INFLUENCE OF SITE EFFECTS AND PERIOD-DEPENDENT FORCE MODIFICATION FACTORS ON THE SEISMIC RESPONSE OF DUCTILE STRUCTURES

A foundation factor, F, is incorporated in the National Building Code of Canada (NBCC) design base shear formula to account for amplificatio n of bedrock ground motions as these propagate upwards through the loc al soil deposit (site effects). In the NBCC, the value of F is specifi ed 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, t he ductility capacity of the structures is taken into account in the c ode by the force modification factor, R, for which values are specifie d depending on the type of the structural system. The current study in vestigates the influence of the ductility capacity of engineering stru ctures in mitigating the site effects. Simple bilinear single-degree-o f-freedom models are used to simulate the seismic response of structur es, underlain by soft or stiff soil deposits and subjected to seismic ground motions. These structural models are also used to investigate t he effects of the period-dependent force modification factors on the s eismic response of structures. The results show that site effects are less significant for ductile structures, as compared with structures t hat respond elastically. The results are then used to evaluate the cur rent code provisions for site effects. The current study also shows th at using period-dependent force modification factors to derive the cod e design base shear not only is recommended for short period structure s but also is necessary to provide a realistic simulation of the seism ic response of these structures.