If it is not too late for input, I agree with PT6FY. I also worked for Butler until very recently and I can tell you that with the software at their disposal it would be much better to have them perform the anaylsis and/or redesign of substitute frames. It will save you time and you will end...
I am wrestling with this same issue, although I am putting a concrete tank on the foundation instead of steel. This adds the additional concern as to whether the membrane slab can handle the radial tension created by the tank. I will continue to reaserch this and let you know if I find anything.