The Influence of Moment Redistribution on the Optimization of Beam Reinforcement in Multistorey Structures

Abstract

The redistribution of design bending moments in continuous reinforced concrete beams is widely acknowledged as a valuable resource for designers of reinforced concrete structures. Moment redistribution is beneficial for practical design, as it provides flexibility in reinforcement arrangement. According to IS 13920:2016, the minimum reinforcement at the bottom face of the beam at support locations must be 50% of the top reinforcement at the same point. At the same time, it should be no less than 25% of the support reinforcement at midspan to meet ductility requirements. In detailing the junctions between columns and beams, the author has noted that congestion of reinforcement at the column faces of the beams poses practical challenges when placing both longitudinal and transverse rebars for the columns. This congestion can lead to inadequate concrete compaction, increasing the risk of honeycombing. By redistributing the top moment in the beam at the support location to the bottom face at midspan, it becomes possible to reduce both the top and bottom reinforcement at the supports, thus optimizing the reinforcement allocation in beams. This study examines a three-dimensional G+20-storey building, which includes both laterally stiffened and unstiffened designs with symmetrical plans. The entire structure is situated in zone III on medium soil conditions, incorporating partial moment releases. The percentages of moment redistributed in the beams were determined at 10%, 20%, and 30% for the laterally unstiffened structure, and 10% for the laterally stiffened structure. Following this, a response spectrum analysis was performed using STAAD Pro. By IS 13920:2016, comparisons were made between the beams' top and bottom-face longitudinal reinforcement across various moment redistribution values.