I'm thinking on putting in my shop a steel I-beam overhead from one end to the other and using a trolley with chain hoist to pick up and move stuff that's heavier than I can lift by myself. Presently, I'm using a gantry style construction of 4" OD square tube, legs and cross beam, 10' wide. I know that will take at least 2 tons and not bend or flex. I have to move the whole thing to use it on the other end of the shop, which is not easy. The setup I'm thinking of would be about 28' long with the 4" square tube (dismantling the present one) at each end for the legs, I could put a temporary support one third of the way in from either end when necessary. My question is: how much weight will a 28' long run of 3.5" I-beam take? an 18' run? and a 10' run?

Not much, probably sag about 4 to 6 inches in the middle from its own weight if it's 28 foot long.

6" or 8" is more realistic.

If you have a copy of Machinrery's Handbook, you can look up the deflection for various standard beam sections. You don't want more than 1/4" deflection under designed loading if you're going to use a trolley on it, or the thing will roll to center and stop. For 28' foot clear span, at 2 tons I would take a WAG and say you'll need something on the order of W16-80 I beam. That's 16" web, wide flange, weighing 80 pounds per running foot in standard beam nomenclature. Check Machinery's, it will have the exact figures. You're gonna be unpleasantly surprised though, I'm afraid.

to span that long of a run, build a truss above it. look at the "red iron" structure in the ceiling of most modern buildings, look at the boom on most wreckers and cranes, this will give you some idea on how to support it. whatever you do, don't stand under it until you have tested it. basically, you need to have a rod coming down to the center from both ends to help carry the load to the end supports. The higher the starting point, from the beam, the more weight it will carry. Think vectors, trig, sine and cosine,etc.

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/ubb/ultimatebb.php/topic/1/14679.html#000000

http://www.practicalmachinist.com/ubb/ultimatebb.php/topic/1/10180.html#000000

http://www.practicalmachinist.com/ubb/ultimatebb.php/topic/1/9899.html#000000

http://www.practicalmachinist.com/cgi-bin/ubbcgi/ultimatebb.cgi/topic/1/5496.html?

http://www.practicalmachinist.com/ubb/ultimatebb.php/topic/1/5323.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.

well, as the saying goes, back to the drawing board. Thanks for everyone's input.