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Beam unbraced length

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Bill Sherman wrote:

"I've recently tried to find if any literature
indicates that web stiffeners reduce effective
unbraced length of beams, under the theory that the
stiffeners prevent twist of the compression flange
relative to the tension flange, and the tension flange
helps keep the beam straight. However, I cannot find
any evidence of this - the literature seems to
indicate that the entire beam cross-section rotates
and thus such stiffeners would not be of benefit
without actual lateral bracing attached."

Bill,
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.


Charlie Carter wrote, in answer to a question:

">It seems to suggest that if the top flange
>is connected to a slab, then a pair of full
>height stiffeners (welded to both flanges
>and web?)can be considered a braced point in
>determining the unbraced length of the beam. Am
>I getting this right and can we extend this to
>a beam connected to a roof 
>metal deck?

Probably, but there are criteria you can use to decide
yes or no."

This would frankly scare me.  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.

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.

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota


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