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RE: Alternate Wind Provision[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Alternate Wind Provision
- From: "Ehrlich, Gary" <gehrlich(--nospam--at)nahb.com>
- Date: Thu, 1 May 2008 12:17:01 -0400
Coming back in to the discussion…Joe V. and Ben are correct, a “rigid” diaphragm under the wind provisions has a different definition than a “rigid” diaphragm under the seismic provisions. It’s a function of the frequency, and most typical low-rise buildings including residences meet that test for consideration of “rigid”. You typically have to get up in the range of 75-foot building heights and 4:1 building width/length ratios before you fall out of the “rigid building” for wind.
(This was one of the concerns over both the simplified methods…they both included building heights up to 100 feet).
As for Joe G.’s comment, the assumption in the simplified methods is that the out-of-plane roof loads on the wall system are transferred vertically (whether through studs, wall panels, masonry wall, etc.) to the floor system, as opposed to horizontally (e.g. through girts) to elements that may be part of the MWFRS. I’m forgetting at the moment *why* that’s preferable (too much travel scrambling the brain), but that’s the primary desire. Also, I believe there is a concern about using the simplified method where you have collectors at floor/roof level that deliver the load to the diaphragms or MWFRS. Anyway, it’s the first assumption that I think the presenter was trying to describe.
From: Joseph R. Grill
Joe, and Ben,
I do agree with you. The presenter does state: “…wind loads are transmitted through rigid floor and roof diaphragms”. His criteria wasn’t the need for a “rigid” building and I agree they are not the same. I’m not sure why he made the statement that the loads had to be transmitted through rigid floor and roof diaphragms. I dug out my copy of the slides to make sure and that is what he states.
All in all it was a pretty good, but short, seminar.
I disagree with the instructor of that class. A typical residence at 30 tall. Ta = .02 x 30^.75 = .25 sec
f= 1/.25 = 4 hz
therefore Rigid per definitionin ASCE-07
Rigid for Wind and Rigid roof and floor Diaphragms are totally different and he is probably confusing the issue.
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