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RE: Rigid vs. Flexible Diaphragms

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Matthew,

	That may be a Miami/Dade County limitation, but I disagree with the
statement that you cannot get rigid diaphragm behaviour out of a metal deck,
under certain circumstances.  The scenario described by Daniel is a little
much, but 3 sided retail box buildings with an open storefront on the 4th
side are pretty normal.  I've run the numbers numerous times and it is
pretty easy to prove, especially if the building is say 70' deep by 200'
wide.

Brian K. Smith, P.E.

> -----Original Message-----
> From: Stuart, Matthew [mailto:mstuart(--nospam--at)schoordepalma.com]
> Sent: Wednesday, October 06, 2004 12:15 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Rigid vs. Flexible Diaphragms
>
>
> No way you're going to get metal roof deck to behave as a rigid
> diaphragm unless you introduce horizontal, diagonal bracing
> members under the deck, which is what you have to do in
> Miami/Dade County, they don't even let you assume that you can
> get any diaphragm action out of the deck.
>
> Matthew Stuart
> Structural Dept. - Manalapan
> 732-577-9889 x1283
>
>
> -----Original Message-----
> From: Daniel Boltz [mailto:dboltz(--nospam--at)1st.net]
> Sent: Wednesday, October 06, 2004 1:04 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Rigid vs. Flexible Diaphragms
>
> Is it possible for an 1.5" Wide Rib - 20 Gage Roof deck that is
> welded to steel joists to act like a rigid diaphragm?  I realize
> that the engineer is supposed to check the deflection of the
> diaphragm and the deflection of the lateral load resisting
> elements to determine if the diaphragm is 'flexible' or 'rigid',
> but how many engineers out there actually run this calculation?
> I generally use a wood truss/plywood roof AND metal deck/steel
> joist roof as a 'flexible' diaphragm, but have seen a reputable
> engineering firm use metal deck/steel joist roof as a 'ridid' diaphragm.
>
> The example building was approximately 100' wide x 200' long.
> The only lateral load resisting system was masonry shearwalls at
> the stairway centered on the right side of the building and an
> elevator shaft centered on the front wall of the building.  This
> building has a 2nd floor and a roof, which I'm sure the 2nd floor
> was a rigid diaphragm, but I don't see how a frame was not
> required in the back left corners of the building to provide
> resistance along all four sides of the diaphragm.
>
> Any help is greatly appreciated.
>
> Dan
>
>
>
>
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