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Vol11-01, March 2006
The Sydney Opera House
by Megan Neill
Originally published in SEAoNY Cross Sections Volume 11 Number 1, this article has been enhanced by its author for web publication.
The Sydney Opera House
On January 7th, Cliff McMillan of Arup recounted to a packed house the story of an unprecedented collaboration between architect, engineer, and contractor – The Sydney Opera House. This article will focus on the structural solution for the roof, the iconic form which allowed Jorn Utzon to be selected from a broad field of competitors to win the architectural competition with a simple hand sketch.
It became evident early on that any major deviation from the Utzon’s original vision would destroy the essential sculptural quality of the scheme, which included a roof design that represented a shell structure of inherently insufficient structural stability. It was therefore decided that all endeavors should be directed to finding a structural solution that would retain the profile and silhouettes initially conceived.
The solution evolved over years of revision and analysis. In the final form, each main shell is regarded as made from a number individual tied arches expressed by concrete ribs. The two legs of each arch lie in different planes, neither of which is vertical. The arch then must be supported at the ridge, and simple tension-compression supports tangential to the ridge circle were used. In addition each rib was supported off the previous rib by tension-compression connections at discrete points between a reinforced concrete pedestal and the ridge in order to reduce the torsional and lateral bending moments. The concrete ribs whose centerlines coalesced into the pedestal provided a common spring point for all precast concrete sections. This rational approach yielded the opportunity to utilize the anchorages for the prestessing cables as aesthetic features of the building’s form – their presence was celebrated rather than concealed. The ribs radiate from the pedestal, becoming wider as they ascend along the shell. The cross sections of each rib transition smoothly from a solid “T” at the pedestal to a solid “Y”, and finally to an open “Y” at its upper reaches. The ridge takes the shape of a prestressed hollow concrete beam of constant cross section whose segments form the keystones between the vertical planes of the half shells. The direct forces in the ridge and in the rib connections were transmitted to the side-shell complex through flatjacks where it is necessary to monitor and control the loads. The centroid of the load on each main shell lies close to a vertical plane through the pedestals so that the net overturning effect on the side shells was relatively small.
After an initial installation of precast concrete panels, over 1 million chevron shaped tiles were manufactured on site and clad to the exterior surface of the roof. In order to better express the anatomy of the roof structure, designers ensured that the joints between the panels coincided with the joints of the main shell ribs. Knowledge gained from previous experience in the use of similar types of tiles was applied, mostly relating to adhesions between tile and backing concrete and the optimum size of panel which could safely be constructed with respect to temperature variations.
During the process, Utzon welcomed geometric discipline, which paved the way for the gradual rationalization of the design needed to cope with the sheer quantity of geometric complexity in the original free form shapes. Sensibly the team relied heavily on computers though not just for the structural analysis. The contractor made use of the computer for the design of the erection arch, statistical control of concrete strengths and job costs. During the erection the roof’s shape had to be precisely controlled by constant surveys. By appropriate use of the computer, complex survey computations were carried out very rapidly, allowing savings in time and cost compared with traditional procedures. The project developed into one of the first large scale applications of computers to a building structure in a era when the capacity of the machines, number of available programs and the sophistication of the languages were a far cry from what is commonplace today.
Additional Figures
| Jorn Utzon was selected from a broad field of competitors to win the architectural competition with a simple hand sketch. |
| After an initial installation of precast concrete panels, over 1 million chevron shaped tiles were manufactured on site and clad to the exterior surface of the roof. |
 
| The cross section of each rib transitions smoothly from a solid “T” at the pedestal to a solid “Y”, and finally to an open “Y” at its upper reaches. |
 
| In order to better express the anatomy of the roof structure, designers ensured that the joints between the panels coincided with the joints of the main shell ribs. |
Knowledge gained from previous experience in the use of similar types of tiles was applied, mostly relating to adhesions between tile and backing concrete and the optimum size of panel which could safely be constructed with respect to temperature variations.
| The concrete ribs whose centerlines coalesced into the pedestal provided a common spring point for all precast concrete sections. |
Additional Resources
In 1973, Arup released an issue of Arup Journal, the company’s quarterly periodical, which was entirely dedicated to the Sydney Opera House Project. The articles published in that issue are available for download below.
Introduction
Sydney Opera House
Design of the Concourse
Glass Walls
Grouting and Prestressing Ducts
Adhesives for Structural Jointing
Influence of Corrosion on the Design
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