Wind Load Calculation Steps — Directional Procedure (Chapter 27)

PRELIMINARY — NOT FOR CONSTRUCTION. All results are for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.

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:

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

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)

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)

External Pressure Coefficients Cp — Roofs

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)

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? Wind speeds in ASCE 7-22 (and since ASCE 7-10) are at strength level (equivalent to the old 3-second gust speed multiplied by the sqrt of the load factor, approximately 1.6 for LRFD). For LRFD, apply wind load W with a 1.0 factor (LC4: 1.2D + 1.0W + L). For ASD, apply 0.6W to convert strength-level wind back to service level. ASCE 7-10 and 7-16 also used strength-level wind; they are superseded by 7-22 and provided here for historical context only. Pre-ASCE 7-10 editions used service-level wind speeds with a 1.6 load factor under LRFD.

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.

ASCE 7-22 Wind Load Procedure (Chapter 26-30)

ASCE 7-22 provides a comprehensive framework for determining wind loads on buildings and other structures. The primary procedure for most buildings is the Directional Procedure (Chapter 27 for MWFRS, Chapter 30 for C&C).

Step-by-Step Procedure

Step 1: Determine Risk Category (Table 1.5-1)

*Applies to wind speed selection, not a separate factor for wind since ASCE 7-10.

Step 2: Determine Basic Wind Speed V (Figure 26.5-1/26.5-2)

Select V from the ASCE 7-22 wind speed maps. The wind speed corresponds to the 3-second gust speed at 33 ft height in Exposure C, with the following mean recurrence intervals:

Step 3: Determine Exposure Category (Section 26.7)

Step 4: Determine Topographic Factor Kzt (Section 26.8)

Kzt = (1 + K1k2k3)^2, where K1, K2, K3 depend on hill shape, height, and distance from crest. For flat sites, Kzt = 1.0.

Step 5: Determine Directionality Factor Kd (Table 26.6-1)

Step 6: Calculate Velocity Pressure qz

qz = 0.00256 * Kz * Kzt * Kd * Ke * V^2    (ASCE Eq. 26.10-1)

Where Ke is the ground elevation factor (Table 26.9-1): Ke = 1.0 for elevations up to 1,000 ft; Ke = 0.94 at 6,000 ft.

Velocity Pressure Calculation: Worked Example

Problem: An office building in Dallas, TX. Risk Category II. Exposure C (open terrain). Flat site. Building dimensions: 60 ft wide x 120 ft long x 40 ft tall.

Step 1: Risk Category II, V = 115 mph (from ASCE 7-22 Figure 26.5-1A)

Step 2: Exposure C, Kzt = 1.0 (flat site), Kd = 0.85, Ke = 1.0

Step 3: Calculate qz at roof height (z = 40 ft)

From ASCE 7-22 Table 26.10-1 for Exposure C:

Kz at 40 ft = 1.04 (interpolated)

qz = 0.00256 * 1.04 * 1.0 * 0.85 * 1.0 * 115^2
   = 0.00256 * 1.04 * 0.85 * 13,225
   = 0.00226 * 13,225
   = 29.9 psf

Step 4: qz at 15 ft (lower portion)

Kz at 15 ft (Exposure C) = 0.85
qh = 0.00256 * 0.85 * 1.0 * 0.85 * 1.0 * 115^2 = 24.4 psf

Step 5: Design pressures (MWFRS, enclosed building, h = 40 ft)

Using ASCE 7-22 Figure 27.3-1 (enclosed simple diaphragm):

Windward wall:  p = qz * GCp - qi * GCpi = 29.9 * 0.85 - 29.9 * (+/-0.18)
  Positive (inward):  25.4 - 5.4 = 20.0 psf (with +0.18 internal)
  Negative (outward): 25.4 + 5.4 = 30.8 psf (with -0.18 internal)

Leeward wall: p = qh * GCp - qi * GCpi
  Suction: qh * (-0.5) = 24.4 * (-0.5) = -12.2 psf (external)
  With internal: -12.2 +/- 5.4 = range of -6.8 to -17.6 psf

Roof (flat): qh * GCp
  Zone 1 (interior): 24.4 * (-0.9) = -22.0 psf (suction)
  Zone 2 (edge):     24.4 * (-1.6) = -39.0 psf (suction)
  Zone 3 (corner):   24.4 * (-2.2) = -53.7 psf (suction)

These pressures are combined in LRFD load combinations (1.0W for wind) to design the MWFRS members. The roof corner Zone 3 pressure of 53.7 psf suction is more than double the interior Zone 1 pressure, demonstrating why corner zone detailing requires special attention.

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.

Try it now: Check your wind load with our free Wind Load calculator →

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 Compression Strength Tool — 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.

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