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Steel moment frames supported on two different ground levels have been widely constructed in the mountainous high-seismicity regions of China. To investigate the seismic collapse behavior of such frames, 3D numerical analysis models were established in Perform-3D and verified to simulate the collapse process. Different structural configurations were considered, including the number of stories below upper ground and the number of spans supported on two different ground levels. Two limit states were defined according to the different structural parts that reach their limitations. Incremental dynamic analyses were performed in both the lateral and longitudinal directions. Seismic collapse fragility curves, collapse margin ratios, and collapse patterns corresponding to eight cases were exhibited. Seismic fragility curves corresponding to four performance levels were used to assess structural vulnerability. Analytical results indicated that two different collapse patterns would exist in the lateral and longitudinal directions. Therefore, two limit states should be considered in the analyses of such steel moment frames. [ABSTRACT FROM AUTHOR]