The peril of consulting an engineer
Bill Tindall, E.Tn.
When I was designing our house in 1991 I had a problem with the support for the first floor complicated by the spans I desired in the shop and family room beneath. I hired a structural engineer to solve the problem. Problem solved with $4,000 (1991 prices) of steel and a crane to handle it.
At the time we had a hardwood lumber business and sold to a cabinet shop whose owner had been a high end home builder before the credit contraction of the mid 80's put him under. While delivering a load of oak I thought to show him the plans and ask his advice. In minutes he suggested running floor joists parallel to the long axis of the house, supported on one end by the basement stair structure. 2 x 12 on 12" centers with bridging solved stiffening the 16' living room span to site made laminated beam on one post that sits out of the way by the jointer. Significant fraction of material cost saved. Glass does not vibrate on the book shelf- design problem solved.
If I had a home construction engineering problem my first source of advice would be an experienced builder. If the builder was unsure of a solution I would ask where they would go for advice. A bridge designer can run off the rails engineering a house.
Houses are way over structurally engineered. Making floors stiff is another matter. You could park a truck on a floor stiff enough to not bounce when walked upon.