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.