---
title: "Wind pressure on roof in Birgunj"
subject: "Design of Steel and Timber Structure"
chapter: 8
key: "semester_6/design_of_steel_and_timber_structure:wind-birgunj-sloping-roof"
source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/wind-birgunj-sloping-roof
site: BCT Engineering
---

# Wind pressure on roof in Birgunj

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

- Asked in: 2070 Magh, 6 marks
- 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-birgunj-sloping-roof
- Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-8

## Question

A building is situated in Birgunj, where the basic wind speed is found to be 60 m/sec. Find the wind pressure for the design of sloping roof of the building having following data:
Angle of slope of roof, $\alpha$ = 28°
Building height ratio, h/w = 0.75
$K_1 \times K_2 \times K_3$ = 0.70

## Short answer

Using the given combined factor, Vz = 60 x 0.70 = 42 m/s and pz = 0.6Vz^2 = 1.06 kN/m^2. For alpha = 28 degrees, h/w = 0.75 and Cpi = +/-0.2, the worst net pressure is 1.06 kN/m^2 suction on slope EG (wind 90 degrees).

## Answer

**Data.** $V_b = 60$ m/s (given), $k_1k_2k_3 = 0.70$ (combined, given), $\alpha = 28^\circ$, $h/w = 0.75$ (so the row $1/2 < h/w \le 3/2$ of Table 5). Permeability is not stated; **normal permeability** is assumed ($C_{pi} = \pm0.2$).

#### Step 1: Design wind speed and pressure
$$
\begin{aligned} V_z &= V_b\,(k_1k_2k_3) = 60\times0.70 = 42.0\ \text{m/s} \\ p_z &= 0.6V_z^2 = 0.6\times42.0^2 = 1058\ \text{N/m}^2 = 1.058\ \text{kN/m}^2 \end{aligned}
$$

#### Step 2: External pressure coefficients (Table 5, interpolated between $\alpha = 20^\circ$ and $30^\circ$)
For $\alpha = 28^\circ$ and $h/w = 0.75$: windward $C_{pe} = -0.7 + 0.8\times0.5 = -0.30$; leeward $C_{pe} = -0.5$; for wind at 90°: EG $= -0.80$, FH $= -0.76$ (interpolated).

#### Step 3: Net design pressure $p_d = (C_{pe} - C_{pi})p_z$
| Surface | $C_{pe}$ | $p_d$ (N/m²), $C_{pi} = +0.2$ | $p_d$ (N/m²), $C_{pi} = -0.2$ |
|---|---|---|---|
| Windward slope EF (wind 0°) | -0.30 | -529 | -106 |
| Leeward slope GH (wind 0°) | -0.50 | -741 | -318 |
| Slope EG (wind 90°) | -0.80 | -1058 | -635 |
| Slope FH (wind 90°) | -0.76 | -1016 | -593 |

(Negative = suction, acting away from the roof.)

#### Answer
Design wind speed $V_z = 42$ m/s; design wind pressure $p_z = 1058$ N/m² ($\approx 1.06$ kN/m²). The most severe net pressure on the roof is **1058 N/m² (1.06 kN/m²) suction** on Slope EG (wind 90°) (net $C_p = -1.00$). On the windward slope for wind normal to the ridge it is $-529$ N/m² with $C_{pi} = +0.2$ and $-106$ N/m² with $C_{pi} = -0.2$.

## Check yourself

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

1. Given Vb = 60 m/s and k1k2k3 = 0.70, what is Vz?
   A. 42 m/s
   B. 60 m/s
   C. 85.7 m/s
   D. 60.7 m/s
   Answer: A. Vz = Vb x (k1k2k3) = 60 x 0.70 = 42 m/s. (Key point: Design speed with combined factors)

2. What is pz?
   A. 2160 N/m^2
   B. 1058 N/m^2
   C. 25.2 N/m^2
   D. 1764 N/m^2
   Answer: B. pz = 0.6 x 42^2 = 1058 N/m^2. (Key point: Design pressure)

3. Which row of the Table 5 is used for h/w = 0.75?
   A. h/w <= 1/2
   B. h/w > 3/2
   C. 1/2 < h/w <= 3/2
   D. h/w = 1 only
   Answer: C. 0.75 lies between 1/2 and 3/2, so that row is used, interpolated by roof angle. (Key point: Table 5 row)

4. Which case gives the greatest roof suction?
   A. Windward EF with Cpi = +0.2, 529 N/m^2
   B. Leeward GH with Cpi = +0.2, 741 N/m^2
   C. Slope FH with Cpi = -0.2, 593 N/m^2
   D. Slope EG at 90 degrees, net -1.00, i.e. 1058 N/m^2
   Answer: D. EG has Cpe = -0.80, and with Cpi = +0.2 the net is -1.00. (Key point: Governing case)

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