Welded crane rails

[SHS456]

I have simple supported crane beams over which sqaure rails need to be welded. Welding rail over the top flange will induce continuity on other wise simple supported beams. How to take care of this induced continuity ? or is it ok to provide rail joints at each ends of the crane beam ? any reference will be apprecaited

 

[graybeach]

Can you use gantrex clips instead of welding?

 

[SHS456]

Rail clips are not an option in this case. In case of welded rails to topflange, can I proceed with non-welded rail joints at ends of each crane beams ?

 

[jrbaus]

First I am no expert on crane runways but from what I have seen, rails with clips that are continuous over simply supported beams will still induce some degree of continuity. If the beams are quite stiff, then the forces in the rail associated with bending of the runway girders will be relatively small (might be worthwhile modeling crane runway + rail in a frame analysis package to confirm). Thermal expansion of the rail on the other hand will likely require a more thorough assessment. There are a several disadvantages that I can see arising from welding the rail directly to the runway:
1) ability to adjust alignment in service
2) no elastomeric strip under crane rail
3) possible serviceability issues arising from increased number of rail splices

Out of curiosity, what is the crane size and frequency of use that will be operating on the runway?

 

[SHS456]

Thankyou jrbause for the comments. I have decided to stiffen the crane beam to limit the crane beam deflection so as to minimize the end rotation of crane beam. Hope this will reduce the induced continuity . The crane in question is 50MT capacity and usage of 50 cycles/day. Any further references in this subject will be apprecaited .

 

[jrbaus]

I design to Australian Standards, so the best references I have seen are "Crane Runway Girders" by Branko Gorenc and "Design of Portal Frame Buildings" published by ASI. Nothing that specifically deals with welding the rail direct to the runway that I am aware of though. your usage equates to approximately 500,000 cycles assuming a 25 year design life which will likely warrant some consideration of fatigue (especially if each cycle is utilisiing a considerable amount of the runway capacity). My limited experience with runways subject to fatigue has yielded the following:

1. Welding = Trouble
2. Bolts in tension = Trouble
3. Fully tensioned bolts in shear = Better

Of course it is all in the detail. Welding of components to the girder is fine so long as you can accurately determine the stress at the weld and calculate the expected fatigue damage over the life of the structure.

Have you seen this thread yet?

http://www.eng-tips.com/viewthread.cfm?qid=309620

It seems as though your situation has been dealt with before. The last post mentions this being completed successfully on a runway 100 feet long.

 

[SHS456]

Thanx jrbaus. Appreciate the comments. Design of crane beam is to AISC and I have checked fatigue stress considerations as per code.
My specific concern is the continuity of rails over simply supported runways and the resulting induced continuity. This is applicable not only to welded rails but also to 'fixed' rails on top of runways ...hence is a common problem and must have been dealt before. need to know how to take care of this situation
Thanks

 

[KootK]

To have continuity, you need to be able to transmit bottom flange forces across the joint. Is the detailing such that that is the case?

The continuity force in the rail will vary about linearly with the depth of the beam. You'd need to add a lot of depth to seriously reduce the continuity force in the rail.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[SHS456]

@KootK...Runway bottom is connected to crane bracket via bolts. This creates continuity at bottom flange... Increasing the depth of crane beam is not a viable option. Already deflection of crane beam is restricted to a minimum(L/1000) so as to reduce end rotations. However, even a small end rotaion will cause huge tension force in rail...Hence it is not vaible to design the crane rail for tension as well. Looking forward for detailing/engineering practices followed in such situations

 

[KootK]

My point wasn't that you should go with a very deep beam. Rather, it was that rail tension depends on depth rather than curvature. Stiffening the beam will only help to the extent that it results in a deeper beam.

Will the bottom flange bolts shear off due to continuity forces in the compression flange? Could that connection be designed to allow axial slip on one side of the connection? If you could do that reliably, it might relieve the continuity as well.

I'm interested to see what comes of this too. As you've mentioned, it must be a fairly common issue.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jrbaus]

@KootK,

I agree with most of your assessment. My understanding is that it is not just the section depth that will make the contribution, but rather the stiffness of the section as a whole (i.e. thicker flange beam will carry more moment and hence reduce axial load in rail). Welding the top rail to the runway girder forms a combined section which will see shear flow between the weld connecting the rail to the girder. The axial force in the rail is influence by more then just the distance between centroids (that is, combined moment of inertia should be computed as there will be a stress gradient in the rail since stress = M*y/I). I think the point you were trying to make here is that despite whether the rail has a joint at the end of each span, there will still be considerable axial compression in the rail.

@SHS456

I have seen runway girders which have slotted ends at one end of the bottom flange to facilitate thermal growth and promote rotation. Such details usually call for snug or hand tight bolts with lock nuts. I did a very rough model of three spans with girder moment released and rail continuous. I found considerable compression to exist at mid span of the rail as expected, but the axial tension in the rail transferred beyond the supports seemed quite manageable. I have attached screenshot showing load location and resulting axial load in the girder and rail. If you are getting very high axial tension at the supports it might be worth checking the end releases of your girders to ensure they are acting as pinned joints (I made that mistake this morning in my haste).

Its an interesting topic - looking forward to hearing your findings.

 

[KootK]

@jrbaus: you've misread me a bit. My point was that tension at the joint will indeed be independent of beam stiffness and wholly a function of the distance between the centroid of the rail and the shear plane between the underside of the beam bottom flange and the top surface of the supporting bracket. If you consider the joint detail in elevation and draw a FBD, I believe that you'll find this to be true. Bernoulli flexural theory doesn't apply at the joint because there is no horizontal shear transfer mechanism (web) coupling the compression and tension forces there. The forces are determined by equilibrium alone.

I agree that, until welds break, the beam and rail will form a composite section everywhere other than the joint. This will see the rail in compression at mid span and create a shear flow demand in the welds.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.