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Proc. Instn Civ. Engrs Structs & Bldgs, 1993, 99 Nov., 377-385

Structural Board Paper 10264

Draft Eurocode 2: Is this the future of concrete design?

A. N. Beal, BSc, MICE, MIStructE

Associate, Thomason Partnership, Leeds

Draft Eurocodes have now been published for comment and trial use in the design of concrete and steel structures. The present Paper reviews Eurocode 2 for concrete (DD ENV 1992-1-1: 1992), considering political and economic aspects, language and quality of drafting in addition to technical considerations. Safety factors, deflection rules, detailing and crack control, column design and the ENV 206 concrete acceptance rules are assessed from technical and practical points of view. Some technical points are found to be satisfactory and improvements on BS 8110 but serious inadequacies and anomalies are identified in a number of areas. The proposal to replace current standard technical English with a completely new set of engineering terms is found to be ill-conceived and poorly executed. It is clear that, although considerable work has gone into preparation of the document, major technical amendments will need to be made and large parts will need to be rewritten for it to be of a satisfactory standard for use. The broader questions of professional responsibility and the purpose of the code also need to be resolved, and the poor quality of the current document makes these issues particularly critical for practising engineers.

Introduction

1. Eurocodes for structural design have been planned for many years and their imminent publication has often been predicted. However, two of them have now finally been published as ‘drafts for development’: Eurocode 2: design of concrete structures Part 1 [1] and the corresponding Eurocode 3: design of steel structures [2]. Eurocode 2 does not give rules for specifying concrete; these are contained in the accompanying draft: Concrete: performance, production, placing and compliance criteria [3].

2. Officially, Eurocode 2 came into effect on 15 May 1992, but the British Standards Institution (BSI) did not have copies available for sale until July 1992, and even then the price of £168 must have deterred most engineers from taking an interest. The price was later reduced but remains excessive for a draft document, and very few copies have been sold to practising engineers. As a ‘draft for development’, Eurocode 2 has been approved for ‘trial use’ and comments on it must be submitted by 15 May 1994. Historically, only a small number of UK engineers have involved themselves in the Eurocode drafting process. In theory, national publication of drafts should bring comment from large numbers of practising engineers, but BSI’s pricing policy is likely to ensure that such comment from the UK is kept to a minimum.

3. Unlike current British codes, which are basically technical in purpose, Eurocodes are intended also to play a role as part of the political and economic programme of the European Community (EC). This seeks the removal of all barriers to free trade within the EC; by replacing national codes. Eurocodes are seen as a way of eliminating technical barriers to free trade in the European construction industry.

Political and economic aspects

4. The idea that engineering codes of practice should also serve political and economic purposes is not new. The development of building regulations and codes of practice in the UK grew out of a desire to ensure that buildings were constructed to good standards, in the interests of both public health and safety, and also of the long-term national economic interest.

5. Their development was given an important impetus in the 1920s by the development of a highly competitive ‘design-and-build’ market in steel and reinforced concrete structural frames, where commercial pressure led designers to prepare ‘cut designs’, reducing their safety margins to try to win contracts with the keenest prices. The national codes of practice which appeared in the 1930s set agreed minimum technical standards, thus protecting the public from the risks of under-designed buildings and also creating a fair and responsible basis for competition between designers.

6. The development of consultant-led design in later years reduced the commercial pressure on standards of design and allowed codes of practice to become less restrictive. However, with the recent resurgence in competitive contractor-led design-and-build contracts in this country, the role of codes in enforcing adequate minimum technical standards is once more becoming important.

7. If codes give minimum requirements, engineers are still free to specify work to a higher standard if they consider that this is appropriate. If an engineer wants, say, some precast concrete flooring to be to a better specification than the code requires, he/she is under no obligation to accept inferior flooring manufactured to code minimum standards. However, if Eurocodes are to succeed in their objective of removing technical trade barriers, they must define not only minimum standards but also, in effect, the maximum standards which engineers can impose. The possible implications of this for engineers and their liability are far-reaching and have not yet been satisfactorily resolved. However, as remarked in BRE Digest 376 [4] ‘. . . as a result of single market legislation, public bodies will find it much more difficult in general, to specify performance that goes much beyond that expressed in European standards’.

Technical barriers to trade

8. Much building work is essentially local in character, tending to rely on locally-sourced labour and materials, using them in ways suited to local economic and climatic conditions. Indeed, for some aspects (e.g. the durability of external brickwork, or alkali-silica reaction in concrete), not enough is known to define requirements fully in terms of test data and specifications and designers rely instead to some degree on past experience of which combinations of local materials are known to have performed satisfactorily. It may not be possible to remove all such ‘barriers to trade’ without increasing costs unduly or compromising performance.

9. In published material about the development of structural Eurocodes, it is difficult to find a clear statement of what are the specific ‘technical barriers to trade’ which have been identified, or how they are to be eliminated. In some cases (e.g. materials standards such as steel), the answer is obvious enough but in structural design codes it is not so clear. Is it differences in technical standards, or is it the way calculations are carried out, or is it differences in language and terminology which cause the trouble?

10. The most plausible idea is that national differences in design and detailing rules inhibit free trade, so the elimination of these will help the creation of a free ‘single market’. As noted earlier, because of local factors it may not be possible for design rules to be completely uniform. But in any case this does not appear to be an objective (at least in the short term) of the current draft documents: they are accompanied by National Application Documents (NADs) in which individual countries set their own preferred safety factors, detailing rules etc.

11. As an example, the British NAD gives different reinforcement detailing rules from the standard Eurocode 2 text: Table 5.1 of Eurocode 2 gives minimum bend radii for high tensile bars as 2Φ (bars less than 20 mm) or 3½Φ (20 mm or larger), whereas the British NAD Table 8 requires radii of 3Φ and 4Φ respectively. On the other hand, for anchorage of shear links, Eurocode Cl.5.2.5 requires a minimum of 10Φ straight bar beyond a bend, whereas the British NAD permits a length of 8Φ. As standard bends and shear links are present in virtually all reinforced concrete structures, these rules ensure that structures designed to the Eurocode 2 standard rules will not be accepted in Britain and that structures designed to the British NAD rules will not be accepted in the rest of Europe.

12. If such technical differences are not regarded as a problem, then presumably there is no objection to national codes of practice remaining in use. What, then, is to be the purpose of Eurocodes? One area where an argument can be made for them is in major projects where bids are submitted internationally by competing design firms or construction consortia. Although these projects have always been an international activity, not seriously inhibited by the existence of differing national codes, it can be difficult to prepare a scheme in a short time when copies of all relevant local codes and regulations must be obtained and fully understood in the time available. A foreign engineer unfamiliar with the British bridge code BS 5400 and all the related Department of Transport memoranda would probably not find it easy at first.

13. However, this problem can be solved without the preparation of a completely new set of codes. The principles of structural engineering do not vary much between countries, so it would not be difficult to prepare a manual summarizing the different permissible stresses, partial safety factors, detailing rules, etc., for the key aspects of design in each material for the various countries of the European Community. With such a manual, engineers would be able to prepare acceptable schemes for structures in other EC countries while basing their work on codes and guidance they were familiar with already.

14. Superficially, the alternative of preparing a series of new internationally-drafted Eurocodes to fulfil the role is attractive. However, in reality, this would be a considerably more complex and difficult task to complete successfully : in addition to sorting out the details of all the different local technical requirements, there would be all the problems of creating completely new texts and ensuring that, despite language differences, these were clear, understandable and free of anomalies.

15. A Eurocode drafted for this purpose would, of course, be quite different from one drafted to eliminate differences in technical requirements or to supplant national codes. Its emphasis would be on simplicity, clarity, ease of adaptation to reflect differing requirements, and on a technical content which did not break new ground but aimed to produce results complying with existing local codes as simply as possible, even if it was sometimes a little conservative. Rather than seeking to define new harmonized technical standards for the whole continent, the objective would be simpler and more practical: ‘a code of convenience’ for engineers to fall back on as an alternative to national codes.

16. Although this has not been clearly stated, it is probably the most plausible purpose for the Eurocodes which are being drafted. However, it is not clear if this is actually the intention. Although Eurocodes are to appear in the Building Regulations as alternatives to national codes, it is proposed that for EC-funded projects the use of Eurocodes will be compulsory and that national codes will not be permitted.

17. This confusion leads to problems. If Eurocode 2 is to act as a back-up to national codes, then its limitations might be accepted but much of its language and presentation is inappropriate. However, if it is to replace national codes, then its technical content is too restrictive. Surprisingly few parts of the code permit the engineer a genuine choice of alternative calculation methods, and the traditional permissible stress method, despite its widespread popularity and proven merit, is not permitted at all. If Eurocodes are to replace existing codes throughout the EC, they should not restrict engineers needlessly; where alternative calculation methods are valid, they should be permitted.

18. One of the more perplexing aspects of the draft is its use of language. Any document drafted by an international committee is always likely to have problems in this area but, for reasons which are not clear, those drafting the Eurocodes appear to have expended considerable effort in seeking to create a completely new technical vocabulary for English-speaking engineers. Forces and moments become ‘direct actions’ and imposed deformations become ‘indirect actions’; there are also ‘permanent actions’, ‘variable actions’, ‘free actions’ and ‘fixed actions’, ‘persistent situations’ and even ‘tangential action effects’ (which, presumably, may be persistent, transient or accidental). Clause 2.3.2.2 Combinations of actions gives an idea of how some of this works out in practice: ‘P(3) Combinations for accidental design situations either involve an explicit accidental situation A (e.g. shock) or refer to a situation after an accidental event (A = 0)’.

19. The language problems in draft Eurocode 2 are dealt with in more detail later and it will be seen that there are many problems still to be resolved. However, even if these did not exist the idea that Eurocodes should require engineers to adopt a new technical vocabulary is still highly questionable. Unless a common language such as Esperanto is to be adopted for all transactions in the EC, language differences will have to be accepted as a fact of life and tinkering with the existing national languages will not really change things.

20. Standard technical English is used worldwide and is the basis of countless thousands of journals, papers and textbooks on engineering, so the idea that it acts as a barrier to free trade is very difficult to take seriously. A project to replace it with a completely new vocabulary of technical terms would be a formidable task, liable to cause all manner of confusion and almost certainly doomed to fail. It is difficult to understand why this should have become part of the Eurocode agenda. The arguments for removing it from the agenda are compelling.

Technical aspects

21. A complete technical appraisal of the draft would be a major project and hence outside the scope of the present Paper. However, detailed comment is offered on some of the aspects which have most effect on design. The areas considered are: safety factors; span/depth limits; column design; nominal reinforcement ; reinforcement detailing; the rules for concrete compliance in the accompanying draft standard ENV 206 [3].

22. Although other areas have not been analysed in detail, a brief review of other parts of Eurocode 2 has identified a number of curiosities and anomalies in both technical content and language which may be of interest. One difficulty in assessing (and using) the code is that many of the recommendations are stated in quite abstract forms, so their meaning and implications are often not obvious; this is not helped by the fact that the design procedure for an element must often be pieced together from clauses dispersed in various parts of the code.

Safety factors

23. Eurocode 2 introduces another variation on the now familiar limit state theme, where the overall safety factor is divided into two partial factors, one applied to loads and the other to materials. However, there are significant changes compared with BS 8110 [5]. In particular, safety factors on dead and live loads are lower than in BS 8110 (partial factors are down from 1.4 and 1.6 to 1.35 and 1.5 respectively) but the partial factor on wind loads is increased from 1.4 to 1.5. The other change is in the treatment of load combinations, where the usual approach of quoting lower values of partial factors (or higher permissible stresses) for combinations including wind loads is not used. Instead, there are factors to reduce loads to ‘combination values’ when two or more types of imposed load are applied simultaneously (the so-called ‘Turkstra principle’).

24. The case for reducing safety margins from present levels is dubious, particularly in a document which, as noted earlier, may be applied to some extent as a maximum standard. However, it is all the more strange when set alongside the increase in the partial factor on wind loads. The curious situation now arises that a structure designed to withstand a 1-in-50 year wind gust must have a higher safety factor than a retaining wall resisting permanent earth pressure, whereas traditionally the opposite has been the case. It is not clear whether this change was intentional or accidental, but it seems wrong in engineering terms.

25. The new treatment of load combinations sounds plausible in theory but can give rather odd results. The major problem is that it is not altogether clear what constitutes a separate loading (or ‘action’), justifying the use of a reducing factor. Are all floor loads classed as one load, or should, say, ‘people’ and ‘stock’ on a shop floor be classed as separate kinds of load? In the latter case, the code implies that we should assume that it would be unlikely for both to attain their maximum values at the same time, so one of them could be taken at a reduced value when they act together. Anyone who has visited a department store during peak Christmas trading will appreciate the fallacy of this idea, yet it is just one of the possible implications of the combination rules proposed in Eurocode 2.

26. Even if it is assumed that the only classes of imposed load which can be considered are imposed floor loads, snow load and wind load, the proposed approach can still produce odd results. For example, consider a member which is being designed against overturning caused by imposed loads and by wind load. For most such loads, the partial factor is 1.5 and the combination factor is 0.7. From the application of these to various ratios of imposed load/wind load, it can be concluded that if the loading is 100% imposed load, or if it is 100% wind load, the safety factor should be 1.5, but if the loading is 50% imposed load and 50% wind load, the safety factor can be reduced to 1.275. Is this correct?

27. A second area where these combination factors replace traditional practice is in calculating live load reductions for columns and foundations in multi-storey buildings. In BS 6399 [6] the total imposed load is reduced according to the number of floors supported, ranging from a 10% reduction for two floors to a 50% reduction for more than 10 floors. In Eurocode 2, this question is again dealt with by these combination factors: one floor is taken at full load and all others are taken at 70% load. The results from these different approaches are shown in Table 1. As can be seen, the Eurocode 2 rules would substantially increase column and foundation loads (and thus costs) in high rise buildings.








8. Viewed as a whole, the proposed changes in safety factors and approach appear rather arbitrary, the safety margins are reduced unwisely in some cases, they are increased unnecessarily in others, and they give odd variations when loads are combined.

29. The same factors and methods, although not considered here, are also supposed to be applied to imposed deformations, such as temperature and shrinkage. It seems likely that when all permutations are considered, some bizarre answers may well be generated. The problems arising from an abstract presentation of design recommendations are clearly apparent here, in that a superficially plausible set of recommendations generates erratic and inappropriate results when applied in practice.

Span/depth limits (Table 4.14)

30. Deflection in reinforced concrete structures is normally controlled by limiting the allowable ratio of span/effective depth, and the rules given for this in a code can have a major effect on structural economy, because they determine slab thicknesses and thereby the weight of the structure. It is important that the rules given are technically correct, and also that they are easy to use and suitable for use as early as possible in the design process. Basic structural dimensions such as slab thicknesses are usually decided early on, when the initial structural scheme is being prepared, and there is often little possibility of altering them later, when the results of more refined analyses might become available.

31. On the face of it, the proposed rules (Table 4.14) are simple but crude (rather reminiscent of CP 114 [7]). Two sets of ratios are quoted: one for ‘highly stressed’ members such as beams (1.5% or more steel) and one for ‘lightly stressed’ members such as slabs (0.5% or less steel). In this form, the rules are simple to use, but two-way slabs and flat slabs are made 20-25% thicker than corresponding designs in accordance with the current Institution of Structural Engineers’ ‘Gold Book’: Recommendations for the permissible stress design of reinforced concrete building structures  [8]. (The Gold Book gives results that are very similar to those in BS 8110 but its recommendations are presented in a format which makes them easier to compare directly with those in Eurocode 2.)

32. For use in Britain, Table 4.14 is replaced by NAD Table 7. This adds a third set of span/ effective depth ratios for ‘nominally reinforced’ concrete (0.15% steel). However, although this amendment solves the problem of over-conservatism, this is achieved at the expense of simplicity. Although Table 7 looks fairly simple, with only three columns (0.15%, 0.5% and 1.5% steel), if any benefit is to be taken from the ‘0.15%’ column, in practice, the appropriate span/depth ratio will need to be interpolated based on the actual slab reinforcement provided.

33. In effect, Table 7 is simply a cruder version of the old CP 110 Table 10 [9]  and it suffers from the same defect. The designer needs to know the allowable ratio of span/effective depth for a slab at the start of design, in order to determine its thickness, but the code allows the ratio to be calculated only once the section design has been completed. Where time (and cost) allow, the optimum thickness and design can be determined by a lengthy process of iteration but normally the engineer would be forced to rely instead on conservative guesswork.

34. There are now better treatments available, which do not compromise accuracy but allow design to be much more simple and direct. For example, BS 8110 bases its rules on M/bd²  rather than on reinforcement percentage, thus allowing the span/depth ratio to be checked earlier in the calculation and the ICE/ IStructE ‘Green Book’ Manual [10] improves on this by adding a table of approximate ratios for initial slab design, based on design superimposed load. Best of all, the Gold Book [8] gives a table of exact ratios based on design total loading; these ratios can be used at the start of design and need no subsequent modification. It seems a shame that Eurocode 2 has not adopted one of these, and has chosen instead an approach which is crude and almost as awkward as the old CP 110 method.












35. In Table 2, the span/effective depth ratios recommended in Table 7 of Eurocode 2 are compared with those from the Gold Book for slabs with a total characteristic load (dead plus imposed) of 10 kN/m². (BS 8110 will give results that are very similar to those from the Gold Book.) As can be seen, in addition to the points raised above, Table 7 is marred by what appears to be an error in the figure for interior bays of continuous slabs with 0.15% reinforcement. (For end bays of these slabs, the quoted ratio is 44.)

Column design (Section 4.3.5)

36. Most codes give similar results for beam design but their recommendations for slender column design often vary widely. The current British code, i.e. BS 8110, is not wholly satisfactory - its additional moment method has been criticized heavily on theoretical grounds and is in need of revision. (The Gold Book [8] uses a different method based on capacity reduction factors.)

37. Eurocode 2 also adopts the additional moment method but there are differences - and also curiosities and anomalies. The clauses are rather complex and also poorly written and laid out. However, once understood, they appear to give an additional moment formula which gives buckling moments approximately 10% higher than those in BS 8110. Also, initial imperfections must be allowed for in all cases (ea = l/400), whereas BS 8110 requires an allowance to be made only if the total load eccentricity (including the allowance for buckling) is less than 0.05h.

38. Table 3 shows the strengths of columns of various slenderness ratios designed to Eurocode 2, expressed as a proportion of section ‘squash’ load; they have been calculated for fcu = 30 N/mm², with either 1% or 4% steel placed as four bars at d = 0.8h. The results are compared with designs in accordance with BS 8110 and the Gold Book and also with the results of an accurate theoretical analysis [11].



 
















39. As can be seen, the results from Eurocode 2 are a considerable improvement on BS 8110, giving close agreement with theory for slenderness ratios up to 20, good agreement for heavily-reinforced columns up to L/b of 40, and although the results for slender lightly-reinforced columns are not accurate, at least they err on the side of caution. However, the Gold Book method is the most accurate of the three and also considerably simpler than Eurocode 2.

40. Although the basic technical content improves substantially on BS 8110, there are a number of quite serious anomalies in the text which need to be resolved before it could be regarded as acceptable for use.

(a)  No guidance is given in the main code for designing columns in unbraced (sway) frames, although these are very common in real structures and have been covered by existing codes of practice for many decades. The reader is referred to Appendix 3, which gives vague recommendations for columns with L/b less than 15; otherwise, ‘appropriate literature’ is to be consulted for guidance.

(b) According to Cl. 4.3.5.6.3(1), the ‘simplified’ method of column design is not applicable to axially-loaded columns. It applies only if the load eccentricity exceeds 0.1h.

(c)  For biaxially-bent columns (which are by far the most common in practice), Cl. 4.3.5.6.4 states that, unless Mx/h ÷ My/b ≤0.2, or My/b ÷ Mx/h 0.2, ‘a refined analysis is necessary’ and leaves the matter at that.

(d) According to Cl. 4.3.5.1(5), second-order effects (e.g. buckling) may be ignored if their effect is less than 10%. As the engineer must first calculate the effect before deciding whether or not to ignore it, this ‘simplification’ seems pointless.

(e) According to Cl. 4.3.5.5.3, buckling effects may be ignored in columns of modest slenderness. However, there is a confusion over sign conventions for moments in the text and diagrams. As a result, for an axially- loaded column, the limit for ignoring buckling may be taken as L/r of 25, 50 or 75, according to taste! The idea that buckling effects might be ignored when L/r is 50 or 75 is bad advice and it is not supported by theoretical or test evidence. The comparison given in Table 3, which gave good results, applied buckling moments at all slenderness ratios.

(f) According to Appendix A.3.4(9), the effect of concrete creep on the strength of a slender column under long-term load ‘may normally be disregarded’, and in edge columns ‘it does not decrease the bearing capacity’. These comments are highly misleading. Creep reduces the effective stiffness of concrete under long-term load (to 40% or less of the short-term value). The strength of a slender column depends almost entirely on its stiffness (remember Euler’s buckling formula ?), and theoretical analysis and numerous tests have shown that under long-term loading the strength of a slender concrete column can be greatly reduced. The comments in this Eurocode clause are wrong and should be withdrawn. According to Cl. 4.3.5.6.3(b)(6), the concrete contribution to total section compressive strength is calculated from the concrete ‘total area’ (ref. definition 1.7.2). It is not clear whether this means the net area of the section (allowing for steel) or its gross area.

Nominal reinforcement

41. The apparently trivial subject of nominal reinforcement requirements can have a surprising effect on the economics of reinforced concrete construction: they can directly affect the quantity of reinforcement in slabs and walls; significant extra cost penalties can result where mesh is used if the values chosen do not fit in with standard mesh sizes.

42. In Eurocode 2, the subject is confused and complicated by the fact that it is covered twice: once in section 4.4.2, which covers cracking and runs to no fewer than 11 pages, and then again in section 5.4, which covers detailing. To make matters worse, the treatment in 4.4.2 is highly abstract : Cl. 4.4.2.2(3) states: ‘Unless more rigorous calculation shows a lesser area to be adequate, the required minimum areas of reinforcement may be calculated from ...’, before proceeding to a formula which requires the engineer to establish the area of ‘that part of the section which is calculated to be in tension just before formation of the first crack’. If a reader can explain precisely what this means for, say, an ordinary suspended slab, their guidance would be most welcome.

43. If it is assumed that the tension depth referred to previously amounts to (say) half the section depth for a suspended beam or slab but the whole section depth for a wall or ground bearing slab, then Cl. 4.4.2 gives minimum reinforcement areas of 0.06% (beam), 0.1% (suspended slab) and 0.5% (wall or ground-  bearing slab). On the other hand, if an effective depth of d = 0.85h is assumed and section 5.4 is used, this gives minimum areas of 0.13% (beam), 0.13%/0.03% (suspended slab main/secondary) and 0.4%/0.2% (wall vertical /horizontal). The lack of any obvious relationship between the two sets of figures and the obvious anomalies and oddities suggest that these parts of the document have not been drafted properly and need to be completely revised.

Reinforcement detailing

44. For many British engineers, the major problem (and surprise) in Eurocode 2 will be the fact that it virtually bans the use of T40 reinforcing bars. There is not an explicit ban on their use, but Cl. 5.2.6.1 P(1) bans their use in sections less than 600 mm deep, Cl. 5.2.6.3 P(1) bans their use with standard bends as anchorages generally and also bans their use with anchorages of any kind in a tension zone, and Cl. 5.2.6.3 P(2) bans their use with lapped joints in either compression or tension zones. Taken together, these restrictions virtually exclude any normal use of T40 bars. In view of their widespread and largely trouble-free use over many years in this country, it is worth asking whether or not there is any good reason for such a ban.

Concrete compliance rules

45. Eurocode 2 does not give any rules for assessing concrete test results but refers instead to the draft: Concrete: performance, production, placing and compliance criteria (ENV 206). Although this is a separate document, it is nevertheless worth examining, as the acceptance rules for concrete can affect the required design safety factors.

46. The first obvious difference between ENV 206 and current British practice is the extremely low sampling rate proposed: only one sample per 75 m³, up to a maximum of 15 samples in any one day. Assessment is on the basis of sets of results: three samples for up to 150 m³ of concrete, and six samples for up to 450 m³ of concrete.

47. The sets of test results are then assessed statistically to determine if they are acceptable, the criteria varying on the basis of whether or not the producer has an approved Quality Assurance system. If the supplier is QA-approved, the criteria are: mean fcu+1.48sd, minimum fcu-3 N/mm² (six results); mean fcu+3 N/mm², minimum fcu-1 N/mm² (three results), where sd is the standard deviation of a set. For a non-approved producer, the criteria are: mean fcu+1.87sd, minimum fcu-3 N/mm² (six results) and mean fcu+5 N/mm², minimum fcu-1 N/mm² (three results).

48. Because of the very low sampling rates proposed, the risks to the customer are high: 450 m³ of concrete (75 lorry loads) may be accepted on the basis of only six cube results, with one test allowed to fall up to 3 N/mm² below the specified strength. However, the risks for the producer are also high, because this huge volume of concrete might also be completely rejected, even if most of the cube results exceed the specified strength. These problems are typical of all assessments of concrete strength based on sets of results [12] but they become particularly severe at low sampling rates.

49. In this case, the assessment rules proposed for sets of six or more results bring back some of the problems which were associated with the similar statistically-based rules found in CP 114 [7] and CP 110 [9], where sometimes a rogue high result could lead to rejection of a set of results (because it increased the standard deviation). These problems are illustrated in Table 4, which is based on cube test results taken from a real job, where the specified strength was 30 N/mm² and the supplier was not QA approved.

50. As can be seen from Table 4, although only one result fell significantly below the specified 30 N/mm², the ENV 206 [3] criteria would have required no fewer than six out of the eight lots to be rejected. If the sampling rate had been 1/75 m³, over 6000 tonnes of concrete would have been rejected, despite the fact that in four out of the six rejected lots there were no below-strength cube results at all.

51. The ENV 206 proposed rules are disturbing from two points of view: the low proposed sampling rates give only limited assurance that all the concrete in the structure is satisfactory, thus calling into question the reduced safety factors proposed in Eurocode 2; yet, on the other hand, the rules are potentially very unreasonable for suppliers. They clearly need to be revised.

















Language

52. A review of the draft Eurocode would not be complete without some discussion of the language used and the quality of the writing. As noted earlier, an attempt is being made to use the document as a vehicle for introducing a new terminology to replace standard technical English. Aside from questions of whether or not this is a necessary or worthwhile project, it is clear that there are serious problems with the text as currently drafted at a much more basic level. Large sections of it appear to be written in ‘English as a second language’ and are simply not good enough for an official code of practice. Some particular problems are listed below.

53. There is a constant muddle about the meaning of the words shall, should, will, must, may, can, might and are. As a result, there is often total confusion between what are mandatory requirements, what are recommendations and what are merely observations. There are also simpler confusions such as between ‘sustain’ and ‘resist’. Accordingly, many clauses are rather quaint and should not be taken too literally, e.g.

(a) Cl. 4.1.1(4) states: ‘For most buildings, the general provisions in this Code will ensure a satisfactory life.’ If only this were true, life would be easy.

(b) Cl. 2.1 P(1) states: ‘A structure ... will sustain all actions and influences likely to occur during execution and use ...’

(c) Cl. 3.4.1.2.2 P(1) states: ‘The strength of the anchorage devices and zones shall be adequate for the transfer of the tendon force to the concrete and the formation of cracks in the anchorage zone does not impair the function of the anchorage.’.

54. The combination of shaky English and the attempt to create a completely new technical terminology leads to some completely unwieldy terms such as: ‘design resisting axial force in compression’ (Cl. 4.3.5.5.3 P(1)), and its possible alternative: ‘resisting design axial compression force’ (Cl. 4.3.5.0). Some parts give glimpses of what sound like exciting new structural concepts, such as ‘brittle failure and hyperstrength’. However, these just seem to fade away without being properly explained.

55. The anomalies and problems of the proposed ‘action’ jargon are too numerous to list here, but they stem from an apparent unwillingness to ‘call a spade a spade’ and from the fact that an ‘action’ as proposed here may be either a load or an imposed deformation - two quite different concepts which engineers usually take care to keep separate.

56. As an example (one of many which could be given), Cl. 2.2.5 ‘Load arrangements and load cases’ reads as follows :

‘P(1) A load arrangement identifies the position, magnitude and direction of a free action.

P(2) A load case identifies compatible load arrangements, sets of deformations and imperfections considered for a particular verification.’

P(1) is wrong: some ‘actions’ are not loads. P(2) is even more obviously wrong: deformations and imperfections are never loads. Presumably, P(1) and P(2) should refer to ‘action arrangements’ and ‘action cases’, perhaps making things sound more exciting but reducing still further the chance that practising engineers would be able to understand the code.

57. Viewed as a whole, a great deal of work needs to be done before the document can be considered to meet the necessary standard of language for a serious statutory document. As currently drafted, apart from its weaknesses in basic English, the document is a hopeless muddle of the proposed new ‘action/situation’ jargon and standard technical English. To redraft the document to an acceptable level of standard technical English would be a major task; if the ‘action/situation’ jargon were retained, the task would be made almost impossible.

Technical curiosities

58. There is a distinctly uneven feel to the technical content of Eurocode 2. Some important matters are treated too briefly and crudely (and some are not dealt with at all), whereas other more minor or academic matters are sometimes covered at length. Furthermore, although most of the technical content is not too controversial, there are occasional surprises - some of them apparently reasonable and some not.

59. Some examples have already been given. However, one of the most astonishing examples, which would profoundly affect the whole of structural engineering, is Cl. 4.3.5.3.2.(1) concerning structural bracing:

‘A bracing element is a structural element which has a high flexural/shear stiffness and which is completely or partially fixed (restrained) to the foundation.’ If this is believed and implemented, it would ban the use of diagonal bracing (which is usually made from elements with very low bending and shear stiffness), and it might also exclude bracing of any of the upper storeys of a structure. This is obviously complete nonsense, but the presence of such a glaring error on a matter as fundamental as structural stability does not inspire confidence in the rest of the code.

General conclusions

60. The BSI has invited comments on the draft Eurocode, submitted as far as possible in the form of specific amendments to particular clauses. However, the problems with the document are clearly too deep to be dealt with fully in this way. The proposed new technical jargon poses all manner of problems and even without it the general standard of writing is not really good enough for a serious official document.

61. This Paper has identified a number of areas where the technical content is in need of review but it is important to note that these are based on only a relatively brief and limited investigation. It is likely that there are further problems in other parts and, because of the abstract way in which many recommendations are presented, it is also likely that some potential problems would become apparent only after a long period of familiarity and use.

62. It is tempting in a technical paper to think only of the technical aspects of the document but, as outlined in the beginning, Eurocode 2 is intended also to serve both a political and an economic purpose. Even if it were perfectly written and beyond reproach technically there would still be serious questions to be resolved about its purpose and the effect it might have on an engineer’s responsibilities. Because the current document is suspect in quality of drafting and content, these questions become even more critical.

References

1.  BRITISH STANDARDS INSTITUTION. Eurocode 2: Design of concrete structures - Part I. General rules and rules for buildings. BSI, London, 1992, DD ENV 1992-1-1:1992.

2.  BRITISH STANDARDS INSTITUTION. Eurocode 3: Design of steel structures. General rules and rules for buildings. BSI, London, 1992, DD ENV 1993-1-1:1992.

3. BRITISH STANDARDS INSTITUTION. Concrete: performance, production, placing and compliance criteria. BSI, London, 1992, DD ENV 206:1992.

4.  BUILDING RESEARCH ESTABLISHMENT. European legislation and standardisation. BRE, Garston, Nov. 1992, BRE Digest 376.

5.  BRITISH STANDARDS INSTITUTION. The structural use of concrete. BSI, London, 1985, BS 8110.

6.  BRITISH STANDARDS INSTITUTION. Design loading for buildings: Part 1 . Code of practice for dead and imposed loads. BSI, London, 1984, BS 6399.

7.  BRITISH STANDARDS INSTITUTION. The structural use of concrete in buildings. BSI, London, 1969, CP 114.

8.  INSTITUTION OF STRUCTURAL ENGINEERS. Recommendations for the permissible stress design of reinforced concrete buildings. IStructE, London, 1991.

9.  BRITISH STANDARDS INSTITUTION. The structural use of concrete. BSI, London, 1972, CP 110.

10.  INSTITUTION OF CIVIL ENGINEERS/INSTITUTION OF STRUCTURAL ENGINEERS. Manual for the design of R.C. structures. ICE/IStructE, London, 1985.

11.  BEAL A. N. The design of slender columns. Proc. Instn Civ. Engrs, Part 2, 1986, 81, Sept., 397-414.

12.  BEAL A. N. Concrete cube strengths - what use are statistics? Proc. Instn Civ. Engrs, Part 2, 1981, 71, Dec., 1037-1048; Discussion, 1982, 73, June, 515-532.

This paper is reproduced by kind permission of the Institution of Civil Engineers www.icevirtuallibrary.com

istbu.1993.25332.pdf

Floors

1

2

3

4

5

10

≥10

BS6399

1

0.9

0.8

0.7

0.6

0.5

0.4

Eurocode 2

1

0.85

0.8

0.78

0.76

0.73

0.73

Table 1. Reduction factors on imposed floor loading



Simply

Cont.

2-way

2-way

Flat

slab

Cant.


supp.


s. supp.

cont.

Drops

No drops


EC2 0.15%

34

38

34

38

41

41

14

EC2 0.5%

25

35

25

35

30

30

10

ISE Gold Book

27

36

31

44

39

36

11

Table 2. Allowable span/effective depth ratios for slabs

30N/mm²

concrete

with 1% steel



L/b

Theory

BS8110

ISE Gold Book

Eurocode 2

0

1

0.89

1

0.87

10

0.89

0.89

0.87

0.87

15

0.78

0.89

0.78

0.82

20

0.64

0.89

0.64

0.63

30

0.38

0.37

0.39

0.27

40

0.22

0.18

0.22

0.14

30N/mm²

concrete

with 4% steel



0

1

0.87

1

0.87

10

0.89

0.87

0.87

0.87

15

0.8

0.87

0.78

0.81

20

0.67

0.87

0.64

0.68

30

0.41

0.51

0.39

0.4

40

0.22

0.28

0.22

0.21

Table 3. Capacity reduction factors for slender columns

Set



Results




Mean

Mean req.

Pass?

1

38.9

47.3

35.9

34.2

40.7

34.6

38.6

39.3

no

2

34.4

43.5

38.1

30.4

36.9

37.7

36.8

38.1

no

3

46.4

30.2

41.9

45.3

37.7

36.7

39.7

41.4

no

4

31.3

39.8

37.2

39.7

40.9

29.8

36.4

38.9

no

5

32

32

35.3

33.4

41.3

41.1

35.8

38.1

no

6

36

41.3

26.4

48.8

35

41.1

38.1

44.1

no

7

40.4

35.2

41.1

34.6

32.2

38.3

37

36.6

yes

8

39.8

42.4

40.7

37.3

39.3

32.3

38.6

36.6

yes

Table 4. ENV 206 compliance rules applied to real test results