Crane Bridge Rail Gap

[NE12345]

I am trying to find out if there is a specification or information on the maximum gap under the crane rail and the girder for an overhead top running bridge crane?

 

[TehMightyEngineer]

I believe CMAA 70 recommends 1/16", though crane manufacturers may require smaller.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[NE12345]

In CMAA 70 (2010) all I can find is in section 1.4.5 which is horizontal rail separation at joints not exceed 1/16", could you please direct me to which section in CMAA 70 refers to the gab between the bottom of the rail to the top of the girder? Thanks.

 

[TehMightyEngineer]

Oh, I misread your first post and thought you were referring to the rail joint separation.

I don't know of a specified limit for the gap between the rail and the girder.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[hokie66]

The rail sits on the girder and is clamped to it. Why would there be a gap? Is there a cushion in play here?

 

[NE12345]

Yes we have rail clips at least every 36 inches but the girder has a camber in it and the rail is not perfect so there are gaps at various point between the two.

 

[Bendog01]

AISE Technical Report 13 Section 5.18.6 addresses crane girder and rail alignment as does AISC Design Guide 7. There is also a good article on Tips for Avoiding Crane Runway Problems by Ricker which is attached.

 

[Duwe6]

Structurally, your gap should not be a problem. With a light load 'on the hook' the slight humps from the camber will not give a problem when trolleying in & out. With a near-maximum load, the humps will be flattened out by the weight transmitted to the trolley wheels.

Ask the crane mfr what is an appropriate maximum gap, and use that value to write your own spec. Shim to close the gap, and/or decrease the clamp spacing. Have any shims used tack-welded to the beam, and try to only use single shims [vs. a stack of shims] at each location. Off the top of my head, I'd space the clamps at 16" to 18" intervals, and shim any gaps in excess of 1/32" 'true gap' - measured by sweeping a 1/32" feeler gage under the crane rail.

 

[NE12345]

I have checked out AISE TR 13, AISC DG7, and Tips for Avoiding Crane Runway problems. I cannot find any thing that correlates to the distance between the bottom of the rail and the top of the girder, at the rail clips there is little to no gap but between the rail clips some locations there is a gap due to the difference between the girder camber and the straightness of the rail cause a gap of 1/8" in 1 location for the max. Any help in locating some specification or guidance would be greatly appreciated.

 

[racookpe1978]

Verify, however, that the structure under the rails has not sagged or been distorted.
If the crane rail is humped up, then it will be forced down as noted above by the load.
If the crane rail supports are sagging,and the unloaded crane rail is "flat", then the rail needs to be properly re-supported with no gaps.

 

[robyengIT]

Hope it can help : see attached file table 4.1 item 51 (page 35 of PDF file) and fig. 9 - 16 - 18

 

[thaidavid40]

In many material-handling equipment rail designs, the rails are supported intermittently on blocks. This leads me to believe that there is no "canned" answer to your question. I believe it is strictly a matter of strength and deflection. If the gap is large enough, then as the rail is pushed down, a non-trivial expansion of the rails occurs lengthwise. This could be problematic if the ends of the rail are restrained against such movement.
Dave

Thaidavid

 

[NE12345]

Thanks everyone with for your responses.
What I was able to find out after contacting our rail manufacturer was that by industry standard any gap greater then 3/16" under the rail to the top of the girder for a bridge crane is considered excessive and that no specification or standard addresses the gap.