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Warping Constant For Back To Back Channels

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tennist

tennist

Structural
Apr 23, 2012
2
I am analyzing an old railroad truss to determine if the bottom chord has sufficient strength to support a work platform. This work platform will not hang at join locations, so I need to check the combined bending and axial strength if the member. The bottom chord is composed of 2 back to back channels with a 14" gap between them. The channels are braced together with sufficient lacing to prevent individual lateral-torsional buckling, however I feel the section as a whole needs to be checked for LTB as well. I am needing a way to calculate the warping constant (Cw) for this check. Does anybody know a formula for calculating this constant for this section? From online research it looks like Roark might have produced this in one of his publications, but I cannot find it. I have found an equation for back to back C shapes with a constant wall thickness, but the flanges and webs of a structural steel channel are different thickness. Any help is appreciated.
 
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AISC Design guide 9 has information for calculating Cw for various shapes.
I`m not sure if this should be calculated as a wide flange (ignoring the space between the two members) or a two individual channels - but the DG is a good place to start
 
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Once20036 thanks for your reply. I got some responses from some other colleagues and found an equation that looks to be very accurate. I also found out that my finite element analysis software could calculate this section property. The equation attached matches fairly close with what our finite element analysis software produces. The variables in the equation are as follows:

swebs = Spacing bewteen webs
tf = flange thickness
tw = web thickness
dw = height of channel
bf = width of flange of a single channel

 
 http://files.engineering.com/getfile.aspx?folder=8e57c981-ee6f-4907-a362-3b9f0efec22a&file=Bac_to_Back_Channel_Warping_Constant_Equation.jpg
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Neat question. A couple of things to consider:

1) If your channels aren't really deep, your composite section Iy might be larger than your composite section Ix. If that's the case, LTB is a non-issue.

2) I suspect that your section will behave a lot closer to an HSS than a wide flange. And that's good news of course. Your St.Venant torsional stiffness should dwarf your Cw stiffness. If the warping business doesn't work out, you could try calculating it as a modified HSS.[pre][/pre]



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
 
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I phrased point #2 badly. Rewrite:

2) With the vertical channel webs and the horizontal lacing, section warping will be insignificant. And what warping there would be would occur about a horizontal axis. LTB checks probably aren't necessary and, if performed, should be dominated by St.Venant torsional flexibility.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
 
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