Structural Concrete, Vol. 6, no. 4, December 2005
Serviceability influences on moment redistribution
in beams
R. Scott, University of Durham, UK
R. T. Whittle, Arup Research and Development, UK
Moment redistribution has traditionally been considered as an ultimate limit state (ULS) phenomenon closely linked to considerations of reinforcement ductility. This paper demonstrates that a significant proportion of this redistribution will frequently occur at the serviceability limit state (SLS) because of the mismatch between the flexural stiffness assumed when calculating moments for the ULS and those actually occurring at the SLS owing to variations in the reinforcement layout along the member and the influence of cracking. This was demonstrated by tests on 37 two-span beams, the parameters investigated being values of redistribution, beam depth, reinforcement arrangements, concrete strength and the effect of brittle reinforcement. The results showed that there is scope for increasing the permissible limits for redistribution from supports into adjacent spans beyond those currently prescribed in design codes. This was supported by numerical modelling. However, considerations of crack width and reinforcement yield at the SLS may be limiting factors. Caution is advised if redistribution from the spans back to the supports is intended since it may prove difficult in practice to achieve the levels of redistribution anticipated in the design.