Yes, it will resemble an I beam. I would also recommend that you strengthen all of the joists in the areas that you notice the bounce. The purpose of jacking, is to reduce some of the load on the joist until you make it stronger. I would cut and glue the 2x4 to the bottom of the existing joist, then jack it up near the center and then screw attach it to the existing floor joist. It is much easier to install it on the "flats" as I stated. this will work.

Is the jacking necessary if the joists are not sagging? And how much do I jack it -- just enough to get some resistance?

STRUCT ENGINEER

I have the same issue as KMAN with floor bounce and noticed your NOV 7 post. I have one question regarding attaching the 2X4 to the 2X10 as you mention. Do we need to predrill for the screws to prevent splitting the 2X10 since will be spacing every 4" ? I plan to use #8 or #10 GRK Fasteners. Is this OK?