Tank Bottom Slope

[xema77]

I have two questions.
1) I am looking for some information on hillside tank bottoms (ie sloping across the diameter of the tank). Can anyone provide some information with respect to the topics below.
a) What are some guidelines as to the largest practical tank diameter that can have a hillside bottom?
b) Can this style of tank bottom be used on concrete ring walls? How is the difference in elevation from side to side accounted for?
c) Any guidelines as to the maximum slope?
d) Any other issues with using a hillside tank bottom?

2) For cone up bottoms, can the slope be as large as 1:50 or 1:12? What are the issues with using this large of slope?

Thanks

 

[IFRs]

I always called them "shovel bottoms". In any case, I don't see why there would be a size limitation. The foundation and bottom ring shell plates will be sloped. API 650 has a minimum slope (when a sloped bottom is required) but no maximum. The bottom has to be uniformly supported, that's all. Issues with steep slopes might be: floating roof legs, maintenance, column bases, needing to slope piping, etc. I have seen many different configurations of bottoms, they all worked!

 

[MJCronin]

xema77..

In the book "Aboveground Storage tanks" by Myers, the three bottom configurations described are "single slope", "cone up" and "cone down". There is little discussion in this handbook of single slope tanks.

I have also have encountered these "single slope bottom" tanks.... but it was in the pulp-and-paper industry.

My recollection is that they were all smaller (shop-built)tanks.

The shell extended down to the floor (a continuous slab was used for support) and inside the tank the flat-bottom was sloped. When the tanks were installed, the space between the bottom and the supporting floor was carefully filled with grout (some holes had to be drilled for grout placement and venting.

In my opinion, only smaller diameter tanks should even consider these bottoms. I do not see how a large diameter tank with a ring wall can be constucted such that the bottom is fully supported.

Because of the odd shell-to-bottom joint configuration, I also do not believe such tanks can comply with API-650...

My opinion only.....

Anyone else have experience on this topic ?/

-MJC

 

[IFRs]

I've seen larger ones - about 100 feet diameter. The ringwall foundation was sloped, the bottom shell course was cut on an angle in the shop. The tank was of course plumb. I seem to remember an 8" difference between the high and low sides. The tank was still API 650.

 

[JStephen]

API really doesn't address this type of construction. If you look at the stress where the shell meets the bottom, you can get a minor bit of uplift force due to the slope, similar to the situation when you have wind or seismic loading. I've seen some larger tanks done like that as IFRs describes, with I believe 12" of slope.

 

[jaydees]

I have had to design tanks with this type of slope on ethanol tanks. You have to make sure that the foundation is correct so that the shell and bottom are completely supported. The bottom of the bottom shell course has to be cut to conform to the slope and you have to make sure the field starts in the right place with locating these shell plates. You also have an issue of sliding with this type of tank. I always anchor a tank of this type no matter how small the slope.

 

[FrCivEng]

For large API 650 tank, 1,6 % cone up compacted backfill slope at construction phase in order to guarantee minimum 1 % slope at long term ; I feel that too much slope is more expensive (backfill, QA-QC, bottom plate, ect) and would be difficult to keep at long term due to product loading / offloading cycles
RC ring wall not mandatory (API 650) but recommended for large tanks, the ring function is to stiffen the foundation area under the shell and to distribute load and deflection, the foundation being in any case an earth foundation ; refer to API 650 Appendix B for typical cross sections