Crane Rail Clip Spacing

[ForrestLowell]

Is there a maximum spacing for crane rail clips? The 1989 version of the AISC ASD manual suggests a spacing of 3', but it is not a hard number. My clips are 9" long and I want to space them 4.5' apart. Strucuturally the crane rail is okay with the lateral loads applied, I am mostly curious if there is an industry rule of thumb or code that limits the spacing of the rail clips. The crane is a large overhead crane that runs on two rails.

Thanks,

Forrest

 

[LSPSCAT]

I am not sure what you mean stating "my clips are 9" long", however, 4.5 ft spacing seems excessive. Structurally, loading needs to be accounted for, however, the dynamic loads are difficult to pin point and vibration can also be a consideration. I would recommended staying below the 36" nominal spacing. Typically we specify 18" through 30" rail clip spacings depending on the type,rail size, and loads.

Check out for basic recommended spacings and allowable lateral load capacities:

Gantrex

http://www.gantrex.com/PDF/Clip Spacing.pdf

LB Foster

http://www.lbfoster.com/Rail/rail_pdf_profiles/LBF_Rail-CraneRailSystems.pdf

Atlantic Track

 

[ForrestLowell]

Thank you for the very useful links. To clarify. My clips are not bolted, they are welded to the girder with 18" of 3/4" fillet weld. The 18" of weld goes in and out so that is how you fit 18" of weld on a clip that is 9 inches long. My wheel load is about 100 kips, and the Iy of the crane rail is about 55 in^4. The allowable stress of the crane rail is 80ksi. My critical condition is during a seismic event where the rail is allowed to yield, just not break, so I am allowed to take a 1.6 increase factor to the 0.75*80ksi for an allowable load of 96 ksi. My moment is figured as if it was a point load on a continuous beam, or PL/8.

Thanks again, and let me know if you have any other thoughts. Some other poeple have suggested keeping it to 3 feet or less, but there are considerable savings for my partiular case if I can make few 3/4" welds 18" long. I am mostly worried about the capacity of the rail not the clips.

 

[ForrestLowell]

One more clarification, the lateral load is 100kips, not the vertical. The lateral load is due to a 3.5 g factor during a seismic event at a critical facility.

 

[LSPSCAT]

Forrest,

I have not had much time to give this more thought on technical/analysis side, however, I may be concerned with those large 3/4" fillets and any distortion you may be getting on the rail girder. This is a finished product once the rail is mounted and flatness needs to be controlled to minimize vibration and rail wear. In the end you will have high localized stress at the rail girder connection points.

(Also, I think you are welding the rail clip to the girder, with this I assume you still have an adjustable part of the clip that then bolts down to your welded clip base.)

Check Whiting's Crane Handbook - it is available on the web.

 

[CTW]

I've never seen a reference that states why 3 ft is considered the maximum spacing for clips. I've searched through my literature and everything simply states a maximum of 3 ft.

Since this is a critical facility and you're worried about the capacity of the rail, I think it's worth staying within the industry recommended spacing of 3 ft. If something ever went wrong with the clips or rail, it would be hard to explain how going beyond industry standard maximum spacing was justified.

 

[FLCraneBuilder]

This topic is a favoite of mine, and now I have an opportunity to challenge yet another "industry standard" that are, at times, ridiculous....

It seems quite logical to me that whatever affixing method and spacing used should be determined mostly by lateral loads induced into the rail the crane's duty cycle.

Example - a small 5 ton capacity crane has a lateral wheel load of about 500#. So, the force trying to displace the rail is about 500#. When one considers the section properties of whatever rail size is used, (often 30# ASCE), you can probably head for 5 ft or much more spacing. I suggest discounting the friction of the rail to top of painted runway beam (although probably a substantial amount in this circumstance)

Conversly:

a 50 ton crane will have 5000# of lateral load at each wheel. Whole different ball game....

Does it seem rational that you would use the same "industry standard" clip spacing for a 5 ton crane compared to a 50 ton crane - Of course not!!!


Another converse:

Duty cycle should be consdered as well: Occasioanlly used cranes do not induce much fatigue into the rail connections, while heavily used and/or high speed cranes tend to cause rail connection problems - here it makes sense to add more connections (reduce spacing)

I've had converstions with major rail & connection device providers. The do not like violating "industry standards" either

You kow why? they get to sell more clips!!!!
and - they dont want to stick their necks out either. Runway system repairs are very costly.

 

[LSPSCAT]

FLCraneBuilder,

It can be a nice challenge to the "standards" to question, however, as you point out, runway repairs can be quite costly.

Additionally, clips are not exactly the money maker for these companies. I am not so sure on that line of thinking, especially, since this is contributes highly to the labor and fabrication cost. If I was building long rail systems for the best price, as a manufacturer, I would be looking to eliminate high quantity components which are labor intensive to install.

Last, sometimes we do not have a good handle on the numbers and we do may have much experience with these systems, so the "standard" should not be overlooked as it has stood for some test of time.

If these clips are spaced at 4.5 ft and there is a problem the first thing the Owner will point out is the industry standard; then you better have some pretty good arguments and mathematics to substantiate your engineering. Then if it is a problem of vibration or weld fatigue and cracks..now where do go? Its great to be a trendsetter and I am also playing a little devil's advocate, however, such judgements to push the envelope can be extremely costly.