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Re: Diaphragm Chords

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Jim Kestner wrote:

> Here is a question that I hope might generate some discussion........
>
> Have any of you ever wondered if a diaphragm can work without having a
> specific edge member, etc. assumed to resist the calculated chord force.
> Can a portion of the diaphragm along the edge actually work as the
> "chord"? It obviously has to be fastened together to resist those
> forces.
>
> This is similar to a large steel plate or concrete wall working as a
> shear panel  with no edge members. The bending stresses are simply
> resisted by the deep rectangular section with a  triangular stress
> distribution which is maximum at the edge (assuming deep member theory
> does not control).
>
> I could see this not working for fluted metal deck in one direction
> where the tensile chord forces would tend to open up the flutes.
>
> To talk in practical terms, let's say we have a building with steel
> joists, metal deck and load bearing masonry walls. The joists span to
> the long walls (span parallel to the short walls).
> The bond beams and the reinforcing are cut at each masonry control joint
> (so that it cannot act as a chord). How does the building work to
> transfer the lateral forces. I believe the deck works as the chord
> parallel to the long wall. The first joist in from the short wall
> (parallel to the short wall) probably works as the chord  in the other
> direction.
>
> I have never tried this but, I know there are buildings out there like
> this that were never detailed properly but nevertheless still work.
>
> Any comments?
>
> Jim Kestner, P.E.
> Green Bay, Wi
>
> I did some diaphragm test on 26 ga. ribbed panel in the sixties and tested
> both conditions.  The edge (parallel to the flutes) without an angle or
> other support showed considerable deformation which was generally
> unacceptable for general service.  It only took a very small member
> (usually a L 3 x 3 x 12ga) attached at the edge to keep the diaphragm edge
> from deforming and not too surprising we got about a 50 to 75% increase in
> ultimate strength.  The edge perpendicular to the flutes should always be
> attached to the supporting member which serves as the chord member.  I
> recommend that you provide a chord member for all of your designs rather
> than depend on joist manufacturers to design the top chord for axial
> forces and who knows what the erector will do it when it goes to the job
> site.  if you have masonry walls, it is convienient to attach a mill angle
> to embeds on the perimeter of the roof at 40 to 48 inches OC to double as
> chord angle and to transfer the forces to your shear walls.