Im not a engineer but ive framed about 300 houses give or take and many houses from the ground up. Those are very strange looking. I only ever built 2 dozen or so ranchers but usually you would have a load bearing beam or a doubled up girder/gable with extra tall bottom runners with metal hangers not 2 giant beams in the middle of a room like that on any modern built home. Basicly ive never ever seen anything like what you got going on and ive built a few custom 4k sqft ranchers with all kinds of crazy cut up roof lines. Id tear it apart myself and see whats in there which it sounds like you already started.

Also i would need to see the roof lines to understand why they would put those beams in there. I see how your foyer entrance is placed and the columns are directly in the middle of the house but without seeing roof lines its hard to say why they would need the columns there.

In your pictures i noticed one thing that stands out. Its this beam with hanger on it. Is the beam directly over those columns?

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Is the upstairs room made from bonus room trusses(trusses that have the room built into them) or hand built with actual floor joists? If there joist's are they 2x10 or 2x12? All you need to do is tell engineer what the joist are and there layout and span and he should be able to tell you the weight they can hold. Also if there just bonus room trusses the original builder should be able to connect you with the truss company to get the info you need or you might be able to crawl in somewhere and find a printed sticker with there info on them.

Subfloor wise is the OSB tonge and groove and glued down? It should be both and will be good to go as thats normal sub floor thickness.