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Re: Texas Twisters

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> From: "Sprague, Harold O." <SpragueHO(--nospam--at)bv.com>

> What got the high rise buildings was missile impact on the glazing (rarely a
> design consideration).  What gets the residential and light commercial world
> is no design consideration for wind uplift.  What gets the light metal roofs
> are local compromises in the envelope due to uplift.  Once that happens, it
> is self exacerbating.

Watch for that note on the drawings that says something to the effect:
... design assumes that all overhead doors, windows, etc. must be
capable of resisting the design wind loads.
Do the doors, windows, etc., as installed, meet that criteria?
What happens if they don't? Who confirms this? What if they fail for
reasons other than wind (e.g. missile impact) in combination with wind
less than design speed?

Regardless of why the roof may become compromised, should the occupants
suffer the ultimate when the integrity of a roof diaphragm is lost?

It is my understanding that some jurisdictions are changing their
attitude toward use of the "closed" category. A prudent engineer should
already know when it is inappropriate, without exceptional code clauses,
but then there's the budget ...

Factory Mutual is forcing industry changes in light metal roof
applications for both snow and wind load designs. I could go on a
long-winded discussion about how necessary this is because "local
compromises in the envelope" are usually the result of inadequate care
given to local details (I've got a story for this one - but another
time). It's part partly due to the, "we've always done it that way",
syndrome.

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>