Jim - sounds like the 3 or 4 inch channels are the best fit considering
the dimensions of the shed.

Hul

Jim Wilkins wrote:
"Hul Tytus" wrote in message
news
Jim - if you can stick with the 500 pound load, try a 16 ft. 2x12.
The
calculator here predicts a max stress of 725 lbs and .323 in.
deflection.
Add a couple of "angle irons" to the sides maybe?
You'd be happier with one of those calculators on your computer.
They
are quite handy for something like this.

Hul

I had cut some wide, 2.0" thick oak planks and did consider them to
strengthen the gantry track.
http://www.woodbin.com/calcs/sagulator/
"This calculator can also be used to measure beam deflection. Because
beams are typically positioned on edge, use "thickness" to represent
beam depth and "depth" to represent the thickness of the beam."
It accepts the non-standard dimensions of unplaned wood from my
sawmill.

The issue I didn't mention here is clearance height under the existing
shed roof beams, as the timbers are stacked almost as high as I could
get them with 3" channel for the gantry track.
-jsw