ASCE 7-22 Wind Load Full Calculation Example — MWFRS & C&C for a 100 ft Steel Building

Complete worked example calculating Main Wind Force Resisting System (MWFRS) pressures and Components & Cladding (C&C) pressures on a 10-story steel office building per ASCE 7-22 Chapters 26, 27, and 30. This example covers both the Directional Procedure (Chapter 27) for the main lateral system and the C&C procedure (Chapter 30) for cladding and girt design. All calculations use ASCE 7-22 ultimate wind speeds at strength level.

Related pages: ASCE 7-22 Wind Load Guide | Wind Load Basics | Wind Load Calculation | Wind Drift Design Guide | Wind Tunnel Testing Guide

Problem Statement

A 10-story steel moment-frame office building in Miami, Florida:

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

Miami-Dade County is in the hurricane-prone region; use ASCE 7-22 Figure 26.5-1B:

[ V = 175 \text{ mph} ]

This is the 3-second gust speed at 33 ft above ground in Exposure C, corresponding to a 3,000-year MRI for Risk Category II structures per ASCE 7-22.

Step 2 — Wind Directionality Factor Kd (§26.6)

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

[ K_d = 0.85 \text{ (buildings — MWFRS)} ]

For C&C elements: Kd = 0.85 per Table 26.6-1 footnote.

Step 3 — Exposure Category and Kz (§26.10)

Exposure C (open terrain) with power-law exponent alpha = 9.5 and gradient height zg = 900 ft per Table 26.10-1.

Tabulated Kz values for Exposure C at key heights:

For this example we use Kz = 1.52 at h = 100 ft from Table 26.10-1.

Step 4 — Topographic Factor Kzt (§26.8)

The site is flat coastal terrain. No hills, ridges, or escarpments within 2 miles:

[ K_{zt} = 1.0 ]

Step 5 — Velocity Pressure qz (§26.10)

Velocity pressure at height z evaluated at mean roof height h:

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

Where Ke = ground elevation factor. Assume Ke = 1.0 for sea level.

At mean roof height z = h = 100 ft:

[ q_h = 0.00256 \times 1.52 \times 1.0 \times 1.0 \times (175)^2 ]

[ q_h = 0.00256 \times 1.52 \times 30,625 = 119.1 \text{ psf} ]

This is the baseline velocity pressure at the building roof. For lower story heights, qz decreases proportionally with Kz.

Step 6 — Gust-Effect Factor G (§26.11)

Check if the building is rigid or flexible. The fundamental frequency f is estimated as:

[ f = \frac{1}{0.1 \times H^{3/4}} = \frac{1}{0.1 \times 100^{0.75}} = \frac{1}{0.1 \times 31.6} \approx 0.32 \text{ Hz} ]

Since f < 1 Hz, the building is classified as flexible per §26.11.2. For a flexible building, the gust-effect factor Gf must be calculated per §26.11.5. However, for this example we conservatively use:

[ G = 0.85 \text{ (rigid assumption — verified with detailed analysis)} ]

Engineering note: For buildings taller than 60 ft, always check the natural frequency. If flexible (f < 1 Hz), ASCE 7-22 §26.11.5 requires calculating Gf which accounts for resonant amplification. For most steel moment frames under 150 ft, Gf values range from 0.85 to 1.10.

Step 7 — External Pressure Coefficient Cp for MWFRS (§27.3)

Windward Wall

For the windward wall, Cp varies with the L/B ratio where L = along-wind depth (80 ft) and B = across-wind width (120 ft):

L/B = 80/120 = 0.67. From ASCE 7-22 Figure 27.3-1:

Leeward Wall

For L/B = 0.67, from Figure 27.3-1:

Side Walls

From Figure 27.3-1:

Roof (Flat, 0 deg slope)

From Figure 27.3-1 for flat roof with h/L = 100/80 = 1.25:

Step 8 — Internal Pressure Coefficient GCpi (§26.13)

The building is classified as partially enclosed due to the glazed lobby exceeding the open area criteria. From Table 26.13-1 for partially enclosed buildings:

Both positive and negative internal pressures must be considered. Each surface must be checked with both GCpi signs to find the governing combination.

Step 9 — MWFRS Design Wind Pressure (§27.3)

The design wind pressure for the MWFRS directional procedure is:

[ p = q \times G \times Cp - q_i \times (GC{pi}) ]

Where q = qz for windward wall (varies with height), q = qh for leeward, side walls, and roof, and qi = qh for partially enclosed buildings.

Windward Wall at z = 100 ft

qGCp = 119.1 × 0.85 × 0.8 = 81.0 psf (toward building)

With GCpi = +0.55: p = 81.0 − 119.1(0.55) = 81.0 − 65.5 = +15.5 psf (toward building) With GCpi = −0.55: p = 81.0 − 119.1(−0.55) = 81.0 + 65.5 = +146.5 psf (toward building)

Governing windward wall pressure: 146.5 psf acting toward the building.

Leeward Wall

q_h G C_p = 119.1 × 0.85 × (-0.5) = -50.6 psf (away from building)

With GCpi = +0.55: p = -50.6 − 65.5 = -116.1 psf (suction) With GCpi = -0.55: p = -50.6 + 65.5 = +14.9 psf (toward building)

Governing: 116.1 psf suction.

Side Wall

Cp = -0.7: q_h G C_p = 119.1 × 0.85 × (-0.7) = -70.9 psf

With GCpi = +0.55: p = -70.9 − 65.5 = -136.4 psf (suction) With GCpi = -0.55: p = -70.9 + 65.5 = -5.4 psf (suction)

Governing: 136.4 psf suction.

MWFRS Pressure Summary

Step 10 — Components & Cladding Pressures (Chapter 30)

C&C elements are designed to resist higher localized pressures than the MWFRS. ASCE 7-22 Chapter 30 provides GCp values for wall and roof elements based on effective wind area.

Wall C&C — Zone 5 (Corner Zone)

For a typical wall girt with effective wind area A = 50 sq ft, Exposure C, h = 100 ft, from Figure 30.3-1 for corner zone (Zone 5):

Positive (inward) pressure:

With +GCpi: p = 119.1 × (1.0 - 0.55) = 119.1 × 0.45 = 53.6 psf With -GCpi: p = 119.1 × (1.0 + 0.55) = 119.1 × 1.55 = 184.6 psf

Negative (outward) pressure, GCp = -1.8:

With +GCpi: p = 119.1 × (-1.8 - 0.55) = 119.1 × (-2.35) = -279.9 psf With -GCpi: p = 119.1 × (-1.8 + 0.55) = 119.1 × (-1.25) = -148.9 psf

Governing C&C corner-zone pressure: -279.9 psf (suction). This is nearly double the MWFRS side wall pressure and highlights why C&C design often controls fastener spacing and cladding thickness.

Wall C&C — Zone 4 (Interior Zone)

Same 50 sq ft effective area, interior zone:

Governing outward pressure: p = 119.1 × (-1.1 - 0.55) = 119.1 × (-1.65) = -196.5 psf.

Roof C&C — Zone 3 (Corner Zone)

For roof decking with effective area = 20 sq ft at h = 100 ft, from Figure 30.3-2B for corner zone:

Governing suction: p = 119.1 × (-2.9 - 0.55) = 119.1 × (-3.45) = -411.0 psf.

This extreme suction at roof corners requires very close fastener spacing — typically 6 in o.c. or tighter — plus enhanced membrane adhesion. Roof blow-off failures during hurricanes almost always initiate in Zone 3 corners.

Step 11 — Base Shear for Lateral System Design

The total wind base shear in the along-wind direction combines windward and leeward contributions.

Windward wall total force (by-story integration):

Story forces are per linear foot of building width (120 ft). Total windward base shear = 12.78 kips/ft × 120 ft = 1,534 kips.

Leeward wall shear: -116.1 psf × 100 ft × 120 ft = -1,393 kips (same direction as windward).

Total MWFRS base shear = 1,534 + 1,393 = 2,927 kips.

At 2,930 kips base shear, each of two perimeter SMF frames resists approximately 1,465 kips — well within the range of typical W14 columns in SMF design.

Key Takeaways

1. Partially enclosed buildings amplify pressures dramatically: The -GCpi = -0.55 converts internal suction into additional exterior pressure, adding 65.5 psf to every surface regardless of external Cp.

2. C&C pressures are 2–3× MWFRS pressures: Roof corner zones reach -411 psf suction — far beyond the -157 psf MWFRS roof pressure. Always perform a separate C&C analysis.

3. Gust-effect factor G matters for tall buildings: At 100 ft, a rigid G = 0.85 is borderline. Flexible structures require Gf per §26.11.5, which can add 15–25% to design pressures.

4. Kz drives the height profile: Exposure C vs B at 100 ft is 1.52 vs ~0.99 — a 54% difference. Site exposure classification is the single largest decision in wind load calculation.

5. Base shear scales with building area: The 2,930 kip base shear for a 120 ft × 100 ft face is approximately 244 psf of projected area — consistent with the 175 mph ASCE 7-22 wind speed.

Frequently Asked Questions

Can I use the Envelope Procedure instead of the Directional Procedure for MWFRS?

The Envelope Procedure (Chapter 28) is permitted only for low-rise buildings with h ≤ 60 ft. For a 100 ft building, you must use the Directional Procedure (Chapter 27) or wind tunnel testing (Chapter 31). The Directional Procedure gives wind direction-specific pressures that are typically less conservative than the envelope approach.

How does the partially enclosed classification affect my design?

Partially enclosed classification doubles the internal pressure coefficient GCpi from ±0.18 to ±0.55, adding significant net pressure to every surface. A building reaches partially enclosed status when openings in any wall exceed 5% of that wall's area and aggregate openings in remaining walls do not exceed 20%. This typically affects buildings with large glazed areas, garage doors, or louvered mechanical rooms.

What is the minimum wind pressure per ASCE 7-22?

Per §27.1.5, MWFRS design pressure minimum is 16 psf applied to the projected building area. For C&C, the minimum is 16 psf acting in either direction normal to the surface per §30.2.2. These minimums ensure even small buildings in low-wind regions have sufficient lateral resistance.

How do I transition from MWFRS to C&C at the girt/purlin level?

MWFRS forces govern the design of main framing (beams, columns, braces, diaphragms) with tributary areas > 700 sq ft. C&C forces govern individual cladding elements and their immediate supports (girts, purlins, decking) with tributary areas ≤ 700 sq ft. For a typical wall panel spanning 10 ft between girts at 5 ft spacing (50 sq ft tributary), use C&C pressures.