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Re: Net uplift on joists and joist girders

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Fellow engineers,

	I can give you one example of a roof structure where the roof decking
actually did act as a membrane and retard collapse long enough to
prevent fatalities and/or serious injury.

	The project was Station Square Shopping Center in Burnaby, B.C. (part
of Greater Metropolitan Vancouver).  The structural system consisted of
concrete filled metal deck supported by open web steel joists which
were, in turn, supported on steel wide flange beams and steel columns.
The design loading included  provisions for car parking on the roof.  On
opening day, when the shopping center was filled to maximum with
shoppers, one of the beams failed by turning over on its side primarily,
I understand, because there were neither web stiffeners above the
columns nor extensions to the bottom chords of joists on the column
lines to stabilize the columns.  There were, of course, a number of
other contributing factors.

	Following the collapse, according to reports presented in a seminar
which I attended, the steel deck actually acted as a catenary and held
the parked cars out of the produce department for FOUR MINUTES thereby
giving shoppers time to escape.

	This failure took place within a few months of the Kansas City Hyatt
Hotel failure which was discussed on this list a few months ago.  Its
impact on the structural engineering profession in the province of
British Columbia has been considerable to say the least.

				Regards,

				H. Daryl Richardson

Michael Bryson wrote:
>
> -----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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