existing slab fot tank support 1

[bhart192]

we have a existing warehouse slab d=8" 4000 f'c , someone decided to put #4's @ 10" c-c on top of slab no bottom steel ( ?) , we ran some numbers and came up with bottom surface concrete tension @ 110 psi looking for allowable concrete tension I seem to remember 75 psi , looking for reference. Thanks Bob

 

[txeng91]

I would avoid counting on the concrete to provide any tensile capacity. How much higher is the weight of the tank compared to the soil bearing? I would check to see what the moment capacity of the slab is counting on the rebar as your tension member, even if it is 2" from the top you still have the same capacity as you would for a 4" slab. If it is not enough, I would consider having them sawcut the slab and install a footing.

 

[hokie66]

You said you have run some numbers, but what assumptions did you make? An 8" slab, reinforced or not, will take a lot of load. Think highways. What are the dimensions of your tank, and how much does it weigh?

 

[bhart192]

25K total tank & contents wgt. 4 legs 10' c-c with 12" sq pads I think the slab works as is

 

[MotorCity]

The #4 @ 10" is most likely for shrinkage. You can calculate the tensile capacity of concrete by using the modulus of rupture which depends on f'c. If your slab is not reinforced or does not contain the minimum required reinforcement, you must check it as plain concrete. If it is plain concrete, you must subtract 2" from the nominal slab thickness to allow for an uneven or variations in the subgrade. You should also check punching shear of the slab for your particular situation.

 

[txeng91]

25K/4 = 6.25 kips per leg. Assuming a soil bearing capacity of 1500 psf, you would only need just over a 2'x2' footing to spread the load out enough to equal the soil bearing capacity. This means slab would need to spread the load out about 6" on each side of the tank pads, which is less than the depth of the slab. This will induce very low, if any, bending moments in the slab, so I don't think it will be an issue.

 

[bhart192]

that's basically what we looked at thanks for the info.

 

[dik]

I usually spec 1-1/2 to 2" top cover for SOG construction. Visible cracking occurs on the top surface. As far as modulus of rupture, for pavement design, it varies from about 8*sqrt(f'c). I recall that the Airport Pavement design publication has it closer to 9... Your load is static and not subject to fatigue and can be reasonably determined to be maximum... your 120 psi is small for flexural tension. If founded on good material, there should be little issue. Are any of the leg loads near a sawcut joint? Could be a little problem to address.

Dik

 

[Nor Cal SE]

I like DNLV's most recent post. You will get some spreading of load. Typically I've seen 45-degree slope as an acceptable assumption for load spreading (whether reinforced or not). And as DNLV notes, you can have less than 45-degree slope spreading to achieve a 2'x2' area and 1500 psf on the soil, which is a conservative allowable soil bearing pressure.

Your reinforcing ratio is 0.0025, well over ACI minimum for temperature steel.

As MotorCity notes, it would be good to check concrete punching shear capacity due to the point load. Your load is not extreme and I expect that check to be adequate.