Consequence of Strength-Dependent Stiffness on Traditional Seismic Design Approach

Traditional seismic design approach uses initially assessed natural periods as fixed designparameters for any structural system ignoring its dependency on original strength providedto meet elastic strength demand computed on the basis of such periods, divided by the response reduction factor. This implies the consideration of a constant stiffness obtained due to initial period while a varying yield displacement inversely proportional with responsereduction factors. However, a series of recent studies shows that the reality being completely otherwise, the yield displacement of any structural system remains almost constant making assigned strength and hence, resulting stiffness both varying inversely with response reduction factor considered in the design. This reality puts a question mark on the validity of the existing approach. The present paper is a limited effort to resolve this issue. To achieve this end, the strength demand, inelastic displacement demand and ductility demand of a large category of systems encompassing all feasible combinations of lateral period and response reduction factors are computed through both the approaches and compared. While computing the inelastic response, elasto-plastic as well as various feasible degrading hysteresis behaviours, have been used. The study indicates that the traditional approach yields a safer estimation of the strength and ductility demand except for system with very stiff periods. On the other hand, inelastic displacement demand is underestimated by this traditional approach, which can be by and large compensated by multiplying this displacement demand by response reduction factor.