The thin-shell concrete dome topping the Sundome in Yakima, Washington, has 24 wedge-shaped segments arranged in a radial pattern like the pieces of a pie. But only six wood forms were needed to cast the 24 segments because of an innovative rotating forming and shoring system. The Sundome roof has a rise of 40 feet and a maximum clear height above the floor of 80 feet. Its 24 identical wedge-shaped segments arch to a compression ring at the crown of the roof and their bases are stabilized by a post-tensioned concrete tension ring supported on 24 reinforced concrete columns. Each segment is doubly curved like a saddle to increase the stability of the dome.
The six forms used to cast the dome segments each consisted of wood saddles supporting a curved deck. To produce the saddle-shaped curvature required, they angled the joists according to detailed design drawings. Aluminum shoring towers supported the forms. Shell segments were cast at intervals of about 60 degrees around the roof to equalize thrusting at the tension and compression rings. Workers pumped concrete for each shell segment in a continuous pour starting at the tension ring and ending at the compression ring. After casting the first six shell segments and allowing them to attain the required strength, workers stripped the formwork and rolled the entire forming and shoring assembly to the next casting position.
