JV,
I built a plywood rack in my pole barn. It is across a 12' span between 2 poles. I used -2x6 yellow pine for all of the wood. Ends look like this:
P
P
P
P
XXXXXXXXXXXXX
X. P X
X. P. X
X. P. X
The P is the pole and the X's are 2x6's I put 2x6's vertical between the 2 frames with 2x6's horizontal on top. I used 20d nails that I predrilled, to prevent splitting, through the 2x6's to fasten them to the poles.
I made it real sturdy and I'm glad because it is holding a lot of weight, it is full of 3/4" plywood.
The top is 48" wide.
I made it so that as soon as I use the plywood I could then use it as a 4'x12' work table.