As you say, the Ringworld is unstable. This was always something I had known, and accepted, but never really knew why. This article came from the problem Bill presented, and then in understanding that better, and making the connection with the Shell Theorem, meant that I can see why it's morally true, and not just a result of "doing the sums."

As lmilcin[0] says[1], the Ringworld was kept stable by artificial means, and that's the major plot point/device in the sequel.

[0] https://news.ycombinator.com/user?id=lmilcin

[1] https://news.ycombinator.com/item?id=19165894

The Ringworld was kept stable artificially if I remember.

Also it rotated extremely quickly which prevented it from getting squashed like the steel ring in the article. The rotation was so high that it created artificial gravity and so was also high enough to counteract star's gravity. In effect, the ringworld was in constant tension.

Since it was made from some pretty exotic stuff it is difficult to say anything about the tensile strength of the material.