Shells as a Universal Structural Type in Nature and Design

Abstract

Universal is this structure or construction which is ubiquitous in the world. It is encountered in living and non-living nature, implemented in various fields of human activity and interpreted appropriately according to the needs and the specifics of the context of use. The aim of the chapter is to verify the universal character of shells as a structural and constructive type. It is discerned that shells exist as a structural type in living and non-living nature. With the progress of civilization, humans have gained experience and mastered their deliberate use for the benefit of the individual and social existence. Therefore, it can be stipulated that research and formalization of shells in scientific and technological aspect will lead to an even wider range of applications in various design fields. In perspective are outlined the emerging form-generation opportunities in engineering, architecture and design. To substantiate the universality, a comparative research method is adopted. The existence of various shells found in nature is compared with the same or similar types applied in design. If hitherto this has happened objectively without interdependence, we demonstrate the possibilities of conscious and purposeful impetus of ideas exchange between the form-finding process, scientific research and technological development of shell structures.

1. Introduction

Universal is this structure or construction which is ubiquitous in the world. It is encountered in living and non-living nature, implemented in various fields of human activity and interpreted appropriately according to the needs and the specifics of the context of use. Universal constructions are the basis for every form-generation, both in nature and in human activity. This universality of constructions is predetermined by the imperative force of physical laws (gravity, symmetry, statics, dynamics, etc.), the properties of materials in which it is materialized and the environmental conditions where it originates or is implemented (wind, temperature, humidity, motion, light, way of life for organic constructions, etc.).

The aim of the chapter is to verify the universal character of shells in particular as a structural and constructive type.

The text will unfold in three directions:

• Presentation of the distribution of shells as a structural type in the living and non-living nature through outlining certain analogies in their universality.

• Analysis on the adoption of shells as a structural type in human activity.

• An overview of the work of the pioneers who ‘discovered’ shells as a structural type and promoted the scientific, engineering and technological research and implementation of shells in various design fields.

To substantiate the universality, a comparative research method will be adopted—the existence of various shells found in nature will be compared with the same or similar types applied in design. If hitherto this has happened objectively without interdependence, we will demonstrate the possibilities of conscious and purposeful impetus of ideas exchange in the form-finding process—the basis for which is the universality of shells.

It can be stipulated that research and formalization of shells in scientific and technological aspect will lead to an even wider range of applications in various design fields. In perspective, the emerging form-generation opportunities in engineering, architecture and design are outlined.

2. Shell structures: definition and differentiation

In our extensive teaching experience, we have used two terms—‘structure’ and ‘construction’ to differentiate two hierarchical concepts covering the semantic field of shells. ‘Shells are a skeletal type of structure—with an external skeleton; The structure consists of a rigid, monolithic, peripheral shell or an elastic, firm skin (shell)—this is the outer skeleton which covers an amorphous, mechanically unstable interior’ [1]. From the point of view of geometry, the shell is a three-dimensional curvilinear structure that can resist loads due to its inherent curvature. Shells can perform different functions according to their expediency: enclosing, limiting, unifying, protecting, preserving, covering, load-bearing, constructive, etc. One and the same shell structure can be realized through different types of constructions and materials—monolithic, cast, adhesive, webbed, knitted, woven, reticulate, rod, ribbed, cable, folded, pneumatic, etc. [1].

However, the analysis of the definitions of the concept of ‘shell’ shows that there are no uniformly accepted meaning, terminology or definitions for this structural type. In the different languages and scientific circles, the name varies from envelopes, membranes and covers to shells, laminas, etc. Furthermore, shells are not unambiguously specified as typology. A wide range of varieties are included: convex, hanging, reticulate, membranous, folded, and all of these are referred to as shells.

Several definitions will be examined to derive the specific properties of shells and what differentiates them as structural systems.

‘The most obvious definition of a shell might be through its geometry… A shell is a structure defined by a curved surface. It is thin in the direction perpendicular to the surface, but there is no absolute rule as to how thin it has to be. It might be curved in two directions, like a dome, or a cooling tower, or it may be cylindrical and curved only in one direction’ [2].

Another definition by the American Concrete Institute states:

‘Three-dimensional spatial structures made up of one or more curved slabs or folded plates whose thicknesses are small compared to their other dimensions. Thin shells are characterized by their three-dimensional load-carrying behavior, which is determined by the geometry of their forms, by the manner in which they are supported, and by the nature of the applied load’ [3].

Both definitions imply the importance of the geometry of shells, and their curved surface in particular, which predetermines the performance and the efficiency of the structure. This central characteristic of shells is considered by Pierre Luigi Nervi as ‘work by form’ or the principle of synthetic resistance of natural forms. Nervi writes that this capacity—resistance and structural strength according to or by the form, is common in nature, in flowers, reed, eggs, insects, crustacean, etc. [4]. The greatest success in achieving high resistance through the form can be obtained using spatial curvilinear systems such as shells. Spatiality and curvature of form are two inherent qualities in all living things in the world. Living systems can increase their size where needed and reduce the cross section of tissues, or save material while increasing their mechanical properties. Nature works primarily with curved surfaces, whose stability is based on their spatial curvature. Therefore, the strength achieved through the form is the most essential of all other means and is the most common in nature [4]. A significant lesson from nature is that natural shells are always double-curved—the most efficient form which avoids bending moments in the material and hence preserves the form regardless of the load condition [5]. E. Torroja also emphasizes the quality of shells to resist load due to its inherent curvature—the best structure is the one whose reliability is ensured by its form and not by the strength of the material [6]. Another specific of shells is that due to the even distribution of stresses through their surface, the thickness can be significantly smaller than the other two dimensions.

According to the above definitions, natural forms such as eggs and man-made concrete thin shells will be obvious representatives of the typology, but tension structures such as spider webs, balloons and tents will be included as well [2]. However, shells and tension structures display quite different properties and structural behaviour. Tension structures, as their name infers, transmit only tensile forces, while shells transmit the applied forces by compressive, tensile and sheer stresses. The first are force-active structures which are made of flexible materials and actively adjust their shape as a result of the applied loads. The latter are force-passive structures which are made of rigid materials, but because the applied forces are redirected through the surface of the structure, their shape is intrinsically related to the structural performance [7]. From the point of view of mechanics, ‘shell structures carry the applied forces mostly by the so called membrane forces’ [8]. F. Candela describes the mechanical behaviour of shells as follows: ‘the external forces, the loads, are transformed into direct or membrane stresses, that is, stresses that at each point of the surface are contained in the plane tangent to it, excluding bending in the sheet and, in this way, the material works in the most efficient way possible’ [9].

This load-resistance mechanism of shells determines their major advantages over the other structural types:

• Efficiency of load carrying behaviour

• High degree of reserved strength and structural integrity

• High strength to weight ratio

• Very small thickness ratio to other dimensions (span, radius of curvature)

• Very high stiffness

• Containment of space [10].

In addition, H. Isler adds that ‘shell structures have an inherent capacity to express structural beauty’ [11].

We can summarize that the advantages of shells consist of their economic, functional and constructive efficiency. For example, with minimal thickness and minimal use of materials, larger spaces can be covered and bigger distances can be spanned. Furthermore, due to the infinite potential for form-finding of shells, maximum functionality of the structure and aesthetic satisfaction are ensured.

3. Research methodology and study of universality

Humankind has constantly developed cognitive skills to adapt to the world through the tools of comparisons, metaphors and analogies. Through these tools of juxtaposition between objects, phenomena and images, the existence of ‘similarity’, ‘resemblance’ and ‘uniformity’ is established, and the cognitive process of transmitting information from one particular subject to another is realized. As a result, comparisons, analogies and metaphors play an important role for the perception and creativity, problem-solving, knowledge gaining, insight, discovery and decision-making. Many researchers postulate the heuristic and productive value of analogies and metaphors for both science and arts. The transfer of meanings from the known to the unknown and the discovery of similarities are a natural mental activity for assimilation of new information. An interesting fact is that psychoanalysts postulate a subconscious basis of the metaphorical thinking. From the point of view of the functional asymmetry of the brain, it has been found that the right hemisphere plays a major role in the understanding of metaphors. The brain works with gestalts. Most probably, it is the right hemisphere which carries the metaphorical (archaic, mythological, complex) consciousness and participates in the decoding of metaphors through the use of the complex, gestalt perception, while the left hemisphere is oriented towards the rationally presented information. The heterogeneity of thinking is determined by the functional specialization of the two hemispheres of the human brain [12].

The specifics of every creative process can be described as metaphorical thinking, which is based on a compound of mental images where image and concept are inseparable. By metaphorical thinking psychologists understand the ability to approximate the semantic differentiation between the individual images of the objects, symbols and concepts, as well as the unexpected connection in an inseparable whole of several normally unconnected parts; the ability to converge concepts and to reach new conclusions. The metaphoric transfer from the known to the unknown with the convergence of different images and the establishment of similarities is actually the way to create new and original ideas. It is quite probable that creative thinking operates with global inseparable visually spatial images [13].

We set the task to trace the cognitive process of adoption of shells in human activity, through analogous models of the structural type found in the living and non-living nature, along with examples from the historical and current human experience. In this way we will verify their objective universality.

4. Distribution of shells as a structural type in the living and non-living nature. Analogies for universality. Interpretations

The meaning of the word ‘discovery’ is that man discovers for himself, for his existence, for his adaptation or for aiding his daily life, something that already exists in the world. This is valid for all non-technical ‘discoveries’ including the subject of our study—the shell as a structural type. Shells can be found everywhere around us, in the living and non-living nature. They have been adopted in their huge variety to serve humans from ancient times to the present day.

Shells in non-living nature for example, are represented by earth’s crust, earth’s mantle, various earth and rock bridges, caves, tunnels, air bubbles, foam, etc. (Figure 1).

Figure 1.

Shells as a structural type in non-living nature. Analogies.

In living nature shells are very common both in the flora and the fauna. The examples are numerous. In the animal world these are skulls, turtle shells, the plates of armadillos, beetles’ elytra, eggshells, molluscs, snails, tusks, skin, biological membranes, etc. (Figure 2).

Figure 2.

Shells as a structural type in fauna. Analogies.

Shells can also be observed in various animal constructions—nests, termite mounds, formicaries, animal shelters (Figure 3).

Figure 3.

Shells as a structural type in animal constructions. Analogies.

In the world of plants shells are found in coconuts and other types of nuts, the skin-shell of seeds and fruits, soft shells of leaves and flowers, etc. (Figure 4).

Figure 4.

Shells as a structural type in flora. Analogies.

5. Mastering and dissemination of shells as a structural type in human activity. Analogies for universality

5.1 Historical retrospective of human experience

In his historical development, man has adapted to the environment as a result of his instincts for self-preservation, home and construction, through cultivating observation skills of the surroundings and imitation of wildlife. Thus personal experience has been accumulated and skills have been developed. A historical fact is that with the progress of civilization humans have gained understanding and mastered both the intuitive and the deliberate use of shells for the benefit of the individual and social existence.

5.2 Areas of application

5.2.1 Pottery

Pottery is one of the most ancient crafts in the world. It is believed that it appeared with the transition to sedentary life, at the end of the Stone Age more than 20,000 BCE. Pottery was used to meet daily necessities in the form of various utensils, tableware, storage containers such as amphorae, silos, depositories for water, olive oil, wine, grain, olives, etc., ritual vessels for weddings and funerals, etc. This is one of the first human activities where shells have been mastered as a structural type. As a result of its strength, resistance to climatic changes, moisture, temperature and other properties, among them their structure and shape, archaeologists have discovered preserved earthenware from prehistory and the late Palaeolithic. In general, early earthenware was deliberately made with rounded bottoms to avoid sharp corners that are prone to cracking. The potter’s wheel predetermined the achievement of a favourable curvilinear, spherical or oval shape which ensured both the functional and constructive pertinence of the vessels (Figure 5).

Figure 5.

Pottery—Shell structures in human daily life.

5.2.2 Basketry

Another traditional craft common for all people around the world is basketry. The process of weaving twigs, reeds, grasses or leaves has been practiced since ancient times to the present. This is one of the most universal arts, which also ranks amongst the earliest industries. In addition, weaving probably indicates the origins of all textile arts. Baskets are containers made of willow, wicker, reed, palm leaves or other vegetal material, intertwined in a variety of shapes and sizes depending on the function. They were used mainly for storage, protection and transportation of different goods. It is worth noting that weaving preceded pottery, if judged by the decoration found on ancient pottery which was derived by the traces left by the shape of the baskets used to make earthenware before the invention of the potter’s wheel. The influence of the art of basketry is clearly traceable in the development of the art of porcelain, as well as the capitals-baskets in Byzantine architecture. In antiquity the shields of the soldiers were also made using the weaving technique. Boats-baskets used on Tigris and Euphrates rivers were mentioned by Herodotus. They were round and covered with bitumen. Boats with such shape can still be found on these rivers and similar types with analogous construction are used on the rivers of India. The methods of construction have not changed significantly [14]. If we study and analyse the shape of the different types of baskets, the complex curvilinear surface typical for the shell structural type can be observed, including the complex shape of the parabolic hyperboloid (Figure 6).

Figure 6.

Basketry—Shell structures in human daily life.

5.2.3 Building construction

In building construction shells have been utilized since ancient times in a wide variety of buildings. At first, these were residential and farm structures, granaries, barns, furnaces, tombs. Subsequently, the functional typology is extended and covered markets, religious buildings, wide-span public buildings, sports facilities, engineering structures such as water tanks, dams, containment shells of nuclear power plants, piping systems and others are included. Today shells as a structural type are ubiquitous.

The initial stage of the distribution of shells is especially interesting. Similar to animals, man possesses building instincts. In the primitive phase of his development, to adapt to the environment, man built shelters using the locally available materials. The efficiency of the natural shells was observed, borrowed from the flora and fauna and imitated in the constructive process. Depending on the availability of materials and the nature of the local climate, shells were constructed either by weaving, gluing, combining both methods or by masonry, intuitively following the imperative role of the physical laws of gravity, the properties and formal qualities of the materials. Among the construction materials used were the traditional stone, wood (cane, reed, rods, etc.), animal bones and skins, straw, mud and clay, ice, etc. Gradually, the construction skills of weaving, knitting, masonry, coating, pressing, moulding, etc. were cultivated. Through intertwining branches, sticks, leaves, bones and other materials at hand, a lattice was created—a kind of reinforcement which was then plastered with mud or clay. Walls were constructed either with stone, compacted snow cut into rectangular blocks or pressed sun-dried mud reinforced with straw. It can be asserted that shells in the form of domes developed as one of the major and most common constructive type in the building tradition worldwide. Indeed, one of the earliest shelters was the small woven dome. ‘The framework was made of pliable branches or saplings, woven together, utilizing the strength inherent in a double-curved surface to span a useful space. It was then covered with leaves, thatch, or animal skins, whatever was locally available’ [15]. This construction type is still implemented today in some ethnic groups in Africa. An impressive example of a vernacular building practice is the tolek of the Mousgoum—an ethnic group living in Northern Cameroon. This thin domed hut (5–7 m in diameter and 7–8 m high) is constructed of mud mixed with straw which sets hard in the sun. The almost perfect parabolic curve eliminates all hoop tensions and ensures the stability of the structure in spite of being extremely thin [16]. The external ornamental ribs are not introduced to strengthen the structure but to protect from local injuries, to serve as water drainage and to allow people to climb atop the house to aid construction or maintain the coating without the use of scaffolding (Figure 7) [17].

Figure 7.

Shell dwellings.

5.2.4 Transportation industry

Alongside these traditional types, shells as a structural type are closely related to the modern world. The industrial era saw a rise in their utilization in various domains of human activity. A huge field of application is the transportation industry. Among the examples are car chassis, boat hulls and airplane fuselages. The so-called monocoque construction consisting of a single shell outer skin which performed the load-bearing function was indispensable for reducing the vehicle weight while providing greater strength and better streamlining. In addition to these properties, monocoque shells increased the safety of the people using the vehicle as it efficiently absorbs the impact and spreads the energy on its surface [18]. The materials utilized initially were plywood and aluminium and later fiberglass and carbon fibres. Moreover, all machines connected to some kind of movement, including such of the military industry such as submarines, missiles or rockets are taking advantage of the shell structure to ensure appropriate weight to strength ratio and aerodynamics (Figure 8).

Figure 8.

Shell vehicles.

5.3 Pioneers: discoverers of shells as structural type. Legacy and analogies for universality

The beginning of the scientific research, engineering implementation, formalization and technological application of shells began in the late nineteenth century. This beginning was set by the Russian engineer and inventor Vladimir Grigoryevich Shukhov (1853–1939), who was the first to introduce grid shell structures in 1896. He patented a number of inventions and laid the foundations of the theory of shells [19]. In 1895, Shukhov submitted a patent application for mesh coverings in the form of a shell. These were meshes made of steel strips and profiles with characteristic rhomboid cells. They were used for lightweight suspended roofs and grid vaults with a large span. The development of these mesh coverings marks the invention of a completely new load-bearing constructive type. For the first time, Shukhov converted a suspended covering into a completed spatial structure which was used again not until several decades later. Compared with the highly developed at that time constructions with metal arches, his lattice shell vaults formed by rods of the same size made a significant progress and replaced the traditional spatial trusses [20]. After two experimental buildings (two lattice arches built in 1890 and a suspended roof built in 1894), Shukhov presented his innovative roofs at the All-Russia Exhibition in Nizhny Novgorod in 1896. The Bari Company built eight pavilions with quite impressive dimensions. Four of them were with suspended roofs and the other four had cylindrical lattice arches. One of the exhibition halls had a load-bearing structure which was not a mesh but a thin membrane—a construction which has never been used before. In addition to the pavilions, a water tower was built in which Shukhov transferred the gridshell to a vertical lattice structure with hyperboloid shape.

Legends are told about the sources of inspiration for his heuristic ideas. Shukhov himself shared: ‘One day I arrived in my office earlier than usual and I saw: my wicker waste bin is turned upside down, and on it stands a rather heavy pot with a ficus. And the future design of the tower was standing clearly in front of me’ [21]. Thus, the waste bin with interwoven wicker twigs became the analogy for the famous Shukhov tower which together with his gridshells became universal and served as models for different types of design (technical, constructive, architectural, furniture, product, etc.) for more than a century. Based on Shukhov’s projects, approximately 200 towers were built in Russia and abroad, including the celebrated Shabolovka Radio Tower in Moscow (1919–1921)—a modification of Shukhov’s huperboloid structures.

According to Shukhov, ‘what looks beautiful, it is also strong, sturdy. Human eye is accustomed to the proportions in nature, and in nature what is strong and appropriate survives’. He designed numerous water towers based on the same constructive principles but though mass-produced all of them featured a striking variety of shapes. Shukhov delightedly used the property of the hyperboloid to take various forms and experimented by changing the position of the connections between the rods or the diameters of the upper and lower ends. Thus, each tower obtained its own appearance, differing from the others and demonstrating its own load-bearing capacity (Figure 9). The complexity of the design task, including in terms of the construction, has always been resolved with a remarkable understanding and sense of the form. The work of Shukhov gained great popularity in the twentieth century, and his lightweight spatial structures have influenced many contemporary architects, among them Sir Norman Foster (Figure 10).

Figure 9.

Shukhov’s gridshell structures.

Figure 10.

Modern interpretations of Shukhof’s gridshells.

Another upsurge in the utilization of shells in architecture took place as a result of the development of reinforced concrete, the advances in the production possibilities and the development of calculation methods and theoretical analysis of shells. Among the most notable early examples exploring the structural strength and formal qualities of concrete, which influenced the building of thin shells were the Centennial Hall in Breslau, Germany (1911–1913) by Max Berg featuring a diameter of 65 m and the Dirigible hangar at Orly Airport, France (1916), by Eugene Freyssinet spanning 75 m. A few years later appeared the first single shell building made of reinforced concrete—the Zeiss Planetarium, constructed in 1923–1924 in Jena, Germany. This was a 16 m concrete dome with an unprecedented thickness of only 3 cm which allowed the reduction of the weight to 1/30 of the weight of a conventional dome structure. This radically new structural type developed by Dr. Walter Bauersfeld—chief engineer at Carl Zeiss AG, together with the structural engineer Franz Dischinger from Dyckerhoff & Widmann AG featured a framework made of steel rods, sprayed with ferroconcrete using the shotcrete technique, later patented as the Zeiss-Dywidag-System [22]. This forerunner of concrete thin shells gave impetus to the design of many remarkable lightweight structures spanning hitherto unthinkable distances with shells’ characteristic slenderness (Figure 11).

Figure 11.

Precursors of concrete thin shells.

One of the major discoverers of shells as a structural type in architecture was Pier Luigi Nervi (1891–1979). The trajectory to reach the application of this structural type in his engineering and later architectural career is very interesting. In the 1940s, Nervi invented and patented a new material called ferrocement. His patent was based on ferciment, a product devised by the Frenchman Joseph-Louis Lambot in the mid-nineteenth century to construct boats. Nervi refined this material to use less steel. His ferrocement was composed of dense concrete reinforced with a fine steel mesh which ensured both lightness and strength of the structure. The difference between ferrocement and reinforced concrete is mainly in the reinforcement—the first consists of a series of layers of wire mesh with a small diameter (0.5–1.5 mm), reinforced with rods which is then immersed in a cement mortar. This allowed the creation of very thin sheets that were very elastic, flexible, malleable, lightweight, resistant to cracking and extremely economical. It was possible to bend the metal mesh in any shape and this liberated the experimentation with the form.

Using this new material Nervi built his first shell boats. He had ambitious plans to build a 400-ton concrete ship which was impeded by WWII. After the end of the war he succeeded in building a 165-ton sailboat with a thickness of the hull 3.6 cm and a 11.6 m ketch with hull thickness just 1.27 cm. The first application of ferrocement in building construction was a warehouse in Rome featuring an undulating roof with a thickness of 3 cm. The structural properties of the material were vital for the creation of his most renowned shells like the Torino Esposizioni, Turin (1949), Palazzo dello Sport, Rome (1956), Palazzetto dello Sport, Rome (1958), Paul VI Audience Hall, Vatican City, Vatican (1971), Norfolk Scope, USA (1971). Analysis of these diverse buildings, differing in scale, construction, function and appearance shows certain sameness. It is exhibited in the ribbing as though the shell (the skin of ferrocement) was stretched and cast over the ribs. The ribs, as those applied in Gothic cathedrals, perform both constructive and decorative function. By applying ribs and curvilinear surfaces, Nervi improved the strength of the structure while making them lighter, spanned greater distances and covered bigger areas without the need of intermediate columns. Impression of continuity with the historical architectural tradition is created. He combined simple geometry with assembly of prefabricated elements to propose innovative designs. This constructive method transfers analogies to both wildlife and some traditional crafts. Undoubtedly, Nervi’s innovative engineering solutions possess a great aesthetical appeal, completely natural and organically interweaved with the visible narrative of the constructive logic. Though he claims that he never thought directly about the beauty of his works, which he believes appears always when the construction is appropriate. Moreover, Nervi gives prominence to intuition which should be used as much as mathematics in design, especially when thin shells are considered (Figure 12) [23].

Figure 12.

Nervi’s shells.

There are many engineers and architects, shell apologists who have contributed with their outstanding work to the theoretical and practical adoption of shells, for their mathematical formalization and universal application.

Among them are:

Buckminster Fuller (1895–1983), who invented the lightweight and durable ‘geodesic dome’—a three-dimensional steel shell made of straight rods, the first self-contained building that can withstand its own weight without any limits in its dimensions (Figure 13).

Figure 13.

Shells by Buckminster Fuller.

Idelfonso Sánchez del Río Pisón (1898–1980)—Spanish engineer who introduced an innovative roof system based on manufactured on site modular corrugated thin shells and lightweight fired clay elements, famous for his innovative prototype for rationalized construction of water tanks (Figure 14).

Figure 14.

Shells by Idelfonso Sánchez del Río Pisón.

Eduardo Torroja y Miret (1899–1961), who advanced the concept of the shell as a structural element bringing its formal expression to new heights in the building of market halls, stadiums, hangars, churches (Figure 15).

Figure 15.

Shells by Eduardo Torroja.

Anton Tedesko (1903–1994)—German-born architect who introduced concrete thin shells in the United States as a representative of Dyckerhoff & Widmann AG and was responsible for the design of more than 60 shells adapted to the American context (Figure 16).

Figure 16.

Shells by Anton Tedesko.

Oscar Niemeyer (1907–2012), who created the modern ensemble Pampulha, which is considered the forerunner of concrete shells in Brazil (Figure 17).

Figure 17.

Shells by Oscar Niemeyer.

Félix Candela (1910–1997) known for his shells with double curvature, namely hyperbolic paraboloid, where the shape was reduced to its pure essence through elimination of the supporting edge ribs allowing its main characteristic—extreme thinness, to be exposed to the attention of the viewer (Figure 18).

Figure 18.

Shells by Félix Candela.

Eero Saarinen (1910–1961)—Finnish-born American architect who gained recognition not only for his innovative buildings using catenary curves, but also for his furniture designs where shells were utilized (Figure 19).

Figure 19.

Shells by Eero Saarinen.

Kenzo Tange (1913–2005)—the Japanese protagonist of shell architecture whose stadiums for the 1964 Olympic Games in Tokyo are referred to as the most beautiful structures of the twentieth century (Figure 20).

Figure 20.

Shells by Kenzo Tange.

Eladio Diestre (1917–2000)—Uruguayan engineer who preferred traditional brick over concrete to design double-curvature masonry shells with spectacular cantilevers showcasing the possibilities of the material (Figure 21).

Figure 21.

Shells by Eladio Diestre.

Frei Otto (1925–2015) famous for his pioneering lightweight tensile and fabric membrane structures advancing the ideas of sustainability even before the term itself was coined (Figure 22).

Figure 22.

Shells by Frei Otto.

Heinz Isler (1926–2009), who rarely used mathematical theories to calculate his shells but relied on experiments with reversed hanging cloth and membrane under pressure physical models in the form-finding process to validate the design of his remarkable thin shells (Figure 23).

Figure 23.

Shells by Heinz Isler.

Among our contemporaries, this group of shell masters includes Frank Gehry (1929), Norman Foster (1935), Nicholas Grimshaw (1939) and Santiago Calatrava (1951).

5.4 Zaha Hadid’s homage to Félix Candela

The influence of the shell masters and their visionary work on contemporary architecture is indisputable. Their revolutionary shells changed profoundly the way structure is understood. They altered once and for all the nature of the architectural form and opened new possibilities for the generations to come. Furthermore, they inspire and encourage experimentation from the perspective of the current day through the use of new media and technologies.

In 2018, during her first exhibition in Latin America ‘Design as a second nature’, Zaha Hadid Architects created an impressive shell installation in honour of the Spanish-Mexican architect and engineer Félix Candela. KnitCandela is an experimental sculpture rethinking his inventive concrete shells by introducing new computational design methods and innovative KnitCrete formwork technology. The dynamic geometry is inspired by the smooth shapes of the colourful traditional Mexican dress sarape and at the same time refers to Candela’s famous restaurant Los Manantiales at Xochimilco. While Candela explored various combinations of hyperbolic paraboloids in his projects to obtain variety of designs, KnitCrete allows the realization of much wider range of anticlastic geometries. The cable network and fabric formwork system enables the efficient building of expressive free-form concrete surfaces without the need for complex formworks. Its thin, double-curved concrete shell with an area of almost 50 sq. m. and weight of 50 tons is made with a formwork of only 55 kg which was transported to Mexico from Switzerland in a suitcase. The experimental structure explored the possibilities for integration of digital production with traditional craft and construction methods. ‘KnitCrete is an innovative material-saving, labour-reducing and cost-effective formwork system for the casting of doubly curved geometries in concrete…KnitCrete formworks use a custom, 3D-knitted, technical textile a lightweight, stay-in-place shuttering, coated with a special cement paste to create a rigid mould, and supported by additional falsework elements such as tensioned cable-net or bending-active splines’ [24]. With this installation Zaha Hadid Architects display not only their admiration for one of the pioneers in the development of shell structures, but also unambiguously declare their creative credo to follow his oeuvre. In fact, Zaha Hadid is devoted to developing curvilinear structures, elevating them to an architectural and design style—the style of Parametricism (Figure 24).

Figure 24.

KnitCandela by Zaha Hadid architects.

5.5 Mathematical, engineering and technological development of shells. Future perspectives

Mathematical, engineering and technological development of shells continues for more than a century. In the beginning of the twentieth century, they were rarely used due to the complexity of their calculation, the increased requirements towards the quality of the materials and compliance with building technologies. An indicative example is the building of Sydney Opera House, whose complex geometrically undefined shape of the roof shells impeded the engineering team to calculate and detail the design. The initially envisioned thin shells were technically impossible to be realized which required rationalization of the form and construction. The challenge was resolved after 15 years of rigorous problem-solving process extensively using the then emerging computer analysis. Subsequently, the structural scheme was fundamentally converted with the introduction of fanlike ribs to support the distinctive roof that we recognize today [25]. In the 1960s came the ‘golden’ period of shells—the heyday of Pier Luigi Nervi, Eduardo Torroja, Félix Candela and the other shell masters. This was the period when structural engineering emerged as a separate discipline, when computation entered design, architecture and engineering to aid structural optimization. Though at the end of the twentieth century, a decline in the construction of concrete thin shells is observed, in the last two decades shells are already mastered and widely used in architecture and design. The invasion of computers in the practice of structural analysis, the emergence of new materials and new technologies has a positive impact on the spreading of shells in all fields of design. The ubiquitous application of shells is also supported by the advent of relatively new branches of scientific knowledge such as bionics, biomimetics, synergetics, etc. The introduction of new knowledge is always ensured by the constant progress of computer technology. The aim of bionics is to search for analogies and know-how in nature and their transfer into human activity. Biologically inspired engineering consists of the application of biological methods and systems for the study and design of engineering systems and technologies. More specifically, biomimetics borrows creative techniques through the use of biological prototypes to derive engineering ideas and solutions. This approach is motivated by the fact that biological organisms and their organs are fully optimized by evolution. Technology transfer between living forms and artificial artefacts is desirable because the evolutionary pressure forces living organisms to be efficient. Synergetics is another interdisciplinary research field with the objective to study natural phenomena and processes based on the principles of self-organization of systems. This refers to synergetics, as defined by B. Fuller, according to his views on nature’s geometry and the consistent self-organization of natural forces (his thesis is that ‘energy has a form’), and the universality of the ideas about the world. The achievements of these sciences, in the context of our study, support the thesis and add evidences for the universality of structures and constructions, including shells as a structural type.

6. Conclusion

Traditionally, the opinion that shells are structures developed and applied mainly in engineering and architecture is popular. In the present study, however, we set the objective to prove that shells are a universal structural type, i.e. that they are ubiquitous in living and non-living nature, that they have been utilized in various spheres of human activity since antiquity, that they are subject of development and application in all fields of design.

Evidences for this universality were traced in the distribution of different shell types, by comparing and extracting similar patterns found in living and non-living nature and in human activity—pottery, basketry, weaving, building construction, mechanical engineering, etc. from antiquity to the present day. An overview was made of the achievements of some of the most prominent pioneers—‘discoverers’ of shells who contributed to their adoption in science and engineering and their practical application, again in various fields of human activity.

We found that the acquisition of shells in antiquity was ubiquitous and was based on instincts, intuition, observation and borrowing from nature during the process of development of practical skills. The adoption of shell structures took place independently in various human activities, organically and naturally, applying one and the same materials and methods adapted to the living environment. Despite the geographical remoteness and the lack of interaction, the structural solutions around the world are surprisingly universal. With the cultivation of various crafts—shell structures were gradually spread through internal and local interactions, thus expanding the universality of their application.

Regarding the pioneers in the professional, scientific, theoretical and practical acquisition of shell structures, a general conclusion related to the form is made: ‘work by form’—stability and structural strength according to or by the form, says Nervi; the most efficient structure is the one whose reliability is ensured by its shape, writes Torroja; geometry is the most important because energy has a form is Fuller’s maxim. In general, the strength of the shell is hidden in its form. It is the form, that is key to the universality of shells, not the size, nor the materials but the spatial curvature determines its efficiency. This is where is implemented the principle of mini-max which is pursued in contemporary design—maximum effect with minimal consumption of materials, energy, labour, etc. a priori embedded in the evolved natural objects and the traditional crafts around the world.

The contribution of the research is in revealing the possibilities for activation of the interactive ideas exchange in the form-generation process between scientists, practicing engineers and designers, as a result of the conscious understanding of the universality of the structural type. If hitherto shell structures have been used intuitively and independently in the various human activities, this research highlights the perspectives for unifying the efforts based on their universality. We can stipulate that the interactive study of shells and their formalization will lead to even greater application in the different fields of design.

Acknowledgments

The publication of this chapter is funded by the Research Department at the University of Monterrey, Mexico.