Normal Strain-Adjusted Shear Strength Expression for Fully Grouted Reinforced Masonry Structural Walls

Since the conclusion of the test programs carried out by the Joint U.S.–Japan Technical Coordinating Committee on Masonry Research (TCCMAR) during the 1970s and 1980s, there has been little progress made to enhance the shear strength expressions used for masonry design. The experimentally-derived expressions currently employed by North American design codes are based on the 45° cracked member assumption and truss analogy, where shear strength is expressed as an algebraic summation of resistance offered by masonry, axial load, and shear reinforcement. By contrast, the modified compression field theory (MCFT), which has gained a wide acceptance within the concrete design community, demonstrates that the 45° cracked member assumption can be overly conservative. Yet the MCFT or similar equilibrium-based approaches have often been thought of as incompatible with masonry due to the latter’s complex anisotropic behavior. However, seismic design detailing requirements within North America typically require the highly-susceptible plastic hinge region of structural (shear) walls to contain vertical and horizontal steel reinforcement in fully grouted concrete block units. Under these circumstances, the anisotropic effects are greatly reduced and the development of a simplified approach to estimate the equilibrium conditions at the shear-critical zone of masonry structural walls is possible. Tests on masonry panels conducted at McMaster University in Canada are used with existing literature to define a set of constitutive relationships for cracked masonry subject to stress states that are typical in reinforced masonry structural walls. Subsequently, a methodology is proposed to accurately estimate the angle of inclination of shear cracking and the shear resistance offered by the horizontal reinforcement and the masonry compression strut accounting for aggregate interlock effects. The proposed normal strain-adjusted shear strength expression (NSSSE) was found to predict the shear strength of 57 wall tests reported in literature with a mean ratio of experimental to theoretical strengths of VExperimental/VTheory=1.16 (C.O.V.=11.4%) and a 95% percentile of VExperimental/VTheory=0.98.