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RE: Beam unbraced length

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>I can see the logic in that the torsionally
>stiff zone, if placed along the unbraced span,
>goes along for the ride. But what if it is
>placed at a support point, or where one flange
>is braced. I haven't been able to find out how
>to quantify the stiffening effect, i.e. by how
>much the critical LTB moment, Mcr, is increased.

I think there is a general solution for lateral-torsional buckling in the
Guide to Stability Design Criteria for Metal Structures. I'm not sure (it's
at the office, I'm at home now) if it gives a way to account for a
non-prismatic or segmented member. This will be extremely complicated
because the segments have such different response. The ends with cover
plates form a closed cross-section, which will have insignificant warping
torsion and be dominated by pure torsion. The bare segment in between will
have both warping and pure torsion.

That's probably a wonderful derivation to do and will require several large
white boards/markers or chalkboards/chalk. In writing the AISC Design Guide
Torsional Analysis of Structural Steel Members, I rederived all the old
Bethlehem equations for prismatic members and I could easily fill a wall
with the derivation of one equation. And each of those was for a prismatic
member.

>From an economic standpoint, I wonder if the framing arrangement is
advisable anyway. It is significantly expensive with all that fitting and
welding. A heavier member or kickers or another means of bracing would
likely be so much more cost effective.

>I have seen one example in a handbook where,
>after calculating Mcr, the author comments that
>if "warping restraint stiffeners" (channels
>welded between the flanges and to the web on
>both sides, thereby creating a closed section)
>are placed at support points, then the elastic,
>critical moment is increased by a factor five
>for that specific case. Nothing is said, however,
>how that result is obtained ...

I'll bet the author started the derivation, ran out of chalk and
chalkboards, and called it quits, guessing 5. (-:

Charlie











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