Support for crawlspace beams
I see no &quot;steel plates&quot; that support the beams. In fact the bottom flat plate is an integral part of the the I-Beam itself.
If you look at the attached enlarged view....

... you can see that the lower flat face of the I-Beam is actually supported on some wooden tapered wedges. These are in turn appearing to be sitting on some whiteish colored bricks. These could be fire bricks or even concrete bricks (it is hard to tell from the pictures).
The floor &quot;bounce&quot; that you are experiencing may very well be very slight deflections of the I-Beam itself. You could experiment with some temporary adjustable support posts under these I-Beams at mid span to see if that offers any significant change in the floor bounce that you experience.