Seismic Torsional Provisions: Influence on Element Energy Dissipation

Code torsional provisions primarily aim to limit the additional response of lateral load-resisting elements arising from plan eccentricity. Intensive research has addressed this issue exclusively by evaluating additional ductility and deformation demands in plan-eccentric or torsionally unbalanced (TU) structural systems, and comparing the results with well-established seismic design criteria. This paper adopts an alternative approach toward providing complementary data for code evaluation by first analyzing the distribution of hysteretic energy dissipated by the inelastic response of the various lateral restraining elements, and assessing the influence of various seismic code torsional provisions on this energy absorption distribution. Second, in order to evaluate the adequacy of code torsional provisions in restricting inelastic damage effects in TU systems, an energy dissipation index based on the equivalent hysteretic ductility is proposed. This index provides a first step in the development of a widely applicable inelastic damage index for such systems. It is concluded that the distribution of seismic energy dissipation demand on the critical edge elements is greatly influenced by the dynamic torsional response arising due to asymmetry, and also by the form of codified torsional provisions. The flexible-edge element in systems designed according to the Uniform Building Code (UBC) and the New Zealand code may incur considerable additional inelastic (hysteretic) energy dissipation demand, compared with the equivalent torsionally balanced (TB) systems. For the stiff-edge element, the New Zealand and European code provisions, which permit large reductions of element strength, may induce exceptionally large hysteretic energy dissipation demand and, hence, this element is potentially more vulnerable to seismic damage. Conversely, the UBC provision, which permits no strength reduction in the stiff-edge element may be overly conservative, with very small amounts of hysteretic energy being dissipated. These conclusions are in close agreement with the results of previous studies by the writers and other researchers, which utilized the peak element ductility and deformation demands as measures of dynamic inelastic torsional effects in plan-asymmetric buildings.