If you use SDS with Ip=1.5 you should good since you would be at the flat part of the spectrum. But you also should check the magnification factor depending on the equipment-foundation frequency ratio (less than 0.7 and more than 1.3 is acceptable).
I would check localized stresses and evaluated the global stress increase. Globally, I do agree with you about stress increase in the minor axis since they design these buildings with no reserved capacity and you should inform the manufactures’ engineer about this modification to his original...
The repair can be done by adding new plates but the tank roof and wall are buckled and at yield. The future Internal and external stresses won’t be uniform. There is still some inelastic capacity left. Try to add plates where material is at yied and use Girts (horizontal Plates) perpendicular to...
