Cracking and tension field action in composite beams
Two aspects of the behaviour of uncased composite 'T' beams of steel and concrete are considered. Flexural cracking in the reinforced concrete slab was studied by subjecting seven 350-mm deep composite beams to hogging bending moment. Measured strains were found to exceed those calculated from an elastic analysis ignoring concrete in tension. Also, the first cracks to form in the slab were much wider than expected. Both of these effects are shown to be due to drying shrinkage in the slab of the composite beam. A formula for calculating the mean surface strain in the slab, which allows for both drying shrinkage and tension stiffening, is proposed. Consideration of the parameters which affect cracking leads to a formula for predicting crack widths in composite beams. Comparison with experimental results shows that it is sufficiently accurate for design purposes. In a second series of tests, three 900-mm deep composite plate girders with slender webs were subjected to combined shear and bending to study their ultimate load behaviour. Failure was due to the formation of a plastic collapse mechanism which closely resembles that observed in plain steel girders under similar loading. The addition of a concrete slab acting compositely with the girder increased the ultimate strength of the beam, but variations in the size of the slab and the strength of the shear connection had negligible effect on the beam's strength. An idealised collapse model, based on the experimental observations, is described and is used in the development of a method to calculate the ultimate strength of composite plate girders. It is shown that a design method for plain steel girders in the draft Part 3, BS5400 may, with small modifications, be applied to composite plate girders.