Gust-effect factor

The gust-effect factor of ASCE/SEI 7-22 26.11. The calculation takes the natural frequency (from your analysis, or the approximate value of 26.11.3 where 26.11.2.1 permits it) and classifies the structure as rigid or flexible. It gives G = 0.85 and the computed G for rigid structures, and Gf for flexible ones (n1 < 1 Hz), with the damping ratio you enter. As an option it also gives the along-wind displacement and acceleration at the top by Commentary C26.11. It is an analysis: the result reads Calculated. Wind loads (wind_loads) covers only rigid buildings: use the factor from here in its place for a flexible one.

Inputs of the gust-effect factor #

• Wind and building:
  • The basic wind speed V (26.5) and the exposure (26.7).
  • The mean roof height h.
  • The width B normal to the wind and the depth L along it.
• Natural frequency n1:
  • From an analysis (26.11.2), with the stiffness and mass expected under the design wind (C26.11).
  • Or the approximate lower-bound na of 26.11.3 for:
    • steel moment frames (Eq. 26.11-2);
    • concrete moment frames (Eq. 26.11-3);
    • other steel or concrete systems (Eq. 26.11-4);
    • concrete or masonry shear walls (Eq. 26.11-5, with Cw entered).
  • The approximate na is permitted only for h ≤ 300 ft and h < 4 Leff (26.11.2.1). Outside those limits the result is NOT FEASIBLE.
  • Low-rise buildings may be taken as rigid (26.11.2).
• Damping ratio β: your judgment of the inherent damping under the design wind (C26.11, Structural Damping). The commentary example uses 1 %.
• Rigid structures: 0.85 (26.11.1), the computed G (26.11.4), or Gf, which C26.11 permits for every case.
• Along-wind response (C26.11):
  • Cfx, the mode shape exponent ξ and the building density (weight per volume).
  • The air density ratio.
  • A serviceability wind speed and averaging time for the acceleration.
  • An acceleration limit, if you want one: ASCE 7 sets none. A limit shows as a recommendation.

Checks performed #

The drawing shows a schematic elevation with the wind profile, the equivalent height 0.6h and the factor.

Limitations #

• Along-wind only. Across-wind loading, vortex shedding, galloping, flutter and torsional response are not covered (C26.11): use the literature or wind tunnel tests (26.11.6, Chapter 31).
• One wind direction per calculation, and the fundamental mode only.
• The acceleration limit has no basis in the code. The 1.5 to 2.5 %g in the hint is from knowledge (to confirm against the code text).
• No auto-design: nothing is sized.

Verification examples #

Verification status: 28 values compared in 5 examples (2 published, 3 hand calculations from the cited equations): 25 within tolerance, 3 recorded deviations. Thin: only 2 published examples. Code basis: the provisions it uses were read in the target edition.

Hand calculations check the approximate frequencies of Eqs. 26.11-2 to 26.11-4.