In any given earthquake event scenario, be it a first shock or an aftershock, the decision to limit traffic or to completely close a bridge in a highway network system hinges primarily on reconnaissance data on visible damage to the bridge. A more quantitative and systematic method for decision making by engineers, owners, and operators alike involves determination of the loss in the bridge load carrying capacity.

In a performance-based earthquake engineering context, the loss of load carrying capacity of a bridge can be described as a earthquake demand parameter. In previously formulated probabilistic seismic demand models, this demand parameter was predicted by a measure descriptive of the earthquake intensity. This method, however, is not the most accurate approach of predicting loss of load carrying capacity. It has been shown that post-earthquake residual displacement is a better proxy for capacity loss than measures of earthquake intensity. The initial load carrying capacity is defined in terms of a static pushover analysis. A comparison of loss in load carrying capacity is made between time history analysis of first shocks and aftershocks, and static analysis including residual displacements and degradation of material stiffness. These demand models are then integrated with estimates of capacity or damage in order to produce traditional fragility curves.