Modified strut-and-tie model for prestressed concrete deep beams

Due to the scarcity of rational models for predicting the shear strength of prestressed concrete deep beams, a modified strut-and-tie model (MSTM) is proposed for simply supported prestressed concrete deep beams in this paper. It is a simplified analytical model and several improvements have been made. The most remarkable one is that the effects of prestressing are represented by equivalent external loads explicitly built in the model. The Kupfer–Gerstle biaxial tension–compression criterion is also utilized to take into account the concrete softening effect, and based on the criterion a formula for shear strength prediction is derived. Besides, when determining the stress distribution factor, kk, both force and moment equilibrium are satisfied. The MSTM is evaluated using 56 test results of both pre- and post-tensioned deep beams, and the results show that predictions are accurate, consistent and conservative. Parametric studies further indicate that the assumptions for determining the factor kk and the stress in the prestressing tendon at ultimate, fps, are sufficiently accurate for the model predictions. It can be concluded that the MSTM is capable of predicting the shear strength of prestressed concrete deep beams and thus can be employed in practical designs.