On Jan 11, 10:57*pm, Limp Arbor wrote:
My house was built in the 1970s when apparently there was a huge
shortage of either lumber or smarts.

2x8 joists, 12" O.C.,14' span
1x3 X bracing in the center of the span
1/2" plywood subfloor nailed with spiral nails - no glue
Oak hardwoord floors nailed perpindicular to the joists - the flooring
is short with most pieces less than 3'
Easy access from below in full basement

Floor bounces noticeably from normal walking, knick knacks rattle,
etc,

I tried on the other side of the beam with 16" O.C. 2x8 joists solid
blocking with little success.

I've seen 3 suggestions and want to know if anyone has tried them and
what the result was

1 * Metal X bracing installed on either side of the current bracing
* * *(splitting the difference between the center and the ends of the
joists)

2 * Sistering the joists with glued & screwed plywood on both sides
* * *(All plumbing & electric are either below the joists or parallel
to them)

3 * Installing a plywood 'ceiling' in the basement glued & screwed to
the bottom of each joist.
* * *(I guess the idea here is to make the whole joist sytem a giant
torsion box)

I'd jack up each one and add a 2nd 2x8. Are both ends on top of
support? If one end is held by joist hangers then you ought to be
able get the full length. The most strength is required at the ends.
If you can't get the full length then use two pieces and overlap at
the center with a strip of 4 ft 1" osb cut the long way. You can get
a ton of them from one sheet. Glue and screw whatever you do.

I have a closet off a landing that I spanned 8' using doubled 2x6's.
Needed to maximize the space under it. It's pretty solid.