A majority of reinforced concrete frame structure failure patterns resulting from the May 12, 2008 earthquake in Wenchuan County, P. R. China were analyzed. It was found that the main failure mode of frame works was “strong beam, weak column”, which conflicts with the design criteria of “strong column, weak beam”. The cause of this failure pattern was analyzed. A new concept is put forward that should be considered in design work. The overstrength caused by slabs connected monolithically to the beams is analyzed. The joint types, transverse beam stiffness and lateral drift influence the reinforcing effect of the sla, Moreover, a reinforced concrete frame model was simulated using the program ABAQUS. The slab reinforcement stress distributions at different lateral drifts were analyzed and the effective flange width value proposed.