NotesThe drawings were used in the London and Sydney offices of Ove Arup and Partners, the wind-test model was used at Southampton University and the National Physical Laboratory (England), the sphere, rib, and segment models were used by Arup engineers in Australia.
There was no prior ownership of these items. They were developed by Ove Arup and Partners, Jorn Utzon, and Hall, Todd and Littlemore, at various stages of the project.
The architect's competition scheme for the roof was that of four main pairs of surfaces for the Major Hall. The proposals for the Minor Hall varied slightly. Each surface was a triangle in elevation, with boundaries formed by curves in space geometrically undefined. In cross-section, a pair of surfaces (or shells) formed a gothic arch. The main shells were connected to each other by a further series of surfaces termed 'side shells', also geometrically undefined.
The terminology used to describe the roof structure has grown with its development but is, strictly speaking, misleading. The term 'shell' stemmed from early hopes that membrane action would suffice to support the roof structure.
The structural implications of Utzon's design were first discussed with him at the first interview after he had won the competition. There were difficulties with Utzon's design that had to be resolved, thus forging a close collaboration between architect and engineer. The chief difficulties, as perceived by Ove Arup and G. J (Jack) Zunz, a senior partner in Over Arup and Partners, London, were:
(1) The interplay of surfaces made an assessment of structural feasibility by normal approximations difficult and of dubious value,
(2) The scale of the structure was misleading. The size of the site and the scale of the harbour and the bridge tended to diminish the building's apparent dimensions,
(3) Not only were the roof shapes geometrically undefined, but external and internal finishes had yet to be chosen, the auditoria ceilings and their acoustic requirements had not been formulated nor were the size and details of the stages and machinery available.
Several solutions to these difficulties were examined in the early stages of the collaboration between Utzon and Arup. Some ideas 'floated' then were the use of non-pointed arches, doubly curved shells covering each hall, and a single roof without discontinuities over both halls. These solutions were regarded as simple structural forms, and crucially, the solutions were meant to eliminate some of the large bending moments inherent in Utzon's roof shapes.
As said, the roof was initially determined by architectural, rather than engineering considerations. It was apparent early in the project, that any major engineering deviation from Utzon's proposal would destroy the sculptural quality of his design. Nevertheless, there was a firm commitment from Arup to finding a structural solution that would retain the roof profile and silhouttes initially conceived by Utzon.
A two-phased programme of architectural and engineering designs for the roof occurred between 1957 and 1963. The first phase was from 1957 to 1961, and the second was from 1961 to 1963. In the first phase, an enormous amount of analytical work and 'model tests' were directed towards finding a solution to the 'roof problem'. In structural engineering terms, the problems to be solved were:
(1) A geometric discipline had to be imposed on the surfaces in a way which would provide adequate clearances for the stage towers, balconies and auditoria roofs, all of which were unknown in any detail (refer to spherical model of roof).
(2) The roof structure had to be proved stable under all possible loads and without undue distortion under normal conditions.(3) The wind loads on the curved surface were unknown and had to be established by wind tunnel tests (refer to large timber model of the Major Hall, Opera House).
(4) A construction method had to be evolved having regard to structure, cladding (tiles) and the variable geometry of any staging system.
It was clear in these early days that to achieve a solution, to make it possible to build the structure, extensive use of electronic digital computers were necessary, to cope with the massive number of geometric problems and the complexity of the analytical work. Utzon was not opposed to geometric discipline. Early geometric systems embodied a system of parabolas and this greatly improved the appearance compared with the original free shapes of the roof. Crucially, the introduction of a geometric discipline, gradually rationalized the design and construction of the entire project, and made possible the factory production of geometrically similar elements.
The second phase, from 1961 to 1963, saw the design of the roof and associated analyses shift from an ellipsoid scheme to a final spherical scheme.
Des Barrett, Curator, Engineering Design.