It is difficult to incorporate multidimensional effect of the ground motion in the design and response analysis of structures. The motion trajectory in the corresponding multi-dimensional space results in time variant principal axes of the motion and defies any meaningful definition of directionality of the motion. However, it is desirable to consider the directionality of the ground motion in assessing the seismic damageability of bridges which are one of the most vulnerable components of highway transportation systems. This paper presents a practice-oriented procedure in which the structure can be designed to ensure the safety under single or a pair of independent orthogonal ground motions traveling horizontally with an arbitrary direction to structural axis. This procedure uses nonlinear time history analysis and accounts for the effect of directionality in the form of fragility curves. The word directionality used here is different from “directivity” used in seismology to mean a specific characteristic of seismic fault movement. 1. Introduction A spatially distributed civil infrastructure system located in a seismically active region is vulnerable to moderate to strong earthquake events. These extreme natural events may cause system interruption over a long period of time and entail substantial costs for postevent repair and restoration. This often results in unacceptable socioeconomic losses and societal disruption. The highway transportation network serving the State of California typically represents such a system. It is then prudent to develop an emergency response strategy for each community served by the system in order to minimize the negative consequences of the extreme earthquake events. For a highway network consisting of a large number of bridges, the initial step toward the development of such an emergency response strategy is to estimate, by means of predictive simulation, the reduction in the traffic flow capability of the network when its bridges are damaged due to regional seismic activity. Such reduction in the traffic flow capability will provide an estimate of network accessibility at distressed conditions. Consequently, the emergency response activities can be designed prior to any earthquake event. Because of the predictive and probabilistic nature of system performance simulation, bridge damage due to seismic activity must be simulated (i) on the basis of the ground motion intensity (typically, peak ground acceleration (PGA)) obtainable from the attenuation equations using the fundamental seismic source parameters and (ii) by
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