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- Date: Sun, 21 Dec 2008 04:00:07 +0000
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From: "Ehrlich, Gary"
Date: Fri, 19 Dec 2008 13:32:41 -0500
Subject: RE: Wind forces
I hate to throw water on everyone, but I wouldn’t necessarily call the old uniform loads “sanity”. We’re debating in the ASCE wind committee right now backing off some on the current minimum load, which is 10psf. The problem is that on a typical low-rise (~35’ mean roof height) gable roof structure, it takes something like a 150mph wind (if I recall correctly) to generate a 10psf load on the vertical projection of the roof using the analytical provisions. And the wind ends up governing over SDC C, and even SDC D level seismic forces.
This explains why all those buildings designed for 15psf, 20psf, or 25psf uniform wind loads don’t seem to have any problems—it would take a Cat 5 or an EF4 to take them down. Regardless of whether they’re built anywhere that could get a wind event of that magnitude.
Gary J. Ehrlich, PE
Attend the 2009 International Builders' Show
Waaaay back when sanity prevailed.
T. William (Bill) Allen, S.E.
Consulting Structural Engineers
Ahhh, for the good ol' days!
From the UBC 1949:
Sec. 2307 (b): For purposes of design the wind pressure shall be taken upon the gross area of the vertical projection of buildings and structures at not less than 15 psf for those portions of the building less than sixty feet (60') above ground . . . ".
Hugh Brooks, SE
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