Ah... Sorry... Here's more information

2x10 floor Joists 16" on center, laid out as below, Hardwood floor ,
the span I'd say is about 15 feet., x represents wood cross bracing.

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x s
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x
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x
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x p
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x
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x
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x s
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The hardwood flooring obviously runs up and down. A person standing in
p can shake the furniture in s.

I have been in the basement with another person shaking the floor above
and I really can't notice any movement. A coworker suggested that the
cross braces (1 per span) is grossly inadequate ana a simple solution
would be to add additional blocking as stiffeners as such...

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| | | x | | |
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| | x | | |
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| | | x | | |
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| | x | | |
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2x10s, circular saw and a framing nail gun should take no more than a
day. I guess I'll try this since there's hardly any cost.

What kind of lumber should I use?
2x10s would be the best right? But wouldn't any stiffening member work
including plywood?
How far apart?
How to get around plumbing + electrical wires? Cut notches in the
blocking?