This question comes up a lot among the machinists. Here are links to the major discussions on it over the last few years.
http://www.practicalmachinist.com/ub...79.html#000000 http://www.practicalmachinist.com/ub...80.html#000000 http://www.practicalmachinist.com/ub...99.html#000000 http://www.practicalmachinist.com/cg...c/1/5496.html? http://www.practicalmachinist.com/ub...23.html#000000
If any of these links are faulty, just do a search in the general forum of:
http://www.practicalmachinist.com
Search on i beam using subject only (not entire message). That will get you these links.
You might also read some posts there on gantry construction. Perhaps a differently designed crane for your shop will do for you.