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Re: Unbraced Length of Cantilever

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This is a tricky situation, not as simple as 2 times cantilever length. That
would make the design overly conservative. The phenomenon calling for the
reduction in compression flange stress is lateral torsional buckling. The beam
would be prevented from lateral torsional buckling even if lateral restraint is
provided on the tension flange along its length. Depending on the type of
member/connection  you provide for lateral restraint, you may satisfy the
requirements of compression flange. (If the lateral restraint is a beam or
channel, it has bending rigidity and provides torsional restraint as well,
irrespective of connection to tension or compression flange).

Regarding Rick's comment, the situations (restraint conditions) used in the
guidelenis are ideal cases, and the choice of the applicable situation
(restraint condition) would depend on the type of detailing actually provided.
Still, since you have a midspan restraint as well, and depending on the type of
restraint offered, you may be able to reduce the effective length. At least to
70 percent of cantilever length, based on Rick's figures. (Logic: Critical
section is at support. Nearest lateral restraint is 0.5L away, and effective
length factor is 1.4.)

Hope that helps.

M. Hariharan
Engineers India Limited, India

David I. Ruby, P.E. wrote:

> Unbraced length of the compression flange is 2 times the length of the
> cantilever
> David I. Ruby, S.E.
> President, Ruby & Associates, P.C.
> 30445 Northwestern Hwy., Suite 310
> Farmington Hills, MI 48334-3102
> Phone:            (248) 865-8855
> Fax:              (248) 865-9449
> Cellular:         (248) 514-2677
> E-mail:           druby(--nospam--at)
> -----Original Message-----
> From: Ricky Leblanc [mailto:Rick.Leblanc(--nospam--at)]
> Sent: Thursday, December 07, 2000 8:37 AM
> To: 'seaint(--nospam--at)'
> Subject: Unbraced Length of Cantilever
> Here is the situation.  Cantilever beam (W36 shape) laterally braced at two
> points on the top flange (free end and midspan).  These two points offer
> bracing against lateral displacement only (no torsional restraint).  At the
> fixed end there is full restraint about all bending axes (including
> torsional).  Full shear restraint as well.  Under gravity load the top
> flange is the tension flange and bottom flange is compression flange (i.e,
> the lateral restraint is at tension flange only).  Major axis bending only,
> no axial load.  Is the unbraced length of the compression flange the full
> cantilevered span, or is it something more?
> Thanks,
> Rick LeBlanc, PE
> Kellog, Brown & Root
> Houston, TX