ASCE 7-22 Wind: Chapter 27 Directional MWFRS for Gable Roof (Low-Rise)

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ASCE 7-22 Wind: Chapter 27 Directional MWFRS for Gable Roof (Low-Rise)

Summary: Main Wind Force Resisting System wind pressures using the ASCE 7-22 Chapter 27 Directional Procedure for a gable roof building where the engineer chooses…

Overview

Building / scenario

  • Occupancy / use: Retail, Risk Category II
  • Geometry / size: 44 ft × 36 ft, h = 20 ft, 6:12 gable
  • Site / exposure: Exposure C, V = 120 mph

Problem statement

A Risk Category II retail building has plan dimensions 44 ft by 36 ft with a gable roof and mean roof height h = 20 ft. Roof slope is 6:12 (approximately 26.6° from horizontal). Ultimate design basic wind speed V = 120 mph. The site is open terrain with Exposure C. The building is enclosed; use GC_pi = ±0.18 per Table 26.13-1. K_d = 0.85 for MWFRS, K_e = 1.0, K_zt = 1.0. Although h is below 60 ft and Chapter 28 could apply, the project structural engineer specifies the Chapter 27 Directional Procedure for MWFRS to align with Figure 27.3-1 roof and wall C_p for wind normal and parallel to the ridge. Determine velocity pressures and design wind pressures for MWFRS.

Workflow in StructSuite

Select Chapter 27 at the top of the wind module. Enter Risk Category II, V = 120 mph, Exposure C, K_d = 0.85, K_zt = 1.0, K_e = 1.0, enclosed, GC_pi = 0.18. Step 4: h = 20 ft and Kh; Step 5: q_h and q_z. Step 6: L = 44 ft, B = 36 ft, Gable, θ = 26.6° (6:12); Step 4 accordion shows Roof angle = 6:12 = 26.60° (or degrees). Step 7: Equation 27.3-1 pressures (G = 0.85 rigid), plus Section 27.1.5 Minimum Design Wind Loads (gable end wall for parallel wind). Step 8: Figure 27.3-8 load cases.

Design considerations (excerpt)

Chapter 27 can be used for low-rise gable buildings when the Figure 27.3-1 geometry matches; Chapter 28 Envelope is often a lower pressure for qualifying low-rise shapes but is not mandatory when Chapter 27 is specified. The engineer should state which chapter governs on the calculation cover sheet. Mixed use of coefficients from different chapters is not permitted.

120 mph with Exposure C is common for open rural or fringe commercial sites. q increases with V²; combined with Exposure C, diaphragm and connection design loads increase versus a suburban Exposure B site at the same wind speed.

Step 4: h=20 ft; Step 6: θ ≈ 26.6° (6:12), L=44, B=36. Step 4 summary shows roof angle after θ is…

Related terms

  • StructSuite
  • structural engineering
  • design example
  • wind
  • ASCE 7-22
  • MWFRS
  • chapter27 gable mwfrs

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