Wind Load Calculation — ASCE 7 MWFRS and Components & Cladding
Wind loads on structures are calculated per ASCE 7-22 Chapters 26–31. The main wind force resisting system (MWFRS) design uses the Directional Procedure (Chapter 27) or Envelope Procedure (Chapter 28), while components and cladding (C&C) use Chapter 30. This reference covers the key equations, wind pressure coefficients, and a step-by-step calculation example.
Wind Load Calculation Steps — Directional Procedure (Chapter 27)
Step 1: Determine basic wind speed V (mph) from Figure 26.5-1 for Risk Category
Step 2: Determine exposure category (B, C, or D)
Step 3: Compute velocity pressure qz or qh
Step 4: Apply pressure coefficients Cp or GCp
Step 5: Calculate design wind pressure p
Basic Wind Speed by Risk Category
ASCE 7-22 provides separate maps for each risk category:
| Risk Category | Use | Typical V (mph) — Coastal SE US | Typical V — Midwest | Typical V — Mountain West |
|---|---|---|---|---|
| I | Low hazard (agricultural) | 110–130 | 90–100 | 100–110 |
| II | Standard (most buildings) | 120–160+ | 95–110 | 105–120 |
| III | High hazard (schools, hospitals) | 130–180+ | 100–120 | 110–130 |
| IV | Essential facilities | 140–200+ | 105–125 | 115–135 |
Note: Coastal hurricane zones have dramatically higher wind speeds. Miami/South Florida: V ≈ 160–185 mph (Risk II). Use ASCE 7 Figure 26.5-1A/B/C for the authoritative values.
Exposure Categories
| Category | Description | Typical Location |
|---|---|---|
| B | Urban/suburban, trees, buildings up to 30 ft for ≥ 1500 ft upwind | Most inland building sites |
| C | Open terrain, scattered obstructions < 30 ft for ≥ 1500 ft | Open fields, coastlines > 1500 ft from ocean |
| D | Flat, unobstructed coastline, water surfaces | Ocean shorelines, lakefronts |
Exposure D produces the highest wind pressures; exposure B produces the lowest.
Velocity Pressure Equation
qz = 0.00256 × Kz × Kzt × Kd × Ke × V² (lb/ft²)
Where:
Kz = velocity pressure exposure coefficient (Table 26.10-1)
Kzt = topographic factor (1.0 for flat terrain, up to 1.58 for hilltops)
Kd = wind directionality factor (0.85 for buildings, 0.95 for chimneys)
Ke = ground elevation factor (1.0 for z ≤ 6,000 ft; slight reduction above)
V = basic wind speed (mph)
Kz Exposure Coefficients (Table 26.10-1)
| Height z (ft) | Exposure B | Exposure C | Exposure D |
|---|---|---|---|
| 0–15 | 0.57 | 0.85 | 1.03 |
| 20 | 0.62 | 0.90 | 1.08 |
| 25 | 0.66 | 0.94 | 1.12 |
| 30 | 0.70 | 0.98 | 1.16 |
| 40 | 0.76 | 1.04 | 1.22 |
| 50 | 0.81 | 1.09 | 1.27 |
| 60 | 0.85 | 1.13 | 1.31 |
| 70 | 0.89 | 1.17 | 1.34 |
| 80 | 0.93 | 1.21 | 1.38 |
| 100 | 0.99 | 1.26 | 1.43 |
| 120 | 1.04 | 1.31 | 1.48 |
| 160 | 1.13 | 1.39 | 1.55 |
| 200 | 1.20 | 1.46 | 1.61 |
Design Wind Pressure — MWFRS (Buildings)
For Enclosed and Partially Enclosed Buildings (Chapter 27)
p = q × G × Cp − qi × (GCpi)
Where:
q = qz for windward wall, qh for leeward/side walls and roof
G = gust factor (0.85 for rigid buildings h/least dimension < 4 and n1 > 1 Hz)
Cp = external pressure coefficient (Table 27.3-1)
qi = qh for enclosed buildings; qz for partially enclosed
GCpi = internal pressure coefficient (±0.18 enclosed, ±0.55 partially enclosed)
External Pressure Coefficients Cp — Walls (Table 27.3-1)
| Surface | Cp |
|---|---|
| Windward wall | +0.8 |
| Leeward wall (L/B = 0 to 1) | −0.5 |
| Leeward wall (L/B = 2) | −0.3 |
| Side walls | −0.7 |
External Pressure Coefficients Cp — Roofs
| Roof Type | Wind Direction | h/L | Cp |
|---|---|---|---|
| Flat roof (θ < 10°) | Normal to ridge | — | −0.3 to −0.7 (suction) |
| Gable (0–5°) | Normal to ridge | ≤ 0.25 | −0.7 windward; −0.4 leeward |
| Gable (10°) | Normal to ridge | ≤ 0.25 | −0.9/+0.2 windward; −0.3 leeward |
| Gable (20°) | Normal to ridge | ≤ 0.25 | −0.5/+0.3 windward; −0.2 leeward |
| Gable (30–45°) | Normal to ridge | any | +0.2 to +0.8 windward; −0.2 leeward |
| Hip roof (20°) | Normal to ridge | any | −0.6 windward; −0.5 leeward |
Components and Cladding (C&C) — Chapter 30
C&C pressures are generally higher than MWFRS because they apply to smaller tributary areas. Used to design roof panels, cladding, windows, and connections.
p = qh × [(GCp) − (GCpi)]
GCp = external pressure coefficient for C&C (from Figures 30.3-1 through 30.3-7)
Depends on: zone (1/2/3 interior/edge/corner), effective wind area, roof geometry
Effective wind area: The loaded area contributing to the force on a component.
- For structural members: span × tributary width
- For cladding panels: span × fastener spacing
C&C pressure is always higher in corners (Zone 3 >> Zone 2 > Zone 1).
Typical C&C Pressure Ranges (90 mph basic wind, Exposure C, h = 30 ft)
| Application | Net Pressure Range (psf) | Controls |
|---|---|---|
| Wall cladding, Zone 1 (interior) | ±15–20 | Inward pressure |
| Wall cladding, Zone 2 (edge) | ±20–30 | Outward suction |
| Roof cladding, Zone 1 | −20 to +10 | Upward suction |
| Roof cladding, Zone 2 (perimeter) | −30 to +10 | High uplift at edges |
| Roof cladding, Zone 3 (corner) | −40 to +10 | Maximum uplift at corners |
| Skylights, windows | ±20–35 | Positive inward pressure |
Worked Example: Simple Office Building
Given: 2-storey office, 40 ft × 60 ft plan, 25 ft eave height, flat roof, Chicago suburb
- Risk Category II → V = 100 mph (approximately; verify with ASCE 7 map)
- Exposure B (suburban)
- Kd = 0.85, Kzt = 1.0, Ke = 1.0, GCpi = ±0.18 (enclosed)
Velocity pressure at roof (z = 25 ft): qh = 0.00256 × 0.66 × 1.0 × 0.85 × 1.0 × 100² = 14.3 psf
Windward wall pressure: p = 14.3 × 0.85 × 0.8 − 14.3 × (−0.18) = 9.7 + 2.6 = 12.3 psf (inward)
Leeward wall pressure (L/B = 60/40 = 1.5, interpolate Cp ≈ −0.4): p = 14.3 × 0.85 × (−0.4) − 14.3 × (0.18) = −4.9 − 2.6 = −7.5 psf (outward/suction)
Total lateral wind force on 25 ft × 40 ft wall: F_wind = (12.3 + 7.5) × 25 × 40 = 19,800 lb = 19.8 kips per frame
Frequently Asked Questions
What is the difference between MWFRS and C&C? MWFRS (Main Wind Force Resisting System) loads are used to design the primary lateral system: moment frames, shear walls, diaphragms. C&C (Components & Cladding) loads apply to individual panels, windows, roof sheets, and their connections. C&C loads are higher than MWFRS for small tributary areas. Use MWFRS for frames and shear walls; use C&C for cladding, purlins, and facade elements.
Why are roof corner zones designed for higher pressures? Wind flow separates at building corners and eaves, creating vortices that generate intense suction in corners. C&C Zone 3 (corner) pressures can be 2–3× higher than interior Zone 1 pressures. Corner and edge zones must use heavier fastening, thicker panels, or additional attachments compared to interior panels.
Is the basic wind speed in ASCE 7 a service-level or strength-level load? Since ASCE 7-10, wind speeds are at strength level (equivalent to the old 3-second gust speed × 1.26). For LRFD, apply wind load W with a 1.0 factor (LC4: 1.2D + 1.0W + L). For ASD, apply 0.6W to convert back to service level. This changed from pre-2010 practice where 0.6×70 mph wind was used with ASD.
Do I need to check wind uplift on roof beams? Yes. Roof purlins and beams must be checked for uplift (negative pressure creates net upward force on the roof structure). LRFD LC5: 0.9D + 1.0W is the governing combination for uplift. Dead load is reduced to 0.9 to find the minimum stabilizing force.
How does exposure category affect design wind pressure? Exposure category controls the velocity pressure exposure coefficient Kz, which directly scales the velocity pressure qz. At a roof height of 30 ft, Kz = 0.70 for Exposure B (suburban), 0.98 for Exposure C (open terrain), and 1.16 for Exposure D (coastal flat terrain). This means an Exposure D building at 30 ft experiences approximately 66% higher velocity pressure than the same building in Exposure B at the same height. Selecting the correct exposure category is therefore one of the most consequential steps in wind load calculation.
What is velocity pressure qz and how is it calculated? Velocity pressure qz is the dynamic pressure exerted by wind moving at speed V at height z, computed as qz = 0.00256 × Kz × Kzt × Kd × Ke × V² (lb/ft²). The constant 0.00256 converts miles-per-hour wind speed to pressure using standard air density at sea level. Kz accounts for height-dependent wind speed profile, Kzt for topographic speed-up over hills, Kd for the reduced probability of the design wind speed occurring simultaneously from the worst-case direction, and Ke for slight air density reduction at high elevations. For a typical flat-terrain suburban building at 30 ft with V = 115 mph, qz ≈ 0.00256 × 0.70 × 1.0 × 0.85 × 1.0 × 115² ≈ 20.2 psf.
Run This Calculation
→ Wind Load Calculator — ASCE 7 / AS 1170.2 wind pressure calculation for MWFRS and components & cladding.
→ Load Combinations Calculator — combine wind loads with dead and live loads using ASCE 7 LRFD or ASD load factors.
→ Portal Frame Calculator — rafter and column design for portal frames under combined gravity and wind loading.
Related pages
- Load Combinations ASCE 7 — how wind fits into LRFD/ASD load combinations
- Snow Load Calculation — companion gravity load for roof design
- Steel Beam Span Guide — span capacity under combined gravity + wind
- Column Capacity Calculator — combined axial + bending under wind
- Deflection Limits Reference — lateral drift limits for wind design
- Reference tables directory
- How to verify calculator results
- Seismic Design Categories
- Floor Live Load Reference
- Steel Roof Framing Reference
- snow load calculator
Wind loads per ASCE 7-22 Chapters 26–31. Basic wind speed must be determined from the project site using ASCE 7 Figure 26.5-1. Local jurisdictions may adopt modified wind speed maps. Always verify with the authority having jurisdiction.
Disclaimer (educational use only)
This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.
All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.
The site operator provides the content "as is" and "as available" without warranties of any kind. To the maximum extent permitted by law, the operator disclaims liability for any loss or damage arising from the use of, or reliance on, this page or any linked tools.