Method of wind load calculation on roof

Wind load on a sloping roof truss is calculated by IS 875 (Part 3):1987 as follows.

Steps

1. Basic wind speed $V_b$ for the site from the wind map (Fig. 1) or Appendix A (3-second gust at 10 m height, Category 2, 50-year return period).
2. Design wind speed at the roof height: $V_z = V_b\,k_1k_2k_3$.
   • $k_1$: risk factor from the design life (Table 1; 1.0 for 50 years).
   • $k_2$: terrain, height and structure-size factor (Table 2). Choose the terrain category (1 to 4) and the building class from its greatest horizontal or vertical dimension: A (below 20 m), B (20 to 50 m) or C (above 50 m). Then read $k_2$ at the height of the roof.
   • $k_3$: topography factor (1.0 for flat sites; up to 1.36 for hills and ridges, Appendix C).
3. Design wind pressure: $p_z = 0.6V_z^2$ (N/m², with $V_z$ in m/s).
4. Pressure coefficients for the roof from Table 5 (pitched roofs of rectangular clad buildings): external coefficients $C_{pe}$ for wind angles 0° and 90°, depending on the roof angle $\alpha$ and the height ratio $h/w$ (interpolate between roof angles). Internal coefficient $C_{pi} = \pm0.2$ for normal permeability (openings up to 5%), $\pm0.5$ for medium openings (5 to 20%) and $\pm0.7$ for large openings (above 20%).
5. Net pressure on the roof surface: $p_d = (C_{pe} - C_{pi})\,p_z$. Take the combination ($C_{pi}$ positive or negative) that gives the more severe effect.
6. Wind force on a roof surface (or truss): $F = (C_{pe} - C_{pi})\,A\,p_z$, where $A$ is the surface area (slope length × truss spacing for one truss). The force acts normal to the roof surface.
7. Joint loads: divide the total normal force on a slope among the purlin positions (panel points of the rafter), with the end joints getting half.
8. Combine with dead and live loads using the limit state load combinations (IS 800 Table 4) and take the worst case, including uplift (suction) where the net wind load is upward.

   wind ->     /\   windward slope: Cpe (EF)
              /  \  leeward slope:  Cpe (GH)
      pressure/    \suction  (net p = (Cpe - Cpi) pz)