Jib Crane Column Loading

[jheydon2]

Hello All,

I have been tasked to see if a 1 ton capacity 10' span suspended boom jib crane can safely be mounted to an existing I beam in our warehouse. I am a young design engineer so this is a little out of my wheelhouse. I have read over the Design Concepts for Jib Cranes paper written by Fisher and Thomas. The beam in question is a W8X24 I Beam 259" long bolted to the floor and bolted to the above truss. Does anyone have any helpful thoughts on how to analyze the column? The paper is a little confusing in some areas.

Anything is appreciated.

Justin

 

[FoxSE14]

It sounds like the "beam" you refer to is actually a column. (Columns are vertical, beams are horizontal. The terminology is important, especially if you are showing your bosses you know what you're talking about) Can you post a photo of the connections at the base and top of the column in question? If it is simply bolted to the floor slab, it may actually be a wind column without a foundation.

Typically, you would hire a licensed Structural Engineer to perform this check, and then to design the new jib beam and any required bracing, reinforcing to the existing column, and foundation (or modification to the foundation - if there is one).

We're talking about a factored moment of ~32 ft*k, this will need professional attention.

 

[dhengr]

Jheydon2:
Are you a young engineer by education and training, or by title only? You should really be talking this over with your boss (who ever tasked you with this task). They should know what you do know and what you don’t know, so they can guide you and keep you and the company out of trouble. They should also be willing to help you learn and see you grow and advance, so that you gain in value to the company. We all had to face this type of problem the first time, and tackled it with various degrees of knowledge and prior experience, so this isn’t something to be particularly embarrassed about, unless you way oversold your abilities to the boss in the first place. And, there is even educational value in having to admit that, and not doing it again. It is what it is. Many boss’s today are also inclined to assume their new/young hires know much more about engineering than they actually do. Finally, truth-be-told, some of them don’t know how to do the problem, so they aren’t very good mentors after all. Look for a good local mentor, and buy him/her a beer once in awhile, and talk engineering, and listen to what they have to say. You should really get that all sorted out.

You really should start with a couple of well proportioned sketches, by hand is o.k. if the proportions are about right, (maybe to scale) showing all the pertinent dimensions, loads, existing framing members, and other design info., you’ll need all of that to visualize the problem, and so we know what you call a beam and a column. Show the details of the jib crane too, so we know how it fits in the picture. What does that paper by Fisher and Thomas look like so we can see where you are starting from and using as a ref.? Can you post that along with your sketches. Then dig out your Engineering Mechanics, Strength of Materials and Machine Design text books and start studying them, as they relate to this problem. The internet can’t do everything for you, despite what they tell you.

 

[DaveAtkins]

This is a problem in axial force plus biaxial bending.

Do you have access to structural analysis software? This will help you to determine the axial force in the column, the two bending moments in the column (strong axis and weak axis), and the rotation at the point where the jib crane connects to the column (this will help you determine how much the jib boom deflects).

Once you have the axial force and moments, you can use AISC Chapter H to check the column.

DaveAtkins

 

[theonlynamenottaken]

Note that torsion can occur depending on how the jib crane is configured. If the crane is mounted to the face of flange (as they are commonly mounted) then you can end up with simultaneous weak axis bending and torsion. See the thread below from a couple of years ago:

Torsional buckling of a column with jib crane

 

[UnusedHandle]

theonlynamenottaken is right about the torsion/weak axis bending. See attached for a quick approximation. Note that in the weak axis the eccentricity will cause the additional torsion. The support conditions for analysis depend on what you have. To be conservative for the column, I assume pin base and lateral restraints at top to maximize the moments. For the base, I redo the analysis with a fixed moment baseplate to determine maximum reactions to the floor/foundation. You don't need a computer for the analysis. In major axis only it's straightforward. In minor axis, you can superimpose the torsion. AISC Design Guide 9 has a good approximation for the warping normal stress in section 4.1.4. You can find that online readily enough. I would also check, for the jib at an angle to get major axis bending, minor axis bending, and torsion, though minor axis typically controls. As a side note, boxing the column in at the connections will only help with local forces, not torsion. If you want to make it a box section to resist the torsion, you must box it in the entire section that is subjected to torsion, or better yet, the whole thing.

 

[racookpe1978]

Good question:

Three important things to remember - all more general than the very detailed comments above.

1. This is an existing building: the column as designed may be either "relaxed" as foundations and slabs and outside sloped dirt has moved over time, or might have been compressed and under more thna design expectations.

2. Building "use" is almost certain to have changed over time: Loads on a second or third floor are likely to have been added, or machinery or storage on a second or third floor might have been removed. Both have to be looked at.

3. Analysis in the slide rule era was conservative, but Codes were followed. You may find that margins "too close" then in terms of bending moments (or the resistance to bending moments then) exceed what a close, fully analytical analysis today shows. Thus, I would "expect" older columns - note the definition above! - would be much more likely to "over-design" a vertical rather than "approach the limit" as a very liberal "under dollar constraint but over-designed" typical now.

But the first thing to do is to exactly check the dimensions and material spec of the existing column and the concrete and columns it is attached to. Then determine what its current loading is. (Worse case, you find it is already overloaded!)

Then you can start looking at what the load from a new jib crane will add to the current loads.