ASCE 7-22 Chapter 27 Directional Procedure — MWFRS (StructSuite Wind Module)
Reference: ASCE/SEI 7-22, Chapter 27 (Directional Procedure), Chapter 26 (wind environment, velocity pressure, gust effects), Chapter 2 (load combinations).
Technical version: 1.0 (March 2026) — aligned with the StructSuite Chapter 27 wizard (Steps 1–8 for directional MWFRS).
1. Procedure selection: Chapter 27 vs Chapter 28
Use Chapter 27 — Directional Procedure when the project specifies MWFRS pressures from Figure 27.3-1 external pressure coefficients C_p and Section 27.3.1 (Equation 27.3-1), or when building height or geometry falls outside Chapter 28 low-rise limits (for example mean roof height h > 60 ft or h > min(L, B) for Chapter 28).
Chapter 28 — Envelope Procedure uses a different formulation (GC_pf zones, Equation 28.3-1, Section 28.3.2 load cases). Do not mix Chapter 28 zone pressures with Chapter 27 C_p figures on the same cover sheet unless the governing code path explicitly allows it.
In StructSuite, select Chapter 27 at the top of the wind module before Steps 1–8.
2. Wizard steps (directional MWFRS)
| Step | Role |
|---|---|
| 1–5 | Risk category, V, exposure and factors, q_h and q_z (Table 26.10-1, Equation 26.10-1). |
| 6 | Building dimensions and roof angle; Figure 27.3-1 C_p for each orthogonal direction. |
| 7 | Equation (27.3-1) — design wind pressure p on each building surface (windward, leeward, sidewall, roof by strip). Subtitle references Equation (27.3-1) for rigid and flexible buildings; StructSuite uses the rigid-building gust shortcut (see Section 3). |
| 8 | Section 27.3.5 and Figure 27.3-8 — MWFRS design wind load cases (Cases 1–4), torsion, and summary horizontal shear / roof uplift from the app’s line-load model. This is not the Chapter 28 “Envelope Procedure”; it is the Chapter 27 load-case figure. |
| (after 8) | Interactive building diagram when results exist. |
Until Steps 1–6 and q_h are complete, the module shows red instructional lines under Steps 7–8; when complete, those lines hide and the blue accordion summaries show computed values only—same pattern as earlier steps.
3. Gust-effect factor G (Equation 27.3-1)
Equation (27.3-1) includes G from Section 26.11.
- ASCE 7-22 defines a rigid building as having fundamental natural frequency n₁ ≥ 1 Hz and a flexible building as n₁ < 1 Hz (see Chapter 26 definitions).
- Section 26.11.1 permits G = 0.85 for a rigid building or other structure.
- Flexible buildings require G_f per Section 26.11.2, not the 0.85 shortcut.
StructSuite assumption: The Chapter 27 Step 7 calculation assumes a rigid building (n₁ ≥ 1 Hz) and uses G = 0.85. If the structure is flexible (n₁ < 1 Hz), the engineer must compute G_f outside this tool and replace G in Equation (27.3-1) for governing design.
4. Section 27.3.5 and Figure 27.3-8 (Step 8)
Figure 27.3-8 defines orthogonal MWFRS cases (full pressure, reduced pressure with eccentricity, etc.) used with the p values from Section 27.3.1. Step 8 tables and summaries implement that figure for the inputs shown—not Chapter 28.3.2 load-case numbering.
Horizontal shear summaries in the app use a line-load model V ≈ f (p_W + p_L) B h over the cases listed; a full 3D or finite-element model may differ. Roof “uplift” summary uses Note 2 representative roof |p| times plan area L × B where applicable (θ ≥ 10°); for very low slopes follow the standard’s parallel-to-ridge roof rules.
5. Chapter 28 cross-references (context only)
- Section 28.3.1.1 (combined GC_pf not separated) applies to Chapter 28, not to Chapter 27 C_p and G in Equation (27.3-1).
- Section 28.3.2 (load cases and torsional exceptions for Chapter 28) is not the same as Figure 27.3-8 cases under Chapter 27.
Use the chapter your jurisdiction and project criteria require; document the choice on the calculation cover sheet.
6. Related design examples
StructSuite includes Chapter 27 directional examples (gable roof, tall building over 60 ft, partially enclosed warehouse). Each example lists Step 7 (pressures) and Step 8 (load cases) in the guided steps when applicable.