I'm building an I-beam gantry. It's not an A-frame; rather, it is the kind with one vertical leg on each end. It will be adjustable for height.
I'm planning to use 4x4x.180" wall square steel tube for the outer leg, and 3-1/2x3-1/2x.180" wall square steel tube for the inner leg. To minimize rattle, I'm going to fix 16 gauge strips to the inside of the outer tube (straddling the weld seam) probably by making small holes in the outer tube and then plug welding the shims in place. I've pretty much investigated this, and I don't see why it won't work.
What I want to know is how far into the outer tube the inner tube must go for safety. Obviously a quarter of an inch is way way too little, and probably three feet is too conservative. The inner leg (the 3-1/2" stuff) pieces are going to be about 69" tall. The top eight or ten inches of each leg will be bolted to a short piece of the 4" tube which is attached to the bottom of the beam. (I want the gantry to disassemble as I can easily envision someone in my metalworking club needing to use it when they bring home a lathe or something.)
I'm planning this gantry to lift 4000 pounds. I really don't know where the safety point is, in how far I can let the inner leg piece stick out of the outer one.