Structural Design of Masonry: Allowable Stress and Strength Design Principles
Design methods
TMS 402 permits allowable stress design (ASD) and strength design. ASD compares computed stresses to code allowable stresses; strength design uses factored loads and resistance factors. Empirical design is a separate, limited method.
ASD essentials
For masonry axial compression, a key allowable stress is: Fa = 0.25 f'm [1 - (h/140r)^2] for h/r <= 99 Fa = 0.25 f'm (70r/h)^2 for h/r > 99 where h is effective height, r is the radius of gyration, and f'm is the specified masonry compressive strength. The modulus Em is commonly taken as 900 f'm.
Worked example
An 8-in. CMU wall has f'm = 1,500 psi, h = 12 ft (144 in.), and r = 2.0 in. Then h/r = 72. Fa = 0.25(1,500)[1 - (72/140)^2] = 375(1 - 0.2645) = 276 psi. For a 96 in.2 gross area per foot, allowable axial load = 276 x 96 = 26,500 lb/ft, or 26.5 kip/ft. If the calculated load exceeds this, the design must change.
Traps
- Using f'm in place of the unit net-area strength, or vice versa.
- Forgetting the slenderness check and the h/r branch at 99.
- Mixing ASD allowable-stress checks with strength-design load factors.
- Applying the 1.33 increase without the code's wind or earthquake conditions.