ASCE 7-22 Wind: Chapter 27 Directional MWFRS for Building Over 60 ft Mean Roof Height

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ASCE 7-22 Wind: Chapter 27 Directional MWFRS for Building Over 60 ft Mean Roof Height

Summary: Main Wind Force Resisting System wind pressures using the ASCE 7-22 Chapter 27 Directional Procedure when Chapter 28 Envelope limits are exceeded (mean…

Overview

Building / scenario

  • Occupancy / use: Office, Risk Category II
  • Geometry / size: 100 ft × 80 ft, h = 75 ft, flat roof
  • Site / exposure: Exposure B, V = 115 mph

Problem statement

A Risk Category II office building has a rectangular plan 100 ft by 80 ft with a flat roof and mean roof height h = 75 ft. The site is suburban with Exposure B. Ultimate design basic wind speed V = 115 mph per ASCE 7-22 Figure 26.5-1 for the project location. The building is enclosed; internal pressure is taken with GC_pi = ±0.18 per Table 26.13-1. Ground elevation is below 1000 ft (K_e = 1.0). No topographic speed-up (K_zt = 1.0). Wind directionality factor K_d = 0.85 for MWFRS per Table 26.6-1. Because h = 75 ft exceeds the 60 ft limit for Chapter 28 (Envelope Procedure) and exceeds the least horizontal dimension constraint for that chapter, MWFRS wind pressures must be determined using the Chapter 27 Directional Procedure with Figure 27.3-1 external pressure coefficients and Section 27.3.1. Determine velocity pressures and design wind pressures for MWFRS design.

Workflow in StructSuite

In StructSuite Wind Loads, select Chapter 27 (Directional Procedure) at the top of the module. Enter Risk Category II, V = 115 mph, Exposure B, K_d = 0.85, K_zt = 1.0, K_e = 1.0, enclosure = enclosed, GC_pi = 0.18. Step 4: Mean Roof Height = 75 ft, Kh from Table 26.10-1. Step 5: q_h and q_z at each height. Step 6: Building Length = 100 ft, Building Width = 80 ft, roof type Flat, roof angle 0°; Step 4 accordion then shows Roof angle = 0.00°. Step 7: Equation 27.3-1 pressures and the Section 27.1.5 Minimum Design Wind Loads card (16 psf / 8 psf vertical projections). Step 8: Section 27.3.5 and Figure 27.3-8 load cases.

Design considerations (excerpt)

Chapter 28 applies only to low-rise buildings with h ≤ 60 ft and h ≤ min(L, B) for flat, gable, or hip roofs. At h = 75 ft, Chapter 28 is not applicable; Chapter 27 Directional Procedure is the standard path for MWFRS. The procedure selection is documented on permit drawings and in the calculation package. Mixing Chapter 28 coefficients with a tall building is a common review comment.

V drives velocity pressure q ∝ V²; 115 mph is typical for many inland Risk II sites. Risk Category and wind map value must match the governing building code edition. Higher V and taller h both increase design pressure; coastal projects may use 130–150 mph or more.

Mean roof height 75 ft sets K_z and K_h at…

Related terms

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

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