Assistance verifying proper thickness of concrete tank pad. 2

[MTUJeeper]

Hey everyone, I'm a Mechanical Engineer and do not have experience with slabs. I've done a lot of digging around the internet and can't find anywhere that says how to calculate required slab thickness for a pad. We have an ammonia tank (77,000 lbs) mounted on an 8" thick slab. I haven't taken overall dimensions to calculate the pressure on the soil yet, but that will be simple enough. I need to explain to our plant manager as to whether or not the pad is properly suited for our application, and why we need 8" thick. The tank is a horizontal tank mounted on a skid constructed of two C-Channels 3" wide running the length of the tank. The contact length is roughly 333" long, giving a contact pressure on the cement of roughly 38.5 PSI. So pretty much I need to know how to calculate the required thickness of the pad to verify proper construction of ours. My initial thoughts were to just do a 2D load analysis on it using the stiffness value of the concrete on both the long and short sides, but I figured you guys may have something more specific to this application. Any help would be appreciated.

 

[JedClampett]

The pad needs to;
1) Meet punching shear criteria.
2) Distribute the tank load over a sufficient area to meet soil loads. The slab then needs to meet bending allowables for the soil pressure pushing up.
3) Be thick enough to develop any anchorage hardware.
I'm not sure if your plant manager thinks the pad is too thick or too thin, but for a design like this, I would never use less than an 8 inch slab. It's likely to work for the parameters above. Slab on grade concrete is very cheap. It's no place to economize.
If you have any strucutral engineers you could reach out to, they can explain the calculations. They're pretty simple, but there's no way to explain them in a forum like this.

 

[MTUJeeper]

Thank you, at least I have a direction to head! Is there anywhere you know of that has this sort of info? #3 is easy as there is no anchoring hardware. #2 is easy enough if there is somewhere that lists the soil pressures/max loads. #1 I would need to find equations for if you know of any place that would have these online. I don't know why he is having me figure this out, he's kind of reserved about stuff like that. We do have insurance doing an inspection of our plant, so it may have something to do with that. I can see about giving one of my buddies that graduated with a civil degree a call if there are no resources online.

 

[MTUJeeper]

I forgot to add that there is a layer of insulation underneath the concrete. Does that affect calculations at all?

 

[JedClampett]

The insulation is not a factor as long as it can support the slab load.
Get a Foundation Engineering book. I have an old one "Foundation Engineering" by Peck, Hanson and Thornburn. Every foundation book is going to have an example of isolated footing with a column on it. Just think of the tank as a very large column. In my reference, it's DP 23-1 on sheet 380. The punching shear and bending is probably going to be OK. I scanned in the example, but there's a lot of code knowledge that goes into it. Plus the fact that this is based on the ACI 318-71, which is 40 years old.