Seismic response of isolated bridges with soil – structure interaction N.P. Tongaonkar, R.S. Jangid * Department of Civil Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India Abstract The modern transportation facilities demand that the bridges are to be constructed across the gorges that are located in seismically active areas and at the same time the site conditions compel the engineers to rest the pier foundation on soil. The purpose of this study is to assess the effects of soil – structure interaction (SSI) on the peak responses of three-span continuous deck bridge seismically isolated by the elastomeric bearings. The emphasis has been placed on gauging the significance of physical parameters that affect the response of the system and identify the circumstances under which it is necessary to include the SSI effects in the design of seismically isolated bridges. The soil surrounding the foundation of pier is modelled by frequency independent coefficients and the complete dynamic analysis is carried out in time domain using complex modal analysis method. In order to quantify the effects of SSI, the peak responses of isolated and non-isolated bridge (i.e. bridge without isolation device) are compared with the corresponding bridge ignoring these effects. A parametric study is also conducted to investigate the effects of soil flexibility and bearing parameters (such as stiffness and damping) on the response of isolated bridge system. It is observed that the soil surrounding the pier has significant effects on the response of the isolated bridges and under certain circumstances the bearing displacements at abutment locations may be underestimated if the SSI effects are not considered in the response analysis of the system. Keywords: Bridge isolation; Elastomeric bearings; Soil–structure interaction; Effectiveness; Earthquake; System parameters 1. Introduction Spectacular failure of bridges due to complete collapse of piers has been observed in every major seismic event. The Hyogo-ken Nanbu (Kobe) earthquake on 17 January 1995 and Gujarat earthquake of 26 January 2001 have demonstrated that the strength alone would not be sufficient for the safety of bridges during the earthquake. For past several years, the research is focused on finding out more rational and substantiated solutions for protection of bridges from severe earthquake attack. Seismic isolation is a strategy that attempts to reduce the seismic forces to or near the elastic capacity of the member, thereby eliminating or reducing inelastic deformations. The main concept in isolation is to reduce the fundamental frequency of structural vibration to a value lower than the predominant energy-containing frequencies of earthquake. The other purpose of an isolation system is to provide means of energy dissipation, which dissipates the seismic energy transmitted to the system. The isolation device, which replaces conventional bridge bearings, decouples the bridge deck (which alone is responsible for majority of the pier base shear) from bridge substructure during earthquakes thereby significantly reducing the deck acceleration and consequently the forces transmitted to piers. There had been several studies in the past for investigating the effectiveness of isolation devices for a sesmic design of bridges. Constantinou et al. [1], Kartoum et al. [2] and Tsopelas et al. [3] conducted experimental and analytical studies on seismic response of bridges using sliding isolation system and found that such devices are quite effective for seismic isolation of bridges. Ghobarah and Ali [4] and Turkington et al. [5] showed that lead-rubber bearings are quite effective in reducing the seismic response of bridges. Saiidi et al. [6] studied the effectiveness of seismic isolators in reducing the force and displacement of the superstructure of a six-span bridge. It was shown that the use of isolators does not necessarily increase the displacement of the superstructure. Togaonkar and Jangid [7] evaluated the performance of three span continuous deck