Example
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.
How StructSuite solves this
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. Enter Building Length = 100 ft, Building Width = 80 ft, Mean Roof Height = 75 ft, roof type Flat, roof angle 0°. Complete Steps 1–5 for q_h and q_z; Step 6 shows Figure 27.3-1 C_p for each orthogonal direction; Step 7 lists Section 27.3.1 design pressures.
Steps
- Wind MWFRS procedure: Chapter 27 (Directional)
Design consideration: 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.
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: 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.
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 115. 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: Mean roof height 75 ft sets K_z and K_h at roof level from Table 26.10-1 and drives q_z at each height in Chapter 27. Plan 100 ft by 80 ft sets orthogonal wind directions and leeward L/B ratios for Figure 27.3-1. Flat roof: use roof C_p for θ ≤ 10° per the figure notes.
In StructSuite: In the Building dimensions section, in Building Length (ft) enter 100; in Building Width (ft) enter 80; in Mean Roof Height (ft) enter 75. These drive Kh from ASCE 7-22 Table 26.10-1.
- Step 3: Wind load parameters
Design consideration: Exposure B (suburban) reduces velocity pressure compared to C or D. K_d = 0.85 for MWFRS is typical. K_e = 1.0 below 1000 ft elevation. K_zt = 1.0 when no hill or escarpment is present; confirm site visits and topo maps before taking K_zt = 1.0 on hillside jobs.
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: Chapter 27 uses q_z at each height z from ground to mean roof height for windward walls and positive internal pressure where applicable; q_h at mean roof height is used for roof and leeward surfaces per Section 27.3.1. Review the Step 5 table for q_z at each elevation before interpreting Step 7 pressures.
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 75 ft and basic wind speed V is 115 mph.
- Step 6: External pressure coefficients
Design consideration: Figure 27.3-1 is evaluated for wind along each principal plan direction (Part 1 and Part 2). Leeward wall C_p depends on L_parallel / B_perp. For flat roofs, roof zones follow the θ ≤ 10° cases. Footnote (b) effective area checks can reduce tributary pressure for some elements; confirm which surfaces govern connections and cladding.
In StructSuite: In Step 6, confirm Building Length (ft) 100, Building Width (ft) 80, Mean Roof Height (ft) 75, roof type Flat, and roof angle 0.0 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: Section 27.3.1 combines external C_p with internal pressure and the appropriate q or q_h. Use governing positive and negative pressures for load combinations in Chapter 2. Compare to minimum design wind load in Section 27.1.5 when required. Document critical zones for MWFRS members, diaphragm chords, and overturning.
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.