Drift spectrum: Measure of demand for earthquake ground motions

A new measure of the demand of earthquake ground motions on structures is presented. This measure, called the drift spectrum, is particularly useful for near-field earthquake motions that contain distinct pulses. Like the well-known response spectrum, the drift spectrum is based on a linear system model. However, the new spectrum differs from the response spectrum in that it is based on a continuous shear-beam model rather than a single-degree-of-freedom model. The continuous model more accurately predicts interstory drift than does a single-degree-of-freedom model for pulse-like excitations. The drift spectrum could be used for design in a manner analogous to the use of the response spectrum. Examples of near-field drift spectra for the Landers, Northridge, and Hyogo-ken Nanbu earthquakes records are provided. The nature of these spectra is discussed and conclusions are drawn regarding the specification of drift demand for structural design.