Wind Load Basics — Engineering Reference

ASCE 7 wind pressure: qz=0.613KzV², windward Cp=+0.8, leeward Cp=−0.5, exposure B/C/D. Interactive MWFRS wind pressure calculator. Free reference.

Overview

Wind loads on steel structures are determined by converting the basic wind speed into velocity pressure, then applying pressure coefficients that account for building shape, surface orientation, and internal pressure conditions. In the United States, ASCE 7-22 Chapter 26-30 governs wind load determination. The Main Wind Force Resisting System (MWFRS) provisions define loads on the primary structural frame, while Components and Cladding (C&C) provisions define higher local pressures on individual elements.

The fundamental velocity pressure equation is:

q_z = 0.00256 x K_z x K_zt x K_e x V^2 (psf, with V in mph)

where K_z is the velocity pressure exposure coefficient (varies with height and exposure category), K_zt is the topographic factor (for hills, ridges, escarpments), K_e is the ground elevation factor, and V is the basic wind speed from ASCE 7 Figure 26.5-1 (maps by Risk Category).

Exposure categories

The terrain surrounding the building determines the exposure category, which affects how quickly wind speed increases with height:

Exposure B is the default when conditions are not clearly C or D. The exposure category must be evaluated for each wind direction independently — a building may be Exposure B for winds from the north (facing a city) and Exposure C for winds from the south (facing open fields).

Pressure coefficients (Cp) for MWFRS

For enclosed rectangular buildings using the Directional Procedure (ASCE 7 Chapter 27):

The design wind pressure on each surface is: p = q x G x C_p - q_i x (GC_pi)

where G is the gust effect factor (0.85 for rigid buildings with natural frequency >= 1 Hz), and (GC_pi) is the internal pressure coefficient (+/-0.18 for enclosed buildings, +/-0.55 for partially enclosed buildings).

Worked example — 3-story office building

Given: 3-story steel office building, 45 ft tall, 100 ft x 60 ft plan, Risk Category II, V = 115 mph, Exposure C, flat terrain (K_zt = 1.0), sea level (K_e = 1.0), enclosed.

1. Velocity pressure at roof height (z = 45 ft): K_z = 1.01 (ASCE 7 Table 26.10-1, Exposure C, 45 ft). q_h = 0.00256 x 1.01 x 1.0 x 1.0 x 115^2 = 0.00256 x 1.01 x 13,225 = 34.2 psf.
2. Windward wall pressure at roof: p_w = q_h x G x C_p - q_h x (GC_pi) = 34.2 x 0.85 x 0.8 - 34.2 x (-0.18) = 23.3 + 6.2 = 29.4 psf (max case with negative internal pressure adding to windward).
3. Leeward wall suction (L/B = 100/60 = 1.67, so C_p = -0.35): p_L = 34.2 x 0.85 x (-0.35) - 34.2 x (+0.18) = -10.2 - 6.2 = -16.3 psf (suction outward).
4. Total MWFRS pressure (windward + leeward combined): 29.4 + 16.3 = 45.7 psf on the projected area in the wind direction.
5. Base shear (simplified, uniform distribution): V_base ≈ 45.7 x 45 x 60 / 1000 = 123.4 kip (approximate; actual calculation varies q_z with height at each floor).

Code comparison — wind load provisions

Note: the wind speed definitions differ significantly between codes. ASCE 7 uses 3-second gust, while Eurocode uses 10-minute mean. A 115 mph 3-second gust is approximately equivalent to a 38 m/s 10-minute mean wind speed.

Key design considerations

• Building classification — enclosed, partially enclosed, or open buildings have different internal pressure coefficients. A building with a large opening on one wall during a wind event (e.g., garage door failure) may become partially enclosed, increasing internal pressure from +/-0.18 to +/-0.55.
• Rigid vs. flexible — buildings with a fundamental natural frequency >= 1 Hz are classified as rigid and use G = 0.85. Taller or more flexible structures require the full gust-effect factor calculation from ASCE 7 Section 26.11, which accounts for dynamic amplification.
• Directional procedure vs. envelope procedure — the directional procedure (Chapter 27) is more general and typically used for steel buildings. The envelope procedure (Chapter 28) is simplified and limited to low-rise buildings (h <= 60 ft).
• Load combinations — wind loads enter LRFD combinations as W. The critical combination for uplift is typically 0.9D + 1.0W (ASCE 7 Load Combination 6), which can produce net tension in columns and uplift on foundations.

Common mistakes to avoid

• Using the wrong wind speed map — ASCE 7-22 has separate wind speed maps for Risk Categories I, II, III, and IV. Using the Risk Category II map for a hospital (Category IV) underestimates the design wind speed by 10-15%.
• Assuming Exposure B everywhere — Exposure B requires obstructions in all directions for a distance of at least 2,600 ft (800 m). Many suburban sites near open fields or water bodies should be classified as Exposure C for those wind directions.
• Neglecting internal pressure — the internal pressure coefficient (GC_pi = +/-0.18 for enclosed buildings) acts on all surfaces simultaneously. For partially enclosed buildings, this increases to +/-0.55, which can double the net pressure on some surfaces.
• Not checking wind uplift on roof connections — net uplift pressures on flat and low-slope roofs can exceed 30-50 psf, requiring positive attachment of roof deck to framing. Gravity loads alone may not prevent roof blow-off.
• Applying wind to projected area only — MWFRS loads require applying windward pressure (positive), leeward suction (negative), and roof pressures simultaneously. Using only the windward pressure underestimates the total lateral force by 30-40%.

ASCE 7-22 wind load procedure overview

The ASCE 7-22 wind load determination follows a structured procedure defined across Chapters 26 through 30. Before calculating any pressures, the designer must establish four fundamental parameters that govern the entire calculation.

Step 1: Determine Risk Category. ASCE 7-22 Table 1.5-1 assigns a Risk Category (I through IV) based on the nature of occupancy. Risk Category I applies to agricultural and minor storage facilities. Risk Category II is the default for typical office and residential buildings. Risk Category III covers schools, jails, and buildings where more than 300 people congregate. Risk Category IV applies to hospitals, fire stations, and other essential facilities. The Risk Category determines which wind speed map to use and the mean recurrence interval: Category I = 300-year, Category II = 700-year, Category III = 1,700-year, Category IV = 3,000-year.

Step 2: Determine basic wind speed V. Read the 3-second gust wind speed from ASCE 7-22 Figures 26.5-1A through 26.5-1D (one map per Risk Category). In non-hurricane regions, wind speeds range from 110 mph in the western US to 130+ mph along the Gulf and Atlantic coasts. Special wind regions (mountainous terrain) may require site-specific studies.

Step 3: Determine exposure category. The exposure category (B, C, or D) characterizes the terrain roughness upwind of the building. It is evaluated for each of the eight cardinal wind directions (N, NE, E, SE, S, SW, W, NW) using the longest fetch of each terrain type within a 45-degree sector centered on each direction. The upwind fetch extends 2,600 ft (800 m) for Exposure B and 5,000 ft (1,500 m) for Exposure D. A building in a suburban area adjacent to open farmland will be Exposure B for winds from the suburb direction and Exposure C for winds from the farmland direction. The designer must check all directions and use the most adverse case for each element under consideration.

Step 4: Determine topographic factor K_zt. ASCE 7-22 Section 26.8 provides the topographic factor for buildings on or near hills, ridges, and escarpments. Wind accelerates as it flows over elevated terrain, increasing pressures on the windward face. The factor is calculated from three multipliers:

K_zt = (1 + K1 x K2 x K3)^2

where K1 accounts for the shape and height of the terrain feature, K2 accounts for distance upwind or downwind from the crest, and K3 accounts for height above ground at the building site. For flat terrain, K_zt = 1.0. For a building at the crest of a 200 ft escarpment in Exposure C, K_zt can reach 1.5 or higher, effectively increasing wind pressures by 50%.

Velocity pressure calculation in detail

The velocity pressure q_z converts the basic wind speed into a design pressure at each height z. The complete ASCE 7-22 equation is:

q_z = 0.00256 x K_z x K_zt x K_d x K_e x V^2   (psf)

The constant 0.00256 reflects the density of air at standard conditions (approximately 0.0765 lb/ft^3) and unit conversions. An equivalent SI formulation uses 0.613 as the constant when V is in m/s and q_z is in Pa:

q_z = 0.613 x K_z x K_zt x K_d x K_e x V^2   (Pa, V in m/s)

Velocity pressure exposure coefficient K_z. This coefficient accounts for the variation of wind speed with height and terrain roughness. It is derived from the power law profile: wind speed increases with height, and the rate of increase depends on terrain roughness. ASCE 7-22 Table 26.10-1 provides K_z values by exposure category and height above ground.

At 100 ft in Exposure C, K_z = 1.27. The same height in Exposure B gives K_z = 0.99, reflecting the 22% reduction in wind speed due to surface roughness in suburban terrain.

Wind directionality factor K_d. ASCE 7-22 Table 26.6-1 provides K_d values that account for the reduced probability that the worst wind blows from the least favorable direction simultaneously. For most building structures, K_d = 0.85. For chimney, tank, or rooftop equipment, K_d = 0.90 to 0.95. Note that K_d = 1.0 is used when checking individual members or connections for specific wind directions (the directionality benefit is not applied to component-level checks).

Ground elevation factor K_e. ASCE 7-22 Section 26.9 introduces K_e to account for reduced air density at higher elevations. At sea level, K_e = 1.0. At 6,000 ft elevation, K_e = 0.785, reflecting the approximately 21% reduction in air density. For buildings below 1,000 ft elevation, K_e = 1.0 is conservative and commonly used.

K_e = e^(-0.000119 x elevation_in_ft)

MWFRS vs C&C: critical distinction

ASCE 7-22 defines two distinct wind loading regimes that apply simultaneously to different parts of the structure. Understanding the distinction is essential for correct design.

Main Wind Force Resisting System (MWFRS) provisions (Chapters 27 and 28) define wind pressures on the primary structural frame: columns, beams, bracing, shear walls, and foundations. MWFRS pressures account for spatial averaging over large tributary areas, which reduces peak local pressures. MWFRS loads are used to design moment frames, braced frames, shear walls, diaphragms, and foundations.

Components and Cladding (C&C) provisions (Chapter 30) define higher wind pressures on individual elements with smaller tributary areas. Because wind pressure fluctuates significantly over small areas, peak pressures on individual cladding panels, purlins, girts, and connections are substantially higher than the spatially averaged MWFRS pressures.

Practical example: Consider a 4-story steel office building, 60 ft tall, V = 120 mph, Exposure C. The MWFRS pressure on the windward wall at the roof level might be 35 psf. The C&C pressure on a 20 sf wall panel at the corner of the same building could be 65 psf (nearly double). The MWFRS pressure designs the moment frame column; the C&C pressure designs the wall girt and its connection to the column.

Pressure coefficients for common building geometries

The following table summarizes external pressure coefficients (Cp) for common steel building geometries using the Directional Procedure (ASCE 7-22 Chapter 27). These values apply to enclosed and partially enclosed buildings with rectangular plan geometry.

Wall pressure coefficients (MWFRS)

Roof pressure coefficients (MWFRS, low-slope roof < 10 degrees)

The corner zone (Zone 3) has the highest suction coefficient at -1.7, which is why roof corners are the most vulnerable to wind damage. For a building with h = 30 ft and q_h = 30 psf, the corner zone C&C pressure can exceed 75 psf of uplift.

Gable roof pressure coefficients (MWFRS, slope 10-30 degrees)

At slopes above approximately 15 degrees, the windward roof surface transitions from negative (uplift) to positive (downward) pressure. This crossover is critical for steel portal frame design because it changes the load direction on the rafters.

Step-by-step ASCE 7-22 Chapter 27 procedure

The Directional Procedure for buildings of all heights (Chapter 27, Part 1) follows these steps:

1. Determine Risk Category from Table 1.5-1
2. Determine wind speed V from Figures 26.5-1A through 26.5-1D
3. Determine wind directionality factor K_d from Table 26.6-1
4. Determine exposure category from Section 26.7 (B, C, or D)
5. Determine topographic factor K_zt from Section 26.8
6. Determine ground elevation factor K_e from Section 26.9
7. Determine enclosure classification from Section 26.12
8. Calculate velocity pressure q_z = 0.00256 x K_z x K_zt x K_d x K_e x V^2
9. Determine external pressure coefficients Cp from Figure 27.3-1 (walls) and Figure 27.3-2 (roofs)
10. Determine internal pressure coefficient GCpi from Table 26.13-1
11. Calculate design wind pressures p = q x G x Cp - q_i x GCpi
12. Apply pressures to the building surface and determine member forces
13. Check all wind directions (0, 90, 180, 270 degrees minimum)

For each wind direction, the exposure category may differ. The designer must repeat the calculation for the minimum of four wind directions (two orthogonal directions, each with positive and negative internal pressure) to find the critical loading on each structural element.

Run this calculation

• Wind Load Calculator
• Section Properties Database

Related references

• Load Combinations ASCE 7
• Live Load Reference
• Wind Load Calculation
• How to Verify Calculations
• structural load path
• seismic design basics
• lateral system connections
• Load Combinations
• Roof Loading
• Steel High Rise Systems

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.

Design Resources

Calculator tools

• Wind Load Calculator

Design guides

• Wind Load Calculator Guide
• Wind Load Worked Example
• Wind Load Step-by-Step Guide