How Do I Calculate Req'd Concrete Thickness?

Gah. Someone trying to do their own structural engineering. Okay. Here's what you don't know: You need to know the bearing capacity of the soil (ground) you are putting the footing (not a pedestal) on. That will tell you how wide the footing should be. I can tell you right now that six inches thick ain't gonna do it. You'll need at least twelve inches thick and that's with steel reinforcing bars in the footing to take the tension put on the bottom of the footing.

You haven't said where in the world you are doing this, or what type of soil you're bearing on, so we can't even take an educated guess as to bearing capacity.

My advice? Find your neighborhood structural engineer and buy him lunch and tell him what you're trying to do.

The structural load and the soil condition are the first to consider. In my area, much of the land is part of a mud flat called "prairie". For most residential homes,a 2500psi concrete mud seal 20" to 44" is poured on top of the chalkline which lies beneath the mud flat. Usually a trench from 36" to 60" will hit the chalk. On top of the seal, a structural footing of min 3500psi continuous steel reinforced concrete can be poured and is usually poured with the slab as a continuous cast. If the chalkline is beyond the 60" depth, I would remove all the dirt from the site to a depth of 60" and then build a pad from approved soil. The pad should be constructed in lifts not to exceed 12" and each lift should be compacted and inspected to 95%. The pad perimeter should exceed the building perimeter by min 60". In this case you will need only a mechanical footing. My advise to you would be to hire your local soil and geologic engineer for your situation or consult builders that have constructed in that area. Good luck

The architect is right, if you do not know the soil capacity that is under the column you will be in big trouble. I have seem a grain storage silo fall over when one of it's legs punched through a weak 4" slab, and had to help clean up about a 1000 bushels of wheat (55,000 LB). Take the time or find someone who knows and carefully think through where and how all the load gets transmitted.

Also, reinforced concrete is a composite structure. Do you really think you are up to this design?

"A man has got to know his limitations." Clint Eastwood as Dirty Harry Callahan.

Hello, I am a Civil Engineer in California. To a degree the Architect is correct you should employ a qualified civil engineer specializing if structural foundation design and/or one specializing in geotechnical for the bearing capacity and the settlement. Actually frequently the bearing capacity is not the problem, but rather the settlement will be your limiting factor. You will frequently have acceptable bearing with excessive settlement for a given column load. These have to be evaluated based on the depth of influence of the foundation and the type of soils underlying the column (fine-grained soils are analyzed differently from coarse-grained). Other factors can also effect the design, like freeze depth, shrink-swell, soil chemistry, etc., depending on the application, soils, etc.. Acceptable bearing capacities published by a geotechnical engineer will typically run from 1 to 3 ksf, higher capacities are rare and take special investigations intended to specifically evaluate for such.

Ried's tipping silo is an instructive story. I have another: A friend's lovely California summer house (circa 1960) had a massive three story rock and concrete fireplace about 6 by 12 feet in cross ection. We noticed that the wall and roof framing around the chimney had distorted over the years. The chimney was well built and perfectly vertical. We finally realized that the massive thing was sinking straight down into subsoil. The builder must have used a rule of thumb to size the footing footprint. While okay for solid soil and small chimney, it wasn't nearly wide enough to carry the 10-15 tons of rock in the chimney especially since he had built it in natural drainage filled with topsoil.

Fireplace -

That's amazing. I wonder how an engineer would go about stablizing such a monster. I suppose it would require big drilled piers at the corners tied into the existing footing with multiple rebar in epoxy. This assumes that the existing footing is thick enough to transfer the load to the piers. Measure twice, cut once.

:)

They joke this way:

Doctor's mistake gets buried 4 feet below.

Engineer's --- Creep down over the years.

We all live in a thumb-rules world - don't we?

That is "joie-de-vivre" .

If everything would have been perfect- you would never RENEW!

So build as per Codes-and you are likely to be "Don't Worry--Be Happy"

I don't know what this thing is supporting but the examples that were offered illustrate that if you mess this up someone could get hurt. Nearly 30k load is significant and the overturning moment adds to the complexity of the design.

Don't do home surgery and don't try to do this yourself.