In this paper, the spatial variability effects of ground motions on the dynamic behaviour of long-span bridges are investigated by a random vibration based spectral analysis approach and two response spectrum methods. The spatial variability of ground motions between the support points is taken into account with the coherency function, which arises from three sources: incoherence, wave-passage and site-response effects. Random vibration analyses are performed on two deck-type arch bridges and a cable-stayed bridge model. Power spectral density function and response spectrum values used in random vibration analyses are determined depending on the recordings of September 20, 1999, ChiChi, Taiwan earthquake. The results strongly imply that the filtered white noise ground motion model can be accepted as a rather convenient model to represent actual earthquake ground motions. It can be also observed that the structural responses for each random vibration analysis depend largely on the intensity and frequency contents of power spectral density functions. (C) 2004 Elsevier Ltd. All rights reserved.