Wind on other structures
Wind forces on building appurtenances and other structures to ASCE 7-22 Chapter 29 (directional procedure): solid freestanding walls and signs, rooftop structures and equipment, chimneys, tanks and similar structures, open signs and single-plane open frames, and trussed towers. The forces are strength level (W in the ASCE 7-22 load combinations). Wind on the building itself (MWFRS and components and cladding) is the Wind loads calculation.
Inputs of the wind on other structures #
- Wind speed and site: V for the risk category, the exposure category, Ke (Table 26.9-1, or 1.0), Kzt (26.8, entered; the Wind loads calculation computes it), Kd from Table 26.6-1 for the structure (0.85 for walls, signs, rooftop equipment, open frames and triangular, square or rectangular towers; 0.90 square, 0.95 hexagonal, 1.0 octagonal and round chimneys and tanks) or entered (the table's note permits 0.95 for round or octagonal structures with nonaxisymmetric systems), and the gust-effect factor: 0.85 for a rigid structure (n1 ≥ 1 Hz; a lower n1 is not feasible with 0.85) or entered (Gf of 26.11.5 for a flexible structure). Rooftop equipment uses (GCr), which includes the gust effect.
- Solid freestanding wall or sign: B, s, h (ground to the top), the solidity ratio ε (more than 0.7; with openings the note 1 reduction 1 − (1 − ε)^1.5 is permitted and is an input), a return corner Lr (Case C) and the double-faced sign options of note 2 (thickness t; Rmin ≤ 0.75 reduces Cf of Cases A and B by 1 − 0.133Rmin, Rmax ≤ 0.4 reduces the Case B eccentricity to (0.2 − 0.25Rmax)B).
- Rooftop structure or equipment: the building's mean roof height, width B normal to the wind and length L, the unit's vertical and horizontal projected areas Af and Ar and its height. The linear (GCr) reduction from 0.1Bh to Bh (lateral) and 0.1BL to BL (uplift) is permitted and is an input.
- Chimney, tank or similar: the cross section (square normal to a face or along the diagonal, hexagonal, octagonal, round), the surface of a round section (moderately smooth, rough, very rough by D'/D), the diameter or least width D, the height h and the base height above the ground (a roof-mounted stack).
- Open sign or open frame: B, s, h, ε ≤ 0.7, flat-sided or rounded members with their typical diameter.
- Trussed tower: square or triangular, one segment: height, base height, ε of one face and the gross area of one face; rounded members (note 3) and the wind along the diagonal of a square tower (note 4).
Checks performed #
| Check | Basis |
|---|---|
| Velocity pressure | qz = 0.00256 Kz Kzt Ke V² (Eq. 26.10-1), Kz by Table 26.10-1 note 1 |
| Signs and walls, Cases A and B | F = qh Kd G Cf As (Eq. 29.3-1), qh at the top of the sign; Cf of Fig. 29.3-1 by B/s and s/h, bilinear (note 4) and held at the table's edges; Case A through the centre, Case B at 0.2B from it (stated as torsion); for s/h = 1 the resultant 0.05h above the centre; Case B need not be considered for B/s ≥ 2 |
| Signs and walls, Case C | B/s ≥ 2: Cf by region from the windward edge, linear between the tabulated B/s along the sign (note 4); the return-corner factor on the asterisked values (Lr/s from 0.3; below 0.3 no reduction is taken); × (1.8 − s/h) for s/h > 0.8 (note 3); region forces, total and resultant |
| Rooftop equipment | Fh = qh Kd (GCr) Af and Fv = qh Kd (GCr) Ar (Eqs. 29.4-2, 29.4-3), qh at the building's mean roof height; the commentary's size note for large units |
| Chimneys and tanks | F = qz Kd G Cf Af (Eq. 29.4-1) summed over height bands of 5 ft or less with qz at each band's centroid; Cf of Fig. 29.4-1 by h/D (linear, note 2), round sections by D √qz; base moment and resultant height |
| Open signs and frames | Eq. 29.4-1 with Af = ε B s and Cf of Fig. 29.4-2 by ε and the members |
| Trussed towers | Eq. 29.4-1 with Af = ε Ag, Cf of Fig. 29.4-3, the rounded member factor (≤ 1.0) and the diagonal factor 1 + 0.75ε ≤ 1.2 |
| Conditions | 29.1.2 stated: regular shape, no across-wind, vortex shedding, galloping, flutter, channeling or buffeting |
The type is an analysis: a result reads Calculated. The drawings show the structure's outline with its dimensions and forces: a sign in elevation and in plan for Case A (and Case B where B/s < 2), and Case C as region pressure bands across the sign in plan; a rooftop unit with Fh and Fv; a chimney or tank with the load per foot by height; a tower segment with its force. Supports, posts, anchorage and foundations are not part of the calculation.
Limitations #
- Not covered: circular bins, silos and tanks of 29.4.2 (roof and underside pressures, grouped tanks), rooftop and ground-mounted solar panels (29.4.3 to 29.4.5), solid signs attached to walls (29.3.2: use the Wind loads calculation's wall C&C with GCpi = 0), components and cladding of rooftop equipment and screens (30.11), tower guys, appurtenances and ice (Fig. 29.4-3 notes 5 and 6), the gust-effect factor of flexible structures (enter it), across-wind response.
- One tower segment per calculation; a tower with several segments is one calculation per segment.
- Case C between B/s = 10 and 13 interpolates point by point along the sign between the two columns' region values, each column's last region continuing to the sign's end.
Verification examples #
Verification status: 47 values compared in 22 examples (17 published, 5 hand calculations from the cited equations): 47 within tolerance, 0 recorded deviations. Code basis: the provisions it uses were read in the target edition.
No published ASCE 7-22 worked example of Chapter 29 is on file (the ASCE wind loads guide is on the documents list). The force coefficients are checked against the tabulated values of the standard and the forces by hand:
| Example | What it checks | Standard / hand calculation | ILX Calcs |
|---|---|---|---|
| F29.3-1-AB | Cf of Cases A and B at (B/s, s/h) = (1, 1), (10, 0.5), (0.5, 0.2), (45, 0.16) | 1.45, 1.70, 1.80, 1.95 | same |
| F29.3-1-C5, -C5LR, -C13 | Case C regions at B/s = 5 and 13, return corner Lr/s = 1 | 3.10 / 2.00 / 1.45 / 1.05; 2.325; 4.00 / 2.60 / 2.00 / 1.50 / 1.35 / 0.90 / 0.55 | same |
| F29.4-1 | Square h/D = 7, round smooth h/D = 25, hexagonal h/D = 1, round D √qz ≤ 2.5 at h/D = 25; Kd | 1.4, 0.7, 1.0, 1.2; 0.90, 1.0, 0.95 | same |
| F29.4-2 | Flat ε = 0.2 and 0.05; rounded ε = 0.5 low and high D √qz | 1.8, 2.0, 1.5, 1.1 | same |
| F29.4-3 | Square ε = 0.3 and the diagonal factor; triangle ε = 0.3 with rounded members | 2.59, 1.2; 1.414 | same |
| O1 | Sign 30 × 10 ft at 25 ft, V = 115 mph, Exposure C | qh 31.98 psf, Cf 1.75, F 12.13 kips at 20 ft, e 6 ft | same |
| O2 | Rooftop unit on a 40 ft building; a large unit with the (GCr) reduction | qh 35.20 psf, Fh 2.729, Fv 2.873 kips; (GCr) 1.85, Fh 33.21 kips | same |
| O3 | Round stack D = 3 ft, h = 15 ft | Cf 0.567, F 0.625 kips, M 4.68 kip-ft | same |
| O4 | Case C, B = 50 ft, s = h = 10 ft (note 3 factor 0.8) | Cf 2.48 at the edge, region forces 5.16 and 3.50 kips, total 14.41 kips | same |