Use of 6" x 4" x ?" Aluminum tube for heavy equipment cribbing 1

[lbelrod]

We typically use 12 x 12" oak timbers as cribbing for heavy equipment maintenance. Typical point load weights = 20,000 lbs . Would like to use aluminum tube instead of the oak due to fire code/ fire loading issues.

Assuming 6" x 4" tube, trying to determine thickness required to support 20,000 lbs point load . Assuming 6" flat on floor, I would treat each 4" vertical leg as a column. Assuming 1/2" wall thickness, compressive stress = 20000/(0.5 x 4") = 10000 psi << 0.6Fy = 0.6 (21000) = 12,600 for T6061 T6 aluminum ... OK ... Agree with this logic ??? Thanks..

 

[JAE]

I think there might be bending in the two vertical tube wall/legs. Depends on the type of wheel/contact.

What sort of point load is it?

 

[ash060]

I would check it for web crippling. Check ADM J.8.1.

 

[MikeHalloran]

Cribbing is a general purpose support, usually installed by unskilled labor without engineering assistance. Wood is kind of a natural choice for that. Maybe you could apply some fire retardant to the wood.

I'd be hesitant to use tubing of any material as cribbing, because there's always some point load that can buckle the walls.

I would prefer oak to aluminum for cribbing, because in a fire, charred oak will be there long after the aluminum has softened, buckled, then melted and pooled. Take a long look at the mechanical properties for aluminum at even slightly elevated temperatures.

You should also talk to your insurance company; they may have some input.



Mike Halloran
Pembroke Pines, FL, USA

 

[PUEngineer]

Fill the tubes with concrete, that should help with the buckling. Might be a little heavier.

 

[JStephen]

That line: 20000/(0.5 x 4") I don't see where it comes from.
You haven't defined what a "point" load is, typically has to be over some finite area.
A tube could conceivably flatten to one side at lower loads than what column failure would indicate, depending on the lateral load applied.

 

[lbelrod]

Thanks for the responses --- we typically use oak timbers but they are trying to eliminate because of NFPA fire loading issues. I dont typically do alumimum design and obviously need to review the Al Design Manual. Thanks again...

 

[dhengr]

The alum. tubes will be very expensive, and the radiused corners on the tubes seriously complicate the transmission of large compressive loads, over some length of tube, from the 6" horiz. surface to the 4" vert. leg, or legs, once the first leg starts to yield or buckle. I understand the 20kips of load, but on what size footprint? Are there three or four of these bearing points under the machine? If the shop floor is irregular do you induce bending in the tube, which might add to the failure mechanism. I’ll bet you can buy a lot of fire retardant treated oak 12 x 12 timbers for the same cost as the alum. tubing. While I don’t discount the significance of fire safety, a few oak 12 x 12's in the shop shouldn’t be worse than the railroad track ties in your plant, or the few sticks of more combustible stuff at the lumber yard down the street. Tell your welders to be careful, and keep some water around. For as long as mill wrights have been in business they have liked at least one layer of oak timber under the equipment, to take up any irregularities and allow some wood crushing instead of damage to the machine. How high is this cribbing? Build the rest of it out of non-burning blocks of some material. Build steel stands to take the 20k load, of sufficient size and design to be stable and put 4"x8" oak planks on top of the stands.

 

[Ron]

agree with dhengr...

Further, the stiffnesses of the two comparatives are nowhere near equivalent. The oak wins.

 

[racookpe1978]

More important, as cribbing, the inherent flexibility and stability of the oak timbers is much, much safer than the very rigid, non-compressible Al (or steel) tube steel.

A slight irregularity on the piece, the floor, the rigging, or the machine forging doesn't distort, scar or damage the (very expensive) part you are supporting. The wood compresses and accepts the change "gracefully". Also, a 12 x 12 or 16 x 16 timiber is very stable against tipping over if bumped or jarred while rigging, or if the load does not land exactly evenly when coming down.

12 x 12 (or 16 x 16) is NOT fireproof per se, but will not burn under any normal circumstance or normal ignition sources. Forest fires, for example, burn right past this kind tree trunk, leaving them only marked on the outside bark. (Then again, when has somebody citing cribbing as a "fire danger" been considering time-until-ignition?)