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???