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Seismic behaviour of a total of 76 bridges, including 4 regular and 72 irregular continuous RC bridges with irregularities in both superstructure and substructure are investigated in this study. Bridges with different span arrangements, various column heights, different support conditions at abutments, different superstructure transverse stiffness were considered in this study. The influence of different seismic intensity levels on the seismic response was also investigated. The bridges were designed and detailed according to American Association of State Highway and Transportation Officials (AASHTO) regulations. Seismic response of bridges was predicted using different methods of analysis including elastic multi-mode and inelastic time history analyses. To investigate the sufficiency of code provisions for predicting the seismic behaviour of irregular bridges, a comparison was made between the results obtained using linear and non-linear analyses in the longitudinal and transverse directions of the bridges and a new relationship was proposed to predict when the results from elastic analyses are not appropriate. The results indicate that in some cases significant deviations can occur between the predictions obtained using linear and nonlinear analyses, when simultaneous irregularities in both superstructure and substructure were considered. Current code provisions do not consider this issue in design of irregular bridges.