Analysis of bridge soil interaction under seismic loading

Abstract

Deformation of soil due to seismic load imposed by the structures above can result in the change in stresses and deformation of the structures. Considering the interaction between soil and structure is important especially for the design of large structures with embodiment such as nuclear power plant containment and foundations of bridges. Engineering experience has shown that under dynamic loading and particularly during earthquakes the bridge-soil systems undergo significant alterations. These alternations cannot be taken into account using the classical methods of design. Consequently, to prevent major disasters and material loss it is imperative to perform adequate soil structure interaction analysis. Furthermore, as some interaction phenomena have favorable effect on the structural resistance, their consideration enables design and construction with economy and elegance. Three multiple span simply supported bridges, representative of typical bridges in Bangladesh, has been analyzed. Soil supports are explicitly modeled with equivalent springs. Radiation damping associated with wave propagation between the masses of the superstructure and foundation-soil is one form of energy dissipation due to soil-structure interaction(SSI). In this study, damping due to material nonlinearly in the foundation-soil is explicitly modeled, while the radiation damping is implicitly accounted for through equivalent viscous damping. Beam column elements are used to model the columns and simple connection elements are employed in modeling bearings and soil-structure springs. Extensive parametric studies are also performed to show the influence of soil shear modulus (400 psi, 4000 psi and 40000 psi), backwall condition (intact and broken), bearing performances (intact and failed with two different coefficient of friction equal to 0.2 and 0.6). Results are presented in terms of time histories of shear force at pier columns, top displacement of pier, gap width of abutment, plastic rotation at base of pier column and deck displacements due to variation of bearing performances, SSI models, boundary conditions of the pier column footings and soil conditions. Time history analysis of bridges has considered both the fixity and flexibility of the supports using 5% damping. Push over analysis and nonlinear time history analyses of bridges have also been performed using both 2-D and 3-D computer models by DRAIN-2DX and DRAIN-3DX. Raleigh's damping proportional to both stiffness and mass matrices is used. Under peak ground acceleration (PGA) of 0.18g and O.4g bridges were analyzed under three different earthquake records (Parkfield, EI-Centro and Nahanni).