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Participant

Join Date: Jan 2010
Posts: 1

Concrete Slab Thickness

01/28/2010 8:40 PM

How do I calculate the required slab thickness,size or rebar, thickness of gravel lift to accomodate 24,500lbs on a 49ft2 area (3000gal 95" round water tank)? This app is for a commercial greenhouse water tank. The soil has a heavy clay contact with random rock outcroppings. Thank you, in advance, for any advice.

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Guru

Join Date: May 2009
Location: Red Hook, New York (Mid-Hudson River Valley)
Posts: 4364
#1

Re: Concrete Slab Thickness

01/29/2010 2:46 AM

Hello develop1,

First, I believe you need to provide some additional information to us so we can steer you in the right direction, okay?

Second, I think you had better recheck your math, as a gallon of water at 60 degrees F weights 8.35 pounds. Therefore, the weight of 3000 gallons of water equals 25,050 pounds. To design the slab, you'll also need to know the Tare weight of the storage tank and add it to the weight of your stored water.

BTW, your slab area of 49 SF that you provided appears to be too small to support your superimposed load. Are you planning on constructing a square or round concrete pad?

If the tank is located outdoors, and depending on your climate, you must include in your design wind loading as well as ice buildup and snow load on the tank roof (if it has a roof). What is the overall tank height? Is there tank overflow piping included, and if so, where does it discharge to? Also, is the water level in the tank maintained either manually or by automatic shutoff using the correct valves and controls? Either way, most likely you'll have to provide an underground valve and metering pit which will greatly effect the design of your tank support pad. this would be especially true if you experience winter temperatures below freezing and your water source is municipal or a privately owned well as you'll need to protect those facilities from freezing.

Depending on where you're located, you may have to include into your design Seismic loadings due to anticipated earth quakes. Most US states now require this design provision be completed by the Design Engineer.

Has a geotechnical investigation been performed, with a Geotechnical Engineering Report generated as a result of the findings of a Licensed Professional Engineer? I would be very concerned about the heavy clay content of the soil, together with all other soils encountered at the tank site w/ soil and rock characteristics & classifications thoroughly identified, and properties provided. Also, what type of rock is encountered on the tank site, extents, randomness of outcroppings, any identified fissures, stratification, depths encountered and its allowable bearing pressure?

I suggest that you obtain the services of a qualified Licensed Professional Engineer that specializes in both Civil and Structural Engineering. Also, You'll need to hire a licensed Geotechnical Engineer and a Materials Testing Agency.

Sorry, we cannot perform the actual design of your water storage tank foundation here in the CR4 Forum. We can only provide engineering advice.

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Guru

Join Date: Oct 2008
Location: I'm outa here
Posts: 1924
#3

Re: Concrete Slab Thickness

01/30/2010 12:17 AM

Captmoosie -- Good engineering answer. For a 3 million gallon tank, 96 feet in diameter.

I'm trying to keep from fits of ROFL's at your expense by surmising that you missed some decimal points here.

This guy has a 3000 gallon water tank!! 8 ft. diameter and 8 feet high. If he pays more than \$1200 for the new tank he's paying too much. He could hardly get a licensed engineer to talk to him for that price. The only possible reason he could need engineering on that project is because the local building officials won't give him a building permit without a PE seal on the plans. (And it's likely that someone's relative is the only engineer in the jurisdiction).

Out our way (7 miles from the hypocenter of the 1989 Loma Prieta, 7.0 magnitude) no permits at all are needed for tanks less than 5000 gallons. And our local county planning department is completely off the wall on the subject of earthquake safe construction. Over 5000 all they really care about is that you have the right setbacks and a 4" fire hose connection. Conventional practice is to level the site within 2 inches. Steel and redwood tanks are put on 4 " thickness of 1" gravel to forestall moisture buildup under the tank. Plastic tanks can go right on the leveled ground. 4" concrete slab standard driveway construction is all that's needed for tanks sitting on that kind of base.

It's recommended that tanks taller than their diameter be cable anchored to the ground at 3 or 4 points from ring lugs normally provided by the tank manufacturer at the top corner.

During the 89 LP event my full 5000 gallon steel tank moved laterally about 3 inches breaking the lower water connection and dumping it's contents. Because so many water tanks in my area had similar failures current practice is to use flexible connections for attached pipes at tank bottoms. This gets a bit pricey (like about \$300) for a very nice 24 inch long convoluted 4" stainless steel hose with NPT male couplings welded at each end for the fire hydrant. Lots less for smaller sizes. Pipes leading to the top of the tank will be inherently flexible due to their length if supported properly. If you don't live in earthquake country this flex pipe stuff is probably unnecessary although building strain relief configurations into threaded steel pipe setups is a always a wise practice.

BTW I'm putting in a new 3000 gallon plastic tank this year because my 30 year old 5000 gallon steel tank is beginning to show signs of corrosion perforation at several points due to my somewhat corrosive well water. It's still sitting where the earthquake moved it 20 years ago and serving as our only domestic water storage.

Ed Weldon

Anonymous Poster
#2

Re: Concrete Slab Thickness

01/29/2010 7:44 AM

Thank you.

4
Guru

Join Date: Oct 2008
Location: I'm outa here
Posts: 1924
#4

Re: Concrete Slab Thickness

01/30/2010 12:19 AM

Before you hire an engineer talk to your local planning/building code officials. Even if you need a building permit you probably don't need to submit engineered plans. Please see my reply above.

Ed Weldon

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