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Modeling Diaphragms

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Well, I came up just about the time that "cheap" computer structural analysis
became available, and so I've often used the "brute force" approach to any kind
of analysis that didn't have an obviously "elegant" closed-form solution ready
to hand.

So I have a question for you guys, especially the grizzled veterans who are
still rather suspicious of any method that doesn't derive easily by hand.

Now, I understand that a flexible diaphragm--of which a steel deck on joists
must be said to be one--behaves rather simply, imparting its load in such a way
to the vertical elements such as shear walls, etc., in such a way that one can
assume a beam analogy.

But doesn't this assume that the flanges of the "beam" itself need to be at
least somewhat consistent stiffness themselves?

For example, what about the not-uncommon situation where you must have a drag
strut, typically a joist girder or beam, perhaps even a frame, across a
substantial opening in one wall?

For example, the typical supermarket which has three relatively unbroken walls
(maybe a loading dock, a few  doors, etc.), but the front wall is pretty much
discontinuous. Then, you would have a plane frame across the opening, taking the
diapragm forces, perhaps even from one shear wall segment to the other.

Now, do you feel "safe" in modeling this as a beam, to determine the Chord
forces? I can see it being a trivial thing for wind parallel to such a frame,
but when the wind is transverse?

I'd like some opinions: How do you feel "safe" in modeling this?

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