Mobile Crane Outrigger Pad Design 3

[ekg517]

I'm designing a wood pad to support an outrigger(24" dia) load of about 120 kips. The soil is pretty weak, about 1000 psf bearing capacity (wood mill environment). This is my first time doing this type of design, and would appreciate any resource/help out there.

Thanks,
Kweku

 

[InDepth]

Locate at Shapiro's book on Crane's and Derricks. They have some examples I believe. I also suggest nailing plywood on top of the wood 12x12 to keep them from rolling over and seperating. I have also seen large/thick steel plates laid down.

 

[InDepth]

Oh, and consult your geotech...your limit on bearing should be based on settlement, not force. You don't want the outrigger to be settling while it is load...keep that crane upright!

 

[ekg517]

Those were very great suggestions, and proved very insightful. The question still hovering in my mind is in regards to the ability of the plywood sheathing to effectively distribute the load evenly to the lumbers/timbers below, especially where the outrigger mat is pretty big, say 10' or 11' square. It seems like a grey area to me.

 

[DTGT2002]

For me, with loads of this size, I would assume you will have many alternating layers of timbers to make your mat. While you may have a large square or rectangular block, the bulk of the work done to engage your soil below will be done with a pyramid shaped bearing "wedge" extending through the layers. Knowing the wood members will not have sufficent bending strength, I make the assumption loads are carried in bearing and with each applied load, the width of bearing widens by the depth of the wood member.

The sheathing serves only to stabilize the adjacent blocking and prevent rolling of members that have a high height to depth ratio.


So, to engage your 10'x12' nominal bearing area, you would need quite a bit of stacked material. I generally consider the "foot" of the outrigger to be capable of engaging its full footprint for the initial load transfer and let my loads transfer from there.


Going by this logic, you need quite a pile of blocking - so must you might not be able to get the outriggers to sit on top without walking the crane up by raising and lowering opposing legs.

Is the 1000 psf a general site condition or might their be some relief and higher capacity exactly where you need to set up?


Daniel

 

[ekg517]

Daniel:
Thanks for your input. The 1000 psf is part of Compliance Standard by the owner/client, to avoid the need for further testing (OSHA Regulations). The site being a timber mill with potential decayed organic material present, I guess the owner is being a little conservative for those two main reasons.

Thanks,
Kweku

 

[InDepth]

ekg, Although messy, if you really need it, you can also put down a mudslab and a gravel base in the crane path to spread the load out beneath.

If the crane is stationary for a long period of time you can also bury a steel Wx in the ground to give a larger bearing area and get to good soil....or go the small micropile or jet grout column route (expensive)....or pour a concrete mat.

 

[DTGT2002]

You beat me to it InDepth, the mud mat was my next thought. If you need a thinner way to gain area, a reinforced concrete slab would be the way to go. If there are any relationships with nearby ready mix suppliers, it might be the way to go.

 

[InDepth]

Rock on DTGT2002