Sizing Caisson or Pier 5

[geterdone]

I need recommendation on how to calculate height and diameter of steel reinforced concrete caisson/pier.

I have searched internet for formulas but to no avail.

I am a Mechanical Engr. assigned to fabricate a concrete pad to set a jet air craft on it. The two rear wheels (main gear)will place about 10,000 pounds (10 kips) on a 1 ft square area (each wheel). Front nose wheel will have a 3 kips load.

A 30 foot diameter concrete pad will sit under the jet. It will be 4 inches thick with 4000 psi concrete with fiber mesh.

Under the concrete pad there will be 3 separate concrete caissons/piers (nder each wheel).

I plan to isolate the concrete pad under the jet by saw cutting around the 3 caissons that support the wheels so that the main slab will float around each of the caissons/piers.

The soil is sandy (assumed firm dry clay with 8 ksf bearing capacity).

I will over-excavate (2 feet)of the native soil on the site directly under the jet and lay over this area 4-1/2 feet of crusher-fine compacted to 95%.

The caisson/pier under the main landing wheel with the 10 kip load will be 36 inches in diameter and 6 feet tall. It is wide because the exact location of the wheel load is not know (because wheel location might move 1 foot to either side due to other considerations). And the 6 foot tall caisson/pier is so that I have about 1-1/2 feet of caisson embedded in native soil.

Question, is there a formula for determining if I have sufficiently sized the caisson/pier?

Thanks

 

[JKStruct]

Are you basing your assumptions on a sealed geotechnical report? You said "Assumed 8ksf" soil bearing capacity. You shouldn't assume anything with regard to the native soil strata or capacity. The geotechnical investigation is the first and most important step.

Beyond that, I would highly suggest hiring a structural engineer to perform the foundation design.

 

[jike]

Why not just pour a slab thick enough to support the jet without the caissons? After all won't the jet need to roll over the slab to position it on the caissons? I assume the jet will not just land on top of the caissons or will a crane lift it into place?

 

[msucog]

i'm with jike unless you've got wicked uplift and you could probably still handle that with the mass of the slab/mat or even large footings. either way, let the geo and structural take care of that. with all crushed aggregate, you should be able to take advantage of some of that material for support. you might end up saving a little money depending on how all the loads end up sizing the slab/mat or footings. by the way, why the need for 8ksf (10k/7sf)? also i'm picturing that you're going to dig down 4.5' over the area + 1.5' in to natural ground for the caisson/ftg, put in a sonotube, backfill a little, pour the concrete, then finish backfilling.? you'd probably end up better supporting the footings in the aggregate layer. i'd look to an engineer to see if they can't get you to a thickened, reinforced slab with integrated column footings sitting on top of the aggregate...there again depending on uplift.

 

[BigH]

geterdone - remember, you shouldn't be designing something unless you have the expertise to do so. Find a geotechnical engineer and have the correct recommendations. (The first maxim of a Professional Engineer)
8 ksf is pretty high for most structures unless you are on dense granular soils. A 4 inch slab is pretty thin. Is this for "show" - i.e., a memorial - so that the jet won't move once it is set?

 

[shin25]

10,000 lbs of force from the wheel does not seem to be too high. I think a 9" thick concrete pad with temp/shrink. reinforcing should be more than enough.

 

[oldrunner]

Don't expect the fiber mesh to do much for you.
You need a geotech as a minimum to provide you with whatever soil bearing parameters are required.

Then you need a structural engineer to design the pad. 10 kips of dead load of the wheels plus whatever an impact load of the landing might be. (small oops crash!)

I don't interpret your problem as a "caisson" or "pier". You may be just designing a round structural slab. There are recommendations in the ACI 617-58.