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.
ASCE 7-22 Chapters 26-30 procedure overview
ASCE 7-22 organizes wind load provisions into six chapters, each addressing a different building type or calculation method. Understanding which chapter applies is the first step in any wind design:
| Chapter | Title | Building type | Method |
|---|---|---|---|
| 26 | General Requirements | All buildings | Definitions, wind speed maps, exposure categories, Kz/Kzt/Kd tables |
| 27 | MWFRS - Directional Procedure | Enclosed, partially enclosed, and open buildings of all heights | Calculate pressures on each surface using Cp and GCpf coefficients |
| 28 | MWFRS - Envelope Procedure | Enclosed and partially enclosed low-rise buildings (h <= 60 ft) | Two-way loading using GCpf coefficients from Figure 28.3-1 |
| 29 | MWFRS - Buildings with Gable/Mansard Roofs | Specific roof geometries | Pressure coefficients for various roof angles and geometries |
| 30 | Components and Cladding | All buildings | C&C pressure coefficients for walls, roofs, and openings |
Step-by-step wind load determination per ASCE 7-22
- Determine Risk Category (I, II, III, or IV) from ASCE 7 Table 1.5-1 based on building occupancy
- Obtain basic wind speed V from ASCE 7 Figures 26.5-1A through 26.5-1D based on Risk Category and location
- Determine wind directionality factor Kd from ASCE 7 Table 26.6-1 (0.85 for buildings)
- Determine exposure category (B, C, or D) from ASCE 7 Section 26.7
- Determine topographic factor Kzt from ASCE 7 Section 26.8 (1.0 for flat terrain)
- Determine ground elevation factor Ke from ASCE 7 Table 26.9-1 (1.0 at sea level)
- Calculate velocity pressure qz using ASCE 7 Eq. 26.10-1
- Determine external pressure coefficients (Cp for MWFRS, GCp for C&C)
- Determine internal pressure coefficient GCpi from ASCE 7 Table 26.13-1
- Calculate design wind pressure p = qz x G x Cp - qi x GCpi (MWFRS) or p = qz x GCp - qi x GCpi (C&C)
Velocity pressure coefficients Kz table
The velocity pressure exposure coefficient Kz varies by exposure category and height above ground. These are the most frequently referenced values in wind load calculations:
| Height z (ft) | Exposure B | Exposure C | Exposure D |
|---|---|---|---|
| 0-15 | 0.57 | 0.85 | 0.98 |
| 20 | 0.62 | 0.90 | 1.02 |
| 25 | 0.66 | 0.94 | 1.06 |
| 30 | 0.70 | 0.98 | 1.09 |
| 40 | 0.76 | 1.04 | 1.14 |
| 50 | 0.81 | 1.09 | 1.18 |
| 60 | 0.85 | 1.13 | 1.22 |
| 80 | 0.91 | 1.20 | 1.28 |
| 100 | 0.96 | 1.25 | 1.33 |
| 120 | 1.01 | 1.30 | 1.37 |
| 160 | 1.09 | 1.38 | 1.44 |
| 200 | 1.15 | 1.44 | 1.50 |
| 300 | 1.26 | 1.55 | 1.60 |
| 400 | 1.34 | 1.63 | 1.67 |
| 500 | 1.41 | 1.70 | 1.73 |
Interpolation between values is permitted. These coefficients are derived from power-law wind profiles: Kz = 2.01 x (z/zg)^(2/alpha), where zg and alpha depend on exposure category.
Topographic factor Kzt
For buildings on hills, ridges, or escarpments, Kzt > 1.0 increases the design wind speed. The calculation uses ASCE 7 Figure 26.8-1:
Kzt = (1 + K1*K2*K3)^2
| Parameter | Definition | Range |
|---|---|---|
| K1 | Factor accounting for shape of topographic feature (hill, escarpment, ridge) | 0.0 to 0.8 |
| K2 | Factor accounting for reduction in speed-up with distance upwind/downwind of crest | 0.0 to 1.0 |
| K3 | Factor accounting for reduction in speed-up with height above ground | 0.0 to 1.0 |
Typical values: flat terrain Kzt = 1.0; building on a 200 ft hill with 1:3 slope Kzt = 1.3-1.6; coastal escarpment Kzt = 1.2-1.5. Engineers should check Kzt whenever the building site is within 10 times the hill height of a significant topographic feature.
Wind directionality factor Kd
| Structure type | Kd |
|---|---|
| Buildings (all) | 0.85 |
| Chimneys, tanks, rooftop equipment | 0.90 |
| Solid freestanding walls and signs | 0.85 |
| Open signs, lattice frameworks, trussed towers | 0.85 |
| Arched roofs | 0.85 |
The factor 0.85 accounts for the reduced probability that the maximum wind speed will occur simultaneously from the least favorable direction. It should not be applied when checking specific directions (e.g., a building with a large opening facing a dominant wind direction).
Velocity pressure calculation example (detailed)
Given: 10-story office building, 130 ft tall, 120 ft x 80 ft plan, flat roof. Located in Dallas, TX. Risk Category II. Exposure C. Flat terrain.
Step 1 - Basic wind speed: From ASCE 7-22 Figure 26.5-1A, V = 115 mph (Risk Category II, 700-year MRI).
Step 2 - Velocity pressure at roof height (z = 130 ft):
- Kz = 1.32 (Exposure C, interpolated between 120 ft = 1.30 and 160 ft = 1.38)
- Kzt = 1.0 (flat terrain)
- Kd = 0.85 (building)
- Ke = 1.0 (Dallas elevation ~500 ft, Ke approximately 0.98; use 1.0 conservatively)
q130 = 0.00256 x 1.32 x 1.0 x 0.85 x 1.0 x 115^2
= 0.00256 x 1.32 x 0.85 x 13,225
= 0.00287 x 13,225
= 37.9 psf
Step 3 - Velocity pressure at mid-height (z = 65 ft):
- Kz = 1.15 (Exposure C, interpolated)
q65 = 0.00256 x 1.15 x 1.0 x 0.85 x 1.0 x 115^2 = 33.1 psf
Step 4 - Velocity pressure at 30 ft:
- Kz = 0.98 (Exposure C)
q30 = 0.00256 x 0.98 x 1.0 x 0.85 x 1.0 x 115^2 = 28.2 psf
Step 5 - Gust factor G: For a rigid building (T < 1.0 sec), G = 0.85 (ASCE 7 Table 26.11-1). Building period T approximately 0.1N = 1.0 sec. Since T is close to 1.0 sec, verify that the building is rigid. If T > 1.0 sec, use the flexible building gust factor procedure.
MWFRS vs C&C: detailed comparison
| Parameter | MWFRS (Chapter 27/28) | C&C (Chapter 30) |
|---|---|---|
| Purpose | Design the lateral force-resisting system (braces, moment frames, shear walls, diaphragms) | Design individual cladding elements, purlins, girts, fasteners |
| Tributary area | Large (entire building face, full story height) | Small (individual panel or framing member) |
| Pressure coefficients | Lower (averaged over large area) | Higher (local peak pressures at corners, edges) |
| Gust factor | G = 0.85 (rigid) or calculated (flexible) | Built into GCp values |
| Critical locations | Overall building overturning, base shear, story drift | Roof corners, roof edges, wall corners near roof |
| Typical pressure ratio | 1.0 (baseline) | 1.5 to 3.5 times MWFRS values at critical zones |
C&C roof pressure zones
ASCE 7-22 Chapter 30 defines distinct roof pressure zones with dramatically different pressures:
| Zone | Location | GCp (typical, h <= 60 ft) | Relative pressure |
|---|---|---|---|
| Zone 1 | Interior roof area (away from edges) | -0.9 to -1.1 | Baseline |
| Zone 2 | Roof edge (strip along perimeter) | -1.3 to -1.7 | 1.3-1.5x Zone 1 |
| Zone 3 | Roof corner (triangular area at corners) | -2.0 to -2.6 | 2.0-2.5x Zone 1 |
The Zone 3 corner suction can be 2-3 times the Zone 1 interior pressure. This is why roof failures in hurricanes typically begin at corners - the local peak suction exceeds the fastener capacity even when the interior roof is adequate.
Positive and negative pressure zones on buildings
Wind creates both positive (inward) and negative (outward/suction) pressures on different building surfaces:
| Surface | Pressure direction | Cp value (MWFRS, rectangular building) | Notes |
|---|---|---|---|
| Windward wall | Positive (inward) | +0.60 to +0.80 | Increases with L/B ratio |
| Leeward wall | Negative (suction) | -0.20 to -0.50 | Depends on L/B ratio |
| Side walls | Negative (suction) | -0.60 to -0.70 | Approximately uniform |
| Flat roof (windward edge) | Negative (suction) | -0.90 to -1.30 | Highest near windward edge |
| Flat roof (center) | Negative (suction) | -0.50 to -0.70 | Lower than edges |
| Flat roof (leeward) | Negative (suction) | -0.30 to -0.50 | Lowest magnitude |
The net lateral force on the building is the algebraic sum of windward positive and leeward negative pressures. For a typical rectangular building with L/B = 2, the net Cp = 0.80 - (-0.30) = 1.10.
Parapet and roof corner loads
Parapets and roof corners experience the highest wind pressures on a building and require special attention:
Parapet pressure (ASCE 7-22 Section 30.9)
Parapets are loaded on both the windward and leeward faces simultaneously. The net pressure is the sum of positive pressure on the windward face and suction on the leeward face:
p_parapet = q_p x (GCpn)
| Parapet zone | GCpn (net) | Typical pressure (115 mph, Exposure C, 40 ft) |
|---|---|---|
| Windward parapet (corner zone) | +1.8 / -2.2 | 48 to 59 psf |
| Windward parapet (interior zone) | +1.3 / -1.6 | 35 to 43 psf |
Roof corner loads
At roof corners, the interaction of windward wall separation and roof edge vortex creates extreme suction. For C&C design:
| Roof zone | Area (ft2) | GCp | Pressure (psf, 115 mph, 40 ft) |
|---|---|---|---|
| Corner Zone 3 (10 ft x 10 ft) | 10 | -2.6 | -70 psf uplift |
| Corner Zone 3 (100 ft2) | 100 | -1.9 | -51 psf uplift |
| Corner Zone 3 (500 ft2) | 500 | -1.4 | -38 psf uplift |
These pressures explain why mechanically attached roof membranes in hurricane zones require enhanced fastening patterns at corners and edges.
Simplified vs analytical method comparison
ASCE 7-22 offers simplified procedures for common building types and analytical procedures for complex conditions:
| Feature | Simplified (Chapter 27 Part 2) | Analytical (Chapter 27 Part 1) |
|---|---|---|
| Building height limit | h <= 160 ft | No limit |
| Building shape | Regular (rectangular) | Any shape |
| Roof type | Flat, gable, hip, mansard (limited) | All roof types |
| Exposure | C or D (Exposure B not permitted) | B, C, or D |
| Topography | Flat only (Kzt = 1.0) | Any topography |
| Flexibility | Rigid buildings only (T < 1.0 sec) | Rigid and flexible |
| Pressure determination | Direct from tables (p_s30, p_net) | Calculate using Cp, GCp, G |
| Ease of use | Simple look-up | Requires multi-step calculation |
| Accuracy | Conservative (10-20% higher) | More precise |
| Best for | Preliminary design, simple buildings | Final design, complex buildings |
For most structural steel buildings, the analytical method (Chapter 27 Part 1) is preferred because it provides more accurate and often lower design pressures, and it accommodates the full range of building geometries encountered in practice.
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Related references
- Load Combinations ASCE 7
- Live Load Reference
- Wind Load Calculation
- Structural Load Path
- Diaphragm Action
- Load Combinations Guide
- How to Verify Calculations
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Frequently Asked Questions
What is the recommended design procedure for this structural element?
The standard design procedure follows: (1) establish design criteria including applicable code, material grade, and loading; (2) determine loads and applicable load combinations; (3) analyze the structure for internal forces; (4) check member strength for all applicable limit states; (5) verify serviceability requirements; and (6) detail connections. Computer analysis is recommended for complex structures, but hand calculations should be used for verification of critical elements.
How do different design codes compare for this calculation?
AISC 360 (US), EN 1993 (Eurocode), AS 4100 (Australia), and CSA S16 (Canada) follow similar limit states design philosophy but differ in specific resistance factors, slenderness limits, and partial safety factors. Generally, EN 1993 uses partial factors on both load and resistance sides (γM0 = 1.0, γM1 = 1.0, γM2 = 1.25), while AISC 360 uses a single resistance factor (φ). Engineers should verify which code is adopted in their jurisdiction.
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