ACI 318 Moment Transfer Strength and Stiffness Considerations

摘要

The 2005 Edition of ACI 318 contains a new provision in Sec. 21.11.5 that is aimed at reducingthe potential for punching shearfailures 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 shearand 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 their relation 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 transferto columns) and Sec. 13.5.3 (reinforcement requirements for moment transferto 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 forthe 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.