They continoulsly weld a bead to the ibeam top side in mobile home plants to prebow the beams before they build the top to make sure everything is level when they are finished. Welding down the length of the tubing or ibeam will bow it. Welding across the tubing at a right angle to it, doesn't cause as much of an issue. When you build a trailer, that's something to keep in mind, when you hang your axles. Only weld the front and rear portion of the hanger. Welding down the side is not permitted in many places. The hanger manufacturers will tell you not to weld the hangers down the side as well as it is not necessary if it is properly welded.