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FW: Collector Elements

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Title: FW: Collector Elements

James and Rick,

Whenever we design steel braced buildings, we've looked at the drag struts as non-composite and have sized the steel to take the bending and axial forces disregarding the concrete.  And since these elements are critical to the lateral system performance, we fully brace the bottom flange to suppress a non-ductile limit state such as buckling.  The bracing usually takes the form of angle kickers or near equal depth perpendicular framing.

If you are looking at an existing building and trying to justify the design, I would look at the connections at each end of the beam first.  If they aren't the controlling limit state, then it may be possible to look at the tension in the steel beam caused by composite action.  I would only use this approach in a wind or low seismic application.  The compressive drag forces would of course be offset, at least partially, by the tension in the beam due to composite action.  Pay close attention to your load combinations in this exercise.  The tensile drag forces would add to the tensile force from composite action, but in this case stability is not a concern.  In deciding what the tensile forces are in the composite beam, I would recommend an LRFD approach for ease of application and understanding.  In the forgoing analysis, the concrete slab only acts as the compression couple for composite action in bending. If it's not perfectly clear, evaluate your bracing by utilizing the stability lecture notes that Charlie mentioned.  I have a set and they have been very usefull.

I hope this was helpful,

Curt La Count
Jacobs Engineering
Portland, OR

-----Original Message-----
From: Rick Burch [mailto:rburch(--nospam--at)]
Sent: Monday, March 10, 2003 6:20 PM
To: seaint(--nospam--at)
Subject: Re: Collector Elements

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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