Wind pressure on roof in Birgunj

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 $p_d = (C_{pe} - C_{pi})p_z$

(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 $C_{pi} = -0.2$.