Influence of Cracking on Equivalent SDOF Estimates of RC Frame Drift

Abstract

Cracking, yielding, and softening of reinforced concrete cross sections influence the response of reinforced concrete structures to strong ground shaking. The accuracy of peak roof displacement estimates made using relatively simple “equivalent” single-degree-of-freedom (SDOF) models is investigated. Peak roof drifts of four multistory frames were determined by nonlinear dynamic analysis using high-fidelity models, in which cracking, yielding, and softening of components are explicitly represented in the multidegree-of-freedom model. Estimates of peak roof drift are made using low fidelity “equivalent” single-degree-of-freedom models, in which nonlinearity is represented using simple stiffness-degrading models that capture yielding and the initial stiffness of the model is set equal to the cracked stiffness. Roof drifts are examined for the four frames, each subjected to a suite of ten ground motion records scaled to five intensity levels, representing ordinary “far-field” motions. As might be expected, the influence of the uncracked stiffness on the accuracy of peak roof drift estimates was found to diminish with increasing roof drift. Of greater significance is that the uncracked stiffness had a negligible effect for roof drifts greater than approximately twice the yield drift. The simple estimates were reasonably accurate at these drift levels.