Moment redistribution limits for steel-concrete composite beams accounting for ULS and SLS requirements

The paper investigates the design of continuous steel-concrete composite beams using the elastic analysis with limited redistribution. The permissible moment redistribution which satisfies the requirements of the ultimate limit state (ULS, collapse) and serviceability limit state (SLS, crack width in the concrete slab) was computed. An advanced finite element program accounting for all mechanical nonlinearities and time-dependent phenomena (creep and shrinkage of concrete) was used. An extensive parametric analysis aimed to determine the influence of several geometrical parameters on the permissible moment redistribution was carried out on propped cantilevers and fixed-end beams. The analyzed parameters include the shape of the steel profile, the ratio between the depths of the concrete slab and of the steel beam, the steel to concrete area ratio and the reinforcement percentage of the concrete slab. The analysis was limited to compact steel sections (class 1 in accordance with the Eurocode 3) and low ductility reinforcing steel (characteristic elongation at maximum load 2.5%). A moment redistribution lower than that recommended by Eurocode 4 was found for high values of the reinforcement to steel area ratio due to limited rotation capacity at ULS. Reduced moment redistribution was also found for low values of the reinforcement percentage due to excessive crack width at SLS.