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Risk Category IV wind speeds v. tornado wind speeds

Risk Category IV wind speeds v. tornado wind speeds

Risk Category IV wind speeds v. tornado wind speeds

(OP)
A client is asking us to evaluate the differences in structure due to the wind loads from typical risk category IV wind speeds [120 mph] and an F2 tornado highest value wind speed [157 mph].
I explained that these are two different things (3 second gust and a possibly longer sustained wind speed).
The IBC wind speeds are ultimate the Fujita scale wind speeds are not.
What would be your approach for trying to evaluate this situation?
Is there a way to bring the tornado wind speeds up to an ultimate value or bring the IBC wind speeds to a nominal value?
I at least would be starting from the same level, but still with different types of wind loads.

RE: Risk Category IV wind speeds v. tornado wind speeds

It is my opinion that you are trying to compare apples & oranges. A straight line wind gust has got to be a different animal than a rotational wind that is also moving forward.

RE: Risk Category IV wind speeds v. tornado wind speeds

Tornado winds are ultimate aren't they?

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RE: Risk Category IV wind speeds v. tornado wind speeds

FEMA 361 and ICC 500 both treat tornado winds as an ultimate effect - using 1.0 as the load factor.

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RE: Risk Category IV wind speeds v. tornado wind speeds

I second the FEMA & ICC documents. Only thing to add is that if you truly are looking at tornado loading than you must satisfy the missile impact criteria not just the wind speeds. Keep in mind that your wall is only as good as your weakest link (doors, door hinges, windows, etc.) Once a projectile penetrations the enclosure then internal pressures can increase causing roofs to uplift or other issues. I only bring this up because I have also seen someone ask a similar question. The situation gets expensive once you start looking into pre-approved wall sections, speciality doors and roof hold downs. Probably worth clarify if they want a tornado shelter or simply a building able to take 157 mph wind speed (but not necessarily tornado resistant).

https://www.fema.gov/wall-sections-passed-previous...

RE: Risk Category IV wind speeds v. tornado wind speeds

(OP)
Thanks all.
Wind gusts and tornado winds are different animals and this was explained to the client.
They asked for a study after hearing our explanation.
Do the FEMA documents use the Fujita scale for the wind speeds?
This is strictly a study at this time. The missiles and internal pressures will all be considered if the client opts for a tornado shelter.

RE: Risk Category IV wind speeds v. tornado wind speeds

FEMA 361 based shelter design on a wind speed map for the USA - not a Fujita scale system.
(FEMA 361 is a free download found Here)

There are, however, various wind speed ranges given in this table from FEMA 361:


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