As mentioned below, it probably isn’t the beam that is doing the bouncing, but the floor joists.
Let’s assume straight off that you don’t want to replace the joists.
So, we are reinforcing the joists. You have three areas you can work from: the top, the bottom, and adding supports.
The top.
- You can double the nailing on the subfloor (provided that you aren’t increasing the nailing to less than a nail every inch). This will allow less flexing of the subfloor, but will only help a little.
- Add another layer of OSB or plywood on top of the subfloor with a similar nailing pattern to the existing subfloor (or replace the existing subfloor with thicker sheeting). This will stiffen the subfloor diaphragm which will reduce the bouncing the thicker you get. But, now you have a change in height of the floor.
The bottom.
- Much like above you can add 5/8″ drywall to the ceiling with screws at 3″ on center or so to reduce flexing. This will be about as effective as adding sheeting on top.
- You can expose the joists from below and add bridging between the joists every 4′ on center. Again, this will help a little.
- You can expose the joists and either sister joist or cap the joists. Sister joists are joists laid next to the existing joists for the full length of the span. Full height joists just need to be nailed in place, less than full height joists need to be nailed to the existing joist 6″ on center along the top and bottom of the sister. One per joist is best, but even one for every three or four joists will improve the bounce by a lot. Capping is like sister joists, but you nail it to the bottom of the joist like a 2×4 nailed on the bottom of a 2×12 like an upside down “T”. They should be nailed 6″ on center and every joist would need this. This will reduce the bounce by a lot.
Supports.
If you want to go with supports you really need an engineer because you can just as easily make things worse on this if you get it wrong.
Hopefully, this helped.
MKDesign 