A simple replacment for the drift spectrum

The elastic drift spectrum displays, as a function of period, the story drift (expressed as the ratio of the story drift to story height) that a ground motion record would cause in a multistory framed building. It is a pivotal instrument for performance-based earthquake engineering. Wave theory for a uniform shear beam is utilized for calculating this spectrum, but it requires that records of the ground velocity and displacement should be available. Alternative but less rigorous expressions of the drift spectrum for the ground story level of a multistory frame are developed. This set of formulations requires only that the displacement spectrum, or equivalently the acceleration spectrum, for which the basic input is the ground acceleration, should be at hand. We compare these expressions against both the rigorous formulation and other simple techniques. The error that the proposed formulation represents appears to be acceptable even when the comparison is done for near-field accelerograms where the drift demand is substantial.