Re-rate of very thin tank

[SEJohn]

I am a Structural Engineer doing foundation and anchorage design for the relocation of an existing tank. The existing tank is 14' dia x 44' tall. Material is unknown SS and wall thicknesses are 0.140", 0.109" and 0.078" from top to bottom. The thicknesses are very consistant, so I don't think there has been corrosion. There is no nameplate. These tank jobs come up every once in a while and I normally run API 650 chapter E calcs on the tank to verify seismic (I'm in seismic country) and to derive anchorage forces and do chair design. Being so thin, this tank is outside of API 650 and 620 and the upper portion is outside of 653. In my analysis I find that if the contents (Water) are held below 35' or so, the tank works (with stiffener rings for wind). I should also mention that the upper potion of the tank has quite a few dents that I would guess were either from a vacuum or from handling.
Questions: What standard allows the design of this thin of a tank? Would it be reasonable to hold the contents(water) down to a certain height (maybe install an overflow) and re-rate the tank for that height? Is there a standard for removing dents?
Thanks in advance for your response!

 

[JStephen]

Are those thicknesses the cylindrical shell or the bottom, the shell, and the roof?
I ask because, looking through UL-142, minimum thickness for stainless steel vertical tank is 0.158" for the bottom plate, 0.115" for the shell plate, and 0.086" for the roof plate, which are not that different from your numbers.

So, that's one possibility, that it was built as a UL-142 tank.

I'm assuming the tank is a welded tank.
Otherwise, API-650 Appendix S was added a number of years ago. Prior to that, there really wasn't a good standard for stainless steel welded tanks of that size. There still isn't for municipal water tanks. In that case, it was likely built without compliance to any particular standard.

It would be reasonable to limit the overflow as described. The catch is, you don't have a good basis for doing so without having some knowledge of the appropriate shell stresses and joint efficiencies. If it was rural, not too critical, didn't need to meet any building code, just storing water, it'd be reasonable to use it as is, where is.

For seismic loading, you may find it difficult to add anchor chairs due to the thin steel.

On the dents, probably just as well to leave as is, working on them will turn big dents into smaller dents that may or may not be worth the trouble.

I'd probably pass on moving it if it needed to meet any kind of code at the new location.

 

[SEJohn]

JStephen, thank you for your response - it is very helpful. Thicknesses given are all wall thicknesses. Shell is welded. I am currently using the lowest API 650 SS stress values and a joint efficiency of 0.7. The tank has already been moved from one industrial plant to another and is currently waiting to be installed. We are in high seismic zone and the plant is in the middle of a city. The tank holds water that is used for dilution of process chemicals. Calculations will be submitted to the city for permit. Does this info change your response any? Thanks again.
Oh, and good point on anchor chairs. I hadn't gotten there yet, but I see that will be a challenge.