Secant Modes Superposition: A Simplified Method for Seismic Assessment of RC Frames

Abstract eng:
Dealing with seismic demands at life safety and collapse prevention limit states implies the necessity of explicitly considering the nonlinear behavior of structures. The nonlinear time-history analysis is the most rigorous procedure to compute the inelastic seismic demand, unfortunately this method is still too complex for widespread professional use. Therefore, the use of nonlinear static procedures (NSP) is going to be favored by the engineering practice. A simplified nonlinear assessment procedure, named Secant Modes Superposition (SMS) method, is presented herein focusing on bi-dimensional frames. The quantities of interest (floor displacements, inter-storey drifts and storey shears) are derived from the combination of the modal contributions as the modal response spectrum method does in the linear case. An iterative procedure is used to define the stiffness of an equivalent linear system and the spectral modification coefficients accounting for the hysteretic energy dissipation. Some response indices are used to check convergence of the method assuring the compatibility between the quantities called into play. Comparing the seismic response of some plane frames as estimated by nonlinear dynamic analyses and by using the SMS method, the simplified procedure seems to be able to reasonably assess their nonlinear behavior. Additionally, the plastic hinge distribution is obtained from the iterative procedure: an useful tool to locate potential local collapse mechanisms. Worth to mention is the extremely reduced computational effort.

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