Daria,
The only suggestion that I would have is that if you can't find anyone to do a 20' radius with a tube, you could build up the section out of flat plates. In fact if you have a steel detailer that you use a bit, they could probably price this for you for nothing: then tell the architect how much it costs and tell them either live with it(i.e. pay for it) or change their detail.
Doug