Atmospheric ice loads
The ice load from freezing rain on a member, guy, cable, chimney, plate or dome, and the wind on the ice-covered element, by ASCE/SEI 7-22 Chapter 10: the design ice thickness with the height and topographic factors, the ice area or volume and weight, and the wind force on the iced projected area with the Chapter 29 force coefficients. It is an analysis: the result reads Calculated. Di and Wi go into the load combinations of 2.3.3 and 2.4.3.
Inputs of the ice loads #
- Nominal ice thickness t and concurrent gust speed Vc: read from Figures 10.4-2 to 10.4-5 (for the risk category) and 10.5-1, 10.5-2, or the ASCE Hazard Tool (Atmospheric Ice Geodatabase). ASCE 7-22 has no ice importance factor: the risk category is in the map you read (Table 1.5-2 now lists the earthquake factor only; C10.4.2).
- Source, special icing regions, exclusions: a site in a special icing region, in complex terrain or where snow or in-cloud icing governs needs a site-specific study (10.1.1); the electric, communication tower and mast structures with their own standards are excluded (10.1.3). Either makes the calculation NOT FEASIBLE until resolved.
- Height z of the element (for fz and qz; one segment per calculation for a tall structure, 10.8 step 3), Kzt (Eq. 26.8-1), exposure, Ke, Kd and G for the wind (26.6, 26.9 to 26.11).
- Ice density: not less than 56 pcf (10.4.1).
- Element: a member of an open frame or lattice (round, rectangular with Dc = √(b² + d²), or Dc and the projected width entered from Fig. 10.4-1) with its length and the frame's solidity ratio with ice; a guy or cable; a round chimney or tank (diameter, height); a flat plate (area of one side, vertical or horizontal, the permitted 0.8 / 0.6 reduction off by default); a dome or sphere (radius).
Checks performed #
| Check | Basis |
|---|---|
| Height factor | fz = (z/33)^0.10 for 0 < z ≤ 900 ft, 1.4 above (Eq. 10.4-4) |
| Design ice thickness | td = t fz Kzt^0.35 (Eq. 10.4-5, 10.4.4) |
| Ice area and weight | Ai = π td (Dc + td) (Eq. 10.4-1), Di = γ Ai; plates and domes Vi = π td As (Eqs. 10.4-2, 10.4-3), × 0.8 / 0.6 for plates where chosen |
| Ice density | Not less than 56 pcf (10.4.1) |
| Velocity pressure | qz = 0.00256 Kz Kzt Ke Vc² (Eq. 26.10-1) at the concurrent gust |
| Wind on ice | Projected width + 2 td (10.5); members: Fig. 29.4-2 flat on the bare width, rounded D√qz ≤ 2.5 on the ice and on round members (10.5.3); guys and cables Cf = 1.2 (10.5.5); round chimneys Fig. 29.4-1 round D√qz ≤ 2.5 by h/D (10.5.1); Wi = qz Kd G Cf × width |
| Combinations | 2.3.3 and 2.4.3 with Di and Wi (stated) |
The drawings show the iced section (the circle Dc around the section and the radial ice) and, for members and cables, the elevation with the ice weight; for plates and domes a schematic section with the equivalent ice layer.
Limitations #
- Wind on iced plates, walls and signs (10.5.2, Fig. 29.3-1 with the iced dimensions) is not computed: use the wind-on-other-structures calculation with the dimensions increased by td. Trussed towers as a whole (10.5.4) are not covered.
- Snow and in-cloud icing loads (site-specific), galloping, ice shedding and aeolian vibration (10.1.2), and the design temperatures of 10.6 are noted, not computed. Partial ice loading (10.7) is the structure's analysis.
- Kd of guys and cables: Table 26.6-1 does not list them; 1.0 is taken (enter another value if you choose).
- Fig. 10.4-1 is a figure in the code: for shapes other than round and rectangular, enter Dc as it shows.
- No auto-design: nothing is sized.
Verification examples #
Verification status: 20 values compared in 6 examples (0 published, 6 hand calculations from the cited equations): 20 within tolerance, 0 recorded deviations. No published example: every value is a hand calculation. That checks the program against this reading of the code, not the reading itself. Code basis: partial. Only some provisions were read in the target edition; the rest rests on older editions, examples or summaries.
No published ASCE 7-22 Chapter 10 worked example is on disk (the Commentary has none). Every value is a hand calculation (closed form) from the equations of Chapter 10 and 26.10 and the Chapter 29 coefficients: a round pipe in an open frame at 50 ft (fz, td, Ai, Di, the ice weight, qz, Wi), a square tube on a hill at 1,000 ft (fz = 1.4, Kzt^0.35, Dc = √32, flat and rounded coefficients), a guy cable at 33 ft (Cf = 1.2), a round chimney (Cf by h/D), a vertical plate with the 0.8 reduction and a dome. An ASCE ice example is listed among the documents needed.