Analytical Fragility Curves for Multispan Continuous Steel Girder Bridges in Moderate Seismic Zones

of bridges are typically based on simplified analyses or empirical data from recent earthquakes. Since earthquake damage data are very scarce in the central and southeastern United States (CSUS), using analytical methods is the only feasible approach to obtain fragility curves. In 1990, bridges in the CSUS began incorporating seismic details. Some of the changes included the component design forces, design of columns and foundation, bearing types, and treatment of liquefaction and liquefaction-induced ground movement (8). A detailed review of bridges in the National Bridge Inventory (9) built after 1990 in the CSUS region shows that multispan continuous (MSC) steel girder bridges account for about 10% of the bridges in the region. The present study focuses on developing fragility curves for seismically and nonseismically designed MSC steel girder bridges in CSUS. Choi et al. examined over 150 bridge plans from this region (10), and details regarding these bridges can be found in Nielson and DesRoches (4). A review of the evolution in seismic design practices in the region reveals that the predominant difference between seismically MSC and nonseismically MSC steel bridges is associated with the detailing aspects in the columns and the replacement of the steel rocker bearings with elastomeric bearing pads with steel dowels. It is well known that transverse reinforcement has a major impact on the shear resistance and ductility capacity of bridges. Several bridge columns experienced flexural shear failure at midheight due to premature termination of longitudinal reinforcement and insufficient spacing of transverse reinforcement during the 1995 Hyogoken Nanbu earthquake (8). Also, CSUS bridges built before 1990 have several known seismic deficiencies, which include nonductile steel bearings, short seat widths, nonductile columns, high pounding potential, and increased potential for the toppling of rocker bearings (11). Bridge piers designed in the CSUS after 1990 have greater splice lengths and reduced transverse reinforcement spacing. Modern seismically designed columns have higher transverse reinforcement ratios in the longitudinal and transverse directions, as a consequence of reduced spacing, when compared with the respective values in columns designed prior to 1990. Flexure thereby becomes the dominant behavior in seismically designed columns, whereas shear often controls in nonseismically designed columns. The steel rocker bearings are replaced with elastomeric bearing pads with steel dowels. The relative flexible nature of these elastomeric bearings allows the superstructure to be decoupled from the substructure, to a certain extent, and hence these bearings are susceptible to large deformations, as illustrated in subsequent sections of the paper. However, these large deformations could cause unseating of the bridge girders and may also result in bearings walking out from under the girders. Analytical Fragility Curves for Multispan Continuous Steel Girder Bridges in Moderate Seismic Zones