Structural needs help w/ WR type problem

[TXengrChickPE]

So, I have this 8000 gallon elevated horizontal tank that is supported on 2 concrete piers. The tank consists of 2 4ft diameter, ~50ft long vessels that are stacked on top of each other. I designed the piers and footings, but now I've been informed that we need to provide secondary containment for the tank. We need to be able to contain at least 50% of the tank volume so that the liquid does not pond under the tank. The other requirements are that the entire containment area must be able to contain 50% of the tank volume + 10 inches of rain OR 110% of the tank volume (higher volume controls).

My slab around the piers will be at grade. So, for the 50% without ponding, I laid out a 58'x10'x1'deep pit (580 cubic ft > 4000 gal --> 535 cubic feet). Then, for the 10" of rain, I said that the wall around the entire containment area has to be a minimum of 10" (we'll probably use 1') above grade. It seems to me that this is all I need to do... Am I missing something? It seems to easy.

Oh, I forgot to mention, the slab at grade will be sloped toward the pit, and the pit will be sloped toward a small sump pit in one corner where a portable pump can be placed as needed.

Thanks.

 

[TXengrChickPE]

110% of tank volume = 8800 gallons or 1177 cf50% of tank + 10 inchs = 4000 gallons + (.833*58*10) = 535 + 483 = 1018 cf

You need to use 110% of tank volume, according to what you said above.

1177 cf / (58*10) = 2.03 feet. You should probably design your basin with the following dimensions:

58ft x 10ft x 2.25ft

I must not have made the situation clear... the 50% has to be stored so that it is not directly under the tank. The entire area is actually 58'x22' where 12' of the 22' is the width of the tank + 4' walkway around the tank. The remaining 10' is the portion that is at a lower elevation that will hold 50% of the tank contents away from the tank. I've attached a sketch (I hope) to sort of show what I'm talking about.



So, the 58'x10'x1' portion contains 50% of the tank, the wall around the entire area should just need to contain the rain, right?

 

[TXengrChickPE]

it looks like it worked, at least from my end, but the text is a little fuzzy.

The text is, from top to bottom,

----------------------------Tank centerline----------------------------

EL. 100.0ft

storage for 50% of tank volume, EL 99.0ft

The dimensions are 12', 10', and 58'

 

[maryannette]

This is very typical of a lot of the problems I saw on the PE in April. It's not really a WR problem. It's a geometry/volume problem that needs some common sense to set it up. Knowledge of WR helps because it makes it easier to interpret the information, but it's really just solid geometry. I was amazed at the number of problems that could have been solved by a sharp high school student. Word problems--learn how to interpret and translate them into simple calculation.

 

[TXengrChickPE]

Believe it or not, this was something that I had to do at work... and my boss kept insisting that the 1' wall would not contain the 10" rainfall because the runoff from the tank would increase the amount of rain going into the containment area. I kept saying that the tank will not increase the amount of rain falling in a given area, and since the footprint of the tank fits within the containment, any rain falling on the tank would have fallen on the containment area anyway. He finally agreed (after doing a bunch of hand calculations that involved precise calculations of the slab slope). I just kept saying "It's conservative to assume that the slab is flat". I don't think my boss likes that I can visualize stuff in 3-D and solve problems just using common sense instead of calculating everything. Oh well, he's the boss, at least til I get my LICENSE!!!