false bottom design

[ird192]

my client has a 8' dia. SA240- 316 API 650 storage tank with what I call a false interior bottom , built in 1985 just started leaking in a crack in bottom plate , the floor is 12" above the bottom of the shell and is 1/4" thick with no grillage or ribbed support just flat plate welded to shell . the tank supports rest on load cells and the tank is loaded by weight . API 650 section 1.7 details grillage support design for a elevated floor. since the grillage or support system cannot touch the slab which would negate the load cell system I'm looking at supporting the floor plate with ribs of some design and attaching them the the shell wall , anyone point me in a direction for help ,the original drawings show tank designed to API 650 , I don't have a old code book but not sure if the unsupported floor met that standard but it work for 39 years .thanks

 

[JStephen]

Going off memory-
I don't think API-650 of 1985 included provisions for stainless tanks.
And I don't think it included rules for grillage-supported tanks, but not sure. I think that may have come in with the leak-detection requirements.
A cone bottom could be designed per API-620 if so desired (which did include SS).
And I think API-12F has provisions for a structurally-supported cone bottom, although it's not necessarily supported off the shell only.
If the tank is very low, the flat bottom with no supports may meet design criteria. Roark's Formulas For Stress and Strain has equations for circular plates with large deflections that could be applied to it.