Wind Load Calculation Example — ASCE 7-22 Chapter 28 Envelope Procedure for a Low-Rise Gable Building

This is a complete, no-steps-skipped MWFRS wind load calculation using ASCE 7-22 Chapter 28 — the Envelope Procedure — on the building the method sees most often in practice: a two-story, 60 ft × 30 ft suburban house with a 4:12 gable roof. We go from the basic wind speed to the velocity pressure q_h (including the Exposure B footnote unique to Chapter 28), through every GC_pf zone, pressure, and surface area for all four load cases of Figures 28.3-1 and 28.3-2 — with the roof-angle interpolation worked out — resolve each zone force into base shear and roof uplift, and finish with the §28.3.6 minimum design wind load check, which for this house governs three of the four cases. 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

#Why the Envelope Procedure applies here

Where each input comes from. K_d = 0.85 for buildings (Table 26.6-1) accounts for the low probability that the strongest wind and the worst pressure coefficient line up in the same direction. Exposure B (§26.7) is urban and suburban terrain with closely spaced obstructions — the correct call for most residential subdivisions, judged by the terrain upwind of the site, not the lot itself. The enclosed classification (§26.12) means openings are small: A_o smaller than the lesser of 0.01A_g or 4 ft², and A_oi/A_gi ≤ 0.2 — giving GC_pi = +0.18 / −0.18 from Table 26.13-1. A failed garage door can reclassify the building as partially enclosed and triple the internal pressure to ±0.55, which is why opening protection is a structural decision, not an architectural one.

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

q_h = 0.00256 · K_h · 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 is applied later, inside the pressure equation (Eq. 28.3-1) — reusing an older spreadsheet that bakes K_d into q_h double-counts it.

K_h comes from Table 26.10-1 at the mean roof height — and here Chapter 28 has a special rule. The rows bracketing h = 20.5 ft, exactly as tabulated:

q_h = 0.00256 × 0.70 × 1.0 × 1.0 × 115² = 23.70 psf

Every pressure in the envelope method uses this single value — unlike Chapter 27, there is no q_z profile up the windward wall. That is part of what makes Chapter 28 fast.

#Step 2 — Internal pressure

The internal pressure term in Eq. 28.3-1 is q_hK_d(GC_pi):

p_i = 23.70 × 0.85 × (±0.18) = ±3.63 psf

Both signs must be considered: +0.18 is the building pressurized (air pushed in through windward openings, pressing outward on every interior surface), −0.18 is the building depressurized (suction through leeward openings, pulling inward). Each load case is therefore evaluated twice — once per sign.

#Step 3 — The edge-zone dimension a

Figure 28.3-1's higher "E" coefficients apply near the corner where the enveloped wind effects concentrate. Their extent is set by the dimension a (Figure 28.3-1 note): 10% of the least horizontal dimension or 0.4h, whichever is smaller, but not less than 4% of the least horizontal dimension or 3 ft:

• 0.1 × 30 = 3.0 ft ← smaller
• 0.4 × 20.5 = 8.2 ft
• Lower bound: max(0.04 × 30, 3) = 3.0 ft ✓

a = 3.0 ft, so the end-zone strips are 2a = 6 ft wide. Every "E" zone in this example lives within 6 ft of the reference corner.

#Step 4 — The zones of Figure 28.3-1

The envelope method names the building surfaces with numbers. For the transverse direction (wind ⊥ ridge, Load Cases 1 and 3): 1 = windward wall, 2 = windward roof slope, 3 = leeward roof slope, 4 = leeward wall, each with an "E" twin in the 2a end strip. For the longitudinal direction (wind ∥ ridge, Load Cases 2 and 4): 5 = windward end wall (the gable), 6 = leeward end wall, 1/4 = the two side walls, 2/3 = the two roof slopes, again with E strips within 2a of the windward end.

#Step 5 — External pressure coefficients GC_pf (Figure 28.3-1)

The transverse table of Figure 28.3-1 is written for roof-angle rows 0–5°, 20°, 30–45° — and a 4:12 roof (18.4°) lands between rows, so the coefficients are interpolated linearly between the 0–5° and 20° rows:

Worked line, zone 1: GC_pf = 0.40 + (18.4 − 5)/(20 − 5) × (0.53 − 0.40) = 0.40 + 0.896 × 0.13 = +0.516. Zones 2 and 2E don't move (−0.69 and −1.07 in both rows).

The longitudinal table (Load Case 2) is a single row — it does not vary with roof angle:

The torsional cases (Figure 28.3-2) reuse these and add reduced-pressure zones on the half of the building away from the reference corner, at 25% of the basic values:

Positive GC_pf is pressure toward the surface; negative is suction away from it. Only zones 1, 1E (transverse windward wall) and 5, 5E (longitudinal windward gable) are positive — everything else on this house is suction.

#Step 6 — Design pressures p (Eq. 28.3-1)

p = q_h K_d [(GC_pf) − (GC_pi)]

With q_hK_d = 23.70 × 0.85 = 20.14 psf, each zone gets two pressures — one per internal-pressure sign:

Worked line, zone 1, +GC_pi: p = 20.14 × (0.516 − 0.18) = 20.14 × 0.336 = +6.78 psf. Worked line, zone 2E: p = 20.14 × (−1.07 − 0.18) = 20.14 × (−1.25) = −25.18 psf — the strongest suction anywhere on the building, which is why roof fastening at corners is always the first components-and-cladding conversation.

#Step 7 — Zone areas, case by case

This is where most hand calculations go wrong, so every area gets its formula. Two facts anchor all of them: the slope length of each roof plane is s = (B/2)/cos θ = 15/cos 18.4° = 15.81 ft, and the E strips run 2a = 6 ft from the reference corner along the building length. Totals must close: side walls 2 × 1,080 ft², gable ends 2 × 615 ft², roof 2 × 948.68 ft².

Load Case 1 (wind ⊥ ridge):

Load Case 2 (wind ∥ ridge): the side walls and slopes keep the same shapes (they run along L; only their coefficients change), and the gable ends come in:

The full gable end is 30 × 18 + ½ × 30 × 5 = 540 + 75 = 615 ft² — wall rectangle plus the 5-ft gable triangle. The corner strip 5E is a width-a trapezoid whose top edge rises from the eave (18 ft) to 18 + 3·tan 18.4° = 19 ft.

Load Case 3 (torsional ⊥): the Load Case 1 pattern on the reference half of the length, 25% zones on the other half:

Load Case 4 (torsional ∥): the Load Case 2 pattern with each gable end split at mid-width — the reference half keeps full pressure, the other half drops to 25%:

Check: 55.5 + 252 + 307.5 = 615 ft² — the corner strip, the reference half, and the torsional half tile the gable end exactly.

#Step 8 — Forces, base shear, and uplift

Each zone force is F = p × A, acting normal to its surface. Resolving into structural actions follows four rules — three geometric, one from the code:

1. Walls carry their full force horizontally. The leeward force adds to base shear — suction on the far wall pulls the building downwind, the same way (p_W − p_L) works in Chapter 27.
2. Side walls parallel to the wind (zones 1/4 in the longitudinal cases) contribute zero along-wind shear — their equal suctions on opposite faces cancel across the building.
3. Roof slopes contribute F·sin θ along the wind only when the wind is perpendicular to the ridge (the slopes face the wind). With wind parallel to the ridge the roof surfaces are parallel to the flow and add nothing to base shear. Vertically, every roof zone contributes F·cos θ to uplift in both directions.
4. §28.3.3 Total Horizontal Load: the total horizontal shear shall not be less than the value found by neglecting the wind forces on the roof. On a pitched roof the windward-slope suction pulls slightly upwind, so including the roof can lower the computed shear — the code refuses that credit. EXCEPTION: buildings using moment frames for the MWFRS. This wood-frame house is not one, so the walls-only floor applies.

Load Case 1 (+GC_pi shown; the −GC_pi case sums to the same shear):

Watch what the roof did: its net along-wind component is −1,444 lb (the stronger windward-slope suction leans the building slightly upwind), dragging the sum below the walls-only value. That is precisely the situation §28.3.3 exists for — the total horizontal shear may not be less than the walls-only 21,287.52 lb, so V_y = 21,287.52 lb. Roof uplift is unaffected by the floor: Σ F·cos θ over the four roof zones = −28,590.86 lb (upward) — on a 4:12 roof the cos θ factor keeps almost all of that suction working vertically.

Load Case 2: only the gable ends act along-wind:

Roof uplift for this case: −26,723.45 lb.

Load Case 3 (torsional ⊥): same rules over the split pattern: Σ including roof = 12,746.75 lb, walls-only = 13,682.40 lb → §28.3.3 governs again: V_y = 13,682.40 lb, uplift −20,689.37 lb. The point of this case is not the smaller shear — it is the twist: full pressure on one half of the building and 25% on the other loads the diaphragm and the wall lines unevenly.

Load Case 4 (torsional ∥): → V_x = 5,718.47 lb, uplift −26,723.45 lb.

#Step 9 — Minimum design wind load (§28.3.6)

The envelope method has the same floor as Chapter 27: the MWFRS load may not be less than 16 psf on the wall area plus 8 psf on the roof area, each projected onto a vertical plane normal to the wind, applied simultaneously and checked per direction.

• Wind ⊥ ridge: walls 60 × 18 = 1,080 ft² at 16 psf, roof band 60 × 5 = 300 ft² at 8 psf → 17,280 + 2,400 = 19,680 lb
• Wind ∥ ridge: the end silhouette is all wall (30 × 18 + gable triangle 75 ft² = 615 ft²) at 16 psf → 9,840 lb

Now the comparison, case by case — this is where the minimum earns its keep:

For this suburban house the minimum governs three of the four cases — and note how the two floors stack: without §28.3.3, Load Case 1's 19,843.18 lb would clear the §28.3.6 minimum by less than one percent; the walls-only floor lifts it to a comfortable 8.2%. That is the normal state of affairs for light-frame buildings in Exposure B, and it is why both checks belong in every submitted package. The torsional cases still must be evaluated for the twist they apply; the minimum replaces the shear magnitude, not the requirement.

#Summary of results

All values above are strength-level (W). For allowable stress design combinations (§2.4.1) the wind term enters at 0.6W — apply the factor after the governing comparison, never before it.

Enter this building in StructSuite's Wind module (/design/wind) — Chapter 28, the geometry, V = 115 mph, Exposure B — and every table in this example appears on screen and in the printed calculation package.

#Frequently Asked Questions

#When can I use the ASCE 7-22 envelope procedure instead of the directional procedure?

Chapter 28 applies to low-rise buildings as defined in §26.2: mean roof height h ≤ 60 ft and h not exceeding the least horizontal dimension, with an enclosed or partially enclosed envelope and a flat, gable, or hip roof. This house qualifies easily (h = 20.5 ft against a 30-ft least dimension). Where it applies, the envelope method is usually faster — one q_h, table-read GC_pf zones, four defined load cases — and its coefficients were calibrated to envelope the worst structural actions over all wind directions, so there is no direction-by-direction C_p bookkeeping. It also gets the Exposure B K_z = 0.70 footnote of Table 26.10-1, which Chapter 27 does not. Buildings that fail the low-rise test (taller than 60 ft, or taller than they are wide) must use Chapter 27, which has no geometric limits. Use one procedure for the whole calculation — the two coefficient systems are not interchangeable.

#Why doesn't internal pressure change the base shear?

Because the internal pressure q_hK_d(GC_pi) acts on every interior surface with the same magnitude, its horizontal resultants on opposite walls are equal and opposite: on this house the +GC_pi case lowers the windward zone-1 pressure to +6.78 psf and deepens the leeward zone-4 suction to −11.99 psf, while the −GC_pi case does the reverse (+14.03 / −4.74 psf) — but (GC_pf,1 − GC_pf,4) is all that survives the subtraction, and both cases sum to the identical 19,843.18 lb (and to the identical §28.3.3 walls-only floor of 21,287.52 lb that ends up governing). Internal pressure absolutely does matter for roof uplift (no opposing surface in the sum), for components and cladding, and for any single wall line — there you keep whichever sign is worse.

#What are the E zones and the dimension a in Figure 28.3-1?

The wind-tunnel studies behind the envelope method found that pressures concentrate near the building corner that faces the oncoming wind — so Figure 28.3-1 boosts the coefficients in strips near a reference corner: zone 1E is windward-wall, 2E/3E roof, 4E leeward-wall, each within 2a of that corner (5E/6E are width-a strips on the gable ends in the longitudinal cases). The dimension a is the smaller of 10% of the least horizontal dimension and 0.4h, but at least 4% of the least dimension and at least 3 ft — here a = min(3.0, 8.2) = 3 ft, so the strips are 6 ft wide. Because the wind can approach any corner, the code requires the pattern to be applied with the reference corner at each corner of the building in turn; for a symmetric building like this one, one orientation per direction captures the envelope.

#Why is the base shear so much smaller with wind parallel to the ridge?

Two reasons, both visible in the Load Case 2 table. First, the along-wind walls are the small ones: the gable ends present only 615 ft² each, versus 1,080 ft² for the long walls in the transverse direction. Second, nothing else helps — the side walls carry equal suction on both faces (net zero across the building) and the roof planes are parallel to the flow, so their large suctions produce uplift but no along-wind force at all. What remains is the gable-end push-pull plus the E-strip terms: 8,939.55 lb, under half the transverse shear — low enough that the §28.3.6 minimum (9,840 lb) takes over. This asymmetry is typical for elongated buildings, and it is why the short-direction lateral system usually ends up the busier design.

#What is the §28.3.3 total horizontal load check, and when does the moment-frame exception apply?

§28.3.3 says the total horizontal shear "shall not be less than that determined by neglecting the wind forces on the roof." On a pitched roof the windward slope's suction acts perpendicular to the slope, which gives it a small upwind horizontal component — so including the roof can reduce the computed base shear, and the code refuses that credit. On this house the roof's net along-wind contribution is −1,444 lb, so Load Case 1's sum of 19,843.18 lb is floored at the walls-only 21,287.52 lb, and Load Case 3 rises the same way (12,746.75 → 13,682.40 lb). The EXCEPTION exempts buildings using moment frames for the MWFRS — their frame design genuinely benefits from modeling the roof pressures as applied. A wood-frame shear-wall house takes no exception; in StructSuite the moment-frame attestation is an explicit opt-in checkbox in the Chapter 28 path, unchecked by default.

#When does the §28.3.6 minimum design wind load govern?

Whenever the computed envelope shear falls below 16 psf on the projected wall silhouette plus 8 psf on the projected roof silhouette, checked per direction at strength level. For this house the floors are 19,680 lb (⊥ ridge) and 9,840 lb (∥ ridge): three of the four load cases fall short — both torsional cases (their 25%-pressure halves shed too much load) and even the basic longitudinal case — while Load Case 1, after the §28.3.3 walls-only floor lifts it to 21,287.52 lb, clears by 8.2% (it would have cleared by only 0.8% without §28.3.3). Suburban Exposure B terrain, moderate wind speeds, and torsional cases are the classic triggers; for light-frame construction the minimum is the rule rather than the exception, the check costs one line of arithmetic, and it belongs in every submitted package. The torsional cases still must be applied for the twist they produce — the minimum replaces the shear magnitude, not the load pattern.

#Where to go next

• The directional companion example on a steeper roof: Wind Load Calculation Example — ASCE 7-22 Chapter 27 Directional Procedure for a Gable-Roof House
• Design the walls for these shears: StructSuite's Wood-Frame Shear Wall module (/design/shear-wall)
• Seismic companion example: Seismic Base Shear Example — ASCE 7-22 ELF for a Two-Story Wood House
• Codes, tools, and learning resources: Structural Engineering Software & Resource Directory