Abstract | Incremental dynamic analysis (IDA) is employed to evaluate the seismic performance of a 20-story steel space frame under biaxial seismic loading. Originally developed for planar frames and uniaxial loading, the IDA framework is now extended to 3D. This involves performing a series of nonlinear timehistory analyses under a suite of ground motion records by equally scaling both components of each record to several levels of intensity and recording the structural response. The structure is thus forced to show its complete spectrum of behavior from elasticity to final global instability for combinations of intensities in the two directions. Using proper intensity measures (e.g., spectral acceleration coordinates of the record components) and engineering demand parameters (e.g., maximum interstory drifts), the familiar IDA curves plus novel IDA surfaces are created, representing the structural response and its statistical summary at any intensity level. These enable a rational definition of limit-states and the calculation of the resulting capacities in a manner consistent with current IDA techniques. A powerful analysis procedure is thus created that is capable of thoroughly assessing the seismic performance of 3D structures and may serve as a solid benchmark for evaluating the accuracy of simpler methods.