Shear and torsion interaction in prestressed hollow core units

Hollow core units are mainly designed to resist bending and shear. There are, however, many applications in which they are also subjected to torsion. The present calculation method for shear and torsion in the European Standard EN 1168 adds stresses from multiple influences without taking into account the softening of cracking concrete; therefore it is probably conservative. The main purpose of this work is to investigate the response of prestressed hollow core units subjected to selected combinations of shear and torsion by using non-linear finite element (FE) analyses. In previous work by the present authors, three-dimensional FE models of hollow core units were developed and validated by full-scale experiments. This paper shows how similar models were analysed for several load combinations of shear and torsion ratios. One load combination was validated by full-scale tests. The results from the FE analyses made it possible to present shear and torsion capacities in interaction diagrams useful in practical design. Moreover, the influences of the shear span and the prestressing transfer zone on the shear and torsion capacity were evaluated. It was concluded that the boundaries affected the capacity: shorter shear span gave higher capacity. The curved interaction diagrams obtained from the FE analyses were compared with linear diagrams from the analytical model in EN 1168. For most combinations the FE analyses showed higher capacities, except for combinations close to pure torsion, and pure shear for one of the geometries investigated.