From 1957 to 1963, the design team went through at least 12 iterations of the form of the shells trying to find an economically acceptable form (including schemes with parabolas, circular ribs and ellipsoids) before a workable solution was completed. The design work on the shells involved one of the earliest uses of computers in structural analysis, to understand the complex forces to which the shells would be subjected. The computer system was also used in the assembly of the arches. The pins in the arches were surveyed at the end of each day, and the information was entered into the computer so the next arch could be properly placed the following day. In mid-1961, the design team found a solution to the problem: the shells all being created as sections from a sphere. This solution allows arches of varying length to be cast in a common mould, and a number of arch segments of common length to be placed adjacent to one another, to form a spherical section. With whom exactly this solution originated has been the subject of some controversy. It was originally credited to Utzon. Ove Arup's letter to Ashworth, a member of the Sydney Opera House Executive Committee, states: "Utzon came up with an idea of making all the shells of uniform curvature throughout in both directions. " Peter Jones, the author of Ove Arup's biography, states that "the architect and his supporters alike claimed to recall the precise eureka moment . . . ; the engineers and some of their associates, with equal conviction, recall discussion in both central London and at Ove's house. "