Shear strength and rationality of strut-and-tie method for design of deep beams

The shear span-to-depth ratio (a/d) is the most important factor influencing the shear strength of reinforced concrete beam and makes the failure mechanism a complex one. As the a/d ratio decreases, the shear strength of beam increases, by altering the mode of failure from diagonal tension to diagonal compression. IS: 456 (2000) and Euro code 2 developed shear strength expression empirically from the experimental results of beams with a/d ratio of more than 2.0, and the same has been adopted with an enhancement factor for the design of beams with a/d ratio of less than 2.0, ignoring the mode of failure. The validity of these modified expressions has been reviewed using experimental database of 227 beam test results with a/d ratio of less than 2.0. In addition, the ACI 318-14 provisions based on the strut-and-tie model (STM) have been reviewed. The STM based approach seems to be more appropriate for the design of beams with a/d ratio less than 2.0. Due to its rationality, the strut-and-tie model for the design of structures with complex failure mechanism need to be recommended in the IS code of practice.