Example
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.
How StructSuite solves this
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. Enter Building Length = 44 ft, Building Width = 36 ft, Mean Roof Height = 20 ft, roof type Gable, roof angle 26.6° (6:12). Complete Steps 5–7 for q, Figure 27.3-1 C_p for each direction, and Section 27.3.1 design pressures.
Steps
- Wind MWFRS procedure: Chapter 27 (Directional)
Design consideration: 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.
In StructSuite: At the top of the wind module, above Step 1, select the Chapter 27 — Wind Loads on Buildings: Main Wind Force Resisting System (Directional Procedure) radio button (not Chapter 28 Envelope Procedure). This example uses ASCE 7-22 Chapter 27 §27.3.1 and Figure 27.3-1 for MWFRS pressures.
- Step 1: Determine risk category of building
Design consideration: 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.
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 §26.2, Figure 26.5-1. If using address lookup, use the address search to auto-fill wind speed.
- Step 4: Velocity pressure exposure coefficient
Design consideration: Gable roof with θ ≈ 26.6° uses the sloped-roof portions of Figure 27.3-1: windward and leeward roof C_p differ from a flat roof. Ridge orientation relative to the building length and width sets which plan dimension is parallel to the ridge for wind-parallel-to-ridge and wind-normal-to-ridge cases in Step 6.
In StructSuite: In the Building dimensions section, in Building Length (ft) enter 44; in Building Width (ft) enter 36; in Mean Roof Height (ft) enter 20. These drive Kh from ASCE 7-22 Table 26.10-1.
- Step 3: Wind load parameters
Design consideration: Exposure C increases K_z relative to B at the same height—often on the order of tens of percent higher q at roof level. Document field verification (open terrain, scattered obstructions) when the jurisdiction questions Exposure C.
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.
- Step 5: Velocity pressure at mean roof height
Design consideration: At h = 20 ft, q_z and q_h are moderate compared to tall buildings; Step 5 still lists q_z at each discrete height for Chapter 27. Confirm the displayed mean roof height matches the architectural roof average used for code.
In StructSuite: In Step 5, verify velocity pressure at mean roof height and, for Chapter 27, the listed q_z at each height z (ft) per ASCE 7-22 Table 26.10-1 and Equation 26.10-1. For this example, mean roof height is 20 ft and basic wind speed V is 120 mph.
- Step 6: External pressure coefficients
Design consideration: For gable roofs, review roof C_p for windward, leeward, and corner zones across both orthogonal directions. Truss heel, wall top plates, and roof diaphragm chords often track peak uplift from roof zones. Compare Part 1 and Part 2 to find governing shear and overturning for the braced line layout.
In StructSuite: In Step 6, confirm Building Length (ft) 44, Building Width (ft) 36, Mean Roof Height (ft) 20, roof type Gable, and roof angle 26.6 degrees. For Chapter 27, review Figure 27.3-1 external pressure coefficients C_p for each orthogonal wind direction (Part 1 and Part 2). If the building is partially enclosed, enter the height z for windward q_z (ft) and the height of the highest opening (ft) where shown for Section 27.3.1 internal pressure.
- Step 7: Design wind pressures
Design consideration: Apply Section 27.3.1 with the correct q for each surface (windward q_z, leeward and roof q_h or q_z as specified). Combine with positive and negative GC_pi as required. Use results for MWFRS and tie to lateral system line forces in the structural model.
In StructSuite: In Step 7, review the Chapter 27 design wind pressure table per ASCE 7-22 Section 27.3.1. Use governing surface pressures for MWFRS load combinations; account for both positive and negative external pressure with internal pressure (GC_pi) from Step 3.
Live design (pre-filled)
The form below is the real StructSuite module with example data loaded. Display only—values cannot be changed.
Steps to Determine Wind Loads on MWFRS
ASCE 7-22 Tables 27.2-1, 28.2-1
User Note: Use Chapter 27 to determine wind pressures on the MWFRS of buildings with any general plan shape, building height, or roof geometry that matches the figures provided. These provisions use the traditional "all heights" method (Directional Procedure) by calculating wind pressures using specific wind pressure equations applicable to each building surface.
User Note: Use Chapter 28 to determine the wind pressure on the MWFRS of low-rise buildings that have a flat, gable, or hip roof. These provisions use the Envelope Procedure by calculating wind pressures from the specific equation applicable to each building surface. For building shapes and heights for which these provisions are applicable, this method generally yields the lowest wind pressure of all the analytical methods specified in this standard.