SEISMIC RETROFIT OF A BRIDGE

Abstract

In this thesis methodology for evaluation of seismic capacity of existing bridge and retrofitting measure for future earthquake motion is studied. The example railway bridge chosen for study is 255.10 meter long having nine simply supported spans on pier well foundation, in seismic zone V. The design of substructure is checked for normal loading that is dead load, live load, water current force, longitudinal braking force and seismic loading. The seismic forces are found by (i) code spectra and, (ii) site dependent spectra. The seismic forces calculated by dynamic analysis using response spectra for design basis earthquake in longitudinal and transverse direction is found to be critical. The seismic capacity of pier and well cap are determined on the basis of capacity/demand ratio based on report ATC-6-2 (1983). The existing bridge pier and well cap are found inadequate to meet the requirement of site dependent spectra. The retrofit scheme is proposed which involves jacketing the pier with reinforced concrete and increasing the thickness to well cap. The stoppers are also proposed on top of pier to restrict the seismic displacement for maximum credible earthquake. In order to check vertical jumping of girder, anchor bolts are provided. The retrofitted structure is reanalysed by dynamic analysis. The analysis of proposed retrofit substructure show that the seismic resistance capacity improves significantly to withstand future earthquake motion.