Load bearing wall: crushing strength and mortar

Data and assumptions

• "250 cm" is read as 250 mm (25 cm) thick, since a 2.5 m thick load-bearing wall is not meaningful. Modular bricks, $k_p=1$.
• Axial load 79.5 kN/m at the base inclusive of self weight ($e=0$). Wall 5 m long between cross walls; clear height 3.5 m between 13 cm RCC slabs, so $H=3.5+0.13=3.63$ m.
• Design code: IS 1905:1987 (Tables 4 to 10); NBC 109 follows the same permissible-stress method.
• Brick masonry unit weight 19 kN/m$^3$ (IS 875 Part 1); modular brick, height/width $\le 0.75$, so $k_p=1$.

Step 1: Effective height, length and slenderness ratio (IS 1905 cl. 4.3, 4.4, 4.6)

Height between slab centres $H = 3.63$ m. Effective height $h_{ef} = 0.75 H = 0.75\times3.63 = 2.723$ m. Effective length (wall supported by a cross wall at each end, Table 5 Sl. 3) $L_{ef} = 1.0L = 5.00$ m. The lesser value governs:

$$SR = \frac{h_{ef}}{t} = \frac{2.723}{0.250} = 10.89\ (<27,\ \text{Table 7, cement mortar})$$

Step 2: Design load per metre run at the base

Load at the base (self weight already included) $P = 79.50$ kN/m.

Stress on the wall:

$$\sigma = \frac{P}{A} = \frac{79.50\times10^3}{250\times10^3\ \text{mm}^2/\text{m}} = 0.318\ \text{N/mm}^2$$

Step 3: Reduction factors

• Stress reduction factor $k_s$ (Table 9, $SR=10.89$, $e/t=0.000$, by linear interpolation) $= 0.868$
• Area reduction factor $k_a$ (cl. 5.4.1.2): area per metre 0.25 m$^2$ is not less than 0.2 m$^2$, so $k_a = 1.0$
• Shape factor $k_p = 1.0$ (modular bricks, height/width $\le 0.75$, Table 10)
• $e/t \le 1/24$: no increase

Step 4: Required basic compressive stress

$$\sigma \le f_b\,k_s\,k_a\,k_p\times1.00 \Rightarrow f_b \ge \frac{0.318}{0.868\times1.0\times1.0\times1.00} = 0.366\ \text{N/mm}^2$$

Step 5: Strength of brick and grade of mortar (IS 1905 Table 8)

Smallest standard unit strength giving $f_b \ge 0.366$ N/mm$^2$:

Answer: use bricks of crushing strength not less than 5 N/mm$^2$ with mortar M2 (cement:sand 1:6) ($f_b = 0.44$ N/mm$^2$ $\ge$ 0.366). Other combinations in the table are also acceptable; a stronger mortar allows a weaker brick.