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

Technical NotesOpen StructSuite

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. 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.

Steps

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

  2. 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 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: 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 entered. Sloped roof uses Figure 27.3-1 sloped roof regions; ridge orientation sets parallel vs normal to ridge cases.

    In StructSuite: In Step 4, enter Mean Roof Height h (ft) and confirm Kh from Table 26.10-1 (this example uses h = 20 ft). Chapter 27 Directional lists Kz at each elevation z on the Step 4 blue summary. In Step 6, enter Building Length = 44 ft, Building Width = 36 ft, roof type, and roof angle θ (26.6° 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 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.

  5. 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 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 20 ft and basic wind speed V is 120 mph.

  6. Step 6: External pressure coefficients

    Design consideration: For gable roofs, review roof C_p for windward, leeward, and corner zones across both orthogonal directions. In ASCE 7-22 Figure 27.3-1, roof strips for wind parallel to the ridge (and other zones with two tabulated C_p values, e.g. pairs such as −0.9 and −0.18 or −0.5 and −0.18) must be read with the figure notes for primary vs. secondary tributary areas. The smaller roof uplift (less suction—the combination that produces a less negative net uplift) can govern when wind is combined with roof live load or snow in the applicable load combinations, so do not assume the largest suction alone always controls every check. 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.

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

    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). This low-rise example assumes a rigid building (n₁ or f₁ > 1 Hz) and G = 0.85 per Section 26.11.1; if n₁ < 1 Hz, use G_f per Section 26.11.2 instead of the default G. Step 7 lists Section 27.1.5 minimum loads (16/8 psf; gable end wall for parallel-to-ridge). Combine with positive and negative GC_pi as required before Step 8.

    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: Figure 27.3-8 Cases 1–4 combine the wall design pressures from Step 7 with the case factors and eccentricities in the standard; they are the Chapter 27 directional load cases, not Chapter 28 Envelope load cases. Check M_z and line-load shear summaries against your structural model boundary conditions. For gable roofs, roof Note 2 representative |p| governs Cases 3 and 4 roof where θ ≥ 10°. Document which case governs diaphragm shear and overturning for each direction on the calculation cover sheet. Step 8 base shear V = V_walls + V_roof: walls use f × (p_W − p_L) × B_normal × h; sloped gable/hip adds V_y,roof = f × (p_rw − p_rl) × A_half × sin θ (A_half = L×(B/2)/cos θ, q_h, windward primary C_p); V_x,roof = 0 for symmetric slopes. Internal GC_pi cancels in (p_W − p_L) for wall shear. Chapter 28 low-rise envelope instead applies p normal to Figure 28.3-1 zones and sums along-wind components—do not add that zone summation and this explicit roof term for the same governing MWFRS direction.

    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)

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.

Figure 27.3-1 external pressure coefficients

Equation (27.3-1) for rigid and flexible buildings

Section 27.3.5 and Figure 27.3-8.