Use of Inelastic Spectra in Aseismic Design

Modal analysis with inelastic response spectra is often used to design multistory frames subjected to earthquake motions. To evaluate the effectiveness of this approach, three different frames were designed using smooth inelastic spectra derived according to the Newmark-Hall rules for ductility factors of two and four. These frames were analyzed with an inelastic dynamic analysis program for three different artificial motions generated to match the design spectra. It was found that this design procedure fails to reflect variation of yielding patterns or local concentration of inelastic action. Curvature ductility factors at several locations exceed significantly the design ductility level. Axial forces in exterior columns produced by inelastic response can be larger than what inelastic spectra modal analysis predicts, leading to reduced plastic moment capacities and increased ductility demands. It appears that spectral modification factors could be used for local ductility control.