Seismic response analysis of isolated and non-isolated bridges

Abstract

Earthquake resistant bridge design must ensure that bridge piers withstand the lateral forces generated during earthquakes. In recent years, base isolation has emerged as an effective technique for reducing the lateral forces acting on bridge piers by placing suitable isoiators in between the deck and pier. Base isolation results in increasing the natural period and damping characteristics of the bridge. In this study the seismic response of bridge structures subjected to moderate and strong earthquakes is evaluated using noniinear finite element method. Time history analysis is performed employing Newmark's step wise time integration. Both conventionai non-isolated and isolated bridges are considered, but emphasis is primarily given on evaluating the seismic response of isolated bridges. A simple dynamic single pier model consisting of deck mass, isolator unit and pier is used after verification with a more detailed modei of a four-span regular bridge. Complete bridge model is used when study is performed on transverse deck vibration. Longitudinal and transverse vibrations of the bridge are studied separately. For most part of the study non-linearity is restricted to the isolation unit, while rest of the structure is assumed to remain elastic at all times. However, non-linearity of pier is considered in the case where ductility requirements for bridge piers are studied, in which case the pier is simplistically modeled as a bi-linear elastic spring element. The study concentrates mainly on evaluating the peak shear forces, which the bridge pier needs to resist during earthquakes, and isolator displacement, which is important for design of isolation system and for allowing relative movement of the deck. Extensive parametric studies are performed to show the influence of key bridge and isolator parameters on the seismic response of bridges. The distributioh of bridge mass over its components, shows significant influence on pier force and ductility requirements for piers. For strong motions, the ductility requirements for piers are found to be reduced significantly with increase in pier period. However, it shows little or no effect on isoiator dispiacement. Among the isolator parameters, isolator yield force and post-yielding stiffness of isolator are found to have considerable influence on peak shear force, ductility demand and transverse mid-span deck displacement. .As a case study, the recently commissioned 4.8 km iong Bangabandhu (Jamuna Multipurpose) Bridge has been considered for seismic response analysis. This bridge has seismic pintles for protection against a design ground acceleration of 0.2g. Analysis results show that the pintles are effective in keeping the pier forces within the elastic limit.