Comparisons of Energy Dissipation in Structural Devices with Foundation Soil During Seismic Loading

Abstract

The effectiveness of structural energy dissipation mechanisms such as passive energy dissipation devices and base isolation methods used in seismic design depends on their capacity, ductility, energy dissipation, isolation, and self-centering characteristics. Though rocking shallow foundations could also be designed to possess many of these desirable characteristics, current seismic design codes often avoid nonlinear behavior of soil and energy dissipation beneath foundations because of concerns about permanent deformations at foundation level. This thesis compares the effectiveness of energy dissipation in foundation soil with structural energy dissipation devices during seismic loading. Numerical simulations of structures with and without energy dissipation devices were carried out to systematically study the seismic energy dissipation in structural elements and energy dissipation devices. The numerical model was validated using shaking table experimental results on model frame structures with and without energy dissipation devices. The energy dissipation in the structure, drift ratio, and the force and displacement demands on the structure are compared with energy dissipation characteristics of rocking shallow foundations as observed in centrifuge experiments, where shallow foundations were allowed to rock on dry sandy soil stratum during dynamic loading. The comparisons of results clearly indicate that foundation (rocking) energy dissipation mechanism is as efficient as structural passive energy dissipation devices. For the structures with energy dissipating devices, about 70% to 90% of the seismic input energy is dissipated by energy dissipating devices, while foundation rocking dissipates about 30% to 90% of the total seismic input energy in foundation soil (depending on static factor of safety). Inclusion of energy dissipating braces increases the base shear force transmitted to the structure, while normalized base shear forces transmitted to the foundation during rocking are smaller than those of the structures with energy dissipating devices because of the isolation effect of rocking foundations. If properly designed (with reliable capacity and tolerable settlements), adverse effects of foundation rocking can be minimized while taking advantage of the favorable features of foundation rocking, and hence they can be used as efficient and economical seismic energy dissipation mechanisms in buildings and bridges.