Design Considerations for Slab-Column Connections in Seismic Zones

The transfer of shearing forces and moments between concrete flat slabs and columns can produce brittle punching failure. Slab-column connections must satisfy adequate strength against punching failure. In seismic zones, the connections are expected to undergo deformations into the inelastic range and, hence, it is necesary to design connections with adequate strength and ductility. In addition, the connections must be able to undergo a specified limit of interstory drift without punching failure. The ductility and drift requirements that must be adhered to in design are discussed. Tests reported in the literature show that the strength under cyclic moment transfer is less than the strength under monotonic loading. For an earthquake-resistant structure, the design must be based on the strength under cyclic loading. Test results are reviewed and discussed. As an application, a hypothetical structure is designed according to American Concrete Institute (ACI) Building Code (ACI 318-89). The structure is subjected to the 1940 Elcentro ground motion, and time-history dynamic analysis is performed. The results show that to obtain adequate strength, drift capacity, and ductile behavior of a slab-column connection without shear reinforcement, it is necessary in most cases to increase substantially the slab thickness in the connection region beyond the minimum thickness required to control deflections by providing shear capitals. The disadvantages of shear captials can be avoided by using shear reinforcement.