Vertical Seismic Forces on Elevated Concrete Slabs

Abstract

The effect of the vertical component of earthquakes on structures has been the source of extensive discussion among structural engineers for a long time. The issue of considering the effect in the design of structures has not yet been resolved. The topic was once again brought into the forefront when extensive damage, which appeared to have been caused by vertical ground motion, was observed in numerous structures after the January 17, 1994, Northridge earthquake. Soon after the Northridge earthquake a major insurance company contracted with our office to initiate a structural investigation of 36 insured subterranean parking structures beneath 2 to 4 stories condominiums located in the northwest Los Angeles metropolitan area. The analytical part of the investigation included the determination of horizontal and vertical seismic loads the structures sustained during the Northridge earthquake. The seismic loads acting on the parking structures, in the horizontal direction, were determined in accordance with the“Minimum Design Lateral Forces and Related Effects” procedure of the “Earthquake Design” section of the 1991 Uniform Building Code (UBC). The seismic forces acting on the parking structures, in the vertical direction, were obtained from a simplified response spectra procedure outlined in this paper as there is no explicit requirement and method for this direction in UBC.