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

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

Summary: Low-rise enclosed building per Chapter 28 Envelope Procedure: velocity pressure, Figure 28.3-1 zone pressures, Load Cases 1–4, and Section 27.1.5 /…

Overview

Building / scenario

  • Occupancy / use: Retail shell, Risk Category II
  • Geometry / size: 48 ft × 32 ft, h = 18 ft, 5:12 gable
  • Site / exposure: Exposure B, V = 120 mph

Problem statement

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.

Workflow in StructSuite

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).

Design considerations (excerpt)

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.

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.

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.

qh from Equation 26.10-1 feeds Equation 28.3-1. Higher qh scales all zone pressures proportionally;…

Related terms

  • StructSuite
  • structural engineering
  • design example
  • wind
  • ASCE 7-22
  • MWFRS
  • chapter28 envelope gable minimum

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