Conical Bottom with supports

[Schuppert]

I have a customer who wants a 28 foot dia. tank x 35 ft liquid level API 304 S.S. tank with a 148 degree included angle bottom head and a small 1-1/2" knuckle radius. He proposes supporting the tank with 8 legs around the outer perimeter. It would also have a rolled vertical C.S. ring at 5'-4" dia. with a top and bottom flanges. To that ring attaches (8) tapering "I" beams out to the outer legs. The smaller ring is also supported with (8) smaller pipe legs. My question is how these bottom stiffeners would reduce the required bottom plate thickness required by API-620 5.10.2.5. Any books with methodology for dealing with this subject would be greatly appreciated. Thank you for your consideration and tips.

 

[MJCronin]

Schup..

I would not rely on the answers that I got from this forum on what is obviously a complex, very wide, very marginal structure.

I belive that a 28 foot diameter is very, very large tank to have legs on and that your client has dreamed up a design that he wants others to take full responsibilty for.

I would guess that 90% of liquid storage tanks with coned bottoms and legs are below 14 ft diameter and you are at twice that...

Contact a long-established and very competent vessel/tank fabricator and start talking.

Perhaps changing the bottom to a fully supported design ( concrete pad ?) might be worth considering.

My opinion only

-MJC

 

[JStephen]

The ideal solution is to vary the form according to your design, rather than the other way around. Consider a skirt-supported tank, omit the knuckle, and omit the inner legs. You can then generally design per the methods in API-620.

API does not have any provision for cones supported on radial beams. In the leak-detection appendix, they have a formula for spacing of grillage members which could be applied, but it will give you more than 8 beams in this case.

Scallopped bottoms have been used on this kind of application, but they have their own design problems.

 

[himat42]

Schupert
Please try the following web site

http://www.kuleuven.be/bwk/materials/Teaching/master/toc.htm

Lecture 15C.1

 

[PEStructural]

I just had to redesign a 304 SS tank with a similar bottom (but much smaller diameter). They had used one central column with 6 radial beams spanning from the center column to the outside legs. Because these beams are taking the full weight of the product, they are going to be large. For my little 12'-0" x 16'-0" high tank, we had to use (6) C8x11.5 radial bottom members with (6) C6x8.2 rim beams. All of these beams were connected to (6) outer columns. The way I designed it was using STAAD.Pro and used the weight of the product (16'-0" x 62.4pcf = 998 psf for my tank) and designed the framework from there.

I basically designed the bottom to be fully supported by this structure and did not design it for membrane action. Normally, to design a cone bottom for membrane forces, you need an internal angle closer to 90 degrees.

You can also design the bottom plates spanning between the radial supports for the product load. It's kind of like designing a rafter supported roof, just upside-down.

Hope that helps.