made a stage

[jackh]

made a 10'x12' stage that sits 18" off the ground the other day out of 2" 14 gauge tubing. when i flipped it over on its feet i noticed that all the main beams had bowed when welding on the center supports, now only the 4 corner feet touch the ground at all times.... when a band and gear is on the stage it sits level and all feet are on the ground. why did the beams bow???

[Jack Olsen]

When you heat steel, it expands and then contracts a little bit more than it had expanded. So the side of the square tubing you heated up got a little smaller. If you're careful, you could cold bend it back. You could also use a torch to heat up the other side to even out the contraction. Or you could just book some heavy-set bands to slowly straighten it back out.

[crabber]

Just looking at your photo, it appears your shielding gas tank is free standing?
Chain, tie, secure that thing before you kill yourself or someone else.

[jackh]

attaching each support leg required 16 welds once you got the 45's in too. i was doing one complete leg at a time. would it not have bowed as much if i had done a couple welds on one leg then moved to another leg and done a couple welds on it instead of doing all 16 at once?

[enlpck]

In this setup it would have probably bowed some, but the appropriate tack and weld sequence can reduce the distortion, as can prestressing the material the opposite way, good fixturing (clamping), and preheating (to reduce differential shrinkage as the weld cools).


For your application, I would have probably have clamped the heck out of it (in your situation, that might have meant a lot of weights, or anchors into the floor), probably put a little prestress (slip a shim under the joint and load it), and sequenced the welds carefully.

The sequence I would have selected would likely have been tack the leg to frame, tack the diagonals to frame, tack the diagonals to leg, check and fix as needed, then run the diagonal to leg weld so the diagonals can limit distortion when the leg is welded up, LIGHT welds for diagonals to frame at the sides (not across the face of the tube), then weld the leg to the frame. The diags will brace against distortion to a point. I would fixture with a little bit of prestress first.

Try a test joint or two to figure out the prestress needed.

[Sberry]

Put a couple little shims on the feet?