Evaluation of ductility and allowable moment redistribution in reinforced concrete structures

Designers can use moment redistribution to reduce the design bending moment envelope. Code provisions for moment redistribution are not entirely rational. They neglect the effects of important parameters on permissible moment redistribution and can be very conservative. To establish a realistic limit on permissible moment redistribution, one needs a rational model for predicting the plastic rotation capacity of critical sections (plastic hinges). This paper presents a model for computing the plastic rotation capacity, theta (p), and permissible moment redistribution, beta, in reinforced concrete beams. Important parameters, affecting theta (p) and beta, are identified and incorporated in the model. The model is validated against experimental results and shows good agreement. A comparison of the moment redistribution limits is made between the model and CSA A23.3-94. Although the code provides a reasonable estimate of beta for unfavourable combinations of parameters, the code can be very conservative when conditions are favourable for moment redistribution. Deeper beams with closely spaced stirrups allow significantly more moment redistribution than that predicted by the code.