Underground Structures

[LIGWY]

I am looking for a reference book for design of underground structures. I am looking at a decant structure that will eventually be 50' tall. The dimensions are 6'square.

 

[SETast]

Use the PCA manual for Rectangular Concrete Tanks. It gives table values for aspect ratios based on depth. If your structure is 50' tall how much is below grade and below water table. For a 6'x6' structure you will have very little bending moment and need to look at shear.

 

[LIGWY]

It will be 40' in the ground. I have a soils report for the loading of the soil and water. I just ordered the PCA manual. Thank you.

 

[LIGWY]

I am looking at the PCA manual and my main concern is that all the tables are for b/a or width/depth for up to 0.5 my ration is 6/40= 0.15. Is there a recomendation when looking at the moment charts. My other question is the baseis of the conditions being hinged or fixed. How do you determine how to model the tank?

 

[LIGWY]

The structure will initally only be 12' tall with 10' all will be below grade with water. As they build up the "pond" the structre will be increased in 10' lifts until it reaches the maximum height of 40'. As I look at it the Horizontal bending moment as 483 in-kip and therefore my horizontal steel is 0.98sqin. This seems high.

 

[JedClampett]

I'm not clear on your exact structure. But your steel is dependent on the wall thickness. If you're using a 12 inch wall, you're going to have a shear problem. I think you need 18 inch thick walls.
The good news is that with 18 inch thick walls, you can use a lot less steel.

 

[LIGWY]

It is a decant structure that will eventually be 40' tall and 6x6 square. There are 18'x18" openings on the structure to allow the tailings to cme in. When I did the analysis referencing the PCA rectangular tanks. There was more of a concern with the bending momoent than the shear. I assumed the structure was fixed at the bottom and free at the top.

 

[LIGWY]

sorry it is an 18"x18" opening

 

[SETast]

The PCA tables are based on two-way slab action for aspect ratios between 0.5 and 3.0. When less than 0.5 or more than 3.0 you have one-way action. So if you tank is 40' deep with 6'x6' walls you can analyze the structure using a triangular soil load distribution plus the hydrostatic loading usually 90 psf/ft of depth.

If you use the table values for 0.5 single wall you will be overly conservative and end up with a very thick wall with lots of rebar because you are assuming your horizontal wall span will be 40'*0.5=20'

You need to look at 1' wide strips at desired locations and determine the uniform load on your horizontal span. If you look at Case 3 tables with fixed base and walls with b/a=0.5 you will see that the maximum bending moment My occurs at the corners at about 0.3a so to be conservative you could take a 1' strip at 0.25a or 0.75*40'=30' from the top.

Personally with a 6' wall span you might consider the walls as hinged, i.e. simply supported and design for My+ and Vu but with a 6' span. At 90psf/ft depth and 30' you will have a uniform load of 2700 lbs/ft. Mw = wl^2/8 yields 12.15 ft kips, Mu = 1.3*1.7*Mw = 26.8 ft kips and Vu = 1.7*w*l/2 = 13.8 kips It looks like 16" walls with #6 horiz @8" ctrs or 18" wall with #6 horis @12" ctrs would work.

You can step your wall thickness down to 12" as they build the structure in the future but no less than 12" and not less than #5's at 12" ctrs in any direction.

 

[LIGWY]

Thank you so much, I have been reviewing the codes and the PCA book to assure that I can explain the steel. You have come up with almost the same thing I did. I just know the client is not expecting this much steel. They were upset when I told them the wall had to be a minimum of 12" thick walls. One more question, I understand the code states double reinforcement is required in the wall because of the height and the loading that may occur on both sides. Am I understanding this correctly?

 

[civilperson]

Yes, two layers of reinforcement are recommended.