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torsional buckling of a column with jib crane

torsional buckling of a column with jib crane

I've been asked to look at the feasibility of mounting a 1-ton jib crane to an existing column. The column is a w10x49#, approx. 35 feet floor to roof, braced in X- and Y-axes at the top. In addition to eccentric loading, as the jib rotates, torsion will be induced in the column. AISC spec E4 refers to Kz, effective length factor for torsional buckling, but I can't find a value anywhere. Can somebody point me in the right direction?

RE: torsional buckling of a column with jib crane

See commentary section E4, "This factor may be conservatively taken as Kz = 1.0. For greater accuracy, Kz = .50 if both ends of the column have a connection that restrains warping... If one end of the member is restrained from warping and the other end is free to warp, then Kz = .70"

I'd be careful about assuming warping restraint. I'd doubt that you had a condition at a conventional warehouse-type column top that would restrain warping. However, I'd think that your torque should be pretty minimal if you have some thrust bearings or greased bushings at the pivot of the jib.

RE: torsional buckling of a column with jib crane

My mistake. The torque would result from the stop force at the end of the range of swing...

RE: torsional buckling of a column with jib crane

Yes, there are thrust bearings at the pivot, so virtually no torque is applied to the column.

RE: torsional buckling of a column with jib crane

We would have to see more details of what you are actually doing, and you’re way short on those so far. But, aren’t you really talking about column cap’y. due to significantly varying Mx and My as the boom rotates or the load moves to the end of the boom? Plus the added ton of vert. load, of course. But, what torsion? You can reinforce the column as needed and you want to put some sort of cushioned stops on swing and trolley movement on the boom. Also, consider that users will reach out and to the sides for picks so consider these possible lateral loadings.

RE: torsional buckling of a column with jib crane

"Yes, there are thrust bearings at the pivot, so virtually no torque is applied to the column."

This doesn't ring true. The pivot will be eccentric of the column, when the crane is swung to the side it will pull the top connection in one direction and push the bottom in the opposite direction. I can't be sure of this because there is, as dhengr noted, a dirth of information.

Timing has a lot to do with the outcome of a rain dance.

RE: torsional buckling of a column with jib crane

With what look to be walls at both ends of range of swing (let's call them 9:00 and 12:00 positions) I think torsion from swing stopping or out-of-swing-range load picking could be neglected. However, you can still have torsion in your column if your jib is the traditional 3-member truss type where you have two discrete connections/pivots at the column, as paddingtongreen is discussing. The torsion would occur between the two connections/pivots when the jib is at the 12:00 position. See attached.

RE: torsional buckling of a column with jib crane

The orientation of the column appears different on the plan and section. Does the jib crane attach to one flange or to a plate between flanges?


RE: torsional buckling of a column with jib crane

Not sure if my last post went thru, so I repeat. The crane attaches to a plate between the flanges.

RE: torsional buckling of a column with jib crane

Torsion is not an issue unless the jib moves really fast and stops. Why would it do that?

RE: torsional buckling of a column with jib crane

Thanks to theonlynamenottaken. I did plenty of number-crunching according to Design Concepts for Jib Cranes. The calc's. appear to be for the case of the jib being mounted to one flange of the column. My case may be to mount to a plate between flanges, due to the orientation of the column. Will this affect the calculations?

RE: torsional buckling of a column with jib crane

Now that I see your details, I agree that there will be a torsional component involved in the design. I do not have the several guides or latest AISC code you are looking at. The torsion is just the lateral force at the pin times the distance to the centroid of the column section, irrespective of which orientation you are connecting to the column. The difference is that you have to consider two different connections, depending upon the orientation, to get that load into the column. Your plates welded to the flanges and the web, maybe with some stiffeners from the hinges base plates and/or to the column, should do the trick. Otherwise, the column doesn’t know the difference.

RE: torsional buckling of a column with jib crane

In short, yes it will. Your maximum torsion will occur in the column with the jib at the 12:00 position, though that may not be worst case overall for your column since that coincides with pure strong-axis bending. In the "Design Concepts for Jib Cranes" discussion paper their maximum torsion occurs where it coincides with pure weak-axis bending in the column - which is what leads to their solving for Theta range 6 to 10 degrees as jib position for worst-case overall stress in the column. Your jib position for worst-case overall column stress may occur more near 10:30 to 11:00 (to use the clock analogy we used earlier), where you have components of torsion and weak-axis bending.

Do you have the ability to "box" the column by welding in plate on both sides in between the flanges, essentially making your wide-flange section into an HSS? If so you could greatly simplify your problem and reduce the possibility of overstress by introducing a closed section. Otherwise you may need to track down the original Jim Fisher & Steven Thomas paper and not the discussion, and re-derive for your specific orientation. Or, you could do a numerical analysis with a spreadsheet.

RE: torsional buckling of a column with jib crane

It may be difficult to weld a plate on the exterior of the column, but it should be possible to weld a continuous plate on the interior corners of the column...not quite such a good torsional shape but better than the WF alone. It also moves the shear center a little closer to the jib connection and improves the weak axis properties.


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