Spanning Beam Loads

An upcoming project will include a roof addition that will span slightly less than 18ft. There will be a professional contractor working with me on the project, but I thought I'd incorporate the knowledge of this group and the resourses of the web in attempt to educate myself a little more on the alternatives.
Does anyone know of a good reference for beam calculations?
Anyone know if I-joists ( http://www.lamwood.com/I%20Joist.html ) are readily available?
clyde
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Try http://www.cwc.ca/design/design_tools/calcs/SpanCalc2002/index.php
- Figmo
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