An analytic study of overload behaviour and load capacity has been carried out on eight reinforced concrete frames, including three simple portal frames, a single-bay two-storey frame, a two-bay two-storey frame and three single-storey two-bay frames. Two different strength designs were undertaken for each frame, one based on an elastic analysis using gross bending stiffness (EI) values for all beams and columns, the other using an elastic analysis with stiffness values of 0.4 and 0.8 times the gross EI values for the beams and columns, respectively. All sixteen frame-designs were then analysed using a non-linear computer simulation procedure to observe overload behaviour and mode of collapse, and to determine the collapse load. The behaviour and strength of each pair of frames were compared, with the purpose of evaluating the adequacy of the two stiffness assumptions for use in the strength design of concrete structures.