Influence of site effects and period-dependent force modification factors on the seismic response of ductile structures

Abstract

A foundation factor, F, is incorporated in the National Building Code of Canada (NBCC) design base shear formula to account for amplification of bedrock ground motions as these propagate upwards through the local soil deposit (site effects). In the NBCC, the value of F is specified as a function of the local soil type and depth, irrespective of the ductility capacity for which the structure situated at the surface of the soil deposit is to be designed and detailed. On the other hand, the ductility capacity of the structures is taken into account in the code by the force modification factor, R, for which values are specified depending on the type of the structural system. The current study investigates the influence of the ductility capacity of engineering structures in mitigating the site effects. Simple bilinear single-degree-of-freedom models are used to simulate the seismic response of structures, underlain by soft or stiff soil deposits and subjected to seismic ground motions. These structural models are also used to investigate the effects of the period-dependent force modification factors on the seismic response of structures.The results show that site effects are less significant for ductile structures, as compared with structures that respond elastically. The results are then used to evaluate the current code provisions for site effects. The current study also shows that using period-dependent force modification factors to derive the code design base shear not only is recommended for short period structures but also is necessary to provide a realistic simulation of the seismic response of these structures. Key words: site, seismic, ductility, structure, foundation, factor, base, shear, amplification, soil, period.