How to reshape square tube after welding a small square bar along one of the sides

I've welded a few 3 m long square tubes out of mild steel angle bars (1.5x1/8) and along one corner side of the tubes welded a 8 mm square bar with one weld along the corner side of the tubes. Now the tubes are bent along the whole length with a deflection in the middle of, I estimate, 0-3/4 inch depending on the tube. I consider the tube as a circle segment as it seems to be uniformly bent along its length.

The next step is to straighten out the tubes and I've prepared a plan for that:

I want to put two tubes vertically along a concrete post, each on opposite sides. The one to be straightened out has both its ends distanced from the concrete post. Then I want to wind a rope under tension around the post with both tubes. The tube directly hitting the post will prevent it from cracking as I doubt it contains enough steel to withstand the forces needed to bend the tube.

The idea is to have the end's distances exactly right for the post to bend as much as needed plus a bit for the elasticity. So the middle of the tube will hit the concrete and will 'spring back' to 'perfectly' straight when we remove the ropes. I guess the rope then exerts a force that's equal to the sum of yielding force and elastic force locally.

My problem is I don't know how to calculate the force I need per winding, or let's say per meter length as the rope thickness is unknown. I also guess this won't be a uniform load, so every winding probably will need a different tension while hoping the friction with the concrete posts' sharp corners will keep the rope from slipping due to the interwinding difference in tension.

No answers are found on google nor here. If anyone has some ideas of how to calculate this, I would be thankful.

Myself I'm not unversed in math but have no idea how to go from Young's modulus of mild steel (210 GPa?) to structural bending and yielding of a square tube.

• You heated then unevenly to bend them, now apply heat to straighten them. – Solar Mike Jul 3 at 7:08
• I am not quite following, with square tube I find cutting and welding is the best way to bend or the walls will wrinkle. – Ed Beal Jul 3 at 16:41
• The square tube is made by welding two angle bars together. After that I welded a frame together from those tubes, of 3mx0.3m as a siding for our truck, but then it warped. Only 1 inch over a distance of 3 m, so wrinkling won't occur if I bend it back. That rope was a bad idea, I used a jack with a chain instead, seems to work. – HarryH Jul 4 at 13:40