Wind Load Calculation Example — ASCE 7-22 Chapter 27 Directional Procedure for a Gable-Roof House

This is a complete, no-steps-skipped MWFRS wind calculation for an ordinary building: a two-story, gable-roof wood-frame house. We follow ASCE 7-22 Chapter 27 (Directional Procedure) from the basic wind speed all the way to the four design wind load cases of Figure 27.3-8 — the part of the chapter engineers most often see for the first time on a plan-check correction. Every number was computed with the same engine that runs StructSuite's Wind module (/design/wind), so you can reproduce any line of this example in the app and get exactly these values.

#The building

#Choosing the input parameters — where each one comes from

Before any equation, four selections set the stage. Each is a code table, and each is worth understanding rather than defaulting:

Exposure category (§26.7). Look at the terrain upwind of the site, not the site itself: B — urban/suburban or wooded terrain with closely spaced obstructions (most residential subdivisions); C — open terrain with scattered obstructions under 30 ft (open country, grasslands, and the default when B and D don't apply); D — flat, unobstructed areas and water surfaces (shorelines, mud flats). We chose C for a house at the edge of open fields. This single choice moves every pressure in the calculation — see the sensitivity box in Step 1.

Directionality factor K_d (Table 26.6-1, excerpt):

K_d accounts for the low probability that the strongest wind blows from exactly the direction that produces the worst pressure coefficient on your building — two maxima that rarely coincide (ASCE 7-22 Commentary §C26.6). Rectangular buildings get the biggest credit (0.85); a round chimney looks the same from every direction, so it gets none (1.00).

Enclosure classification → GC_pi (Table 26.13-1):

The classification is about openings — doors, operable windows, vents — and it is a design decision with teeth: a garage-door failure can turn "enclosed" into "partially enclosed" and triple the internal pressure. In hurricane regions this is why opening protection matters as much as member sizing.

Topography and elevation. K_zt = 1.0 unless the building sits on the upper half of an isolated hill, ridge, or escarpment (Figure 26.8-1 has the K₁K₂K₃ procedure); K_e = 1.0 at sea level and may be reduced with altitude (Table 26.9-1) — taking 1.0 is always conservative.

#Why Chapter 27 (and when Chapter 28 applies)

#Step 1 — Velocity pressure q_z (Eq. 26.10-1)

q_z = 0.00256 · K_z · K_zt · K_e · V² (psf, with V in mph)

Note what is not in this equation in ASCE 7-22: K_d. The directionality factor moved into the pressure equation (27.3-1) — a frequent source of double-counting errors when engineers reuse old spreadsheets.

K_z comes from Table 26.10-1. The rows bracketing our heights (values exactly as tabulated):

Between tabulated rows, interpolate linearly. At the mean roof height h = 22 ft:

K_h = 0.90 + (22 − 20)/(25 − 20) × (0.94 − 0.90) = 0.916

Hand-substitution at z = 15 ft (Exposure C → K_z = 0.85):

q₁₅ = 0.00256 × 0.85 × 1.0 × 1.0 × 100² = 21.76 psf

The full profile at the heights this building needs:

q_h = 23.45 psf is the workhorse value: leeward wall, sidewalls, roof, and the internal pressure all use it. Only the windward wall uses the height-varying q_z.

#Step 2 — External pressure coefficients C_p (Figure 27.3-1)

Walls. Windward is always C_p = +0.8; sidewalls are always −0.7. The leeward wall depends on how deep the building is along the wind (L/B ratio):

A deeper building lets the flow partially reattach, so its leeward suction is weaker — that is what the interpolation captures.

Roof, wind ⊥ ridge (θ = 26.6° ≥ 10°). With h/L∥ = 22/32 = 0.69, the windward slope carries two coefficients — Figure 27.3-1 tabulates both a suction and a pressure value between θ = 20° and 30°, and both must be checked as separate load cases: C_p = −0.33 and +0.15 (interpolated at θ = 26.6°). The leeward slope: C_p = −0.6.

Roof, wind ∥ ridge. The slope doesn't matter when the wind runs along the ridge — the roof behaves like a flat roof, loaded in strips measured from the windward edge (h/L∥ = 22/48 = 0.46 < 0.5): C_p = −0.9 from 0 to h/2 (0–11 ft) and h/2 to h (11–22 ft), −0.5 from h to 2h (22–44 ft), −0.3 beyond (44–48 ft) — each paired with −0.18 as an alternative that must also be checked.

#Step 3 — Design pressures p (Eq. 27.3-1)

p = q · K_d · G · C_p − q_i · K_d · (GC_pi)

with q = q_z for the windward wall, q = q_h everywhere else, and q_i = q_h for an enclosed building. Every surface is computed twice — once with +GC_pi (internal pressure pushing out) and once with −GC_pi (internal suction) — because the interior can do either, and different elements are governed by different signs.

Hand-substitution, windward wall at z = h = 22 ft:

p = 23.45 × 0.85 × 0.85 × 0.8 − 23.45 × 0.85 × (±0.18) = 13.56 ∓ 3.59 = +9.97 psf (+GC_pi) or +17.14 psf (−GC_pi)

And once more for a suction surface — the leeward wall, wind ⊥ ridge (C_p = −0.5, q = q_h):

p = 23.45 × 0.85 × 0.85 × (−0.5) − 23.45 × 0.85 × (+0.18) = −8.47 − 3.59 = −12.06 psf (+GC_pi)

Notice the sign logic: on the windward wall, internal pressure pushing outward (+GC_pi) fights the external push, so the −GC_pi column gives the bigger wall load. On suction surfaces, +GC_pi adds to the pull. That's the whole reason every surface carries two columns.

The complete tables, exactly as StructSuite's Step 7 prints them:

Wind ⊥ ridge (loads the 48-ft face):

Wind ∥ ridge (loads the 32-ft gable end):

Sign convention: positive pressure acts toward the surface, negative (suction) acts away from it.

#Step 4 — Base shear and the Figure 27.3-8 design wind load cases

For the lateral system you need the story shear the MWFRS must resist in each direction. The horizontal drag comes from the signed sum of windward and leeward wall pressures — internal pressure acts on all interior surfaces at once, so GC_pi cancels from the total shear — plus, for a sloped roof with θ ≥ 10°, the horizontal component of the windward-minus-leeward roof pressures.

But one direction at full pressure is not the whole story. Section 27.3.5 requires the MWFRS to be designed for the four load cases of Figure 27.3-8:

Why do the diagonal and torsional cases exist? Real wind is not obliged to blow along a principal axis. An oblique (quartering) wind loads two faces at once — Case 3 — and gust structure plus plan asymmetry twist the building even when it looks symmetric — Cases 2 and 4. The elements Case 1 alone would miss are corner columns (75% on two axes combined can exceed 100% on one) and the end walls of torsionally sensitive layouts. ASCE 7 exempts some buildings from the torsional cases via the Appendix D criteria referenced in §27.3.5 (including certain one- and two-story light-frame buildings) — verify the conditions before dropping Cases 2 and 4.

StructSuite's Step 8 computes all four cases in both directions. The summary for this building (torsion is reported per foot of wall height; e_x = 0.15 × 32 = ±4.8 ft, e_y = 0.15 × 48 = ±7.2 ft):

Reading the table:

• The 48-ft face direction governs the shear (19,963 lb) — more sail area. Your shear wall lines perpendicular to the ridge must resist it.
• The minimum design wind load (§27.1.5: 16 psf on the wall projection, 8 psf on the roof projection) is a floor on the analytical result — here Case 1 exceeds it in both directions, but for light buildings in low-wind regions the minimum often governs; always show the check.
• ASD level: wind enters the allowable stress design combinations as 0.6W — 19,963 × 0.6 = 11,978 lb is what the NDS shear wall design actually sees. As with seismic, labeling the force level is the difference between a clean review and a correction letter.

#Summary of results

Enter this building in StructSuite's Wind module (/design/wind) — Chapter 27, the geometry, V = 100 mph, Exposure C — and every table in this example appears on screen and in the printed calculation package, including the Figure 27.3-8 case summary above.

#Frequently Asked Questions

#What are the four MWFRS design wind load cases in ASCE 7-22 Figure 27.3-8 — and why do the diagonal cases matter?

Section 27.3.5 requires them for the Directional Procedure: Case 1 — full design pressures on each principal axis, one at a time; Case 2 — 75% of the Case-1 pressures plus a torsional moment from shifting the resultant by 15% of the face width; Case 3 — 75% of the Case-1 pressures on both axes simultaneously (the quartering-wind proxy); Case 4 — 75% of Case 2 on both axes, i.e. 56.3% of full pressure plus torsion. They exist because oblique winds load two faces at once and asymmetric pressure distributions twist the building — effects Case 1 alone cannot capture. Corner columns and torsionally sensitive wall layouts are the classic elements governed by Cases 2–4. In this example, Case 1 governs the overall base shear (19,963 lb ⊥ ridge) while Cases 3 and 4 set the governing roof uplift (21,125 lb) — a typical pattern for symmetric low-rise buildings, and exactly why the code makes you check all four. Certain buildings qualify for a torsional-case exemption through the Appendix D criteria referenced in §27.3.5 — verify before omitting.

#Where to go next

• Seismic companion example: Seismic Base Shear Example — ASCE 7-22 ELF for a Two-Story Wood House
• Design the walls for these shears: StructSuite's Wood-Frame Shear Wall module (/design/shear-wall)
• The NDS side of the story: Wood Design with the NDS Supplement and Engineered Wood Products
• Codes, tools, and learning resources: Structural Engineering Software & Resource Directory