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Re: Collector Elements

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Last week Jim Kestner asked a good question about collectors in steel
frames that did not receive any response on the list. I have wondered
about related issues, so I would like to ask again if anyone has any
answers. In addition to Jim's original questions, consider these:

1. When you have a composite steel floor beam which is a collector
element, or even the actual horizontal member in a braced bay, what is
the laterally unbraced length of the member for axial design? It's
unlike a typical column, since the top flange is totally braced, and yet
the bottom flange is (or sometimes is) totally unbraced.  

2. In another case, say that the member is part of a moment frame,
instead of a braced frame line. Part of the bottom flange is in
compression and part of it is in tension due to reverse curvature
bending. For flexural design, what is the unbraced length Lb for the
bottom flange?  

Rick Burch
Columbia, SC


"Kestner, James W." wrote:
> When you have a collector element which is a composite beam, what is the recommended procedure for combining normal bending stresses in the composite beam with  those produced by the collector (axial plus flexural to do the axial load acting at the steel beam top flange with an eccentricity of 1/2 beam depth). Should the collector force be applied to the non-composite beam section or to the composite beam section? If using the composite section for the collector force, should extra rebar be added over the beam to handle the tensile collector forces or let the steel beam carry it all? How should the stresses be combined?

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