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RE: Net uplift on joists and joist girders

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-----Original Message-----
From: Effland, Greg [mailto:geeffland(--nospam--at)butlermfg.com]
Sent: Wednesday, January 16, 2002 1:43 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Net uplift on joists and joist girders

Lets assume that you would be correct in the fact that the roof would act
like a tension membrane under uplift even after the bottom chord and bar
joist failed (which I don't agree with 100%)... what happens after the wind
load is gone and a live/snow load is placed on the now failed truss/bar
joist.

Reply: I doubt you will get uplift if you have a snow or live load acting at
the same time, except maybe for very high wind areas. After the chord
buckles elastically, I see it simply snapping back straight when it goes
back into tension. Don't we assume this when designing tension only bracing
for our lateral system?

If the roof is such a good tension membrane then you could also argue that a
building would never fall down due to a snow overload event because the roof
would prevent the purlins/joists from failing completely especially since
the roof is most likely hooked to the compression chord (top chord in snow
loads)... Can't say this never happens (wood, steel, or other).

Reply: If you designed your roof membrane forces for the anticipated
stresses then you're right, it would not fail because of a snow overload.
While I don't think a roof diaphragm is ordinarily capable of handling a
15psf DL + 20psf LL gravity load, I am not sure that it could not handle a
5psf wind uplift.

Aside from failures, the positive benefit of lateral bracing can be seen in
multiple research reports from various authors/researchers as well in real
live demonstrations.  Anytime you can increase the mode of buckling the
compressive strength increases (i.e. 1 mode c-shape to 2-mode s-shape).  The
difficuly there is in the fact that for each mode of buckling you want to
increase the stiffness of the braces needs to be increased.  If you have
ever designed anything with K not equal to 1 then you have used this theory.
Good articles on this can be found written by Yura, Fisher, and others (even
in the LRFD or ASD manuals to some extent).

Reply: I agree completely that bracing gives you a stronger structure. I am
just wondering if the added strength is warranted given the very light and
temporary nature of the loading.


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