Parametric study on moment redistribution in continuous RC beams using ductility demand and ductility capacity concept



Experimental studies show that an indeterminate structure or a continuous concrete beam does not fail when critical sections reach their ultimate strengths. Therefore, if a structure has adequate ductility, stress and moment redistribution will take place in the flexural members by developing plastic hinges at critical sections. This causes the other points of beams to achieve their ultimate strengths and capacities. Besides, moment redistribution allows designers to adjust the bending moment diagram computed by elastic analysis. The usual result is a reduction in the values of negative moments at the support face as well as an increase in the values of positive moments along the span.
In the current investigation, a parametric study on moment redistribution in continuous RC beams with equal spans under uniform loading was performed. First, the governing equation for the allowable percent of moment redistribution was extracted using ductility demand and ductility capacity concepts. The effects of different parameters such as the concrete compressive strength, the amount and the strength of reinforcing steel, the magnitude of elastic moment at the support and the ratio of the length to the effective depth of the continuous beam on moment redistribution were then investigated. Furthermore, the allowable moment redistributions were calculated according to the regulations of different codes in each case. The results showed that, whereas the permissible moment redistribution in continuous reinforced concrete beams based on the relevant rules in the current codes is not in a safe margin in some cases, it is rather conservative in most cases.