Cantilever Crane Rail 1

[abuchman]

I have an interesting situation. I've been asked to look at a crane rail that cantilevers off a beam by approximately 14". The rail is 60 lb rail and carries carts weighing approximately 25,000 lbs (DL + LL) - each wheel then is 6,250 lbs and rides on two rails spaced about 4 feet.

The carts travel in/out of a pressure vessel. When they come out, they meet up to a lifting platform. Currently the rail is supported on one end by a beam welded to the vessel and on the other side by a beam connected to a lifting platform (i.e. the rail spans 14"). What the client is interested in doing is removing the support on lifting platform end end and basically having the rail cantilever out. I've never seen a situation where crane rail is anything other than continuously supported, and I'm not sure if the bending induced into a rail like that is even permissible for that material.

 

[r13]

I don't understand the situation described. Can you provide a sketch?

 

[ldeem]

It sounds like a straight forward problem but time consuming calcs. You will need I and S on the rail to calculate bending stress and deflection. I don't know about material but I assume it should be documented somewhere.

Don't forget to check your local elements for buckling. Beyond AISC Blodget can help you with looking at buckling of the compression elements.

On the cart check if it is realistic to except the cart to be evenly loaded.

 

[abuchman]

@r13, I've attached a quick sketch.

@Ideem, I guess what I'm wondering is if it's even permitted, given that rail steel is a hardened steel, and therefore more brittle, especially because it will be loaded cyclically. I've never seen a crane rail that is anything other than continuously supported. I did some research, but couldn't find anything that discussed the permissibility of using crane rail as a bending element.

 

[sbisteel]

Any appreciable deflection in that rail and the wheel won't be able to bridge the gap. If I was forced into this position, I might take a look at moving the gap between the rails over the slab, but even that is suspect.

 

[r13]

If I understand correctly, the sketch shows the end condition that the client wants - 14" cantilever without support beam (removed). I would check the cantilever against allowable stress, and fatigue. I think it is conservative to include impact factor (25% of wheel load). You may want to check CMAA Specification 70 for further guidance.

 

[r13]

I estimated the deflection is quite small, and would be kept within tolerance. However, for repeat travels between two supporting structures that having drastically different stiffness, the tear and wear of the wheels could be substantial. The suggestion of relocate the joint is quite plausible.

 

[abuchman]

The deflection would be minimal. I'm estimating less than 1/16".

Not to get too far into the weeds, but the reason they want to leave this as a cantilever is because they are having a problem with the existing set up. The left hand side with the slab sits on a hydraulic lift. It comes up to meet the rail on the right. Currently, when the lift comes up, the rail sits on that slab and acts as a simple span - so the joint is over that point. The problem they are having is that, through a combination of factors, when the lift comes up to meet the rail, it's actually pushing the rail up and causing a problem with the support steel inside the vessel. The lift has a limit switch, but it's a travel limit, not a pressure limit. What they are trying to do is have a complete break in the rail so the one doesn't impact the other.

 

[r13]

Ok, better than situation I've guessed .If the 14" space beneath the rail was not required for something else, would the owner objects to a short bracket?

 

[abuchman]

They wouldn't object, but it's basically impossible. These rails stick out of a pressure vessel. There's a door that closes over it, with two pockets to capture the rails. The only possible solution would be temporary stands that they put underneath each rail. This isn't impossible, but would be a significant hurdle because it would slow their production.

 

[r13]

I see. Can any change be made to the left side platform? I am thinking cutting the rail to a very short distance beyond the vessel, and extend/elongate the lifting platform.

 

[abuchman]

That's a good point. I'll have to look into that. Thanks for the input!

 

[ldeem]

I don't know for certain about the heat treat aspect but it seems like if the calculations pass it would work. I would first talk with some crane rail suppliers and possibly AISC. Calculation wise the heat treatment will bring your yield and tensile limits closer so be sure to consider tensile failure modes.

As others have pointed out getting the two ends to lineup looks challenging. Maybe hinging the rail such that it hinges up during movement and then down and spanning the gap for when moving the cart.