I'm an EE, so I'm out of my area here. But, I have some cohorts who have been using a portable home brew tripod for lifting small (750 lbs and under) stones during cemetery repairs. They have been using this for years, no one has died -yet-... But being an engineer, I have to know...
The design uses schedule 40 black steel pipe, 1 1/2 inch. To make it car portable, they use two 5 foot sections per leg, with a rigid conduit coupling (two fittings welded together, to allow the two pipe sections to fully thread and end butt against each other). The top ends of the pipes are hinged- a pin thru the leg into two angle irons for each leg, spaced 120 degrees apart; effectively, the legs are only free in that plane, there is some lateral play, but not much.
I poked around my old texts and on the net and found and ran Eulers equation for all of the end connections I could find- fixed-fixed, pinned-fixed, pinned-free etc. I also assumed single piece 10ft legs, since I don't know the effect of the coupled 5 foot sections. And I used a 5:1 safety factor (what we use in aero-space) and A53 steel, 29.5E6 for E.
BUT- that's for 1 piece legs, not two piece.
So, my (initial) questions I'm trying to find answers for-
Does their threaded/butted two piece leg behave like a continuous leg? I suspect not, but I'm just an EE.
If not, can it be treated like two 5 foot columns with the common ends considered pinned (or fixed, or ...)?, or what correction factors should be made?
What are the proper end conditions to use for calculating the effective length? The bottom end is usually on dirt, of varying hardness. They use a load plate to prevent "sinking" (4" square), and sometimes use a rope to "prevent" spreading.
What would the effect be of adding braces (uni-strut?) between each adjacent leg pair just above or below the joint, using drilled holes/pins for leg spread adjustability?
My goal is not to give them a blessing to keep using this thing. I want to know if I should cut it up the next chance I get, or give them some recommendations on what to do to make it safer, or just shut up and go away.
Thanks for any and all advice!