Evaluating rotational restraint at beam supports

[canwesteng]

Is anyone aware of any way to evaluate the strength and stiffness requirements for a beam support? Eurocode or some paper maybe? I'm thinking specifically to evaluate unstiffened beams over supports, underhung monorails, etc...

 

[canwesteng]

For reference, in case anyone stumbles across this thread later - Bradford, M. A., and Trahair, N. S. (1983), “Lateral Stability of Beams on Seats,” ASCE Journal of Structural Engineering, for unstiffened beams on seats. AISC has also confirmed appendix 6 can be used to calculate the requirements for end restraint.

 

[KootK]

Thanks for reporting back on this. Classy. I'm sorry that you had to suffer in silence here. Some thoughts:

1) The place I see this come up most is with temporary works beams. There, I've seen some charts that deal with this by designing the beam for LTB assuming an exaggerated length to account for the lack of support restraint.

2) You'll want to have one of our antipodean friends fact check me on this. Some time ago, I was involved in an LTB thread per the Aussie code. My recollection is that their LTB methods do take some account of the rotational support conditions.

3) I've been checking this condition using side-sway web buckling per AISC in many practical situations. I feel that is quite salient and speaks a bit to the "web distortion" issue mentioned in the abstract for the paper that you referenced.

In your situation, is the top flange restrained at the supports?

I believe that using appendix six would require one of two approaches, both of which introduce annoying complexity in my experience:

4) Assuming that the column, and it's connection, torsionally bracke the beam or;

5) Assuming that the compression flange itself braces the beam as a sort of girt.

If you've evaluated this, can you describe the method that you went with?

If you feel that "the moment has passed" on this, don't feel any pressure to respond to my questions. That said, if you remain interested in further discussion, I'm game. Your ongoing contributions here entitle you a seat in first class when it comes to having your own questions discussed. If I can help, I will.

 

[canwesteng]

This is mostly for temp steel beams, in which case the Bradford and Trahair paper just lets you bump up the unbraced length, with L.u=L+(d/6)*(t.f/t.w)^3. This satisfies the construction use case for temp steel, so I wouldn't take the appendix 6 checks any further, but otherwise I would have just derated the moment capacity of the beams until the web provided enough stiffness and 0.2% of the moment capacity at the support.

For appendix 6 checks, I would be using the strength and stiffness provisions to check if the rotational restraint is adequate - my thoughts here are to try and actually quantify if monorails are properly rotationally supported at the ends when considering the load is below the shear centre, but probably a weekend project for later. I think your questions relate more to a question of how the column itself is able to provide the rotational restraint - in which case, since this check isn't strictly required by any code yet, I'm happy to ignore it. But the case of a long, slender, unbraced column idealized as only in compression could possible not quite meet the stiffness and strength requirements.