Arch bridges are generally statically indeterminate and could be vulnerable to differential support motions. This report describes seismic analyses that were performed on the world's longest steel deck arch bridge, the New River Gorge Bridge (NRGB) in West Virginia. Elastic time history solutions were derived with input consisting of artificial ground acceleration histories applied first uniformly and then unequally to the arch and deck abutments. The greatest effect occurred under longitudinal motion which gave increased arch axial stresses 50 to 159% greater under unequal arch abutment motion compared with uniform motion.