In the 1994 Northridge earthquake in California, parking structures constructed using precast concrete diaphragms appeared to suffer more damage than other structures. The presence of ramps and openings combined with long floor spans led to complex load paths and interdependencies in the flow of lateral forces through the floor diaphragms to shear walls. This paper examines in-plane strength demands, interstory drifts, and force distributions in the diaphragms of parking structures using three- dimensional, inelastic dynamic analyses. The parameters considered in the dynamic analyses included various degrees of diaphragm flexibility, shear wall overstrength, and number of stories. The effects of shear-wall layout and earthquake direction on the seismic response of diaphragms were investigated. The analytical study showed that current code provisions underestimate diaphragm force demands and interstory drifts for common structural configurations. It also showed that the current code provisions are insufficient for representing the complex in-plane force paths associated with the shear-wall layout andearthquake direction. Finally, design recommendations to provide sufficient strength and stiffness for diaphragms were suggested.
