Interior cross wall design: stiffened, 130 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$^3$.
• 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 3.2 m long and stiffened by cross walls at its ends (effective length $1.0L=3.2$ m, Table 5), which is greater than $h_{ef}$, so height governs the slenderness. Slab 130 mm, ceiling 2.8 m, slab span 2.0 m each side (tributary 2.0 m).
• Finish 1.0 kN/m$^2$, live load 1.5 kN/m$^2$ (roof) and 2.0 kN/m$^2$ (floor) assumed as per IS 875 Part 2 for residential use.

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 $H=2.80+0.130=2.930$ m.

$$\begin{aligned} h_{ef} &= 0.75\times2.930 = 2.197\ \text{m}\\ SR &= \frac{2.197}{0.23} = 9.55\ (<27) \end{aligned}$$

$k_s = 0.903$ (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 $f_b=0.233$ 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 = 9.55 < 27, OK).