10 PUNCHING SHEAR STRENGTH - HSE

Transcription

10PUNCHING SHEAR STRENGTH10.1INTRODUCTIONThe failures at H2, I2 and J2 at Pipers Row all showed the classic features of punching shearfailure, as do most of the subsequent column failures. In this section comparisons have beenmade between the original CP114 design estimates of punching shear strength in 10.2, and theBS8110 design and appraisal procedures in 10.3. More fundamental estimates of the strength,as built and with deterioration and repairs at the time of failure, have been carried out basedon research on punching strength in 10.4 and on detailed finite element analyses in 10.5.There is a body of specialist literature [e.g. 40-42] on the evolution of the shear design rulesin CP114, CP110 and BS8110 and on the testing flat slabs to induce punching shear failures.BRE have carried out a detailed review [H216] of these publications and the source researchas it relates to the particular features at Pipers Row and Prof Regan has carried out acomplementary survey [H222]. From this, BRE have developed a best estimate for thestrength of the Pipers Row column heads ‘as built’, with the effects of deteriorationdeveloping progressively down from the top of the slab and with repairs.The strength of the slab in punching shear has been determined using CP114, the originaldesign basis, and BS8110 the current design code and also the basis on which assessments ofcurrent strength are made when following IStructE ‘Appraisal of Existing Structures’ [32]approach. These calculations have been carried out by BLS in 1964 [H190 - 201] for theoriginal design, H&S for their report [H2] to NCP after the collapse in May 1997 and insubsequent calculations [H5 - 9] August 1997, by AV for this study and by SS&D. [H209 –213 & 215]One objective has been to identify the range of interpretations of the code rules to assess themagnitude of ‘the inaccuracies in assessment of the resistance of sections’, which is one ofthe uncertainties covered by the γm in BS8110. This should lead to improved guidance on themost appropriate basis for future appraisals of flat slab and deteriorating concrete structures.The codified rules are relatively straightforward to apply to a conventional in-situ flat slabwith uniformly spaced reinforcement. With the lift slab configuration, a range of more or lessappropriate interpretations can be made which can give rise to ‘inaccuracies’ which erode thepartial factor. Many of these interpretations are not inconsistent with the wording in CP114and BS8110, which can be ambiguous or ill defined when applied to unusual structures orthose which were not designed to meet BS8110 layout and detailing assumptions.The factor of safety γm also needs to cover variation in concrete strength and, with γf, the‘inaccuracies’ which arise from the geometric differences between the ‘as-built’ structure andthe ‘as-designed’ structure shown on the drawings. Design stresses are calculated on the basisof the nominal geometry, but the specification tolerances give a scatter on strength. The γmfactor also needs to cover the wider variation found on site.115

10.2DESIGN FOR SHEAR TO CP114.When CP114 was written, relatively little experimental work on shear had been carried out and simpleempiricism was used in calculating the strength in shear and in particular for punching shear strengtharound columns in flat slabs. The deficiencies of CP114 in shear became a cause célèbre in structuralengineering and major programmes of research were carried out by Regan[41], Chana [42] and othersin the UK which provided the basis for the progressive development of more reliable strength clauses inCP110, BS8110 and BS5400. CP114 shear rules should never be used in appraisal as the basis fordetermining the strength of a flat slab in shear.The CP114 check is based on comparing the column reaction V under working load with the strength ata working stress of 100lb/in2 (0.69N/mm2) for the 3000lb/in2 (20.5N/mm2) concrete specified on thearea of depth equal to the lever arm around the shear perimeter at 0.5 x the thickness of the slab fromthe column or drop.The principle limitations in CP114 for punching shear are:1.CP114 ignores the transfer of moment from the slab into a column, which causes the shearstress on one side to be increased. This is explicitly considered in BS8110.2.CP114 ignored the effect of flexural reinforcement on shear strength, which it related simplyto concrete strength. BS8110 relates the permissible shear stress to the area of steelcrossing the shear perimeter, slab depth and concrete strength.3.CP114 perimeter at half the slab thickness from the column and the use of the lever arm forthe effective depth carrying shear has been superseded in BS8110. The effective shear depthof slab and the shear perimeter around the column were redefined in BS8110 withadjustment to the permitted shear values and there is an additional check close to thecolumn.4.The particular requirements for edge column shear design were not covered in CP114, butare given specific treatment in BS8110.5.The treatment of holes in the slab adjacent to a column is not included in CP114, but there isa requirement in 329 that “an opening should not encroach upon a column head or drop”10.2.1British Lift Slab calculationsIn the original BLS design to CP114 in 1964 punching shear was checked for the four types of shearhead. Only Type 1 (at J2) and Type 4 (at H2 and I2) shear heads were used in the area that collapsed.The 10” x 10” columns (254mm x 254mm) Types A and F, as at J2, had a rectangular 18” by 16.5”(457mm by 419mm) Type 1 collar shear head. The column load for roof design was 43,200lbs,(192kN) which BLS calculated gave a working shear stress of 67lb/in2 (0.46N/mm2) compared to the100lb/in2 (0.69N/mm2) permitted. This corresponds to a ‘working stress’ strength of 287kN in theabsence of holes. In the available calculations there are no checks on the holes adjacent to J2, perhapsthe reserve of strength was considered adequate to permit this.116

The 12” x 12” columns (305mm x 305mm) Types E & I, as at H2 and I2, had a 36” by 18.5” (914mm by470mm) Type 4 collar shear head with angles protruding 9.75” (248mm). The column load for servicedesign for the roof was 97,000lbs (431kN) which BLS calculated gave a working shear stress of 97.5lb/in2 (0.67N/mm2), compared to the 100lb/in2 (0.69N/mm2) permitted. This corresponds to a ‘workingstress’ strength of 443kN.In these calculations BLS took 8.25” (210mm) as the thickness of the slab for determining the perimeterand calculated the lever arm as 0.85 x 8.25”. The 8.25” is not the thickness of the slab, actually 9”(229mm ), nor the slab depth to the top of angle at 8.125” (206mm) , nor the distance from the averagedepth of the T1 and T2 reinforcement at I2 to the soffit at 7.5” (190mm) , nor the distance from theaverage depth of the reinforcement at I2 to the top of the angle at 6.625” (168). The value of 0.85 forcalculating the lever arm is a typical value within the range of those derived using methods permitted byCP114.BLS did not calculate the effect of misfit on the height of the supports on the column reactions, but thereare calculations dated 15/8/64 [H193] on the effect of a 3/8” (10mm) differential in level during liftingon the moments in the slab.10.2.2Interpretations of CP114 for Lift SlabsDespite its simplicity CP114 strength calculation is open to a wide range of interpretations when appliedto Pipers Row. These are of importance as some clients have instructed their engineers to carry outassessments on the basis of the original design, rather than the current codes which IStructE [32]recommends as the appropriate basis for strength assessment.In interpreting codes, designers tend to adopt the most economic interpretation, ie that which maximisesthe calculated strength of the section. This is balanced by a tendency to use the code simplificationswhich are conservative. If a section fails in appraisal on the basis of the simple method, then it is oftenreappraised to give a ‘better’ answer. It is often the interpretation of the wording in the code, rather thana consideration of the underlying principles of structural behaviour, which guides the engineer’sapplication of a code. Increasingly code checks are based on entering numbers in boxes in generalisedsoftware which gives added scope for mistakes when used on an unconventional structures.In evaluating CP114 and reviewing the interpretations which H&S adopted in their analyses after thefailures and which AV used in their checks for this study, the objective has not been to decide that oneparticular interpretation of the flawed CP114 is ‘right’, but to examine how wide a range of strengthestimates might be reasonably derived from the code.The H&S report to NCP in May 1997 included checks on the slab at J2 and H2 to CP114, wherecompliance was reported, and BS8110 checks which are discussed below. Following further discussionswith HSE, H&S carried out some further CP114 checks in August 1987 to examine the sensitivity todifferent assumptions and to include the full set of 12 columns in the area of collapse.Amey Vectra reported, in their calculations 1300-218-C100[H210], their CP114 assessment of the slabin flexure and shear. AV were left to develop their own interpretations of the code and their assumptionscover a wide range. In making comparisons and drawing conclusions from these studies a limitednumber of examples have been recalculated on a standard reference set of assumptions.117

The thickness of a normal flat slab is unambiguous and the treatment of drops is clear in CP114.However the setting of the collar angle with its top surface 7/8” (22mm) above the slab soffit, 8.125”(206mm) below the top of the slab and 6.625” (168mm) below the average centroid of the top steel, hasled to four interpretations in sizing the perimeter at ‘thickness’/2 from the ends of the shear head anglesbased on:i)full slab thickness at 9” (229mm)ii)‘thickness’ to the bottom of angle at 8.5”(216mm)iii) ‘thickness’ to the top of the angle at 8.125”(206mm)iv) ‘thickness’ as 200mmIn determining the lever arm la , which CP114 uses as the effective depth in shear, it must be decided ifthe shear fracture surface depth should be related to the overall depth of slab or the depth to the top ofthe angle from which the shear fracture originated (HSE sketch in Appendix B of H&S report May1997). Figure 10.2 - 1 compares the shear fracture positions for a normal in-situ detail with Pipers Rowlift slab.Figure 10.2 - 1 Comparison of normal in-situ detail with Pipers Row lift slab shear head.The value of the lever arm is based on effective depth d1 which has variously been taken for I2, andsimilarly for other locations, as ranging fromd1 8.25” (210mm), as assumed by BLSd1 7.5” (191mm), the distance from the average depth of the T1 and T2 reinforcement at I2to the soffitd1 6.625”(168mm), the distance from the average depth of the T1 and T2 reinforcement atI2 to the top of the angleAV calculated d1 and lever arm la separately for the T1 bar face and the T2 bar face.118

The lever arm la in CP114 can be calculated from d1 on a range of different bases using either themodular ratio or load factor method. These can be based simply on the modular ratio and maximumpermitted concrete and steel stress giving la 0.89d1. If la is calculated on the load factor basis withthe variation of the steel As/bd on the effective width considered, together with the moments applied tothe section, the lever arm can be in the range la 0.75d1 and 0.95d1. AV pursued this in considerabledetail in their calculations with la ranging from 138 to 194mm.The treatment for holes and for edge columns, which are critical in the evaluation of J2 are not clarifiedin CP114.The projecting angles of the Type 4 shear head collar at H2 and I2 extend 7” (178mm) beyond the liftangles. They are more flexible than a column support of the same area and so may be less than fullyeffective, making it prudent to reduce the shear perimeter. It has also been argued [44] that theprojecting angles create stress concentrations which could trigger the brittle failure mode. These aspectsare evaluated in detail by BRE DIANA analysis in 10.5. In appraisal both H&S and AV haveconsidered the influence of curtailment of the angles. H&S reduced the effective length of the angles by21mm at each end reducing the shear perimeter by 2.5 %. AV in CP114 calculations reduced theeffective angle length by 90 mm each end, reducing the shear perimeter and slab strength by 10%.CP114 permitted the use of the age factor for the increase of concrete strength with time, which was acharacteristic of cements at that time. This would permit for concrete of over 1 year old an increase off cu from 1000 to 1240lb/in2, with a 14.4% increase in the permissible shear stress from 100 to 114.4lb/in2 (0.69 to 0.79N/mm2 ). Normally this enhancement would be checked by core strength tests beforeit was used in assessment. As noted in 8.3.1 a typical set of cores from the car park as a whole wouldhave justified the use of this factor. However, a set of cores specifically from the more deterioratedareas in the repaired slab in 1996 would have shown concrete weaker than originally specified.10.2.3Comparison of CP114 Estimates of Reactions and Punching StrengthThe range of permissible shear strengths calculated to CP114 are compared to the working load columnreactions Vt in Table 10.2.3 - 1. This gives the vertical reaction Vt, as distinct from the BS8110 Veffwhich includes the effect of moments as used for the comparisons in Section 9.2.In these comparisons the strength has been related to the reactions Vt from the AV CP114 sub- frameanalysis, after the redistribution which reduces the reactions at H2 and I2. The review shows that theoriginal shear design at H2 and I2 broadly complied with the CP114 design requirements, exceptmarginally at I2 if the most rigorous assumptions are made.At J2 there was a substa

5. The treatment of holes in the slab adjacent to a column is not included in CP114, but there is a requirement in 329 that “an opening should not encroach upon a column head or drop” 10.2.1 British Lift Slab calculations In the original BLS design to CP114 in 1964 punching shear was checked for the four types of shear head. Only Type 1 (at J2) and Type 4 (at H2 and I2) shear heads were used in the area