Cross wall design, 125 mm slab, 3.2 m

Data and assumptions

• Design code: IS 1905:1987; NBC 109 follows the same permissible-stress method.
• Interior cross wall 230 mm thick (one brick, Nepal practice), modular bricks, $k_p=1$. Brick masonry 19 kN/m$^3$, RCC 25 kN/m$^3$.
• Stated slab width is taken as the tributary width of slab per metre run of wall (each side contributing half of its span). Finish applies to roof and floors; live load not reduced.
• Sections are checked at the base of each storey (wall height for weight = clear ceiling height).
• Wall unstiffened, so slenderness is governed by height. Slab 125 mm, ceiling height 3.2 m, tributary width 2.5 m.

Step 1: Loads per metre run of wall

Unit weights (IS 875 Part 1): RCC 25 kN/m$^3$, brick masonry 19 kN/m$^3$.

Step 2: Slenderness ratio and factors

Interior wall with slabs on both sides: load is axial ($e=0$, IS 1905 App. A-4). RCC slab bearing on the wall gives full restraint (Table 4 Sl. 1): $h_{ef}=0.75H$, with $H=3.20+0.125=3.325$ m.

$$\begin{aligned} h_{ef} &= 0.75\times3.325 = 2.494\ \text{m}\\ SR &= \frac{2.494}{0.23} = 10.84\ (<27) \end{aligned}$$

$k_s = 0.869$ (Table 9, $e=0$, interpolated), $k_a = 1.0$ (area 0.23 m$^2$/m $\ge 0.2$), $k_p=1.0$.

Step 3: Stress and required $f_b$ at the base of each storey

Step 4: Brick strength and mortar (IS 1905 Table 8)

Smallest standard unit strength for each section:

Answer: 230 mm thick wall; the ground storey governs, requiring 3.5 N/mm$^2$ brick with M2 (1:6) mortar (required $f_b=0.292$ N/mm$^2$; $f_b$ provided 0.35). The same brick and mortar can be used throughout. As a practical minimum, adopt bricks of not less than 5 N/mm$^2$ with 1:6 cement mortar (SR = 10.84 < 27, OK).