Theory of Plasticity
The truss analogy for shear analysis of RC beams relies on the lower-bound theorem of plasticity, which states that a lower bound to the true failure load can be found from any stress field in equilibrium that does not violate the yield condition. From this point of view, the inclination angle of the inclined struts can be freely chosen as long as yield conditions are not violated. These yield conditions are usually taken as the shear strength controlled