I downloaded some info on I-joists, and I wonder why you couldn't make your own? They look pretty simple- OSB, let into a 2X4 top and bottom. I assume they are glued? I wouldn't try to sneak them past a building inspector, but I don't have to have an inspection of my barn, as long as it is for 'agricultural purposes'. I could simply copy the info on the manufacturers specs, and maybe go up a size or two for good measure. Or are they cheap enough to buy that it wouldn't be worth it? Anyone got any idea what 16" I-joists cost per foot? I get these crazy ideas form time to time! d*
You could but... the top and bottom flanges really need to be effectively continuous chords or very well spliced. But sure it is a form of truss, the deeper you make it the lower the stresses are. I don't think you are going to do it as strong cheaper than they make em.
This is a good article on structural (and non structural) adhesives
http://www.apawood.org/pdfs/download_pdf.cfm?PDFFilename=managed/Y391.pdf
Continuing to edit...
Design and fabrication of all plywood beams... a joist is simply a small beam;
http://www.apawood.org/pdfs/download_pdf.cfm?PDFFilename=managed/H815.pdf
Design and fabrication of plywood/lumber beams;
http://www.apawood.org/pdfs/download_pdf.cfm?PDFFilename=managed/S812.pdf
Just remember that while an I-joist is lighter, but still as strong as a regular 2x4 or what have you, that should your structure catch on fire, the risk of collapse arrives much sooner than a solid piece of wood.
Sometimes I-joists show up on Craigslist around this neck of the woods, good deals can be had if you do your homework on prices and look them over carefully for damage, rot, evidence of too-long exposure to the elements, etc.
G/L!