James wrote:

Well, I must respectfully disagree. In addition to assurances from folks
here WHO HAVE DONE IT, as stated in my OP, I have walked on a floor made
with 2 X 10's spanning 16 feet. There was no bounce. Surely I can go
to 2 X 12's and span 4 more feet.

Respectfully disagree with what or whom? The Canadian Wood Council?
Let me know what they think of your research - walking on an entirely
different floor system and asking people on usenet.

If you want to revise your criteria to justify what you seem set on
doing, please feel free. Instead of having 40 PSF as your live load,
go with 30 and there won't be a problem. Just be aware that storage
can frequently exceed live loads commonly found in a house, and rather
quickly.

The chart you pointed to is neat !! Thanks !! It does indicate I would
have to use 12 inch centers.

Right. Or go with a higher grade of wood. Before you count your
chickens though, find out if you can get SYP framing lumber where you
are for a reasonable price. In my neck of the woods framing lumber is
either Hem Fir or Doug Fir. You can get 2x12 in pressure treated
lumber but you do NOT want to do that. Besides the greatly increased
cost, for no reason, you would have to use different connectors and
fasteners and there would be substantially increased shrinkage.

Instead of going through all of that extra effort, why not rethink your
objective, realize that indeed it has all been done before, and select
a commonly available wood such as Hem Fir Select Structural on 16"
centers. Most likely that will be your best alternative.

You should also check out some of the other design tools online at CWC.
For example, they have DimensionCalc which will tell you that your
2x12 framing lumber will shrink about 1/4" as it dries. That is area
of construction commonly ignored by all too may professional designers
and all amateurs.

R