Wind Load Worked Example — MWFRS per ASCE 7-22 Directional Procedure

Problem: Calculate the design wind pressures for the Main Wind Force Resisting System (MWFRS) of a 40 ft × 80 ft, 30 ft tall office building located in an open suburban exposure in Miami, Florida. Use the ASCE 7-22 Directional Procedure (Chapter 27, Part 1). The building has a flat roof with a parapet height of 3 ft.

Step 1: Building Parameters

Step 2: Basic Wind Speed (ASCE 7-22 Chapter 26)

Miami is in a hurricane-prone region. From ASCE 7-22 Figure 26.5-1A (Risk Category II):

V = 170 mph (ultimate design wind speed for Miami coastal area)

Adjustment for Risk Category II: For MWFRS, use V = 170 mph directly.

Step 3: Wind Load Parameters

Wind directionality factor (Kd): Table 26.6-1 → Kd = 0.85 (MWFRS, building)

Exposure category: Exposure B (suburban terrain with numerous buildings, trees, etc.)

Topographic factor (Kzt): Assume site is not on a hill, ridge, or escarpment → Kzt = 1.0

Gust Effect Factor (G): For rigid buildings (h < 100 ft and h/least width < 4), ASCE 7-22 allows G = 0.85 default. Our building: h = 31.5 ft < 100 ft, and 31.5/40 = 0.79 < 4, so it qualifies as rigid. Use G = 0.85.

Enclosure classification: Enclosed building (typical office with windows and doors)

Internal pressure coefficient (GCpi): Table 26.13-1 → GCpi = ±0.18 (enclosed)

Step 4: Velocity Pressure Exposure Coefficient (Kz)

From ASCE 7-22 Table 26.10-1, for Exposure B:

For the mean roof height h = 31.5 ft, use Kzh = 0.71.

Step 5: Velocity Pressure (qh)

ASCE 7-22 Equation 26.10-1:

qh = 0.00256 × Kz × Kzt × Kd × V² × (1/G for certain cases, but G applied separately)

Actually, the velocity pressure at mean roof height is:

qh = 0.00256 × Kzh × Kzt × Kd × V²

qh = 0.00256 × 0.71 × 1.0 × 0.85 × (170)²

qh = 0.00256 × 0.71 × 0.85 × 28,900

qh = 0.00256 × 17,441

qh = 44.7 psf

Step 6: External Pressure Coefficients (Cp) — Walls

From ASCE 7-22 Figure 27.3-1, for MWFRS:

Windward Wall:

Cp = 0.80 (for L/B > 1.0, which is the case with wind perpendicular to the long side)

Using L/B = 80/40 = 2.0 → Cp = 0.80

Leeward Wall:

Cp depends on L/B:

• L/B = 2.0 → Cp = -0.30 (from Figure 27.3-1)

Side Walls:

Cp = -0.70 (both sides, suction)

Step 7: External Pressure Coefficients — Roof (Flat, h ≤ 60 ft)

For flat roofs with parapet, ASCE 7-22 Figure 27.3-1 gives pressure coefficients for zones:

Without Parapet (wind parallel to ridge):

With a 3 ft parapet, internal pressures are reduced in corner zones, but for a simplified example we use the without-parapet coefficients as a conservative approach.

Step 8: Design Wind Pressure (ASCE 7-22 Equation 27.3-1)

p = qh × G × Cp - qh × (GCpi)

Windward Wall (at height h):

For windward wall at h = 31.5 ft: p = 44.7 × 0.85 × 0.80 - 44.7 × (±0.18) p = 30.4 - (±8.0) p = +22.4 psf (internal pressure adds to inward force)

p_max = 30.4 + 8.0 = 38.4 psf (when internal suction helps) p_min = 30.4 - 8.0 = 22.4 psf (when internal pressure pushes outward)

Use the more critical value for design: p_max = 38.4 psf for windward wall.

Leeward Wall:

p = 44.7 × 0.85 × (-0.30) - 44.7 × (±0.18) p = -11.4 - (±8.0) p = -19.4 psf (suction, more critical)

Side Wall:

p = 44.7 × 0.85 × (-0.70) - 44.7 × (±0.18) p = -26.6 - (±8.0) p = -34.6 psf (suction)

Roof (Zone 1, interior):

p = 44.7 × 0.85 × (-0.90) - 44.7 × (±0.18) p = -34.2 - (±8.0) p = -42.2 psf (uplift)

Step 9: Summary of Design Pressures

Step 10: Load Application

For MWFRS design, these pressures are applied simultaneously:

• Windward wall: Triangular distribution varying from 0 at base to 38.4 psf at roof (or use uniform at qh for simplicity per code allowance)
• Leeward wall: Uniform -19.4 psf over full height
• Roof: Uplift pressures distributed per zone widths equal to h/2 = 15.75 ft from edges

Total Horizontal Force at Roof Level:

F_windward = 38.4 psf × (31.5/2 ft) × 80 ft = 48,384 lb (simplified triangular) F_leeward = -19.4 psf × 31.5 ft × 80 ft = -48,888 lb

Net shear at base ≈ 97,272 lb (97.3 kips) per frame line.

This base shear must be distributed to the lateral force-resisting system (moment frames, braced frames, or shear walls).

Step 11: Check Serviceability

For service-level wind (approximately 0.6 × ultimate for 50-year return):

Service wind speed ≈ 170 × √0.6 = 131.7 mph

Service pressure = 44.7 × 0.6 = 26.8 psf at roof height

Service drift limit: H/400 = 31.5 × 12 / 400 = 0.95 in per ASCE 7-22 Table CC-1 for building content damage.

The frame must be proportioned to limit interstory drift to 0.95 in under service wind loads.

Try the Calculator

Use the Wind Load Calculator to compute wind pressures per ASCE 7-22 for your own building parameters, exposure, and roof geometry.

Frequently Asked Questions

What is the difference between MWFRS and C&C wind loads? MWFRS (Main Wind Force Resisting System) loads apply to the primary structural frame — beams, columns, and lateral bracing. C&C (Components and Cladding) loads apply to individual elements like roof panels, purlins, and window mullions. C&C pressures are higher (up to 2-3× MWFRS) because they account for localized peak suction at edges and corners.

When should I use the Directional Procedure versus the Envelope Procedure? The Directional Procedure (Ch 27 Part 1) is the standard method for buildings of any height with regular shape. The Envelope Procedure (Ch 28) is limited to low-rise buildings (h ≤ 60 ft) and is simpler but generally less conservative for some load cases. For the example building (31.5 ft tall), both procedures could be used.

Does ASCE 7-22 account for hurricane versus non-hurricane regions differently? Yes. ASCE 7-22 Figure 26.5-1 provides separate maps for hurricane-prone regions (Gulf and Atlantic coasts, Hawaii, Puerto Rico) and non-hurricane regions (continental interior, Alaska). Hurricane regions have higher basic wind speeds for the same Risk Category. The 170 mph used in this example is specific to coastal Miami (Risk Category II).

How do I handle wind loads on open structures like canopies? ASCE 7-22 Chapter 27 Part 2 covers open buildings and other structures. The key difference is that internal and external pressures are combined differently — for an open building, internal pressure acts on the projected area rather than just the enclosed volume. The wind load calculator supports both enclosed and open building configurations.

See Also

• Beam Capacity Calculator
• Beam Deflection Calculator
• Steel Beam Sizes Reference
• Beam Design Guide
• Beam Span Reference