Crane Support Column Bracing

[SteelPE]

I am in the process of providing engineering for a facility with multiple 10 ton 60' bridge cranes. The buidling is 40' tall with the crane girders being supported by a bracket at elevation = 25'-0". The crane girders are anticipated to be W24x55 + C12x20.7's spanning 30'-0" +/-. The column in questions have 10 ton cranes on each side of the column. I seem to remember my old boss saying the column was braced against lateral torsional buckling (LTB) by the crane girders. So in this instance he would consider the unbraced length of the column in strong axis bending at 25'. I had a discussion with a colleague regarding my old bosses design approach and he agreed that the girders will brace the column against LTB.

So my question to the community is, will the crane girders brace the column against LTB?

 

[SwinnyGG]

Note: I am not an SE. I do, however, have a lot of experience with cranes.

Whether or not a bridge crane girder (assuming you are using the term 'girder' in the conventional sense, i.e. the lateral, moving, load-bearing HSS or fabricated member of the crane that the hoist hangs from and moves on) provides any lateral support to a column would depend heavily on the crane type.

Some bridge cranes use single-flange wheels, like a train; cranes that use double flange wheels have very small non-steering (outer) flanges. In either case, the runway trying to move away from the centerline of the crane bay (because of deflection in the column the runway is mounted on) is not going to get much assistance from the crane girder- there's no rigid load path to develop the strength of the girder itself.

As I said above- I'm not an SE but I would be shocked were I to find out that structural engineers frequently allow the crane itself to share any building load.

 

[jayrod12]

I believe SteelPE is talking about the runway beams bracing the column for LTB. I would be inclined to agree with that, provided at some point along the line the load could make it's way to the ground. Otherwise technically you could have all columns buckle in the same direction at the same time.

 

[WARose]

I've seen a lot of people assume that......but at the end of the day, bracing against LTB isn't a mystery. It can be quantified as per Appendix 6 in AISC (or whatever the latest is). It depends on the loads involved, details of the connections, and so on. And something a lot of people miss with that method: you have to account where all the net load is going with the bracing system. All the members could buckle at once (as another poster said).

 

[SwinnyGG]

Having read the post again... It does seem that OP was asking about the beams supporting the runway, not the crane assembly itself.

Apologies for this confusion- in crane world, the girder is very specifically the structural part of the moving assembly.

 

[SteelPE]

Yes, I am referring to the runway girders that support the wheels of the crane. These girders impart a whole host of loads in my columns which are quite large. The runway girders rest on a bracket/outrigger and are usually bolted to this girder. We then typically attach an angle to the top of the runway girder to satisfy the support assumptions in the AISC. Each girder receives their own separate angle. In order for the column to rotate it has to bend these girders in the weak axis.

I have always found Appendix 6 of the AISC straight forward yet confusing as I am always confused on what makes up a nodal brace and a torsional brace.

 

[KootK]

- With respect to AISC Appendix six, this is definitely a torsional brace (or two). For me personally, once I've paused to question something like this, I feel as though I'm on the hook to run the numbers for justification.

- I agree that your torsional brace(s) are you girders acting in weak axis bending. Your cap channels are going to help quite a bit here so long as they're continuous within individual girder spans. You'll be predominantly using the channels in strong axis and I expect that they'll do the lion's share of the work.

- I expect that this will work handily given that L/d on the channel caps is only about thirty and I would expect your column section to be fairly stocky even with the large applied moments.

- We'll not rely on it explicitly but the system will benefit a good deal from the fact that the cap channels will have a significant degree of rotational fixity wherever the gantry is located.

Long live the mentor.

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.