The threaded rods are placed at reasonably regular intervals around the circle. The outer helix has an outer radius of about 33 inches and a perimeter of about 207 inches. I ended up with 9 pairs so the supports are placed about 23 inches apart. The supports are fashioned out of 1/8 inch thick, 3/4 inch wide steel stock. The bar is cut to length and then drilled with a 1/4 inch hole on each end to fit over the 1/4 inch all-thread rod. The threaded rods are placed to fit the holes in the supports and are wide enough to not cause clearance problems. [I used an 85 foot Amtrak Superliner, an 89 foot three tier auto rack and a double stack well car with two standard containers to check clearance - both side and top.] One of the holes is then opened up to form a slot so the support can be put on one threaded rod and then swung under the roadbed and slid onto the other threaded rod.
The steel supports are attached using a hex nut and a fender washer below and another fender washer, a lock washer and another hex nut above.
Here are some photos of the helices going up.
At first I just roughed it in using a block for vertical clearance and a special grade tool. Bob Hamm sent me the grade tool which is quite simple. The tool is made from the same steel bar stock as the roadbed supports. At 12" from one end a hole is drilled and threaded to accept a short threaded section with a lock nut. The threaded section is adjusted to provide for the desired grade and then a short level is placed on top. When the level is level you have your grade.
Here is a photo of the tool itself and in use on the helix along with a photo of the double stack car to show vertical clearance.
I am currently working on extending the masonite spline roadbed from the helix around the second level. I should post about that and the wye at Surf in the next few days. Then I start laying track for the second level. Stay tuned.