ACI 421.2R-10

Brittle punching failure can occur due to the transfer ofshear forces combined with unbalanced moments betweenslabs and columns. During an earthquake, significanthorizontal displacement of a flat plate-column connectionmay occur, resulting in unbalanced moments that induceadditional slab shear stresses. As a result, some flat platestructures have collapsed by punching shear in past earthquakes(Berg and Stratta 1964; Yanev et al. 1991; Mitchellet al. 1990, 1995). During the 1985 Mexico earthquake(Yanev et al. 1991), 91 waffle-slab and solid-slab buildingscollapsed, and another 44 buildings suffered severe damage.Hueste and Wight (1999) studied a building with a posttensionedflat plate that experienced punching shear failuresduring the 1994 Northridge, CA, earthquake. Their studyprovided a relationship between the level of gravity load andthe maximum story drift ratio that a flat plate-columnconnection can undergo without punching shear failure. Thedisplacement-induced unbalanced moments and resultingshear forces at flat plate-column connections, althoughunintended, should be designed to prevent brittle punchingshear failure. Even when an independent lateral-forceresistingsystem is provided, flat plate-column connectionsshould be designed to accommodate the moments and shearforces associated with the displacements during earthquakes.