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RE: Beam unbraced length

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I can see the logic in that the torsionally stiff zone, if placed along the unbraced span, goes along for the ride. But what if it is placed at a support point, or where one flange is braced. I haven't been able to find out how to quantify the stiffening effect, i.e. by how much the critical LTB moment, Mcr, is increased.
I have seen one example in a handbook where, after calculating Mcr, the author comments that if "warping restraint stiffeners" (channels welded between the flanges and to the web on both sides, thereby creating a closed section) are placed at support points, then the elastic, critical moment is increased by a factor five for that specific case. Nothing is said, however, how that result is obtained. I think that Eurocode 3 (the steel code) says something about the effect of end restraints, but not how to evaluate the degree of restraint acquired by using such stiffeners. Have there been published any practical design methods for this, or is this still just an interesting theory.

Gunnar H. Isleifsson
Denmark


-----Original Message-----
From: Carter, Charlie [mailto:carter(--nospam--at)aisc.org]
Sent: Monday, March 17, 2003 22:15
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Beam unbraced length 


>Normal plate stiffeners aren't effective in preventing
>relative flange twist because they have such low
>torsional stiffness.  If you're really stuck, use
>something with torsional stiffness.  The Guide to
>Stability Design Criteria (Galambos--I think 4th Ed.)
>mentions the use of split HSS members welded all
>around.  For example for a W24x55 you could split a
>TS6x6x1/4 and weld it as a vertical stiffener on
>either side of the web, each end, taking care to weld
>the 6 inch edge to the flanges.  Then the flanges
>can't twist.  Unfortunately, I don't think they gave a
>lot of advice on quantifying this effect. 
>Practically, it's more interesting than useful, but it
>is at least theoretically useful.

With all due respect, and I hate to disagree with you and the Guide, that
detail isn't much better than the stiffeners. The split tee will only create
a torsional rigidity for the width of the beam it covers. Outside of that
zone, there is still as little at there was before, and the torsionally
stiff zone created is along for the ride if nothing can provide the
restraint at one flange to be extended down through the detail to brace the
other flange.


<snip>

Charlie



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