Assistance pls with likely distortion direction

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Hi.    In a clothing shop a steel rhs tube prob 32 high by maybe 15 0r 20 wide is 2 m long and sagging badly with more weight than anticipated    I can only guess the wall thickness at 1  or 2 mm or somewhere in between     The shop is happy with my suggestion of welding a solid bar 50 by 6 underneath so that when viewed from the side as a customer would be browsing you are now looking at a total height of 82 mm.   Am confident that done right this will b strong enough but as I will be welding staggering either side maybe every 300 mm .     My question is     Is there enough heat generated to put a curve in my 50 mm flat bar because I will only b welding at the top and nothing at the bottom ?    It may be difficult to answer how much it's likely to deflect but if it does deflect am I going to be looking at a bar that is distorted up ( I can live with that ) ?    But if it distorts down and when loaded up with stock the customer may not understand the distortion factor and just think that I have used an unsuitable reinforcement piece and be un satisfied     If the distortion is likely to be down am I likely to successfully counter it by setting it all up in a pre cambered jig and allowing it to cool naturally.?       Or more likely with no patience at all hose the bugger down to room temperature
Reply:If I understand your predicament, welding at the top of your introduced support will tend to shorten the top. It will also tend to shorten the bottom of the member above. In fewer words it'll sag. Give it a bit of arch ahead of time. I'd use a hydraulic press to bend it ever so slightly, or if it is material you can get away with heat the side that will be down to a dull red with a acetylene torch first.An optimist is usually wrong, and when the unexpected happens is unprepared. A pessimist is usually right, when wrong, is delighted, and well prepared.
Reply:Thank u.   Rather than just jump on here for a one off I will actually post what I do and the result    It all adds up to us constantly learning from our projects rather than just discussing what might work before we start
Reply:My son did a dump trailer. The dump box involved a square tube frame, plated with 11 gauge hot rolled. it involved one seam in the floor. The prudent thing to do would have been a series of skip welds around the perimeter, with an overlap instead of a butt weld at the seam where one sheet wouldn't cover it all. The trailer owner, my friend since childhood got involved. He wanted to try his hand with the welder. The entire perimeter of the first sheet got welded down. then the entire perimeter of the second sheet also. It was as flat as a potato chip! I ended up heat straightening the frame with a rosebud tip. I spent hours! The floor was bigger than the frame. After hours of heat shrinking, and realizing I'd need more hours, I gave in and cut two X cuts to relieve the hump in the middle. The final product looked good except the curious "Why did you weld it there?" X shaped welds. I should say it looked pretty good the first week until he started throwing chimneys, and toilets out third floor windows into it. Oh well.An optimist is usually wrong, and when the unexpected happens is unprepared. A pessimist is usually right, when wrong, is delighted, and well prepared.
Reply:By the way, it is that stupid metric system that kept much of the world in the middle ages 2 centuries longer than USA.An optimist is usually wrong, and when the unexpected happens is unprepared. A pessimist is usually right, when wrong, is delighted, and well prepared.