Factors in design of masonry

Design of masonry (IS 1905) is based on working stresses, so the following factors are to be considered:

1. Strength of masonry unit and mortar grade: the basic compressive stress $f_b$ depends on both (Table 8); stronger mortar and unit give higher $f_b$.
2. Slenderness ratio: effective height or length divided by effective thickness, whichever is less; limited to 27 (cement mortar) for walls and 12 for columns (Table 7).
3. Effective height and length: depend on end restraint, such as RCC slab (0.75 H), timber floor (0.85 H or 1.0 H), free top (1.5 H) (Tables 4 and 5).
4. Eccentricity of loading: eccentricity ratio $e/t$ reduces the permissible stress (Table 9); App. A gives guidance.
5. Area of section: for sections smaller than 0.2 m$^2$ the area reduction factor $k_a$ applies.
6. Shape of masonry unit: height-to-width ratio of units gives shape factor $k_p$ (Table 10).
7. Type of loads: vertical load, wind, earthquake; no tension in masonry normally; limited flexural tension (0.07 N/mm$^2$ for M1 mortar) for lateral loads.
8. Openings and stiffening: openings reduce the effective area; stiffening by cross walls, piers or buttresses increases effective thickness (Table 6).
9. Workmanship and quality control: raked joints reduce thickness; curing, bond and supervision.
10. Seismic zone: buildings in zones III to V need special strengthening (IS 4326 / NBC 105, 109).