Experimental analysis of shear-lag effect in reinforced concrete T-beams

Abstract

The non-uniform distribution of normal stresses within wide flanges of beam sections is typically referred to as shear-lag effect. Shear lag is a result of the interaction of normal and tangential stresses or correspondingly by the influence of shear strains on longitudinal strains. In structural engineering, the shear-lag effect has been a concern in the flanges of thin-walled metallic structures or the slab of composite steel–concrete elements, among others. Though existing codes of practice provide a simplified way to address shear lag by means of the effective width concept, such provisions can be insufficient for concrete structures, for which a deeper knowledge on the real influence of shear lag is necessary due to the current trend to design wide concrete bridge girders, with geometric configurations of the cross-sections which can be sensitive to shear lag (i.e. box- or T-sections). Moreover, the influence of the behaviour of structural concrete (cracking, rheology, yielding of reinforcement) has not been dealt with in detail so far. In the present paper, an experimental campaign on two reinforced concrete T-beams is presented. The beams have been subjected to two types of tests: firstly, to time-dependent effects governed by concrete shrinkage and creep; secondly, to the application of increasing direct loads to cover the whole concrete behaviour (uncracked, cracked and ultimate stage). The strain distribution at the top slab of the T-sections has been studied with extensive strain measurements at different cross-sections. The experiments have shown a distinct intensity of shear-lag effect depending on the load type. Moreover, the shear-lag impact varies longitudinally for each cross-section. In case of direct load tests, it has been also found that the strain distribution on the top slab changes as a function of the load level, which indicates that the different behavioural stages of concrete affect the shear lag, especially cracking. The experimental results have been analyzed with the help of analytical and numerical models, which have allowed understanding the progressive modification of the strain distribution within the top slab and the shear-lag intensity.