ASCE 7-22 Wind Load Worked Example — MWFRS for a 60 ft Steel Building

Complete worked example for calculating Main Wind Force Resisting System (MWFRS) wind loads on a low-rise steel building per ASCE 7-22 Chapters 26 and 27. This example walks through the Directional Procedure from basic wind speed selection through velocity pressure calculation, external and internal pressure coefficients, and final design pressures at each building surface. All calculations follow the ultimate-strength (strength-level) wind speeds of ASCE 7-22.

Related pages: ASCE 7-22 Wind Load Guide | Wind Load Basics | US Load Combinations | Wind Load Calculator | Seismic Load Calculator

Problem Statement

A single-story steel-framed office building with the following geometry:

Step 1 — Basic Wind Speed V (ASCE 7-22 Section 26.5)

The basic wind speed V is the 3-second gust speed at 33 ft (10 m) above ground in Exposure C (open terrain), corresponding to a 3,000-year mean recurrence interval (MRI) for Risk Category II structures.

From ASCE 7-22 Figure 26.5-1A:

[ V = 115 \text{ mph} ]

For Risk Category II, the wind speed map provides ultimate (strength-level) wind speeds. No importance factor is applied to the wind speed in ASCE 7-22 — the hazard maps embed the required MRI directly.

Step 2 — Wind Directionality Factor Kd (Section 26.6)

The wind directionality factor accounts for the reduced probability that the maximum wind speed and the worst-case aerodynamics occur simultaneously from the same direction.

From ASCE 7-22 Table 26.6-1 for buildings — MWFRS:

[ K_d = 0.85 ]

Step 3 — Exposure Category and Velocity Pressure Coefficient Kz (Section 26.10)

Exposure B applies to urban and suburban areas with closely spaced obstructions the size of single-family dwellings or larger. The velocity pressure exposure coefficient Kz is tabulated in ASCE 7-22 Table 26.10-1.

For Exposure B, the power-law exponent alpha = 7.0, and the gradient height zg = 1,200 ft.

Velocity pressure coefficient at mean roof height z = h = 20 ft:

[ K_z = 2.01 \times \left(\frac{z}{z_g}\right)^{2/\alpha} = 2.01 \times \left(\frac{20}{1200}\right)^{2/7.0} ]

[ K_z = 2.01 \times (0.01667)^{0.2857} = 2.01 \times 0.3197 = 0.643 ]

Interpolated from Table 26.10-1 for Exposure B at z = 20 ft:

[ K_z = 0.62 \text{ (0–15 ft: 0.57, 20 ft: 0.62, 25 ft: 0.66) } ]

We use the tabulated value Kz = 0.62 at h = 20 ft.

Exposure B Kz values at key elevations for this building:

Step 4 — Topographic Factor Kzt (Section 26.8)

The building is on flat terrain with no hills, ridges, or escarpments within 2 miles.

[ K_{zt} = 1.0 ]

Step 5 — Velocity Pressure qz (Section 26.10-1)

The velocity pressure at height z combines the wind speed, exposure coefficient, topographic factor, and directionality factor. The equation per ASCE 7-22 Eq. 26.10-1:

[ qz = 0.00256 \times K_z \times K{zt} \times K_d \times V^2 ]

Where:

• 0.00256 = mass density of air for standard atmosphere (0.0765 lb/ft^3 / 2g), units: (lb-s^2/ft^4)/(mph^2)
• V is in mph
• qz is in psf

At mean roof height z = h = 20 ft:

[ q_h = 0.00256 \times 0.62 \times 1.0 \times 0.85 \times (115)^2 ]

[ q_h = 0.00256 \times 0.62 \times 0.85 \times 13,225 ]

[ q_h = 0.00256 \times 0.527 \times 13,225 = 0.00256 \times 6,965 = 17.83 \text{ psf} ]

Velocity pressure at selected heights:

For MWFRS design of a low-rise building (h < 60 ft), ASCE 7-22 Section 27.3.4 permits using qh (velocity pressure at mean roof height) as a constant value over the full building height for simplicity and conservatism. We adopt qh = 17.83 psf for the remaining calculations.

Step 6 — Gust Effect Factor G (Section 26.11)

For a rigid building (fundamental natural frequency >= 1 Hz), the gust effect factor may be taken as:

[ G = 0.85 ]

This value accounts for gustiness in the wind and is applicable to most low-rise steel buildings. A 20 ft-tall steel moment frame building typically has a fundamental period well under 1 second and easily satisfies the rigid-building criterion.

Confirm rigidity: For steel moment frames: [ T_a \approx 0.028 \times h^{0.8} = 0.028 \times 20^{0.8} = 0.028 \times 11.0 = 0.31 \text{ s} ] [ n_1 = 1 / T_a = 3.2 \text{ Hz} > 1 \text{ Hz} \implies \text{rigid building} ]

Step 7 — External Pressure Coefficients Cp (Figures 27.3-1 and 27.3-2)

For MWFRS, the Directional Procedure uses external pressure coefficients that depend on the building's aspect ratios and wind direction.

Building Dimensions and Aspect Ratios

Windward Wall (Figure 27.3-1)

For windward wall: Cp is positive (pressure toward the surface). Value depends on L/B ratio.

Interpolation: at L/B = 0.75 between 0.8 (at L/B = 0.5) and 1.0 (at L/B = 1.0): [ C_p = 0.8 + (0.75 - 0.5) \times \frac{1.0 - 0.8}{1.0 - 0.5} = 0.8 + 0.25 \times 0.40 = 0.90 ]

For conservatism and simplicity, use Cp = 0.8 (the code minimum prescribed for all aspect ratios in simplified analysis, ASCE 7-22 Chapter 30 envelope procedure).

Leeward Wall (Figure 27.3-1)

Leeward wall Cp depends on L/B ratio:

Interpolation at L/B = 0.75: [ C_p = -0.5 + (0.75 - 0.5) \times \frac{-0.2 - (-0.5)}{4.0 - 0.5} = -0.5 + 0.25 \times \frac{0.3}{3.5} = -0.5 + 0.021 = -0.48 ]

Use Cp = -0.48 (leeward wall suction).

Side Walls (Figure 27.3-1)

Side wall Cp = -0.7 (suction) for all L/B ratios.

Flat Roof (Figure 27.3-2)

For flat roof (roof slope < 10 degrees) with h/L = 20/60 = 0.33:

For MWFRS, we use area-averaged values:

• Windward roof half: Cp = -0.9
• Leeward roof half: Cp = -0.5

Step 8 — Internal Pressure Coefficient GCpi (Section 26.13)

Internal pressure coefficient depends on the building enclosure classification and opening distribution. For MWFRS calculations, GCpi is applied directly because ASCE 7-22 defines design wind pressure with gust factor already combined.

From ASCE 7-22 Table 26.13-1 for an enclosed building:

[ GC_{pi} = \pm 0.18 ]

Both the positive and negative sign must be considered — positive internal pressure pushes outward on all surfaces (adding to external suction), while negative internal pressure pulls inward (reducing net outward pressure on leeward surfaces but increasing the differential on windward surfaces).

Step 9 — Design Wind Pressure p (Section 27.3-1)

The design wind pressure for MWFRS per ASCE 7-22 Eq. 27.3-1:

[ p = qh \times G \times C_p - q_h \times (\pm GC{pi}) ]

Or equivalently: [ p = qh \times [(G \times C_p) - (\pm GC{pi})] ]

Where qh = 17.83 psf, G = 0.85.

Windward Wall Pressure

Cp = +0.8, with both GCpi signs:

With +GCpi = +0.18 (internal pressure adds to external suction): [ p_w = 17.83 \times [(0.85 \times 0.8) - 0.18] = 17.83 \times [0.68 - 0.18] = 17.83 \times 0.50 = 8.92 \text{ psf} ]

With -GCpi = -0.18 (internal suction adds to external pressure): [ p_w = 17.83 \times [(0.85 \times 0.8) - (-0.18)] = 17.83 \times [0.68 + 0.18] = 17.83 \times 0.86 = 15.34 \text{ psf} ]

Governing windward wall pressure: pw = 15.34 psf (positive, toward building).

Leeward Wall Pressure

Cp = -0.48, with both GCpi signs:

With +GCpi = +0.18: [ p_l = 17.83 \times [(0.85 \times -0.48) - 0.18] = 17.83 \times [-0.408 - 0.18] = 17.83 \times (-0.588) = -10.49 \text{ psf} ]

With -GCpi = -0.18: [ p_l = 17.83 \times [(0.85 \times -0.48) - (-0.18)] = 17.83 \times [-0.408 + 0.18] = 17.83 \times (-0.228) = -4.07 \text{ psf} ]

Governing leeward wall suction: pl = -10.49 psf (suction, net effect).

Side Wall Pressure

Cp = -0.7, with both GCpi signs:

With +GCpi = +0.18: [ p_s = 17.83 \times [(0.85 \times -0.7) - 0.18] = 17.83 \times [-0.595 - 0.18] = 17.83 \times (-0.775) = -13.82 \text{ psf} ]

With -GCpi = -0.18: [ p_s = 17.83 \times [(0.85 \times -0.7) - (-0.18)] = 17.83 \times [-0.595 + 0.18] = 17.83 \times (-0.415) = -7.40 \text{ psf} ]

Governing side wall suction: ps = -13.82 psf (suction).

Roof Pressure

Windward roof half: Cp = -0.9

With +GCpi = +0.18: [ p_{r,w} = 17.83 \times [(0.85 \times -0.9) - 0.18] = 17.83 \times [-0.765 - 0.18] = 17.83 \times (-0.945) = -16.85 \text{ psf} ]

With -GCpi = -0.18: [ p_{r,w} = 17.83 \times [(0.85 \times -0.9) - (-0.18)] = 17.83 \times [-0.765 + 0.18] = 17.83 \times (-0.585) = -10.43 \text{ psf} ]

Leeward roof half: Cp = -0.5

With +GCpi = +0.18: [ p_{r,l} = 17.83 \times [(0.85 \times -0.5) - 0.18] = 17.83 \times [-0.425 - 0.18] = 17.83 \times (-0.605) = -10.79 \text{ psf} ]

With -GCpi = -0.18: [ p_{r,l} = 17.83 \times [(0.85 \times -0.5) - (-0.18)] = 17.83 \times [-0.425 + 0.18] = 17.83 \times (-0.245) = -4.37 \text{ psf} ]

Step 10 — Summary of Design Wind Pressures (MWFRS)

Key observations:

• The -GCpi case (+0.18 internal suction) produces the highest positive pressure on the windward wall because internal suction adds to external pressure, increasing the net push on the wall.
• The +GCpi case (+0.18 internal pressure) produces the highest suction/uplift on all other surfaces because internal pressure pushes outward against already-negative external pressures.
• Roof uplift is the numerically largest pressure (-16.85 psf), which controls roof framing and anchorage design.

Step 11 — Total Base Shear Calculation

Windward wall area = height x width = 20 ft x 80 ft = 1,600 ft^2 Leeward wall area = 20 ft x 80 ft = 1,600 ft^2

Total horizontal force on windward wall: [ F_w = p_w \times A_w = 15.34 \times 1,600 = 24,544 \text{ lb} = 24.5 \text{ kips} ]

Total horizontal force on leeward wall (absolute value): [ F_l = |p_l| \times A_l = 10.49 \times 1,600 = 16,784 \text{ lb} = 16.8 \text{ kips} ]

Total base shear (wind in N-S direction, building depth = 60 ft): [ V_{base} = F_w + F_l = 24.5 + 16.8 = 41.3 \text{ kips} ]

This total base shear is distributed to the moment frames resisting lateral load. For a building with 5 identical moment frames at 16 ft spacing:

Per-frame shear: V_frame = 41.3 / 5 = 8.3 kips

Step 12 — Check Against Envelope Procedure (Optional)

For low-rise buildings (h <= 60 ft), ASCE 7-22 Chapter 28 Envelope Procedure provides an alternative simplified approach. The Envelope Procedure pre-calculates pressure coefficients for low-rise rectangular buildings and may produce different results than the directional procedure.

Comparing our directional procedure result for transverse direction (wind normal to 80 ft face):

Envelope Procedure (h <= 60 ft, enclosed, flat roof):

• Windward wall pressure coefficient: 0.40 to 0.69 (varies by zone)
• Our directional result: 0.86 (0.85 x 0.80 combined with -GCpi)
• The directional procedure is more conservative in this case, as expected for a building well within the low-rise threshold.

The directional procedure controls for this building.

AS/NZS 1170.2 and EN 1991-1-4 Comparison for Reference

For engineers working across international codes, the equivalent parameters are:

Frequently Asked Questions

Why use the Directional Procedure instead of the Envelope Procedure for a low-rise building?

The Directional Procedure (Chapter 27) provides wind-pressure results specific to each wind direction, which may be less conservative than the envelope approach. The Envelope Procedure (Chapter 28) bundles results from all wind directions into a single envelope. For buildings where the framing layout is sensitive to wind direction (e.g., a long narrow plan), the directional procedure can produce more efficient designs. Both are acceptable per ASCE 7-22.

How do I account for the parapet in wind load calculations?

ASCE 7-22 Section 29.6 requires parapets to be designed for a net pressure coefficient GCpn = +1.5 on the windward face and GCpn = -1.0 on the leeward face. For a 3 ft parapet, the design pressure is p_parapet = qh x (1.5 + 1.0) = qh x 2.5 applied to the parapet projected area. This is a C&C check, separate from the MWFRS calculations shown above.

When should I use C&C pressures instead of MWFRS pressures?

C&C (Components and Cladding) pressures apply to individual structural elements with tributary areas less than 700 ft^2 — roof decking, wall girts, purlins, fasteners, cladding, and individual mullions. MWFRS pressures apply to the main structural frame (beams, columns, braces) and diaphragms that distribute wind forces through the building. C&C pressures are invariably higher because they capture localized peak suctions at edges and corners. Always check both MWFRS and C&C for a complete design.

How does Exposure B affect the final numbers compared to Exposure C?

Exposure B reduces Kz by approximately 30-40% compared to Exposure C at the same height. For our 20 ft building: Kz_ExposureB = 0.62 vs Kz_ExposureC = 0.90. This means the design pressures would be 45% higher in Exposure C (open terrain). For example, windward wall pressure would be approximately 22 psf in Exposure C vs 15 psf in Exposure B. Suburban offices in developed areas reliably qualify for Exposure B, but always confirm the upwind terrain meets the 2,600 ft fetch requirement for the exposure in the approach direction.

What is the most common mistake in MWFRS wind calculations?

The most frequent error is misapplying the internal pressure coefficient GCpi. Many engineers incorrectly multiply GCpi by the gust factor G again, or apply it with the wrong sign. Per ASCE 7-22 Eq. 27.3-1, GCpi is already a combined gust-and-pressure coefficient and is added algebraically to GCp. The +/- sign on GCpi means you must run both cases — internal pressure adding to external suction (+GCpi) AND internal suction adding to external pressure (-GCpi) — and take the worst case for each surface independently.

Reference only. Verify all values against the current edition of ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures. This worked example does not constitute professional engineering advice and should be verified by a licensed Professional Engineer for project-specific conditions.