Tornado loads
Tornado pressures on a rectangular building by ASCE 7-22 Chapter 32, for Risk Category III and IV in the tornado-prone region: whether design is required, the main wind force resisting system, and components and cladding. The same building's wind pressures stand beside them, since the structure takes the greater. The type reads Calculated.
Inputs of the tornado loads #
- Risk Category, tornado-prone region, Essential Facility. Essential Facilities take KdT 1.0 on C&C and need protected glazing.
- Tornado speed VT: from Figures 32.5-1 and 32.5-2 or the ASCE 7 Hazard Tool, for your site and Ae. Interpolation by the logarithm of Ae is permitted, and it can decide whether design is required. The type does not read the maps.
- Effective plan area Ae: a record of what VT was read for (32.5.4).
- Basic wind speed V and exposure: for the 32.5.2 threshold and the wind pressures. Tornado pressures use neither exposure nor topography (32.7, 32.8).
- Ground elevation factor Ke: Table 26.9-1, or off for 1.0.
- Plan dimensions, h, roof, θ. A pitched roof's ridge runs along y.
- Tornado enclosure classification: enclosed, partially enclosed, partially open. 32.12 counts unprotected glazing as openings on each assumed windward wall: this often makes a building partially enclosed for tornado and not for wind. You decide and enter it.
- zop, Ri: partially enclosed only.
- GT: 0.85 prefilled (32.11).
- C&C zones: each with its effective wind area.
Checks performed #
| Result | Basis |
|---|---|
| Design required | 32.1.1, 32.5.2 |
| KzTor, qhT | Table 32.10-1, Eq. 32.10-1 |
| MWFRS, both directions, both signs of internal pressure | Eq. 32.15-1, Figure 27.3-1, KdT 0.80, KvT 1.1 on roof uplift |
| C&C, h up to 60 ft | Eq. 32.17-1, Figures 30.3-1, 30.3-2 |
| C&C, h over 60 ft | Eq. 32.17-2, Figure 30.4-1 |
| Wind pressures beside them | Chapter 27 and 30 with Kd 0.85, G 0.85, the wind GCpi, and 16 psf at least on C&C |
Which load governs may differ by surface: wind often governs the lateral system and tornado the interior of the roof. Tornado loads act as WT in Chapter 2 (the Load combinations type has it).
Limitations #
- Rectangular, rigid buildings. No open buildings (Eq. 32.15-2), elevated buildings, parapets, overhangs, canopies, rooftop equipment, solar panels, bins, tanks or other structures (32.16, 32.17.3 to 32.17.5). No Appendix G longer return periods. No wind tunnel.
- Over 60 ft the C&C here covers roofs of 7 degrees or less.
- For a building over 200 ft, the pressure is at h: walls lower down have a larger KzTor up to 1.0 and the table does not list each level.
- The wind pressures are a side-by-side for the same simple building: use the Wind loads type for design wind.
- VT is your input: see the first line above.
Verification examples #
Verification status: 55 values compared in 6 examples (6 published, 0 hand calculations from the cited equations): 54 within tolerance, 1 recorded deviations. Code basis: the provisions it uses were read in the target edition.
| Example | Source | Values |
|---|---|---|
| 1-story school, Evansville, VT 80 mph, partially enclosed | NCSEA Wind Engineering Committee, Tornado Loads on Schools and Hospitals (11/2023), pp. 3 to 8 | qhT 16.2 psf; MWFRS tables; C&C at 10 and 100 ft² |
| 4-story hospital, Oklahoma City, VT 104 mph, enclosed, Essential Facility | The same, pp. 14 to 21 | qhT 26.5 psf; MWFRS tables; C&C at 10 and 100 ft² |
One recorded deviation: Zone 1' of the school at 10 sq ft, where the source uses a directionality factor of 0.75 against the table's 0.90, so the source prints 12.5 and the program reads 13.3.
The building dimensions are read through their L/B ratios, as the figures of the example are not used. The wind pressures of the NCSEA tables print no derivation (they do not follow from the stated data by a simple check), so they are not compared. The wind columns here are hand checked only. The NCSEA equation lines above its tables have typing errors ("18.0 Cp" where 11.0 is meant in the school); the tables themselves match Eq. 32.15-1.