This is an unusual dome in that it is not based on a sphere like geodesic domes.
My biggest project yet built with struts. The struts are made from what is called 1" EMT. This is steel tubing about 1.162" outside diameter and about 0.055" wall thickness.
This dome was first assembled at Burning Man in 2009. The observation deck floor is about 20 feet above the ground. The floor of the second story is 10 feet above the ground. The bottom is 20 feet in diameter.
The shape of the outer surface of the dome is based on a catenoid. This is a surface formed by rotating a catenary curve, in this case about its axis.
More about the mathematics here.