TomC, I remember Max Gasman advocated one end only, before I even made my first grate - so I welded both ends 
I’ve noticed no warping. I think if the rods sit inside a slice of a round tubing like that, it can easily flex into an oval shape if needed.
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I just have a lot of respect for Max’s knowledge. I had an experience of angle bars buckling in my outside boiler, so without proving either way, I have accepted the idea. If you say welding both ends doesn’t cause a problem, I will accept that also. TomC
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I plan to use part of a car rim for my hoop on the angle irons. I’m hoping that it cups the ends of the bars so when it warps it has an edge to rest on and not just collapse.