Return Period of Design Ground Motions

Abstract

Risk assessment and/or mitigation are the ultimate goal of any hazard analysis. Scientists engaged in the analysis of seismic hazard should know the end use of their analysis in calculating the design strength of buildings, and the risk to life and property from the code-designed buildings. Figure 1. The 5% damping, 500-year return period acceleration response spectrum for a site. Buildings designed according to the seismic provisions of 2O03 IBC (ICC, 2003) or ASCE 7 (2002) are expected to protect lives and keep damage at repairable levels when subjected to the design ground motions (BSSC, 2001). The return period of the design ground motions is not apparent in the building codes, however. Therefore, the risk (frequency of losses) from the code-designed buildings cannot be readily assessed. The seismic loads in 2003 IBC and ASCE 7 are based on probabilistic seismic hazard analysis (Cornell, 1968; Reiter, 1991; McGuire, 2004), but the probabilistic values are adjusted by several factors to obtain the design loads for buildings. The effect of these factors on the return period of design loads is examined in this paper. The following two sites are used to facilitate discussion: Both sites are assumed to be on firm rock (National Earthquake Hazard Reduction Program [NEHRP] Site Class B). The site-specific response spectrum of ground motion is an important tool in the seismic design of buildings. It allows an engineer to estimate the forces in a building due to ground motions caused by future earthquakes. Figure 1 shows a 500-year return period, 5% damping acceleration response spectrum for a site. (The acceleration response spectrum is a plot of peak acceleration induced in an elastic single-degree-freedom [SDF] system of period T and specified damping.) An SDF system of mass m , natural …