This technical note explores the physical and mathematical basis for the drift demand spectrum proposed in the 1990's by Iwan, and reveals several unexpected features of the original mathematical formulation. It is shown that the solution presented by Iwan does not provide the drift response of an isolated uniform shearbeam; instead, the solution corresponds to an analytical model in which the cantilevered shearbeam is attached to external springs and dampers anchored to a fixed reference point. Two alternative mathematical formulations are presented based on more realistic analytical models, and the results are compared with those obtained by Iwan's formulation. Advantages and disadvantages of all three formulations are discussed.
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