Lateral Load Stiffness and Strength Requirements for Slab Structures

Abstract

ACI 318-05 contains a new provision 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 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. The reason for this new provision is described along with recommendations as to how to calculate the lateral load stiffness of two-way slab structures in accordance with ACI Code 318-05 requirements. Also described is 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 can be satisfied. Finally, the relation between these provisions and those of Sec. 21.12.6 for intermediate moment frames consisting of two-way slabs without beams is discussed.