Seismic assessment of curved bridges using modal pushover analysis
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The assessment of existing bridge structures against earthquake threat has become a major issue lately, motivated by the maturity of seismic design of new structures, on one side, and by the recognition of the inadequate level of seismic protection, the aging and the constant need of maintenance of the existing ones, on the other. While nonlinear time history analysis (NL-THA) is the most rigorous procedure to compute seismic demands, many seismic-prone countries, such as United States, New Zealand, Japan and Italy, have recently released standards for the assessment of buildings, all of which include the use of the non-linear static analysis procedure (NSP), the so-called pushover. Recently Chopra and Goel (2002) proposed the modal pushover analysis (MPA) procedure that considers the effect of higher modes on the behavior of buildings.
This research investigation is intended to evaluate the accuracy of the modal pushover analysis (MPA) procedure in estimating seismic demands for curved bridges after proposing some modifications that would render the MPA procedure applicable for bridges. For verification purpose, the nonlinear time history analysis (NL-THA) is also performed in order to quantify the accuracy of MPA. Three bridges were analyzed using both the MPA and NL-THA in addition to the standard pushover analysis (SPA). Maximum Demand displacements, total base shear and plastic rotations obtained from SPA and MPA are compared with the corresponding values resulting from the NL-THA. Comparison shows a good agreement between MPA and NL-THA results and MPA is deemed to be accurate enough for practical use. Furthermore, to evaluate the applicability of the MPA method for a wide range of bridges, a parametric study using both the MPA and NL-THA is performed. Results from the MPA for demand displacement and base shear are compared with results from the NL-THA. Also, the influence of different parameters on the behavior of curved bridges is studied. Parameters included the girder cross section (steel I vs. steel BOX), span length, number of spans, radius of curvature, and pier height. Pier height is found to have the most significant effect on bridge behavior as well as span length, while radius of curvature is found to have less influence on the behavior of curved bridges.