ASCE 7-22 Wind: Chapter 28 Envelope MWFRS for Low-Rise Gable (qh, Zones, Minimum Loads)

Example

A Risk Category II retail shell is 48 ft long by 32 ft wide in plan with mean roof height h = 18 ft and a gable roof at 5:12 slope (roof angle approximately 22.6°). Ultimate design wind speed V = 120 mph. The site is Exposure B (suburban), ground elevation below 1000 ft (K_e = 1.0), no topographic speed-up (K_zt = 1.0). The building is enclosed with GC_pi = ±0.18 per Table 26.13-1. Wind directionality factor K_d = 0.85 for MWFRS per Table 26.6-1. The building satisfies Chapter 28 low-rise limits (h ≤ 60 ft and h ≤ min(L, B)). The engineer uses the Envelope Procedure to obtain zone design pressures and must verify that MWFRS design is not less than the Section 27.1.5 minimum wind loads (16 psf on wall vertical projection and 8 psf on roof vertical projection, applied simultaneously) as implemented with Section 28.3.6. Determine qh, zone pressures, load cases, and review minimum design wind load cases in StructSuite.

How StructSuite solves this

Select Chapter 28 — Envelope Procedure. Enter Risk Category II, V = 120 mph, Exposure B, K_d = 0.85, K_zt = 1.0, K_e = 1.0, enclosed, GC_pi = +0.18 / −0.18. Step 4: Mean Roof Height = 18 ft, Kh from Table 26.10-1. Step 5: qh from Equation 26.10-1. Step 6: Building Length = 48 ft, Building Width = 32 ft, roof type Gable, roof angle = 22.6° (5:12); Step 4 accordion shows Roof angle = 5:12 = 22.62°. Step 7: Review Equation 28.3-1 pressures for each zone and load case; expand Minimum Design Wind Loads and compare computed base shear to the minimum for each direction (Section 27.1.5 / 28.3.6).

Steps

1. Step 1: Determine risk category of building

   Design consideration: 120 mph with Risk II is common for inland retail shells. Verify the wind map and risk category match the adopted code and project-specific geotechnical and architectural data.

   In StructSuite: In Step 1, use the Risk Category dropdown to select I, II, III, or IV (e.g., II for offices, residential). In the Basic Wind Speed (mph) input box enter 120. Per ASCE 7-22 Section 26.2, Figure 26.5-1. If using address lookup, use the address search to auto-fill wind speed.

2. Step 4: Velocity pressure exposure coefficient

   Design consideration: h = 18 ft and min(L,B) = 32 ft satisfy h ≤ min(L,B) for Chapter 28. Step 4 lists h and Kh; after Step 6, Step 4 shows roof angle. Plan 48×32 ft sets zone dimension a and tributary widths for envelope zones.

   In StructSuite: In Step 4, enter Mean Roof Height h (ft) and confirm Kh from Table 26.10-1 (this example uses h = 18 ft). Chapter 27 Directional lists Kz at each elevation z on the Step 4 blue summary. In Step 6, enter Building Length = 48 ft, Building Width = 32 ft, roof type, and roof angle θ (22.6° for this example). After θ is entered, the Step 4 accordion line includes Roof angle = …° or Roof angle = rise:run = …°.

3. Step 3: Wind load parameters

   Design consideration: Exposure B reduces velocity pressure relative to C or D; document surface roughness if the jurisdiction questions suburban vs open terrain. K_e = 1.0 below 1000 ft; K_zt = 1.0 when no hill feature applies.

   In StructSuite: In Step 3, use the Exposure dropdown to select B, C, or D. If ground elevation > 1000 ft, use Table 26.9-1 for Ke. For topographic features (hills, ridges), use the Kzt section and enter parameters from ASCE 7-22 Figure 26.8-1. Kd = 0.85 for MWFRS is typical.

4. Step 5: Velocity pressure at mean roof height

   Design consideration: qh from Equation 26.10-1 feeds Equation 28.3-1. Higher qh scales all zone pressures proportionally; minimum design wind loads in Step 7 are independent of qh (fixed 16/8 psf on projected areas).

   In StructSuite: In Step 5, velocity pressure qh is computed from ASCE 7-22 Equation 26.10-1. For Chapter 28 Envelope, continue to Step 6 for zone dimension a and GCpf from Figure 28.3-1; Step 7 shows zone pressures, load cases, and Minimum Design Wind Loads per Section 27.1.5 and 28.3.6 when applicable. For Chapter 27 Directional, Step 5 lists qz at each z; then Step 6–8 follow Figure 27.3-1 and Section 27.3.1.

5. Step 6: External pressure coefficients

   Design consideration: Figure 28.3-1 gives GC_pf for gable roofs by zone for wind perpendicular and parallel to ridge. Load Cases 3 and 4 apply when Section 28.3.2 torsional cases are required unless an exception is attested. Confirm roof type and θ match the figure ranges used in the app.

   In StructSuite: With Chapter 28 Envelope selected, in Step 6 enter Building Length = 48 ft, Building Width = 32 ft, Mean Roof Height = 18 ft, roof type Gable, roof angle = 22.6°. Confirm zone dimension a and external pressure coefficients (GC_pf) from Figure 28.3-1 (and 28.3-2 for torsional cases if required). Attest torsional exceptions per Section 28.3.2 when applicable.

6. Step 7: Calculate wind pressure, p, on each building surface

   Design consideration: Section 28.3.6 references Section 27.1.5 minimums: 16 psf × wall vertical projection + 8 psf × roof vertical projection for enclosed/partially enclosed buildings. Parallel-to-ridge wind on a gable uses the gable end wall projected area (rectangle below eave plus gable triangle), not a single B×h rectangle. StructSuite shows separate Minimum Design Wind Loads cases and max(computed, minimum) where listed. Use governing shears for diaphragm and foundation design.

   In StructSuite: In Step 7, review design pressure p from Equation 28.3-1 for each zone and load case. The tables include comparison to Minimum Design Wind Loads (Section 27.1.5 / 28.3.6): 16 psf × wall vertical projection plus 8 psf × roof vertical projection for enclosed and partially enclosed buildings (applied simultaneously); open buildings use 16 psf × Af. Governing base shear uses the larger of computed and minimum where shown.

Live design (pre-filled)

The form below is the real StructSuite module with example data loaded. Display only—values cannot be changed.