Earthquake performance assessment and retrofit investigations of two suspension bridges in Istanbul

This paper presents the results from earthquake performance assessment and retrofit investigations for Fatih Sultan Mehmet and Bosporus suspension bridges, with main span lengths of 1090 and 1074 m in Istanbul. In the first part of the study, sophisticated three-dimensional finite element model of two suspension bridges were developed and the results of the free vibration analysis were presented. The models contain detailed structural components of the bridges and geometric non-linearity with cable sagging and stress stiffening, cumber of the deck and set-back of the towers. These components affect the natural frequencies and the corresponding mode shapes of the bridges. In the second part of the study, the seismic performance evaluation of two suspension bridges was undertaken. For performance assessments, non-linear 3-D finite-element time history analysis of with multi-support scenario earthquake excitation was used. Displacements and stresses at critical points of the bridges were investigated. Their earthquake performance under the action of scenario earthquake (site-specific ground motion that would result from the Mw=7.5 scenario earthquake on the Main Marmara Fault) were estimated and comparison with actual design data were also presented. Although both suspension bridges were originally designed for much lower earthquake loads they exhibited satisfactory performance. Finally, suggestions for retrofit need were made and retrofit design with hysteretic dampers for the Bosporus suspension bridge was calculated.