--- title: "Method of wind load calculation on roof" subject: "Design of Steel and Timber Structure" chapter: 8 key: "semester_6/design_of_steel_and_timber_structure:wind-load-method-roof-truss" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/wind-load-method-roof-truss site: BCT Engineering --- # Method of wind load calculation on roof Design of Steel and Timber Structure · Chapter 8: Design of Roof Trusses - Asked in: 2074 Bhadra, 4 marks; 2073 Bhadra, 5 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-load-method-roof-truss - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-8 ## Question Explain the method of calculation of wind load on roof truss (sloped roof as per IS 875). ## Short answer Determine design wind pressure pz = 0.6(Vb×k1×k2×k3)². From IS 875 Table 5, obtain external and internal pressure coefficients for roof angle and h/w ratio. Net pressure = (Cpe - Cpi)pz, acting normal. Multiply by slope area to get wind force on truss, then combine with other loads per IS 800. ## Answer 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. ```text wind -> /\ windward slope: Cpe (EF) / \ leeward slope: Cpe (GH) pressure/ \suction (net p = (Cpe - Cpi) pz) ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In Vz = Vb k1 k2 k3, what does k2 represent? A. Risk factor from design life B. Topography factor C. Terrain, height and structure-size factor D. Internal pressure factor Answer: C. k1 is risk (design life), k2 is terrain/height/size (Table 2), k3 is topography. (Key point: Design wind speed factors) 2. What Cpi applies for a building with normal permeability (openings up to 5%)? A. +/-0.2 B. +/-0.5 C. +/-0.7 D. 0 Answer: A. Cpi is +/-0.2 for low openings, +/-0.5 for medium (5-20%) and +/-0.7 for large openings. (Key point: Internal pressure coefficient) 3. How are net pressure and the wind force on a roof surface found, and in which direction does it act? A. p = Cpe x pz; parallel to the roof B. p = (Cpe - Cpi) pz; F = p x A, acting normal to the roof surface C. p = (Cpe + Cpi) Vz; vertical always D. p = pz; acting horizontally Answer: B. Net pressure is (Cpe - Cpi) pz and the force F = (Cpe - Cpi) A pz acts normal to the surface; the more severe Cpi sign is taken. 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