You don't mention a budget but we've done it with pre-cast double-t beams. There used to be a pre-cast outfit down your way. They use these things for the decks on parking garages among other things. http://www.tpub.com/content/engineering/14069/css/14069_253.htm
If it's a "do it yourself" and cost is a big issue then some kind of steel truss is probably going to be the most inexpensive way. For that matter you may be able to find some a couple of steel joist sitting in a yard somewhere like these.
http://www.vulcraft.com/downlds/catalogs/joistcat.pdf

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