structural steel I beam question.

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I am fabricating a gantry/A-frame to lift a maximum of 4tons 12"x5"x3/8  I- beam for the cross member and 6"x3/8 pipe for the legs,I believe this should be plenty if not overkill for the structural strength of the frame,but my steel supplier in town is asking what[ #p/square inch]do i need for the I beam,not sure what i need for this amount of weight. can anyone help.
Reply:How long is the span between your a-frames?Miller TB Pro 350DMiller 12VS ExtremeMillerSpoolMatic 30AMiller Spectrum 2050MillerMatic 251Look Out for 79 Year Old Ladies Runnin' Late for A Funeral
Reply:span is 17' 6"
Reply:If you are gonna make something that holds heavy weight above your head, and you are not an engineer, copy it from somebody that already paid an engineer.Like these guys-http://www.wallacecranes.com/triste3.htm#trist3_specsThey make them to sell, so you figure they have done the calcs.For a 5 ton, in your span range, they seem to be going with at least a 15" beam, and 18" would probably be better.
Reply:Lincoln Electric offerred a beam deflection program somewhere on their site.Any of the Blodgett texts would give you the info.If you have a Machineries Handbook it will walk you through the calculation.  Four tons at center span on a 17ft 6in. span will need a pretty big beam.  Excessive deflection will force your pipe legs outward unless they are track mounted.What you need to figure out is the "section modulus" required to take the load with adequate safety margin.  2 to 1 safety not nearly enough.  I would be using four to one to yield strength.      Your steel supplier's book will give you section modulus for each size beam.
Reply:Thanks for the info. I will contact a engineer if I decide to proceed.
Reply:Just thought i would throw this in, I just recently did a job installing an addition on some overhead cranes for a mobile home plant.  We added in a couple of 25ft bridges for a 4ton capacity overhead.  The bridge was a 10" x25.4 lbs/ft beam (roughly 3/8" thick) and the span across the supports were a touch over 20ft.  So your 12" beams should be plenty.  Most beams are sized by the height and the weight per foot that is what your supplier is wanting to know.  Be careful on overheads. If its not put together right someone can get killed so take your time and prepare your joints for full penetration and make good clean welds.  good luck  ~JacksonI'm a Lover, Fighter, Wild horse Rider, and a pretty good welding man......