Inelastic seismic torsional response of simple symmetric structures

The inelastic seismic response of simple single-story structures, symmetric in plan in the elastic domain but having lateral load resisting structural elements (LLRSEs) of unequal yield strengths, has been studied. When yielding is initiated in such structures, an instantaneous state of torsional coupling in plan induces an additional torsional component to the response of the system. This torsional effect produces, under some circumstances, a magnification of the ductility demand of the LLRSE having lesser strength as compared to what would otherwise be expected of a single-degree-of-freedom. A parametric study has been conducted to investigate the circumstances where this amplification becomes significant, and the results of this research are presented herein. The rotational inertia is shown to have a significant influence on this transient state of inelastic torsional response. A limited investigation of single-story multi-element structures, single-story structures with complex force–displacement relationships, and simple multistory structures demonstrates that the findings and observations noted from the parametric study are equally applicable to more elaborate structures. Implications on Canadian design practice are discussed. Key words: seismic response, torsional coupling, structural symmetry, inelastic response, ductility demand, parametric study, code implications, rotational inertia.