Seismic Fragility of Multispan Simply Supported Steel Highway Bridges in New York State. I: Bridge Modeling, Parametric Analysis, and Retrofit Design

Abstract

This paper studies the dynamic seismic behavior of a typical highway bridge in New York State. The topological layout and structural details of this multispan simply supported steel-girder bridge are identified as the most typical of the New York State Department of Transportation bridge inventory database. Three-dimensional finite-element models of the bridge are established considering the nonlinear behavior of critical bridge components. An in-depth parametric study is carried out to evaluate the sensitivity of the bridge’s seismic response to variations in its structural parameters. The parametric analysis determined that uncertainties associated with the steel reinforcement’s yield strength, the superstructure’s weight, the expansion joints’ gap size, the friction coefficient of expansion bearings, and the concrete compressive strength should be considered during the fragility analysis of the bridge system. The Latin hypercube sampling (LHS) approach is used to obtain representative samples for the fragility analysis based on the mean values and probability distributions of each critical random variable. The LHS is thus used to create a set of nominally identical but statistically different bridge samples for performing the fragility analysis. The individual bridges from this statistically representative set are matched with earthquake samples of various intensities for the nonlinear seismic demand analysis. The seismic capacity of critical bridge components are estimated for each bridge sample from published experimental data. Through extensive numerical simulations, the sensitivity analysis identified the most vulnerable bridge components. Two seismic retrofit strategies for reducing the seismic risk of multispan simply supported steel bridges are studied: (i) steel bearing replacement by elastomeric bearings and (ii) deck/girder-splicing (continuity) with steel bearing replacement by elastomeric bearings. The analysis verified that retrofit option ii is the most effective. The finite-element model of the bridge samples, along with the assembled data on parameter uncertainties and member capacities, as well as a simulated set of ground motion records are suitable for use during the development of fragility curves for bridges with and without retrofit. Detailed description of the fragility analysis is presented in the companion paper.