Drift Spectrum vs. Modal Analysis of Structural Response to Near‐Fault Ground Motions

Abstract

A new measure of earthquake demand, the drift spectrum has been developed as an adjunct to the response spectrum, a central concept in earthquake engineering, in calculating the internal deformations of a structure due to near‐fault ground motions with pronounced coherent pulses in the velocity and displacement histories. Compared in this paper are certain aspects of the elastic structural response to near‐fault and far‐fault ground motions. It is demonstrated that (1) the difference between drift and response spectra are not unique to near‐fault ground motions; these differences simply reflect higher‐mode response, which is larger due to near‐fault ground motions; (2) response spectrum analysis (RSA) using existing modal combination rules can provide an estimate of structural response that is accurate to a useful degree; (3) these modal combination rules are similarly accurate for near‐fault and far‐fault ground motions although the underlying assumptions are not satisfied by near‐fault excitations; and (4) RSA is preferable over the drift spectrum in computing structural response because it represents standard engineering practice and is applicable to a wide variety of structures.