4.1. Introduction
The Sydney Opera House is one of the most iconic buildings of the 20th century, nominated as a UNESCO World Heritage Site in 2007. Its design process is also a very interesting case study in the use of references to nature, as will be shown below.
When in 1956 Jørn Utzon developed his proposal for the International Architecture Competition for a new opera house in Sydney, he was very clear about both his working method and his references. As for the latter, he himself has stated in numerous interviews his predilection for natural references, with a strong preference for organic forms, thus distancing himself from the Modern Movement, the architectural style that clearly prevailed at the time.
The duality between the organicist (Art Nouveau, Organicism) and functionalist (Modern Movement) currents was evident in those days. The organicists tried to transfer elements directly from nature to be applied to buildings, opting for fluid geometries. On the other hand, the functionalists rejected the use of organic forms in favor of more Cartesian geometries and for giving total primacy to programmatic and functional aspects, perhaps to the detriment of other more “sensorial” ones.
In any case, in the 1950s there was a certain revival of organic architecture, encouraged by the strong development of reinforced concrete technology during the Second World War, with references such as Pier Luigi Nervi, Eduardo Torroja, and Félix Candela exploring new formal possibilities for the use of concrete.
Thus, the design proposed by Utzon for the Sydney Opera House competition can be clearly framed in this context. It is important to underline that this set of experimentations in the field of the formal possibilities of concrete are not produced as a mere “formalism” but are developed in the context of rigorous geometric studies (minimum energy surfaces) and form-finding (3D antifunicularity studies) to progressively refine the tools that ensure geometries that favor an optimal use of materials. In this context, mimesis with natural forms was also an important design tool.
4.2. Inspiration by Nature in the Competition Proposal
In 1956, after the recent Olympic Games for Melbourne, Australia was trying to maintain its momentum on the international scene by launching an international ideas competition for the construction of a large opera house in Sydney. Thus, on 29 January 1957, the competition panel ruled in favor of the entry presented by a very young and inexperienced Danish architect, who decades later would establish himself as one of the most relevant figures in his field, upon receiving the Pritzker Prize in 2003: Jørn Utzon.
Utzon developed his proposal mainly through physical models, trying to emphasize the three-dimensionality of his building, something that at that time could not be done in any other way, due to the lack of current digital design tools. He himself declared after winning the competition that he had worked as a sculptor, physically “shaping” his building. This idea of an object, a sculpture, a building in which the facades and roofs are diluted and intermingled, is also evident in his famous quote:
“God sees from everywhere” [
17].
Utzon’s search for natural references for the development of his project was a constant: the shape of the roofs, the geometry of the facade, the chromatic range selected for the cladding, etc. Nature was his main source of inspiration, but he was neither focused on the forms produced by nature nor tied to organicist aesthetics but rather interested in the generating principles of nature [
18]. As he later stated in several interviews:
“I looked at flowers and insects, at organic forms. I wanted something that was growing out” [
19].
He himself acknowledged that it was the study of Sydney’s navigational maps that was his starting point and initial inspiration. In any case, regarding the possible source of inspiration for his formal proposal, in an interview published on 31 October 1992 in
Good Weekend magazine [
20], Utzon refutes the hackneyed explanation of the sails of the ships sailing in the bay (
Figure 4):
“Many people say my design was inspired by the sailing yachts in the harbour or by seashells. This is not the case. It is like an orange, you peel an orange, and you get these segments, these similar shapes. It was like this in my models. It was not that I thought it should be like sails in the harbour. It just so happened that the white sails were similar. I was influenced by the sails only to the extent that my father was a naval architect, and I was familiar with big shapes” [
19].
It should be noted that these explanations seem unlikely in any case, given that it seems to be proven that the orange as a source of inspiration for the resolution of the concrete shell roofs did not occur until 1962, 5 years after the competition proposal. In any case, Utzon also pointed out:
“It is fine that people find what things are from what they see. Of course, they are like sails but this is not what we meant here, but I am very happy people think this” [
19].
As mentioned, Utzon began working on his proposal for the Sydney Opera House competition strongly influenced by the recent popularization of reinforced concrete shell roofs. It is therefore a design framed within what could be considered the “organicist alternative” to the prevailing mechanistic and rationalist current represented by the almost hegemonic Modern Movement within the architectural panorama of the time [
1].
It is important to note that he worked during the competition phase without advice from structural engineering consultants. The competition proposal consists of 3-inch-thick reinforced concrete shells for the main roofs. As explained, is a very efficient typology in terms of material consumption and consequently allowed roofs to be built for large spaces at very low costs. The competition panel itself adduced as one of the main reasons for choosing Utzon’s proposal that it had been considered the most economical option among those analyzed.
Utzon therefore designed the curved surfaces that would form the roofs of the different parts of his building, imagining that they would be made of concrete, using shell structures a few centimeters thick, as eggshells. This is reflected in the competition drawings (
Figure 5). However, as opposed to the aforementioned examples of Torroja, Candela, Nervi, etc., Utzon did not take geometry as the starting point for his creative process, but rather drew a series of organic forms, inspired by nature, that did not obey any geometrically known or mathematically defined figure. He himself referred to this fact in his competition report, stating that he had refused to use orthogonal forms in order to create what he called sculptures, large sculptures. These first natural references seem to be applied as formal analogies from a purely artistic point of view, without any great technical rigor to support them.
The general profile of the roof pieces in the competition proposal is markedly horizontal, except for the larger pieces covering the auditorium areas, which are more markedly vertical. The surfaces of the roofs have smooth, rounded shapes, trying to provide an image of lightness despite the material with which they were conceived (reinforced concrete). For Utzon, again drawing on natural references, the roofs were like clouds of concrete (
Figure 6), which should float above the landscape, contrasting with the heaviness of the base on which they rested. In Utzon’s words, the base was anchored to the earth and the roofs connected with the sky. Between these two zones, an area was established which in the drawings of the competition proposal appears apparently empty and diaphanous, in order to emphasize the tension between the two antagonistic concepts mentioned above. To this end, Utzon initially conceived all the building’s enclosures in glass, a large proportion of which were also mobile, thus maximizing the lightness of the hypothetical concrete shell.
The only condition imposed by the design of a roof of this type is to be absolutely devoted to the antifunicular geometries that would ensure membrane behavior without the presence of significant bending moments. However, Utzon did not take geometry as the basis of his creative process, but “sculpted” through his physical models certain organic forms for the roofs. These shapes did not obey any geometrically known or mathematically defined form, and this aspect greatly conditioned the course that the development of the design was to take [
19].
Ultimately, the great power of Utzon’s idea won the architectural competition, not without controversy: the press announced its surprise, and great figures of the international architectural scene such as Frank Lloyd Wright publicly expressed their rejection.
4.3. Inspiration by Nature during the Project Development
Aware of the construction challenge that lay ahead and of Utzon’s total inexperience, the committee appointed the engineer Ove Arup to take over the structural design. Arup was, at 64, a celebrity in the field of structural engineering. His Danish origin and his extensive experience in singular architecture projects led the Committee to hire his engineering firm.
The initial challenge for Arup and his team was not even trying to check through structural calculations whether Utzon’s proposal was feasible; the first challenge was “simply” to be able to translate these shapes into drawings. As indicated, the sketches that Utzon presented in the competition panels were based on the direct translation of the forms that he had arrived at through physical models. These drawings of plants, elevations, and sections were made freehand without any geometric rigor, so they actually wanted to express approximately certain shapes, but they were neither coincident with each other (plants, elevations, sections) nor reproducible through known mathematical expressions (circles, ellipses, parabolas, etc.) nor, therefore, ultimately, constructible [
22].
This problem, difficult to understand nowadays due to the proliferation of all kinds of digital tools, was brilliantly expressed in 1983 by the architect Enric Miralles in his essay “How to Lay Out a Croissant” [
23], in which he reflected on the enormous complexity of such a curious task with the tools available at the time.
A few decades later, Frank Gehry solved an analogous situation in the case of the Guggenheim Museum in Bilbao with the help of 3D laser scanners and three-dimensional digital design software. With these new tools, he could directly transfer the geometry of his physical models to representable and buildable digital models, also using the so-called NURBS (non-uniform rational B-splines), curves that allowed the representation of free forms and that had been discovered a few years before, in the context of the automotive industry. However, in 1957 none of these digital tools existed, so the first thing that seemed clear was that the geometry of the roofs had to be transformed into some of the mathematically known forms at the time.
The great task in which everyone was involved at that time was to find a mathematical definition that reasonably resembled the natural shapes conceived by Utzon. The objective was clear: it was essential to find an analytically definable form that would also behave as closely as possible to that of a membrane, thus eliminating undesirable bending moments and making it possible to build a concrete shell with a reduced thickness. Utzon was obviously not aware of these constraints when he drew his proposal for the competition. He envisaged a beautiful sculpture, not a structurally efficient form.
Throughout this process of searching for a feasible solution, natural references were also recurrent on Utzon’s side. Thus, Utzon, fascinated by images of the Soviet Union’s launch of Sputnik on 4 October 1957, immediately contacted Arup to express his interest in the silhouettes of the decks having the same geometry as the space rocket had described in its trajectory into space: sections of parabolas and ellipses, with a noticeably vertical outline at the start from the podium and gradually more horizontal as it ascended to its crown (
Figure 7).
Although Utzon was unfamiliar with the geometrical fundamentals of correctly representing a parabola, he drew it freehand as best he could and sent it to Arup with a message indicating that this was the shape he wanted for the roofs. Based on the drawings in the 1958 “Red Book”, a new set of plans was drawn up, finalized in December 1960, in which both the ridge tiles and the cross-sections (hypothetical ribs) of the roofs were parabolic in shape. The drawings already showed a structural scheme consisting of two concrete sheets joined together by two families of beams in perpendicular directions. The two halves of each roof were tied together by concrete walls at the end facades. In this way, all the roof parts were interlocked with each other. As can be seen from the drawings presented, the total thickness of the concrete sheets was kept at approximately 15 cm (7.5 + 7.5 cm, including the ceramic pieces that would make up the exterior finish), the total thickness of the roof being 1.5 meters [
24].
This typological solution was soon discarded due to its impossibility of being structurally analyzed, even with the help of the first computers available at the time. Circular and elliptical shapes were also tested, considering both metal and concrete structures, to form a graphic catalog of up to 12 solutions studied. In general, these are attempts to force the structure into a known shape.
Thus, over 5 long years, from 1957 to 1962, Arup and his team tried unsuccessfully to establish a valid geometry for the different volumes of the building [
25]. Parabolic, elliptical, and similar geometries were studied (
Figure 8). Systematically, the proposals were rejected by Utzon, who viewed with great suspicion that the sculptural forms inspired by nature to which he had arrived with his initial models would be modified. In the face of the despair of the Arup team, Utzon vehemently persevered with his idea, with lapidary phrases such as:
“We can go to the Moon... of course we can build this building” [
19].
The situation was very tense. Ronald Jenkins, the project manager within the Arup team, a few months before he resigned from further involvement in the project in 1961, stated:
“We went to all this trouble because of the shells being the wrong shape as we pointed out to you right at the beginning” [
19].
His major objection to Utzon’s original design was that the shapes of all the shell parts were different from each other. In addition to this, the fact that they had no defined geometry made it impossible to reuse the formwork, which in turn drove up construction costs.
A few months later Arup presented two alternatives to Utzon; the first consisted of “V”-shaped concrete ribs, taking up the idea used on the ground floor beams, consisting of folds in a continuous concrete surface, but with the added complexity of having a two-way curvature in the case of the roof beams. The use of steel was thus dispensed with, returning to a solution of reinforced concrete only.
Utzon received Arup’s proposal loud and clear:
“I don’t care what its costs. I don’t care what scandal it causes; I don’t care how long it takes, that is what I want” [
19].
Utzon was not very much in favor of the use of a “hidden” steel structure inside the concrete structure as he considered it to be a “dishonest” gesture. On the contrary, the triangular concrete beams seen from the inside clearly showed the load path, which he considered essential for an acceptable solution for the roof structure. Thus, the family of concrete beams arranged in a fan-shaped pattern, concentrating at their confluence at the supports, was completely to his liking and immediately met with his approval.
4.4. Inspiration by Nature in the Final Solution: The Orange Analogy
Having established, at the typological level, the fan-shaped concrete rib scheme as the solution agreed as valid by all parties, the geometrical part of the problem remained to be solved: what shape should be given to the surfaces so that they would be geometrically representable, provide a correct structural performance, and also provide a simple construction?
Utzon did not have the technical background to solve a problem of this magnitude, yet it seems that he was the one who quite by accident came up with the solution to a problem that some of the leading engineers of the day had been grappling with for several years. It certainly seems clear that Utzon would have been unable to find the solution, even if this discovery had been largely fortuitous, without the “training” that the Arup engineers had unwittingly given him through the countless meetings they had held on the subject to try to make Utzon clearly understand the problem they were facing. It is well known that the correct formulation of a question is the best basis for finding an answer to it, and this part had certainly been to the credit of the Arup team.
At this time of maximum pressure, when the urgency to finally find a viable solution for the roof construction was pressing, the search for inspiration in nature remained the main driving force for Utzon.
Several years ago, Eero Saarinen, during a breakfast with Utzon, had explained the behavior of his concrete shell roofs for the TWA building by cutting a grapefruit and showing the shapes of its envelope.
There is no unanimous version of these facts, but apparently, Utzon, very angry with the way things were going and frustrated that his life’s project was doomed to failure, went into the factory in Hellebaek where all the models of the project were displayed. He began to stack the shells of the large model to make space when he noticed how similar the shapes appeared to be. He noticed that they fit together perfectly, like a Russian doll. As a result of this fortuitous discovery and asking himself about the geometric shape that generates different curvatures from the same radius, he realized that this geometric figure was the sphere. So, each roof could perhaps be derived from a single, constant form, such as the plane of a sphere (
Figure 9).
Moved by the great excitement that this discovery had generated in him, he immediately began to experiment with spheres. Given his limited knowledge of geometry, he began by working with his children’s plastic beach ball. He used water to draw different shapes on the surface of the ball, which, when dry, became visible through a noticeable change in color. In this way, he was able to experiment with the possible surfaces that could be generated from the same sphere. Once he was satisfied with his experiments with the beach ball, Utzon went to the shipyard in Helsingor, where he usually made his models, to build one with his new spherical shapes, and at the same time, he called an urgent meeting with Arup.
Ove Arup summarizes in the following words, taken from a conference for the Prestressed Concrete Development Group read in London in 1965, the changes on the shell roofs proposed by Utzon:
“Then Utzon called from Copenhagen saying that he had solved the whole previous problem. The point was to change the whole shape of the shells by the cut generated by the sphere itself. So now all the shells were spherical, and their ribs followed the meridian curves, on the sphere, of the same radius, 246 feet” [
26].
Arup immediately accepted the proposal and agreed to give the sphere a radius of approximately 74 meters (246 feet), which was the distance between the outer faces of the extreme ribs [
27]. The adoption of a spherical shape allowed the use of a common formwork family for all parts, which would simplify and therefore greatly reduce the cost of roof construction. In addition, the calculation was also greatly simplified.
The explanation of how Utzon was able to come up with the idea of spherical geometry is, in his own words, as follows:
“I’ve grown up in big shipyards and I had at Elsinore, close to my office, all the possibilities I wanted for studying the production of big, curved shapes.... Also, I had developed various systems for prefabrication in the building industry before the Opera House” [
28].
Given Utzon’s familiarity with prefabricated systems, he immediately proposed that the family of circular ribs be prefabricated in reinforced concrete, to which Arup, very sensitive to finding a simple and economical construction process, immediately agreed.
In January 1962, Utzon submitted his Yellow Book defining the new geometry of the roofs, details of the precast ribs, and the tiling. The proposal was very radical since, after long years of stubbornness, it represented a very strong geometric modification with respect to the competition proposal (
Figure 10). On the other hand, it also included some positive aspects such as the relative ease of graphically representing the surfaces of the different volumes, as well as the possibility of prefabricating both the structure and the roof coverings, as there was already a single curvature.
The final solution based on triangular-shaped folded concrete ribs was most likely influenced by the work of the Italian engineer Pier Luigi Nervi, and in particular by the building he constructed in 1960 for the Olympic Games in Rome: The Palazzetto dello Sport. This building covered a span of approximately 100 meters with folded, V-shaped reinforced concrete ribs only 9 cm thick.
Utzon himself reflected on this as follows:
“Through my work with curved shapes in the opera house I have been inspired to go further into free architectural forms, but at the same time to control the geometry which makes it possible to erect the building out of mass-produced components. I am fully aware of the danger of using curved forms in contrast with the relative security of basing architecture on rectangular forms, but the curved form world offers something which one will never find in rectangular architecture. The ships’ hulls, the caves and the sculptures prove it” [
19].
This finding also allowed Utzon to move away from the expression of a style, in this case the concrete shell architecture, so fashionable at the time, and towards something more timelessly universal, based on purely geometric concepts inspired by natural forms.
Thus, the final roof structure was based on large precast concrete ribs, up to 3 m deep, joined together by post-tensioning and epoxy resins [
29]. Obviously, the image of the roof structure had nothing to do with Utzon’s initial images, neither from a geometric point of view nor from a typological point of view. Both public opinion and the Australian press began to form a very critical current with the project, mainly due to the change in its forms. In this context, on February 28 of 1965, Utzon sent a letter of resignation to the Committee, and a few days later he left Australia, a country to which he would never return, not even to see his building finished. Finally, on October 20, 1973, more than 11 years behind schedule and with a budget deviation of more than 1,000%, Queen Elizabeth II of England inaugurated the building.
Once the roof structure had been structurally resolved and was self-supporting, the facades were freed from bearing any kind of stress other than that due to wind loads. With the concrete rib solution, it was no longer necessary to use the facades as tie rods to avoid the deformation that would occur when considering the roofs as concrete slabs, and the feeling of transparency that Utzon had longed for could be achieved [
28].
Once again Utzon turns to analogies with natural forms for his facade design. At first, Utzon proposed glass enclosures in vertical planes, but he soon realized that this type of enclosure would not produce the effect of transparency he was looking for, so he began to break up these vertical lines in search of more organic solutions, apparently inspired by the image of a bird in flight (
Figure 11).