As you know the problem stems from the fact that the 12' span is on the
long side and 8" is on the short side for depth of joists. You are on the
wrong sides of both dimensions for any solidness of the structure so guess
work by non-professionals will not work at all.

I am a civil engineer but not a structural one. I took the structural
courses and work with structural guys all the time. For them, it is a
simple calculation to determine the degree of stiffness you would get with
either #2 or #3 approaches. Try a structural engineering consulting firm
and pay them to do the calcs for you - might be a few hundred bucks but it
would save you a lot of money over making a mistake with the wrong guess.
Note - you need to ask the right question to get the right answer. You want
to know the amount of deflection in the new situation versus the deflection
in the old case. Normally, they design for a structure that won't collapse
or flex too much. You already know it won't collapse, you just want to
improve the deflection which relates directly to the amount of bounce.
Bounce will be twice or more the deflection so if you want minimal bounce
you need very low deflection.

I don't think #1 will help at all and I think that #3 by itself will not
work because it would place too much stress on the bottom plate. There is
no top plate solidly fastened to the joists. The floor wood is not parallel
to the joists and, even if it were, the pieces are short and thus not
contiguous along the joist line. Thus there is no 'box' - no stresses in a
top plate to balance the stress in the bottom plate. Need a structural
engineer to confirm. You may need to do both #2 and #3 - this would be a 3-
sided channel.

Also, ask how many screws are required to transfer the stresses - there are
tables of allowable stress per screw.

You probably shouldn't use plywood. If you use 1" by 8" boards you save a
lot of cutting and may get a better final product. Wood strength differs
considerably so ask about the type of wood you should use - the calcs would
be based on an assumed type of wood.

Just a guess but a structural engineer may well tell you that deflection
(bouncing)will be too large for anything that is only 8" deep and spans 12
feet. They may suggest that the depth has to be increased. Prepare yourself
to answer that question - can you go to 12" depths? Would that create
problems of headroom or conflict with plumbing or wiring?

The reinforcing boards should be one-piece for best strength, thus 12'
long. Can you get such long pieces in place? Shorter lengths can be done
but require proper design or you won't get the improvement you need. One
idea is 6' pieces with 6' overlaps at the seam, all glued and screwed.

The existing joists are already sagging due to overloading. Ask the
structural engineer if they would carry their share of the load in a new
reinforced structure. Also, ask if the joists should be jacked up to be
straight while the reinforcing boards are screwed and glued in place. If
you just add new boards to the existing amount of defection you may not get
what you want. Or you may have to use stringer reinforcing. This question
may depend on the condition of the walls and floors above these joists. Are
the walls OK? Are there cracks, etc, that you would fix after the main
structural problems were solved?

I should also point out that glue is only as good as the dryness and
cleanliness of the surfaces glued and a proper gluing procedure. Unless you
do a lot of very good cleaning the glue won't hold. If the wood is damp for
some reason glue won't hold at all. Also, for glue to hold the surfaces
must be flat so there will be no significant gaps. This is probably
achievable with thin plywood but not with thicker plywood. You need thicker
for strength. Maybe a lot of screws will pull the sheets close enough
together, maybe not. Use lots of glue so it will soak into the wood and
span gaps.

Do both glue and screw but use enough screws to transfer the stresses in
case the glue doesn't hold. If the glue does hold it will be an extra
bonus.


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)