How to Calculate Wind Loads — Directional Procedure (ASCE 7-22 Chapter 27)

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 Loads module, 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 9 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 10 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):

Where the ⊥-ridge shear comes from. The walls contribute Σ(p_W − p_L) × strip area = 18,218 lb (the detailed calc lists each strip). The sloped roof adds a horizontal component, V_roof = (p_rw − p_rl) × A_half × sin θ, where A_half = 48 × 16 / cos 26.57° = 858.7 ft² is one sloped half and sin 26.57° = 0.4472. Because the windward slope carries two C_p values (Step 2), Figure 27.3-1 Note 3 requires the roof to be designed for both conditions, so the term is run twice and the larger total governs (+GC_pi column; the internal pressure term is in both p_rw and p_rl and cancels in the difference, so the −GC_pi column gives the same shear):

• C_p = −0.33: (−9.21 − (−13.75)) × 858.7 × 0.4472 ≈ 1,744 lb
• C_p = +0.15: (−1.07 − (−13.75)) × 858.7 × 0.4472 ≈ 4,869 lb — governs

With the pressures carried at full precision instead of the 0.01-psf values printed in Step 3, StructSuite's detailed calc reports 1,745 and 4,871 lb; V_⊥ = 18,218 + 4,871 = 23,089 lb, the figure the table carries. The roof adds to the wall shear here, so Figure 27.3-1 Note 6 (except for a moment-resisting-frame MWFRS at the roof, the total shear shall not be less than with the roof forces neglected) is satisfied. Wind ∥ ridge has no roof term — the two slopes oppose each other and cancel.

Reading the table:

• The 48-ft face direction governs the shear (23,089 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, and in sheltered terrain the floor takes over. The full check — projected areas, figures, and numbers — is worked in the next step.
• ASD level: wind enters the allowable stress design combinations as 0.6W — 23,089 × 0.6 = 13,853 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.

#Step 5 — The minimum design wind load (§27.1.5), worked in full

Every analytical wind calculation ends at the same code checkpoint. §27.1.5 (and its Envelope-Procedure twin, §28.3.6) sets a floor under the answer: the MWFRS wind load shall not be less than 16 psf multiplied by the wall area of the building plus 8 psf multiplied by the roof area of the building, both projected onto a vertical plane normal to the assumed wind direction, with the wall and roof loads applied simultaneously. (Open buildings: 16 psf on the whole projected area A_f.)

The rule exists because the Chapter 27 pressures can legitimately come out small — a low mapped wind speed, sheltered Exposure B terrain, favorable C_p — and the code wants a hard lower bound tied to nothing but geometry (Commentary Figure C27.1-1). Two things engineers miss on the first pass:

1. It is a projected-area load, not a surface pressure. You don't chase the wind around the building. You flatten the silhouette onto a vertical plane facing the wind, take 16 psf on the wall part and 8 psf on the roof part, and push horizontally.
2. It is a per-direction check. Each principal direction's computed base shear is compared with that direction's minimum — a comfortable margin in one direction cannot excuse a shortfall in the other.

#Wind ⊥ ridge — wall strip + roof strip

Looking along the ridge (left panel), the wind sees the 48-ft-long face. The silhouette splits at the eave, with the roof's vertical rise = (B/2) · tan θ = (32/2) × tan 26.57° = 8.0 ft:

• Wall projection: A_wall = L × h_eave = 48 × 18 = 864 ft²
• Roof projection: A_roof = L × rise = 48 × 8.0 = 384 ft²
• F_min,⊥ = 16 × 864 + 8 × 384 = 13,824 + 3,072 = 16,896 lb

#Wind ∥ ridge — the gable-end triangle is wall

Looking across the building (right panel), the wind sees the 32-ft gable end — and here is the subtlety worth a highlight: the triangle between eave and ridge is the gable end wall, a vertical wall surface, not roof. The whole silhouette therefore takes 16 psf. The roof planes do project onto that same triangle, but the area is counted once — Commentary Figure C27.1-1 draws the along-ridge slab as a single 16-psf area:

• Wall projection: A_wall = B × h_eave + ½ × B × rise = 32 × 18 + ½ × 32 × 8.0 = 576 + 128 = 704 ft²
• Roof projection: 0 ft² (already inside the wall silhouette)
• F_min,∥ = 16 × 704 = 11,264 lb

(StructSuite carries θ = 26.57° in full precision, so its detailed calc prints the ⊥-ridge split as 13,823 + 3,073 — the same 16,896 lb total.)

#The check, per direction

Both sides of the comparison are strength-level (W) loads: check first, then apply 0.6 to whichever governs for the ASD combinations.

#Summary of results

Enter this building in StructSuite's Wind Loads module — 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

#When does the ASCE 7-22 minimum design wind load (§27.1.5) govern?

Whenever geometry beats pressure. The minimum is 16 psf × the wall area + 8 psf × the roof area, projected onto a vertical plane normal to the wind, applied simultaneously and checked per direction — so it scales only with the building silhouette, while the analytical pressures scale with V², exposure, and C_p. For a gable building there is one subtlety: with wind along the ridge, the gable-end triangle is wall area, so the entire end silhouette takes 16 psf (Commentary Figure C27.1-1 draws it as one 16-psf slab). In this example (V = 100 mph, Exposure C) the computed shears clear the floor by 22–37%; rerun the identical house in Exposure B and the Case-1 shears drop to 15,783 / 9,376 lb while the minimums stay at 16,896 / 11,264 lb — the minimum governs both directions. Low mapped wind speeds, Exposure B terrain, and small-C_p geometries are the classic triggers, and both sides of the comparison are strength-level (W) values — apply 0.6W afterward for ASD.

#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 (23,089 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.

#Does this example cover components and cladding (C&C)?

No. Everything on this page is the main wind force resisting system (MWFRS): the pressures that load the diaphragms, shear walls and foundation as a whole. Components and cladding — roof sheathing, siding, windows, doors, and the individual studs, rafters and fasteners that hold them — are designed for the Chapter 30 pressures instead. Those start from the same velocity pressure but use their own external pressure coefficients by surface zone and effective wind area, and they are highest at roof edges and corners, so a C&C pressure is normally larger than the MWFRS pressure on the same surface. StructSuite's wind module computes the MWFRS pressures only; C&C is a separate calculation.