---
title: "Wind pressure on high-rise building, 50 m"
subject: "Design of Steel and Timber Structure"
chapter: 8
key: "semester_6/design_of_steel_and_timber_structure:wind-high-rise-50yr-kathmandu"
source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/wind-high-rise-50yr-kathmandu
site: BCT Engineering
---

# Wind pressure on high-rise building, 50 m

Design of Steel and Timber Structure · Chapter 8: Design of Roof Trusses

- Asked in: 2075 Bhadra, 8 marks; 2073 Magh, 10 marks (asked 2 times)
- Video: [Calculation of Wind load  | Design of steel structures and timber | IOE III/II PU MU |](https://www.youtube.com/watch?v=32ze2SwlHXk) — Parash Joshi - Civil Construction and Tutor
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/wind-high-rise-50yr-kathmandu
- Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-8

## Question

The high rise building at Kathmandu is to be constructed for a 50 years life; the size of the building is over 30 m. The height of the building is 50 m. Determine the wind pressure at the site and force on the truss. Where basic wind speed of Kathmandu is 47 m/sec. (Building size 40×30 m², at Sundhara, Kathmandu.)

## Short answer

Given Vb=47 m/s, k1=1, k2=1.05 (Category 4, Class B, 50 m), k3=1: Vz=49.35 m/s, pz=1.461 kN/m². For h/w=1.67, net coefficient -1.0 gives suction pressure 1.461 kN/m². Force on one slope (area 15.23×5=76.2 m²): 1.0×76.2×1.461=111.3 kN; whole truss 222.5 kN.

## Answer

**Assumptions.** $V_b = 47$ m/s (given). Kathmandu high-rise: terrain **Category 4** (city centre). Height 50 m, so the greatest dimension is 50 m, which does not exceed 50 m: **Class B**. Life 50 years: $k_1 = 1.0$; flat site: $k_3 = 1$. Plan 40 m × 30 m, so $w = 30$ m and $h/w = 50/30 = 1.67$. The roof truss is taken to span the 30 m width with a low slope $\alpha = 10^\circ$, truss spacing 5 m and normal permeability.

#### Step 1: Design wind speed (IS 875 Part 3:1987, cl. 5.3)

$$
V_z = V_b\,k_1k_2k_3
$$

- Basic wind speed $V_b = 47$ m/s.
- Risk coefficient $k_1 = 1.00$ (design life 50 years, Table 1).
- Terrain, height and size factor $k_2$: Category 4, Class B, at height 50 m: $k_2 = 1.05$ (Table 2, read directly).
- Topography factor $k_3 = 1.000$ (flat site).

$$
\begin{aligned} V_z &= 47\times1.00\times1.050\times1.000 = 49.35\ \text{m/s} \\ p_z &= 0.6V_z^2 = 0.6\times49.35^2 = 1461\ \text{N/m}^2 = 1.461\ \text{kN/m}^2 \end{aligned}
$$

#### Step 2: Pressure coefficients (Table 5, pitched roof of rectangular clad building)

Height ratio $h/w = 1.67$ (band $3/2 < h/w < 6$), roof angle $\alpha = 10.0^\circ$.

Internal pressure coefficient $C_{pi} = \pm0.2$ (normal permeability), cl. 6.2.3.

| Surface | $C_{pe}$ | $C_{pe} - C_{pi}$ ($C_{pi}=+0.2$) | $C_{pe} - C_{pi}$ ($C_{pi}=-0.2$) | $p_d$ (N/m²), $C_{pi}=+$ | $p_d$ (N/m²), $C_{pi}=-$ |
|---|---|---|---|---|---|
| Windward slope EF (wind 0°) | -0.70 | -0.90 | -0.50 | -1315 | -731 |
| Leeward slope GH (wind 0°) | -0.60 | -0.80 | -0.40 | -1169 | -585 |
| Slope EG (wind 90°) | -0.80 | -1.00 | -0.60 | -1461 | -877 |
| Slope FH (wind 90°) | -0.80 | -1.00 | -0.60 | -1461 | -877 |

Design pressure normal to the roof: $p_d = (C_{pe} - C_{pi})\,p_z$ (positive = towards the surface, negative = suction). The most severe case is slope EG (wind 90°), with $C_{pe} - C_{pi} = -1.00$, so $p_d = -1461$ N/m² $= -1.461$ kN/m².

#### Step 3: Wind force on the roof / truss

$F = (C_{pe} - C_{pi})\,A\,p_z$, acting normal to the roof surface. The most severe net coefficient, $-1.00$, is used for the maximum force.

- One slope of one truss (slope length $15/\cos10^\circ = 15.23$ m, spacing 5 m): $A = 76.2$ m², $F = 1.00\times76.2\times1.461 = 111.3$ kN (suction, i.e. acting away from the roof).

- Whole truss (both slopes): $A = 152.3$ m², $F = 1.00\times152.3\times1.461 = 222.5$ kN (suction, i.e. acting away from the roof).

#### Answer

- Design wind speed $V_z = 49.4$ m/s; design wind pressure $p_z = 1461$ N/m² (1.46 kN/m²).
- Net design pressure on the roof: up to $1461$ N/m² (1.46 kN/m²), suction.
- Force on one slope of one truss (slope length 15.23 m, spacing 5 m): 111.3 kN.
- Force on the whole truss (both slopes): 222.5 kN.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The building is 50 m high. Which class is used for k2?
   A. Class A
   B. Class B, since the greatest dimension is not greater than 50 m
   C. Class C, since it is over 30 m
   D. No class applies
   Answer: B. Class B covers 20 to 50 m; exactly 50 m is not above 50 m, so Class B and k2 = 1.05 at 50 m. (Key point: Class of building)

2. With Vb = 47 m/s, k1 = 1, k2 = 1.05, k3 = 1, what is pz?
   A. 1237 N/m^2
   B. 1325 N/m^2
   C. 1461 N/m^2
   D. 2922 N/m^2
   Answer: C. Vz = 47 x 1.05 = 49.35 m/s and pz = 0.6 x 49.35^2 = 1461 N/m^2. (Key point: Design wind pressure)

3. Which case governs the roof pressure?
   A. Windward slope EF at 0°, net -0.90
   B. Leeward slope GH at 0°, net -0.80
   C. Slope EG at 90°, Cpe - Cpi = -1.00 giving -1461 N/m^2 suction
   D. Windward slope with Cpi = -0.2, net -0.50
   Answer: C. The largest magnitude net coefficient is -1.00 (Cpe = -0.80, Cpi = +0.2) on EG/FH for wind at 90°. (Key point: Governing roof case)

4. With slope length 15.23 m and truss spacing 5 m, what force acts on one slope and the whole truss?
   A. 111.3 kN per slope; 222.5 kN for both slopes
   B. 222.5 kN per slope; 445 kN for both
   C. 76.2 kN per slope; 152.3 kN for both
   D. 1461 kN per slope
   Answer: A. A = 15.23 x 5 = 76.2 m^2, F = 1.00 x 76.2 x 1.461 = 111.3 kN; doubling for two slopes gives 222.5 kN. (Key point: Force on truss)

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