lVI1NVW NDISaa
••
3DQI)IH
1996 ISSUED UNDER THE AUTHORITY OF THE GENERAL MANAGER ROAD DEVELOPMENT SRI LANKA
9
AUTHORITY
FOREWORD
This Manu~1 is intended essentially to introduce basic bridge design concepts and to present guide lines in the technique or bridge design (or highway bridges.
,
This manual has been studied and approved by the following committee.
01.
Mr. P.B.L Cooray
General Manager Committee]
02.
Dr. G.LA.J. De Silva
Director (ES) [Committee
03.
Mr.·Lionel Rajapakse
Director (MM&C) [Committee Member]
04.
Mr.IlVV.Fernando
Director (P&PM) [Committee
05.
Mr.S. VVecrathunge
Director (T) [Committee
06.
Mrs. H.Y. Fernando
Dy. Director (BD) Committee]
07.
Mr. Asoka \iVeeraratne
Dy. Director (CM)
08.
Mr. T.L Chandrasiri
Dy. Director (P&PM) Mcmbcr]
[Committee
09.
Mr. D.IlR. Swarna
Senior Engineer (BD) Member]
[Committee
10.
Mr. R.A.D.S.1l Ranathunge-
Executive Engineer (MM&C) Member]
[Committee
11.
Mr. M. Chandrasena
Bridge Consultant Chandrasena & [Committee Member]
12.
Mr:J. Zavesky
Bridge Desi.gn Expert (MS. Renardet Consulting Engineers) [Committee Member]
This manual has been drafted by the following members. 01.
Mrs. H. Y. Fernando
02.
Mr. D.IlR. Swarna
03.
Mr. VV.E.S.1l 1i'ernando
04.
Mrs. VV.B.S.H.Fernando
05.
Mr. P.S. Sadadcharan
06.
Mr. C.C.VV.Jayasuriya
[Chairman
of
the
Member]
Member]
Member] [Secretary of the
[Committee Member]
(MS. Partners)
INDEX
1.0
SCOPE AND GENERAL ..
01
2.0 2.1 2.2
DESIGN CO;')E
02 02 02
2.2.1 2.2.2 2.2.3
2.2.4 2.2.5
2.2.6 2.2.7 2.2.8 3.0
General Loads .. General Dead Loads .. .. , Live Loads Breaking and Traction Horizontal Forces due to Water CU1Tent, Debris, & Log Impact Wind Loads .. Temperature Siress in Concrete Bridge Decks Creep and Shrinkage
INVESTIGA T.~ON Geological Investigation
3.1 3.1.1 3.1.2 3.1.3 3.2 3.3
Topographical :.~wvey Hydrological Survey _ Technical Surv-y & Details of the Existing Bridge Geotechnical Lrvcstigation .. Waterway and Length of Bridge
4.0
ALIGNMENT
5.0
SELECTION
AND GEOlVIETRICAL CONSIDERATION
6.0
DESIGN OF SUBl\1ERSIBLE BRIDGES
6.1 6.1.1
Scope
6.2
03
05 05 08
09 09 10 10
11 14 16
OF BRIDGE TYPES & DESIGN CONSIDERATION 18
Foundation Substructure .. 5.2 -' 5.2.1 Abutments 5.2.2 Wing Walls 5.2.3 Piers 5.3· Super Structure 5.3.1 Design of Super Structure 5.4 Bridge Bearing Other Features of Super Structures 5.5 ~.1
02 03 03 03
Introduction
Bridge Location, Proportioning & Orientation 6.2.1 Location 6.2.2 Proportioning Bridge & Approaches 6.2.3 Deck Level & Trafficability 6.2.4 Vertical Alignment 6.2.5 Horizontal Alignment 6.2.6 Deck Crossfall 6.3 Analysis 6.3.1 Uplift and Instalility
19 19
20 21 21
22 23 23
24 25 25
25 25
25 26 26
26 26 26
;
•
--
--
---
__
6.3.2 6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5
Critical Flood Levels & Velocities Suitability of Alrernative Structures Kerbs and Ban iers .. Super Structure Bearings and Hold Down Restraints Substructure .. Batter Protection
01
GENERAL Scope Introduction
.1 1.2
2.0 2.1 2.2 2.3 2.4 2.5 2.6
1
~
27 27 27
27 28 28 28
30 30
BRIDGE LOCATION, PROPORTIONING Location Proportioning Bridge and Approaches Deck Level and Trfficability Vertical Alignment Horizontal Alignment Deck Crossfall
3.0
ANALYSIS
3.1 3.2
Uplift. .and Instability Critical Flood Level and Velocities
4.0
SUITABILITY
4.1 4.2 4.3
OF ALTERNATIVE
Kerbs and BaIT--;--r Super Structures Dearing and Hold Down Restraints '~~.4 Substructure - -If-.5 Batter Protection
•
ill
_~ __ __
& ORIENTATION
30 31 31 31 32 32 /
32 33
STRUCTURES 34 34
35 35 36
1
1.0
SCOPE & GENERAL: : Availability of construction materials & equipment, less maintenance and long life span are the main factors in choosing concrete bridges abundantly in Sri Lanka As the other types such as steel bridges, arch bridges & timber bridges are limited in number, this note mostly covers the design aspects for concrete bridges. Bridge Design Manual is to supplement the Bridge Design Code adopted by the Road Development Authority, the British Standard 5400, for the loadings and effects where the local conditions require different provisions than those included in the British Standard. These include but are net limited to the provisions related to design live loading and to the local climatic conditions. This is to provide a guidance to the designer in the interpretation of some of the provision of the standard and in calculation of the effects prescribed by the standard and to summarize and to advise the designer on the design practices adopted by the Road Development Authority in terms of selection of substructure and superstructure types. It is recommended that there guide lines are used by other authorities for design of highway 1__ = .l .. __
••
2
2.0
DESIGN CODE:
2.1
General Design of Bridges and ether related structures is carried out in accordance with the B.S. 5400 with certain modifications to suit local conditions as stipulated herein. Permissible stresses to be adopted are to be in conformity with Part 4 of BS 5400. However in mass concrete substructure the following criteria could be adopted. Where overturning effects are considered in substructures, at any level, always Factor of Safety should be greater than 1.00 Where F.O.S. ~=-
Stability Moment Overturning Moment
When 1.0 < F.O.S. < 1.5 permissible tensile stress = 0.24 Nzmnr' When F.O.S. > :.5 permissible tensile stress = ~6 N/mm2
NOTE:
But it i~.a good practice to have the F.O.S. of 1.3 always to cater for constructional deficiencies.
Capping beams are designed for bending moments and shear forces due to loads acting on them. Ballast wall in ab..tment capping beam is designed to take up horizontal pressure created by wheel load hohind the capping beam. Ref
- Reynolds Hand Hook
A 40 nun thick bearing seat is provided for the bearing pad. Sufficient reinforcement is provided under the seat to resist the splitting forces .
• 2.2
Loads-
2.2.1 General Bridges in Sri Lanka de not need to be designed for effects due to earthquakes as Sri Lanka is not in a zone affected by earthquakes. Generally the loading is to conform and applied according to BS 5400 part 2. Bridges should be able to resist tle effects of the loads & actions as listed below. (1) Dead Loads (2) Earth Pressure t (3) Live loads (4) Braking & Tractl,m of vehicle (5) Water current (6) Floating debris 8: Impact (7) Wind (8) Temperature (9) Shrinkage
3
2.2.2 Dead Loads the case of precast slabs and beams, adverse stresses during handling, transporting and stacking should be considered.
I'll
In the case of submersible bridges, the effect of horizontal forces due to water and impact of debris and buoyancy should be considered. Dead Load includes self weight, kerbs, sidewalks, handrails, uprights, wearing surface and weight of water mains and lamp posts when applicable. 2.2.3 Live Loads The following loads given in part 2 ofBS 5400 are used for design of bridges in the local highway network.
(a)
All bridges should be d~'~.;ignedto resist the effect ofHA loading specified in the relevant code.
(b)
Bridges should be able ;:0 resist the effect of 30 units of Hls loading for A & B class of roads. However the following condition is to be applied to suit local conditions. (i)
Always the Hli vehicle is to straddle two national lane widths.
2.2.4 Braking and tractionThe following factors a.e to be applied to the full tractive force decided according to the code in designing subs.ructures for simply supported bridges to suit local conditions. For Abutments ••
For Riers
0.6 X Tractive applied 0.8 X Tractive applied
force, at bearing level. force, at bearing level.
The bridge is to be designed for HA Loading with HA tractive force and checked for adequacy to carry the nllocated HB Loading. In checking for HB, it is permissible to decrease the HB Tractive force by 25% to allow for an permissible overstress. However the live load surcharge should be limited to 10 kN/m2. 2.2.5
Horizontal (a)
Forces duv to 'Vater Current
& debris and Log impact-
Horizontal Forces due to Water Current Any part of a bridge structure which may be submerged in running water should be designed to sustain safety the horizontal pressure due to the force of the current
4 On piers parallel to the direction of the water current, the intensity of pressure is given by;
P
=
KW (v:'2/2g)
P
-
W
-
V
-
g K
-
intensity of Pressure in kg/m/\2 due to the water current unit weight of water in kg/rn/\3 velocity of current in rn/sec. at the point where the pressure .ntensity is being calculated acceleration due to gravity in m/sec.oz ;1 constant depending on the shape of pier as follows
with the normei values for W & g equation reduces to P = 52 Kv/\2
Type c;~'Pier square ended pier Circular piers or semi circular cutwaters Triangular cutwaters Trestle type piers
I
II
k 1.5
0.66 0.5 to 0.9 1.25
The velocity V :s assumed to vary linearly from Zero at the point of deepest scour to a maximum at the free surface. The maximum velocity at surface for the purpose is to be taken ';2 times the maximum mean velocity of the current. To provide for the possible variation of the direction of the current from the direction assumed in the design allowance should be made in the design of the piers for an ex.ra variation in the current direction of 20 degrees. In this case velocity is resolved into two directions, parallel and normal to the pier with k assumed as l.5 for all except circular piers. Ref. : Essential. of Bridge Engineering - D.S. Victor (b)
Horizontal Forces due to floating Debris and Impact-
(i)
DebrisWhere debris i:, likely, allowance shall be made for the force exerted by a minimum depth of 1.2 m debris. The length of the debris applied to anyone pier shall be one half of the sum of the adjacent spans with a maximum of 22.0 m where the deck is not submerged. For debris the formula for water current shall be used the value of the constant K being 1.0.
(ii)
Log Impact When there is a i'JOssibilityfor driftwood and other drifting items to collide with a bridge, collisio.: force shall be calculated from equation. ~
•••
5
Ref:
F
=
0.1 W.v
Where P
=
Collision force (t)
\V
=
Weight of drifting item (t) (2 t log is assumed)
v
=
Surface velocity of water (m/s)
Specification for Highway Bridges Part I - Common Specifications by Japan Read Association
2.2.6 Wind LoadsThe mean hourly wind s;:-eedis determined for the location of the bridge, from the Wind Loading zone map for ~::riLanka given in Fig. 2.1. This mean hourly wind speed, to be used when calculating wind pressures using BS 5400 Part 2, is found from th« following table.
II
I~'
;==:
ZON',3
11EAN HOURLY WIND SPEED
1 I'
2 3
I
75 m.p.h. (33.0 mls) 65 m.p.h, (28.9 m/s) 50 m.. h. ;22.2 mis,
II
2.2.7 Temperature Stress in Concrete Bridge Decks There are three causes resulting temperature stresses in concrete bridge decks. (a)
Effect of change (rise or fall) in the 'Mean Temperature of the body of the deck. 11
For the purpose cr'this effect, it is assumed that the temperature of the entire body of the deck has one 'mean' value at any instant of time and that this 'body mean temperature' rises or falls over a long period of time, thereby wanting the structure to 'heave'. If the structure is free to permit this 'heave' ie, is free to expand or contract (e.g. simply supported beam or a continuous beam), this causes no thermal stress. However, if the structure is unable to permit such a heave (e.g. arch, frame. fixed beam) ie, offers constraint to its desire to heave, moments etc., are then caused; which create stresses (thermal stress type 1). These moments can be evaluated by the usual methods of theory of elasticity. (b)
Temperature GradientMinimum and Maximum shade air temperaturesFor all bridges, extremes of shade air temperature for the location of the bridge shall be obtainec from the maps of isotherms given in figure Nos. 2.2 & 2.3. These values have been obtained from extracts from Department of Me tea logy.
6
Adjustment for height above mean sea level The values of shade air temperature shall be adjusted for heights above 300 m above sea level by subtracting 0.5 C per 100 m height. Effective bridge temperatures The effect.ve bridge temperatures for different types of construction shall be derived fr.-rn the shade air temperatures by reference to table No. 2.1. The different types of construction are as shown in figure No. 2.4.
Ef~.:~ctiveBridge Temperature
Table No. 2.1
-
Shade Air Temperature
f.---.
Gt::UD 1
13 14 15
16
I I
I
I
I
!.
17 18 19 20 21 22 23 24 25 26 27 28
Group 2 16 17 18 18 19 20 20 21 22
19 19 20 21 22 23 23 24 25 26 27 27 28 29 30 30
08 09 10 11 12
"
Type of Superstructures
•
22 23
24 25 25 26 27 27 28 29 29
31 32
33 34 34
30 31
29
35
30
36
31
31 32
37
33 34
38 39 40
32 33 33 34 35
35
38
..
,
7 (c)
The effect of Non linear Distribution of temperature across the Deck-Depth. If the top surface of the concrete deck is hotter than it's soffit surface, the ordinate of the thermal ~~radientat any intermediate depth follow a nonlinear variation. Considering be build up of the total thermal gradient, it's uniform part at the instant of consideration, is akin to the 'body mean temperature', the effect of change in which over a long period of time, is already taken case of in case (a). However, the •.:ariable part, better called the 'differential thermal gradient' would heat each fibre r-..0 a different degree, the variation being non linear. If the fibers were free of each other (i.e. unrestrained) then they could accept the corresponding non linear thermal strains xi (x being the coefficient of expansion/contraction). But since their deformations must follow a linear law (plane sections must remain plane), they will not accept these non linearly related strains, and the difference between the final 'linear' strain gradient and the 'unrestrained' s.rain gradient will represent the uneven 'internal disturbance'. It's strain effect m.ry be called the 'Eigenstrain' and its stress effect may be called the Eigenstress, beth of which would be zero if only the thermal gradient were linear (which is not). This Eigenstrcss and the Eigenstrain, as can be seen, is purely an internal entity, not associated with any support reactions. Eigenstrcs, or: its own, may be small or significant, depending on (i) (ii) (iii) (iv) (v) (vi)
(vii) (viii)
depth of section thickness & colour of pavement wind speed orientation of bridge and incidence of sun rays. ambient temperature material properties thermal conditional specific heat thermal diffusiniry coefficient of thermal expansion and contraction coefficient of absorptivity coefficient of surface - heat transfer surface temperature shape of thermal gradient
The distribu'on of Eigenstres, not being linear, when added to the thermal 'continuity' stress [see under (C)] may show significant stress not only at extreme fibers but als. at intermediate fibers (e.g. mid height portion of webs) which are heavily loaded under shear. This can produce longitudinal cracks in webs. (d)
Effect of Intc.mediate - support resuaint on the Free Hogging (or Sagging) Desire ofthe structure caused by unequal Extreme Fibre Temperatures - 'The continuity effect'. In 8 beam-type deck, the difference oftempemture between the extreme surfaces causes hoggiag (or sagging) of the beam. If the beam.is simply supported, it merely hogs (or sags) as its supports do not
8 prevent rotation, This free deformation is not a 'moment induced' deformation, but merely a 'Strain induced' deformation, and no moment is caused. However if the beam is continuous, its aforementioned desire to freely hog (or freely sag) wiil be 'constrained' at the intermediate supports (presence of dead load reactions wilt prevent it from lifting up and presence of supports will prevent it from going down at their supports. This 'continuity' effect sets up moments that cause additional stresses called 'continuity stresses'. Ref:
Concrete
Bridge Practice
by Dr. V.K. Raina
Stress due to «emperature should be calculated as per BS 5400 cl. 5.4. The shade air temperature referred to in the clause should be taken from the tables given for different districts in Sri Lanka. For minimum effective bridge temperature ofBS 5400.
2.2.8 Creep and Shrinkage
same pattern is assumed as per table I I
-
Creep and Shrinkage .mly have to be taken in to account when they are considered to be important Obvious srruations are where deflections are important and in the design of the articulation for a bridge. Loss of prestress due to creep & shrinkage can be calculated using BS 5400 : Part 4. Shrinkage per unit length is obtained for normal exposure of 70% relative humidity. Stress due to shrinkage in reinforced concrete can be calculated using following method.
(a)
Shrinkage restrained by the reinforcement; • Stress in reinforcement = f", = (compression;
Ecs'
E.
1+ Ue. (Aj~) Stress in concrete (tension)
= fel
= A. .
f'lC
Ac Where;
EroS
free shrinkage strain refer fig. 2.5
Es
modulus of elasticity of steel
As
area of tension reinforcement
Ac
area of concrete
~"e
modulur ratio
9 (b)
Shrinkage fully restrained; Stress in concrete (tension) Where ~
NOTE,'
3.0
,.,
.-' -'0
= fel = tcs .
E,
Static secant modulus of elasticity of concrete
The value of €C3 to be obtained either from BS 5400 : Part 4 : Appendix Cor BS 8110,' Part for 80% relative humidity, (Fig. 2,5)
INYESTIGA TIONS
:
3.1 Geographical Investigation A detail survey should be carried out at the proposed location to cover topographical hydrological and technical details.
3.1.1 Topographical Survey (a)
A minimum length of 150 m on both ends of the bridge or the selected location of the bridge should be considered for detailed survey (i.e. Chain Survey. including all the permanent & temporary features and levelling) unless there is a curve encountered in e .e close proximity of the bridge beyond this length. If there is a curve the Engineer has to justify the situation and survey should be extended.
(b)
Chain survey need not be a close traverse unless it is a very important location but the levelling should be a close survey.
(c)
The chainage marked should be always in the direction of the road, (i.e. In Colombo - Kaney Road chainage 00+00 m should be started in the Colombo end of the bridge) The 00+00 m chainage should be tied.
(d)
" Longitudinal sections along the centreline of the road and cross sections should be recorded systematically with the chainages and the distances from the centre line.
(e)
At lease 05 cross sections should be taken at intervals of 05 m close to the bridge on both ends of tle bridge and the balance should be at 10 m intervals and 15 m intervals.
(f)
On a curve of the mad also the cross sections should be taken at intervals of 05 m.
(g)
The levels & chaiaages of every expansion joint of the bridge at the L.HS .• centre and R.H.S. should be taken. Also the invert levels of the waterway should be taken.
(h)
Cross sections should be taken to a distance at least 15 m from the centre line of the road on either side unless there are considerable changes in the levels. In case if there is a possibie deviation of the existing road is involved, the cross section should be taken as necessary.
10 If considerable level differences are encountered cross section should be extended
as necessary. (i)
The site survey should include the river banks to a distance of30 m. If there is a change in the direction of the stream the length should be extended as necessary.
G)
The reduced level of the M.S.L. also should be taken if it is marked in the close proximity of the oridge by other organisations such as the Survey Department, Irrigation Department etc .. T.B.M. must be en a permanent structure in close proximity of the bridge.
(k)
The direction of ~,orth should be marked.
(I)
If there are services crossing the river or carried by the bridge the necessary details such as size of the pipe, the distance from the bridge to the pipe line, type & number of supports etc. should be taken.
(m)
High tension power lines or any other structures closer to the bridge which can be affected during ccnstruction should be noted down. The possibility of cetouring and accommodating traffic during construction should be found out. SUI'. ey & levelling should cover the detour area. Possible alternative locations for the bridge apart from the existing bridge) to be considered and thcr merits/demerits noted.
_.1.2 Hydrological Survey(a)
The flow directioi. of the waterway over which the bridge is to be constructed should be clearly marked. The banks of the waterway also should be marked.
(b)
Bed level and cross sections of river on up stream and down stream sides should be taken, to a distance of 30 m approximately .
(c)
• The lowest water level, the duration of the same and high flood level and frequency of floods should be gathered from flood gauges and the natives. The flood marks on the: existing structure should be noted where ever possible.
(d)
Scouring of river be.' & river meandering patterns should be checked & any local scour patterns documented.
(e)
The approximate noted.
SI;-_
of the floating debris if there are any should be inquired &
3.l.3 Technical Survey & Detacs of the Existing Bridge
~
fa)
Type of bed material, rock out crop/boulders etc. should be noted down.
I b)
Environmental condition. sal inc/rnari nc atmosphere windy condition etc. shoul- i I)"~ taken
11 (c)
Any visible settlement of the existing structure should be marked. In doing so particular attention to be given for alignment of parapetslhandrails, kerbs etc.
(d)
Sketches of the ~!ridgefoundations, substructure and superstructure must be given with all dimensons. Where ever possible existing bridge foundation type should be indicated th-ough inspection or from data collected by the neighbours. Conditions of existing structures nearby to be noted if any.
3.2
(e)
Bearing points (1.1 the existing capping beam of the bridge should be marked clearly with dimension..
(f)
Details of existing bridge should be taken in the form of photographs.
Geotechnical Investigation (a)
-
Subsurface Invr stigation Detailed sub sur.ace investigations are carried out in the form of bore holes using rotary core percussion drilling machines. In certain cases where good soil conditions or bed rock are expected at shallow depths, soil investigations may be carried out by digging test pits, Bore holes shou: ,i be carried out at suitable intervals in the form of a grid covering the entire area. The spacing of the grid is decided on the nature of the structure and the variation of soil conditions at the site. The Geotechnical Repoi t prepared by the Geotechnical Consultant completion of t.e geotechnical investigation should include:
at the
Description of i'ie geotechnical investigation undertaken .. Dctai led assessment of stratigraphy and subsurface condition .
•
Site plan and longitudinal profile/profiles of stratigraphy. Datum for bore ;'Ioles and co-ordinates of the location of boreholes.
It is desirable to sink c.l the bore holes to bed rock in order to obtain ail necessary information unless bed reck is at a large depth and bridge could be founded at a shallow depth. Additional boreholes ma be required at sites where the bores indicate variability of subsurface conditions. The site investigation should include: In situ field test: 'which may include standard penetration tests or static cone penetrometer so .ndings. Definition of be.irock properties, where applicable.
12 Colour photographs of cores. Laboratory classification of main soil types.
The following soil conditions should be determined as appropriate. Strati graphy Physical description and area distribution of each stratum. Thickness and devation at various locations of top and bottom of each stratum.
For each Stratum of C.<);besiveS
or previous soil
Swelling characteristic, Factors affecting rimetable of consolidation such as internal stratification, especially thin sand members not otherwise identified.
For each stratl1IDof G[a!.l\.dar Sojl In situ density, average and range. Grain size distributions. Grain shape uniformity etc. Shear strength c.iaracteristics, internal friction.
which usually may be expressed as an angle of
Presence of orgauc or other deleterious materials. Ground Water Piezometric surface over site area, existing, past and probable range in future. Permeability.
13
Sources of inflow to each aquifer, where determinable. Temperature. Bedrock Depth over enti re site. Type of rock and physical properties of intact rock. Extent and character of weathering.
Joints including distribution, spacing, and whether open or closed. Faults. Solution effect in limestone or other soluble rocks. Rock quality designation, Spycial considerations Chemistry of so.l or ground water as it would affect buried structures, e.g. sulphate attack on concrete, or acids as encountered in industrial areas. Dynamic soil parameters, if required. Ambient vibration levels, if such could be a source of distress to the proposed structure or to t5e public. Problem soils
0\
conditions.
The underground investigation that have been obtained by the Engineer should be made available to the Geotechnical Consultant. Notwithstanding the above, the responsibility for the location of underground utilities lies with the geotechnical Consultant who should make all appropriate arrangements to ensure that underground utilities will not be damaged in the course of geotechnical investigation. (i)
Standard Penetration Test. Due to the extreme difficulty in obtaining undisturbed samples from granular soils, their strenghs are determined by taking disturbed samples and carrying out standard penetration tests. Penetration tests should be done not less than every 1.5 m and at least one in each of the different soil strata Once the bedrock is reached, to ensure the rock formation the bore hole is carried to a depth of another 3.0 m.
14 (ii)
RQD, Core recovery & fracture index Once the bed rock is reached RQD, core recovery and fracture index values are obtained to classify the quality of rack.
(b)
Estimation 0;' Allowable Bearing Pressure Foundation and other recommendations appropriate:
should include the following
as
Assessment cf alternative foundation systems. Allowable bearing capacities and the appropriate levels of the foundation system. Estimates of settlements and lateral stability. Relevant sou-rock design parameters. Comment on relative cost of alternatives. Assessment of possible construction problems (e.g. dewatering, usery/permanent casing, preboring, excavation stability). Empirical Charts arc referred to obtain shear strength parameters (C,
3.3
Design of Pile Foundations by MJ_ Tomlinson. Foundation Analysis & Design by EJ_ Bowels
Waterway & Length of Bridge " Piers and abutments should be so located as to make the best use of foundation condition available and also not to obstruct water flow. Decision on the number of spans depends not only on the requirement of the waterway, but, economy too should be considered for reduction in wingwali lengths for increased number of spans. A cost comparison has to be carried out to decide on the span length and the number of supports, whether to select less number of supports with large span lengths or to increase the number of supports and provide small span lengths. (a)
Determination oflength and level of Bridge For bridges across water courses or rivers the length has to be decided, taking into consideration the nature of the water course. A knowledge of the waterway required at site is also necessary. In general the bridge is constructed to provide this waterway. The width of waterway provided is the distance between the abutments less the total width of the piers with allowances made for the effects due to the edges. !(the waterway provided by the bridge is restricted, it will create an afflux upstreac. and also cause additional sCOlfydue to increased velocities. The
15 afflux created sl.ould not have any harmful effects in the region prepared for the bridge. The effect of the additional scour or the stability of the foundation should also be checker'. In determining i~lewaterway requirements of bridges, the streams/rivers should be divided into three groups. (i) (ii) (iii)
Stream/river those banks and bed with hard and inerodible. Stream/river with inerodible banks but with erodible beds. Stream/river with erodible banks and bed.
As the extra waterway required during the floods may create partly by rising water level and partly by the flood water causing scour in the bed and banks, it is necessary to study the nature and the action of the scour in river/stream banks and bed. Depth of scour should be determined, and if the bridge is founded above the scour depth a SI) itable protection for bridge piers/abutments should be done. While designing the water way through bridges. "afflux" has to be calculated in case of bridges where there is a reduction in the overall width ofthe waterway over the natural width. The afflux should be kept minimum and limited as far as possible to 150 mm, otherwise when there is a higher affiux, the more will be the velocity produced through the obstruction. Hence an estimation of the afflux is necessary in de.ermining the soffit of the deck. Ref:
(b)
Considerations in the Design &. Sinking of Well Foundations, for Bridge Piers by B. Balwant Rao & C. Muthuswamy.
Determination Of Design Maximum DischargeA number of empirical formulae are available for estimating the maximum discharge at a point on a river or any other water course. These formulae cannot be applied indiscriminately as they have been derived for specified condition. Hence for a particular bridge site the formula applicable to specific conditions of the catchment hes to be selected and used. This is very important as otherwise spurious result may be obtained. It is also necessary to check the validity of the results thus obtained by the following commonly used methods for estimation of maximum flood discharge. (i)
Area Velocity Method.
(ii)
Using Rational formulae involving the rainfall and other Characteristics.
Using recorded d:ta on existing structures on the same waterway in the vicinity or by collecting dat.. through inspection and investigation. (i)
Area Velocity Method: The cross sectional area of the river is measured in a straight reach and this is rnultipl.ed by the velocity calculated from the Maning's Formula.
16 (ii)
Rational Formula : The catchment is the area upstream of a point in the river from which all rain water falling in that area will tend to flow to that point. This is computed with the help of Topography sheets.
The rainfall records in the catchment are obtained from the Irrigation Department or the Meteorological Department. Ref:
01. 02. 03.
Bridge Engineering by S. Ponnuswamy Essentials of Bridge Engineering by D.l Victor Design ofIrrigation head works for small catchments by - A.lP. Ponrajah
Spacing and Location of Piers & Ahutments :
(c)
The positioning [;;ld spacing of the piers and abutments are finally decided taking into consideration the requirements of the waterway outlined above and the result of the bore hole investigations and available standard beam lengths after considering the e. onomic aspects for alternate proposals for different type of super structure as well as substructures.
4.0
AUGNMENT
AND G~:QMETRICAL CONSIDERATION:
The superstructure is the visible feature of the bridge. By selecting the correct shape for me superstructure aesthetic appearance of the bridge is enhanced. Superstructure consists of the deck, kerbs. hand rails, uprights and lamp posts. Service ducts are also provided in the superstructure as a means for carrying service mains across the river. A bridge may be right or ;:kew. Skew angle is defined as the inclination of the abutment to the perpendicular to its rree edge. A bridge with a skew angle of zero degrees is a right bridge. In simply suppor ed bridges the effect of skew in general is neglected up to 20 degreesand if the skew angle is more than that. bridge deck should be designed to resist • the effects. The bridge deck may be e.rher of reinforced or prestressed concrete. Factors that affect the choice of deck are the spans. foundation condition, aesthetics etc .. Alongthe Road Network ~,II bridge decks should provide for carriageway widths. to suit the traffic requirements as given below. A minimum width of7.4 m for Class A roads and a minimum width of6.8 171 for Class Broads should be maintained with adequately wide foot walks on either side. ';"'heminimum foot walk width adopted is 1.2 m which varies to a larger width depending on the specific location with respect to the pedestrian volume . .__
.
i ADT PCU/day
I
4000v 72000
-
25000 40000
}2
2(2x7.0)
--=
18000 25000 .12x3,7
300 18000
<300
2x3.4
3.7
17 For bridges over highway and railway a minimum provided.
vertical clearance
of 5.25 m should be
The approach road layout along with the bridge centre line should be designed in accordance with the ; Iighway Designs Manual which includes design of horizontal and vertical alignments. !.ongitudinal camber also depends on the aesthetic requirements and type of construction. Cross camber is so designed (to a slope of 1:60 for concrete bridges) to lead the water to t:le lower kerb which selves as a surface drain leading water through the rainwater outlets.
5.0
SELECTION OF 1SRIDGE TY!)ES ... \ND OE£HGN CONSIDERATION: Types of bridges are , lassificd depending on the material used and the type of construction adopted'. The common types of bridges are :1)
2) 3) 4)
5) 6)
Concrete Bri.lges Steel Bridge= Stone or bride masonry Concrete arcl. Bridges Timber Bridges Box culverts
arch Bridges
Concrete bridges are used in most of the places, because oflong life span and speed of construction. The materials for construction of concrete bridges are readily available. It has also been found tl.u the maintenance of concrete bridges is less costly than for other types of bridges. On account of the shortage of steel involvement of foreign exchange and non availability of rolled sections, it is preferable to avoid use of steel bridges as far as possible. However, where steel trusses in ::ood condition from dismantled bridges are available, these can be used on certain class ,:f roads. A disadvantage in using steel bridges is the high cost of maintenance .
•
For small culverts and ondges of moderate span, where the available headway is adequate. stone or brick masonry arches can be used with advantage when bricks or stones are locally available. Services of skilled workmen me required for this type of construction. Concrete arch bridges are generally used in places to fit into the aesthetic appearance of the area. In hilly areas. where the velocity of flowing water is such that it is not possible to construct any intermediate piers, a concrete arch bridge is advantageous and convenient. Use of timber bridges i,: limited to areas \•.,hen timber logs are found in plenty. As timber gets easily deteriorated under normal weather conditions, such bridges are generally built for temporary construction and for light loadings. At places where the flood spread is large, since providing a all weather bridge is uneconomical, a submersible bridge is acceptable. It effects great economy of construction. However the formation level of the structure should be fixed depending on the period of inundatic " of the structure. In addition, the type of ~'ridge, to be provided
at a site is generally
decided
on economic
18 analysis, availability o: materials and CIlSC' of construction. Use of precast prerensioned beam in the bridge decx hac; the advantage of case of construction due to the f~ryffiade product as well as the low thickness of deck in the case of design of the vertical profile for low level approaches. However the diDicu:ties that may have to be encountered in the transportation of prer ast beam with respect to the location of the bridge should be considered. Before decision is taken to adopt a post tensioned beam deck for the bridge, the possibility of pro" ding a beam casting yard close to the bridge location should be looked into. Standard nrccast prctcnsioned beams are available from 4.3 m upto 16.23 m. A combination of pretcnsioned/post tensioned beam with precast, 16.23 m unit is available for spans of 19 m. The commonly used bridge types and components arc described here in detail. It is the decision of the desig.ier to adopt or to deviate from the types indicated herein as appropriate to the circumstances
5.1
Foundations: Types of foundations
(a)
.xnnrnonly
used ar
foundations
Shallow
Spread footing iounded on rock or
(b)
0n
suitable
soil strata.
Deep Foundations (l) Pile Fotndations
(i) Cw;t Insitu or Bored f'tb (ii) Precast driven Pi!.:;s (iii) Ti-nber Piles (2)
Caisson
Foundations
(i)
Cii cular
(ii)
Rc 'tangular
9
The choice of the type of foundation
depends
(a)
Nature of Soil Sl rata
(b) (c)
Magnitude of Site conditions
(d)
Economy
(e)
Availability of Construction Techmques, Maximum likely scour depth
(I) (g)
Minimum
grip
primarily
on following;
tl. loads to be carried
length required In the case of deep foundations
Foundation types can be- classified as shallow & deep. Spread footing & side by side cassons can be consider: .d as shallow ff'und.'l:iom and spread footing can be provided ~Jcre a suitable soil strar.m can be found ;:11 a shallow depth within about 3-4 m below groundlbcd level. Where •.he founding layer is b(·twcen 4.0 - 6.0 In side by side caissons, an improvise method of ~::read foundation could be adopted. Where water table is high, the problems of cofferdai.. ing and de-watering should be considered when adopting these types of foundations,
19
Pilefoundations may be adopted when suitable bearing strata are found deeper than 6.0 m from ground level. Bored piles should be adopted when driven piles are liable to damage existing structures. Pre-castpiles or cased c.ist in situ piles are preferred where peat over layers are found or when foundations have to be constructed in water. Timberpiles are used where there is no risk of decay of timber and loads to be transmitted fromthe structure are not excessive. In the design of driven ;Iiles in particular, two different capacities should be taken into account. (a)
The capacity of the pile as a structural member
(b)
The capacity of the pile to transmit loads to the foundation material.
the case of piles on good rock the capacity is governed by condition (a). Where significant horizontal forces are present, raked piles could be used. III
Caissonfoundations am~large diameter bored piles are used when heavy loads have to be transmitted and when tue foundations have (0 be carried very deep. 5.2
Substructure -
Thesubstructure mainly consists of three components, abutments and wingwalls and piers. Abutments,wingwalls arid piers must be so proportioned so as to satisfy both the practical as well as theoretical consideration S. Selection of the type of substructure should be carriedout to suit the par: icular site conditions. Alternative proposals should be considered for economic feasibiliry. The overall dimensions are first determined from practical considerations and components are designed to resist the various forces acting on them. The height of abutment and/or piers should be selected to give a sufficient clearance betweenthe highest flood level and the bearing level) unless designed as a submersible ,. bridge. This free board t.';, usually taken as 1.0 ill, or a minimum of 0.6 m due to restricted conditions. Partial or ~jl! flooding is not acceptable on Class A & B roads unless a considerablesavings on construcrlon may be achieved on bridges oflesser classification h!' providing reduced wa.erway area and accepting short duration flooding by floods with shorterthan the full Design Recurrence Interval. The selection depends on the number of parameters, including availabil ity of alternative routes, short term inconvenience connected withshort duration floodng weighed against [he cost saving. The width and length of the substructures are governed by the loads to be carried (both vertical and horizontal) self weight(necessary to redi.ce loads on foundation), economy in construction, use of local material to a maximum ;"Indpossible obstructions of waterway in the case of piers. 5.2.1 AbutmentsDifferent types of abutments commonly used are; (a) (b)
Mass concrete Reinforced conCi~te -
1. Reinforced concrete wall 2. Reinforced concrete column (open
abutments)
20 Assessment of Loads: Openabutments are more economical if there IS no risk of earth fill being washed away as in the case of flyover or bridges with protected banks. (1)
Vertical Loads (i) (ri)
(di) (iv) (2)
Horizontal Load s
Dead Load reactions From Superstructure Live Load reactions From Superstructure Self Weight Buoyancy (i) (ii) (iii) (iv) (v)
Earth Pressure Pressure due to Surcharge Tractive Force Temperature effects Shrinkage effects.
Earth pressure and pressure due to surcharge are determined from Rankine formula. For tractive force and live loads reference should be made to the notes given under loads.
In addition, abutment should be checked for vertical and horizontal forces acting during construction stage. Propped abutment type ~:~TUcturesoften provide an economical solution for single short span bridges, provided that complete scouring of the till behind abutments can be eliminated. Significant part of the horizontal forces is transferred through the deck betweenthe abutments wid only the unbalanced horizontal forces need to be resisted by the foundations.
5.2.2 WingwallsDifferent types of wingv.alls commonly used are; (a) (b) (c)
(d)
Wingwalls cantilevered from abutments Mass concrete wngwalls Reinforced conc.ete wingwalls. Sheet pile wingwalls.
Forshort wingwalls of m-sdi urn heights and where there is no risk of scour cantilever type maybe used. Wingwalls are to be designed as earth retaining structures subjected to active earth pressure. The mass concrete stepped section is to be designed as sloped back retaining wallon the stepped side. However to be conservative the vertical component of the earth pressure may be ignored and the full pressure assumed to act horizontally. Sheet pile wingwalls are designed according to standard design practice.
~\~f ~evrO\ds Ti~~Boo"
21 Weep holes are provided in abutments and wingwalls to reduce the build up of pore water pressure in the earth rill behind. They are usually spaced at 1.5 m centres horizontally and vertically as appropnate. The lowest row of weep holes is provided at 0.3 m above the normal water level. When abutments anu wingwalls are founded at different levels on soils of different bearing capacities, a slip joint between the abutment and the wingwall from top to bottom is provided. 5.2.3
Piers Different types of ;:iers commonly used are; (a) (b) (c)
Mass concrete stems Reinforced concrete walls Reinforced concrete colwnns
Piers should be checked for the following loads. (1)
Vertical Leads -
(i) (ii) (iii)
(iv) (2)
Horizontal "Forces Longitudinal Direction
Transverse Direction •
(i) (ii) (iii) (iv) (v) (i) (ii) (iii)
Dead load reaction Self weight Live load reaction (a) Both spans loaded (b) One span loaded Buoyancy
Tractive force Force due to water current Force due to floating debris & impact. Temperature effects Shrinkage effects Force due to water current Force due to floating debris & impact Wind
The design criteria adopted for mass concrete piers is given under stresses in the note. 5.3
SuperstructureWhen selecting tne type of bridge superstructures, span, length, location of bridge, maximum deck tlickness that could be accommodated etc. should be considered. For short spans, up to about 6 m Rfc slabs or pre-tensioned rectangular units can be provided and for spans of 6m - 19m m decks with pre-tensioned P.S.c. beams placed side by side with insitu infiller concrete deck and for spans more than 19 m post tensioned P.S.C. beams with post tensioned or reinforced concrete deck slab can be provided When deciding en the length of precast beams consideration should be given to the transportation capability and if post tensioned beams are used the space required for post
22 tensioning bed and its proximity to the bridge site should be considered. The common types of structural arrangement of bridge decks are; (a)
Plain Slab Deck, R.C. or precast P.S.C. units
(b)
Beam & Slab Decks.
(c)
Steel concrete composite deck
(a)
Plain Slab Dr.cks : Mostly used for short spans. They are also used for larger spans where limitations on construction depth and economy are governing factors of design. The different '~pes that have been adopted are ;
(b)
(1)
Plain concrete slab - Precast or Insitu
(2)
Composite construction of precast prestressed concrete beams with or without insitu infiller concrete to form the deck slab. Type precast prestressed beams of standard lengths are available for this type of construction.
Beam & Slab Decks: The deck comprises of several longitudinalbeams and transverse diaphragms with a concrete deck slab. The beams may be either ofT,! or Box Section. The different types that are being used are; (1) •. Reinforced concrete beam and slab (2)
Prestressed concrete beam with end and intermediate diaphragms with (i) (ii)
(c)
Prestressed concrete deck slab or R.einforced concrete deck slab
Steel concrete.composite deck The deck comprises of several longitudinal steel beam with concrete slab on top.
5.3.1Designof Superstructure
-
Analysisof superstructure is carried out using the following methods. 1.
'Elastic Analysis Theories
ego Load Distribution
23 02.
Plastic Analysis : eg. Yield line theory
Permissible stresses to be adopted are to be in conformity with Part 4 ofBS 5400. In prestressedconcrete decks in general, permissible stresses to be in accordance with Class IIrequirements ofBS 5400. (a)
Slab Decks Slab decks are designed as one way spanning, either simply supported or continuous.
(b)
Beam and Slab DecksReinforced concrete deck slab is designed in the same way as the slab bridge.
InthePost tensioned slab prestressing steel is designed using empirical method proposed by Guyon in "Prestressed Concrete", However nominal reinforcement of 0.1% is also providedin the direction of prestress. In the perpendicular direction steel is provided to resistthe Bending Moments and Shear forces in that direction. Compositeaction of the slao may be taken into account in the design of the longitudinal beams.Beam is designed hi cater for the portion of lane load depending on the spacing. Thediaphragms are placed. at supports, mid span and quarter span points. Transversemoments and lcngitudinal moments due to HB loading is worked out using deck analysis. Ref. : Concrete Bridge Design by SA
R.E. Rowe ~
Bridge Bearings Loadsimposed by the vehicle on the superstructure are transmitted to substructure through thf; bearing. ••
Bearingsmay be of steel concrete or rubber. Bearingsshould designed in accordance with BS 5400 Part 9.1.
S.5
Other Features of Superstructure
-
Handrails, Uprights & Parapets Parapets and Handrails may be of masonry or concrete. Shape of hand rail and upright or parapet is an architectural feature. The minimum height of the railing or parapet of a high' ..vaybridge should be 1.0 m. Parapets are not normally designed for collision loads and may be designed for pedestrian loads. Where the situation demands crash barriers should be used.
24 End Pilasters End pilasters may be of masonry or reinforced concrete. The size and shape is designed to give a good appearance to the bridge. Kerbs Kerbs are provided at the edge of carriageway to deflect the vehicle back. Kerbs should be a solid section not less than 225 mm wide at base and not less than 225 mm high above the adjacent road surface. Service ducts Service ducts are provided under the sidewalks to carry cables and water mains. Lamp Posts -
Spacing of lamp posts, height of lamp posts, etc. are designed according to the illumination requirements. Provision shall be made in the deck or the sub-structure to accommodate the lamp post. Expansion Joints In the case of simply supported spans there is a complete separation between abutting spans wuich permits change in length of superstructure due to temperature variation. The gap in-between should be sufficient to accommodate the expansion of the deck within temperature range expected. Expansionjoints should extend over the entire width of the deck. It should not allow penetration of water to capping beams. As it is very difficult to achieve full water proofing of the expansion joints -md particularly to maintain through out the life of the structure, it may be acceptable on some bridges and much more economical and easier to allow the surface water to penetrate the joint and to make appropriate provisions for its drainage.
Rain water outlets Rain water outlets are usually kept at 4.5 to 6 m centres. Rainwater outlets should project out side the deck sufficiently to prevent water dripping on any part of deck or substructure.
6.0 DESIGN OF SUBME1~SmLE BRIDGES 6.1 Scope TheseGuidelines are intended to be used for low level bridges subjected to submergence by floods frequently. TheseGuidelines are not .ntended for use as a code of practice or a design code. It is the responsibility of the designer to consider and evaluate all aspects relevant to the bridge underconsideration. The design process may need to include seeking the opinion of all theusers of the bridge. o~~questions such as the level of service. location and choice of barriers.
2S 6.1
Introduction
BridgeType Low-levelbridge
T-r Flooding
I 1
Frequency
Frequently submerged 100 year
IHigh-levelbridge.l
'I
Bridges subjected to sui.mergence are usually adopted for reasons of economy often where the difference between ~')rrna1water level and the flood level is large but the floods are of relativelyshort duration, or where it is impractical to raise the bridge and approaches above floodlevel because of the resulting backwater effects. This type of crossing may also be chosen where usage is limited and flood free alternatives exist.
6.2 BRIDGELOCATION, PROPORTIONING
& ORIENTATION
6.2.i Location
Submersiblebridges are suitable for flat and arid areas inland where large floods occur infrequentlyor in remote forested hilly areas where flash floods could be frequent but last onlya short time. Submersible bridges are also suitable on large flood plains as flow over the approach roads is often acceptable and velocities are generally not high. 6.2.2 Proportioning Bridge & Approaches Theentire bridge structure should be designed to minimize catching debris or at least to alloweasy removal of any debris caught. Bridges with short individual spans tend to catch moredebris than those w~::nlonger individual spans. Where debris loading is significant spanlengths less than 10 metres should be avoided. The span lengths chosen should at leastexceed the expected lengths of debris to be passed. Acompromise tor the calculated backwater, and drag effects must be made between the advantagesof longer span: with fewer piers (but incurring a deeper superstructure) and shorterspans with more piers (but allowing a shallower superstructure),
6.2.3 Deck Level & Trafficability Ifstaticplus velocity head at the crown or highest edge of a carriageway exceeds 300 mm overtoppingflood depths must always be indicated with gauge markers. Thusovertopping of submersible bridges and their road approach embankments may be toleratedto provide traffical.ility for a river crossings subject to low serviceability floods. Alternatively,the bridge deck could be placed above low flood level (but for economic reasons,below larger flood levels while the approaches, set at a lower predetermined level, couldpermit overtopping by the low flood and yet still remain trafficable.
26 6.2.4 Vertical Alignment A level grading should be provided for the full length of bridge so that the bridge acts as a weir when the rising upstream water surface just over-tops it. If only portions of the structureare overtopped 6e pattern of flow in the stream could be severely disturbed. A deckon a grade or vertical curve is also a hazard to traffic, because the water depth is not constant Drivers negotiating a flooded crossing should not encounter an unexpected increase in depth of water.
6.2.5 Horizontal Alignment Submersiblebridges should be on a straight alignment and located as square as possible tothe most common direction of flood flow.
6.2.6 DeckCrossfall As the water subsides, debris and silt will tend to be left on the upstream side of submersible bridge with normal two-way crossfall. For this reason, the preferred deck sectionwith one-way falling cross fall toward the downstream side is preferable.
6.3 Analysis 3.1 Uplift & Instability
The stability of submerged bridge should be assessed for both uplift and overturning effectsfrom the following simultaneously occurring forces due to the stream flow, viz: (a)
Buoyancy uplift
(b)
Hydrodynamic dra-; forces resulting from stream flow past the superstructure andlor on debris ceught against the upstream edge of the superstructure;
(c)
Unbalanced hydrostatic pressures acting on the upstream sides of the bridge from ponding ~ux effect), aggravated by the collection of debris;
(d)
Floating object impact forces
(e)
Some superstructures may also trap debris, which if buoyant, will create further uplift forces.
or
It isessential that all parts the structure are considered for reduction of dead load due to buoyancy when the bricge is submerged. The possible use of concrete aggregates lighterin weight than the assumed design value causing over estimates of the stabilizing deadload of the structure should also be considered. The possibility of air pockets being trappedunder the deck creating destabilising buoyancy should also be considered. Calculationof the destabilisi vg forces listed above on bridges from stream flow effects are oftensomewhat uncertain due to lack of data on actual flood flow and debris as well as knowledgeof appropriate drag factors or hydraulic flow patterns to be used.
27 To account for uncertainties, the bridge must have a very large factor of safety against instability under flood submergence. Unless the stabilizing restraint provided by gravity force provides a factor of safety of at least 02 or more on unfactored loads the superstructure must be securely tied down to the substructure.
6.3.2 Critical Flood Levels & Velocities It is suggested the following load case categories for submersible bridges be considered. (a)
Partial submergence of superstructure
(b)
Overtopping o f superstructure
(c)
Deep submergence of superstructure
Stream velocities as well as frequency, magnitude and duration of the respective submergence calculated for various flood flows will determine the required scour protection of the eml.ankrnents for the road approaches and abutments. Where road approach or bridge abutment embankment is overtopped by flood waters special protection works are required. Additional culvert openings may be required
6.4
Suitability of Alternative Structures
6.4.1 Kerbs & Barriers This section deals with some aspects of the choice of appropriate kerbs and barriers. The requirements are sometimes contradicting, e.g. a grill type pedestrian or combined traffic/pedestrian barrier is not suitable for sites with substantial amounts of debris, yet it may be required if the !,ridge is located in town and there is heavy pedestrian usage. The choice of the appropriate kerb or barrier depends on circumstances. Where deep water is present use os collapsible railing should be considered. If it is considered that kerbs are warranted, they should either be castellated (short sections of kerb sepasated by full-depth gaps not exceeding 200 mm) or if continuous, provided with slots to allow water to drain freely from the deck and to aid the manual removal of debris. Small diameter drainage holes and scupper pipes are prone to clocking with debris.
6.4.2 Superstructure It is desirable to select a form of superstructure with a shallow depth, to minimize hydrodynamic force, be__ ckwater and scour effect particularly at the critical flood height when the bridge and approaches are about to be overtopped. Closed cell structures si-ch as single and multicell steel or concrete box girders, steel trusses or composite steel or co.crete through girders, multicelled decks formed from linked broad flange concrete girders, voided concrete slabs, and other voided concrete girders are generally considered unsuitable. All parts of concrete '",uperstructures subject to frequent flood submergence must be considered to be 'in contact with water' for calculation of required cover to prestressed and non-stressed reinforcement. This may lead to slightly greater cover requirements for PSC
.~
28 girders, especially in rockets where silt may be trapped and remain damp after submergence. Ajudgement must be made whether siltation due to submergence will occur frequently or only rarely in the life of 1 he bridge. 6.4J Bearings & Hold-down "Restraints It is usual to provide separate anchor bars. Typically 24 mm diameter galvanized steel holding down bolts are specified between each P.S.C. plank on composite slab structures while more substantial angle brackets may be specified to straddle each P.S.C. girder on composite decked structures. Shear blocks or dowels connected between end diaphragms andabutments or piers will prevent a submerged superstructure from lateral displacement. Ifelastomeric bearings are used, the holding-down bolts should be suitably de-bonded and anair space left above the heads of the bolts, to allow the bearings to deform under live load. Frictional restraint against creep of elastorneric bearings is diminished during flood submergence due to reduced bearing reactions from superstructure buoyancy. Under the influenceof rotation from hogging effects this creep usually moves the bearings towards the centre of the girder spans. 6.4.4 Substructure Thefoundations for piers and abutments of all bridges crossing fast flowing waterways shouldeither be keyed on ti: rock or cast on piled footings. This avoids the possibility of underminingof the base material under the foundation footing pads by the formation of adjacent local scour holes. Eddies also tend to wash out fill behind abutments. Local scourfrom turbulence around piers may be reduced by hydraulically shaping the pile caps andcolumns. 6.(5 Batter Protection
Protectionworks of embankments adjacent to abutments will be similar to those for high levelbridge and may include rock fill, rip-rap enclose, concrete revetment mattresses or rigidreinforced concrete slabs as well as sheet pile toe walls. However abutment batter protectionof submersible bridges not only must be secured against stream bed scour but must be secured against SCOtt! effect during overtopping. Grassingof batters may be adequate when the velocity of water flow over the embankment islessthan 1.5 metres/sec. Generally this is the case when tailwater levels are not more thanaround 300 mm below tl.e downstream edge of the road formation when overtopping firstoccurs. Grass batters may not be suitable where frequent overtopping occurs for a periodof more than two or three hours during floods. Grass batters are not suitable for shadedareas under the bridge superstructure. Moreelaborate protection wo.ks against scour are detailed similarly to the requirements forcauseways. It is essential ~;l the following protection works are either taken below the anticipatedstream bed general scour level or local scour hole depths using cut-offtrenches, orareprovided with level aprons extending into the waterway to accommodate scour erosion.In particular. batter protection works subject to flood submergence must also be anchoredalong their upper edges to resist scour erosion during overtopping.
29 Nominalcover to all reinforcement (including links) to meet durability requirements - Adopted fromBS 5400 for Sri Lankan Practice.
Environment
Nominal cover (in any case
Examples
should notbe less than the d!a. of the bar) (mm) , ,-
Concrete Grade
i (
,
25
Extreme Concretesurfaces expo red tosbrasi ve action by sea
30
40
50 and over
65
55
50
40
Parts of structure in contact with sea water
water
Very Severe Concretesurfaces directly affectedby sea water spray
I
Severe Concrete surfaces exposed todrivingrain or alternativewetting and dryirig
Concrete adjacent to the sea -.
50
45
35
30
50
40
30
25
\Valls and structure supports romote form the carriageway Bridge deck soffits Buried E:U1:s of structure
I Moderate
Concretesurface above groundlevel and fully shelteredagainst all of the followingrain sea water spray
-
Surface protected by bridge deck water proofing or by . permanent form work Interior surface of pedestrian subways voided superstructure or cellular abutments concrete t..:!,rmanently under water
--
30 01. GENERAL
1.1 Scope TheseGuidelines are intended to be used for low and intermediate-level bridges subjected to submergence by floods with Average Recurrence Intervals (ARl) less than 100 years. This document is not intended for use as a code of practice or a design code. It is the responsibilityof the designer to consider and evaluate all aspects relevant to the bridge under consideration. The design process may need to include seeking the opinion of all the users of the bridge,on questions such as the level of service, location and choice of barriers. 1.2 Introduction A bridge with a superstructure
which may be partially or fully submerged by any flood with respectto Average Recurrence Interval CARl) is specified below. This return frequency was generallyadopted as the design frequency for bridges and large culverts.
BridgeType
r 'Flooding
Frequency
Typical ARI
Low-level bridge
Frequently flooded
< 20 *-
Intermediate-level bridge
Rarely flooded
20>11to < 100
Clearance above 100 year flood
> 100
Hgh-level bridge
*
The20 year ARI demarcation between frequently and rarely flooded bridges is indicative only anda reduced value may be more appropriate for some bridges that are intended to provide lowerlevels of service.
A lowor intermediate-level type of crossing is usually adopted for reasons of economy often wherethe difference between normal water level and the flood level is large but the floods are ofrelativelyshort dilration, or where it is impractical to raise the bridge and approaches above floodlevel because of the resulting backwater effects. This type of crossing may also be chosen whereusage is limited and ficod free alternatives exist. 02. BHIDGE LOCATION, PROPORTIONING
& ORIENTATION
2.1 Location Low-levelbridges are suitable for flat and arid areas inland where large floods occur infrequently orin remote forested hilly area, where flash floods could be frequent but last only a short time. Large food plains are also su: table as flow over the approach roads is often acceptable and velocitiesare generally not high. TIleideallow-level bridge site is where the stream bed is broad and shallow, with gently sloping banks. The bridge should be located on a straight reach of the stream, not subject to scour or siltation. Ifthebridge is located in a posit (on where the road approaches or bridge itself cannot be suitably protectedagainst a moving cha.inel, scour, debris or siltation, the economical advantage of the low-levelcrossing may be outweighed by maintenance costs.
31 2.2 Proportioning Bridge &
,
pproaches
Ideally,any low-level bridge should cross the majority of the stream channel to minimize the length of approach embankments which would constrict the flow of water. This may be impractical for wide flood pl•.•ins. Where submerged bridge structures and embankments do constrictthe stream channel tho:effects of backwater, local stream velocity increase and scour mustbe considered.
The batters of the approach embankments should, as far as possible, follow the contours of the streambanks. If the banks are steep, the embankment should be smoothly transitioned into the channelto prevent sudden changes in the stream velocity pattern. Otherwise the embankment willform a backwater with resulting siltation which will cause constant maintenance problems. Eddiesand turbulence around rhe ends ofuntransitioncd embankments will cause local scour requiring further maintenance
The entire bridge structure should be designed to minimize catching debris or at least to allow easy removal of any debris caught. Bridges with short individual spans tend to catch more debris than those with longer individual spans. Where debris loading is significant span lengths less than 10metres should be avoided. The span lengths chosen should at least exceed the expected lengthsof debris to be passed. A compromise for the calculated backwater, and drag effects must be made between the advantagesof longer spans with fewer piers (but incurring a deeper superstructure) and shorter spans with more piers (but allowing a shallower superstructure).
2.3 Deck Level & Trafficabiliey The deck level should be kept ss low as possible, having regard to the needs of traffic and the frequencyof overtopping, so that the structure creates the least restriction to the passage of debris attimesof high flood. Ifa stream tends to pick up and carry large driftwood at a certain flood level,the deck level should preferably be lower than this flood level. Nevertheless, the clearance abovethe stream bed should not be reduced to such an extent that drift carried by intermediate flowsis trapped beneath the deck. (Staticplus velocity )lead) at tile crown or highest edge of a carriageway exceeds 300 mm (Ref.14.) Overtopping flood depths must always be indicated with gauge markers.
Thus overtopping of low level bridges and their road approach embankments may be tolerated to provide trafficability for a river crossings subject to 20 year (AIR)serviceability floods. Alternatively, the bridge deck could be placed above the 20 year (AIR) flood level (but for economicreasons. below larger .lood levels while the approaches. set at a lower predetermined level,could permit overtopping by the 20 year (AIR) flood and yet still remain trafficable. Floodsof larger return intervals may require closure ofthe crossing.
1.4 Vertical Alignment Alevelgrading should be provided for the full length of a low-level bridge so that the bridge acts asa weir when the rising upstream water surface just over-tops it. If only portions of the structureare overtopped the pattern of flow in the stream could be severely disturbed. A deck ona grade or vertical curve is also a hazard to traffic, because the water depth is not constant. Driversnegotiating a flooded crossing should not encounter an unexpected increase in depth of \""li~r
32 2.5 Horizontal Alignment Low-levelbridges should be straight and located as square as possible to the most common direction of flood flow. A skewed or horizontally curved bridge superstructure, when submerged, directs the flow sideways towards the downstream abutment. The resulting turbulencemay cause serious »cour at this abutment. Moderate skews or very large horizontal radiimay be tolerated provided that the downstream abutment and adjacent embankment batters are adequate protected against scour. 2.6 Deck Crossfall Asthe water subsides, debris and silt will tend to be left on the upstream side of low-level bridge with normal two-way crossfall. For this reason, the preferred deck section in the past has adopted a slight one-way crossfall falling toward the downstream side. However such a superelevateddesign of the bridge a.id matching adjacent road approach may incur greater damage to the upstream edge of road p.rvements from increase in flow velocity. Super-elevated bridges with soffits falling downstream may trap debris underneath the superstructure and be more susceptibleto vertical 'lift' effects. Also, it is considered safer to drive across an overtopped bridgewith crossfall falling, instead, towards upstream. Therefore, cross-sections with normal two waycrossfall are probably the best compromise although the choice of deck crossfall will dependon the type of bridge. structure proposed and local conditions for stream velocity and debris.
03. ANALYSIS 3.1 Uplift & Instability TIle stabilityof submerged bridge should be assessed for both uplift and overturning effects from ihe following simultaneously occurring forces due to the stream flow, viz: (a)
Buoyancy uplift
(b) Hydrodynamic drag forces resuJting from 'form' stream flow past the superstructure and/or • on debris caught against the upstream edge of the superstructure; (c) Unbalanced hydrostatic ;.:;ressuresacting on the upstream sides of the bridge from ponding (afflux effect), aggravated by the collection of debris; (d)
Floating object impact f.)rces
(e) Vertical Tift' forces actir-g under superstructures with soffits inclined to the stream flow.
(0 Some superstructures may
also trap debris, which ifbuoyant, will create further uplift
forces.
Notethat super-elevated bridg-es with superstructures soffits falling on a constant gradient towards downstream \,.111 be pcrticularly susceptible to these latter two effects(e) and (f). If s essential that all parts of ~he structure are considered for reduction of dead load due to buoyancy when the bridge is s.ibrnerged. Attention is drawn to the possible use of concrete
33 aggregates lighter in weight than the assumed design value causing over estimates of the stabilizing dead load of the structure and also to the possibility of air pockets being trapped under the deck creating destabilising buoyancy. Calculation of the destabilisine forces listed above on bridges from stream flow effects are often somewhat uncertain due to lac': of data on actual flood flow and debris as well as knowledge of appropriate drag factors or hydraulic flow patterns to be used. For calculation purposes damage to piers from log impact is estimates at levels in other countries. It is cautioned that the 2 tonne mass still specified for log impact in the AUSTROADS Bridge Design Code may not represent a realistically sized log and the stopping distance assumed in calculations significantly affects the forces generated. Damage to bridge piers from log impact with an estimated 5 to 6 tonne mass has recently occurred in NSW. Current practice within many UK consultancy firms is to assume a 10 tonne mass for log impact. Also a debris mat collected against the side of the bridge may offer a cushioning effect to impact from large floating objects, an uncushioned blow could displace a near buoyant. Buoyancy of the superstructure reduces the effectiveness of the lateral restraints. Therefore, to account for these uncertainties, the bridge must have a very large factor of safety against instability under flood cubmergence. Unless the stabilizing restraint provided by gravity force provides a factor of safety of at least (say) 03 or more on unfactored loads the superstructure must be secur ely tied down to the substructure. This may be achieved by a suitable arrangement of bolts (,:'bars with positive end anchorages or the design may incorporate bearings with hold down restr.iinr. The small additional expense for provision of hold -down restraints should be evaluated against the great expense of possible Joss of the whole bridge struct.ure from instability. Naturally the substructure and bearings must be designed for the overturning and uplift effects mentioned above. 3.2 Critical Flood Levels & Velocities It is suggested the following load case categories for low level bridges be investigated: (a)
Partial submergence of substructure
(b)
" Partial submergence of superstructure
(c:
Overtopping of superstructure
(d)
Deep submergence of suporstructure
Stream velocities as well as frejuency, magnitude and calculated for various flood flows will determine embankments for the road approaches and abutments, embankment are overtopped by flood waters special
duration of the respective submergence the required scour protection of the Where road approach or bridge abutment protection works are required.
34 04. SillTABILITY
OF ALTERNATIVE STRUCTURES
4.1 Kerbs & Barriers This Clause deals with some aspects of the choice of appropriate kerbs and barriers. The requirements are sometimes contradicting, e.g. a grill type pedestrian or combined traffic/pedestrian barrier is no; suitable for sites with substantial amounts of debris. yet it may be required if the bridge is located in town and there is heavy pedestrian usage. The choice of the appropriate kerb or barrier depends on circumstances. Ifit is considered that kerbs are warranted, they should either be castellated (short sections of kerb separated by full-depth gaps not exceeding 200 mm) or if continuous. provided with slots to allow water to drain free I',· from the deck and to aid the manual removal of debris. Small .' diameter drainage holes and scupper pipes are prone to clocking with debris. Consideration may be given to capping the caste'lations with steel channel fenders to provide a continuous tyre rubbing strip. In this case the 200 rom limit on gaps between castellations does not apply.
4.2 Superstructure It is desirable to select a form of superstructure with a shallow depth. to minimize hydrodynamic force. backwater and scour ei .ect particularly at the critical flood height when the bridge and approaches are about to be overtopped. 9lm.P2§ite decked bridges. ~:!singeither prestrxssed concrete or steel girders, have been suc~s~f~Iy,_llSed for s!:!h.t!\$J1:'~d.~ti1~~ prf~I>~Qi2r~9_(,'{)'J~~rders have utilized 'inverted ~ and standard AtJSTRO·\l)S .c "kd girders have utilized rolled or welded plate sections. These ~(;ction" allow .loodwaters to rise between the girders. UH
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Slender steel girders may requi- e additional midspan lateral bracing as strengthening to resist log impacts against the bottom flange. If used, cross bracing should be orientated to avoid trapping debris. For corrosion resistance, steel components must be detailed to avoid trapping ponds of water after submergence. Closed cell structures such as s.ngle and multicell steel or concrete box girders, composite steel or concrete trough girder, multcelled decks formed from linked broad flange concrete girders. vo.ded concrete slabs. and other voided concrete girders are generally considered unsuitable and should only be used with grea: caution when subject to submergence. The greater buoyancy forces acting on these closed cell structures may require the use of significant hold-down restrain-s. Experience has shown it is impossible to hermetically seal concrete closed cell structures against the ingress of water whether rhese structures are considered to be submerged or not. The exception is those voided concrete structures using polystyrene void former which is left permanently in place. Otherwise noisture enters the air filled internal voids through cracks formed in the surrounding cone; ete by either temperature, shrinkage cracking or tearing of the plastic concrete during curing. Thus the cross-section must be detailed to facilitate draining of nay water trapped in pockets, tycically with 25 mm diameter vertical drainage pipes placed at the girder ends. Due to the provision of these drainage pipes in such structures the possibility always exists that the internal voids wil. be filled completely during submergence. If the voids do not empty again at the rate of flood recession (perhaps due to siltation blocking the drainage holes) severe loads would be applied io the bridge from water trapped inside.
35 Even if the steel or concrete members of closed cell structures are modified with large openings to permit rapid filling and emptying, when submergence occurs, it is anticipated significant silt and fine debris will be deposited inside when such submergence is frequent. This will have an adverse effect on the durability of the structure and may require significant future maintenance. Note that access would therefore be required inside the closed cells for maintenance inspection and works. This cost could d-: tract from any economic advantage of this form of construction. All parts of concrete superstn ....tures subject to frequent flood submergence must be considered to be 'in contact with fresh '.'ater' for calculation of required cover to prestressed and nonstressed reinforcement. This may lead to slightly greater cover requirements for PSC girders used in low-level bridges, especially in pockets where silt may be trapped and remain damp after submergence. A judgement must be made whether siltation due to submergence will occur frequently or only
rarely in the life of the bridge. For example, submergence ever fifty years of an intermediate-level bridge may be acceptable whereas submergence every second year of a low level bridge would be totally unacceptable. Steel trusses and suspended decks are considered unsuitable for low or intermediate level bridges as these structures readily collect debris mats to a greater depth than solid section decks while debris or floating objects may damage slender bracing or support components. Such superstructures should only b} use for high-level bridges. 4.3 Bearings & Hold-down Hestraints Generally, the size of bridge ~:.1anschosen for typical low-level bridges will permit the use of elastomeric bearings without v.arranting use of the higher load capacity of more sophisticated spherical, rol.er or pot type ber.rings. However, pot type bearings may be sometimes appropriate for intermediate-level bridges os these bearings can be economically provided with hold-down restraints. This may prove advantageous where high lateral loads must be resisted by submerged bridges incurring reduced overturning stability.
It is usual to provide separate anchor bars. Typically 24 nun diameter galvanized holding down bolts are specified between each P.S.C. plank on composite slab structures while more substantial • angle brackets may be specifie -,i to straddle each P.S.c. girder on composite decked structures. Shear blocks or dowels connected between end diaphragms and abutments or piers will prevent a submerged superstructure from lateral displacement. If elastomeric bearings are usee the holding-down bolts should be suitably de-bonded and an air space left above the heads oftP.~ bolts, to allow the bearings t deform under live load. Frictional restraint against creep of elastomeric bearings is di minished during flood submergence due to reduced bearing reactions fron. superstructure buoyancy: Under the influence of rotation from hogging effects this creep usually moves the bearings towards the centre of the girder spans. I
It4 Substructure The foundations for piers and ~~'Iutmentsof all bridges crossing fast flowing waterways should either be keyed on to rock or cr.' t on piled footings. This avoids the possibility of undermining of the base material under the f .undation footing pads by the formation of adjacent local scour holes. Eddies also tend to wasp. out fill behind abutments. Local scour from turbulence around piers may be reduced by hydraulically shaping the pile caps and columns.
36 In particular, the piers and abetment structures of low-level bridges will be subject to a greater risk ofundennining of the foundations by scour. Where the substructure restrains the superstructure from overturning and uplift effects the piles may require design for tension and uplift resistance. If driven piles are used they should be driven well below local scour level to a sufticient depth and sufficiently bard set to resist uplift. Where vertical piles are placed in closely spaced groups account must be taken of the soil acting as a solid block around the incividual piles. Spread footings or bored piles on rock should have either a sufficient socket length into rock or be provided with rock anchors. Solid rather than framed piers may be chosen to provide an advantage of greater mass to resist overturning effect. Framed piers also tend to collect more debris. Solid circular or elliptical pier columns could be used to avoid the horizontal 'lift' force effects generated on blade type piers angied to the direction of stream flow. Spill-through type abutments are generally preferred because of the smoother shaping effect of the front and side embankment batters in the stream flow. Portions of embankment washed out can always be replaced after tile floods recede, whereas damage to the abutment structure itself is to be avoided. These spill-through abutments must be adequately drained with weep-holes, geofabrics and gravel drainage layers, and as well, the embankment batters protected against scour. Adequate drainage works are especially critical for the stability of cantilever, cell (boxed) and Reinforced Earth-type abutments subject to flood submergence. However the use of Reinforced Earth type abutments is not recommended where stream velocity is such that scour could undermine the base material or eddies could wash out the infill. Abutment details should include a sill drain behind the bearings if staining from water seepage over the front face of the abutments is of concern. Abutment approach slabs subject frequent submergence should comprise reinforced concrete slabs laid on a free draining grovel base. Geofabrics should be used to prevent drainage layers clogging with silt. The embankments adjacent to the abutment must be properly drained and batter slopes reduced where possible to avoid instability effect with rapid draw-down offlood waters. Internal granular lenses may be incorporated to assist in drainage after flood submergence. Highly expansive clay fill must not be used under approach slabs to avoid flexing orthe slabs after periods of flood submergence.
4.5 Batter Protection Overtopping flow oflong duration at frequent interval is likely to cause failure of pavements as well as scouring of embankments, especially adjacent to the bridge abutments. Stream velocities over embankments adjacent to the abutments are increased where the road approach embankments direct flood waters towards the bridge opening. More substantial, but costly, embankment batter protection may be required at the abutments than that used along the road approaches, to protect the bridge structure from flood damage. Protection works of embankments adjacent to abutments will be similar to those for high level bridge and may include rock fill, rip-rap enclose din wire cages, concrete revetment mattresses or rigid reinforced concrete slabs as well as sheet pile toe walls or spur dykes. However abutment batter protection of low-Ievel bridges not only must be secured against stream bed scour but must be secured aga.nst scour effect during overtopping,
37 Selection of the form of embankment protection against scour is governed by : (a)
Whether flow across the embankment is free or submerged.
(b)
Under free flow conditions, whether plunging or surface flow occurs on the downstream embankment batter.
(c)
The relative cost of protection works against the degree of protection required.
Grassing of batters may be adequate when the velocity of water flow over the embankment is less than 1.5 metres/sec. Generally this is the case when tailwater levels are not more than around 300 mm below the downstream edge of the road formation when overtopping first occurs. Grass batters may not be suitable where frequent overtopping occurs for a period of more than two or three hours during floods. Grass batters are not suitable for shaded areas under the bridge superstructure. More elaborate protection works against scour are detailed similarly to the requirements for causeways. It is essential all th~ following protection works are either taken below the anticipated stream bed general scour level or local scour hole depths using cut-off trenches, or are provided with level aprons extending i-rto the waterway to accommodate scour erosion. In particular. batter protection works subject to flood submergence must also be anchored along their upper edges to resist scour erosion during overtopping. Geotextile filter cloths are required beneath either flexible or rigid forms of protection to avoid leaching of fine material underlying the protective layer by piping, jets or eddies. Rock fill or hand packed rock placed on batters is the oldest type of embankment protection. uhough costly it provides a flexible treatment which is capable of deforming without loss of integrity. Rock fill is now preferred as it is cheaper to place and accommodates embankment displacement more easily than handpacked rock without sacrificing protection capacity against scour. Rip-rap enclosed in wir..: cages also provides a flexible treatment and permits the use of stone of small size. In highly corrosive conditions such as salt water, PVC coated wire is used but if the stream carries a bed load of boulders, which may damage the wire cages, this type of protectionmay not be suitable. Siltation within the cages and the growth of a protective cover assists in stabilizin& the rip-rap. Rigid reinforced concrete slabs are suitable for extreme conditions such as a very low tail water depth at overtopping, and highly erosive material at the toe of the embankment. Care is required in their design and construe ..:ion to resist cracking induced by temperature changes and embankment deformations. N. well, their design must ensure that adequate open joints or weep holes are provided to relieve ~ydrostatic pressure and reduce uplift forces. The selection of the type of protection to be used will depend on cost as well as the degree of protection required The depth of cut-off'trenches and the length of apron, choice of geo-textiles and drainage layers, whether more substantial protection is required behind or adjacent to the abutment must be determined {or each job. Finally, design technique is sometimes used in flood plain crossings with long approaches. The level of the bridge and the entire road approach embankment, is placed above (say) the 20 year (ARl) serviceability flood level hut a chosen section of the approach embankment is placed at a slightly lower level. When overtopped by rising floodwater, this section of the embankment is designed to be scoured away, the breach thus acting as a 'fuse-plug' and preventing submergence of the adjacent bridge structure.
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.
o. _
Ar:
I!''-"I'ln ;) .\I "...~,- )II\ I c-
r-
'. -,
.
.J
'" i· ,- I.::1 I"() r
1 c' .:J 35 "0 ,. ~'r,
.
0.817.611.7 6.0.1 10"0 "-I'O-I~3 11.5
~ 1.5
_________
8,4
i.:,)
1.0
0.5,1.0 ---J
O.
16.5
_
4"4
.r
---------------------
30 year shrinkage
x
t; )
150
300
GC:J: J
- .-
f-3C·j· 400 -350 ·350 ·300
300
lndoor
exposure
in the UK
~'-
I
I'
.. _.-l
_.. .,------. --1
!~ i 25
1---:
150 150
lor .. 100
100 '-50-
--0 - 1-0-
-51
'----'
I
:
i-O
I
62.5 25100
75
-1-1~50
~',)
I
40
30
Arnuient
rr!l:ni\'<,
o
4
70
tllJll
i
80
4.3
The
100
90
oidi t y (%)
Drvu .• shrinkage of n orm al-vvc ujt i t conc.rric (I""" ,. 'o tcs to concrete of rior m.r] W()rk.llir!tty Wlill water content of al/lut 190 11m) ShrinkaGe may',,· r"'Jd"lt-d .," pr oporuo oa: vvuhrr : range of 150 to :~30 ilrll' Fiaur'c
8110)
10
i
!
S)!ri}ka~c
i~--' 50 GO
15·-
25 12.51-5
II~ I
37.5 25.0
1f---{
I ~t--j--j'--
20
50.0 20
Swelling
200
30
125
r---1~-
: i
I
35 75.0
=ti -I JJ 1_
- 15(
200
40
150
-1--1---,---1
250 -20l
45
100 87.5
~
0.-
GOO
150 300
I I __ [-200 I
:
2S
-200
bo
section thickness (nim) of
Outdoor exposure
---=-r--J'~'·-'--Ili
250
f-50-
6 month shrinkage X 106 for an effective
graph
llS ;;
.!~"
«
----------
•
EFFECTIVE AREA
SECTION
DIVIDED
THICKNESS
BY TIlE
eXPOSED
IS
Ti\Kr~N
lJERlIvlETCH.
-
r\s
TV·nCE
THE
CROSS
SECTIONAL
