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RE: Collector Elements

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Answering both of the posts on this in one reply ...


>From: Kestner, James W.
>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? 

I asked around on this one and got some feedback from Lanny Flynn of PCSA,
Inc. as follows:

Typically the collector force is so large that the section will be designed
non composite (since concrete isn't too good in tension). If the force is
small some supplemental slab reinforcing is usually used.

>From: Rick Burch
>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.  

You have a column braced at one flange and unbraced at the other. This
condition can be addressed with a specially derived column buckling formula
that is presented n the lecture notes for the AISC-SSRC stability bracing
short course. If you have them, great. If you don't, just contact the AISC
Steel Solutions Center (1.866.ASK.AISC or solutions(--nospam--at) and we will
send you the equation.

>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?

Lateral torsional buckling is unaffected by this tension -- unless there is
an actual point of direct or torsional bracing. A direct brace is like a
kicker angle. A torsional brace is like a pair of stiffeners that extend the
restraint of the other flange connected to a slab down to the flange tending
to move laterally. But if there were such bracing present, you probably
would not be asking if the tension affects the LTB strength. This is also
addressed in the lecture notes I referenced above.

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