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RE: Beam unbraced length

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There is another solution.  I have a January 1972 AISC Engineering
Journal article titled "The case fo rthe Semi-Box Girder".  The author
describes using a wide flange or plate girder with diagonal plates from
the flange tip to the web (at a 45 degree angle)

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This creates a torsionally closed section at the top (compression)
flange, which radically increases the buckling resistance.  The noted
article has a good set of references and a design example.  I have used
this method and it works!


Duane Siegfried, P.E., S.E.
Associate Manager, Structural Engineering
Horner & Shifrin, Inc.
5200 Oakland Avenue
St. Louis, MO  63110
Phone:  314/531-4321
FAX:  314/531-6966
Visit us at http://www.hornershifrin.com to see why we are one of the:
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-----Original Message-----
From: Michael Hemstad [mailto:mlhemstad(--nospam--at)yahoo.com] 
Sent: Monday, March 17, 2003 2:01 PM
To: seaint(--nospam--at)seaint.org
Subject: Beam unbraced length 


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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