so... you ask at a good time. I'm building a 15' wide X 22' long bridge crane in my shop right now. The engineering is a bit tricky, but we can discuss some of the details here...
John N has built several bridge cranes and has more practical experience than I do. My cousin has been buiding bridge cranes for 45 years, and has helped me refine some of my lifting ideas. My Dad used to maintain the 200 and 400 ton bridge cranes at Stelco Hamilton, so I got a little info from him as well as I was growing up.
My design is using a 3X3X 3/16 wide flange aluminum I beam for all of the support members. I got the material for a steal from another member... The entire design stems from what capacity your trusses will hold at the rails. The rails must be perpendicular to the trusses, or you are in for a lot of restucturing. My trusses will hold 250 lbs on the bottom of the chord quite safely - we tested this with myself and my daughter on the same truss several times while insulating the ceiling. You have to determine what is safe for your situation.
If your requirement is only to lift that amount (such as in my case, 250lbs) then you need to then size your I beam accordingly. For a 250lb lift I'd make the rails out of 2X2X3/16 wide flange I beam. You have to use wide flange I beam for all structure, as the trusses aren't good at bracing the load at unexpected angles. A stiffer I beam will also help spread the load across I beams.
Next go to an I beam calculator and use the load and span to find out what I beam you need to support your design load. Here again you must use wide flange I beams. When a load is moving or seinging in a random direction the normal flange I beams will fail a far below the calculated load. Make sure that your bridge will support at least 2 - 2.5 times the max load.
Both rails need a novel mounting scheme, as they just cannot screw into the trusses, weakening their bottom chord, and risking pull out. I'm using a wooden beam above the trusses to further average out the load, but my lifting requirements are much higher than a single truss will tolerate.
I'm being cagey about the details here because doing this wrong or at loads that are too high can damage your trusses. Every truss system has weaknesses and I cannot generalize here. In the late 1970s I designed and implemented truss design software, so I can be sure that what I'm doing on my trusses will work. Depending on the number of chords (diagonal members) and design limits, your trusses might be stronger or weaker than mine. Note that stiffness does not mean strength. You can infer the strength of your trusses by measuring deflection under load, as long as that deflection is very small.
There are tests you can do to help you, but you should not deform the bottom chord of your truss by very much at all. This is because deflecting this puts tension stresses in your top chord connector plates that were designed for compression loads. At the place where your rails join to your trusses, you want a deflection less than .250 IF you are near the centre of the truss. Depending how how far you are from the wall, you will find this to be in the range of 200-400 lbsf. You should never exceed this value during any lift. NOTE THIS IS A VERY BROAD DESCRIPTION THAT CAN BE COMPROMISED BY A MEASUREMENT AT THE WRONG PLACE. iF YOU ARE WITHIN 24" OF A WALL THE .250 DEFLECTION IS NONESENSE. READER DISCRETION IS STRONGLY ADVISED.
This is just an overview of the 50 or so hours of design work I did for my bridge crane - but my gantry crane had as much or more design effort. I'm not trying to scare you here, but I'm trying to say that lifting equipment can be tricky, and it has to be thought through carefully - for safety's sake.
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