Seismic behavior of precast parking structure diaphragms

Six parking structures collapsed during the 1994 earthquake in Northridge, California. Most of the damaged parking structures employed precast concrete systems in which the floor system is intended to serve as a stiff diaphragm between shear walls. However, large distances between shear walls and the flexibility of joints between precast elements may result in a floor system that is quite flexible. This paper presents a study of the role of the diaphragm deformations in the seismic performance of these structures. Several significant issues regarding the behavior of the parking structures were identified. These include large drift demands on columns due to excessive deformations of the diaphragms; shear wall configurations that cause the diaphragm to twist in plan amplifying these deformations; and cross-sections in critical locations that have insufficient strength to meet the earthquake demands. Possible retrofit concepts are presented.