International Concrete Abstracts Portal

This paper describes and evaluates the punching shear provisions of ACI318-05, CSA A23.3-04, Eurocode 2 (2003), DIN 1045-1 (2001), BS 8110-97 and CEB-FIPMC 90 for interior columns without moment transfer and interior, edge and cornerconnections with moment transfer. All code column slab punching shear predictionswith moment transfer are extensions of the no-moment transfer provisions - any lack ofconservatism in the no-moment transfer provisions are reflected in the moment transfercomparisons.The validity of the interior connection, no moment transfer, punching shear provisionswere evaluated using the information from the data bank developed by the PunchingShear Working Group of fib Commission 4. The code provisions for interior columns withmoment transfer and edge columns connections under gravity loads were comparedwith data taken from the University of Alberta Research Report No.223 (Afhami,Alexander and Simmonds 1998). The code provisions for corner column slabconnections were compared with data from Zaghool (1971) and Walker and Regan(1987).BS 8110, DIN 1045-1 and CEB-FIP MC 90, which have size effect and reinforcement ratioterms and use control perimeters 1.5d, 1.5d, and 2.0d from the column, have smallercoefficients of variation than ACI 318 and CSA A23.3 for interior column slabconnections. However, these codes do not have general provisions for punching undercombined shear and moment transfer for edge and corner connections. Their simpleshear stress multipliers are limited to gravity load moments. Calculated statisticalindicators show that only ACI 318 satisfies the requirement of a 5% fractile valuegreater than one for concentric punching shear. The shear resistance coefficients of BS8110, CEB-FIP MC 90, CSA A23.3-04, and DIN 1045-1 should be reduced to meet a 5%fractile value of unity.ACI 318 and CSA A23.3 should be modified to include size effect terms and use the cuberoot of the concrete strength. A minimum flexural reinforcement ratio of 0.75% shouldbe specified in regions susceptible to punching shear. The consequences of permittinguse of a rectangular control perimeter have to be acknowledged.

This paper starts with a review of the major international codes with regardto the effect of the flexural reinforcement on the design equations for punching shearof two-way slabs. A review of experimental programs in which the main parameter isthe flexural reinforcement ratio follows. In addition to the flexural reinforcement ratio,the type and grade of reinforcement, the arrangement of the reinforcement and theconcrete cover are also investigated.Evaluation of the parameters investigated shows a distinct decrease of the punchingshear resistance with decreasing reinforcement ratio. If shear failure occurs beforeflexural yielding has developed, the value of the yield strength does not affect theshear resistance of slabs. A concentration of the flexural reinforcement in the vicinity ofthe column resulted in a strength increase only if the reinforcement was well anchored,otherwise the reduced bond of closely spaced reinforcing bars will not lead to anincrease in the punching shear strength. The test results on concrete cover are not fullyconclusive, but it is argued that it is the effective depth rather than the slab thicknessthat governs the shear and flexural behavior of a slab.

This paper presents some code expressions for the punching shear strengthof slab-column connections. The influence of slab thickness (size effect), columnaspect ratio and concrete compressive strength are investigated by examiningexperimental results.

The 2005 Edition of ACI 318 contains a new provision in Sec. 21.11.5 that is aimed at reducing the potential for punching shear failures at columns in two–way-slabs located in regions of high seismic risk or in structures assigned to high seismic performance or design categories. The provision requires that slab-column connections in two-way slabs without beams, where the slabs are assumed not to contribute to the lateral resistance of the structure, shall be provided with slab shear reinforcement unless: (1) it is shown that punching will not occur under the design shear and the moment induced between slab and column at the design displacement; or (2) the design story drift does not exceed a limiting value that is a function of the ratio of the design shear to the nominal shear strength of the connection under gravity loading. This paper presents an overview of the ACI 318-05 provisions for moment transfer to columns, summarizes background information on the existing provisions and the irrelation to the new provision and summarizes existing information on how to calculate the lateral load stiffness of two-way slab structures in accordance with ACI Code 318-05 requirements. The paper describes how to utilize the provisions of Sec. 11.12.6 (shear resulting from moment transfer to columns) and Sec. 13.5.3 (reinforcement requirements for moment transfer to columns and moment redistribution between columns) to determine if the first alternative for application of the new provision can be satisfied. The paper summarizes the background information for the new provision and also discusses the relation between these provisions and those of Sec. 21.12.6 for intermediate moment frames consisting of two-way slabs without beams.

Reinforced concrete slabs exhibited interesting behavior under severedynamic loads associated with impact and explosions. During the 1980s, high explosivesimulation techniques (HEST) were used to study the behavior of shallow-buriedreinforced concrete boxes under the effects of nuclear explosions. Those tests showedthat, under specific conditions, the roofs could fail in a direct shear mode within aboutthe first millisecond of loading. Those incidents were studied also numerically, andgood behavioral models were developed to predict this phenomenon. The behavior ofstructural concrete slabs under localized impact was studied more recently bycombining numerical simulations and precision impact tests, as well as by addressingthe pressure-impulse characteristics for such structures. These studies enabled abetter understanding of structural concrete slabs behavior under severe dynamic loads,as presented and discussed in this paper.