Regarding Lateral Torsional Buckling 1

[allimuthug]

Hi Everybody,

I have a channel simply supported between the columns as shown in the figure attached. The channel is connected to the column flange by a shear tab or fin plate.

(i)As per AISC 360-05 clause F1 "The provisions in tis chapter are based on the assumption that the points of support for beams and girders are restrained against rotation about their longitudinal axis" It means the fin plate or the shear tab should be restrained against rotation or should have torsional capacity ?
(ii) It the fin plate doesn't have any torsional capacity this AISC 360 is no more applicable to the channel since it doesn't quality the assumption of AISC?
(iii) How much is the torsional rotation the fin plate is subjected to?

Please clarify me I am totally confused, because if this doesn't work then the whole world cannot use fin plate connection at all.

 

[SocklessJ]

I would think that with a low enough rotational stiffness, the effective length for LTB could become infinite. For example, if you try to pick up a girder by it's ends at mid-height, it will roll over under it's own self-weight.

 

[KootK]

I'm not sure that clears up my confusion as to the justification for your feeling "that the unbraced length is the bolt line to bolt line distance", which would, by your subsequent statements, be an assumption yielding a higher LTB resistance than we agree a detailed analysis would show for common connections of that type.

Nope, not quite that either. My stance is that the unbraced length of the channel should be taken as the bolt line to bolt line distance but, in addition, the ability of the shear tab to perform the bracing function MUST be evaluated. The evaluation could be numerical per AISC appendix 6 provisions or, as I would normally handle it, by inspection. For me, there is no practical path forward with this that does not involve the evaluation of the shear tab for it's capacity to perform the bracing function.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BridgeSmith]

"...in addition, the ability of the shear tab to perform the bracing function MUST be evaluated."

Got it. Makes sense. We don't concern ourselves with LTB for diaphragm members, since they only see axial load and small end moments, so I hadn't really processed how to go about it. Thanks for taking the time to help me understand it.

 

[allimuthug]

Hi,
So it will be nice if somebody gives an example how to evaluate the shear tab as a bracing.

 

[KootK]

Thanks for taking the time to help me understand it.

You're most welcome. You've been making a substantial contribution here lately. I expect that you'll return the favor in short order.

So it will be nice if somebody gives an example how to evaluate the shear tab as a bracing.

The sun has probably set on this but, since I had the sketch lying around on my desk, I figured that I might as well post it for any interested parties. It shows a concept for treating the knife plate as an equivalent rotational spring. You're on your own for the detailed mathematics I'm afraid.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BridgeSmith]

"You've been making a substantial contribution here lately."

Thanks for the encouraging words. I've had my doubts about whether I was actually being of any help. You and some of the other contributors have far more experience than I. I feel like an EIT again sometimes.