. The list is by no means exhaustive, but they represent core data obtained in the early 199Os, used in developing assessment methods for some of the guidance documents in category (a) above.
c) Diagnosis, test methods, expansion and restraint, etc. The four references in this give detailed information on test methods during the diagnosis stage. Of particular importance is the estimation of expansion, as affected by restraints in the actual structure, and as measured on cores stored a t different temperatures. This information has been integrated into the general assessment procedure contained in Reference 9.9.
d) Synergetic effects Where more than one deterioration mechanism is acting, one will usually dominate, and the diagnosis phase should clearly identify which one. However, the effects that the primary mechanism can produce may be exacerbated by defects due to other causes. This is treated in Reference 9.9 for combinations of ASR, corrosion and freeze-thaw. References 9.23 and 9.24 give further information.
9.2.3 Key points in StrUCtUra[ assessment
This Section merely highlights some of the important points from the reference documents in the previous sub-section. This Guide deals with Highway Structures, and therefore, in terms of detail, the emphasis is on relevant BDs and BAs (BA52/94; BD21/01; BA44/96 and BD44/95). For structural assessment, the general approach will be analytical (Chapter 6) or will involve the re-evaluation of critically loaded sections using modified design models. The
primary objective, based on input from the Inspection and Testing stages, is to devise assessment parameters for analyses, or modifications for design models, which represent the physical damage due to ASR and the effect of the estimated expansion. In the first instance, this involves estimating any reductions in mechanical or geometrical properties, which may be followed by introducing any further necessary modifications to the design models themselves.
9.2.4 StrUCtUra[ SenSltlVity: elements most a t risk
Clarkgloindicated that the structural elements or forms most a t risk to the effects of ASRwere: 0 Over-reinforced beams 0 Columns and other compression members
I
103
9 Speciffic materia! and assessment ffaaors
U Anchorage zones of both reinforced and prestressed members, where there is insufficient restraint provided by links c] Unreinforced or lightly reinforced members U Decks composed of sub-elements of webs, flanges and diaphragms 0 Supporting elements with poor reinforcement detailing, which does not restrain expansion. In assessing these and other types of element, it is also important to consider sensitivity to structural form and to reinforcement detailing. Restraint to the expansive action is significant, whether created by structural redundancy or by confinement provided by reinforcement. Reference 9.9 provides data on how mechanical properties may be affected by different levels for free expansion, and adds modifying factors for a range of restraints. In addition to reductions in mechanical properties, it may also be necessary to consider geometrical properties, where physical damage has led to severe cracking, spalling or delamination.
9.2.5 Indicative modifications to design models for strength assessment
Most approaches to strength assessment are based on modified design models. The approaches in References 9.7 to 9.10 are broadly the same in principle, but differ in detail, including some assumptions made, in the interests of conservatism. Table 9.Ig9 gives an indication of the type of amendment proposed. The specific case of Highway Structures, covered by BD44/95, is dealt with in a later Section. Possible limitations to assessed strength, unique to ASR-affected elements In general, the approach to assessment, via analysis or modified design models, is focussed on a re-evaluation of the primary action effects such as bending, shear, axial compression, torsion, etc. Bond and anchorage may be more significant in assessment, due for example to reductions caused by severe cracking or delamination. It may be necessary also to consider alternative load bearing mechanisms, induced uniquely by the deterioration, which may lead to reductions in strength, stiffness, stability or serviceability. For ASR in particular, the following possibilities are identified and may require consideration: U Possible buckling of cover concrete if this becomes delaminated. 0 Possible buckling of longitudinal bars in compression members, if the cover concrete
becomes delaminated. This is only likely in old structures, with very few links which may be anchored inadequately (link spacing greater than 44 diameters for mild steel and 32 diameters for high yield steel. U Possible instability effects in heavily over-reinforced beams. 0 Crushing of concrete, if the difference between the failure strain in the concrete and the yield strain in the compression reinforcement, is less than the ASR expansion strain U Some of the indicative amendments in Table 9.1 are dependent on the anchorage bond being unaffected. A careful study of deterioration in anchorage zones of both reinforced and prestressed elements is required.
104
Table 9.1 Indicative amendments to be made t o structural codes t o allow for the effects of ASR.
Flexure
U Use ASR-affected concrete strengths derived from uniaxial (core or cylinder) compressive strengths
0 Include ASR-induced expansion such that reinforced members are treated as prestressedmembers
0 Only use 60% of the ASR-induced (restrained) expansion in determining the additional strain in the reinforcement or tendons Column behaviour
0 Use ASR-affected concrete strengths derived from uniaxial (core or cylinder)
Shear
0 Use ASR-affected concrete strengths derived from uniaxial (core or cylinder)
compressive strengths compressive strengths
Ci Include ASR-induced expansion such that reinforced members are treated as prestressedmembers. For some codes this will mean using the prestressed concrete shear rules 0 Only use SO% of the ASR-induced (restrained) expansion in determining the additional strain in the reinforcement or tendons U Ascertain the effects of any reductions in anchorage bond strength on shear strength Torsion
0 Use ASR-affected concrete strengths derived from uniaxial (core or cylinder) compressive strengths
0 Carry out an extra equilibrium check to ensure that the stress in the concrete compression struts is not exceeded Punching shear
0 Use ASR-affected concrete strengths derived from uniaxial (core or cylinder) compressive strengths
0 Include ASR-induced expansion such that reinforced members are treated as prestressed members. For some codes this will mean using the prestressed concrete shear rules U Only use SO% of the ASR-induced (restrained) expansion in determining the additional strain in the reinforcement or tendons 0 Ascertain the effects of any reductions in anchorage bond strength on shear strength Bearing
0 Use ASR-affected concrete strengths derived from uniaxial (core or cylinder) compressive strengths or (preferably) direct tensile strength measurements
Fatigue
0 No guidance can be provided yet
Bond
U Use ASR-affected concrete strengths derived from uniaxial (core or cylinder) compressive strengths or direct tensile strength measurements
il Use the Tepfers bond expression, but see Section 9.12
9.2.6 Assessment Of Highway Structures - BD44/95 and BA44/96; BA52/94; BD21/01
The principles and general approach in the above sub-sections apply, but detailed requirements are contained in the above Highways Agency documents. The indicative amendments in Table 9.1 then become particular. Table 9.2 lists relevant equations for assessment in BD44/95, with suggestions being made for reductions in assumed material strengths, as affected by ASR
9.3 Aggressive chemical attack 9.3.1 Background
In the UK, sulfates in soil and groundwater are the chemical agents most likely to attack concrete. Another possible cause of concrete deterioration is groundwater acidity, and this is sometimes linked with the presence of sulfates. The extent to which sulfates and acidity affect concrete is linked to their concentrations,the type of ground, the presence of groundwater (and its mobility), the type of concrete, and the form of construction.
105
Table 9.2 Suggested material strengths for use in equations given in BD44/95. for ASR.
Uniaxial Compressive Strength (fc)
indirect or Splitting Tensile Strength V,)
f,, replaced by fJO.8
If,,replaced by fJO.3
f,, replaced byf,/O.ll
4.3.2
Material Properties
Figure 1
5.3.3.1
Shear stress
0.92J V c J Y m J
5.3.3.2
Shear capacity
Equation for V,
5.3.2
Resistance moment of beams
Equations 2, 3, 4, 5 and 7
5.5.3
Short columns subject to axial load and bending about the minor axis
Equations 14 and 15
5.5.4
Short columns subject to axial load and either bending about the major axis or biaxial bending
Equation 17
5.6.1.1
Definition (Reinforced concrete wall)
0.1f'"AC
6.3.3
Ultimate limit state: flexure
Equation 27
6.3.4.3
Sections cracked in flexure
Equation 29A
5.3.4.3
Stresses and reinforcement
Equations for V,, and Vt,
5.8.6.3
Anchorage Bond
P,ifcJYm,
6.3.4.2
Sections uncracked in flexure
Equations for M,,and maximum principal tensile stress
6.4
Slabs
Equation for maximum principal tensile stress
6.7.4
Transmission length in pre-tensioned members
kt@/,ifO
5.8.6.9
Bearing stress inside bends
Equation for bearing stress inside bends
7.2.3.3
Bearing stresses
Equations for bearing stresses
Cross references Subclauses 5.3.2.l(b), 5.5.3.2(b) and 6.3.3.1(b)
First component of the equation only
If reinforcement is present that crosses the potential splitting cracks then the fcU value taken as the value for the ASR concrete should be used
f,, is the concrete strength at transfer
Conventional sulfate attack has been recognised for many years, and dealt with by defining classes of aggressivity and recommending concrete specifications for each, in terms of ingredients and mix proportions. As more has become known, these have been refined in successive British Standards; BS85OOgz5,is the most recent example. In recent years, the situation has become more complicated by several developments: 0 The 'discovery' of new forms of sulfate attack, particularly thaumasitegz6. 0 The greater use of brownfield sites, possibly increasing the range of aggressive chemicals. U Recognition that some sites may contain sulfides, such as pyrite, which may increase sulfate content from oxidation following ground disturbance.
106
-?.
Specific materia! and a
This has led to a re-classification of aggressive chemical
with
corresponding material specifications.
All of this means that, while provisions to deter sulfate attack have existed in specifications for many years, they have not always worked or have been misapplied. Therefore, assessment engineers may be faced with deterioration due to some form of sulfate attack, and the need to take this into account in assessment. The purpose of this Guide is to outline the key features involved.
9.3.2 ‘COnVentiOnal’ form O f su[fate attack
This term applies to sulfate attack, which leads to the formation of ettringite and gypsum in susceptible concretes. For this to occur, the following must be present: 0 A source of sulfates, generally from sulfates or sulfides in the ground 0 The presence of mobile groundwater
0 Calcium hydroxide and calcium aluminate hydrate in the cement matrix In the alkaline pore solution provided by the sodium, potassium and calcium hydroxides, liberated during the cement hydration reactions, the sulfate ions from the ground react with calcium hydroxide to form calcium sulfate (gypsum). This, in turn, reacts with calcium aluminate hydrate to form calcium sulfo-aluminate hydrate (ettringite). In Sulfate-Resisting Portland Cement (SRPC), the tricalcium aluminate (C,A) level is kept to a minimum, so reducing the extent of this reaction. As for all deterioration mechanisms, careful diagnosis is important; if the dominant
mechanism is not identified, the assessment of the likely consequences may be wrong, leading to inappropriate remedial Sulfates in the ground are usually of natural origins and are those of sodium, calcium and magnesium. Usually they occur in significant quantities, in weathered or disturbed zones (possibly also in backfill). This means that the most likely locations for sulfate attack are in concretes from
I
1
approximate ground level to a depth of about 10 m. The extent of the damage, and the rate of the reaction, is dependent on the porosity and permeability of the concrete, and on the mobility of the groundwater. The first effect of the reactions is to increase the strength and density of the concrete, as the reaction products fill the pore space. When it is filled, further ettringite formation induces internal stresses in the concrete, which may disrupt, causing expansion of the affected region. Together with white crystalline accumulations, this is characteristic of the ettringite/gypsum form of sulfate attack. As a result, affected concrete elements may suffer from:
0 Disintegration of the concrete due to expansion, followed by erosion 0 Softening of the concrete 0 Increase in ingress of moisture and aggressive salts, possibly leading to corrosion of any reinforcement present.
I
For assessment purposes, these effects may lead to: 0 Loss of effective concrete cross-sectional area 0 Loss of concrete strength
107
8 Specific materid and assessment ffaaors
U Loss of cover t o reinforcement 0 Loss of bond between concrete and reinforcement 0 Possible loss of reinforcement cross-sectional area, due t o subsequent corrosion
The significance of this will depend on the type of construction, and on the nature, size and shape of the structural elements. This can only be fully assessed by a risk analysis typified by that in Reference 9.26.
9.3.3 Thaumasite f o r m O f sulfate attack (TSA)
This form of attack is dealt with in detail in References 9.26 and 9.27. Generally, several factors must be coincident for TSA t o occur in susceptible concrete: U A source of sulfates, generally from sulfates or sulfides in the ground. 0 The presence of mobile groundwater.
0 A source of calcium silicate hydrate, mostly derived from cementitious calcium silicate phases present in Portland cements. 0 The presence of carbonate, generally in coarse and/or fine concrete aggregates. U Low temperatures, since thaumasite formation is most active below 15°C. The significant difference from 'conventional' sulfate attack is the presence of carbonate and the involvement of calcium silicate hydrates with the sulfate ions, to produce thaumasite. The calcium silicate hydrates provide the main binding agent in Portland cement, so that this form of attack weakens the concrete, and, in advanced cases, the cement paste matrix is reduced eventually t o a mushy incohesive white mass. SRPC concretes can be vulnerable, but concretes containing ground granulated blastfurnace s(ag have better resistance. Other factors known t o influence the severity of TSAgZ8include the concrete mix design, workmanship, changes t o ground chemistry and water regime, and the type, depth and geometry of the buried concrete. The key effects of thaumasite on structural strength are: 0 Loss of concrete cross-section and strength. This is the most critical feature. Concrete
affected by TSA should be disregarded in strength calculations. This may lead t o overloading in the remaining section, or t o instability and buckling. 0 Loss of cover t o reinforcement. Most substructures are in compression. If the depth of the affected concrete is less than the cover, generally compression bars will reach their ultimate design strength provided that the detailing of the links is sufficient t o provide restraint. 0 Loss of bond. In compression members, provided that buckling of the main steel is avoided, this may be less important. For other elements, including piles and pile caps, both flexure and shear will require consideration
0 Synergetic effects, including a possible increase in the risk of corrosion, may also require consideration.
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9.4 Freeze-thaw action
Concrete exposed t o cycles of alternative freezing and thawing can undergo progressive deterioration, which starts a t the surface and moves deeper into the concrete with increasing number of cycles. Its primary effects are surface scaling and internal mechanical damage due t o the expansive action of ice formation. Water expands on freezing and in concrete this creates pressure that causes internal damage when greater than the tensile strength of the concrete. Nevilleg2’ identifies two mechanisms by which this occurs; the freezing of water in voids and of that diffused into small pores by osmotic pressure generated by the process of freezing. The latter is thought to be particularly important in causing damage t o concrete because of the greater pressure that is generated in the small pores. Osmotic pressure can also arise in concrete that has been exposed t o de-icing salts. Repeated cycles of freezing and thawing cause cumulative damage. The destructive expansion of water turning into ice in each cycle allows water in the following cycle t o migrate t o new locations and t o cause further damage t o the concrete on freezing. The factors affecting the severity of damage can thus be summarised as follows: 0 The degree of saturation; fully saturated concrete is most vulnerable 0 The level of exposure t o the atmosphere
0 The frequency and number of freeze-thaw cycles 0 The level of exposure t o de-icing salts 0 The quality of concrete.
A bridge is more susceptible t o damage by freeze-thaw action the further north it is
located in the UK because of the increasing severity of the environmental factors. In structural assessment terms, the key factors to be considered are as follows:
0 Reductions in concrete cover due t o surface scaling. In its simplest form, this may increase the risk of subsequent corrosion. In more severe cases, as scaling and crumbling of cover extends laterally and into the concrete, bond and anchorage may be affected, particularly in negative movement regions or in reinforcement designed for punching shear. 0 Internal damage will affect (reduce) the mechanical properties of the concrete. Guidance on this is given in Reference 9.30. If there are other destructive mechanisms acting simultaneously, such as the penetration of chlorides, then the different types of interaction between them may need t o be consideredg30. It is suggested that the assessment of frost-damaged structures should be divided into two. Firstly, a qualitative one based on visual inspection t o estimate the aggressiveness of the environment and consequences of structural failureg3’, and secondly, a quantitative one t o analyse structural capacities of the critical elements using lower bound value of concrete properties. In the latter, it is important t o assess the present and future structural capacities and safety levels t o determine a strategy covering inspection, further investigation and future repair and strengthening workg32.
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9.5 High Alumina Cement (HAC)
High Alumina Cement (HAC) is manufactured from limestone and bauxite and contains principally alumina with smaller amounts of ferrous and ferric oxides, silica and traces of titanium oxide, magnesia and alkalis. HAC concrete (HACC) has a very high rate of strength development, achieving 80% of its ultimate strength within 24 hours of placement, and, because of this, was widely used in the 1950s and 1960s to maximise the output of precast concrete building components. Because production runs were small, HAC was less widely used in the manufacture of precast bridge beams a t this time. HAC had been used as early as the 1920s for its sulfate resisting properties in the construction of bridges and was known then as ‘fondu’ or ‘lightning cement’. In the early 1970s a number of roofs made with HACC collapsed. Following this, the Building Research Establishment carried out a major investigation into HACC and established that under normal service conditions the compressive strength of this concrete could be halved over a long period of time. The investigation confirmed previous findings that the hydrate produced by the hydration of HAC is chemically unstable a t normal and high temperatures and converts to a more stable form. This conversion results in a greater porous structure of the concrete and leads to a loss of strength. Other facts about HACC that have been noted include: 0 The loss of strength is greatest for mixes made with high water/cement ratios and
exposed to high temperatures. 0 The general pattern of loss of strength versus time is similar for any mix proportion of
concrete. 0 The minimum strength reached is dependent on the water/cement ratio used and can be as low as 0.4 times the strength a t day one for a free water/cement ratio of 0.6. Because the problem of HACC is one of greatest concern to the building industry, little work has been done on its effect on bridges. However, because bridges in general are more conservative in their design than that of buildings, the loss of strength of HACC has less impact. The process of conversion can take many years to complete, but can be assumed to have been fully completed in bridges in the UK where the structural use of HAC was withdrawn from British Codes in the early 1970s. It is recommended, because of the large variability in the strength of HAC concrete, that cores are taken for the measurement of strength. The estimate of cube strength from core tests is considered to be conservative but offers the best method for HACC because of the background experience in its interpretation.
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The risk and the rate of corrosion of reinforcement are determined as in Portland cement concrete mainly by a combination of the depth of carbonation, chloride and sulfate levels and environment. However, because of its greater porous structure, these could be higher in converted HAC concrete. Some guidance is given in References 9.33 to 9.38.
Salts Contact with salt solutions is more likely to cause deterioration through corrosion of reinforcement rather than attack of the concrete itself.
Carbonation HACC are subject to carbonation in the same way as concrete made with Portland cement. However, because the chemistry and mechanism of carbonation is different from that in Portland cement concrete, the method of testing for carbonation in HACC is different. Petrography is considered to be the only cost effective way to establish the depth of carbonation, as the phenolpthalein test for assessing the depth of carbonation in Portland cement concretes is unreliable for HACC
Sulfates HACC is more vulnerable to sulfate attack if its conversion has been rapid, thus substantially increasing its porosity and permeability, and if its water/cement ratio is high.
9.6 Supersulfated Cement
(ssc)
Supersulfated cement (SSC) is manufactured from granulated blastfurnace slag (80% to 85%), calcium sulfate (10% to 15%) and Portland cement clinker (up to 5%). It is resistant to attack in aggressive conditions and, consequently, has been used in the
manufacture of concrete pipes for placement in contaminated, acidic or sulfate-rich ground, and in the construction of bridges over railway lines during the steam train era. Because of its low heat of hydration, it has been used in large concrete pours to minimise thermal cracking. Because of its high slag content, concrete made with SSC requires to be wet cured for a significant time to prevent exposed surfaces from becoming friable or powdery. Carbonation causes a greater loss of strength in concrete made with SSC than Portland cement and can also lead to friable or powdery surfaces. SSC has a low alkali content and therefore any reinforcement, embedded in concrete
made with it, will be more susceptible to corrosion if chlorides are present or if carbonation has taken place. Lime released during hydration of SSC made with higher contents of Portland cement will interfere with the reaction between the calcium sulfate and the slag.
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Lime-base sealers, known to have been applied as standard practice in the past, and lime from adjacent concrete made with Portland cement have been observed to damage concrete made with SSC.
SSC is no longer made in the UK but is quite commonly used in Belgium and to a lesser extent in France. Carbonation of Portland cement concrete results in an increase in strength, whilst that made with supersulfated cement on carbonation suffers a loss. However, since this applies only to the outer skin of the concrete the effect is not normally structurally significant. The use of supersulfated cement does not affect the assessed capacity of reinforced concrete.
9.7Chlorides
Sections 9.7, 9.8 and 9.9 are related, in covering different aspects of the major cause of the deterioration of concrete bridges - reinforcement corrosion. This Section deals with possibly the prime cause of corrosion - chlorides from various sources; similarly, Section 9.8 covers the other significant cause - carbonation of the concrete. Section 9.9 then relates to the corrosion process itself. Finally, the possible structural consequences of corrosion are covered in Section 9.12, in terms of bridge assessment. The mechanism of corrosion, in aqueous media, is electrochemical in nature; zones of different electrochemical potential develop (anodes and cathodes). The corrosion process mainly proceeds by the formation of numerous microcells. The nature of these cells can vary, and, under certain circumstances, macrocell effects can develop; this depends on the resistivity of the concrete, and, in the case of chlorides, on the uniformity of the penetration of the chlorides through the cover concrete. Any investigation of corrosion needs to look at the nature of the cells involved, since the galvanic current in macrocell action can lead to local pitting, rather than general corrosion, which is potentially more serious in structural terms. Local pitting is more likely where corrosion is initiated via chlorides, but it is by no means universal. The essential action of chlorides, when they have penetrated the concrete and reached the reinforcement, is to disrupt the protective passivity layer of oxide on the surface of the reinforcing bars, which was formed by the hydrating cement paste, and, in the presence of moisture and oxygen, to create corrosion cells. For concrete bridges, the sources of chlorides can be either internal or externai. Internally, they can emanate from contaminated aggregates or water. Until the
mid-I 970s, they could also come from admixtures such as calcium chloride, which was used as an accelerator, until banned in Codes and Standards. Internal sources of chlorides are much less likely in modern structures due to improved standards and controls for the production of concrete. External sources come from the use of de-icing salts during the winter months in the UK, and from salts in marine and maritime environments.
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Before the action of chlorides can come into play in creating corrosion cells, they must first penetrate the concrete cover and reach the reinforcement. There are three features which are important in this process: 1, The build up of chloride concentration in the surface of the concrete. This depends on the concentration and continuity of the chloride sources. It also depends significantly on the local environment and on the microclimate that is created due t o the location and orientation of the structural element, e.g. some surfaces are directly washed by driving rain and run-off, others are not. Cycles of alternative wetting and drying are also important. 2. The penetration of the chloride ions through the cover concrete. In submerged zones or in fully saturated concrete, chlorides penetrate by diffusion. In other cases, particularly where wetting and drying cycles are involved, capillary absorption may be involved, and this is a faster mechanism. A great deal of research effort has gone into the prediction of this process, usually involving some form of diffusion model; the essentials of these may be found in References 9.39, 9.40 and 9.41. 3. When the chlorides reach the reinforcement, a critical amount has t o accumulate, t o induce the onset of corrosion; this is known as the threshold level. This is not a fixed single value, but depends on: 0 Type of cement: fineness, amount of C,A, amount of gypsum, presence of pfa or
slag, etc. Watedcement ratio (porosity) 0 Curing and compaction (porosity)
0 Moisture content and variation 0 Type of steel, surface roughness and condition 0 Oxygen availability.
The same concrete may have very different threshold levels therefore in different situations, and this has been found in practice. One approach has been t o assume a critical chloride threshold level, conservatively at that given in codes for mixing water (0.4% of cement weight), but values in excess of 1.O% of cement weight have been measured in The above brief review of the action of chlorides has been presented t o make the point that, in an assessment situation, where the presence of chlorides is suspected but corrosion has not started, in-depth investigation may be worthwhile (via measurement and diagnosis) t o predict how long it might be before corrosion might start.
9.8Carbonation
The alkalinity of hydrated cement paste is important in providing protection t o reinforcement against corrosion. This is reduced by carbonation of the concrete. Carbonation of concrete occurs when carbon dioxide from the atmosphere reacts with the calcium and alkaline hydroxides and cement phases, leading t o a reduction of the pH value in the pore solution t o values near neutrality. As the carbonated front reaches the reinforcement, the protective passive film on the steel surface may break down, and the corrosion process can start, in the presence of water and oxygen.
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8 Specific material and assessment factors
Carbonation is a diffusion process, and therefore similar models can be used t o predict its advance as those used for chloride penetration 9 3 9 , 9 4 0 , 9 4 1 . A general review of carbonation in concrete is provided in Reference 9.43, and Reference 9.44 provides data on carbonation depths measured on site. The rate of carbonation depends on the characteristics of a particular concrete. It does not occur in fully saturated or very dry concrete, and is probably most intense at a RH value of about 50%. Alternative wetting and drying is another critical environmental condition, in alternatively providing conditions for high rates of carbonation and those for higher corrosion rates. The soundness and depth of cover influences the rate of carbonation. Older structures with low covers to reinforcement and low-strength concretes inadequately cured are more susceptible t o the effects of carbonation. Concrete made with blended cements if not properly compacted and cured can sustain rapid rates of carbonation. Carbonation is more rapid in the presence of fly ash but is counteracted by the dense structure of the hardened cement paste produced by the reaction between the pozzolanic silica and calcium hydrate. Concrete made with sulfateresisting cement sustains a higher rate of carbonation than one using Portland cement. Carbonation takes place also in high alumina cement paste with carbon dioxide reacting with the calcium aluminate hydrates. Conversion of HAC increases the rate of carbonation. The rate of carbonation is not uniform and is slowed by the reduced porosity of carbonated concrete; carbonation produces products of increased volume, and water then enables the hydration of hitherto unhydrated cement to take place. Shrinkage due t o carbonation may contribute t o crazing (shallow cracking) that allows in moisture, contaminates and accelerates carbonation ingress. Loss of strength does occur with carbonation but, because this applies only t o the surface zone of structural members, is not significant. Corrosion caused by carbonation is general with expansive rust products causing delamination and spalling. Carbonation accelerates chloride-induced corrosion of reinforcement. It has been shown that sequential carbonation and chloride attack can be a very much worst degradation problem than either process alone. Corrosion caused by carbonation not only cause reinforcing bars to lose their cross-sectional area but their cover and bond through the spalling of concrete. Further discussion on the effects of corrosion on reinforcement is given in Section 9.9. The key effects on structural strength are: U Loss of concrete cross-section area 0 Loss of reinforcement cross-sectional area
0 Loss of cover t o reinforcing bars U Loss of bond between concrete and reinforcement in affected area
These are discussed in Section 9.1 2
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9.9Steel corrosion 9.9.1 Introduction and background
The mechanism of corrosion is an electrolytic process, which, in concrete structures, is initiated by carbonation of the concrete or by the penetration of chlorides. A brief description is given in Section 9.7, which stresses that different types of corrosion cell can occur, leading t o either general corrosion around the perimeter of the bars or t o local pitting which penetrates more deeply into the bar cross-section. In assessment, therefore, the nature of the corrosion cells has t o be looked at closely. General background t o all of this is provided in References 9.29, 9.39, 9.40 and 9.41. The significance of corrosion in engineering terms, was first given shape by Tuuttig4’, who proposed a two-phase model: 1. An initiation period, which consists of the time from the erection of the structure until
the aggressive agent (chlorides or the carbonation front) has reached the reinforcement and the conditions are right for corrosion t o start (see Sections 9.7 and 9.8). 2. A propagation period from the depassivation of the reinforcement until a certain unacceptable level of deterioration has been reached. For this period, corrosion rate is of great importance in assessment, in estimating the nature and magnitude of rebar section loss - and, hence, its effect on residual structural capacity.
9.9.2 Effects O f COtTOSiOn
The effects of corrosion in structural assessment terms are as follows: 1. Reduction ofrebar cross-section: This may be either general, around the bar perimeter,
or local, penetrating into the cross-section. The structural effect is t o reduce capacity (bending, shear, etc.), although this is not always proportional t o the loss of rebar section, depending on detailing and on where the corrosion is occurring. 2. Ductility ofreinforcement: There is evidence that corrosion can reduce rebar ductility, which may be important if plastic or non-linear analytical methods are used for assessment. 3. Cracking, spalling, delamination: The formation of expansive corrosion products can lead t o cracking along the line of the reinforcement. At corner sections, this can lead t o local spalling. In extreme cases, delamination of concrete cover can occur over significant areas, either for tension or compression steel. In compression members, the concrete cover can be rendered ineffective in load carrying terms; in extreme cases, short elements may be rendered slender. 4. Loss ofbondandloranchorage: There are two aspects here: 0 Formation of expansive corrosion products may alter the basic bond characteristics. 0 Spalling and delamination reduces the effective bond perimeter, and residual bond
will depend on reinforcement detailing (especially of links). Loss of anchorage is more critical than loss of bond.
The development of corrosion, and the magnitude of its effects, may also depend on the general quality of the design and construction, and, in particular, on the presence or otherwise of defects due t o other causes. Cracking, in general, is particularly significant in this regardg46,9 47.
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Assessment models have been developed t o predict all of the effects itemised above. These are a t different stages of development, and have varying amounts of calibration against test data. Most are based on the principle of modifying the corresponding design models.
9.9.3 COrrOSiOn rate
This is a highly variable phenomenon, much influenced by the moisture state and the availability of oxygen, in the inner concrete environment where the reinforcement is located. This, in turn, is affected by the local microclimate at the perimeter of the concrete element, which is subject t o daily and seasonal variations in moisture levels and temperature. How this outer environment affects the inner environment depends on the quality and thickness of the concrete cover. Methods of varying effectiveness do exist for estimating corrosion rates in the field, either directly or indirectly. However, unless they are used t o take measurements under the different local internal and external environments that might exist, they may not give values for loss of rebar section, which are representative over significant periods of the remaining life of the structure. This is a major dilemma, since an underestimate of the rate could lead t o excessive damage prematurely, and (more likely in practice) an overestimate can lead t o premature remedial measures. Until knowledge and technology improves, the solution has t o lie in a mix of good site measurements and risk analysis, involving probability distributions for sensitive locations. A method of obtaining representative values for corrosion rate is given in Reference 9.40.
9.9.4 Prestressed Concrete
Pre-tensioned concrete elements, typified by standard bridge beams, have a good track record over periods up t o 40 years, in terms of steel corrosion. Possibly, this is due t o high cover values and t o concrete quality dictated more by strength at transfer rather than strength in service. The situation is different for post-tensioned concrete bridges, where corrosion has occurred a t poorly formed joints and a t poorly protected anchorages. Inadequate grouting of tendon ducts has been another major factor. Local corrosion leads t o the breaking of individual wire and eventually failure of tendons. The effects of tendon failure vary from one structure t o another and are influenced by:
0 Bridge deck geometry 0 Prestressing system 0 Integrity of grout within the duct 0 Extent of tendon corrosion.
Because it is difficult with present inspection methods t o discover tendon failure and t o determine adequacy of grouting, and then t o model these defects analytically, the assessment of post-tensioned bridges is problematical.
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Recent work by Cavell and Waldron a t Sheffield U n i v e r ~ i t yhas, ~ . ~however, ~ provided a better understanding of the behaviour of corrosion-damaged post-tensioned concrete bridges. This found that: 0 Significant levels of deterioration do not always compromise flexural strength to the
same degree. 0 Presence of grout voids alone may not significantly affect flexural strength. 0 Presence of voids a t or near to a tendon failure will affect the ability of a failed
tendon to re-anchor fully and thus profoundly affect residual strength, particularly when a duct contains long or continuous voids. 0 Length and distribution of grout voids along the beam are important factors when considering the loss of strength. Stress corrosion cracking (SCC) is another form of corrosion of tendons that has been reported. It is a brittle failure a t relatively low constant tensile stress of an alloy exposed to a corrosive environment.
In a survey of French bridgesg4’, SCC was found to only affect quenched hot-rolled wires containing more than 0.1% of copper that were used from 1950 to 1965. SCC has not been reported as a problem to bridges in the UK. Fortunately examples of post-tensioned bridges collapsing in the UK are rare, because most have been designed as ‘fully’ prestressed and would have to lose a substantial percentage of their tendons before they became unsafe. The assessment of concrete structures affected by steel corrosion is discussed in Section 9.12.
9.10 Fatigue
Fatigue performance depends upon the strength of the member and the load spectra. The key elements of the load spectra are the amplitude and frequency of load applicationgs0.Investigation of actual fatigue parameters can be undertaken using weigh-in-motion measurements to provide information on the loading and frequency of heavy loading a t specific sites. Fatigue damage is particularly a problem with short span bridges, where a high proportion of the induced stresses in the reinforcing bars will be due to live loading. The location of the reinforcing bars is also important with those located beneath a slow lane being subjected to a greater amount of heavy vehicle loading. The prediction of fatigue life can be undertaken using the rigorous method given in BS 5400 Part 10. However, the Advice Note BA38/93”’ provides guidance on a simplified method, which should cover most cases. The latter document notes that corrosion or damage results in shorter fatigue life of reinforcing bars. The Appendix in this document is useful in determining the loss of section of corroded bars based upon the observable surface.
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8 Specific materia[ and assessment factors
BD44 does not include a requirement to undertake a fatigue check, as this is considered to be a serviceability check, and concentrates on analysing load carrying capacity a t ultimate limit state. BA38/93 states that fatigue assessments are only appropriate for bridges that are less than 25 years old. This is because for structures older than this, the loading and fatigue history is uncertain and the future life may be limited. Where bars are mechanically damaged, then the assessment of the fatigue life may be based upon the same principles as for corroded bars. The strength and modulus of elasticity of concrete can reduce significantly under cyclic loading. In reinforced members, concrete fails in the compression zone through micro cracks. Whilst in steel bridges fatigue changes can be monitored by observation of visible cracking, those in concrete bridges cannot, as cracking is internal.
9.1 1 Sub-standard reinforcement deta i1ing
Sub-standard reinforcement detailing is a particularly difficult subject to consider in the assessment of bridges. One of the most common reasons for a concrete bridge to fail its assessment is the inadequate detailing of the reinforcement. Usually, this is associated with an apparent bond or shear failure. Fortunately, the incidences of bridges failing in service as a consequence of inadequate detailing are rare, and this suggests that the current assessment code is conservative in dealing with this problem. The current assessment code (BD44) is based almost in its entirety on the design code with some technical changes to the clauses associated with shear, bond and detailing to make it less conservative where possible. The design code covers complex structural behaviour with empirical formulae derived from strictly controlled test situations, for example in the anchorage of reinforcement. These formulae do not allow for interaction between concrete, longitudinal and shear reinforcement. Whilst in the design of a new structure the conditions of these tests can be replicated, they are unlikely to be so in an older structure being assessed. Consequently, the application of these formulae in their assessment format is restricted to a limited number of situations.
Bond failure is an extremely complex phenomenon being a function of many variables including, in particular, cover. The assessment code does acknowledge this, but gives no guidance, except to give direction to a number of referenced papers. It has been suggested as a consequence of testing undertaken by Clarkgs2a t Birmingham
University, that the effective area of an inadequately anchored bar is a function of the ratio of the actual lengths, and not of their squares, as implied in the bridge assessment It is assumed in both papers code. This conclusion is also reached by Cleland that this effective steel area can be used in the standard assessment formula for shear in BD44/95. However, this formula is taken from the design Standard BS 5400, and applies to the effect of fully anchored reinforcement, which is largely governed by the stiffness
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of the steel rather than by the ultimate bar force. The approach of reducing the effective area in the formula to allow for reduced anchorage is inconsistent therefore with the rationale behind the formula. For loads that are applied close to a support, there is usually an enhancement in shear
capacity to model the effect of the steeper angle of compression. Clark et a/954 recommend that the ‘shear enhancement’ factor in BD44/95 for short shear spans be allowed, even for inadequately anchored reinforcement. This contrasts with the existing rule in BD44/95 that an anchorage length of 20 bar diameters must be present for shear enhancement to be used in assessment. The decrease in anchorage length was also shown by Clark to be accompanied by a reduction in reliability of the anchorage. Clark suggests that allowance for this could be made by deducting an unreliable length of bar, this being a function of the number of bars a t the section under consideration. A new approach that deals with the mechanics of the problem in a more rational way
has been developed by Shave e t a / 9 5 5based on test results and theoretical analyses. The effect of the anchorage length on the shear capacity is modelled by considering the anchorage force that may be developed in the steel. The shear capacity is then reduced from its standard fully anchored value by a factor that is a function of the anchorage force. The existing approach in BD44/95 assumes that shear reinforcement is totally ineffective a t low anchorage lengths, and does not allow enhancement of shear a t short shear spans unless an anchorage length of at least 20 bar diameters has been provided. Shave’s method is based on the fully anchored capacity, including the effects of shear reinforcement and enhancement of shear a t short shear spans, and so these effects are considered even a t low anchorage lengths. The proposed method is based on the following expression:
where Vu =shear capacity V,,, = fully anchored shear capacity including shear reinforcement and shear enhancement as calculated using BD44/95 a, = a factor to allow for enhancement of bond due to transverse pressure F,, =total ultimate anchorage force in the longitudinal reinforcement and the other parameters are as defined in BD44/95. Shave’s method agrees well with test data and is considerably less conservative than BD44/95, especially when the effects of shear reinforcement and shear enhancement are considered. A large number of reinforced concrete bridges in the ownership of local authorities were
built in the earlier part of the twentieth century when there was an incomplete and,
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in some situations, incorrect understanding of the factors that affected strength. The detailing of reinforcement in these structures also differed significantly from current practice. Bussellg 56 describes the various propriety reinforcement systems in use a t the beginning of the twentieth century. Interestingly, the introduction of the first British codified design and construction guidance for reinforced concrete a t the end of the Great War led to a period of poor quality construction. The contractors were said to be less aware of good design and working practices than the specialist concrete firms who built many of the pre-war buildings and bridges. This highlights the need to identify as far as possible all details of a reinforced concrete bridge before commencing its assessment. The empirical code rules are severely limited in their applicability and a more realistic assessment approach is required that looks a t the detailing and design of bridge individually. The use of plastic theory as an alternative to the empirical rules, which, more adequately, would consider the influence of detailing on the load capacity of bridges, has been proposed. This requires a detailed knowledge and understandingof plastic theory that many practising engineers may not possess or may be reluctant to use because it is not covered by the current code. Consequently, there is a need to improve the current bridge assessment code to ensure that its conservatism is reduced significantly while remaining safe, particularly in the consideration of sub-standard reinforcement detailing.
9.12 Deteriorated reinforced concrete structures 9.12.1 Background and introduction
Much work has been done on the mechanisms of the different deterioration processes, and most are now well understood. Less work has been done on the effects that deterioration can have on structural capacity; the result of this is that assessment methods in the literature are highly variable, ranging from theoretical reliability methods to assuming that structural capacity is somehow proportional to the general condition of the structure. The first extreme has its difficulties, primarily due to the confidence levels in the essential input and the problems of taking account of structural sensitivity and reinforced detailing. The second extreme can be seriously misleading, depending as it does on qualitative judgement, with reductions in structural capacity often not proportional to loss of section. The general approach to assessing the effects of ASR is now fairly well established
(Section 9.2),in terms of using modified design equations, with inputs derived from test data. A similar approach seems valid for the effects of freeze-thaw action (Section 9.4). The major difficulty is with corrosion. While the same approach is again valid in
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principle, the effects of corrosion are more wide-ranging, and there is a dearth of valid test data, to permit proper calibration of the modified design models. There is the
further complication that the deterioration itself can, in some circumstances, create failure mechanisms where no corresponding design models exist. In this Section, only a brief issue of the key issues can be given. This is focused on:
0 Effects of deterioration, and principles for assessment 0 Sources of detailed information 0 Procedures in accordance with Highways Agency documentation
9.12.2 Effects O f deterioration, and Drincides for assessment I
I
For corrosion, these are itemised in Section 9.9, and listed here for convenience: 0 Reduction of rebar cross-section 0 Ductility of reinforcement 0 Cracking, spalling, delamination
0 Loss of bond and/or anchorage.
The real effect of corrosion on all of these will depend on the quality of the design and construction, on structural sensitivity and reinforcement detailing, on where the corrosion has occurred, and on the type of structural element involved. It will also depend on the particular action effect under consideration. Structural assessment will be initiated generally by some form of structural analysis a t a level described in Section 1.3, with reference to BA79; available analytical methods are covered in Chapter 6. The first task in this procedure is to derive assessment parameters as inputs, which take account of the corrosion effects listed above. In general, this will involve reduced values for geometrical and mechanical properties, ideally derived from the inspection and testing phases on site - but making use of general guidance in available authoritative documents as back up. It will then be necessary to look a t individual action effects at critical sections, such as shear, bond, anchorage, axial compression - inevitably using modified design models. Strength will be the dominant concern, but some issues such as spalling may require consideration where safety is involved. There are techniques and models available for doing this (see the References in the following Sections), which are at different stages of development, with varying amounts of calibration against test data. In applying these, it is important to develop an understanding of the structural behaviour, and of how this might be modified by the effects of deterioration - and of the sensitivity of the structures to these effects.
1
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9 Specific material and assessment ffaaors
9.12.3 Sources O f detailed information
In offering these sources, the emphasis has been put on consensus documents, where the guidance and the proposed models have been calibrated against test data. There four primary sources: 1. HighwaysAgencydocumentation: This is dealt with in more detail in the following sub-section, since it is the source that will generally be used in assessing concrete bridges and highway structures in the UK. 2. fibsources: The starting point here is Reference 9.57 This updates, and summarise: the work of the former CEB and FIP over a period of 20 years, with greater detail being contained in the references to the report. This is strong on technical issues a modelling. 3. BRlME sources: Reference to this is made in Section 1.8. There is a strong emphasis European bridge management systems. 4. CONTECVET: Following an earlier European-funded project (BRITE 4062), on the science of deterioration, CONTECVET focused on the development of models for structural assessment. For corrosion, the details are in Reference 9.40, with companion manuals on ASRg9 and on freeze-thawg30.Currently, the work is continuing, in linking assessment to optimising repair and strengthening solutions (acronym - REHABCON). CONTECVET technology is more focussed on bridges in Reference 9.58.
9.1 2.4 Procedures in accordance with Highways Agency docu mentat io n
As may be seen from Section 1.4, Volume 3 of the DesignManualforRoadsandBridgc contains a number of Standards and Advice Notes that relate to the assessment and
repair of bridges. On assessment, these are either: 1. General. BD21; BA16; BD and BA 44; BA79, or 2. Related to a particular form of deterioration. BA52; BA51; BA38, or 3. Related to a particular type of element. BA55; BA39.
All of this represents the current methodology for the assessment of concrete bridge: As with other approaches, it is constantly being updated, as more test data appears, i as experience is gained. From 1997 to 1999, a comprehensive test series was conduc a t the University of Westminster in association with W S Atkins. The results are recor
in References 9.59 to 9.73. As a result, amendments have been proposed for BD44, b details of which are given below. Until BD44 is formally updated, the use of any of th would only be by agreement between the approval authority and the assessing engini Key proposed amendments are:
0 Making an allowance for the future trends of deterioration in assessments (this is achieved by the ongoing monitoring and modelling of deterioration). 0 Reducing the contribution of concrete to shear strength by 25% in the presence of cri up to 2 to 3 mm in width (for wider cracks it is suggested that shear strength be base( on either the tensile strength of adjacent stirrups or dowel action of the main bars). 0 Limiting the scope for redistribution of moments because of the reduced ductility ( deteriorated members.
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0 Taking account of deterioration in determining effective sections. 0 Taking account of corrosion in calculating the cross-sectional area of reinforcement. 0 Determining the effectiveness of compression reinforcement (it is suggested that the surface layer be ignored and other layers be considered only 80% effective if the section is delaminated in the compression zone and corroded cross-sectional area of shear reinforcement does not meet minimum r e q u i r e m e n t s ) . 0 Ignoring shear and torsion links that are lapped in locations that are d e l a m i n a t e d . 0 Taking the effective loss of the area of a shear link to be half of the actual area, providing that the beam depth is adequate to develop bond along half the leg of the link. 0 Describing a more rigorous method of calculation to the shear strength of defective shear l i n k s . 0 Doubling the limiting bond stress for the leg of l i n k s . 0 Modifying the method of calculating enhanced shear strength of sections close to supports consideringthe applied force in the main reinforcement and the type of l o a d i n g . 0 Using the varying angle truss approach to assess shear strength. 0 Checking torsion in combination with s h e a r . 0 Discounting longitudinal reinforcement in columns where links fail to meet minimum design r e q u i r e m e n t s . 0 Using Regan’s method to calculate forces in bars due to the combined effects of bending and s h e a r . 0 Modifying the calculation of bond stress to account for the presence of links, position of bars in relation t o links, amount of delamination (flush with the top or mid barrel of the reinforcement), transverse pressure and combinations of bars. Regan and others have carried out flexural testing of reinforced concrete beams with unbonded main reinforcement and, from this, have suggested formulae to calculate ultimate moments similar in form to that in B D 4 4 . These formulae, which take account of arching action, are applicable to simply supported beams that are adequately reinforced against shear and anchorage f a i l u r e s .
9.13 References
9.1 BuitDiNG RESEARCH ESTABLiSHMENT (ERE), Alkali-silica reactioninconcrete, Digest 330 (4 parts), ENE, Garston, UK, 1997.
9.2 THE CONCRETE SOCIETY, Alkali-silica reaction.Mmimising the risk ofdamage to concrete, Technical Report 30 (3rd Ed ), The Concrete Society, Camberley, 1999
9.3 BRITISH CEMENT ASSOCIATION, The diagnosis ofalkali-silica reaction: Report ofa Working Party, Report 45.02 (2nd Ed ), BCA, Camberley, 1992
9.4 HOBBS, DW, Alkali-silica reactionin concrete, Thomas Telford Ltd. London, 1988 9.5 INSTITUTION OF STRUCTURAL ENGINEERS, Structural effects ofalkali-silica reaction: Technics/ guidance on the appraisal ofexistingstructures. (2nd Ed.), IStructE. London, 1992.
9.6 INSTITUTION OF STRUCTURAL ENGINEERS, Appraisalofexistingstructures (2nd Ed.), IStructE, London, 1994. 9.7 HIGHWAYS AGENCY, BA52/94, The assessment of concretestructures affected by alkali-silica reaction, 1994. 9.8 CANADIAN STANDARDS ASSOCIATION INTERNATIONAL, Guide to the evaluation andmanagementofconcrefe structures affectedby alkali-aggregate reaction, Document No CSA-A86A-00. CSA, Ottawa, Canada, February 2000. 9.9 BRITISH CEMENT ASSOCIATION et a/ A validatedusers manualforassessing theresidualsemcelife ofconcretestructures affectedby ASR, EC Project Ref. IN 309012, CONTECVET, BCA, Camberley, 2001
9.10 CLARK, LA, Criticalreview of the structuralimplications of the alkali-silica reaction in concrete, Transport Research Laboratory (TRL), Contractor Report 169,TRL. Crowthorne. 1989
9.11 CHANA, PS. Bondstrength ofreinforcement in concrete affected by alkali-silica reaction, Transport Research Laboratory (TRL), Contractor Report 141, TRL. Crowthorne, 1989.
9.12 CHANA, PS and KOROBOKIS, g, The structuralperformance ofreinforced concreteaffected by alkali-silica reaction, Phase I., Transport Research Laboratory (TRL), Contractor Report 267, TRL, Crowthorne, 1991.
9.13 CHANA, PS and KOROBOKIS, g, The structuralperformance ofreinforced concreteaffectedby alkali-silica reaction. Phase 11, Transport Research laboratory (TRL), Contractor Report 233, TRL, Crowthorne, 1991
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9 Specific material an8 assessment ffaaors
9.14 CHANA, PS. and KOROBOKIS.g. The structuralperformance ofreinforcedconcrete affectedby alkali-silica reaction, Phase Ill, Transport Research Laboratory (TRL). Contractor Report 311, TRL, Crowthorne, 1992 9.15 COPE, RJ and SLADE L. The shear capacity of reinforced concrete members subjected to alkali-silica reaction, Structural EngineeringReview. No 2, 1990, pp 105-112. 9.16 NG, KE and CLARK, LA. Punching tests on slabs with alkali-silica reaction, TheStructuralEngineer, Vol. 70, No. 14, July 1992, pp 247-252. 9.17 COPE, RJ, MAY, IM and WEN, H, Prediction ofstress distributions in reinforcedconcrete members affectedbyalkaliaggregate reaction, Transport Research Laboratory (TRL). Contractor Report, TRL, Crowthorne, 1993 9.18 CAIRNS, J and ZHAO, 2. Behaviour of reinforced concrete beams with exposed reinforcement,Proceedingsofthe InstitutionofCivilEngineers. StructuresandBuildings, Vol. 99, May 1993, pp 141-154. 9.19 JONES,AEK and CLARK, LA, The practicalities and theory of using crack width summation to estimate ASR expansion, Proceedingsof the Institution of Civil Engineers; Structures andBuildings, Vol. 104, 1992, pp 183-1 92. 9.20 JONES.AEK and CLARK. LA, The effects of restraint on ASR expansion of reinforced concrete, MagazineofConcrete Research. December 1995. 9.21 CHANA, PS and HOBBS, DW, The suitability ofcoresfor determining expansion rate andfuture expansion in concrete adverselyaffectedby ASR. BRITE-EURAM Project 4062, Contract BREU-CT92-0591,January 1995. 9.22 FRENCH, WJ, Concrete petrography a review, Quarterlylournal ofEngineering Geology. Vol 27, 1991, pp 17-48. 9.23 KAWAMURA, M, TAKEMOTO, K and ICHISE. M. Influence of the alkali-silica reaction on the corrosion of steel reinforcement in concrete, Proceedingsof the 8th InternationalConference on Alka/i-Aggregate Reaction, Kyoto,Japan, Elsevier Applied Science, London, 1989, pp 115-1 20. 9.24 BOLTON, RF and WANE, J, Secondary effect of ASR on the durability of concrete Freeze-thaw, Proceedingsofthe9th InternationalConference on Alkali-Aggregate Reaction, London, 1992. Vol. 1, The Concrete Society. Carnberley, pp 117-1 26. 9.25 BRITISH STANDARDS INSTITUTION, 858500-1:2006, Concrete - comp/ementaryBritish Standardto BS EN206- 1 . Part 1, Methodofspecify,ng, andguidancefor the specifier, BSI, London, 2006. 9.26 DEPARTMENT OF ENVIRONMENT, TRANSPORT AND THE REGIONS, The thaumasiteform ofsulfateattack. risks, diagnosis, remedialworks andguidance on new construction, Report of the Thaumasite Expert Group, DETR, London. January 1999. 9.27 BUILDING RESEARCH ESTABLISHMENT (BRE), BRE Special Digest 1, Concretein aggressive ground (in 4 parts), BRE, Carston, 2001 9.28 HIGHWAYS AGENCY, IAN48/03, Measures to minimise the risk ofsurfate attack (including thaumasite) - New structures andstructuresunder construction. 2003 9.29 NEVILLE, A, PropertiesofConcrete, Longman, 1999 9.30 BRITISH CEMENT ASSOCIATION et a/ A validatedusers manualforassessing the residualservice life ofconcretestructures affected byfrost action, EC Project Ref. IN 30901 2, CONTECVET, BCA, Camberley, 2001 9.31 CONCRETE BRIDGE DEVELOPMENT GROUP, Guide to testing andmonitoring the durabilityofconcretestrucfures, Technical Guide No. 2, CBDC, Camberley, 2002. 9.32 PEARSON, S and PATEL, R, Repair ofconcretein highway bridges -apracticalguide. Application Guide AC43, Transport Research Laboratory, Crowthorne, 2002. 9.33 DEPARTMENT OF THE ENVIRONMENT (DOE), Highalumina cement concrete, Technical Memorandum BE5/74. DOE, London, 1974 9.34 INSTITUTIONOF STRUCTURAL ENGINEERS, Reportofa Working Pariyonhigha/umina cementconcrete, IStructE, London, 1976. 9.35 CURRIE, Rand CRAMMOND, N, Assessment of existing high alumina cement construction in the UK, Proceedingsofthe Institution ofCivil Engineers -StructuresandBuildings, ICE, London, February 1994 9.36 CURRIE, Rand CRAMMOND. N, Assessment of existing high alurnina cement construction in the UK. Discussion on Reference 9 35. Proceedingsof the Institution of Civil Engineers - StructuresandBuildings, ICE, London, February 1995. 9.37 BUILDING RESEARCH ESTABLISHMENT, Assessment ofexisting high alumina cement constructionin the UK. BRE Digest 392, BRE, Garston, March 1994. 9.38 THE CONCRETE SOCIETY, Diagnosis of deterioration in concrete structures, Technical Report 54, The Concrete Society, Camberley, 2000 9.39 BUILDING RESEARCH ESTABLISHMENT (BRE), Corrosion ofstee/inconcrete, Digest 444 (4 parts), BRE, Garston 9.40 CEMENT ASSOCIATION et al. A validatedusersmanualforassessing the residualservice life ofconcretestructures affected by corrosion. EC Project Rei IN 309012, CONTECVET. BCA, Carnberley, 2001. 9.41 BAMFORTH, P, Guidance on theselection ofmeasuresforenhancingreinforcedconcrete durability. Report for DETR PI1 Project C139/3/376, Department of Environment,Transport and the Regions, London, 1999. 9.42 PETTERSON, K, Chloride threshold value of the corrosion rate in reinforced concrete, Proceedingsoffhe International Conference on corrosion and corrosionprotection ofsteel in concrete, University of Sheffield, 1994. (Also available from the CBI, Stockholm, Sweden) 9.43 PARROTT, LJ,A review of carbonation in reinforcedconcrete, British Cement Association, Camberley, 1987. 9.44 BUILDI NG RE S EA RC H ESTA B LI S H ME NT, Carbonation depths in s tructural quality concrete: an assessment of evidence from investigation ofstructures and othersources, Paper BR75, ERE, Garston. 9.45 TUUTTI. K. Corrosion ofsteel in concrete, Swedish Cement and Concrete Research Institute (CBI), Stockholm, Sweden, 1980. 9.46 THE CONCRETE SOCIETY, The relevanceofcracking in concrete to the corrosion of reinforcement, Technical Report 44, The Concrete Society, Camberley, 1995 9.47 THE CONCRETE SOCIETY, Non-structuralcracksin concrete, Technical Report 22 (3rd Ed.), The Concrete Society, Carnberley. 1992
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9.48 CAVELL, D and WALDRON. P, Parametric study of the residual strength of deteriorating simply supported posttensioned concrete bridges, Proceedings ofthe Institutionof CivilEngineers- Structures andBuildings, ICE, London, November 2001 9.49 HIGHWAYS AGENCY et a/, Post-tensionedconcretebridges. Anglo-French liaison report, Thomas Telford Ltd. London, 1999. 9.50 LAMAN, JA and NOWAK, AS, Fatigue Load Models for Girder Bridges,ASCEJournal ofStructuralEngineering, Vol 122, NO.7, 1996. 9.51 HIGHWAYS AGENCY, Advice Note BA38/93, Assessment ofthefatigue life ofcorrodedordamagedreinforcing bars, 1993. 9.52 CLARK, LA, Assessment of Concrete Bridges, National workshop on new conceptsfor the management ofhighways structures. Institution of Highways and Transportation. Leamington Spa, 1990. 9.53 CLELAND, DJ,CUMMINCS, SJ, RANKIN, GIB, TAYLOR, Sand SCOTT, RH, Influence of reinforcement anchorage on the bending and shear capacity of bridge decks, JheStructuralEngineer,Vol 79, No 16, 21 August 2001, pp. 24-31 9.54 CLARK, LA. BALDWIN, MI and CUO. M, Assessment of concrete bridges with inadequately anchored reinforcement. Bridge Management 3, University of Surrey, April 1996. 9.55 SHAVE, JD, IBELL, TJ and DENTON, SR, Shear Assessment of Concrete Bridges with Poorly Anchored Reinforcement, Proceedings Structural Faults andRepair2003, Engineering Technics Press, Edinburgh, 2003. 9.56 BUSSELL, MN, The era of the proprietary reinforcingsystems, Proceedings ofthe InstitutionofCivi/ Engineers - Structures dndBuildings. ICE, London. August/November 1996. 9.57 fib, Management, maintenance andstrengthening ofconcrete structures. fib bulletin 17, fib, Lausanne, Switzerland, 2002. 9.58 WEBSTER. MP, The assessment of corrosion-damaged concretestructures, PhD thesis, University of Birmingham, July 2000. 9.59 WS ATKINS. DeterioratedStructures Phase 2A Report, Document Reference AR40980/1/4/1, March 1997. 9.60 REGAN, PE, Tests ofslabs withfullyorpartlyexposedmain reinforcement. University of Westminster, July 1997. 9.61 REGAN, PE, Further tests ofslabs with exposedmain steel, University of Westminster, November 1997 9.62 RECAN, PE, DeterioratedStructures Jests ofRectangular Beams with Partially ExposedMain Steel, University of Westminster. November 1997. 9.63 RECAN, PE, Effects ofDelamination ofcover on the Shear Strengths ofReinforced Concrete Beams Jest Series S G T, University of Westminster, January 1998. 9.64 RECAN, PE, Flexure of UnbondedReinforcedConcrete Beams, University of Westminster, January 1998 9.65 RECAN, PE, Jests ofReinforcedConcrete Beams with Partially ExposedCornerBars, University of Westminster, July 1998 9.66 RECAN, PE, Jests ofReinforcedConcrete Columns with loss ofcover. University of Westminster, July 1998. 9.67 RECAN. PE, ExploratoryJests ofthe Punching ResistanceOfSbbS with Partially ExposedReinforcement, University of Westminster, July 1998. 9.68 RECAN, PE, Effects of Prior Cracking on the Shear Resistanceof Reinforced Concrete Beams. University of Westminster, December 1998 9.69 REGAN, PE, Jests of Beams with Partially ExposedMainSteel Series F Cantilevers under Distributedloading. University of Westminster, November 1998. 9.70 RECAN, PE, Shear Strengths ofRCBeams with Defective Stirrups, University of Westminster,January 1999. 9.71 RECAN, PE, Jests ofBeams with Partly ExposedMainSteel Series C, RestrainedBeams,University of Westminster, January 1999. 9.72 RECAN. PE, Jests ofDelaminatedAnchoragessubject to Transverse Pressure. University of Westminster, March 1999. 9.73 RECAN, PE, Tests ofBeams with Partly ExposedMainSteel Double Cantilever Beams, University of Westminster. May 1999
Further reading D DALY, A, Bridge Management in Europe Modelling of Deteriorated Structures, Bridge Management 4 (Ryall, MJ, Parke, CAR and Harding, JE, eds), Thomas Telford, 2000
0 KENNEDY-REID, I, The Strength of Deteriorated Reinforced Concrete Bridges, Bridge Management 4 (Ryall, MJ,Parke, CAR and Harding, ]E. eds), Thomas Telford, 2000
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10. Load testing 10.1 General principles
10.1. I Background
The load testing of bridges is carried out in many countries and for a variety of purposes This document is concerned only with its use in assessment in the UK Although concrete bridges are the subject of this document, the principles apply to other types except where there are specific references to concrete details and behaviour such as cracking The urge to carry out load tests for assessment is based on a common view that analytical methods often underestimate bridge strength. Load tests on redundant bridges to collapse show that this impression is not without foundation. However, it probably applies less to modern bridges than to older short span bridges, which are less likely to have proper bearings and well-defined load paths. Older bridges are often much stiffer in response to live loading than indicated by calculation. Although they can also be stronger, the ratio of stiffnesses is not a reliable guide to the ratio of strengths. These points are discussed by Cullington and Beales”.’. Load testing can be regarded as supplementary to the normal process of assessment, used when calculation alone is not enough to demonstrate an adequate load-carrying capacity. Although this intention is clear enough, opinions are divided on how it should be applied. The assessing engineer should be aware of that fact when considering load testing Opinions on load testing are divided because assessment is carried out a t the ultimate limit state. (An exception to these principles is the load testing of bridges in which cast iron members are the critical elements, which are assessed for resistance to fatigue under working stresses.) To be directly relevant to assessment, a load test must provide additional information about that limit state not obtainable by other means. The applied test load can be high enough to show directly that the uls is satisfied or the uls behaviour must be inferred from the response at a lower load. Purely exploratory site testing is not recommended. Calculations should be provided in advance to estimate the range of performance expected under the test loads and ensure that brittle failure is avoided. Load testing has been the subject of study and debate in the National Steering Committee for the Load Testing of Bridges (which was originally set up informally by a group of bridge owners, consultants and specialists and later formalised with the support of the Building Board of the Institution of Civil Engineers) and has occupied engineers in the papers and proceedings of several conferences.This Chapter explains the current situation as it appears to the Task Group.
10.1.2 Reasons for presence of reserves of strength
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The ultimate capacity of a bridge or component under live loading may be greater than obtained by calculation for two main reasons which should be borne in mind when considering bridge load testing:
1. The critical load effects may be less than calculated. 2. The strength of a member or the bridge may be higher than calculated
Clearly, the possibility that it may sometimes be less than obtained by calculation cannot be ignored. In some circumstances, it may be possible t o address both cases using test loads less than uls loads provided data are available, or can be obtained, on the behaviour of similar bridges or components beyond the test loads. Unless the original model gives a greatly underestimated capacity, non-linear behaviour is likely t o occur before the uls loads are reached. For the first of the two cases, it is necessary t o show that the structural model adopted remains valid or overestimates load effects occurring at higher loads. For the second case, the resistance model must be valid or underestimate the resistance. In theory, if the load model is inadequate, the uls may occur at a lower load than calculated particularly for brittle types of failure, and there are examples of the measured resistance being lower than the calculated unfactored resistance.
10.1.3 Types O f load test
This Section deals with the main types of load testing that the assessing engineer may wish t o consider. These are: 0 Supplementary Load Tests 0 Proof Load Tests (Section 10.3) (Note that the terms Proof Load Test and Proving Load Test mean the same thing in this document) 0 Tests t o ultimate 0 Hybrid and other load tests U Dynamic tests. The Supplementary Load Test is a type of static load test most easily identified as conforming with the assessment documents in the Design Manualfor Roads andBridges. The use of other types is currently at the discretion of individual Technical Approval Authorities.
10.1.4The aims of load testing
Possible aims for an in-situ load test for assessment are:
0 To provide information at working load levels that can be used t o revise the structural model for further assessment calculations. U To provide a direct indication of the lower bound ultimate capacity by applying test loads factored for the ultimate limit state (Proof Load Test). 0 To give confidence in the performance of the structure under normal working loads. 0 To provide information that can be used for a monitoring regime. In addition: U Load tests that directly establish the ultimate limit state performance may be carried
out on models in the laboratory or on redundant structures on site.
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U In-situ dynamic testing may be helpful when an aspect of the dynamic performance is in question. It has also been studied for its potential t o give information about
assessed capacity or changes in capacity. For a static load test, the main ways in which it is believed a load test can help directly are: U To show that the distribution of load within elements is more favourable than deduced
from the assessment model - i.e. it produces lower load effects in critical members. 0 To show that restraints are present that reduce the load effects on structural
members - e.g. dispersal through fill, end restraints. 0 To show that the capacity of members is greater than calculated.
10.1.5Documents in the Design Manualfor Roads and Bridges
The Standards BDZIlo3and BD341°4 from the DesignManualforRoadsandBridges require more refined methods t o be applied when simple methods of assessment fail t o demonstrate an adequate capacity. BD2l is the primary reference and is not particularly encouraging. It states (Clause 3.29) that: “The objective of load testing shall be to check structural behaviour under load andlor verifv the method of analysis being used, i.e. to prove the accuracy and suitability of the assessment model of the structure. ’I
Further guidance is given in BA54. This defines Supplementary Load Tests and Proving Load Tests. Supplementary Load Tests receive a qualified acceptance, whereas Proving Load Tests are not recommended. Definitions of the tests are extended in Guidelines for the Supplementary Load Testing of Bridgeslos (NSCLTB, 1998 - see Section 10.2) t o include Proof Load Testing and Dynamic Load Testing. It is important t o note that in this Section the terms Proof Load Test and Proving Load
Test are used interchangeably. The specialised definition of Proving Load Tests given in the NSCLTB publication is useful in the context of that document but at odds with other countries practising Proof Load Testing.
10.1.6Safety
Safety is an important issue and it is accepted that there is an element of risk in any load test associated with the possibility of damage t o the structure and personal injury. The higher the load in relation t o the day-to-day service loads and a realistic assessed capacity the greater the risk. The NSCLTB p ~ b l i c a t i o nprovides ’~~ a framework for assessing risk that leads t o a ranking for the level of expertise required for the test according t o scores for probability and consequences. Some bridges, it suggests, are unsuitable for testing and others should only be tested by specialist consultants with considerable structural knowledge and testing experience of the particular type of structure. Proof Load Tests are expected t o fall into that category.
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10.2 Supplementary load testing
10.2.1 Status
This method of testing is described and recommended in qualified terms in BA54. It is the type of testing referred t o in BD2l. The Supplementary Load Test is an expression used in the UK. In other countries, the term diagnostic test is sometimes used t o mean a similar test.
10.2.2 Methodology
The loads applied are intended t o be of a magnitude producing load effects that do not exceed those occurring on a day-to-day basis. Loads can usually be applied using vehicles. They should be applied in a way that produces effects in the structure that replicate the effects found in the assessment calculations: e.g. transverse and longitudinal bending. The structural model used in the assessment is loaded using identical load cases, and then revised until correspondence with the test is achieved. This requires sufficient measurements t o be made, for instance of strains and displacements, for those comparisons t o be made effectively. The revised model is then used with the assessment live loading t o produce a revised assessment. However, the equilibrium model that results from the test must be checked carefully to ensure that it is valid up to ultimate loads. For example, connections and restraints used explicitly or implicitly in the model need t o be assessed. If their contributions cannot be sustained t o the uls, changes must be made t o the model accordingly. This is expected t o require the results of tests t o ultimate carried out on similar bridges or models (see BA54’06). It is important t o note that a linear extrapolation from the test loads t o the uls loads is
not the intention. Unless the original assessment is highly conservative, non-linear behaviour is likely t o occur before the uls loads are reached. It is necessary t o show that the structural model or resistance model, as appropriate, remains valid or conservative a t the higher loads
10.2.3 Further guidance
Detailed guidance is given by the NSCLTB’07.Reference should also be made t o BD21’08 and BA54. The use of testing must be approved by the Technical Authority.
10.3 Proof load testing 10.3.1 Background
There are no guidelines for Proof Load Testing available in the UK except for those given briefly in BA54, which does not recommend the method. In principle, loads have t o be applied t o the bridge t o represent directly and fully all load cases that are critical in the assessment (and critical to the structure in practice, if these are different). Provided the structure remains in a satisfactory condition under all combinations of assessment live loading, it is concluded t o have sufficient load carrying capacity.
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L
Inclusion of this Section in this document does not constitute a recommendation for its general application.
10.3.2 Methodology
In order t o satisfy the uls, it is necessary at least t o apply loads that are factored for the uls. However, it may be sufficient t o adopt values of unity for yt,for dead load and yt3 (using standard terminology, e.g. as in BD21) on the basis that the structure is fulfilling this function directly and requires no additional allowance for variation. This assumption relies on ensuring that the dead and superimposed loads are not allowed t o increase over time and during the test loading, all critical combinations of load are faithfully replicated. Loads are applied incrementally. If the load test has t o be stopped short of the required loading, a reduced live loading capacity is obtained by applying the partial factors for load. Then a reduced loading is selected (from BD2l for highway bridges) consistent with the loads and load effects proved t o be acceptable in the test. Reference is made above to the proof loads being a t least those factored for the uls. It can be argued that applying the uls factored loads is not completely sufficient because the theoretical probability of these loads being exceeded in service is higher than the acceptable probability of reaching the uls in service. This is because in an assessment by calculation, partial factors are applied t o the resistance side of the assessment equation. However, provided the structure is showing no signs of distress a t the maximum applied load it may be acceptable to assume intuitively that the probability of failure at a somewhat higher load is low. The observed performance of the structure is a substitution for the resistance/material partial factors.
10.3.3Application
Attention must be given t o the criteria for stopping the test short of the factored assessment loading. As there is no established practice for Proof Loading in the UK, there are no criteria for stopping the test. Selection of these criteria is structure dependent and requires knowledge of structural performance beyond the experience of most nonspecialist engineers. Another important consideration is the form of loading t o be adopted. If this replicates the loading configuration of the assessment loading directly, the test may be referred t o as a Load-Type Proof Load Test. If not it is a Load-Effect Type Proof Load Test. In the latter case, a theoretical structural model is required to convert the loads t o load effects. Loading is applied to replicate the load effects that would arise in the structure if the assessment-loadingconfiguration had been applied. It is considered important to derive this model from load test results (strains, deflections, etc.) otherwise the assumed model may be sufficiently incorrect to invalidate the test. This issue must be carefully considered, for example, if end restraints are thought t o have a significant strength-enhancing effect in the performance of the bridge. In that case, the uniformly distributed load (UOL) should be obtained using the cusp method given in BD37, which compensates for the errors arising when a UDL is substituted for the axle configuration from which it is derived.
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10.3.4 Current status
Although tests of this type are reported t o be used in some c ~ u n t r i e s ’ ~ ~ ,‘ they ~’~’’ appear t o be the province of specialists. Proof Load Testing is not currently in use in the UK because there are reservations about its adequacy, safety and long-term effects, and there are no fully researched guidelines. In 1998, the NSCLTB issued an invitation t o bridge owners (with the support of the Highways Agency) t o undertake Proof Load Tests on a trial basis under the supervision of the committee. There have been no takers t o date. It was intended that guidelines would be produced by the Highways Agency tailored t o the bridges in question. A structure accepted for a trial Proof Load Test had t o pass onerous qualification conditions that were supplied with the invitation. Issues t o be addressed in the guidelines included the distinction between a Load-Type Proof Load Test and a LoadEffect Type. In the event that any bridge owner or assessing engineer wishes t o follow this line of enquiry, the NSCLTB, the Highways Agency or TRL should preferably be contacted directly. References t o load tests carried out in other countries, notably Canada, are available.
10.3.5 Safety
The safety of the structure, the test team and the public is crucial in a Proof Load Test. All methods of load application, measurement and monitoring during the test must be devised t o manage the risk t o an acceptable level. It is recommended that Reference 10.5 should be consulted and the most qualified level of specialists engaged t o do the test. See also the paragraph directly above.
10.4 Full-scale or model-scale testing t o failure
10.4.1 Background
There are numerous published examples of tests t o failure of bridge elements or models. Generally, these are the basis of methods of design or assessment contained in Standards. Their use directly for the assessment of a particular structure should not be overlooked, however. Provided the structure is of sufficient importance, or present in sufficient numbers, the use of model or component tests can be cost effective. Published papers can be consulted t o establish what should be done and what can be achieved. Because of the nature of this type of test, a specialist is normally required if the assessing engineer does not have a suitable qualified person in the team. This type of test is primarily applicable t o cases where the load effects are known for the structure and load case(s) in point, but the calculated resistance is insufficient and there is reason t o believe the calculation gives an underestimate.
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It may also be of assistance when the behaviour of a whole structure or large section of
it is in question a t the uls. This addresses the structural model, the load effects and the resistance in combination. A laboratory model can be constructed, a t either full scale or reduced scale, and loaded in the service range culminating in a test to failure.
10.4.2 Broad principles
The test specimen should be designed to reproduce faithfully the structure or component being assessed. For example, reinforcement details should be reproduced rather than simplified unless they lie outside the failure region. Often, for reasons of cost and practicality, it is necessary to use a scale less than unity. As a general principle, it is recommended that the scale adopted is as large as possible, particularly when shear is being investigated. The availability of reinforcement of the desired size or sizes may influence the precise scale chosen. Often it is preferable to select the scale so that a crucial reinforcing bar can be reproduced exactly to scale, enabling the principal features of detailing to be reproduced accurately. It is acceptable to model a component or part of the structure provided proper attention
is given to the boundary conditions. All load effects should be reproduced in the combination(s) present in the structure being assessed. Particular attention should be given to free rotation and sliding a t supports if this is the configuration being evaluated. Sufficient measurements should be taken to demonstrate that this is the case in the test. Applied loads and reactions are the most important quantities generally to be measured in a test to failure. Where practical, sufficient load measurement points should be used to provide an audit and check load distribution. Control of the test may be by load or displacement. Commonly, load control is used for the initial stages of the test, but displacement control has advantages in the latter
stages. It is less likely to cause a sudden failure or allow creep to failure while measurements are being made. It may give a better indication of behaviour if the reducing capacity and ductility are of interest. It is common practice to record the development of cracking by marking the structure at intervals during the test. Displacement measurements are straightforward - strain measurements on the concrete, less so in regions of tension and cracking. Reinforcing bars can be strain gauged internally if required, but interpretation of the results can be difficult between the loads to cause concrete cracking in the vicinity of the gauge and yield in the bar. Specialist knowledge is required if these results are to be used in analysis. In designing the specimen, it is appropriate usually to carry out strength calculations using the model dimensions, as if it were the structure being assessed. Sometimes this results in the proposed specimen having a theoretical failure in a different mode from the structure being assessed - a fact that is important to account for in the
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development of the test. For instance, in a small model the presence of the depth factor in shear may increase the resistance t o shear above the resistance t o flexure. Consequently, the test may not achieve its objectives because the model fails in the wrong mode. It is sometimes acceptable t o test specimens at a relatively young age (e.g. less than 28 days), particularly when several tests are required and the programme has t o be developed and specimens cast as work progresses. However, some properties of concrete, such as bond, are known t o develop at a different rate from compressive strength. If it is considered that a concrete property, significant t o the mode of behaviour in question, is affected in that way, the age of the specimen should not be significantly less than 28 days. Otherwise, ages less than 1 4 days are not advisable. In assessing the test results, it is customary to use the mean measured strength of concrete and steel elements from tests on specimens, cubes in the case of concrete. If Code expressions are being used as the basis for the evaluation, all partial factors are removed to produce the theoretical resistance. If there are implied safety factors, these must be removed also. In BD44, the principle has been to revise the resistance equations so that partial factors are given explicitly. This aids the process. However, it is advisable t o examine the derivation of the formulas when this process is carried out. BA44 is an appropriate reference as this not only explains BD44 provisions, but also provides background references.
10.4.3 Extent O f testing
An important decision that may affect the practicality of testing is the number of specimens that have t o be tested for each required loading configuration. Generally’
random factors give rise t o a spread of test results that should be taken into account, particularly if the tensile strength of concrete contributes t o the capacity being measured. Guidance is given on testing in BS 5400: Part 1 (and Draft Eurocode prEN.1900) for design purposes, but for the assessment of existing structures, the tendency is t o reduce the number of specimens t o a minimum. One approach is t o test one specimen in the required configuration and make a decision based on the margin of safety it demonstrates. Some repetition is always desirable. When it is necessary t o test several load cases, for example, different shear spans in tests for shear resistance, this can be used t o reduce the number tested for each load case, in some cases t o one. This is because the validity of the data can be assessed by taking the results as a set and checking that the trends are systematic and not subject t o wide scatter from the theory proposed t o fit the data. If the testing is intended t o apply t o a family of structures containing parameter variations that are t o be modelled, savings in specimen numbers may arise by planning the test programme with that in mind. However, it is generally more difficult t o interpret test data when several parameters are varied together.
Denton eta1”12 report a short series of tests directed a t one type of reinforced concrete slab in which a combination of proof tests and failure tests was used to advantage in tests carried out in the laboratory.
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10.4.4 Application
10.5 Hybrid and other forms of load testing
10.5.1 Examples
As the testing operation itself generally requires a testing laboratory to carry it out, advice can usefully be sought from the laboratory operators on the specimen design and testing configuration as part of the package. This assumes that the testing organisation can provide the necessary experience and expertise, a factor that is a significant consideration in selecting them.
This refers to testing which does not fall wholly within one of the main types Supplementary or Proof Testing. Examples arise because, for instance, the load applied does not comply with the main types or the interpretation of the results is different. It must be emphasised that these are theoretical possibilities that have not necessarily been used in practice, a t least in the UK.
One hybrid form of load test is a type of Supplementary Load Test in which the loads are applied incrementally beyond the day-to-day loads. This enables the structural model to be checked a t a load closer to the ultimate factored loads and, therefore, reduces the difference between the load test and the assessment condition. If loading continues to a sufficiently high load, the condition for a Proof Load Test may be achieved. Another form may occur when the test is approached from the other direction, i.e. from the Proof Load Test. In this case, the test is planned as a Proof Load Test but the instrumentation and load application initially conforms to the Supplementary Load Test requirements. If the load test proceeds satisfactorily to the Proof Load Test values of applied load, the test is interpreted as a Proof Load Test. If, however, it is necessary to stop the test before the Proof Loads are reached, it may be possible to interpret the results as if for a Supplementary Load Test. Bridge owners sometimes call for testing that resembles Supplementary Load Testing in respect of the conduct of the test, instrumentation and level of loading applied. However, a formal assessment is not carried out a t the ultimate limit state, because, for instance, there are insufficient data from bridges tested to collapse to enable the behaviour of the model to be established with confidence. The best that can be achieved with this type of test is confidence in the behaviour of the structure under service loads. The results can be used for monitoring as described in the next Section.
10.5.2 Load testing for monitoring
This type of test originates from a desire to adopt a monitoring regime in accordance with BA79. If it is considered necessary to understand how the structure is carrying the load without distress, a form of test similar to a Supplementary Load Test may be considered. The intention in this case is not to re-model or re-assess the structure, although that could be done in addition. Using an understandingof the behaviour of the bridge based on a load test enables a more rational decision to be taken on the monitoring regime and the nature, location
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and alarm levels required to indicate a significant behaviour change. For instance, if there is moment end restraint present that is contributing to a high observed stiffness of the deck, it may be possible to provide gauges to detect any reduction, or identify the potential location of future cracking.
10.5.3 Reliability and updating
Reliability considerations may be applicable to load testing and interpretation of the results. For example, a paper by Fu’O13 explains this in the context of testing in the USA. If a full reliability analysis is being carried out for an assessment (Level 5 in BA79), it is in principle possible to introduce the results of a load test to improve the reliability estimate obtained by calculation alone. A paper by Das and C ~ l l i n g t o n ’ proposes ~’~ the use of Bayesian updating methods with
load testing. This form of testing has not been used in practice at the time of writing. In principle, this entails loading being applied as if in a Proof Load Test. However, it is not necessary to apply the full uls loads provided information is available about the behaviour of similar bridges. It depends on a relationship being established between the load to cause a low level of structural distress and the ultimate load - for instance between the loads to cause non-linearity and failure. Provided the structure does not reach the distress level under the test load, a higher test load may be inferred having only a small probability of causing failure. If the probability of failure under the required assessment loading is judged low enough, the structure may remain in service with an appropriate level of monitoring if necessary. The level of monitoring is likely to be significantly less onerous than would be the case without the testing.
10.6 Dynamic load testing
10.6.1 Structures where dynamic response is critical
Dynamic testing is occasionally required when the dynamic behaviour of the structure is a critical factor. Examples are excitation of long span bridges by wind, and the pedestrian excitation of footbridges. The vibration of footbridges is not a strength issue. It has been known for the vibration of concrete bridges to be reported as questionable, but when dynamic measurements are recorded, the stresses involved are generally small. When dynamic testing is necessary, a specialist consultant/testingorganisation should be contacted at an early stage. Some large consultants have in-house specialists in the subject. Measurements of the structural response can be made during service loading or under test loading. Typical measurements include acceleration of the superstructure and
sometimes displacement at joints and bearings. Analysis of the data is likely to require dedicated software directed, for example, to the computation of frequencies and mode shapes. In order to understand the behaviour of a bridge, it is helpful to carry out dynamic calculations for comparison. Frequencies are relatively straightforward to obtain by
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calculation. It is more difficult t o calculate the amplitude of response. Structural damping values can be obtained in free decay or less reliably from the analysis o f the frequency-domain response in the mode required.
10.6.2 Dynamic tests for health monitoring
The basis for this form of testing is that physical changes t o a structure generally change the dynamic characteristics. Natural frequencies, modes shapes and damping may all be affected. If changes occur t o the dynamic characteristics, it may be taken as a signal therefore that structural changes have also occurred. Routine determination of the dynamic characteristics will bring these changes t o the attention o f the maintaining authority and may be useful for the management o f the structure. Alternatively, changes t o the measured dynamic characteristics may indicate that boundary conditions or environmental changes have taken place. When environmental changes are responsible, there is a tendency for genuine structural effects t o be obscured. In themselves, changes caused by environmental factors are seasonal and generally of little structural consequence. The potential for testing o f this type has been recognised for many years. Early practical applications occurred with offshore oil installations. Compared with bridges, the inspection of those structures is more difficult and expensive, and the consequences o f structural failure are more critical in terms o f financial and safety respects. For bridges, generally, the cost o f monitoring has been t o o high for the benefits that accrue. Experience has shown that many structures are excited sufficiently by traffic or wind. Processing of data from accelerometers enables frequency and mode shape t o be determined. Damping can also be estimated but with less confidence.
10.6.3 Applications for assessment
It is important t o recognise that changes t o frequency and mode shape are related t o
stiffness and not strength. Direct relevance t o the assessed capacity at the uls is limited therefore. Such changes are relevant t o assessment only if it can be shown that the source o f the changes is detrimental t o strength. This requires a secondary evaluation, probably involving inspections and other forms o f testing. For instance, if the shear connectors in a composite bridge have failed, it may reduce the local stiffness, which in turn could reduce the section stiffness over a length of superstructure and affect frequency and mode shapes. Knowing that the connectors have failed will enable a revised assessment t o take place. Naturally, it is necessary t o show that the dynamic changes will be detectable when the extent of connector failure becomes significant. Another example is the development of cracking in a reinforced concrete member. It is well known that as applied load increases, cracking may develop and spread. This happens in laboratory tests and can happen under increasing service loads o n a bridge particularly in flexure. There is an accompanying loss of stiffness that can be detected by measuring changes of frequency (at least under laboratory conditions). However, the ultimate capacity is not directly affected. At the uls, usually, the concrete will be
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extensively cracked in flexure even when the failure is finally in shear. This is a case where stiffness changes are not accompanied by a reduction in strength. It is not directly relevant therefore to assessment at the uls. Nevertheless,the bridge manager may find the information useful if cracking is difficult to detect by visual inspection. A case where strength changes are not accompanied by stiffness changes is local
corrosion of reinforcement. This can reduce the areas of reinforcement significantly over a short length and reduce the uls capacity without significant global stiffness change. Reliance on dynamic monitoring would not be appropriate in that case. Calculations can be carried out to estimate the effect of structural changes on the stiffness and mode shape. Studies have shown that, generally, the structural deterioration has to be large and extensive to affect frequency by as much as 5%, and environmental effects can cause more change than this. Structural changes of this magnitude may be visible by inspection in a bridge. Large, long span bridges with clean lines and efficient bearings may be reasonably stable and/or behave predictably in the presence of temperature changes. Structural changes from strengthening may increase stiffness sufficiently to be detected above the environmental changes. Nevertheless, in practical terms, the detection of strength changes resulting from deterioration that could not be detected by other means is not considered likely. Smaller bridges such as motorway overbridges are more likely to be affected by thermal changes, particularly when the surfacing is a substantial thickness.
10.6.4 Case studies and research in the literature
Conference papers and journal articles can be found that describe techniques of measurement and analysis, and the results of testing on a large number of structures. For health monitoring applications, what are absent are clear-cut examples where: 0 The testing was for bridge management and not a research exercise; and was not
associated with a problem of dynamic ‘behaviour. 0 Changes to dynamic characteristics were the first indications of a potential problem 0 The changes were structurally significant and not detectable by other means, for instance visual inspection. 0 Dynamic response was able to identify and quantify structural deterioration that involved loss of capacity - and this was subsequently corroborated, and remedial action taken.
I
A conference paper by B r o w n j ~ h n ’ provides ~’~ a useful summary and introduction to
the topic and an extensive bibliography
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10.7References
10.1 CULLINGTON, DW and BEALES, c, Is your strengtheningreally necessary’ Proceedings oftheConferenceonBridge modifcation. Institution of Civil Engineers, 23-24 March 1994, Thomas Telford. 1995, pp 270-284. 10.2 HIGHWAYS AGENCY, BA 54, Load testing for bridge assessment, Design Manualfor Roads andBndges. 10.3 HIGHWAYS AGENCY, BD 21, The assessment of highway bridges and structures, DesignManualforRoadsandBridges. 10.4 HIGHWAYS AGENCY, BD 34, Technical requirements for assessment and strengtheningprogramme for highways structures Stage 1 - Older short span bridges and retaining structures, Design ManualforRoads andBridges. 10.5 NSCBLT 1998, Supplementaryloadtesting ofbridges. The Institution of Civil Engineers National Steering Committee for the Load Testing of Bridges, Thomas Telford. 1998, p 73 10.6 HIGHWAYS AGENCY, BA 54, Load testing for bridge assessment, DesignManualforRoadsandBridges. 10.7 NSCBLT 1998, Supplementary load testing ofbridges, The Institution of Civil Engineers National Steering Committee for the Load Testing of Bridges, Thomas Telford. 1998, p 73. 10.8 HIGHWAYS AGENCY, BD 21, The assessment of highway bridges and structures, DesignManualforRoadsandBridges. 10.9 MINISTRY OF TRANSPORTATION,ONTARIO, Ontario highway bridge design code, 1992 10.10 NATIONAL CO-OPERATIVE HIGHWAY RESEARCH PROGRAMME, Manualfor bridgerating through loadtesting, Research Results Digest Number 234, , Transportation Research Board, National Research Council. Washington DC, November 1998 10.11 NEW ZEALAND STANDARDS, ConcreteStructuresSrandardPart I
- thedesign ofconcretestructures,1995.
10.12 DENTON, SR, IBELL, TJ and POSNER, CD, Full-scale model testing to investigate the shear capacity of concrete slabs with inadequately anchored reinforcement,In Bridge Management4 (Ryall, MJ, Parke, CAR and Harding, JE. eds), Thomas Telford, 2000, pp 55-62 10.13 FU, C, Highway bridge rating by non-destructive proof-load testingfor consistent safety, Research Report 163, Transport Research and Development Bureau, New York State Department of Transportation, 1995. 10.14 DAS, PC and CULLINCTON, DW, Proof load testing and Bayesian updating, Bridge rehabilitationin the UK;review ofthe currentprogramme andpreparingfor thenext, Institution of Civil Engineers, 2-3 October 2000. 10.15 BROWNJOHN,JMW, Dynamic performance and characterisation of highway bridges in Singapore, In Currentandfuture trends in bridge design constwctionandmaintenance. Singapore, Thomas Telford, October 1999.
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11. Re iabil ty and risk-based techniques This Chapter provides a guide on the use of reliability and risk-based techniques for the assessment for bridges.
11.I Background
The process of assessment is of crucial importance for maintaining highway bridges in a safe and serviceable condition. The objective of assessment is t o evaluate the safety of an existing bridge quickly and with a minimum of effort. However, the assessment rules and criteria need to be established rigorously and judiciously. If assessments are unduly conservative, structures will be unnecessarily strengthened, or needless load restrictions will be imposed. Conversely, if the rules are too lax some bridges could actually fail during service”
’.
Codes and standards for design and assessment employ partial safety factors t o ensure an appropriate level of safety for bridges. These factors guard against extreme variations in design parameters (e.g. material properties, applied loads, etc.), which could occur during service. In order t o ensure that the design rules are simple for routine use the values of the partial factors are chosen such that they cater for a wide range of structure/component types and failure modes. O f necessity, therefore, the rules tend t o be conservative for the majority of bridges and the level of conservatism varies considerably from structure t o structure. In current assessment practice, the safety is checked mainly at a component level using in most cases elastic methods of analysis and, therefore, does not take account of the reserve strength of the whole bridge system. Only in the case of failure of reinforced concrete slabs is the use of yield-line analysis t o qualify the ultimate strength becoming more common. Also the assessment criteria do not take into account the consequences of failure and so, for example, a bridge on a motorway and a small culvert on a local road are assessed t o the same criteria” ’. There have been no reported cases in the UK of highway bridge collapses as a result of traffic overloading, however element distress has been observed in some instances. The design rules are considered therefore t o provide an adequate level of safety. However, this does not necessarily mean that the safety level is optimal; it may well be conservative and the extent will vary for every structure. For many bridges that have been in service for a number of years and have shown no signs of distress, later assessments have shown low or no apparent live load capacity! In order t o ensure that the assessment rules provide a more consistent level of safety across the bridge stock and t o avoid unnecessary strengthening costs due t o over-conservative assessments, it is necessary t o develop criteria which can be tailored to each bridge by taking into account its particular safety characteristics” 3. To address these issues, the Highways Agency’s Advice Note BA79/98 sets out a
philosophical approach t o assessment and defines the principle of different levels of assessment” as follows:
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U Level 1 assessments are carried out using simple analysis methods, in accordance with
the assessment Standard BD21 and the accompanying Advice Note BA16. 0 Level 2 assessments involve more refined analysis and the use of characteristic strengths of materials. 0 Level 3 introduces the concept of Bridge Specific Assessment Live Loading and makes use of material test results t o determine worst credible strength values. 0 In addition Levels 4 and 5 were included which were based on using structural reliability methods. However, these specific methods have'been dropped in the draft amended version of BA79 (see Section 12.1 and Reference 12.5) and reference made t o the general use of structural reliability methods. Whilst structural reliability has been used in other fields, notably for offshore structures, there have been very few applications for bridges in the UK.
11.2 The appropriate application of reliability and risk-based assessments
In general, reliability and risk-based assessments should be considered only after generic assessments have been completed and the bridge fails t o meet the general assessment requirements. A reliability analysis can also be performed t o provide further assurance on the results from lower levels of assessment. An assessment that makes use of bridge specific partial safety factors can provide particular benefit in the following cases: U The consequences of failure of a bridge are significantly lower compared t o a major
bridge on a busy motorway or a bridge over a busy railway line. U The bridge is known to have carried unrestricted traffic over a reasonable period of time (e.g. 5 years) without damage/distress but the calculated live load capacity factor, K, suggests a significant shortfall of capacity (e.g. K < 0.7). In addition t o the above, a detailed reliability analysis may be beneficial in the following cases: U Bridge specific data are available on material properties, dimensions and traffic loads. 0 Reliable information is available on abnormal heavy vehicles passing the bridge. 0 Results of supplementary or proving load tests are available. 0 The critical component is known t o have suffered deterioration but the extent of deterioratioddamage cannot be estimated accurately. 0 When weight restrictions suggested by more general assessments need t o be relaxed t o minimise traffic disruption. 0 Results of model tests of adequate scale with similar section details and support conditions t o that of the bridge element under consideration are available.
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11.3 Overview of reliability-based assessments 1 1.3.1 General
11.3.2 Uncertainties In bridge assessment
The risk of structural failure depends on the uncertainties in load and resistance parameters as well as other factors such as gross errors. Quality control and verification are used to control gross errors, while limit state designs, or alternatively, structural reliability index can be used to mitigate failure due to normal variability. An overview of the application of structural reliability methods is given in the following Sections.
A number of uncertainties influence the assessment of safety of a bridge and its
components. These uncertainties arise from inherent variability of loads and material resistance parameters, uncertainties in deterioration over time, uncertainties in the analysis models used for determiningthe load effects and capacities, and the uncertainties in measurement and inspection techniques. In view of these uncertainties, it is possible that an element could fail from an adverse combination of extreme values of the variables. This is illustrated in Figure 11. I , in which the overlap between the loading and resistance distributions represents the failure region and the area of overlap between these distributions is related to the probability of failure. In a deterministic method of assessment, the safety of a bridge component is ensured by using a number of partial factors to guard against extreme variations of the random parameters. In a reliability analysis, which is based on a probabilistic approach, the partial factors are not used. Instead, the uncertain parameters are modelled using appropriate probability distribution functions, and the probability of failure of the component is calculated.
Figure 11.1 Illustration of the concept of probability of failure.
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.-
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11.3.3 Reliability Index
In order t o carry out a reliability-based assessment of a bridge, all the relevant failure modes for each critical component of a bridge are identified and a reliability analysis is carried out separately for each failure mode. It is common t o express the result of a reliability analysis in terms of a ‘Reliability Index’,
p, which is mathematically related t o the probability of failure, P,:
where #J ( . ) is the cumulative distribution function of a standard normal variable. This relationship is shown in Figure 11.2. Note that P,decreases as P increases. Figure 11.2 Relationship between reliability index and probability of failure.
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Probability of failure
The reliability index, or probability of failure, should be defined with respect t o a specified time interval. When the reliability is evaluated for a reference interval of one-year, it is called an ‘annual reliability index’; and when it is evaluated for the lifetime of the structure, it is called a ‘lifetime reliability index’. The annual reliability index is used in determining the adequacy of a bridge. The corresponding annual probability of failure represents the likelihood of the failure mode occurring in any one year. For a non-deteriorating element where traffic live load is the only time varying load, the annual reliability index is calculated considering the maximum live load event that could occur within any one-year interval. In order t o determine the structural adequacy of a component the calculated reliability index needs t o be compared against a specified minimum acceptable value called the ‘target reliability index’. It should be noted that the results of reliability analysis are very sensitive t o the resistance and load effect models used, the probability distributions for the basic random variables and, t o a lesser extent, the method for calculating probability of failure or reliability index. To ensure that assessments are carried out on a consistent basis, it is important that a standardised procedure is followed and for the assessment of Highways Agency structures this might be as given below.
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1. Identify critical component(s) in a bridge based on previous assessment results. Identify the relevant failure modes for this component. 2. Select a failure mode and identify the relevant basic random variables. 3. Where the structural response is linear, compute the load effects a t the critical section using a linear-elastic grillage or finite element analysis separately for structural dead load, superimposed dead load and surfacing load components. These results can be taken from previous assessments, where available. 4. Evaluate the load effects a t the critical section due t o the ‘Basic Static Live Load’. 5. Choose appropriate probability distributions for the basic random variables. 6. Select an appropriate method for reliability analysis and any one of the available commercial software systems, Develop sub-routines for the failure mode function and link it with the selected computer program, as required. (This needs t o be done only once for each bridge and failure mode.) 7. Perform an initial reliability analysis for the selected failure mode using the probability distributions from Step 5 and any other deterministic parameters as required. Determine the annual reliability index, P, and the sensitivity factors, Lyi, for each random variable. 8. Design a ‘calibrator’ element such that the element satisfies the assessment requirements exactly. Perform reliability analysis of the calibrator element using the probability distributions from Step 5 and any other deterministic parameters as required. Determine the annual target reliability index, PO.Modify this value based on consequences of failure t o obtain the final target reliability index, PT. 9. Collect additional datahnformation on material strengths, loads, etc. specific t o the bridge being assessed. The results of sensitivity factors from Step 7 should be used t o identify what data it would be most beneficial t o collect. 10. Re-calculate the reliability of the component by incorporating bridge specific datahnformation using Bayesian updating methods. 11. The component passes the assessment for the selected failure mode if P > &. If not, consider if additional data collection would be beneficial and refine the reliability estimate as in Step 10. 12. Repeat Steps 2 t o 11 for each selected failure mode and component of the bridge.
11.3.4 lnterpretatiOn O f Reliability Analysis Results
The probability of failure computed from a reliability analysis should be treated as a ‘notional’ value t o be used in a relative sense t o compare reliabilities of different structures. It should not be interpreted as a measure of the frequency of failure that could be expected in service, i.e. in the sense that one out of so many bridges would fail. This is because each structure is unique and, hence, a population cannot be readily defined. Furthermore, the probability distributions used in a reliability analysis are intended t o represent inherent variability in design parameters that could be expected for structures that are designed, constructed, operated and maintained t o good engineering practice. They do not account for ‘gross errors’ in design (e.g. incorrect calculations of loads or capacities); construction (e.g. the use of wrong steel reinforcement, missing bars, poor quality of construction); or in operation (e.g. gross overloading of vehicles) which are seen as the cause of most structural failures observed in practice.
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aa
ReLlabiRity and risk-based techniques
Nevertheless, by treating the underlying uncertainties on a consistent basis the reliability method gives a rational basis for comparing different components and different structures. Guided by engineering judgement and experience, the method provides a rational basis for evaluating the margin of safety for a given failure mode and t o demonstrate structural adequacy. This provides a formal link between Codes of Practice and reliability requirements. The reliability analysis can also be used as a decision tool t o compare technical alternatives in terms of their cost and risks. Flaig and Lark” propose a framework for the incorporation of such an approach into a decision suppor;t system ._ for bridge management. Reliability-based techniques provide a greater flexibility for incorporating service data (e.g. load testing, material testing, measurement of dimensions, deterioration, etc.) and any additional safety characteristics (e.g. warning of failure, consequences of failure) for a specific structure and thus provide a more rational assessment of its adequacy.
11.4 Acceptance Criteria
In order t o judge whether the calculated value of a reliability index for a particular bridge is adequate, it is necessary t o specify the minimum acceptable value of the reliability index, often termed the ‘target reliability index’. A number of Codes and Standards and other published works specify target reliability
values for bridges and other types of structures. However, it is necessary that the specified target values are consistent with the method of reliability analysis, in particular the probability distributions used for the various basic variables and the models for the calculation of capacity and load effects. For this reason, published values cannot be readily used. In choosing a value for target reliability, one is essentially faced with the question of How safe is safe enough? This involves complex technical, social and economical issues and requires value judgements. Generally, there is a consensus among researchers that the following factors should have an influence on the choice of target reliability index: 1. Consequences of failure 2. Reserve strength and redundancy 3. Warning of failure 4. Inspection and monitoring measures 5. Marginal cost of increasing the reliability 6. Acceptability of risk by the society. It is important t o consider the consequences of both element and overall collapse of a
bridge. The collapse of a bridge can result in fatalities and injuries, disruption t o traffic, environmental damage in some cases, loss of confidence in the profession, adverse publicity, enquiry and litigation, changes t o Codes and Standards, assessment and strengthening of other similar bridges. Element failure or distress can lead t o temporary closure of the bridge, thereby causing traffic disruption.
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Factors 2 and 3, in turn, influence the consequences of failure. Inspection and monitoring procedures help t o detect signs of distresddamage a t an early stage, thereby preventing complete collapse of a bridge. They are only effective if significant warning is available following an element failure. Factor 5 brings the economic reality into the picture. The marginal cost of increasing the reliability is considerably higher for an existing structure compared to new build. This supports the use of marginally lower target reliability for existing structures compared t o the target used for the design of new structures of the same type. The issue of social acceptability of risk is very complex; but is known t o be influenced by factors such as: how frequently the failures occur; the number of fatalities and injuries; the scale of resulting disruption t o public; whether the risk is voluntary; whether the failure is due t o man-made or natural causes; publicity from the media; the resulting costs t o the public of increasing safety; etc. It is believed that society expects a higher level of safety for buildings and bridges compared t o risks accepted in other aspects of daily life. One view is that the safety level implicit in current design/assessment practice is about right since this has been a result of years of experience, both good and bad. Therefore when a new approach for design/assessment is introduced, it is strongly desirable that the resulting reliability levels in the new method are on average of the same level as those implicit in current Codes and Standards. Any changes in reliability, where appropriate, should be introduced in a gradual manner backed-up by careful monitoring of selected structures over a period of time. In principle, three approaches can be recognised for establishing target reliability values, see for example Ditlevsen and Madsen” 6 : 1. Socially acceptable risk levels from historical data 2. Calibration t o existing Codes and Standards 3. Economic optimisation.
The first approach is limited in that it cannot easily be related t o the ‘formal failure probabilities’ computed from a reliability analysis. Furthermore, as bridge failures due t o overloading have not occurred in UK,direct calibration against bridge failure statistics is difficult. In using the economic optimisation approach, the difficulty is in the accurate evaluation of all direct and indirect consequences of failure. This approach is satisfactory where economic losses dominate over life, limb and social consequences; otherwise the approach becomes controversial. The second approach, although not free from difficulties, seems t o be the only practicable way forward at the present time. This approach was used for deriving partial safety factors in BS 5400: Part 3 (steel bridges) and for a number of North American Codes for bridges and buildings.
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It is strongly suggested that results from a reliability-based assessment should not be
used in a strict pass/fail manner and Lark and Flaig” discuss the use of such results to aid bridge management. The results should form inputs to the decision-making process for choosing between alternative remediaVmitigation options for the bridge. An example of the application of such an approach is described in Reference 11.8.
11.5 Bridge Specific risk-based assessments
In addition to utilising bridge specific material strengths and loads in an assessment of a bridge, there may be justification for further rationalising the assessment criteria by taking into account factors such as age of the bridge, loading history, consequences of failure, reserve strength and redundancy, warning of failure, and inspection and monitoring regime. In the following paragraphs, modifications to the assessment criteria are given for consequences of failure and loading history.
Modification based on consequences of failure Bridges with low consequences, for example those carrying a minor road over a small span, could have a lower target reliability than a bridge carrying a motorway so that the resulting risks are the same for the two bridge types. By maintaining the same level of target risk, R,, the target probability of failure, P,, for a bridge with consequences of failure, C,, can be determined. By developinga relationship between reliability index, p, and Live Load Capacity Factor, K, the corresponding modification of the K-factor can be determined.
Modification based on loading history Similarly, if the bridge has been in service for a sufficiently long period of time, it can be reasonably expected that the bridge would have been subjected to some extreme loading. Using Bayesian reliability analysis procedures, the variation in annual reliability index for bridges of different age (or service exposure) and with different calculated K values can be developed. The typical result is shown in Figure 11.3. Figure 11.3 Typical variation of Kfactors based on the age of a bridge.
KMcd
;
t . .....
iK ,
.: , .... ,
, ,, ,
Live load capacity factor K
146
....
, , , , ...,
,
,
,, ,,,,
,
, , ,
The lower curve uses no information about the service history of a bridge, while the upper curve assumes that the bridge has been carrying unrestricted traffic for over five years at a level similar t o the present-day traffic. Although the bridge may have been in service for longer than five years, it is likely that the traffic in the past has been lighter and hence this is n o t taken into account. It can be seen from Figure 11.3 that significant improvement in K can be obtained when
the K,,,, value is low. The benefit of Bayesian Updating reduces as the calculated K,,,,,,, value approaches 0.7. Therefore, the increase in the K factor obtained using this method may not be adequate in most cases for a bridge t o pass the assessment. However, this may help in justifying a higher weight restriction o n a bridge than what would otherwise be dictated by a more general assessment. Reliability and risk-based assessments require considerable statistical expertise and, in general, expert help should be sought along with client approval before using either of these techniques.
11.6References
11.1 DAS, PC, Development of bridge-specific assessment and strengthening criteria, in SafetyofBridges, (Das, PC, ed.), Thomas Telford, London, 1997. 11.2 SHETTY, NK, CHUBB, MS and HALDEN, D, An Overall Risk Based Assessment Procedure of Substandard Bridges, Safety ofSridges, ICE, London,July 1997 11.3 SHETrY, NK. CHUBB, MS and MANZOCCHI,CME, Advanced methods of assessment for bridges, International Conference on Management ofSridges, ICE, London, 1998 11.4 DAS, PC, Development of a comprehensive structures management methodology for the Highways Agency, Management o,fh;qhwaystructures, (Das, PC, ed.), Thomas Telford Publishing, London, 1999. 11.5 FLAIC, KD and LARK, RJ,The Development of a Risk-Based Decision Support System for Bridge Management, Proceedings of the Institution of Civil Engineers- Bridge Engineering, 2005. 11.6 DITLEVSEN, 0 and MADSEN. H 0. StructuralReliabilityMethods, Chichester, John Wiley, 1996. 11.7 LARK, R.J.and FLAIC, KD, The use of reliability analysis to aid bridge management, TheStructuralEngineer,Vol. 83, No 5, March 2005 11.8 CHUBB M and SHETTY. NK, Reliable Bridge Assessment, Concrete Engineeringinternational July/August 2000.
12 Bridge management and assessment ~
12. Bridge management and assessment 12.1 General PritlCipkS
This Chapter considers how the assessment process can be integrated within bridge management activities, by providing advice on the management of sub-standard bridges and the influence of assessment results on maintenance activities. An introduction to the Assessment Concept was set out in Chapter 1 where the Need for Assessment, Staged Assessments and Assessment and Asset Management are particularly relevant to Bridge Management and Assessments. In this Chapter, only general principles will be established in relating bridge management to assessment; these should be read alongside the guidance given in Chapters 1 and 2, with relevant Standards (BDs) and Advice Notes (BAs) being listed in Section 1.4. Reference should also be made to the Code of Practice for the Management of Highway Structures published in September 200512’.
12.1 .I Managing sub-standard bridges
The Highways Agency Advice Note BA79, The Management of Sub-standardHighway Structures, (the Advice Note) was published to provide guidance on the implementation of interim measures for structures which were assessed to be sub-standard, or provisionally sub-standard, in order to maintain the safety of the highway network. Its use is applicable to virtually all highway bridges. The substantial completion of the assessment programme has left a large number of bridges that have failed the assessment process, but there is little prospect of these being strengthened in the near future. The assessment code, BD21, requires that in the event of an assessment failure, one of a series of formal interim measures is imposed on the bridge. Many bridges are unsuitable for propping, and weight restrictions or lane closures would often cause significant disruption to traffic flows. The diversion of traffic away from a restricted bridge may create additional problems for safety on diversion routes. Consequently, many Authorities had been operating monitoring schemes with the intention of allowing bridges to remain open to unrestricted traffic. The publication of the Advice Note provided, for the first time, a rational method for considering the whole concept of sub-standard bridges and provided a framework for instigating a robust monitoring regime in approved cases’22.
12.1.2 The BA79 Advice Note
The Advice Note is divided into five main chapters: 1. Assessment (see Chapter 1) 2. ‘Immediate Risk Structures’ 3. Interim Measures During Assessment 4. Interim Measures on Completion of Assessment 5. Prioritisation for Strengthening (not considered in this Guide).
This document makes more reference to the role of the Technical Approval Authority (TAA) than any other Highways Agency Standard or Advice Note (other than BD2). It
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also leaves a number of areas open for interpretation. It is necessary, therefore, for bridge owners, Technical Approval Authorities and bridge managers t o consider their respective duties. In practice these three roles are often combined. Some of these area for interpretation are discussed further below (refer also t o Chapter 2). It is also important that, for Local Authorities, locally elected members are aware of the steps that need t o be taken t o ensure the safety of the highway network. Diagrams, derived from Appendix A of the Advice Note, illustrating the procedures are given in Figures 2.1 and 2.3. Experience with the use of the Advice Note is contained in a document produced for the Highways Agency by Parsons Brinckerh~ff’‘~.This showed that there had been an inconsistent application of the advice and that further guidance was required.
12.1.3 Managing the results of assessments
A key aspect of the Advice Note is the need t o keep adequate records throughout the assessment process, with details of the decisions taken at each stage. This is done by the use of a proforma (known as Form E l ) throughout all of the processes shown on the flow chart (Figure 2.3). The use of this proforma is recommended for use by a Highway Authority irrespective of the ownership of the bridge. The Form E l should be kept on the bridge file at each stage or level of the assessment process. Where the structure is considered t o be ‘monitoring-appropriate’ then a monitoring procedure document (see below) should be appended t o the Form E l . The following procedures are recommended: 0 The level of assessment completed should be shown on the assessment certificate 0 Amendments t o Approval in Principal (AIP) forms are required for each level of
assessment proposed by way of an addendum t o the original AIP. 0 Sub-standard bridges must be restricted t o Authorised Weight Regulations (AWR) vehicles if no lower limit is imposed. One objective of the Advice Note is t o limit the disruptive effect of weight restrictions by the use of monitoring on appropriate structures. The use of abnormal indivisible loads (AIL) on Special Type General Order (STCO) vehicles in the highway network is increasing and the selection of suitable routes is not made any easier by the large number of sub-standard bridges that exist. In particular circumstances, this problem can be overcome by a special assessment for a specific vehicle carried out in accordance with BD86/01. The existence of one AWR restricted bridge on a motorway link can create significant problems for the alternative AIL route on the parallel local road network. Annex D t o BD86/01 provides advice on the management of abnormal loads.
12.1.4Immediate risk structures
These structures are where an immediate and unacceptable risk t o public safety is identified (BA79). The diagnosis should be confirmed by the TAA. The Advice Note recommends that “factors such as the nature of the structural weakness, any corresponding signs of distress, the recent load history of the structure and the level of assessment completed should be taken into account”.
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1 2.1.5 Interim meaSUreS during assessment
The assessment process can involve several stages of work and be extended over a considerable period of time. It is recommended, therefore, that interim measures are implemented whilst the assessment process is completed, unless the structure is considered t o be ‘low risk’. The use of this procedure might require, for example, the erection of weight restriction signs and their subsequent removal without any change in the condition of the bridge. The bridge owner should be prepared t o explain these essential safeguards t o elected Councillors or Members of Parliament. The definition of ‘low risk’ requires that there is likely t o be a considerable reserve of strength inherent in the structure. It should be remembered that at this stage of the process it will not have been possible t o have completed the full range of options open t o the assessment engineer. Therefore, the definition can only be subjective. The Advice Note also suggests that structures can be considered t o be ‘low risk’, “where consequences of failure are low or where live load capacity factor is greater than 0.7”.
12.1.6 Interim MeaSUreS On comp[etion of assessment
The purpose of these measures is t o reduce the risks of a failure t o acceptable levels until strengthening is carried out. The Advice Note introduced the definition of ‘Monitoring - appropriate Structures’. In order t o qualify as ‘Monitoring - appropriate’, structures need t o satisfy all of the following criteria (except in the case of small spans generally less than 5 m): 0 No significant signs of critical distress 0 Hidden problems unlikely t o be present
U Predictable and gradual failure mode 0 Monitoring should be meaningful and effective.
The use of monitoring is treated as a ‘Departure from Standard’. Further guidance is contained in Appendices C and D of the Advice Note. The assessment engineer may be able t o offer alternative Formal Interim Measures. For example, a bridge may be assessed at 7.5t with three lanes of traffic but at 18t with only two lanes of traffic. The lower Assessment Live Loading (ALL) will handicap buses and most commercial vehicles but permit unrestricted use by private cars. Conversely, the higher ALL is suitable for most buses and a large number of commercial vehicles but the reduction in traffic lanes may cause queuing of private cars a t peak times. The bridge manager will need t o discuss the options with the highway manager and record the outcome on Form E l . The lower the level of posted weight restriction, the more likely it is that it will be ignored. Very low levels of weight restriction (3t) allow an additional width restriction t o be used. Generally, this ensures enforcement of the weight limits but the physical restriction may need t o be substantial t o deter drivers determined t o avoid using a diversion route. Higher levels of weight restriction are more difficult t o enforce and a risk management exercise should be carried out t o consider the effects of possible repeated abuse. Monitoring and further assessments may also be required.
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Weight restrictions are imposed by the use of temporary traffic regulation orders. These are normally only rated for 18 months unless extensions (up t o six months) are granted by the Secretary of State. Failing this, they either have t o be removed or made permanent. Objections may be raised t o a permanent order that could, in an extreme case, result in a public inquiry with the obvious programming and resource implications. It is important that legal sections preparing these orders are aware of the structural problems and do not include exemption clauses that are common in environmental weight restrictions.
12.1.7Assessment and maintenance
Section 1.1 introduced the concept of Asset Management, i.e. the need t o increase the overall value of the bridge stock, the asset, but a t lowest whole life cost. The concept is developed further in Section 1.5, which referred t o the work carried out by Flint and DaslZ4and the publication of a draft advice note (BA81) by the Highways Agency . The assessment rating of an individual bridge should be included in the calculation of a bridge condition index, which can then be used t o monitor the effectiveness of maintenance over a total bridge stock. Discussion of these techniques is beyond the scope of this Guide but the reader is referred t o the output of the BRIME project (see Section 1.8) for further details. The reader should also refer t o the Code of Practice for the Management of Highway Structures”’ which includes many recommendations for good highway structure management practice set out in an asset management framework. In response t o the findings of Parsons Brinckerhoff‘s review of the application of BA79 123, BA79 is currently being updated in consultation with structure owners and other stakeholders. The Advice Note BA79 will be republished as a new Standard, BD79, incorporating much of BA79. The background t o this new Standard is described by Shave et aP2 The new Standard establishes mandatory, process orientated, requirements together with guidance on how these can be fulfilled. Guidance on record keeping is improved, and management processes and responsibilities have been clarified. The following guidance is based on the requirements of the current Advice Note, BA79. Much of the guidance will remain broadly valid when the new Standard is published. In addition, many of the issues identified with BA79 in the text will be addressed in the new Standard.
’.
12.2 Monitoring
Monitoring is a process that is instigated following concern over the performance or integrity of a structure following observations made from a routine or special inspection and/or the outcome of an assessment. The transition from inspection t o monitoring is often blurred; due t o the dilemma over what frequency of monitoring is appropriate for a specific set of circumstances. Monitoring is distinctly different however t o routine inspection and should comprise two principle components; it must be either more frequent than the norm or it must involve special targeted scrutiny. Useful general guidance on monitoring is given in References 12.3 and 12.6.
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As risk is related to the probability of an event occurring and time, the frequency of monitoring is a key parameter that needs to be determined and this is an area that warrants careful consideration. The factors that should be taken account of include: 0 Load assessment capacity rating U Structural and material condition
U Deterioration rates 0 Interim measures 0 Road or rail classification
0 Possible failure mechanisms 0 Obstacles being crossed. Based on these factors, a monitoring interval needs to be identified, which should range between immediate action to a maximum monitoring interval of six months. The monitoring frequency should be subject to review by the engineer and adjusted, either from the outset, based on engineering judgement, or as an action following from the results of monitoring.
12.2.1 Appropriate monitoring
BA79 Appendix D’27,goes into some depth into the forms of bridge or bridge elements that are ‘monitoring appropriate’, but in summary the following structures are likely to meet the relevant criteria:
0 Reinforced concrete slab bridges or composite steel and composite slab bridges with theoretical longitudinal or transverse flexural inadequacy, especially where adequate continuity exists over the supports. 0 Bridges in which the structural inadequacy is rooted in an element or connection whose failure would not precipitate sudden collapse and whose failure can be observed by monitoring. The crucial feature is that the bridge will retain a substantial proportion of its load carrying capacity following element/connection failure until the failure is detected and safeguarding measures are implemented. 0 Bridges of small span, generally less than 5 m, which are in sound condition and where the consequences of failure in terms of death and injury or traffic delay costs, etc., are low. Those bridges that are not normally ‘monitoring appropriate’ include bridges “that are sub-standard by virtue of tension, shear or anchorage inadequacies where failure in tension, shear or anchorage would precipitate collapse of the bridge”. However, as recognised in Appendix D in BA79, in reality the situation is more complex. So, these basic criteria should only be used as a guide to inform the assessing engineer’s judgement, as to whether an appropriate monitoring regime can be initiated that is both meaningful, effective and that will give adequate forewarning of distress or failure. The following sections outline some of the key issues relevant to determining appropriate monitoring.
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12.2.2Classes and frequency of monitoring
Appendix C of BA79 states that the frequency of monitoring should be as follows:
Class 7 - “observations are normally carried out at intervals of weeks ora few months andshould normally be more frequent than fora structure which meets the requirement of BD2 1 I’
Class 2- “depending on the bridge, from periodic visits at intervals of several months, to
morefrequent visits or to continuousmonitoring” Class3- ‘yrequent or continuous” Although there is no maximum interval specified in BA79 for any of the classes of monitoring any interval greater than six monthly should be carefully considered; as it is not within the spirit of the advice note and poses the question whether such inspections are distinguishable from the normal scope of a superficial or general inspection. Specific care should be taken to examine the structure for signs that the identified structural inadequacy is causing distress. The frequency of monitoring inspection should be appropriate to the mode and likelihood of distress or failure. Experience from testing and in-service performance of similar structures can be a particularly useful guide in this context. As well as considering the various timings, e.g. weekly, monthly, quarterly, etc.,
monitoring inspection during or likely unusual loading or environmental events should also be considered.
12.2.3 Application O f monitoring
The application of monitoring either in isolation or in combination with other measures is the most commonly used method of managing sub-standard structures. However, as is recognised in BA79, ensuring the safety of a sub-standard structure through monitoring is a complex process and requires in-depth knowledge of its structural behaviour. The process of monitoring should aim to verify structural behaviour characteristics by using a variety of investigative techniques to evaluate:
U Loading 0 Structural response to loading U Ground or foundation movement 0 Structural response to ground or foundation movement 0 Environmental factors, e.g. temperature 0 Structural response to environmental factors U Defects, deterioration and their development 0 Bearings and other restraints 0 Structural response to bearing actions and other restraints. Using these techniques, the monitoring process must also be properly evaluated by: 0 Gathering and reviewing records, facts and data (including in-service performance)
0 Assessment of the theoretical and actual structural response
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0 Forming an hypothesis 0 Checking results and observations 0 Drawing conclusions. What is learnt from the above evaluation will need to be compared with the known service and theoretical performances, whether any loading restrictions or interim supporting measures need to be implemented and the development of a monitoring specification. The form of monitoring must be appropriate however to enable a sound assimilation of the performance and functionality of the bridge to be formed.
12.2.4 Permanent monitoring
Permanent monitoring should be used for selected, severely deteriorated bridges and for critical bridges. Generally, such bridges would fall within BA79 Class 2 and 3, where safety of the structure is of paramount concern. Safety monitoring is appropriate when failure may occur through the worsening of damage. Growth of the damage should be reliably measured by an instrumentation system, and the measurement must allow a simple interpretation of damage severity. Overall, the monitoring system must offer an assurance of safety. The implementation of a safety monitoring regime requires a preliminary, five-stage analysis: 1, Analysis of the bridge condition. 2. Study of the various structural responses and possible failure paths.
3. Choice of the failure path(s) having the greatest probability of occurring. 4. Selection of the measurable quantities that reveal an adverse evolution of damage in the bridge. 5. Selection of threshold values of the physical effects that will require intervention in terms of emergency repairs, restriction of loads or closure of the bridge.
12.2.5 Monitoring specifications
Within Appendix C of BA791Z7,it is stated that for each sub-standard structure being managed through monitoring, a clear, unambiguous procedural plan should be prepared detailing the monitoring specification. Unless the monitoring is merely intended to check that other forms of interim measures are continuing to function satisfactorily, the plan should address the following: 0 The basis of the assessment inadequacy, clearly and concisely stated. 0 An appraisal of the reasons for the observed satisfactory service performance. 0 The anticipated mode or modes of failure. 0 A description of the parameters to be monitored and their relationship to the predicted mode of failure and progression to that state, together with a required frequency of observation and frequency of monitoring.
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0 A description of the ranges of observations which are acceptable and the values, or other features which constitute alarm or warning levels requiring action. U A clear set of procedures to be implemented if alarm or warning levels are reached.
0 Recording and reporting requirements. 0 Provision for review of the monitoring regime or procedures following observed
behaviour of the structure. The absence of a clear monitoring specification will lead to the following problems, which have implications for the safe management of sub-standard structures:
0 Inspectors are less likely to be able to target the focus of their monitoring to particular areas of structural inadequacy. 0 Inspectors are less likely to be aware of what range of observations are acceptable. 0 Inspectors and engineers are less likely to have or be aware of what procedures should be followed if the observations are not deemed acceptable. 0 Information is more likely to be lost on organisational changes and particularly changes managing agents. This is especially relevant where monitoring is by means other than visual inspection, where any development work carried out by the managing agent could be deemed to be Intellectual Property.
12.2.6 Methods and took for monitoring
Visual inspection A substantial part of monitoring concrete bridges will comprise visual observations, the
measurement of cracks; size, geometry, direction, number and any associated leakage of water and other substances. This regime (Class 1) is likely to be appropriate for monitoring structures that are sound with no signs of distress and where the likely mode or modes of distress are likely to progress slowly and that are easily measurable. The following types of distress may be appropriately monitored in this fashion:
0 Bulging 0 Corrosion 0 Delamination U Distortion 0 Flexural cracking U General cracking 0 Inadequate concrete repairs 0 Loss of section 0 Settlement U Spalling 0 Tilting.
Other testing and forms of monitoring When Class 2 and 3 frequency of monitoring is required then visual monitoring will need to be supplemented by appropriate testing and permanent forms of monitoring.
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Bridge management and assessment
An overview of these is given in the CBOG Technical Guide 2, Guide to testing and monitoring of concrete xtructure~’*8, including: U Testing within the structure maintenance process
0 General aspects of testing 0 Site tests for physical structure and response 0 Site tests for corrosion and probability of corrosion 0 Laboratory tests for physical structure and response 0 Laboratory chemical tests U Automated monitoring of physical structure and response Cl Automated monitoring corrosion and probability of corrosion
Monitoring reviews The need and frequency of monitoring is appropriate for a specific set of circumstances and any monitoring regime should be reviewed in the light o f the observed deterioration rates; experience and knowledge that emerges from other bridges and loading events such as exceptional loading from abnormal loads.
12.3 Cracking and crack widths
The appearance of cracks in hardened concrete always gives rise to comment but, irrespective of this, the structural significance of cracks, and hence their relevance to assessed capacity, needs to be considered carefully. To enable this consideration, as much as possible of the following information should be gathered and recorded during an inspection: 0 Location, frequency, orientation and pattern of cracking
0 Length, width and general appearance of individual cracks 0 Estimated age of concrete when cracking first appeared.
12.3.1 Crack widths
Cracks rarely have a constant width throughout their length and generally only become noticed when they exceed 0.1 m m in width. Crack widths can aid in diagnosis of their cause and can also become of concern in respect of durability of the concrete. Many cracks are considered ‘live’, i.e. their width varies over time, and this may be due to structural effects, thermal effects or a combination of those effects. Crack widths in excess of 0.5 mm are probably structural cracks, and crack widths in excess of 1.Omm are possibly indicative of the reinforcement having yielded and should be investigated further.
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12.3.2 Types O f crack and diagnosis
Cracks fall into t w o general categories, structural and non-structural. The significance of any cracks observed during a bridge inspection should become apparent by careful diagnosis of those cracks. Guidance o n a provisional diagnosis of the cause of cracking and other physical defects is given in CBDG Technical Guide 212*,and Appendix D of that document contains reference photographs and diagrams.
Structural cracks Structural cracks are those caused by load effects or other external influences such as movement or support settlement. They may be due t o flexure, shear or punching shear and indicate that assessment should be carried out at the serviceability limit state in accordance with Section 6.7. They will be recognised generally by their location in the structure, e.g. across the soffit near the mid span of a slab deck or diagonally in the web of a beam near a bearing, and may be reasonably wide. Consideration should be given t o the repair or remediation of these cracks t o restore as far as possible the durability of the concrete.
Non-structural cracks Concrete Society Technical Report 22, Non-structural cracks in c ~ n c r e t e ’ ~ ’classifies , three main types of non-structural crack: 1. Plastic
2. Early thermal contraction 3. Long-term drying shrinkage
An additional type of crack, and one which is of particular relevance t o assessment, is that caused by reinforcement corrosion. Such cracks are characterised by their width, location and associated rust staining. A typical example is given in Cracking Reference 1 of Appendix D in Technical Guide 2’28. Plastic cracks, arising from plastic shrinkage and plastic settlement, occur in the fresh concrete before hardening, whereas cracks due t o early thermal contraction and longterm drying shrinkage occur after the concrete has hardened. Figure 1 of Technical Report 22 gives a ‘family tree’ of crack types and Figure 2 shows examples of those crack types on a hypothetical concrete structure. A classification o f these crack types is given in Table 1 of the document. The information obtained from the inspection should be used in conjunction with Table 1 t o classify the cracks. Cracking, especially crazing, may be indicative of poor curing, but it may also be indicative of alkali aggregate reactions, especially alkali-silica reaction. ASR cracking is usually parallel t o the direction of stress. Further information o n ASR is given in Section 9.2 and in Reference 12.10. The significance of non-structural cracks in respect of assessment is their potential effect o n the durability of the structural element, which may need t o be taken into account as part of the assessment of that element. Information o n the assessment of deteriorated structures is given in Section 9.12 and methods of enhancing the durability of concrete are given in Concrete Society Technical Report 61 12.11.
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12.4 Deterioration rates
The main objective of programmes of assessments in the past has been t o evaluate the load capacities of structures designed t o earlier codes and rules of thumb. Deterioration, where pronounced, was usually taken into account but its progression and future influence on the load capacity of a structure was rarely considered. It became apparent t o engineers that many structures had durability and service problems that eventually led t o reductions in load carrying capacities, and that for more cost effective management of structures the future performance of structures had t o be determined by bringing together the results of condition and structural assessment.
This requires knowledge of: 0 The causes of deterioration 0 The effects of deterioration on the load carrying capacities of structures 0 The timescales and rates of deterioration.
It must be emphasised that there can be considerable uncertainty associated with the modelling of deterioration. The approaches described here can be helpful in informing decisions about the management of structures, particularly where they are supported by correlations between test results and predictions. Technical Report 61 l 2 provides more detailed guidance on deterioration models.
’’
12.4.1 The causes of deterioration
Of the causes of deterioration of reinforced concrete, the penetration of chlorides from de-icing salt and the advancement of carbonated zone (carbonation) leading t o the depassivation and subsequent corrosion of reinforcement are the most important factors. Detailed descriptions of the processes involved in deterioration are given in
Chapter 9 and how these affect the load carrying capacities of structures in Chapters 6 and 7. This Section considers the penetration of chloride only as this is the most likely form of deterioration encountered by engineers in the UK, concentrating on timescale and rates of deterioration.
12.4.2 Timescales and rates of deterioration
The determination of timescales involved and deterioration rates requires the corrosion of reinforcement t o be considered in two stages as follows: 1. Initiation period 2. Propagation period.
Initiation period This is the period that an aggressive front of chloride penetration takes t o advance t o and then t o initiate corrosion of reinforcement, frequently assumed t o occur when 0.3% chlorides by weight of cement a t the depth of the reinforcement are present. It is calculated mostly from models based on the mathematics of diffusion.
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Older structures, particularly those built before the 1950s and 60s, were built with far lower covers than considered necessary today and consequently have had far shorter initiation periods and correspondingly far greater deterioration problems when compared with more modern structures. Concretes in these older structures will often also be more permeable with low cement contents.
Rate of advance calculations A number of models have been suggested t o predict the advance of an aggressive front
but most have adopted the same general form based on Fick's 2nd Law of l 2 13. The relevant equation, assuming one-dimensional diffusion, is: SC -(x,t)
6t
S2C
= D-(x,t)
Sx2
where C(x,t) = chloride concentration at a distance x from the surface after time t. D = diffusion coefficient The commonly used solution assuming constant chloride concentration at the surface of the concrete is: C(x,t)= C,[I- erf ( x / ( ~ , / ( ~ t ) ) ]
where erf = the error function C, = chloride surface concentration, which is assumed constant, though in practice surface concentrations can vary and will influence the calculation) More involved analyses have been developed but in most situations a pragmatic and balanced approach using the basic solution t o Fick's 2nd Law has been found sufficient t o support the technical and financial decisions that need t o be taken. More modern structures may have been subjected t o surface treatments for preventative maintenance reasons and this will need t o be taken into account in the value used for the diffusion coefficient. Buenfeld and Zhang1214provide guidance on this.
Propagation period This is the period from the end of the initiation period t o the time of assessment of the structure. The initiation and propagation periods are illustrated diagrammatically in Figure 12.1. For the propagation period t o commence, the chloride concentration at the surface of ~ the reinforcement must exceed a threshold value Cth;Vu and S t e ~ a r t " . 'provide guidance on this. Thus, during the period from when the threshold value is exceeded t o the time of assessment a representative corrosion rate value (an estimate of metal loss per unit of surface and time) is calculated. It should be noted that along with the onset of delamination the corrosion rate changes.
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Figure 12.1 Initiation and propagation periods.
Initiation period
Propagation period
Lifetime or time before repair
Corrosion rate Corrosion rate is usually expressed as an attack penetration depth, either uniform or localised, expressed as a depth of steel lost over one year or as a measure of electrical current over a unit area. The former is difficult to measure where, for example, pitting corrosion caused by chlorides has occurred and the reinforcement remains embedded in concrete. There are several methods used to measure electrical current but the most widely used of these is the linear polarisation resistance method”’6. In this, a small electrical current or voltage is applied to the reinforcement and the corresponding response respectively in voltage or current is measured. From this the polarisation resistance and, in turn, instantaneous corrosion rate is determined. The instantaneous corrosion rate does vary with changes in the climate and therefore to obtain a representative value it is recommended that measurements are taken during: U Dry periods with low temperatures 0 Periods of low temperatures after raining continuously during a t least one or two
days 0 Dry periods and high temperatures 0 Periods a t high temperatures one week after raining continuously during two to three days. The results obtained are then averaged to give the representative value.
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Where only single measurements are possible, the manual for assessing corrosion-affected concrete structure^'^'^ provides guidance on how t o use these with laboratory testing of cores taken near to the points of measurement to derive the representative values. Different locations in a structure and different deterioration processes will also require the calculation of different representative values. From the representative values, penetration depths of corrosion are determined and in turn the residual or remaining diameters of the corroded bars are assessed taking into account whether the corrosion is uniform or pitting.
12.4.3 StrUCtUra[ assessment
The consequences of corrosion of reinforcement results in: 0 Reduction of rebar section and ductility
0 Reduction of bond between reinforcement and concrete 0 Loss of concrete integrity due t o cover cracking and/or spalling This is illustrated in Figure 12.2 Having knowledge of when corrosion commenced and also the rate at which the steel is corroding at a particular section in a structure, enables one t o determine the loss of steel, the loss of concrete and, in turn, the level of performance of that section. This is illustrated in Figure 12.3. When determining the time a structure reaches a non-acceptable level of performance, it is necessary t o appraise the effects on the concrete of the different levels of corrosion of the reinforcement. The assessed capacities for different penetration depths using the appropriate geometrical and mechanical characteristics relevant t o the different levels of corrosion are estimated and plotted as shown in Figure 12.4. From this ‘deterioration’ curve the predicted time when the structure reaches a non-acceptable level of performance and the residual life of the structure are determined.
Figure 12.2 I
I
Consequences of corrosion.
Loss of concrete
Delamination
J
Bond deterioration
A
/-
Mechanical properties (ductility)
I
Reduction of rebar area
V
I Reduction in Load capacity I
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Bridge management and assessment
Figure 12.3 Determination of level of performance.
Structural assessment
Condition data
Estimate start of corrosion
Estimate corrosion rate
Determine critical
*
Level of performance of section
Current deteriorated section
Figure 12.4 Deterioration curve.
Present
/
ranariiu
Assessed capacities at different levels of corrosion
Minimum acceptable performance
,
Time
Residual period
t Present time
--
I-
Initiation period
Propagation period
tl
The above is based on a deterministic approach but because of the large amount of variability in the factors considered a stochastic approach is t o be preferred. Although the paper by Tantele et aP2 concerns the effectiveness of preventative maintenance, it provides useful guidance on a stochastic approach relevant t o the assessment of deterioration rates.
12.4.4 StrUCtUra[ apprakal
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Atkins propose a general corrosion model for reinforcement based on that developed for the Midland Links. An indication of the threshold corrosion level for delamination a t Midlands Link is given.
Bridge management and assessment 12
Hughes eta112,20 in their paper on the site testing and monitoring carried out on the pier crossheads of the M4 Elevated Viaduct in West London noted that the average losses of reinforcement for the solid, delaminated and spalled surfaces were respectively 3.1 %, 12.2% and 16.9%. However, there appears to be little information published about the quantitative interpretation of the effects of corroded reinforcement on concrete structures. What information exists appears to be restricted to a few organisations that have undertaken this type of work in the past.
Duty of justices of peace to investigate "all manner of annoyancesof bridges broken in the highe wayes to the damage of the Kynes liege people" Statute of Bridges 1530
The assessment programme has encompassed bridges owned by various organisations that either come under the category of a Highway Authority or person/body that are liable by virtue of statue or of historical liability to be responsible for maintaining bridges on public highways.
12.5.1 Historical background
The responsibility for building and maintaining bridges in medieval times was part of the feudal obligations placed upon landowners. Many bridges were built with money raised through indulgences by the church; the belief being that the purchase of an indulgence would save one from a period of time in purgatory before entering heaven. However by the sixteenth century, with the dissolution of the monasteries, the role of the church in building and maintaining bridges declined and this, together with other factors, led to many bridges being in a poor state of repair. It was recognised that bridges were important to the economy as a whole, that the wider community should take greater responsibility for their maintenance. The Statute of Bridges, 1530, placed responsibility on the inhabitants of the counties to maintain bridges of public utility and gave power to justices of peace at the Court of Quarter Sessions to administer the extent of this liability. However, others remained Liable because of statute or historical liability as illustrated by the Bridge House Estates, a charity formed over 900 hundred years ago for the 12.22. managing and maintaining of bridges across the Thames in the City of Throughout succeeding centuries further legislation was introduced that defined in everincreasing detail the Liability to maintain bridges. Interestingly, the Bridges Act 1803
163
-=
12 Bridge management and assement
required all bridges built for public utility in a county to be so under the direction or to the satisfaction of the County Surveyor, who prior to the local government reforms of the 1880s was an officer of the Court of Quarter Sessions and, subsequently, an employee of the newly created county council; this is an early form of technical approval procedure so familiar to bridge engineers today. By the Highways Act 195912.23, the county council was made the Highway Authority for all 'county bridges' and with the Local Government Act 197212.24; the distinction between the authority liable for the maintenance of the bridge and for the highway over it to all intents and purposes disappeared. The Highways Act 198012.25 does, however, recognise that not all bridges in the highway are the responsibility of the county council.
12.5.2 Current situation
The determination of this liability to maintain bridges is a major concern of highway law therefore with respect to bridges and for assessment this is particularly important in establishing the load bearing obligations of the owners. Table 12.1 summarises the liability to maintain bridges. Thus, for bridges not the liability of Highway Authorities to maintain it is necessary to consider legislation existing at the time of their construction and that brought in subsequently to define the responsibility of their owners. The Transport Act 196812.26, defined the responsibilities of the inland waterway and railway companies for their highway bridges many of which were built at the time of the construction of the canals and railways. Significantly, this Act recognised that the highway load bearing obligations differed from those for Highway Authorities. Section 117 of the Act states that these shall be in accordance with an order made by the appropriate minister, this being the Statutory Instrument 1972/1705 which refers to Technical Memorandum BE4, which limits the load bearing obligation to 24 tons.
Table 12.1 Bridge maintenance liability.
Publicly maintainable bridges a
b c
All bridges built since 1959 and approved and adopted by the Highway Authority All bridges built before 1960 and 'of public utility or benefit' except those bridges which are privately maintainable All bridges built at any time and expressly adopted by the Highway Authority by agreement or under any statute or statutory process
Privately maintainable bridges a b c d e
164
All bridges built since 1959 and not approved or adopted by the Highway Authority Any bridge built by a person or organisation who interfered with the highway and caused the need for a bridge Any bridge built by a person or organisation who by prescription remains liable to maintain it Any bridge built by a person or organisation who by reason of tenure remains liable to maintain it Any bridge built under a statutory power which requires the bridge to be privately maintained
Section 7 of the Trunk Roads Act 1946lZz7 and later Section 5 5 of the Highways Act 1980’225 led to the adoption of all private bridges on roads that became trunk in accordance with these acts. Subsequently, roads losing their status as trunk roads became with the affected bridges the responsibility of local Highway Authorities in accordance with Section 2 of the Highways Act 1980; this has led to the anomalous situation where some bridges having been built and formerly maintained by the railway companies not being subject to the requirements of the Transport Act 1968. For highway bridges owned by other persons or organisations, it is even more important to investigate their histories and consider both the common law liabilities and legislation existing a t the time of their construction. It is generally recognised in law that the owners of these bridges need only maintain them to standards adequate a t the time of their construction and are under no obligation to bring them up to standards commensurate with the volume and weight of modern traffic. A Highway Authority, however, does have the power under Sections 93 to 95 of the Highways Act 1980 to take responsibility for a bridge in agreement with the owner if it wishes to bring the bridge up to the current load bearing standard. For bridges in unknown ownership the situation is unclear. However, Highway
Authorities have a duty in accordance with Section 41 of the Highways Act 1980 to maintain their highways to an adequate standard, and in the event of bridges falling into a state of disrepair and/or are inadequate to carry the existing traffic they would not be fulfilling this. Therefore, it would appear that these bridges would have to be considered as bridges owned by Highway Authorities. The Road Traffic Regulation Act 1984’228in its Section 122 states that Highway Authorities have a duty to:
0 Secure and maintain reasonable access to premises U Preserve or improve the amenities of areas through which roads run This together with the duty imposed by Section 41 of the Highways Act 1980 imply that Highway Authorities should ensure that all bridges carrying roads are capable of carrying all vehicles designed in accordance with construction and use regulations. Clearly, this presents problems to Highway Authorities because of: 0 The continually increasing weights of heavy goods vehicles brought about by commercial and political factors. U The differing load standards that other parties are legally required to provide. U The ever increasing ages of bridges caused by the failure by central and local government to fund an adequate bridge replacement programme. 0 The failure of central and local government to adequately fund a bridge strengthening programme. The Highway Authorities are left with the choice, therefore, of imposing weight restrictions on bridges correspondingto the results of the programme of assessments or accepting the risks of allowing unrestricted use of weak bridges with or without other
165
12 Bridge management and assessment
measures such as monitoring or lane restrictions to continue. Bridge engineers are under considerable pressure from politicians to do the latter, but must use their own judgement and experience to do the former where in their judgement the public is put a t too great a risk. The imposition of a weight restriction can imply that the Highway Authority is not fulfilling its duty to maintain the highway to an adequate standard, and consequently can in theory be served notice under Section 56 of the Highways Act 1980 to restore the highway to a satisfactory standard by strengthening or replacing the weak bridge.
Culverts Although not strictly part of the assessment programme a Highway Authority should assess, following the outcome of the case Bybrook Barn Garden Centre-v-Kent County Council, whether its culverts are large enough to take existing flows and, if not, whether it has a duty to enlarge them.
Accommodation and occupation bridges A Highway Authority has no duty to maintain bridges over or under the highway unless
the bridge or underpass is itself a public highway carrying vehicular traffic, or public rights of way on foot or horseback, or has been provided by the authority for pedestrian safety. However, a Highway Authority must ensure that these structures have been adequately designed for required loadings before granting licences for new bridges under Section 176 of the Highways Act 1980 and ensure that the design loadings are adhered to. For older structures where design loadings are unknown a Highway Authority may have to assess these to ensure that the safety of the highway is not impaired. The legal obligations for accommodation and occupation bridges, particularly with regard to Network Rail, are subject to contractual agreements and reference to statute necessitating site-specific advice. Generally, accommodation bridges were constructed at the request of landowners whose lands were otherwise severed by the construction of railways, canak and highways. As such, they can only be legally used by the successors in title to the original landowner(s),although in some cases public footpath and bridleway rights may also have been acquired across the bridge. Occupation bridges carry or pass over private roads which existed before the construction of the railways and highways, and which serve adjoining lands. In principle they can only be legally used by the subsequent owners/occupiers of the lands originally benefiting from access, which may now be open to interpretation in the light of changing land use. In both cases, those entitled to the use of the bridge can invite others to exercise similar rights provided that in so doing, the burden on the owner of the bridge to repair and maintain the structure is not increased. As a result of recent case law, Network Rail are obliged to maintain private bridges to the maximum load bearing capacity of the original bridge, calculated five years after the
166
date of its construction. A bridge’s load bearing capacity is the maximum weight that it can bear without causing it to fail or be damaged, whether immediately or through repetitive use. Section 6 of the Locomotive Act 1861’229may only be used to warn users that the capacity of the bridge is insufficient. The capacity may be expressed as axle weight or gross laden weight.
Access to property and powers of entry Almost invariably, a Highway Authority will require access through adjacent property to carry out the required inspection for the assessment of a structure. Sections 289 and 291 of the Highways Act 1980 provide the powers to a Highway Authority to do this, but Sections 290, 292 and 294 also place certain obligations on it when exercising these powers. When inspection of a structure that is over or adjacent to operational railways, canals or navigable waterways and to watercourses or flood defences is required then Network Rail, British Waterways, Environment Agency or other relevant body must be consulted well in advance of it being undertaken.
12.6 The assessment of bridges for abnormal Loads
12.6.1 Abnormal load categories
There are two main categories of Heavy Load Vehicle, both of which are allowed to operate under the provisions of Section 44 of the Road Traffic Act 1988. There are those which move under Motor Vehicles (Authorisation of Special Types) General Order 1979 (STCO), weighing up to 150 tons, and those which exceed this limit and move under an special individual order (SO) issued by VSE Division in DETR. It is estimated that there are some 200,000 STCO movements on trunk roads each year, and 200-300 SO movements. Abnormal Indivisible Loads (AILS)are moved in England after prior notification under the provisions of Section 44 of the Road Traffic Act 1988. Notifications are made to DETR, Highway Authorities, bridge owners and police forces. The notification requirements for heavy load vehicles are shown in Table 12.2 Under the current Regulations, Highway Authorities have little control over STCO movements. For example, there is no control over the time of day a t which a movement can take place, so there is considerable potential for disruption to traffic from large slow-moving loads.
12.6.2 Management O f abnormal indivisible load movements
Currently, the Highway Authorities utilise a variety of adhoc methods to manage notifications and check bridges for each notified AIL movement. It is common practice to compare the AIL with a known HB assessed capacity of the bridges on the proposed route. Some Highway Authorities maintain a grid of Abnormal Load routes on which the bridges have been assessed or they have been known to have safely carried AILS.
167
Department of Transport Special Order plus eight weeks clear notice to the HA five clear days notice to Police and five clear days notice with indemnity t o Highway and Bridge Authorities
I
Width exceeding 2 9 m up t o 8 0 m
Two clear days notice to Police Department of Transport form VRI plus ten clear days notice to the HA and t w o clear days notice to
Police and five clear days notice with indemnity to Highway and Bridge authorities 7 4 m rigid
]Vehicle combination exceeding 25 9 m
I
Two clear days notice t o Police Two clear days notice t o Police
A recent audit of AIL procedure carried out for the Highways Agency showed that, although a generally professional approach was being adopted by the Highway Agency’s Maintenance Agents, the systems and procedures needed to be improved and harmonised.
Separation of responsibilities for the maintenance of motorways and Trunk Roads from local authority roads and the introduction of DBFO roads have significantly increased the number of notifications that have to be made. It has been accepted generally that in managing the Trunk Road Network, the Highways Agency needs to make effective provision for the necessary movement of Abnormal Indivisible Loads (AILS) on the network. This has to be achieved with minimal adverse impact on the network infrastructureand other users. Much the same applies to other Highway Authorities and bridge owners. Increasingcongestion on the nation’s road system will make the movement of AILS more problematic.
This will worsen Highway Authorities’ assessment and clearance difficulties associated with these movements. These difficulties are exacerbated because AILSare usually wide, long and slow. Highway Authorities need an efficient and effective system for administering AIL notifications, assessments and clearances, so that these initiatives are not jeopardised. The current adhoc systems are potentially unsafe; do not facilitate a comprehensive database of bridge capacity Loads, which can readily be interrogated, and there are risks that key data could be lost when the responsibilities for highway bridges are periodically changed.
168
..
~
~-
The Highways Agency has commissioned the development of a new system called ESDAL (Electronic Service Delivery of Abnormal Loads) using a software package designed for MS Windows NT operation that can be accessed on its website w.esdal.com, which will provide the facilities required to receive, check and administer AIL notifications. The following principal functions will be provided by the system: W Fax and imaging software to receive and respond to AIL notifications, with facilities
for displaying, printing, logging and archiving. W On-screen mapping, allowing the notified route to be selected, with facilities for
enlargement, on screen selection of routes and search and find tools. W Database, maintaining records of AIL loads and movements, standard routes,
operators (includingtheir indemnity/insurance details) AIL vehicles, bridge details and their capacity. Mapping, gazetteer and place name data, standard routes. W Bridge analysis software, enabling each bridge on the route selected to be checked by a process of comparative assessment (i.e. based on imposed weight limits and preassessed capacities) for the capacity for the vehicle axle configuration notified. W Height checks for overbridges. W Reporting facilities that enable more detailed statistical examination of the database. Restrictions: e.g. global, local, position of AIL, restrictions applicable with other traffic, speed restrictions. The development of ESDAL has been divided so far into four phases as shown in Table 12.3. The first phase came on line in March 2006 and the final ones are due at the end of 2007/early 2008. A guide for the owners of structures.can be accessed via http://www.esdal.com.
Table123
-p
'hase
I Description -
169
12.7 References
12.1 THE STATIONERY OFFICE, A Code of Practicefor the Management ofHighway Structures, September 2005. 12.2
COLE. C, Managing sub-standard bridges, Bridge Management 4, (Ryall, MJ,Parke, CAR and Harding, /E, eds), Thomas Telford, 2000
12.3 PARSONS BRINCKERHOFF, HBR80616 - Highways Agency Contract 21419, TechnicalAudit oftheapplicationofBA79 and A review of bridge assessment failures on the Motorway and Trunk Road network, Final Reports, Parsons Brinckerhoff, December 2003. 12.4 FLINT, AR and DAS, PC. Whole Life Performance Based Assessment Rules - Background and Principles, SafetyofBridges Conference. Thomas Telford. London, July 1996. 12.5 SHAVE, J, DENTON. Sand JANDU. A, Management of Sub-standard Structures - A new Standard, Proceedingsofthe First lnternationalConference on Advances in Bridge Engineering, Brunel University, 26-28 June 2006. 12.6 ManualofBridgeEngineering: Thomas Telford Books ISBN 0 7277 2774 5 12.7 HIGHWAYS AGENCY, BA 79/98 -Amendment No 1, The Management of Substandard Highway Structures, Design Manualfor Roads and Bridges. 12.8
CONCRETE BRIDGE DEVELOPMENT CROUP, Guide to testing andmonitoring the durabilityofconcretestructures, Technical Guide 2, CBDC, Camberley, 2002.
12.9 CONCRETE SOCIETY, Non-structuralcracks in concrete, Technical Report 22, 3rd Ed., The Concrete Society, Camberley, 1992. 12.10 INSTITUTION OF STRUCTURAL ENGINEERS, Structuraleffects ofalkali-silica reaction - Technicalguidance on the appraisal ofexisting structures, The Institution of Structural Engineers, London. July 1992. 12.11 CONCRETE SOCIETY, Enhancingreinforcedconcrete durability, Technical Report 61, The Concrete Society. Camberley, 2004. 12.12 COLlEPARDl, M, MARCIALIS, A, and TURRIZIANI, R, Kinetics of penetration of chloride ions in concrete, IlCemento, 1970, pp. 157-164 (in Italian) 12.13 COLLEPARDI, M. Penetrationofch/oride ionsfnto cementpasteandconcrete,American Ceramic Society, 1972 12.14 BUENFELD. NR and ZHANC. JZ, Chloride diffusion through surface-treated mortar specimens, Cement andconcrete Research, Elsevier Science Ltd, Vol. 28, Issue 5, 1998. 12.15 VU, KAT and STEWART, MG. Structural reliability of concrete bridges including improved chloride-inducedcorrosion models, Structuralsafety, Elsevier, Vol 22, Issue 4, 2000. 12.16 CONCRETE SOCIETY, Nectrochemical tests for reinforcement corrosion, Technical Report 60, The Concrete Society, Camberley, 2004. 12.17 BRITISH CEMENT ASSOCIATION et al. A validatedusersmanualforassessing the residualservicelife ofconcretestructures affectedby corrosion. EC Project Ref. IN 309012, CONTECVET, BCA. Camberley, 2001. 12.18 TANTELE, E, ONOUFRIOU, T and MULHERON. M, Effectiveness of preventative maintenance for reinforced concrete bridges - a stochastic approach, BridgeMangement5, (Parke. CAR and Disney, P. eds), Thomas Telford, 2005. 12.19 ATKINS, C, HOCC, V, MIDDLETON, C and ROBERTS, MB, A proposed empirical corrosion model for reinforced concrete, Proceedings ofthe lnstitution of Civil EngfneersStructures andBuildings, 2000, Vol. 140, pp 1-1 1 12.20 HUCHES, C, Smith, JSC and Warren, CP. Site testing and monitoring of pier crossheads suffering from chloride induced reinforcement corrosion, Bridge Management 4, (Ryall, MJ,Parke, CAR and Harding, JE, eds), Thomas Telford, 2000. 12.21 PIERCE, P, Oldlondon Bridge, Review, 2002. 12.22 SAUVAIN, SJ, Highwaylaw, 2nd Ed.. Sweet & Maxwell, 1997. 12.23 Highways Act 1959, HMSO, http.//www.tsoshop CO u k l bookstore 12.24 Local Government Act 1972, HMSO, http.//www.tsoshop co.uk/ bookstore 12.25 Highways Act 1980, HMSO, http://www.tsoshop.co.uk/ bookstore 12.26 Transport Act 1968, HMSO, http://www tsoshop.co.uk/ bookstore 12.27 Trunk Road Act 1946. HMSO, http.//www.tsoshop.co.uk/ bookstore 12.28 Road Traffic Regulation Act 1984, HMSO, http.//www.tsoshop.co.uk/ bookstore 12.29 Locomotive Act 1861, HMSO, http://www tsoshop.co.uk/ bookstore
170
Appendix. Relevant historica I references t o design and materials specifications and standards used in concrete bridge construction The following is indicative of the information that may be useful in bridge assessment
Bridge design
0 LECAT, AW, DUNN, C and FAIRHURST, WA, DESIGNAND CONSTRUCTION OFREINFORCEDCONCRETEBRIDCES. Concrete Publications Ltd, London, 1948 (Revised 1957).
0 MINISTRY OF TRANSPORT, Memorandum 577, Bridgedesignandconstruction,HMSO, London, 1945,1952. 0 MINISTRY OF TRANSPORT, SpecificationforRoadandBridge Works. HMSO, London, 1951,1987,1963,1976,1986 (Amendment No 1, 1988). 0 MINISTRY OF TRANSPORT, Memorandum S77/1, Rulesfor the design anduse ofFreyssinet hinges on highwaystructures, HMSO, London. 1966.
0 CONCRETE AND CONSTRUCTION ENGINEERING, 1906-67, Various appropriate articles on
http.//w.lStructE org/Library/advanced search.asp, Search term - conc cons engg + search term.
0 BRITISH STANDARDS INSTITUTION, BE5/75, Rulesfor the design anduse OfFreyssinethinges inhighwaystructures.1975. 0 DEPARTMENT OF TRANSPORT, DesignManualfor RoadsandBridges,HMSO, 1992, 1997.
0 DEPARTMENT OF TRANSPORT, BD24/92, Design ofhighway bridges andstructures.HMSO, London, 1992. 0 DEPARTMENT OF TRANSPORT, BD15/92, Ceneralprinciplesfor the design andconstruction ofbridges. HMSO, London. 1992. DEPARTMENT OF TRANSPORT, BDS7/94, Designfordurability, HMSO, London, 1994.
0 DEPARTMENT OF TRANSPORT, BD58/94. Designforconcrete highway bridgesandstructureswith external and unbondedprestressjng.HMSO, London, 1994.
0 DEPARTMENT OF TRANSPORT, BD42/96, Design ofintegral bridges, HMSO, London, 1996.
ConCrete
The first national design‘code for concrete structures was introduced in 1934. Subsequently new codes have been introduced, as indicated below (with the date of first publication). Invariably there was some overlap a t each transition between an old and a new code. In addition, codes are subject to revision before they are eventually replaced. 0 REYNOLDS, CE and STEEDMAN, J.C,ReinforcedConcrete Designer’s Handbook, E&FN Spon, London, 1932,1939, 1946, 1948 (revised 1951, 1954), 1957, 1961 (revised 1964), 1971 (revised 1972). 1974, 1981, 1988, 436pp.
0 BRITISH STANDARDS INSTITUTION,CP114, Thestructuraluse ofnorrnal reinforcedconcretein buildings. 1948, First revision 1957, Reset and reprinted 1968 (including Amendment No 1). Amendment No. 2 published 25 May 1967. 0 BRITISH STANDARDS INSTITUTION, CP115, Thestructuraluse ofprestressedconcreteinbuildings. 1959.
0 BRITISH STANDARDS INSTITUTION, BS1926:1962, specificationfor ready-rnixedconcrete. 0 BRITISH STANDARDS INSTITUTION,CP116, Thestructuraluse ofprecast concrete, 1965. 0 MATTHEWS, DD et al. The draft unifedcode ofpractice forstructuralconcrete (CP7 70) Concrete, Vol 4, No. 2, February 1970, pp 66-70. Three papers covering: The new code its background and purposes, by Matthews, DD A discussion on the draft code Slab design and the draft code, by Taylor, R, Hayes, B and Mohamedbhal, GTG. 0 BRITISH STANDARDS INSTITUTION, CP110, Code ofpracticeforthestructural use ofconcrete, 1972.
0 DEPARTMENT OF TRANSPORT, Technical Memorandum (Bridges) BE 1/72 Reinforcedconcretehighwaystructures,BE 2/73 Prestressedconcretehighwaystructures,1973
0 BRITISH STANDARDS INSTITUTION, BS5328:1976. Methodsforspecifying ready-rnixedconcrete. 0 BRITISH STANDARDS INSTITUTION, 555328.1981, Merhodsforspecifyingconcrete, includingready-rnixedconcrete. 0 BRITISH STANDARDS INSTUTITION, BSS400. Steel, concrete andcompositebridges, Part 4. Code ofpracticefordesign ofconcrete bridges. 1984. 0 BRITISH STANDARDS INSTITUTION, 5881 10-1.1985, Structuraluseofconcrete, Part 1: Codeofpracticefordesignand construction.
171
0 BRITISH STANDARDS INSTITUTION, 855328.1 1991 (revised 1997). Concrete- Part 1: Guide tospecifyingconcrete. 0 BRITISH STANDARDS INSTITUION, 855328-2 1991 (revised 1997), Concrete- Part 2,Methodsforspecifyingconcrete mixes. 0 BRITISH STANDARDS INSTITUTION,855328-3.1990, Concrete- Part 3 Specificationfortheproceduresto be usedin producingand transporting concrete. 0 BRITISH STANDARDS INSTITUTION, 855328-4:1990, Concrete - Part 4. Specificationfor theprocedures to be usedin sampling, testing and assessing compliance of concrete. 0 BRITISH STANDARDS INSTITUTION, B5 EN206-1.2000 (Incorporating Corrigenda Nos 1 and 2 and Amendments 1. i 3). Concrete- Part 1. Specification, performance, production and conformity. 0 BRITISH STANDARDS INSTITUTION, 858500 2002 (revised 2006), Concrete- ComplementaryBritish Standardto ES EN206- 1 Part 1 - Method of specifying andguidancefor the specifiers Part 2 - Specificationfor constituentmaterials and concrete. 0 DEPARTMENT OF TRANSPORT, IAN 52/04. Changes to Aggregate andConcretespecification affecting MCHWSeries 500,600, 700, 1700and Notes for Guidance NC 100, NC 500, NC 600, NC700, NC 800 and NC 1700, Highways Agency, London, January 2004. 0 DEPARTMENT OF TRANSPORT, IAN 58/04. Changes to concretespecificationaffecting MCHWlnterim Advice Note 52/04 guidanceforstructuralconcrete, Highways Agency, London, May 2004
0 DEPARTMENT OF TRANSPORT, IAN 74/06, Revisedguidanceregarding the use ofES8500for the design and constructionofstructures using concrete, Highways Agency, London, 2006
Reinforcement
0 MINISTRY OF TRANSPORT, Memorandum 785. Permissibleworking stresses in concreteandreinforcing barsforhighwaj bridgesandstructures,HMSO, London, 1961
0 STEEL REINFORCEMENT COMMISSION, UK reinforcement standards 1938-1990, Concrete, Vol. 24, No. 3, March 1990, pp 40-41 0 BRITISH STANDARDS INSTITUTION, 65785.1938 and BS785:Part 1 1967, Hotrolledbarsandharddrawn wireforthe reinforcement of concrete. 0 BRITISH STANDARDS INSTITUTION, BS4449:1969, 1978, 1988, 1997, Speci~cationforcarbonsteelbarsforthe reinforcemen t of concrete. 0 BRITISH STANDARDS INSTITUTION, 8S11441943, 1967, Coldworkedsteel barsforthereinforcement ofconcrete. 0 BRITISH STANDARDS INSTITUTION, 854461.1969. 1978, Specificationfor cold workedsteelbarsfor the reinforcemen! of concrete
POSt-tenSiOning Systems
The commercial use of prestressing in the UK began just before the Second World War. During and just after the war it was used to overcome material shortages. The early designs were mainly by refugee European engineers and the systems manufactured wen not British. After about 1950 various British systems were developed. Detailed information on systems available between 1940 and 1985 may be found in ClRlA Report 106, Post-tensioning systems for concrete in the UK: 1040-1985, published in 1985. A number of the systems described ceased to be used after about thc 1960s. Thus when assessing an existing structure, identification of the type of anchoragi used may give an indication of the age of the structure ANON, Test of prestressedconcrete railway bridge, Concrete andConstructionEngineering, Vol. 46, No. 5, June 1951, pp 186-188. MORICE. PB, The theoryofdesign offullyprestressedbeams. Report CACA 130, Cement & Concrete Association (now British Cement Association),Camberley,July 1953 MORICE, PB,A test on a 55ftspanprestressedconcrete beam, Report TRA/144. Cement & Concrete Association (now British Cement Association), Camberley. March 1954 MORICE, PB, Concentratedloadonprestressedconcrete bridge decks, Report TRA/175, Cement & Concrete Associatioi (now British Cement Association), Camberley,January 1955. MORICE, PB. The ultimate strengthof two-span continuousprestressedconcrete beams as affected by tendon transformation andun-tensionedsteel,Report TRA/186, Cement & Concrete Association (now British Cement Association), Camberley, May 1955. ANON, A new type of prestressed bridge deck, ConcreteandConstructionEngineering, Vol. 53, No. 3, March 1958, pp 145-146. ANON, An investigation of prestressed bridges, Concrete andConstructionEngineering, Vol. 53, No. 9. September 1958, pp 351-354 CIRIA, Post-tensioningsystemsforconcretein the UK.1940-7985, Report 106, 1985
172
Precast beams
0 PRECAST CONCRETE DEVELOPMENT GROUP PC1, Standardbeam sectionsforprestressedconcrete bridges ( I ) , InvertedTbeamsforspans25 to 55ft. Cement and Concrete Association, October 1963 Revised as Report CSC1, Concrete Society, Camberley, October 1967
0 PRECAST CONCRETE DEVELOPMENT GROUP, PC4, Standardbeam sectionsforprestressedconcrete bridges (2). Box section beamsforspans 40 to 85ft, Cement and Concrete Association. November 1963, Revised as Report CSC2, Concrete Society, Camberley, October 1967
0 PRECAST CONCRETE DEVELOPMENT GROUP, PC6, Standardbeam sectionsforprestressedconcrete bridges (31 Box section beamsforspans 85 to l20ft, Cement and Concrete Association, April 1964, Revised as Report CSC3. Concrete Society, Camberley, October 1967
0 PRECAST CONCRETE DEVELOPMENT GROUP, PC8, Standardbeam sectionsforprestressedconcrete bridges (4), Isection beamsforspansfrom 40 to S i f t , Cement and Concrete Association, July 1964, Revised as Report CSC4, Concrete Society, Camberley, October 1967
U PRECAST CONCRETE DEVELOPMENT GROUP, PC9, Standardbeamsectionsforprestressedconcrete bridges (S),Isection beamsforspansfrom 85 to l20ft, Cement and Concrete Association, August 1964, Revised as Report CSCS, Concrete Society, Camberley, October 1967
Highway loading
0 BRITISH STANDARDS INSTITUTION, 85153 Part 3A 1954, Speci~cationforsteelqirderbridges-Loads, 1954
0 MINISTRY OF TRANSPORT, Memorandum 771, Standardhighwayloadings, HMSO, London, 1961 0 BRITISH STANDARDS INSTITUTION, 855400 Part 2, Specificationforloads, 1978
U DEPARTMENT OF TRANSPORT, BD14/82 (Amendment No l ) , Loadsforhighwaybridges,1982 0 DEPARTMENT OF TRANSPORT, BD37, Loadsforhighwaybndges 1988, Clause 6 8 1 replaced by BD48 93
0 DEPARTMENT OF TRANSPORT, BD60/94, Design ofhighway bridgesfor vehicle co//isionloads, 1994 U DAWE, P, Traffic loading on highway bridges,Thomas Telford, London, 2004, 168pp
173
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