EC2 and BS8110 compared Dr R M Moss BSc PhD CEng MICE MIStructE, Building Research Establishment Rod Webster CEng FIStructE, Concrete Innovation & Design This paper is intended for publication in the Structural Engineer Synopsis This paper is intended to raise awareness amongst the Structural Engineering Profession of the forthcoming Eurocode for the Design of Concrete Structures EC2 which will in a few years replace the existing British code BS8110. The two codes are compared in the context of design of primary structural elements and information is given on the availability of design aids to assist the practitioner in becoming familiar with and using the new code. Keywords Concrete, structure, design, codes of practice, standards, aids Introduction The ENV version of Eurocode 2 Part 1 General Rules and Rules for Buildings has been around for some years as a draft for development. A National Application Document (NAD) was prepared to be used in conjunction with the ENV and together with the main document was published by BSI in 19921. The intention was that this document would be trialled on real structures and it is referred to in Approved Document A of the Building Regulations. However use of the existing ENV is believed to have been very limited. As part of the wider European Harmonisation process and linked to the requirements of the Construction Products Directive, considerable effort has been expended in recent years in converting the ENV version of the code into a full EN. The EN status means that eventually the code will become normative if and when it is accepted by formal voting. For practical purposes normative in the UK means that it will have the same status as and eventually replace BS8110.
Timetable for the introduction of EC2 The EN versions of Part 1 and 1.2 of the code dealing with fire have been through several draft revisions and are now being finalised1. The target date for publication of these Parts of the code is Summer 2003. There will be National Annexes to accompany each part of the code, which will include values for what are called Nationally Determined Parameters. In a similar way to the NAD the intention is that the code will be used with the appropriate National Annex in each member state. These National Annexes are in course of preparation and it is intended to make them available for Public Comment through BSI when the main code is issued. Grades of concrete EC2 allows benefits to be derived from using high strength concretes, which BS8110 does not. Concrete strengths are referred to by cylinder strengths, which are typically 10-20% less than the corresponding cube strengths. The maximum characteristic cylinder strength fck permitted is 90N/mm2, which corresponds to a characteristic cube strength of 105N/mm2. Materials and workmanship Part 1 of EC2 specifically does not cover this and a separate standard (termed an Execution Standard) has been prepared. This is currently in ENV form and a national document based on the existing National Structural Concrete Specification2 is in preparation and is expected to be available towards the end of 2003. One issue, which does need to be considered as part of the main code, is the tolerance on cover. Cover to meet durability and bond requirements is specified as a minimum value with a tolerance of up to 10mm to be added on top. This is in contrast to BS8110 where cover is specified as a nominal value and a tolerance of 5mm accepted. In situations where good quality control is exercised there is scope for reducing the tolerance. Note that cover for fire requirements needs to be considered separately. Design for fire This paper does not consider this topic, and in making comparisons between BS8110 and EC2 assumes that covers and dimensions of members are largely unaffected by the changed design process.
1
The full references are EN1992-1-1 Eurocode 2: Design of concrete structures - Part 1: General rules and rules for buildings and EN 1992-1-2 Eurocode 2: Design of concrete structures Part 1.2: General rules – structural fire design
In Part 2 of EC2 there is a prescriptive method encompassing simplified approaches based on covers and member dimensions, which is broadly similar to the approach taken in BS8110. There are however also more sophisticated performance based methods that can be used and the information is much more extensive than in Part 2 of BS8110. It is envisaged that a specific How to design leaflet4 will be prepared to assist engineers in relation to this topic. Some further guidance on this topic can be found in a paper prepared by Professor Narayanan3. Design for durability In making comparisons between BS8110 and EC2 this paper assumes that covers are largely unaffected by the changed design process. Simplified guidance to enable engineers to specify appropriate grades of concrete for particular exposure conditions with appropriate covers is in course of preparation and will be included within the National Annex to the code. There is a difference in approach with theoretically at least durability issues being considered more explicitly. For example the code has classifications based around potential deterioration mechanisms and the designer should identify the most severe conditions in any particular case, rather than simply assessing the environmental exposure. The concept of an explicitly defined design life and the recognition of the need to take additional measures if this design life is required to be significantly exceeded must be seen as a positive step forward. Material Partial Safety factors As with BS8110 EC2 uses a basic material partial safety factor γm for concrete of 1.5. Several years ago the material partial safety factor for reinforcing steel in BS8110 was reduced from 1.15 to 1.05. EC2 uses a value of 1.15 although this is subject to a National Annex. This is unlikely to have any practical impact however as steel intended to meet the existing yield strength of 460N/mm2 assumed by BS8110 is likely to be able to meet the 500N/mm2 assumption made by EC2, so that the design yield strength fyd will be virtually identical. Design values for loading In due course these will be given by EC1. The comparisons made in this paper in general consider only the resistance side of the equation although some mention is made of the partial load factors to be used. It is worth noting that a value of 25kN/m3 is taken for the density of normal weight concrete as opposed to the currently assumed value of 23.6 kN/m3.
The combined impact of the partial load factors in conjunction with values for basic design loads and other items such as column load reduction factors and the assessment of slenderness in column members has been considered as part of a separate small-scale study in relation to a whole building design. The building studied was a typical RC framed flat slab structure. The conclusions from this study were that, at least for the particular building studied, the overall impact of using EC2 instead of BS8110 was minimal. Design of flexural elements at the ultimate limit state The design of flexural elements to EC2 is in practice very similar to that of BS8110. Where EC2 differs, as with the ENV, is that it does not generally give element specific design guidance, but more the general principles to be applied. This approach should be welcomed, as it is less restrictive and may encourage innovative design methods. Several options are given for the type of stress-strain relationship that may be assumed for concrete. In many cases the designer is likely to opt for the simple rectangular stress block. The stress block used in EC2 is compared with that in BS8110 below.
EC2 stress block
BS8110 stress block
There has been some debate as to what is the most appropriate value to take for αcc. The recommended value in the code is 1.0 but it is likely that the UK National Annex will require a value of 0.85 to be used. The parameter η has been introduced into EC2 and in combination with modification of the value for λ has the effect of reducing the allowable concrete force for higher strength concretes (above C50/60). For concrete strengths up to this value λ=0.8 and η=1.0. The following basic equations may be derived for the design of elements in flexure.
K= z=
M bd 2 f ck
K '=
α cc xmaxηλ λx d − max 2 2 d γc
γc d (min K , K ') ≤ 0.95d 1 + 1 − 2 2 ηα cc
M 2 = bd 2 f ck (K − K ') ≥ 0 M2 As2 = f sc (d − d 2 ) f M −M2 As = + As 2 sc f yd z f yd In these equations xmax is the maximum neutral axis depth permissible before compression steel is to be provided. This in turn depends on the amount of redistribution assumed. The effect of redistribution is dependent on the concrete strength with one set of values up to and including C50 and a differing set of values for higher concrete strengths. The values are subject to a National Annex, and the UK recommended values for strengths up to C50 lead to the following equation: xmax = (δ - 0.4)d where for example δ =1.0 means no redistribution and δ = 0.8 means 20% redistribution. This is basically the same equation as in BS8110. It may also be considered advisable to set some upper limit on xmax regardless of the amount of negative redistribution (i.e. redistributed M being greater than elastic M). The effect of redistribution is also dependent on the ultimate compressive strain of the concrete, which for strengths above C50 reduces from 0.0035. M2 is the additional moment to be carried by the compression steel. fsc is the design stress in the compression steel, which for concrete strength grades up to C50 may be calculated from: fsc = 700((x-d2)/x) ≤ fyd
Parametric studies have been carried out looking at the impact of the different stress block on the design of rectangular beams using linear elastic analysis with limited redistribution. In these studies αcc was taken as 1.0 and the redistribution formula was taken as xmax = (δ - 0.4)d with xmax limited to 0.6d. The conclusion from this study was that there was very little practical difference between EC2 and BS 8110. This conclusion can also be reasonably extended to solid slabs designed using linear elastic analysis with limited redistribution. Span/depth ratios In both BS8110 and EC2 the allowable span/depth ratio depends on concrete strength and tension and compression reinforcement ratios. The attached flowcharts show how the permissible span/depth ratio is arrived at in each case. A detailed parametric study on span/depth ratios has been carried out comparing the provisions of the two codes in relation to the minimum permitted depth of rectangular beams for a given span. The influence of increasing the allowable tension steel was considered by allowing a maximum increase of 100% (i.e. double) that required for the ultimate limit state, although there is no upper limit stated in EC2. 20% redistribution was assumed for all continuous spans. The study showed that EC2 tended to be more conservative at low concrete strengths. However EC2 permits much higher span/depth ratios for cantilevers where a low reinforcement percentage is used, even restricting the maximum enhancement in steel area. In practice however, economic rather than minimum permissible depths will generally be used, and these are very similar in both codes. Shear When checking normal shear, EC2 is the same as BS8110 in that there is a shear stress below which only minimum shear reinforcement need be provided. In EC2 as in BS8110 this shear stress depends on concrete strength, effective depth and tension steel ratio. The recommended design shear stress of the concrete alone for comparison with the values of vc given in Table 3.8 of BS8110 is: v Rd ,c =
0.18
γc
k (100 ρ l f ck )
1
3
≥ 0.035k
Where k = 1 + √(200/d) ≤ 2
ρl = As /(bd) ≤ 0.02
3
2
f ck
The value 0.18/γc and the expression for the minimum concrete shear stress are subject to the National Annex. Choosing a value of 0.12 for 0.18/γc in the above equation, and the expression for concrete shear stress as given above, BS8110 generally allows a higher shear stress before shear steel is required. Because of the minimum shear stress that can be carried, EC2 can however allow higher shear stresses for low reinforcement percentages and this effect is accentuated the higher the strength of the concrete. EC2 differs from BS8110 in that above the limit at which the concrete alone has sufficient capacity, the designed shear steel to be provided is determined ignoring the contribution from the concrete. The design method used is known as the variable strut inclination method and is based on a truss model.
For members not subjected to axial forces the required area of shear steel needing to be provided in the form of links at a distance d from the support face is given by: Asw /s = VEd /(0.9d fywd cotθ)
This compares with the BS8110 equation: Asv /sv = bv(v-vc) /fyvd The designer should choose an appropriate angle θ (the angle between the assumed concrete compression strut and the main tension chord) to use in the model. The limits on θ are between 22 and 45 degrees such that cot θ is greater than or equal to1 but less than or equal to 2.5. The maximum shear capacity depends on θ. The lowest possible value of θ (maximum cot θ) should therefore be chosen within the limits above. As with BS8110 there is also an upper shear stress which cannot be exceeded. In BS8110 this limit is 0.8√fcu ≤ 5 N/mm2. In EC2 this corresponds to taking θ = 45 degrees, which gives a recommended upper limit to the shear stress for non-prestressed members of: 0.45 ν fcd where ν = 0.6(1-fck/250) fcd = αccfck/γc If the design stress of the shear reinforcement is below 80% of the characteristic yield stress fyk, ν may be taken as:
ν = 0.6 up to C60 ν = 0.9 - fck/200 > 0.5 for grades above C60 The two codes have been compared choosing values of αcc = 0.85 and γc =1.5 and ignoring the increase allowable for ν if the stress in the shear steel is restricted. EC2 will allow a smaller maximum shear capacity at low strengths but a higher capacity at higher strengths principally arising from the cut off of 5 N/mm2 in BS8110. The increase in the allowable shear stress becomes quite significant when increased values of ν are permitted even ignoring the cut-off in BS8110 as illustrated in Figure 1.
Figure 1: Comparison of maximum permissible shear stresses 14
12
2
Shear stress (N/mm )
10
8
EC2 EC2 shear steel stress restricted BS8110 (40N/mm2 limit removed)
6
4
2
0 0
10
20
30
40
50
60
70
80
90
100
fck
For a given required shear capacity the amount of shear steel to be provided when designing to EC2 is dependent on cot θ which should be maximised as stated above. In practice the following inequality needs to be satisfied: 1 ≤ cot θ =
ω + ω2 −4 2
≤ 2 .5
0.9b w dνf cd V Ed Indirectly the concrete strength can therefore influence the amount of shear steel provided if cot θ needs to be less than 2.5 to satisfy the criterion on maximum shear capacity.
where ω = cot θ + tan θ =
The two codes can in general be expected to give similar results in terms of the number and spacing of links to be provided. Design of compression elements at the ultimate limit state EC2 does not give separate guidance on the approach to be used to designing a column under a known combination of moment and axial force. For practical purposes as with BS8110 the rectangular stress block used for the design of beams may also be used for the design of columns. However unlike with BS8110 the maximum compressive strain when designing to EC2 will be less than 0.0035 if the whole section is in compression and will fall to half this value (fck ≤ 50N/mm2) if the section is subject to pure compression as illustrated below. This will affect the steel strains and hence forces which the steel can carry.
N-M interaction charts for a 300mmx300mm section with these assumptions have been produced taking a value of αcc= 0.85 and give close agreement between EC2 and BS8110 as illustrated in Figure 2. The horizontal cut-off line on the EC2 curve has little practical effect, as it will normally fall within the zone of minimum applied moment.
2
N/bh (N/mm )
Figure 2: N-M Interaction charts for C35/45 concrete d/h = 0.82 (alpha cc = 0.85)
40 35 30 25 20 15 10 5 0
BS8110 4T32 EC2 4T32
0
2
4 2
6
8
2
M/bh (N/mm )
. Dealing with slenderness The first step in deciding whether a column is slender is to determine the effective lengths in both directions. The effective lengths are in turn dependent on whether the column may be assumed to be braced or unbraced (“nonsway” or “sway” in EC2 terminology). BS8110 provides tables of values of β with assessment of the end conditions that are appropriate. β can range from 0.75 to 2.2. EC2 appears more complicated in that an assessment needs to be made of the relative flexibilities of the rotational restraints at each end of the column. However this process can be simplified by making conservative assumptions.
Having determined the effective lengths the slenderness ratios can then be calculated. In BS8110 the limits on slenderness ratio lex/h and ley /b are 15 (braced) and 10 (unbraced). In EC2 the allowable slenderness ratio λ is calculated from l0/i where i is the radius of gyration of the uncracked cross section. For a rectangular section ignoring the reinforcement this simplifies to λ =3.464 l0/h where l0 is the effective length. The slenderness should be checked in both directions. Where the column is slender when designing to EC2 and using the nominal curvature method which it is probably the most straightforward, the final design moment is increased by the additional moment to account for second order effects. Once this adjustment has been made the N-M interaction charts may be used as before. The same approach is used for BS8110 except that the second order moments will be calculated differently. Biaxial bending EC2 states that a separate design may initially be carried out in each principal direction. Imperfections need be taken into account only in the direction where they will have the most unfavourable effect. No further check is necessary if: λy / λx ≤ 2 and λx / λy ≤ 2 and (ey/h)/(ex/b) ≤ 0.2 or (ex/b)/(ey/h) ≤ 0.2 ex and ey are the effective total eccentricities including second order effects. If biaxial bending needs to be considered the following simplified criterion may be used: (MEdx/MRdx)a + (MEdy/MRdy)a ≤ 1.0 MEdx,y = Design moment of resistance in the respective direction including second order effects MRdx,y = Moment of resistance in the respective direction a = exponent dependent on geometry When it is necessary to consider biaxial bending BS8110 states that symmetrically reinforced rectangular sections may be designed to withstand an increased moment about one axis. It is known that this approach can be unsafe in extreme circumstances, so the introduction of the above equation in EC2 should be welcomed.
Strut and Tie models These are beyond the scope of this paper. However it is hoped to include some guidance on this in a future paper underpinning the provisions within the National Annex for the code. Robustness and tying requirements This is currently covered in the section dealing with detailing requirements. The UK has pushed for and has had accepted National Annex provisions for all forms of ties except vertical ties, allowing the requirements to be brought into line with BS8110. This issue will need to be revisited in the light of the current proposed revisions to Approved Document A of the Building Regulations. Flat slabs EC2 Part 1 now has an Informative Annex dealing with flat slabs which was noticeably absent from the ENV version. The widths of column and middle strips are the same as in BS8110. The percentages of moments carried by these strips are given as ranges but the BS8110 values fall within these ranges and hence may still be used. The other major issue when designing flat slabs is dealing with punching shear. The code provisions in EC2 dealing with this topic have recently been revised and it is believed worthwhile to revisit the implications of these. Initial indications are that EC2 is marginally more economic, mainly because the link arrangements are more efficient. Detailing of links should also be easier. Simplified load combinations and load cases The complete set of possible load combinations and load cases is obtained from EN1990 Basis of Structural Design. In practice these can be simplified greatly for the design of everyday building structures. For practical purposes the UK National Annex is currently permitting the simplified load combinations of all spans and alternate spans loaded as per BS8110 to be considered sufficient in the majority of cases. For slabs the UK National Annex is currently permitting the all spans loaded condition to be considered sufficient subject to the restrictions as currently imposed in BS8110. A major difference between the two codes is the partial safety factor appropriate to the dead load for unloaded spans. Three load combination equations are permitted in EN1990 dubbed 6.10, 6.10a and 6.10b. Which equation is used has a bearing on the load factors and the more complicated expressions 6.10a and 6.10b can offer some additional potential economies to the designer.
In the simplest case using the basic equation 6.10 the values may be summarised as below. In the table γG is the partial load factor appropriate to dead loads and γQ that appropriate to imposed (live) loads. Loaded spans: Unloaded spans:
EC2
BS 8110
γG = 1.35, γQ = 1.5 γG = 1.35
γG = 1.4, γQ = 1.6 γG = 1.0
Strictly speaking the above table relates only to the design of loaded spans. The design of unloaded spans should theoretically be considered separately taking γG = 1.0 on all spans, but in practice this is very rarely likely to prove the governing load case. Detailing issues It is believed that spacing rules may lead to more and smaller bars, unless crack widths are checked. There is a requirement that beam top steel should be distributed across flanges (both tension and compression). Availability of design aids A suite of practical design aids to assist practising engineers to become familiar with and apply the code is currently in course of preparation. These include: 1. A set of Excel based spreadsheets, to complement the existing highly popular set of spreadsheets to BS8110 produced by the Reinforced Concrete Council (RCC) 2. A series of How to Design Leaflets explaining the basic design concepts for primary structural elements available on-line and to be freely distributed. 3. A concise code summarising the key information within the code required for everyday use and appropriate values from and references to other supporting codes 4. Worked Examples for the Design of Concrete Buildings A helpline facility is planned to be set up so that frequently asked questions can be answered and a dedicated website www.eurocode2.info is now on-line and will be expanded to provide links to available sources of information. This will complement other activities such as the RCC’s Calcrete Computer Aided Learning package.
Concluding remarks 1. The advent of EC2 as for the other Eurocodes will have a big impact on the design of all types of structures. There will be a learning curve associated with gaining familiarity and using the new code. 2. To make this as painless an exercise is possible, the concrete industry in conjunction with BRE, are producing design aids and information to assist the profession, and can answer detailed queries, by way of answers to frequently asked questions posted on the above website. 3. In general EC2, used in conjunction with the National Annex, is not wildly different from BS8110 in terms of the design approach. It gives similar answers and offers scope for more economic structures. 4. There will be an opportunity for comment on the values proposed for the Nationally Determined Parameters to be included within the National Annex, before it is published. 5. Overall EC2 is less prescriptive and its scope is more extensive than BS8110 for example in permitting higher concrete strengths. In this sense the new code will permit designs not currently permitted in the UK, and thus give designers the opportunity to derive benefit from the considerable advances in concrete technology over recent years. The authors believe that, after an initial acclimatisation period, EC2 will be generally regarded as a very good code and a step in the right direction. Acknowledgements The authors would like to acknowledge the funding for this work provided by the ODPM and the BCA. The paper is endorsed by the Concrete Industry Eurocode 2 Group (CIEG) referred to in Reference 4. References 1. DD ENV 1992-1-1:1992 Eurocode 2: Design of concrete structures Part 1. General rules and rules for buildings, BSI 1992. 2. National Structural Concrete Specification for Building Construction, Second Edition, BCA Publication 97.378, July 2000. 3. EUROCODES, Proceedings of the Institution of Civil Engineers, Civil Engineering Special issue Two, November 2001, Volume 144. 4. Pal please provide final reference for your paper
