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RE: Cantilevered flexible diaphragm

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> I have a masonry bearing wall/ shear wall building with a metal
> deck roof on steel joists. A portion of the roof must extend
> beyond an end shear wall to a row of columns with glass walls
> (no bracing or shear walls permitted). If the metal deck roof is
> considered to be flexible, how are lateral loads assumed to be
> distributed at the cantilevered extension of the roof diaphragm?

I don't know the exact reference, but I seem to remember a code provision
allowing a "flexible" horizontal diaphragm to cantilever past the last
support some distance.  I don't remember if it was a set distance, a
span/depth ratio, or a combination of the two.  The loads would be
distributed just like with a regular cantilever beam, so the load in the
nearest shear wall would be greater than the load caused by the overhanging
portion.  The ends will have compression tension forces into and out of the
plane of the main roof.

Other options:
1. Moment frame at the end (relatively expensive).
2. Horizontal tension-only diagonal bracing.  This can be done with straps
(rolled bar or angles with coped flanges) under the deck welded to the top
chord of each joist.  This is fairly common in the drive-through canopies of
"big box" stores like Home Depot.

Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

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