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RE: Beam unbraced length

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Duane,
Likewise with angles which I have used to reinforce 
lightly loaded crane runways where we could not remove 
rail and conductors were not a problem.

Gary

On 20 Mar 2003 at 7:12, Duane Siegfried wrote:

> 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)
> 
>  =======
>   \    |    /
>     \  |  /
>       \|/
>        |
>        |
>        |
>        |
>  =======
> 
> 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:
> - Top 500 Design Firms (Engineering News Record) - Top 25 Best Civil
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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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