Originally Posted by Todd-Beaulieu
When we walk, there's quite a bit of bounce.
Each quadrant's span is about 14' - a few inches over. The joists are 2x8 (true size), 18" O.C.
As a test, I added a 4x6 beam under the living room with two jacking stands. I only applied a little pressure - wasn't trying to jack it up, just support it. It's now rock solid.
Borrow a book, through interlibrary loan if necessary, on "Lumber Engineering." There are probably several on this subject.
It should confirm what you are seeing and what is necessary to get the center-span-deflection down to acceptable levels.
Or look at the latest version of the International Residential Code.