Crane Loading 1

[Joe Grant]

Hi I am just curious about the loading from the footprint of a crane (track dimensions: 11m x 1.5m). When discussing the spreading of the loading from this crane through regular fill would it be standard to take it as a wheel load with a 30 degree spread angle against depth. Or because the dimesions are so large would it be more accurate to take it as an area load (udl) therefore the pressure will remain constant with depth?

Any advice is welcome

Thanks

 

[TehMightyEngineer]

If cranes are at max load they're usually controlled by either tipping of the crane or the strength of the hydraulics/equipment. In the load tables for the equipment (many of these can be found online) it should tell you which controls. In the past I've always assumed a uniform load distribution to tires/pads/tracks if it's not heavily loaded or controlled by other than tipping. If controlled by tipping I assume a triangular load distribution.

Yes, I've typically taken the wheel load as spreading out at 30 degrees for the soil pressure. I use actual track dimensions or calculated wheel contact areas.

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

http://www.americanconcrete.com

 

[Lomarandil]

Agreed, it's appropriate to consider load spreading (on most scales, perhaps not for a very thin layer of soil) -- either by a rule of thumb method, Bousinessq solution or the like.

A few cautions and general rules of thumb about crawler crane track load distributions (since they came up):
[ul]
[li]Most crawler cranes are designed so that about half the track width bears on hard surfaces. When used on soft surfaces, the crane sinks in to engage the full width of the track[/li]
[li]In the situation mentioned by TME with a heavy pick over the front, you'll often end up with a triangular distribution over some portion of the track length, but not the full length (often 1/2L to 2/3L)[/li]
[li]If the crane has a large counterweight and the boom is in tight with no load on the hook, the crane is unbalanced in the opposite direction. This happens alot while the crane is mobilizing around a site, and can have nearly as pronounced an effect for track loads[/li]
[li]When a crane is swinging over the side, you can typically count on a uniform load in each track, but different percentages of the weight in the two tracks -- 80% and 20% isn't unheard of[/li]
[/ul]

Crawlers with variable-position counterweights or super lift trailers perform better in this regard -- I've heard that you can get close to uniform loading with those, but not quite.

Also, I apply similar considerations for RT and Hydro cranes on tires or outriggers -- but I won't get into those here.

 

[JStephen]

Example of soil failure:

https://www.youtube.com/watch?v=xz71eVreMDc

 

[LSPSCAT]

I second and highlight Lomarandil's post:

*** If the crane has a large counterweight and the boom is in tight with no load on the hook, the crane is unbalanced in the opposite direction. This happens alot while the crane is mobilizing around a site, and can have nearly as pronounced an effect for track loads

It is imperative that this load condition be checked. This is many times the controlling load case for heavy lifts. Large crawlers that are fitted with backstay, loaded up with counterweight, and boomed up tight and unloaded generate very high ground bearing pressures with the triangular load distribution opposite of what you see when loaded.

 

[thaidavid40]

In addition to track pressures while mobile, are you needing to account for plate pressures on the outriggers when it is set up stationary and lifting?
Dave

Thaidavid

 

[Lomarandil]

Dave, (most?) cranes with tracks do not use outriggers (except during self-assembly).. at least not the heavy civil construction variety cranes.

 

[thaidavid40]

@Lomarandil,
I was thinking about the cranes used along the pier at NUCOR, in Decatur, Alabama. These do set up on outriggers. @Joe Grant didn't specify his crane usage in his post, so I offered my thoughts on speculation. Apparently these were off the mark for his application.

Thaidavid

 

[Lomarandil]

You could be entirely right. I don't know the usage either.

Do you happen to have a photo of the mentioned crane? I've not seen one like that. Sounds a little like a dock-side "whirly-bird" crane, but all of those I've seen have either been on rail or on wheels.

 

[thaidavid40]

Don't have any photos available. You might be able to see something on Google Earth, if you have the high resolution membership.
Dave

Thaidavid