ACI 318 Moment Transfer Strength and Stiffness Considerations


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Title: ACI 318 Moment Transfer Strength and Stiffness Considerations

Author(s): N.M. Hawkins

Publication: Special Publication

Volume: 232


Appears on pages(s): 23-38

Keywords: concrete design codes; lateral loading; moment transfer;punching shear; seismic effects; slab-column connections

Date: 10/1/2005

The 2005 Edition of ACI 318 contains a new provision in Sec. 21.11.5 that isaimed at reducing the potential for punching shear failures at columns in two–wayslabs located in regions of high seismic risk or in structures assigned to high seismicperformance or design categories. The provision requires that slab-columnconnections in two-way slabs without beams, where the slabs are assumed not tocontribute to the lateral resistance of the structure, shall be provided with slab shearreinforcement unless: (1) it is shown that punching will not occur under the designshear 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 theratio of the design shear to the nominal shear strength of the connection under gravityloading.This paper presents an overview of the ACI 318-05 provisions for moment transfer tocolumns, summarizes background information on the existing provisions and theirrelation to the new provision and summarizes existing information on how to calculatethe lateral load stiffness of two-way slab structures in accordance with ACI Code 318-05requirements. The paper describes how to utilize the provisions of Sec. 11.12.6 (shearresulting from moment transfer to columns) and Sec. 13.5.3 (reinforcementrequirements for moment transfer to columns and moment redistribution betweencolumns) to determine if the first alternative for application of the new provision can besatisfied. The paper summarizes the background information for the new provision andalso discusses the relation between these provisions and those of Sec. 21.12.6 forintermediate moment frames consisting of two-way slabs without beams.