Crane Column Support Conditions 2

[SteelPE]

I have a project that is an industrial facility with a 15 ton bridge crane. We have decided to install the crane on its own separate column system as the loads are getting pretty high (42 kips down). The distance from the center of the crane to the center of the support column is currently 1’-6” (this may change). The tractive forces induced into the system put quite a bit of torsion into the support column.

I am currently designing the base of the column to be fixed against torsion. Now the questions is what do how do I design the connection at the roof? Is this connection considered fixed or pinned torsionally? I have attached a quick partial roof framing sketch.

 

[TehMightyEngineer]

Personally I'd run it both ways and get a design envelope. In reality I would expect you would see a reasonable amount of fixity.

Maine EIT, Civil/Structural.

 

[jimstructures]

SteelPE,

We normally put the crane bracing in the plane of the separate craneway columns so the tractive force does not induce any torsion into the building support columns. This implies separate bracing for the crane way and the building. the attachments of the separate crane columns to the building columns serve mainly to provide brace points for the weak axis of the crane columns.

Hope this helps.

Jim

 

[SteelPE]

Jim,

I understand your point, and if the crane were much larger I would add additional bracing. The layout if the building does not allow for the bracing you describe. Another question might be how many columns do you distribute this force to. One, three, all the columns attached to the crane girder? If you were to apply bracing you certainly wouldn't have it in every bay.

The column can support the torsional force, however, it's the deflection due to rotation that I am concerned about. I don't remember seeing anything in the AISC DG regarding this deflection. It might be because it was a pain to calculate back when the DG was written. If I fix the top (even partially) we are talking about 1.5 ft kips of torsion. I don't think that a lot.

 

[JAE]

I would always do what jimstructures suggests. Putting torsion in a WF column isn't the best - the WF has very little torsional stiffness so the rocking you would get would be very large.

If you used tube columns for the crane columns then you could perhaps pull it off.

Usually we would X-brace the same bays (building and craneway) so there would only be one bay on each side that would be interrupted with braces.

 

[SteelPE]

JAE, i contemplated using tube columns but for some reason i have a feeling that they should not be used due to fatigue at the crane connections.

 

[JAE]

I guess I typically place the crane beams over a cap plate on the columns with the crane beams running over the cap plate. Then we have a splice/gap in the crane beams directly over the column.
Each beam can then rotate freely (slotted splice plates).

We also then use plate tie-backs (2 of them - one for each beam separately) to the building column to transfer the lateral crane forces back to that column.
The tube column doesn't get affected with fatigue in that system.

 

[jimstructures]

SteelPE,

You are not putting torsion into the crane column, you are merely putting a bending force the top of the column. Which you resolve by putting bracing in the plane of craneway, beams and columns. You are putting torsion into the building columns by the moment arm of the distance from the center of the crane rail to the centroid of the building column. I sections have all the torsional stiffness of a wet noodle. The torsional deflections/rotations of the building columns will be substantial unless you provide a load path for the tractive force to be taken by the craneway bracing. With separate columns that is what the crane way bracing is supposed to do.

42 kips is not an excessive force, albeit large, to handle with brackets, tiered bracing struts at the bracket centroid height and angled braces to provide a load path into the tiered brace strut and then by angles, rods or I sections to the base of columns.

As others have said there is a reason why they do that way, it works, or provide bracing in the plane of the crane way on each side of the crane aisle.

Jim

 

[paddingtongreen]

This was confusing, the description doesn't seem to match the sketch. I conclude that the columns shown on the sketch were the ones that were added. I don't know what you would have done without them. Bracket out to support the crane girder. Brace the top flanges of the girders back to the column without making the girders continuous, add another beam at the same level, between the columns and introduce plan bracing, (make it a horizontal truss). Now the columns don't get torqued, they get horizontal loads in both directions, vertical load and bending moment from the brackets. The horizontal loads go into the building frame at the top of the columns. You might be able to brace the columns above the crane rail but I'm not sure that would help.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[SteelPE]

paddingtongreen,

Since my concern was in regards to how to model the support at the roof, I only provided a sketch of the roof framing. Attached you will find a quick sketch I did of what the connection at the crane girder will look like. Hopefully this will clear up some of the confusion.

 

[JAE]

SteelPE -that is pretty close to what we've typically done - we just include craneway longitudinal bracing in line with the lower crane columns to avoid torquing the building columns.

 

[SteelPE]

JAE,

I would think it would be the same as it pretty much comes out of the AISC DG.

How often do you need to provide the bracing? In going back and looking at the architectural layout, the only location that I may be able to apply bracing is in the last bay of the crane runway which is 8 bays at 30 feet/bay. So the last 30' bay of a 240' long crane runway is the only possible location. The client fabricates and installs equipment on the back of trucks, they need to drive the truck under the crane to install the equipment, so they need access to drive under the crane.

 

[JAE]

I think the best concept is typically placing the bracing near the mid-length (center) of the runway. That way any thermal or load induced deflections work outward.

We always avoid placing X-braces at each end as the two braces then fight each other under thermal loads. If this is in a temp. controlled building then the thermal loads may not be that big of an issue.

Placing a brace at a far end may work depending on how long the runway is - the "stretch" of the crane beams would have to be looked at (for the example of a bridge suddenly stopping at the far end away from the brace). That would induce a PL/AE stretch at the far end with a potentially very big "L" value.

 

[SteelPE]

In this instance, we are looking at a traction force of 3.6 kips which would give a "stretch" of approx 0.013 inches in 240 feet (A=27.5in^2). I don't think really amounts to much.

 

[JAE]

I would agree.

 

[jimstructures]

SteelPE

Portal frames with the portal beam in line with the crane runway beam and X-rods, angles or I sections crossing above the Portal beam.

What others have suggested, put the crane/building bracing in the middle od the building lengthwise.

Jim,