Choice of a Vector of Ground Motion Intensity Measures for Seismic Demand Hazard Analysis

Among the challenges of performance-based earthquake engineering is estimation of the demand on a structure from a given earthquake. This is often done by calculating an “Intensity Measure ” (IM) for a given earthquake ground motion, and then computing the probability that the earthquake will cause a given level of demand in the structure as a function of this IM. Traditional IMs include peak ground acceleration and spectral acceleration at the first-mode period of vibration. These IMs consist of a single parameter. In this paper, IMs consisting of two parameters are proposed: spectral acceleration at the first-mode period of vibration along with a measure of spectral shape (the ratio of spectral acceleration at a second period to the original spectral acceleration value). A method for predicting the probability distribution of demand using a vector IM is presented. This method accounts for the effect of collapses on the distribution of demand. Two complimentary methods for determining the optimum second period at a given intensity level are described, and an improvement in the accuracy of demand predictions is shown