A new approach is presented for evaluating seismic responses of arch-type long-span structures. The responses are estimated using a series of pushover analyses utilizing multiple load patterns as linear combinations of dominant modes. The representative displacement and acceleration are defined in a general manner without resort to the base shear or roof displacement. The eigenmodes of initial elastic structure are assembled to the equivalent static loads to take the snapshot of the deformed structure at its maximum deformation. The damping due to plastic energy dissipation is evaluated through equivalent linearization for inelastic systems. Accuracy of the proposed method, especially for force and stress responses, is demonstrated in the numerical example of an arch-type long-span truss.