Water Tank Foundation Design Help

[Kevin Lu]

Dear Professional Engineers,
I'm a student intern at a medium sized design and build firm.
My boss gives me another task on Outdoor Fire water tank foundation calculation.
My boss already hired a structure engineer to design the tank foundation, but seems like my boss doesn't like that version. (Design attached)
The tank is 185,700 Gallons 33' 0"D x 30'0"H 3308D Corrugated Steel Water Tank with Dome Roof.
Where can I start with?
I haven't learn anything on Anchors and tank foudnations at school.
I read the IFC (international Fire Code Sections 5704.2.9.1 through 5704.2.9.7.9, and NFPA 30 Chapter 22.5. But none of them provide me calculation steps or any equations. The NFPA 30 direct me to IBC code, but I didn't find anything there either.
Can you give me any hint to start with? Or any code section name to read?
Thanks,

 

[JedClampett]

This a weird one. Why is your boss giving a messy design issue to an intern? Is it just to keep you busy? Is he expecting a big savings if you do it?
You're not going to find code direction for this. It's statics, but not even regular statics. The tank rings should take all the stresses. The anchors only take wind and seismic, which are small. I'd get the hoop stresses from a portion of the bottom tank ring. How much? Who knows? Put in the anchor bolts to take the hoop stresses in shear. The good news is that anchors in shear can be designed per ACI 318. Then use that portion of the hoop stresses to design the reinforcing in tension. Use a low allowable (16,000 psi), as all that tension will like to crack the concrete.
Hopefully, your design is three times the other guys, and your boss will stop delegating inappropriate work to interns.
PCA has a publication called "Circular Concrete Tanks without Prestressing". it's apples and oranges, but maybe it can get you started.

 

[Kevin Lu]

Thanks for your reply, Mr. JedClampett.
I just talked with my boss about the workload and my struggle. (The comany is actually a small firm, I'm the first structural aspect student intern to them)
He said that I missunderstanded what he said.
He want me to calculate the estimated load quantity of the tank (full-filled) and based on the structure engineer's drawing to design a belled-caisson foudnation system. (illustration attached)

Should I start with the tank live and dead load first? Then start looking on the concrete slab thickness?
btw, how to design the this slab? Use ACI 318-19 Chapter 13? or other Building Code?

 

[TigerGuy]

Do you have a geotechnical report on the project? Sorry, I have to plug for geotechnical investigations

The geotechnical report should provide a net bearing capacity for the caissons. From there, it's just a matter of calculating what diameter the bells need to be based on the load from the tank and contents. Remember the bells need to have some clearance from each other (typically 2.5 diameters, center to center).

Sounds like you landed in a great learning environment, the more difficult the challenge, the more you learn and remember. Not knowing your boss, I am hoping he is available to help guide and mentor you through the process, while letting you struggle a bit to figure things out.

 

[Kevin Lu]

Thanks for your reply,
I do have the geotech report in my hand,
As written in the report, the caisson has a recommend depth of 12 ft under ground level and allowable bearing of 4500 psf (for live and dead load).
There is a recommendation of 3 ft soil backfill. Since the surface soil is not good for big load, and the deep soil type is clay.

 

[fel3]

Have a look at PIP STE 030202, "Guidelines for Tank Foundation Designs" (see attached).

Other relevant documents and websites include:
-- ASCE 7, "Minimum Design Loads and Associated Criteria for Buildings and Other Structures." I use the 2022 edition, but the 2016 version is still being used by some agencies and some engineers.
-- ACI 318, "Building Code Requirements for Structural Concrete."
-- Your local building code (for me it's the California Building Code).
-- Your project's geotechnical report.
-- Even though your tank is corrugated, you will find these two AWWA water tank standards to be useful: AWWA D-100-21, "Welded Carbon Steel Tanks for Water Storage," and AWWA D-103-19, "Factory-Coated Bolted Carbon Steel Tanks for Water Storage." Both of these standards include a procedure for estimating the sloshing wave height due to seismic activity and from that determining the required minimum freeboard. I don't know if there is a corrugated tank standard because I have never designed a corrugated tanks.
-- ASCE Hazard Tool,

https://asce7hazardtool.online/

This is where you get some of the data needed for the freeboard calculation.

Please note that your tank is relatively tall (30' vs a diameter of 33'), so the wind load while empty might require anchorage. If you tank is in a high seismic area, it will certainly require anchorage. In my neck of the woods (California), I try to avoid relatively tall tanks so I can avoid anchorage. All but one of my tanks have had a diameter that was at least twice the height. My tallest tank (43' dia x 32' tall) had to be anchored.

***EDIT*** I should have read your subsequent posts. My post no longer applies to your situation. However, if you ever need to design a tank ringwall foundation, my post will get you started.
============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[Kevin Lu]

Thank you Fel3,
I'll start looking on the PIP STE 030202 guidelines, AWWA D-100-21, and AWWA D-103-19. I have ASCE 7 and ACI 318-19(22) in my hand. Which specification section would you recommend me to look?
Thanks again for your tips.