Seismic Ductility Demands Predicted by Alternate Models of Building Frames

Peak values of story ductility demands produced by earthquakes on several models of multistory rigid frames are evaluated by means of step-by-step nonlinear dynamic analysis of structural response to simulated ground motion time histories. The models studied include the basic rigid frame model with nonlinear behavior concentrated at plastic hinges eventually forming at the ends of beams and columns, as well as two approximations, the conventional shear-beam system and a shear-frame system constituted by ordinary reinforced concrete columns and infinitely stiff and strong beams. Two families of models were studied, including seven-story and fourteen-story systems, respectively. The contributions of soil-structure interaction, stiffness-degradation and P-delta effects were analyzed. The equivalent shear-beam model produced results clearly different from those of its rigid frame counterpart: while peak values of story ductility demands vary gradually along the height of rigid frames, they concentrate at the bottom story of shear-beam systems, where they are significantly greater than for the corresponding rigid frame. The responses of both, shear-beam and shear-frame systems, displayed a more pronounced sensitivity to variations in the lateral strengths of structural members than that observed in rigid frames.