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%.

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.