In this paper an intrinsic arch, which will create an arch action in straight concrete dams, is revealed from the results of a back-analysis on a 4m thick concrete core with straight axis in a rockfill cofferdam. The concrete core has stood for two years functioning as the cofferdam to withstand upstream water and sediment pressure after all the material supporting the core was washed away during overtopping. The arch action existing in straight concrete dams challenges the traditional two-dimensional cantilever theory (gravity method) for the design of gravity dams. Results of the three-dimensional finite element back-analysis are compared with those observed on site, and agreement of the results confirms the arch effect. The conditions for forming the arch action in straight concrete dams are discussed, on the basis of which new design and construction philosophy is proposed, considering the arch action for straight concrete dams. This may result in project cost savings and/or increased dam safety.