On the Seismic Design of Inelastic Asymmetric Systems

Abstract eng:
Design codes incorporate seismic torsional effects via elastic static or dynamic analyses, and seem to favor three-dimensional elastic modal procedures. However, several studies concluded that current code provisions may overestimate or underestimate the design shear forces, leading to inconsistent ductility demands. In particular, it has been reported that the modal method underestimates the design forces for members affected favorably by torsion. This paper presents an analytical study of a model consisting of a rigid deck with uniformly distributed mass, resting on three vertical bilinear inelastic elements in the direction of the input ground motion. The input consists of 87 records from hard to medium hard Californian sites, and also 66 records from the soft lakebed of Mexico City. The parameters studied are: ratio of uncoupled torsional to translational frequencies, Ω; normalized static eccentricity, es/r; elastic symmetric natural period, T; and design target ductility, µt. The IBC and the Mexico City Codes are evaluated by statistical analyses of ductility demands in asymmetric systems. It is found that on average the IBC provisions overestimate all design forces while the static regulations of Mexico City overestimate design forces for the flexible side elements, but underestimate the forces for the stiff side elements. The three-dimensional modal analysis always underestimates the design shear for elements on the rigid side. New amplification and reduction factors for the static design eccentricity are proposed that lead to ductility demands which closely approximate the target ductility in all elements.

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