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The Structural Engineer Vol. 64A No. 8, August 1986

Viewpoint

The new concrete Code - is it all it’s cracked up to be?

A. N. Beal BSc(Eng) CEng MlStructE MICE  R. H. Thomason & Partners

The recent history of Codes of Practice for structural concrete work in the UK has hardly been happy. For evidence of this, one need look no further than the sorry case of CP110 [1], which was introduced as a new Code in 1972 and withdrawn in 1985, while the predecessor it was meant to replace, CP114 [2], is still in use today. It is widely accepted that CP110 did not live up to expectations, earning a reputation of being cumbersome to use and requiring over-complex calculations for simple problems. CP110’s successor, BS8110, has been a long time in preparation - partly because a completely new text was prepared, with the intention of producing a more acceptable document. Has it succeeded? Despite its substantial improvements in presentation, the omens are not promising. Already, a ‘design manual’ has been published by the Institution [4] and a draft revised CP114 has been published by the Campaign for Practical Codes of Practice [5], both claiming to be simpler to use and more in line with practising engineers’ needs. Only time will tell whether BS8110 will gain the general acceptance that eluded its predecessor.

However, the debate over presentation and approach has tended to divert attention from technical content. In fact, the technical content of BS8110 is less than perfect and it may be advisable to pay more attention to this - particularly if, as intended, it becomes the only Code available.

1. Although the rules for accepting concrete strength are now completely changed from those in CP110 and the purposes of the materials factor have been altered somewhat, the value of γm in BS8110 is unchanged at 1.5. This does beg the question of whether there was any scientific basis for the value in CP110 - or whether there is one now in BS8110.

2. Table 2.1

The overall safety factor for earth retaining structures has been reduced from 1.6 x 1.15 = 1.84 (CP110) or 1.8 (CP114) to 1.4 x 1.15 = 1.61 in BS8110. In practice, earth pressures tend to be among the least well-defined of all the loads considered by the designer - is it logical to give them the lowest safety factor? Conversely, where stress is due to combined dead and wind loads, the required safety factor has been increased from 1.8/1.25 = 1.44 in CP114 to 1.61 in CP110 and BS8110. On the one hand, is there any evidence that a safety factor of around 1.4 is inadequate for wind load? On the other, should the permissible stress due to permanent earth pressure be the same as that for 1-in-50 years 3s wind gust? Most curiously of all, the presence of imposed load is normally taken to justify increasing the overall safety factor from 1.61 (dead load only) to 1.7 or 1.8; however, when wind is involved the presence of imposed load is taken to justify reducing the overall safety factor from 1.61 to 1.2 x 1.15 = 1.38. Furthermore, the safety factor required against overturning by wind is 1.4/1.0 = 1.4 when only dead load is present, yet for when dead, imposed, and wind load are present, it reduces to 1.2/1.2 = 1.0!

This table is at the heart of BS8110, defining safety levels throughout. Some of the safety factors it gives are difficult to reconcile with past practice - or even with one another.

3. Cl. 3.3.5

Durability is an area where great advances have been claimed. However, the most serious changes in BS8110 concern indoor concrete, where remarkably few problems have occurred in practice. For indoor work, the minimum cover and concrete strength have been raised from 15 mm and 20N/mm² in CP114, through 25 mm and 20 N/mm² or 15 mm and 30 N/mm² in CP110 to 25 mm and 30N/mm² or 20 mm and 35N/mm² in BS8110. Although BS8110 claims that cube strength is the best way to specify durability, the paper [6] its recommendations were based on showed that specifying grade 30 concrete can result in cement contents anywhere between 250 kg/m³ and 430 kg/m³. It generally results in a figure in the region of 300-350 kg/m³ - much richer than most present indoor concrete, which tends to have 240-300 kg/m³. These increases in cover and the richness of mixes for indoor work will substantially increase costs of buildings - sample designs suggest that plain continuous slabs designed to BS8110 will actually be heavier and more costly than designs to the current CP114, despite the lack of durability problems in the latter. Is this necessary, and is it progress?

4. Cl. 3.4.5.8 concerns shear near supports: ‘Account may be taken of enhancement in any situation where the section considered is closer to the face of a support or concentrated load than twice the effective depth, d . . . to be effective, tension reinforcement should extend on each side of the point where it is intersected by a possible failure plane for a distance at least equal to the effective depth, or be provided with an equivalent anchorage’. Does this mean we can reduce links close to supports (although we know this is the failure zone in tests)? What anchorage is ‘equivalent’ to the effective depth? Why is this anchorage satisfactory in beams, while in pilecaps ‘full anchorage’ is required (cl. 3.11.4.4.)? When asked to comment on this BS8110 clause, a leading expert on shear described it as ‘a terrible mess’ [7].

5. Cl.3.5.2.3 gives a simplified method for slabs, allowing design and detailing to be based an ‘all spans loaded’ for certain slabs. It allows this to be applied where there are cantilevers of length up to one-third of the adjacent slab span, with live load up to 1.25 times the dead load. If detailing is based on this, the cantilever will be adequate only under perfectly uniform loads and it will fail if the load on the cantilever is greater than that on the adjacent span; a live load of 1.25 times the dead load applied to the cantilever but only dead load on the back span would mean a factor of safety of 1/(1 + 1.25) = 0.44 against failure! Even if the detailing rules given in cl. 3.12.10.3 were followed, for a cantilever of one-third the length of an adjacent single-span the safety factor (LL = 1.25 DL) would be only about 1.1; if the other end of the single span was continuous or if it had a similar cantilever, the factor of safety against cantilever failure would be only about 0.8.

The recommendations given in this clause are potentially extremely dangerous.

6. The standard moments for flat slabs (Table 3.19) are now restricted in application; they also reduce the top reinforcement in column strips at internal columns from the traditional 46% of panel moment to 33% of panel moment. Not only will this increase difficulties with shear (reducing the strength of a given slab by 10%) and make cracking more likely, but it also implies a moment redistribution of at least -34 %, which is outside the limit of 30% specified in cl. 3.2.2. Also according to a recent paper [8] the recommendations for designing shear reinforcement in flat slabs could result in insufficient links in some situations.

7. As in CP110, BS8110 tells us (cl. 3.8.2.3) that column reactions can be calculated ignoring elastic shear, even though this can easily increase column loads by 10-20%. Either some columns must be unsafe or else most are overdesigned. Which is it?

8. The design of slender columns in BS8110 (cl. 3.8.3) is still a complicated iterative process. Recent research by the author [9] has shown that the results from the CP110/BS8110 method are not very accurate - and that a simpler method can give more accurate results. Detailed discussion of this must await publication but in the meantime it may be useful to note that BS8110’s method gives more accurate results if the factor K given in eqn. (33) is set as 1.0 in all cases.

9. The design of prestressed beams and slabs with unbonded tendons (cl. 4.3.7.3), as in CP110, appears to provide an overall safety factor of only 1.4-1.6 against bending failure, as γm is not brought into play; indeed, excessive loss of prestress could reduce even this figure. Surely an overall safety factor nearer the traditional 1.8 or 2.0 is necessary?

BS8110: Part 2 has its flaws too:

11. Cl. 2.2.2 allows safety factors to be derived by statistical methods but then requires these to lead to probabilities of failure ‘similar to those implicit in the use of the factors given in BS8110: Part 1’. However the safety factors in BS8110: Part 1 are based on ‘calibration with preexisting practice, together with a subjective assessment of the relative uncertainties inherent in the various aspects of loadings and strength’ - they have no defined probability of failure. Therefore, the first condition cannot be satisfied. What is the point in saying design may be based on statistical methods when it is clearly not possible?

12. The limiting lateral deflection of frames under wind loads given in cl. 3.2.2 (H/500) is much stricter than the figure used in steel designs to BS449 (H/325). Is such a strict limit necessary?

13. A procedure for fire engineering calculations similar to those in the Institution’s ‘red book’ is given in section 4.5. In this method, design is based on calculating reduced strengths of sections in a fire. The Institution’s publication gives a set of charts for estimating the temperature of reinforcement in a fire and another for estimating the effect these temperatures have on strength; BS8110: Part 2 cl. 4.5 gives only the latter - only half of the information needed. It is thus virtually useless.

The above is not simply an exercise in ‘nitpicking’ - the points noted are substantial and serious and will affect designs in many situations. The confusion and inconsistency in safety factors noted in (2) is particularly bad, as the complication of the partial factor format can really be justified only if it leads to results that are better and more logical than normal permissible stress design; in view of the time and effort that has gone into developing limit state Codes and the claims that have been made for their system of partial safety factors, it is surprising to find that BS8110 gives results that make less sense than the Codes it is intended to replace (CP114, CP115, and CP116). Perhaps before we rush to write computer programs, books or Eurocodes - or to withdraw CP114 - we should try to resolve at least some of the problems in BS8110.

References

1.  CP110 The structural use of concrete, London, British Standards Institution, 1972.

2. CP114 The structural use of reinforced concrete in buildings, London, British Standards Institution, 1969.

3.  BS8110 Structural use of concrete, London, British Standards Institution, 1985.

4. Manual for the design of reinforced concrete building structures, London, Institution of Structural Engineers, 1985.

5.  CP114 revised A draft Code of Practice for the structural use of reinforced concrete in buildings, London, Campaign for Practical Codes of Practice, 1985.

6.  Deacon, C. and Dewar, J.: ‘Concrete durability - specifying more simply and surely by strength’ Concrete, February 1982.

7.  Regan, P. B.: letter to the author, 15 March 1985.

8.  Regan, P. E.: ‘Shear combs, reinforcement against punching’, The Structural Engineer, 63B, No. 4, December 1985.

9.  Beal, A. N.: ‘The design of slender columns’, forthcoming paper in ICE Proceedings, Part 2.

10. Design and detailing of concrete structures for fire resistance, London, IStructE and Concrete Society, April 1978.

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