Gust-Effect Factor G (ASCE 7 §26.11)
The ASCE 7 gust-effect factor G for a rigid building, instead of the flat 0.85 default.
Example
You enter
- Exposure category C
- Mean roof height h (ft) 30
- Building width B, normal to wind (ft) 100
You get
- Gust-effect factor G 0.863 (above the 0.85 default)
- Turbulence intensity Iz 0.2213
- Background response factor Q 0.8801
- Lz (ft) 442.9
Checked against published worked examples from: ASCE 7 §26.11.4 (zbar floored at zmin).
Details, formula, and sources
ScopeASCE 7 §26.11.4 rigid form G = 0.925 (1 + 1.7 gQ Iz Q)/(1 + 1.7 gv Iz), gQ = gv = 3.4, with Iz = c (33/zbar)^(1/6), Lz = l (zbar/33)^eps, Q = sqrt(1/(1 + 0.63 ((B+h)/Lz)^0.63)), zbar = max(0.6h, zmin), and the Table 26.11-1 exposure constants. A 30 ft tall, 100 ft wide building in Exposure C gives G 0.86 (the 0.85 default is slightly unconservative); a taller, narrower building or a smoother exposure raises it. Rigid buildings only; a flexible building needs Gf with the resonant response R. The engineer of record governs.
Formula or tableG = 0.925 (1 + 1.7 gQ Iz Q)/(1 + 1.7 gv Iz), gQ = gv = 3.4; Iz = c (33/zbar)^(1/6); Lz = l (zbar/33)^eps; Q = sqrt(1/(1 + 0.63 ((B+h)/Lz)^0.63)); zbar = max(0.6h, zmin).
Source and editionThe ASCE 7 §26.11.4 rigid-building gust-effect factor G, with the ASCE 7-16 Table 26.11-1 turbulence constants (c, l, eps, zmin), by name. ASCE 7-22 adjusted Table 26.11-1; the 7-22 values of these four have not been verified against the standard, so the 7-16 set is used and disclosed.
Free accessASCE 7 is available through the ASCE Library at ascelibrary.org; the §26.11 gust-effect-factor provisions and the Table 26.11-1 exposure constants are public. B and h are the user's own building dimensions.
What governsEstimate. AHJ and licensed professional govern.
API field namesexposure, h_ft, b_ft, g_factor, iz, q, lz_ft
- Rigid gust factor G = 0.925 (1 + 1.7 gQ Iz Q)/(1 + 1.7 gv Iz), gQ = gv = 3.4ASCE 7 §26.11.4
- Turbulence and length scale Iz = c (33/zbar)^(1/6); Lz = l (zbar/33)^eps; zbar = max(0.6h, zmin)ASCE 7 §26.11.4 / Table 26.11-1
- Background response Q = sqrt(1/(1 + 0.63 ((B+h)/Lz)^0.63))ASCE 7 §26.11.4
- Scope rigid buildings only; a flexible building needs Gf with the resonant response RASCE 7 §26.11.5
- Edition of the constants c / l / eps / zmin are the ASCE 7-16 Table 26.11-1 values (B 0.30 / 320 ft / 1/3 / 30 ft; C 0.20 / 500 / 1/5 / 15; D 0.15 / 650 / 1/8 / 7); ASCE 7-22 adjusted that table (SEAOG 2025 7-22 wind seminar) and its values are not verified here -- the code's G = 0.85 option for a rigid building stays availableASCE 7-16 Table 26.11-1; SEAOG, Wind Loads: What's New in ASCE 7-22 (2025)