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RE: Beam unbraced length

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>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.  

Stiffeners themselves do nothing to prevent twist and therefore alone will
not be of benefit for restraining lateral-torsional buckling. For example,
in a bare beam, you'd use the full span as unbraced regardless of how many
stiffeners are welded into it. But for a beam in a moment frame with a floor
slab at the top flange and reverse curvature bending putting the bare flange
in compression, stiffeners could be designed to activate the strength and
stiffness of the floor slab to restrain the bottom flange. The 1999 LRFD
Specification gives the equations for required strength and stiffness to
consider such an arrangement braced.


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