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Hello, how are you doing? I'll be happy to assist you.
The live load rating of the existing slab is 60 psf?
Do you know anything about the structure? Is it a one way beam and slab? flat plate? precast? post tensioned?
What is the size of each tank?
I can provide you with the beam size to carry this load on a 15' span, but you will need to ensure it's compatible with the existing structure. Sometimes adding beams (especially with concrete construction) can change the behavior and cause more harm than good.
You can plan for two W8x18 beams ASTM A992 Gr. 50. They are 8" deep and weigh 18 pounds per foot.
The loads you have exceed 60 psf. Therefore the prudent thing to do is to add a new beam to support the new load and to bypass the existing structure as much as possible.
The beams were sized to support ONLY the weight of the 4 tanks.
I understand that. That is why I made the statement above.
Your load is greater than the 60 psf allowable live load.
Is the beam concrete or steel? You can certainly make modifications to use it, but you can't use it as-is
There is probably no footing below.
Why would they add a footing where one is not needed because a post doesn't exist?