Behaviour of FRP-PC members subjected to combined torsion, shear and bending

International design codes contain detailed provisions for determining the ultimate capacity of conventional Reinforced Concrete (RC) and Prestressed Concrete (PC) members subjected to combined bending, shear and torsion. Similar provisions lack for members with Fibre Reinforced Polymer (FRP) reinforcement. In general, there is paucity of research on the treatment of torsion combined with other stress resultants in FRP-PC structures. Critical issues such as the appropriate strength and inclination of the diagonal struts and failure criteria need to be further investigated. In the new edition of the Canadian Standard CSA S806 the compression field theory is applied for FRP-PC structures. In this paper this method is described and its results are compared with available experimental data regarding FRP-PC beams subjected to combined torsion, shear and bending. For comparison, another approach, which is based on the space truss model, is also used to evaluate the capacity of the same tested beams. Based on the critical analysis of the results, suggestions are made regarding the design of FRP-PC members against combined actions.