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

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. 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.

Steps

1. 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 Section 27.3.1 and Figure 27.3-1 for MWFRS pressures.

2. 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 Section 26.2, Figure 26.5-1. If using address lookup, use the address search to auto-fill wind speed.

3. 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 Step 4, enter Mean Roof Height h (ft) and confirm Kh from Table 26.10-1 (this example uses h = 75 ft). Chapter 27 Directional lists Kz at each elevation z on the Step 4 blue summary. In Step 6, enter Building Length = 100 ft, Building Width = 80 ft, roof type, and roof angle θ (0.0° for this example). After θ is entered, the Step 4 accordion line includes Roof angle = …° or Roof angle = rise:run = …°.

4. 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.

5. 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. Step 4 blue summary lists h, Kh, and Kz at each listed z for tall buildings. 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 qz 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.

6. 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.

7. Step 7: Calculate wind pressure, p, on each building surface

   Design consideration: Section 27.3.1 combines external C_p with internal pressure and the appropriate q or q_h. Assume rigid response (n₁ or f₁ > 1 Hz) so G = 0.85 per Section 26.11.1; if flexible (n₁ < 1 Hz), determine G_f per Section 26.11.2—confirm period for tall frames before relying on Step 7 alone. Step 7 includes a Section 27.1.5 Minimum Design Wind Loads card: 16 psf×wall vertical projection + 8 psf×roof vertical projection (enclosed/partially enclosed), with parallel-to-ridge gable wall treated as rectangle plus gable triangle. Use governing pressures for load combinations in Chapter 2.

   In StructSuite: In Step 7, review design wind pressure p on each building surface per ASCE 7-22 Equation 27.3-1 and Section 27.3.1 with Figure 27.3-1 C_p. StructSuite uses G = 0.85 for a rigid building (n₁ ≥ 1 Hz) per Section 26.11.1; confirm rigidity or use G_f manually if n₁ < 1 Hz. Use governing surface pressures for MWFRS load combinations; account for both positive and negative external pressure with internal pressure (GC_pi) from Step 3. Review the Section 27.1.5 Minimum Design Wind Loads card (16 psf / 8 psf wall and roof vertical projections, or 16 psf × Af for open buildings; gable end wall for wind parallel to ridge).

8. Step 8: Evaluate design wind load cases

   Design consideration: At h = 75 ft, Chapter 27 Figure 27.3-8 still applies for MWFRS case combinations; this is not Chapter 28. Review Cases 1–4 and ±GC_pi columns for the lateral system and foundations. Line-load shear summaries combine wall strip loads with sloped-roof horizontal shear where θ ≥ 10° (V_y,roof term)—compare to project finite-element or line-element results for diaphragm flexibility. Document governing case for each orthogonal direction and connection design. V = V_walls + V_roof (walls: f × (p_W − p_L) × B_normal × h; roof horizontal per Step 8 detail block). Internal GC_pi cancels in (p_W − p_L) for wall shear. Chapter 28 envelope (where it applies) obtains along-wind shear by resolving normal pressures on Figure 28.3-1 zones—including sloped roof—without a second parallel roof strip; tall buildings like this example stay on Chapter 27 only.

   In StructSuite: In Step 8, review MWFRS design wind load cases per ASCE 7-22 Section 27.3.5 and Figure 27.3-8 (Cases 1–4 and internal pressure ±GC_pi columns) after Step 7 pressures are established. Horizontal base shear is V_walls + V_roof: walls use f × (p_W − p_L) × B_normal × h (signed Equation 27.3-1); for sloped gable/hip and θ ≥ 10°, add roof horizontal along the axis normal to ridge as f × (p_rw − p_rl) × A_half × sin θ (A_half = L×(B/2)/cos θ, q_h, windward primary C_p); wind parallel to ridge uses V_x,roof = 0 (symmetric cancellation). Internal GC_pi cancels in (p_W − p_L) for global wall shear. Chapter 28 Envelope instead sums along-wind components of normal pressures on Figure 28.3-1 zones—use one MWFRS procedure per governing calculation, not both combined.

Live design (pre-filled)

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