How is velocity pressure qz calculated in ASCE 7-22?

Velocity pressure qz = 0.00256 × Kz × Kzt × Kd × Ke × V² (ASCE 7-22 Eq. 26.10-1) in psf when V is in mph. The 0.00256 constant converts the kinetic energy ½ρV² to units of psf (0.00256 = ½ × 0.0765 lb/ft³ / 32.2 ft/s² × (1.467 ft/s per mph)²). Kz is the velocity pressure exposure coefficient from Table 26.10-1: for Exposure B, Kz = 0.70 at 0-15 ft, 0.70 at 20 ft, 0.77 at 40 ft, 0.89 at 80 ft, 1.04 at 150 ft; for Exposure C, Kz = 0.85 at 0-15 ft, 0.90 at 20 ft, 0.98 at 40 ft, 1.09 at 80 ft, 1.25 at 150 ft; for Exposure D, Kz = 1.03 at 0-15 ft, 1.08 at 20 ft, 1.16 at 40 ft, 1.27 at 80 ft, 1.42 at 150 ft. Kzt is the topographic factor: Kzt = (1 + K1 × K2 × K3)² for hills/escarpments; Kzt = 1.0 for flat terrain. K1 is from Fig. 26.8-1 based on H/Lh ratio, K2 from distance to crest, K3 from distance to half-height. Kd is the wind directionality factor: 0.85 for MWFRS of buildings, 0.85 for C&C, 0.95 for round HSS, 1.0 for signage. Ke is the ground elevation factor: Ke = e^(-0.0000362×zg) where zg is ground elevation in feet above sea level; Ke ≈ 1.0 for elevations below 1,000 ft, reaching about 0.96 at 5,000 ft. For a building at sea level in Exposure C with V=115 mph: qz(at roof height 40 ft) = 0.00256 × 0.98 × 1.0 × 0.85 × 1.0 × 115² = 0.00256 × 0.98 × 0.85 × 13,225 = 28.2 psf.

What is the difference between MWFRS and C&C wind design?

Main Wind Force Resisting System (MWFRS) design addresses the overall structural frame — columns, beams, braced frames, shear walls, and diaphragms that resist wind as a complete system. Components and Cladding (C&C) design addresses individual building envelope elements: wall panels, roof panels, girts, purlins, fasteners, windows, doors, and their immediate supports. The two use different pressure coefficients because C&C elements see higher localized peak pressures than the building-average pressures used for MWFRS. For a 60 ft tall building: MWFRS uses GCp values from ASCE 7 Chapter 27 (average 0.8 windward, -0.5 leeward, -0.7 sidewall); C&C uses GCp values from Chapter 30 Table 30.3-1 through 30.3-6, where corner zones can have GCp values of -2.8 to -4.0 (suction), 2-4 times the MWFRS values. The effective wind area for C&C is the tributary area of the component, not the full wall surface — smaller tributary area means higher peak pressure (the gust doesn't cover the whole wall simultaneously). For a girt spanning 20 ft between columns with 6 ft tributary height: effective area = 20 × 6 = 120 sf. C&C is almost always more critical than MWFRS for cladding attachments — a roof panel rated for -30 psf C&C uplift may experience only -12 psf MWFRS uplift at the same location.

How is the gust effect factor G calculated?

The gust effect factor G accounts for the dynamic amplification of wind loads due to turbulence and structural response. For rigid buildings (fundamental frequency n₁ ≥ 1 Hz, which covers most buildings under about 150 ft height), ASCE 7-22 Section 26.11 permits G = 0.85 for the simplified directional procedure — this is the default for most steel buildings. The full gust effect factor for rigid structures per Eq. 26.11-6: G = 0.925 × (1 + 1.7 × gQ × Iz × Q)/(1 + 1.7 × gv × Iz), where Iz = c × (33/z̄)^(1/6) is the turbulence intensity at equivalent height z̄ = 0.6h, c = 0.30 (Exp B), 0.20 (Exp C), 0.15 (Exp D); gQ = gv = 3.4 (peak factor for background response); and Q = √(1/(1 + 0.63×((B+h)/Lz̄)^0.63)) is the background response factor, with Lz̄ = l × (z̄/33)^ε̄ being the integral length scale of turbulence. For a typical 80 ft × 60 ft building in Exposure C at h=60 ft: z̄ = 36 ft, Iz = 0.20 × (33/36)^(1/6) = 0.196, Lz̄ = 500 × (36/33)^(1/5) = 519 ft, Q = √(1/(1+0.63×((80+60)/519)^0.63)) = 0.82, G = 0.925 × (1+1.7×3.4×0.196×0.82)/(1+1.7×3.4×0.196) = 0.925 × (1+0.929)/(1+1.134) = 0.925 × 1.929/2.134 = 0.84. For flexible buildings (n₁ < 1 Hz, tall/slender), a resonant response factor R is added and G can reach 1.1-1.3 — the full equations are in Section 26.11.5.

When should the directional procedure vs envelope procedure be used?

ASCE 7-22 provides two main procedures for MWFRS wind loads. The Directional Procedure (Chapter 27) requires analyzing wind from each direction separately: p = q × G × Cp − qi × (GCpi) for each wind direction, where Cp values differ for windward (+0.8), leeward (−0.5 for L/B=1, −0.3 for L/B≥2), sidewall (−0.7), and roof (varies by slope). This gives the most accurate results and is preferred for irregular buildings and those sensitive to wind direction. The Envelope Procedure (Chapter 28) provides a single envelope of the worst-case pressure from all directions combined, simplified to p = qh × (GCpf) for low-rise buildings (h ≤ 60 ft, h/d ≤ 1). The envelope procedure is simpler but conservative — it produces higher total loads because it combines the worst windward + worst leeward + worst roof pressures, even though those peaks don't occur simultaneously from the same wind direction. Use the directional procedure when: building is taller than 60 ft, building has unusual shape or response characteristics, optimizing for economy (less conservative), or when torsional wind loads need to be captured (Case 1 and Case 2 torsional load cases per Figure 27.3-8). Use the envelope procedure for simple low-rise buildings where quick conservative results are acceptable.

How does wind tunnel testing modify ASCE 7 wind loads?

ASCE 7-22 Section 26.12 explicitly permits wind tunnel testing to determine design wind loads. Testing is required or strongly recommended for: buildings taller than 400 ft, buildings with height-to-width ratio > 4, buildings on complex topography (ridges, escarpments), buildings with unusual shapes or aerodynamic features, buildings where wind loads are the dominant lateral load (> 70% of total base shear), and buildings where a 15% reduction in wind load would save enough structure to pay for the test. Wind tunnel tests measure synchronous pressures at 200-500+ pressure taps across the building surface model (typically at 1:300 to 1:500 scale), capturing the correlation between pressures that generic ASCE 7 coefficients cannot capture. Tests provide: (1) floor-by-floor wind force time histories for the structural analysis model, replacing MWFRS coefficients; (2) peak cladding pressures by zone with higher spatial resolution than the three C&C zones in ASCE 7, typically reducing C&C design pressures by 15-30% compared to code-default values; (3) pedestrian wind comfort assessment for plazas and entrances. A typical wind tunnel study costs $50,000-150,000 and takes 4-8 weeks. The structural benefit: wind tunnel loads are typically 10-25% lower than code analytical loads for tall buildings because the tunnel captures the imperfect correlation of peak pressures across the building face — the code conservatively assumes perfect correlation of worst-case pressures.

How to Calculate Wind Load — ASCE 7-22 Step by Step

Q: When should the directional procedure vs envelope procedure be used?

ASCE 7-22 provides two main procedures for MWFRS wind loads. The Directional Procedure (Chapter 27) requires analyzing wind from each direction separately: p = q × G × Cp − qi × (GCpi) for each wind direction, where Cp values differ for windward (+0.8), leeward (−0.5 for L/B=1, −0.3 for L/B≥2), sidewall (−0.7), and roof (varies by slope). This gives the most accurate results and is preferred for irregular buildings and those sensitive to wind direction. The Envelope Procedure (Chapter 28) provides a single envelope of the worst-case pressure from all directions combined, simplified to p = qh × (GCpf) for low-rise buildings (h ≤ 60 ft, h/d ≤ 1). The envelope procedure is simpler but conservative — it produces higher total loads because it combines the worst windward + worst leeward + worst roof pressures, even though those peaks don't occur simultaneously from the same wind direction. Use the directional procedure when: building is taller than 60 ft, building has unusual shape or response characteristics, optimizing for economy (less conservative), or when torsional wind loads need to be captured (Case 1 and Case 2 torsional load cases per Figure 27.3-8). Use the envelope procedure for simple low-rise buildings where quick conservative results are acceptable.

Q: How does wind tunnel testing modify ASCE 7 wind loads?

ASCE 7-22 Section 26.12 explicitly permits wind tunnel testing to determine design wind loads. Testing is required or strongly recommended for: buildings taller than 400 ft, buildings with height-to-width ratio > 4, buildings on complex topography (ridges, escarpments), buildings with unusual shapes or aerodynamic features, buildings where wind loads are the dominant lateral load (> 70% of total base shear), and buildings where a 15% reduction in wind load would save enough structure to pay for the test. Wind tunnel tests measure synchronous pressures at 200-500+ pressure taps across the building surface model (typically at 1:300 to 1:500 scale), capturing the correlation between pressures that generic ASCE 7 coefficients cannot capture. Tests provide: (1) floor-by-floor wind force time histories for the structural analysis model, replacing MWFRS coefficients; (2) peak cladding pressures by zone with higher spatial resolution than the three C&C zones in ASCE 7, typically reducing C&C design pressures by 15-30% compared to code-default values; (3) pedestrian wind comfort assessment for plazas and entrances. A typical wind tunnel study costs $50,000-150,000 and takes 4-8 weeks. The structural benefit: wind tunnel loads are typically 10-25% lower than code analytical loads for tall buildings because the tunnel captures the imperfect correlation of peak pressures across the building face — the code conservatively assumes perfect correlation of worst-case pressures.

Wind load is one of the three primary environmental loads on structures (along with seismic and snow). In the United States, ASCE 7-22 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) governs wind load determination.

This page walks through the complete procedure for calculating wind loads on buildings using the directional procedure (Chapter 27) and components and cladding (Chapter 30).

Wind Load Basics

Wind Pressure Concept

Wind creates both positive pressure (on windward walls) and negative pressure (suction on leeward walls, roofs, and sides). The net effect depends on:

Wind speed: higher speed = much higher pressure (pressure scales with v²)
Building geometry: height, shape, roof slope, openings
Exposure: terrain roughness affects wind turbulence
Topography: hills and escarpments accelerate wind

Key Equation

Wind pressure: p = qz × (GCp) - qi × (GCpi)

where:

qz = velocity pressure at height z (evaluated at the point of interest)
GCp = external pressure coefficient (from tables)
qi = velocity pressure at building height (for internal pressure)
GCpi = internal pressure coefficient (±0.18 for enclosed buildings)

Step-by-Step Procedure

Step 1: Determine Risk Category

From ASCE 7 Table 1.5-1:

Risk Category	Description	Examples
I	Low risk to human life	Agricultural, storage
II	Standard occupancy	Residential, office, commercial
III	Substantial hazard	Schools, assembly >300, power stations
IV	Essential facilities	Hospitals, fire stations, emergency shelters

Step 2: Determine Basic Wind Speed (V)

From ASCE 7 Figures 26.5-1A through 26.5-2 (wind speed maps):

Category I: V = 105-150 mph (depending on location)
Category II: V = 110-160 mph
Category III: V = 115-170 mph
Category IV: V = 120-175 mph

These are 3-second gust speeds at 33 ft height in Exposure C.

Special wind regions: Mountainous terrain, coastal areas, and hurricane-prone regions may have higher values. Always check local amendments.

Step 3: Determine Exposure Category

From ASCE 7 Section 26.7:

Exposure	Description	Surface Roughness
B	Urban, suburban, wooded	Closely spaced obstructions > 30 ft
C	Open terrain, flat, grassland	Scattered obstructions < 30 ft
D	Flat unobstructed, water	Mud flats, salt flats, water surfaces

Default: Exposure C. Use Exposure B only if the upwind terrain meets the criteria for at least 2,600 ft (or 10× building height, whichever is greater).

Step 4: Calculate Velocity Pressure (qz)

qz = 0.00256 × Kz × Kzt × Kd × Ke × V²

where:

Kz = velocity pressure exposure coefficient (from Table 26.10-1)
Kzt = topographic factor (1.0 for flat terrain)
Kd = wind directionality factor (0.85 for buildings)
Ke = ground elevation factor (1.0 for sea level, < 1.0 for higher elevations)
V = basic wind speed (mph)

Velocity Pressure Exposure Coefficient (Kz)

For Exposure C (most common):

Height z (ft)	Kz
0-15	0.85
20	0.90
25	0.94
30	0.98
40	1.04
50	1.09
60	1.13
80	1.21
100	1.27
120	1.32
160	1.40
200	1.47

For Exposure B:

Height z (ft)	Kz
0-15	0.57
20	0.62
25	0.66
30	0.70
40	0.76
50	0.81
60	0.85
80	0.93
100	0.99

Step 5: Determine Building Geometry Parameters

h = mean roof height
L = building dimension parallel to wind
B = building dimension perpendicular to wind
θ = roof slope (degrees)
Enclosure classification: Enclosed, Partially Enclosed, or Open

Step 6: External Pressure Coefficients (GCp)

MWFRS — Walls (Chapter 27)

Windward wall: GCp = +0.80 (positive, toward the surface)

Leeward wall: GCp depends on L/B ratio:

L/B	Leeward GCp
0-1	-0.45
2	-0.45
≥ 4	-0.45

(Note: simplified. See ASCE 7 Figure 27.3-1 for full table.)

Side walls: GCp varies along the length:

Zone	Side Wall GCp
0 to h from corner	-0.90
h to 2h from corner	-0.60
> 2h from corner	-0.60

MWFRS — Roofs

For flat or low-slope roofs (θ ≤ 10°):

Zone	Roof GCp
0 to h from windward edge	-1.3 (or +0.5 uplift)
h to 2h from windward edge	-0.7
> 2h from windward edge	-0.5

For gable roofs (10° < θ ≤ 25°):

Zone	Roof GCp (Pressure)	Roof GCp (Suction)
Windward slope	+0.3 to +0.4	-0.8 to -1.0
Leeward slope	—	-0.7 to -0.9

See ASCE 7 Figure 27.3-1 for complete tables based on θ.

Step 7: Internal Pressure Coefficient (GCpi)

Enclosure Classification	GCpi
Enclosed buildings	±0.18
Partially enclosed buildings	±0.55
Open buildings	0.00

Internal pressure acts simultaneously with external pressure. For enclosed buildings, check both +0.18 and -0.18 to find the worst case.

Step 8: Calculate Design Pressures

Wall pressure:

p = qz × (GCp) - qh × (GCpi)

For windward wall: qz varies with height
For leeward wall: qh at mean roof height
For side walls: qh at mean roof height

Roof pressure:

p = qh × (GCp) - qh × (GCpi)

All pressures use qh at mean roof height (except windward wall which uses qz at each height).

Worked Example: 30 ft Steel Office Building

Problem

A 60 ft × 100 ft steel frame office building in Dallas, TX. Mean roof height h = 30 ft. Flat roof (θ = 0°). Exposure C. Enclosed building. Risk Category II.

Step 1-3: Parameters

Risk Category II → V = 115 mph (Dallas)
Exposure C
Kd = 0.85 (building)
Kzt = 1.0 (flat terrain)
Ke = 1.0 (sea level approximation)

Step 4: Velocity Pressure

At roof height (z = 30 ft): Kz = 0.98

qh = 0.00256 × 0.98 × 1.0 × 0.85 × 1.0 × 115² = 28.1 psf

At z = 15 ft (windward wall): Kz = 0.85

q15 = 0.00256 × 0.85 × 1.0 × 0.85 × 1.0 × 115² = 24.4 psf

Step 5: Pressure Coefficients

Wind direction: assume wind blows along the 100 ft dimension.

Windward wall: GCp = +0.80
Leeward wall: GCp = -0.45 (L/B = 100/60 = 1.67)
Side walls: GCp = -0.90 (near corners), -0.60 (interior)
Roof: GCp = -1.3 (edge zone), -0.7 (transition), -0.5 (interior)
Internal: GCpi = ±0.18

Step 6: Design Pressures

Windward wall at z = 15 ft (with GCpi = +0.18 for worst case):

p = 24.4 × 0.80 - 28.1 × 0.18 = 19.5 - 5.1 = 14.4 psf (inward)

Windward wall at z = 15 ft (with GCpi = -0.18):

p = 24.4 × 0.80 - 28.1 × (-0.18) = 19.5 + 5.1 = 24.6 psf (inward) ← governs

Leeward wall (with GCpi = -0.18 for worst case suction):

p = 28.1 × (-0.45) - 28.1 × (-0.18) = -12.6 + 5.1 = -7.6 psf (suction)

Leeward wall (with GCpi = +0.18):

p = 28.1 × (-0.45) - 28.1 × (0.18) = -12.6 - 5.1 = -17.7 psf (suction) ← governs

Roof edge zone (with GCpi = +0.18 for worst uplift):

p = 28.1 × (-1.3) - 28.1 × 0.18 = -36.5 - 5.1 = -41.6 psf (uplift)

Roof interior zone (with GCpi = +0.18):

p = 28.1 × (-0.5) - 28.1 × 0.18 = -14.1 - 5.1 = -19.2 psf (uplift)

Components and Cladding (C&C)

C&C loads are for individual elements (wall panels, roof decking, windows, cladding) rather than the main structural frame.

C&C vs MWFRS

Aspect	MWFRS	C&C
Purpose	Main frame design	Individual elements
Pressure coefficients	Lower (averaged)	Higher (localized peaks)
Effective area	Large	Small (individual element)
Application	Columns, beams, bracing	Panels, purlins, girts, glazing

C&C Pressure Coefficients

For enclosed buildings with h ≤ 60 ft, use ASCE 7 Chapter 30, Part 1:

Wall C&C:

Zone	GCp (positive)	GCp (negative, A ≤ 10 ft²)	GCp (negative, A ≥ 500 ft²)
4 (corner zone)	+1.0	-1.1	-0.90
5 (interior)	+1.0	-0.9	-0.70

Roof C&C (θ ≤ 10°):

Zone	GCp (positive)	GCp (negative, A ≤ 10 ft²)	GCp (negative, A ≥ 500 ft²)
1 (corner)	+0.3	-1.7	-0.90
2 (edge)	+0.3	-1.5	-0.80
3 (interior)	+0.3	-1.2	-0.60

The effective area A determines the pressure coefficient. Smaller elements get higher pressures.

Frequently Asked Questions

How do I calculate wind load on a building? Determine the basic wind speed from ASCE 7 maps, select exposure category, calculate velocity pressure qz = 0.00256 × Kz × Kzt × Kd × Ke × V², then multiply by pressure coefficients (GCp) for each surface.

What is velocity pressure? Velocity pressure qz is the theoretical pressure exerted by the wind at height z. It depends on wind speed (scales with V²), terrain roughness (exposure category), and height above ground.

What is the difference between MWFRS and C&C wind loads? MWFRS (Main Wind Force Resisting System) loads are for the overall structural frame, using averaged pressure coefficients. C&C (Components and Cladding) loads are for individual elements, using higher localized pressure coefficients that capture peak suctions at corners and edges.

What exposure category should I use? Most buildings use Exposure C (open terrain). Use Exposure B only for buildings in dense urban or suburban areas with buildings taller than 30 ft on all sides for a distance of at least 2,600 ft upwind.

What is the internal pressure coefficient? Internal pressure is the pressure inside the building. For enclosed buildings, GCpi = ±0.18. For partially enclosed buildings (large openings on one side), GCpi = ±0.55. Both positive and negative values must be checked.

Do I need to consider wind from all directions? Yes. ASCE 7 requires checking wind from all directions. For rectangular buildings, check two orthogonal directions. For irregular shapes, check additional directions.

How does roof slope affect wind load? Low-slope roofs (θ < 10°) experience primarily suction (uplift). As slope increases, windward slopes transition to positive pressure. Steep slopes (θ > 45°) act more like walls.

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Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.