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RE: Net uplift on joists and joist girders

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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.

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).

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).

My $0.02
Greg Effland, P.E.
KC MO USA

-----Original Message-----
From: Michael Bryson [mailto:MBryson(--nospam--at)mhpse.com]
Sent: Wednesday, January 16, 2002 2:52 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Net uplift on joists and joist girders


Do you recall when and where you saw this bottom chord failure? Was it 100%
attributable to inadequate bottom chord bracing?

Also, do you design your beams for localized cladding wind pressures? For
90mph wind and 15psf DL I would design the beams for:
 W - 0.9DL = (31psf)(0.7)(1.23) - (0.9)(15psf) = 5psf uplift

You are right about satisfying Factory Mutual requirements, but would you
say the same thing if the roof was wood construction?


-----Original Message-----
From: Sprague, Harold O. [mailto:SpragueHO(--nospam--at)bv.com]
Sent: Wednesday, January 16, 2002 11:30 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Net uplift on joists and joist girders

 In the land of higher wind loads, I
have seen buckled bottom chords that resulted in a general failure in wind
uplift.  That is why Factory Mutual got so active in the roofing throughout
the US.  The insurance losses were real.



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