Performance Evaluation of Special RC Moment Frames against Collapse Considering Soil-Structure Interaction

In this paper the study of FEMA-P695 on collapse performance of moment frames is extended to take soil-structure interaction into account. Special reinforced concrete moment frames being 4, 8, 12, and 16 stories tall resting on firm, medium, and soft soils are considered. The incremental dynamic analysis is performed under several strong ground motions to determine the fragility curves of the same structures both on rigid and flexible bases. Modeling of the flexible base is performed using the beam-on-nonlinear-foundation method in which the underlying soil is replaced with materially nonlinear no-tension springs. The comparative study reveals that for keeping the collapse probability lower than a certain value, the maximum acceptable design spectral acceleration including soil-structure interaction has to be considerably smaller than that on a fixed base. The reason is shown to be happening because of larger drifts in the lower stories of a building when its base is flexible. Moreover, modified fragility curves are developed for fixed-base structures in which effects of soil-structure interaction are included. A simple tool for performing such a correction is also presented. The correction is shown to be quite distinctive in certain cases.