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RE: Beam unbraced length

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>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.


>A top flange with studs
>reaching into a concrete slab is very stiff (both
>translationally and torsionally), and in my opinion
>placing stiffeners attached to the top flange at a
>location qualifies that location as braced for the
>bottom flange.
>However, the same scenario with metal deck is much
>less secure.  The decking will brace the top flange
>translationally (because it's very stiff axially and
>in shear), and if the top flange is in compression and
>the ends of the beam are prevented from twisting,
>you're all set.  But if you get compression in the
>bottom flange, those few little puddle welds or screws
>holding that nice thin metal deck to your top flange
>don't offer very much torsional resistance (because
>the deck is not very stiff in bending, and the
>fasteners aren't worth much either, and besides they
>may only be a few inches apart).  So, the discrete
>torsional load placed by the stiffeners trying to
>restrain the bottom flange is not going to find the
>resistance it needs.

I don;t disagree with your concerns and points here. To cover that in what I
posted, I referenced the equations that you can use to establish what
strength and stiffness is needed to consider a point braced. If you have it,
you can. If you don't you can't.

>If you're stuck in an as-built situation, you're
>better off trying to X-brace to the next member. 
>Otherwise, as was pointed out, it's better and cheaper
>to design a wider flange.

Excellent point. Direct bracing is often more economical than stiffening.
When possible, heavying the beam may be the cheapest solution in many cases.

Charlie




















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