Analysis of the equal principal angles assumption in the shear design of reinforced concrete members

Several Compression Field Theories have been developed for the design of structural concrete members to resist shear. These theories adopt the simplifying assumption that the direction of principal tensile stress and the direction of principal tensile strain coincide. This assumption is known as the Equal Principal Angles assumption (EPAs), or Wagner’s hypothesis. Experimental evidence indicates that this is a reasonable simplification. This paper investigates potential differences in the directions (or angles) of principal stress and principal strain and evaluates the relative influence of design parameters on the divergence of these angles. Within practical ranges of these design parameters, the transverse reinforcement content is found to have the greatest influence on the applicability of the EPA assumption.