On May 9, 4:01*pm, stryped wrote:
...
Will a heavy 6 foot by 12 foot square stucture made out of 3x3 tubing
warp much when welding if I tack everything first?

That exceeds my limited experience, which is why I describe what I've
already done and how well or badly it worked rather than give general
advise. I'm not qualified to design and fabricate a roadworthy
trailer; I don't even trust my welding skill enough to fix one for
someone else.

The lengthwise rails on the sawmill are 1-1/2 square tubing with 1/8"
walls. When I welded in the 1/4" wall tubing crosspieces near the
center the free ends of the rails pulled inward and I had to jack them
back out parallel. The crosspieces had been milled square on the ends,
then beveled about half way. Clamping and tacking didn't help.

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I think the rails bent because all the beads were all on one side.
Steel shrinks a lot as it cools. Usually I make mitered corners or add
gusset plates and the weld beads are distributed more evenly across
the width of the structural members, so they hold shape better.

Jim Wilkins