As a professional mechanical engineer I have worked on high quality steel tubes (nuclear submarines) before. The immediate thing that sticks out to me in this proposal are the tight mechanical tolerances that have to be maintained. Talking about tens of thousandths of an inch tolerances on a 10' diameter tube is not to be dismissed lightly. That is going to be tough to maintain - especially with welding heat distortion. I would image the tubes will be joined with automated friction stir welding or something similiar, but that will still require a fair amount of post weld machining which has its own pitfalls. Not to mention simple thermal expansion and contraction as the temperature changes could change the circularity and inner diameter.

I would be more interested to see a tolerance stack up of those considerations than an FEA model of the concrete pylons. I can gaurentee that we can build concrete pylons capable of holding up a steel tube, that is done all over the country dozens of different uses cases. But can we build a multi-hundred mile long steel tube to the required tolerances?

I would be inclined to trade off efficiency for manufacturability. I.e. maybe a higher internal pressure or larger diameter to make it less sensitive. There should be plenty of power from the solar panels so it doesn't have to be perfectly efficient.

I'm also surprised that the I-5 plan is cheaper than buying private land. I may be naive here, but the pylons really do take away most of the objections from farmers and installing tubes over farmland has to be a lot cheaper than doing construction above a highway. I just look at boondoggle that was the SkyTrain in NYC (tram running over a highway out to JFK airport) and wonder if that is a great option.