Comparison of Procedures for Computing Seismic Displacement Demand

For the seismic design of pier and wharf structures, there are multiple methodologies permitted for determining displacement demands. The current edition of MOTEMS and ASCE 61-14 permits use of an iterative substitute structure approach for nonlinear static demand analysis. The proposed 2016 revisions to MOTEMS will also permit a simplified, non-iterative Coefficient Method to estimate nonlinear displacement demands.

This paper compares displacement demands computed from the two methods for three selected structures. The presented results indicate that the substitute structure approaches generally provide larger displacement demands compared to the Coefficient Method. A detailed examination of the background for these two methods indicates that the Coefficient Method was extensively evaluated recently in a FEMA 440 study, but the substitute structure approaches were subject to only a very limited evaluation more than 40 years ago against results from “exact” nonlinear dynamic analysis. Therefore, there is an urgent need to comprehensively evaluate the “accuracy” of the substitute structure approaches before arriving at more definitive conclusions about relative “accuracy” of these two approximate procedures.

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