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#### beetlebomb

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I'm going to pour a concrete slab 10' X 15'. Its purpose is to provide a secure pad for a rectangular shaped 3000 pound water boiler that will be holding 400 gallons of water(3,340 lbs) max. while boiling it off. The question has to do with thickness of the pour, rebar size and lay pattern. Three thousand pounds of the unit plus 3,340 pounds of water... let's round the maximum total weight the pad will ever need to support, up to 7,000 lbs.

So how thick should the pad be, what size rebar size and lay pattern would be necessary to support the given maximum weight, or how would I calculate it?

#### Daniel Holzman

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You would calculate the required thickness and rebar spacing based on the most restrictive failure condition. In your case, you need to evaluate diagonal shear in the concrete, punching shear through the concrete, and direct compression failure of the concrete. Since it is a slab, you also need to look at soil bearing capacity, and the potential for tension failure of the slab if voids develop under the slab and the concrete needs to bridge the gap.

The actual loading depends on the specific details of the water tank support, i.e. is it on feet, or is it in a cradle, or is it supported some other way.

By the way, slab design for heavy loads is what structural engineers get paid to do, but if you want to do it yourself, those are some of the design parameters you need to look at.

#### concretemasonry

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Depending on the size of the heavy equipment, you may be better off supporting it on a separate foundation (different thickness and amount of reinforcement than the rest of the slab) and locate it to create joints in the normal slab to prevent cracking. - It all depends on where the legs (or loads are).

The alternate is to over-build the entire slab and place the load anywhere you want to. A 10x15 slab at 6" thick only takes 3 yards of concrete, which is the minimum in most areas of the world.

The 7000# is not that much load if it and its location are defined. For that load, you only need about 4-6 square feet maximum with decent soil.

It is difficult to save even pennies on a small job.

Dick

#### beetlebomb

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Thanks for the guidance. Based on the responses, here’s a bit more info. The unit is about 6’ X 4’. It does not have legs. It rests on three metal channels 3” X 4” X 4’; one at each end and one in the center. The pad will be poured on a prepared bed of compacted gravel and dirt. The soil is hard and clay like.

The unit will be bolted to the cement pad. The location is California so extreme cold is not a factor. Seven thousand pounds is not a lot of weight in this application. The average half ton pick-up truck has a curb weight of about 10,000 pounds. Given the purpose of the subject pad, it will not have any vehicles driving on and off. Once the unit is put in place, the only activity will be a human or two for servicing and maintenance.

I don’t mind paying an engineer to calculate the load requirement. It seems, however, that a 6” thick pad on a compacted base with crisscross (horizontal and vertical) ½” rebar spaced 1 foot apart, should be more than sufficient. Whadayathink?

#### jklingel

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A new Ford 150 pick-up truck weighs about 11,500 pounds.
Typo, or are you thinking of an F650 (maybe)? An F150 is more like 6500 lbs. That aside, overkill like you plan to, and compact gravel under the slab, as per code in your area. I don't see where you need vertical rebar, but N/S and E/W ought to hold an Abrams tank. I was going to install a boiler that holds 1400 gallons, and the engineer said my 4" slab w/ #4 N/S and E/W, 12" OC, would be fine (boiler was on two, 6" wide skids the whole 10' length).

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