Permissible compressive stress of masonry as per IS 1905

IS 1905 (Code of practice for structural use of unreinforced masonry) uses the working stress method. The permissible stress is the basic compressive stress multiplied by factors for slenderness, area and shape:

$$f_c = f_b \times k_s \times k_a \times k_p$$

Basic compressive stress $f_b$

It is the compressive stress that a short, concentrically loaded masonry element can carry safely. It is given in a table against the compressive strength of the masonry unit and the grade of mortar (cement–sand, cement–lime–sand or lime mortars). Higher unit strength and stronger mortar give higher $f_b$. For ordinary brick masonry it is of the order of 0.5 to 2 N/mm².

Modification factors

1. Stress reduction factor $k_s$: accounts for slenderness ($SR$) and eccentricity of load. It is 1.0 for low slenderness (about 6 or less) and reduces as $SR$ rises. Typical values for axial load:

2. Area factor $k_a$: small sections have a higher chance of defects. For a loaded area $A < 0.2$ m², $k_a = 0.7 + 1.5A$ (with $A$ in m²); $k_a = 1.0$ for $A \ge 0.2$ m².
3. Shape modification factor $k_p$: applies to units whose height-to-width ratio differs from that of the standard brick; for bricks $k_p = 1.0$. Squat blocks give higher values, which are taken from tables.

Increase for wind and earthquake

When wind or earthquake loads act together with gravity loads, the permissible stresses may be increased, commonly by one-third (33 percent) as in working stress design (some editions of the code quote 25 percent, so check the edition in use). The section must also be safe for the gravity load alone.

Other points

• The permissible stress is checked against the actual stress $P/A$ (plus bending effects if the load is eccentric).
• The slenderness ratio of a load-bearing wall should not exceed 27.
• Permissible tensile and shear stresses are given separately and are small; masonry is not designed to take tension.