I build steel and oak frames in a small workshop, and most of the failures I get asked to repair are not the frame itself, they are the hardware. Notes from the bench, in case they help someone planning a build.
Anchor points. An 8 mm bolt through a 2 mm wall tube will oval the hole under load. Weld a 5 mm doubler plate where the bolt passes and the same bolt lasts. If you cannot weld, use a through-bolt with a wide washer on both sides, never a self-tapping screw.
Pins and adjustment. A 10 mm pin in a 10.5 mm hole sounds fine until it is loaded from one side. Go 10 mm pin in a 10.1 mm reamed hole, and put the load on the pin in double shear, not single shear. If the frame adjusts in steps, drill the steps at 5 cm and add a second pin hole as a safety so a single pin cannot drop the bar.
Hinges. Cheap butt hinges have a thin leaf and pull out of hardwood in a season. Use a hinge with a 2.5 mm leaf and through-bolt it, or use a removable pin hinge so nothing is left under tension when stored.
Coating. Powder coat is not a rust barrier at a cut end or a drilled hole. Plug weld, or fill with wax after drilling, then coat. I have seen a frame rust from the inside out in 18 months in a humid room.
Floor contact. A frame that slides is a frame that fails. Rubber feet around 60 shore A, 5 mm thick, glued and screwed.
Load test. Before trusting anything, load it to twice what you expect, leave it an hour, then check every joint for a hairline crack in the coating. A cracked coating at a weld toe means the weld is moving.
Sizes I work with: width 60-120 cm, height 90-200 cm, 32 mm tube with 2.0 mm wall, 30 mm oak crossbars with hardwax oil.
Happy to answer build questions.