Minimum and Maximum Reinforcement Ratios for Out-of-Plane Flexural Design of Masonry Walls by the Strength Approach

The strength design provisions of the 2008 MSJC Code include requirements for minimum and maximum flexural reinforcement. In this section, the implications of those requirements for the out-of-plane flexural design of masonry walls are addressed.

Minimum Flexural Reinforcement by 2008 MSJC Code

The 2008 MSJC Code has no requirements for minimum flexural reinforcement for out-of-plane design of masonry walls.

Maximum Flexural Reinforcement by 2008 MSJC Code

The 2008 MSJC Code has a maximum reinforcement requirement (Sec. 3.3.3.5) that is intended to ensure ductile behavior over a range of axial loads. As compressive axial load increases, the maximum permissible reinforcement percentage decreases. For compressive axial loads above a critical value, the maximum permissible reinforcement percentage drops to zero, and design is impossible unless the cross-sectional area of the element is increased.

For walls subjected to out-of-plane forces, for columns, and for beams, the provisions of the 2008 MSJC Code set the maximum permissible reinforcement based on a critical strain condition in which the masonry is at its maximum useful strain, and the extreme tension reinforcement is set at 1.5 times the yield strain.

The critical strain condition for walls with a single layer of concentric reinforcement and loaded out-of-plane is shown in Fig. 6.21 along with the corresponding stress state. The parameters for the equivalent rectangular stress block are the same as those used for conventional flexural design. The height of the equivalent rectangular stress block is 0.80 fm′, and the depth is 0.80 c. The tensile reinforcement is assumed to be at fy. Locate the neutral axis using the critical strain condition