The simplified equations available at present to predict the collapse loads of single-cell concrete box-girder bridges with simply supported ends are based on either space truss analogy or collapse mechanisms. Experimental studies carried out by various researchers revealed that, of the two formulations available to predict the collapse load, the one based on collapse mechanisms is found to be more versatile and better suited to box sections. Under a pure bending collapse mechanism, the existing formulation is found to predict collapse load with high accuracy. However, in the presence of cross-sectional distortion, there are significant errors in the existing theoretical formulation. This paper attempts to resolve this problem, by proposing a modification to the existing theory, incorporating an empirical expression to assess the extent of corner plastic hinge formation, under distortion-bending collapse mechanism. The modified theoretical formulations are compared with the experimental results available in the literature. New sets of experiments are also conducted to validate the proposed modified theory to estimate the collapse load. In all cases, it is seen that the modified theory to predict the collapse load match very closely with the experimental results. © 2007 ASCE.