Interior cross wall design: three-storey, 150 mm slab

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$^3$.
• The stated slab width is taken as the tributary width of slab per metre run of wall (each side contributing half of its span). The finish applies to the roof and floors; live load is not reduced.
• Sections are checked at the base of each storey (wall height for self weight = clear ceiling height).
• Mortar 1:4 cement:sand = grade H2 (IS 1905 Table 1) in all storeys.

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: the load is axial ($e=0$, IS 1905 App. A-4). An RCC slab bearing on the wall gives full restraint (Table 4 Sl. 1): $h_{ef}=0.75H$, with $H=3.50+0.150=3.650$ m.

$$\begin{aligned} h_{ef} &= 0.75\times3.650 = 2.737\ \text{m}\\ SR &= \frac{2.737}{0.23} = 11.90\ (<27) \end{aligned}$$

$k_s = 0.842$ (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)

Mortar fixed as H2 (cement:sand 1:4):

Answer: 230 mm wall, mortar H2 (1:4). Minimum brick strength: top storey 3.5 N/mm$^2$, second storey 5 N/mm$^2$, ground storey 7.5 N/mm$^2$ (slenderness ratio 11.90 < 27 in all storeys). For simplicity, the whole wall may be built with 7.5 N/mm$^2$ bricks in H2 mortar, which also gives extra safety in the upper storeys.