# Plasticity of Composite Beams at Serviceability Limit State

*Usually, structures are considered to be elastic at the serviceability limit state. Evidence collected from previously reported large-scale experiments, supplemented by analysis, is used to show that plasticity can sometimes be developed in composite beams before the beam deflection reaches the serviceability Code limit. This phenomenon does not accord with the traditional concept of the serviceability limit state, for which elastic behaviour is assumed and elastic analysis methods are adopted in design Codes when predicting deflections. In order to limit plasticity at serviceability, tighter limitations on beam span/deflection ratios are required. The derivation of such limitations is presented herein. The beam span/deflection ratio
corresponding to first yield of the beam is shown to be dependent on the steel grade, neutral axis position, and beam spaddepth ratio. Ifa small degree of plasticity is allowed at serviceability, a multiplication factor is needed to adjust the deflection obtained from an elastic analysis so as to allow for the efect of plasticity. The derivation of the multiplication factor is also presented.
Professor D.A. Nethercot, T.Q. Li and B. Ahmed*

Author(s): Nethercot, D A;Li, T Q;Ahmed, B

Keywords: beams;composite construction;serviceability;limit state methods;design;analysis;testing;research