Standards & Codes
Design Wind Speed
Also known as: 3-second gust, design wind, shelter wind rating
Why 250 mph
The 250 mph design wind covers the upper end of recorded tornadoes. ICC 500 picks a single conservative number nationwide rather than a regional grid because a tornado can deliver upper-EF intensities anywhere in the affected zones.
'F5 rated' and other marketing terms
Phrases like 'F5 rated' or 'tornado proof' are marketing language, not engineering language. The defensible question is whether the shelter has a test report showing it survived the 250 mph design event and the matching missile impact test under ICC 500.
Distinguished from
- Sustained wind speed
- Hurricane reporting often uses sustained 1-minute winds. ICC 500 uses 3-second gusts, which are higher. The two numbers are not directly comparable.
Authority: ICC 500-2023. Last reviewed 2026-06-04.
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