Application of the Period Dependence of Stability Coefficient in Enhancement of Seismic Design and Evaluation of Seismic Response under Biaxial Excitation

Present study tries to simplify seismic resistance design process based on the stability coefficient (SC) and /or displacement (rotation) limits defined by practicing codes. To this end, a constant ductility response spectrum, derived for the first story with total mass acting on it, and known height, in which the P-delta effect and period dependent feature of the SC is exclusively inherent, is used. The mentioned spectrum plots spectral acceleration, or yield displacement (rotation), versus SC, and hence, is referred to as the “stability-coefficient-response-spectra”. The notion of the “first storey-single-degree-of-freedom” (FSSDOF) system is presented next. The FSSDOF is intended to set the minimum necessary lateral stiffness complying with a pre specified ductility level at early stages of design. Moreover, the pendulum model is used to evaluate effects of the geometric-nonlinearity and vertical-component-of ground-motion (VCGM) on the global-inelastic-response of non-deteriorating bilinear single-degree-of-freedom (SDOF) systems. Results indicate that both VCGM and geometric nonlinearity affect the inelastic response, which are significant for certain combinations of the initial period of vibration, strength reduction factor and ductility demand. In addition, effect of the VCGM is not limited to near fault regions. Furthermore, it is shown that the VCGM could cause dynamic instability of SDOF systems; hence, given the importance of SDOF systems in evaluation of existing structures, considering the VCGM for collapse assessment is suggested.