ASCE 7-22 · CHAPTER 29 · OTHER STRUCTURES
Wind Loads on Specialty Structures
Chimneys, tanks, rooftop equipment, freestanding walls and signs, open frames, and trussed towers don't follow the building chapters. They use a projected-area force method instead.
WHAT COUNTS AS A SPECIALTY STRUCTURE
Non-building structures, shaped by the wind
Each catches wind on its projected area rather than enclosing pressurized space. A tank, a trussed tower, and a rooftop unit are three common cases.
THE METHOD
One force equation, four terms
Wind force is the velocity pressure times a gust factor, a shape coefficient, and the projected area normal to the wind.
qz
Velocity pressure at height z: qz = 0.00256 Kz Kzt Kd Ke V².
PRESSUREG = 0.85
Gust-effect factor for a rigid structure (Section 26.11.1).
GUSTCf
Force coefficient set by shape and aspect ratio (Figs 29.4-1 series). Read it from the figure for your geometry.
SHAPEAf
Projected area normal to the wind — the silhouette the wind actually sees.
AREADIRECTIONALITY · TABLE 26.6-1
Each shape carries its own Kd
Unlike buildings (a flat 0.85 everywhere), specialty structures take a wind-directionality factor that depends on cross-section.
| Structure type | Cross-section | Kd |
|---|---|---|
| Chimneys, tanks & similar | Square | 0.90 |
| Chimneys, tanks & similar | Hexagonal | 0.95 |
| Chimneys, tanks & similar | Octagonal | 1.0 |
| Chimneys, tanks & similar | Round | 1.0 |
| Solid freestanding walls / signs | — | 0.85 |
| Rooftop equipment | — | 0.85 |
| Open signs & single-plane open frames | — | 0.85 |
| Trussed towers | Triangular, square, rectangular | 0.85 |
| Trussed towers | All other cross-sections | 0.95 |
| Circular domes | — | 1.0 |
KEY DIFFERENCE
Why these aren't buildings
Buildings use enclosure pressures (external GCp plus internal GCpi) over wall and roof surfaces. Specialty structures instead resolve a single net force on a projected area — there's no internal pressure to add, and Cf is read from a shape-specific figure rather than a pressure-coefficient table. Directionality (Kd) is no longer a fixed 0.85; it tracks the cross-section, so a round tank or octagonal stack carries Kd = 1.0 while a trussed tower stays at 0.85.
KEEP GOING
Take it to a real calculation
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