Abstract | The seismic performance of a horizontally curved highway bridge is examined using Incremental Dynamic Analysis (IDA) in 3D. IDA has been successfully applied to two dimensional structures with a single horizontal ground motion component. An extension to three dimensions and two components of motion is needed to capture the performance of this asymmetric structure. It involves performing a series of nonlinear timehistory analyses under a suite of ground motion records, equally scaling both components of each record to several levels of intensity and monitoring the resulting structural response. The complete spectrum of structural behavior is thus recovered, from elasticity to final global instability for combinations of intensities in the two directions. Using a single intensity measure or a vector of two (e.g. spectral coordinates of the record components) and appropriate engineering demand parameters (e.g., maximum peak column drift), the familiar IDA curves plus novel IDA surfaces are created, clearly showing the bridge response and its statistical summary at any intensity level. This is a powerful, albeit resource intensive, analysis procedure that is well suited to evaluate the performance of asymmetric structures under seismic loads and may serve as a solid benchmark for evaluating the accuracy of simpler methods.