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Re: Lateral Torsional Buckling

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Daryl and Paul,

Thanks for the comments and input.

The answer to FAQ 4.3.1 in AISC knowledge base web site states that it is not necessary to prevent both the lateral movement of the compression flange and the twisting of the cross section. If a brace satisfies one of the two requirements, it is an adequate brace. The 6X6 bear on the top flange of W36. In addition, the 6X6 are all connected together by bolting them to the horizontal leg of the L5X3 and, the vertical leg of the angle section is bolted to clips which, in turn, are welded to the flange of W36. The clips are spaced at 30". I believe the detail is sufficient to qualify as adequate lateral bracing for torsional buckling of W36.


Paul Ransom <ad026(--nospam--at)> wrote:
> From: Padmanabhan Rajendran

> As you may notice from the rudimentary sketch, I provided earlier, the
> flange of W36 is connected to the 6X6 only at the "outer" end of the
> flange. Therefore, the restraint to lateral relative movement between the
> flange and the 6X6 comes through the friction between the two surfaces. I
> cannot see how the warping or the twisting of the section could be
> prevented. If my apprehension is well founded, is it appropriate to assume
> an allowable bending stress of 0.66Fy?

I was unable to visualize your sketch. However, the 6Xs are probably
bearing on the top of the W36. This provides some torsional restraint.

Unfortunately, the design standards are written without explicit
consideration of torsional restraint for bending strength. See SSRC
Guide to Stability Design Criteria for Metal Structures for more
rigorous analysis (e.g. Lx, Ly, Lt different).

I believe that it is more critical, in your situation, to confirm that
the top diaphragm is adequate for shear transfer between 6Xs. If the
design is that close to the edge that it could not tolerate reduction to
even 0.6Fy and still be marginal, then it may be better to reconsider,
anyway. Over time the wooden members will loosen up and you may lose
some of that residual capacity.

> From: Daryl Richardson

> Someone has mentioned lateral support via friction. It seems to =
> me that for some conditions such as bridges with three or more =
> longitudinal girders (yours has only two) that continuity of the 6x6 =
> elements could result in uplift (or at least lack of contact) between =
> the 6x6 and some of the girders. In such cases friction could not be =
> relied on.

But then there would not be compression in the top flange of that member
due to applied loads. Top flange stress state would have to be reviewed.


R. Paul Ransom, P. Eng.
Burlington, Ontario, Canada

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