Risk assessment of infrastructure systems plays an increasingly important role in their plan, design, maintenance, retrofit and life-cycle cost evaluation taking into consideration their vulnerability to natural, technological and terrorist hazard. This paper develops a mechanistic model for seismic damageability of concrete bridges in the form of fragility curve in such a way that the model can be calibrated with the empirical fragility curves constructed on the basis of the damage data from the 1994 Northridge earthquake. As often observed from this and other destructive earthquakes, bridges are one of the most vulnerable components of a highway network system subjected to earthquake ground motion. For this reason, bridge damageability information in a succinct from as fragility curve is needed to pursue the seismic risk assessment of a highway networks consisting of as large as thousands of bridges that can be affected by a high magnitude earthquake with in and near the service area of the network. (c) 2007 Elsevier Ltd. All rights reserved.