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SUMMARYThe dynamic responses of three deck-type arch bridges, with main spans of 59,213 and 518 m, are presented. The ratios of natural period to arch span were found to be quite close for all three bridges. Artificial ground motions were applied separately in three directions to finite-element models of each bridge. Three acceleration levels were considered4.09g, 0.22g and OSOg-corresponding to the AASHTO Seismic Risk Zones I, I1 and 111, respectively. Responses to uniform lateral motion were generally the largest, while the responses to vertical motion were generally lower than those due to lateral or longitudinal motion. In all cases considered, none of the total stresses in the main members exceeded the yield stress. Connection and secondary member responses were also calculated and are presented. In addition, the effects of unequal motions at the supports were sampled by various deterministic inputs. While the effects of such motions in the vertical and lateral directions were less than the responses to uniform motion, the effects of unequal longitudinal motions at the supports were substantial.