Wind Loading on Steel Structures — ASCE 7, AS 1170.2, EN 1991-1-4
Wind load determination: velocity pressure (qz), exposure categories, MWFRS vs C&C pressures, directional procedure, and cross-code comparison.
Wind load fundamentals
Wind load on a building is the result of air flowing around and over the structure, creating positive pressure on the windward face, negative pressure (suction) on the leeward and side faces, and uplift on the roof. The magnitude depends on wind speed, terrain exposure, building height, shape, and the tributary area of the component being designed.
Wind loading governs the lateral design of most low-rise and mid-rise steel buildings in non-seismic regions. It also governs the design of roof cladding, purlins, girts, and connections in virtually all buildings regardless of seismic zone.
ASCE 7-22 velocity pressure formula
The fundamental equation for velocity pressure at height z is:
qz = 0.00256 x Kz x Kzt x Kd x Ke x V^2 (psf)
where:
- V = basic wind speed (mph) — 3-second gust at 33 ft in Exposure C. ASCE 7-22 maps provide V for different risk categories (MRI 300, 700, 1700, 3000 years).
- Kz = velocity pressure exposure coefficient — increases with height and varies by exposure category.
- Kzt = topographic factor — 1.0 for flat terrain, up to 1.5-2.0 for hilltops and escarpments.
- Kd = wind directionality factor — 0.85 for buildings (accounts for reduced probability that maximum wind comes from the critical direction).
- Ke = ground elevation factor — 1.0 at sea level, decreasing at higher elevations (lower air density). New in ASCE 7-22.
Exposure categories
| Category | Description | Example | Kz at 33 ft | Kz at 100 ft |
|---|---|---|---|---|
| B | Urban, suburban, wooded | City center, dense residential | 0.70 | 0.90 |
| C | Open terrain, scattered obstructions | Flat farmland, airport | 0.85 | 1.04 |
| D | Flat, unobstructed waterfront | Coastal, lake shore | 0.99 | 1.16 |
Exposure B produces the lowest wind loads; Exposure D the highest. Most building sites default to Exposure C unless the engineer can demonstrate that sufficient upwind roughness exists for Exposure B (requires 2,600 ft of roughness in all directions for MWFRS).
MWFRS vs C&C
ASCE 7 distinguishes between:
- MWFRS (Main Wind Force Resisting System) — the overall structural frame, bracing, diaphragms, and foundations. Uses lower pressure coefficients because the wind load is averaged over large tributary areas. Designed using the Directional Procedure (Chapter 27) or Envelope Procedure (Chapter 28).
- C&C (Components and Cladding) — individual elements (purlins, girts, cladding panels, fasteners) with small tributary areas. Uses higher pressure coefficients (1.5-3.0 times MWFRS values) because wind pressures are not averaged over large areas — local peak pressures at corners and edges are much higher than area-averaged pressures.
This distinction means a purlin designed for MWFRS pressures is under-designed. Always use C&C pressures for individual framing members.
Worked example — MWFRS wind pressure on a 3-story building
Building: 3-story office, 40 ft (12.2 m) tall, 100 ft x 60 ft plan, flat roof. Location: V = 115 mph (ASCE 7-22, Risk Category II). Exposure C. Flat terrain (Kzt = 1.0). Sea level (Ke = 1.0). Kd = 0.85.
Velocity pressure at roof height (z = 40 ft): Kz = 0.87 (from ASCE 7 Table 26.10-1, Exposure C, interpolated). qz = 0.00256 x 0.87 x 1.0 x 0.85 x 1.0 x 115^2 = 0.00256 x 0.87 x 0.85 x 13,225 = 25.0 psf.
Design wind pressure on windward wall (MWFRS, Directional Procedure): p = q x G x Cp - qi x (GCpi). For windward wall: Cp = 0.8, G = 0.85 (rigid building gust factor). p_windward = 25.0 x 0.85 x 0.8 = 17.0 psf (external). For leeward wall (L/B = 100/60 = 1.67): Cp = -0.35. p_leeward = 25.0 x 0.85 x (-0.35) = -7.4 psf (suction).
Total frame pressure at roof level = 17.0 - (-7.4) = 24.4 psf (net, windward to leeward). For a 3-story braced frame, base shear is approximately: V = average pressure x tributary height x building width = 22 psf (averaged over height) x 40 ft x 60 ft = 52,800 lb = 52.8 kips.
Internal pressure: For an enclosed building, GCpi = +/- 0.18. Internal pressure = 0.18 x 25.0 = 4.5 psf. This adds to or subtracts from the external pressure on each surface.
Code comparison — wind load provisions
| Aspect | ASCE 7-22 | AS/NZS 1170.2 | EN 1991-1-4 | NBCC 2020 |
|---|---|---|---|---|
| Reference wind speed | 3-second gust at 33 ft | 3-second gust at 10 m (regional) | 10-min mean at 10 m | Hourly mean at 10 m |
| Speed conversion | V_3s (direct) | V_R (regional) | V_b x c_dir x c_season | q = CV^2 (tables) |
| Pressure formula | qz = 0.00256 Kz Kzt Kd Ke V^2 | qz = 0.5 x rho x [V_des x M_z,cat]^2 | qp(z) = 0.5 x rho x v_m^2(z) x [1 + 7Iv(z)] | p = Iw x q x Ce x Ct x Cp |
| Exposure categories | B, C, D | Terrain categories 1-4 | Terrain categories 0-IV | Open, rough |
| Internal pressure | GCpi = +/- 0.18 (enclosed) | Cpi (Table 5.1) | cpi (Table 7.1) | Cpi (Table) |
| C&C method | Chapter 30 | Cl. 5.4 (local pressure) | EN 1991-1-4 Cl. 7.2 | NBCC Commentary |
The wind speed definitions differ significantly between codes. ASCE 7 uses a 3-second gust, EN 1991-1-4 uses a 10-minute mean, and NBCC uses an hourly mean. A 3-second gust of 115 mph corresponds to approximately an hourly mean of 80 mph. Converting between codes requires careful attention to the gust factor and averaging period.
Common pitfalls
- Using MWFRS pressures for cladding and purlin design. C&C pressures at roof corners (Zone 3) can be 2-3 times the MWFRS roof pressure. Purlins, girts, clips, and fasteners must be designed for C&C pressures, not MWFRS.
- Claiming Exposure B without verifying upwind roughness. ASCE 7 requires 2,600 ft of continuous Exposure B roughness upwind for MWFRS (1,500 ft for C&C). A building on the edge of a suburb facing open farmland must use Exposure C for that direction.
- Ignoring internal pressure in enclosed buildings. Internal pressure (+/- 0.18 x qh) acts on every surface. For roof uplift, internal pressure adds to external suction, increasing total uplift by 20-40 percent. Missing this means under-designed roof connections.
- Not checking the partially enclosed condition. If a large opening (garage door, loading dock) can be breached by windborne debris, the building becomes partially enclosed with GCpi = +/- 0.55 instead of +/- 0.18. This triples the internal pressure and can double the design uplift on the roof.
Run this calculation
Related references
- Load Combinations ASCE 7
- Live Load Reference
- Wind Load Calculation
- Structural Load Path
- Diaphragm Action
- Load Combinations Guide
- How to Verify Calculations
Disclaimer
This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.