Modelling of Substructure of Bridge for Seismic SSI

Abstract

Bridges are integral to any country's civil infrastructure and the backbone of contemporary transportation networks. The heart and soul of a bridge are the substructures. Initially, it is believed that both the foundation and the soil beneath the foundation are perfectly rigid. However, in actual cases, neither the foundation nor the soil is fixed. Here comes the essential role of soil structure interaction. When an external force such as earthquake excitation acts on the bridge, structural and ground displacement will be developed and mutually dependent on each other. The process of soil response influences the structure's motion and vice versa is termed as soil-structure interaction (SSI). The Substructure and the Direct methods have been used to study the influence of SSI where soil and structure response can be obtained simultaneously. Thus, Soil structure interaction plays a significant role in the case of a multi span bridge design founded on the weak soil. In this context, this dissertation aims to discuss the analysis of a 2-span simply supported bridge consisting of a substructure with a pile foundation in elastic response spectrum method. Linear as well as boundary nonlinear time history analysis with introducing Winkler’s springs and damping dashpot, which carries the soil responses in the nodes of piles as a grillage model, has been carried out. For this purpose, the SSI model whole bridge of two spans with the finite elements in 3D using the solid beam and plate elements has been made the results of seismic responses due to earthquake excitation are evaluated in detail. The Winkler’s springs, i.e., p-y, t-z & q-w curves, are obtained by the finite difference analysis (FDA) in a cyclic loading action considering inertial interaction. Four earthquakes like Kern (South California, USA,1952), Tabas (Iran, 1978), Darfields (New Zealand, 2010) and Gorkha (Nepal, 2015), are considered by spectral matching and deconvoluted to the base of soil model as an input motion. The damping dashpot and nonlinear Winkler’s springs from 1D ground response analysis have been introduced as a spring and dashpot to the pile nodes to perform boundary nonlinear time history analysis. The influence of the soil-structure interaction in the dynamic behaviour of the bridge is reflected in an increase in the natural period and an increase in the system damping compared with the fixed-base model or cantilever base model, which does not consider the supporting soil. By comparing responses with the fixed base solution, SSI's influence becomes relatively prominent on the response like bending moment, shear force and deflection of substructure components in abutment and pier when the soil stiffness is introduced. Conventional fixed-base analysis ignoring the effect of soil flexibility carried out for the seismic design of bridges may result in an uneconomical design. Therefore, the effect of SSI is an important issue from the viewpoint of design considerations. Thus, the soil-structure interaction (SSI) effect shall be incorporated into the design to evaluate the realistic structure behaviour.