--- title: "Cross wall design, 125 mm slab, 3.2 m" subject: "Concrete Technology and Masonry Structure" chapter: 8 key: "semester_5/concrete_technology_and_masonry_structure:interior-cross-wall-125mm-3-2m" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/interior-cross-wall-125mm-3-2m site: BCT Engineering --- # Cross wall design, 125 mm slab, 3.2 m Concrete Technology and Masonry Structure · Chapter 8: Design of masonry walls for gravity loads - Asked in: 2073 Shrawan, 10 marks; 2068 Chaitra, 10 marks (asked 2 times) - Video: [2015 fall PoU| Design interior cross wall supporting unequal slab| Concrete Technology & Masonry St.](https://www.youtube.com/watch?v=i5t-6Krbkdw) — Civil Calculation - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/interior-cross-wall-125mm-3-2m - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-8 ## Question Design an interior cross wall of a two-storeyed building to carry 125 mm thick RCC slab with 3.2 m ceiling height. The wall is unstiffened and supports a 2.5 m wide slab on both sides. Assume necessary data relevant to Nepal. Live load on roof = 1.5 kN/m$^2$; live load on floor = 2.0 kN/m$^2$; floor finishing = 1.2 kN/m$^2$. [Figure: brick wall between slabs with 3200 mm storey heights, on a floor.] ## Short answer 230 mm thick interior cross wall, effective height 2.494 m, slenderness ratio 10.84. Required basic compressive stress 0.292 N/mm²; adopt bricks of crushing strength not less than 3.5 N/mm² with M2 mortar (1:6 cement:sand). ## Answer #### 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$^3$. | Item | Working | kN/m | |---|---|---| | Slab dead + finish | $25\times0.125 + 1.2 = 4.325$ kN/m$^2$ | - | | Roof slab reaction | $(4.325 + 1.5)\times2.5$ | 14.56 | | Floor slab reaction | $(4.325 + 2.0)\times2.5$ | 15.81 | | Wall per storey | $0.23\times3.20\times19$ | 13.98 | #### 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 | Section | Load $N$ (kN/m) | $\sigma=N/(t\times1000)$ (N/mm²) | Required $f_b=\sigma/(k_sk_ak_p)$ (N/mm²) | |---|---|---|---| | Top (2nd) storey base | 28.55 | 0.124 | 0.143 | | Ground storey base | 58.34 | 0.254 | 0.292 | #### Step 4: Brick strength and mortar (IS 1905 Table 8) Smallest standard unit strength for each section: | Section | Required $f_b$ | M2 (1:6) | M1 (1:5) | H2 (1:4) | |---|---|---|---|---| | Top (2nd) storey base | 0.143 | 3.5 | 3.5 | 3.5 | | Ground storey base | 0.292 | 3.5 | 3.5 | 3.5 | **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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the floor slab reaction per metre? A. 4.325 x 2.5 B. (4.325 + 1.5) x 2.5 C. (4.325 + 2.0) x 2.5 = 15.81 kN/m D. (1.2 + 2.0) x 2.5 Answer: C. Dead load with finish plus floor live load times 2.5 m. (Key point: Floor reaction) 2. What is the wall weight per storey? A. 0.23 x 3.20 x 19 = 13.98 kN/m B. 0.23 x 3.2 x 25 C. 3.2 x 19 D. 0.125 x 3.2 x 19 Answer: A. Brick masonry weighs 19 kN/m^3. (Key point: Self-weight) 3. What effective height is used? A. 3.2 m B. 1.5 x 3.325 m C. 3.325 m D. 0.75 x 3.325 = 2.494 m Answer: D. RCC slab on the wall gives full restraint. (Key point: Effective height) 4. Which section governs and what is the result? A. Top storey, requiring 15 N/mm^2 B. Ground storey, requiring 3.5 N/mm^2 brick with M2 mortar C. Ground storey with 10 N/mm^2 D. Top storey, requiring 7.5 N/mm^2 Answer: B. Required f_b = 0.292 N/mm^2 and 0.35 is provided with 3.5 N/mm^2 units. (Key point: Governing storey) ## Close variants set in other exams - [Cross wall design, 120 mm slab, 3 m wide](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Ainterior-cross-wall-120mm-3-0m-slab) — Concrete Technology and Masonry Structure, chapter 8, asked 4× ## Related in Concrete Technology and Masonry Structure - [Interior cross wall design: stiffened, 130 mm slab](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Ainterior-cross-wall-stiffened-130mm) — Concrete Technology and Masonry Structure, chapter 8 - [Cross wall design, 120 mm slab, 3.0 m](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Ainterior-cross-wall-120mm-2-5m-slab) — Concrete Technology and Masonry Structure, chapter 8, asked 2× - [Interior cross wall design: three-storey, 150 mm slab](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Ainterior-cross-wall-three-storey-150mm) — Concrete Technology and Masonry Structure, chapter 8, asked 2×