Actually Mike they come bigger than that. Many years ago I managed construction on the new overseas terminal at Philadelphia Int'l Airport. The terminal meet/greet area had two steel trusses that were 400 tons of deadweight lift. We had to bring in a special 500 ton ringer crane of which there is only two in the whole USA plus two 150 ton units to make the lift. These don't travel whole but in pieces (for those who don't work with this stuff). The amazing part was is took 62 tractor trailers, lowboys, and specialty transports to get this unit on site, and 8 days just to assemble. Take Brenden's stuff and double in size and you get the idea. Looks to me that if we needed to have a longer turntable, then most of the work would be to remake an outer ring rail to the radius needed. The inner bearing ring should be able to stay as it is. It would be very cool to see one built in either Head Tide or Wiscasset or both. (wishful thinking)
