Stiffness & Drift for Cantilever Columns

[bookowski]

When designing a cantilevered column (steel) is the base assumed to be fixed for drift calculations? Is there any consideration for rotation or rocking of the footing? This will be supporting an overhead crane runway - I am worried that the assumption of fixity at the base is underestimating the drift too much... but any guidance on how much that is?

 

[rowingengineer]

yes normally if drift is important rotation of the foundation is taken into account. What type of foundation are we dealing with?

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[bookowski]

Spread footings, so pretty flexible.

 

[structSU10]

see this thread

http://www.eng-tips.com/viewthread.cfm?qid=351084

Its outlined procedure can be used iteratively to determine the proper spring constant to use. It required an estimate of footing and baseplate, and redesign of those for the moments, and messing with the model until applied P & M can match a FEM model by adjustment of the spring constant.

 

[SteelPE]

What size crane? I don't know if I would be comfortable designing a cantilever column system with any type of crane. Would it be possible to brace the columns back to the existing building (if there is one)?

 

[bookowski]

SteelPE - That brings up another question. What I have is: 10T crane, the building will be open (no roof or walls, just a frame), the building is "L" shaped in plan and they want to keep that "L" clear (no columns). So over the turn in the "L" we have a 60ft. span. This is being accomplished with a heavy WF and there will be a catwalk immediately adjacent which has plan bracing to take lateral from the 60ft span. On the opposing wall there are typical 30ft spans - so there is only one cant column and it aligns with midspan of the 60ft span. See attached sketch.

There are two phases to the job, phase 2 is that it someday gets enclosed. So in the current scheme (phase 1) there is very little wind/seismic load, the lateral from the crane will control. This brings up another question which is can I design the horizontal catwalk on the one side to take 100% of the crane lateral load? Maybe I'd do that but still design the cantilever for 50% of the load as a belts & suspenders... thoughts?

 

[UcfSE]

Look in the PCI, as mentioned in the referenced thread. They have a procedure to estimate base connection fixity, including foundation rotation.

 

[SteelPE]

Now, I don't have much experience with cranes. If it was my building do design I would not attach the crane girder to column A-2. This is because the deflection of the beam along line B will be at the greatest at B-2 when the crane is at line 2. This will rack the bridge crane and causing premature wear of the wheels and future maintenance problems. When ever I have run into something similar, I have skipped the column attachment and made the beams the same size for this reason. At least this is how I was taught to handle the problem.

It also appears from your description, that column A-2 is the cantilever column. Again, in my opinion, this is just another reason to "skip" this column all together. If that is not possible, I would probably design the column as a A frame installing another footing to the north of line A and not rely on the column cantilevering up from the footing. Just something I would not be comfortable with.

Just my $0.02

 

[bookowski]

SteelPE: I agree - I've been thinking about that as well. The column would eventually need to go in, in the phase 2 of the project there will be a concrete slab on deck roof over the whole thing so I'd need it for that. In that case though I'd have a diaphragm to get loads over to other frames. Since it has to go in eventually I'd been thinking I'd try to get it in now and save on the beam... but I see your point.

If using two 60ft. span runways beams and only one has a horizontal truss can lateral crane loads be distributed based on stiffness, i.e. most of it will go to the truss? If not then I need to add another lateral element along line A.

 

[SteelPE]

The column can be added in the future with no attachment to the crane girder. I have a building that is under construction now where we are up against the existing building. The crane is supported by girders at 30-35 feet (depending on where you are in the building). The new exterior line has "wind columns" at 15-17.5 feet o.c. To keep the crane girder deflection the same, I skipped attaching to the wind columns.

You will get some differential deflection due to the position of the picked load on the crane..... but that's not my problem and there is nothing I can do about that. I just do the best I can with the problems I have control over, and in this instance I made all of the columns align to give me equal span crane girders on each side of the building.