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40 CHAPTER 3-HISTORIC DEVELOPMENT OF GLASS FAÇADES

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“All architecture is great architecture after sunset; perhaps architecture is really a nocturnal art, like the art of fireworks.” Gilbert K. Chesterton

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Historically, the relationship between glass and architecture is at its most sophisticated when transcending technical limitations, notably those imposed by load-bearing masonry construction which restricted the width of window openings. When glass was first used in architecture and construction, more than 2000 years ago, the limitations of masonry and weaker building materials meant that its prominence was restricted to small windows. The first break with convention was the Gothic “exoskeleton”; the stone frames and flying buttresses of medieval cathedrals made possible unprecedentedly tall, arched windows composed of myriad fragments of glass (Fig. 1). The role of illumination was spiritual as well as literal; the stained glass panels efficiently disseminated Biblical narratives to a largely people. The architectural quest for transparency, weightlessness and luminosity began, in effect, with the radiant membranes of coloured light in the Medieval Era. The trend for tall, stone Gothic churches facilitated the use of elaborate stained glass windows made up from fragments of coloured glass and depicting striking Biblical scenes [27].

In Gothic cathedrals, the glass is the main protagonist, since the whole architecture seems to be designed as a function of the stained glass windows. "Light is the essential

complement of the glory of God”: this was said in the Middle Age, and this was the basis to

organize the internal spaces and to capture the light with all its symbolic value [84]. The 13th century is the heyday of the art of stained glass window, because the gothic architecture offers immense opportunities to decorating glass (Fig. 2). The architecture of the great Gothic cathedrals gradually replacing the glass to the wall becoming itself a “wall of light” that changes fascinatingly at any time of day. The cancellation of the wall as a support and

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the ability to use large glass surfaces capable of light to flood the interior of the cathedrals contributed as well to create a new conception of space.

From an historical viewpoint, then, it is possible to identify a dual function for glass façades: on one hand, stained glass windows define and close an internal space, protecting it from the weather, on the other hand they serve as lighting, letting the light penetrate. While in Romanic church the closing function was entrusted to the wall and the lighting function was related to the window, in the Gothic cathedral architects perfected the system of pillars and vaults, replacing walls and windows with a single element, the stained glass façades, capable of perform both functions [16].

Fig. 1 Chartres Cathedral. La belle verrière (1180-1225)

Fig. 2 Reims Cathedral. Stained glass facade above the choir (13th century)

Perhaps the most distinctive feature of Gothic cathedrals is how the architectural structure has been adapted to meet the needs of stained glass [6]. The use of a three-part elevation with external buttressing allowed for far larger windows than earlier designs, particularly at the clerestory level. Most cathedrals of the period had a mixture of windows containing plain or grisaille and windows containing dense stained glass panels, with the result that the brightness of the former tended to diminish the impact and legibility of the

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latter. With the architectural evolution of the façade construction, windows become real glass partitions, the main source of illumination and allowing depiction of vast iconographic cycles of Christian thought (Fig. 3).

Fig. 3 Chartres Cathedral. Detail

The Middle Age, to which many attribute the definition of the historical darkest season, revealed instead a triumph of light and colour. Gothic stained glass windows related the stories of the Bible to an illiterate people and promoted the architectural trend for transparency, luminosity and weightlessness through glass. For Gothic artists and craftsmen the symbolic meaning of the space had a very important value: the light entering through the windows is something more than just a physical medium that allows you to see.

Outdoor natural light, passing through the coloured glass, it turns into light of a transcendent and mystical, symbolizing God's light illuminating the faithful. The façades, the windows, the portals, capitals, all possible usable spaces became instruments of teaching, performing the function of didactic “books”. For those kind of façades, the term "painting with light" can be used: those façades must be seen against the light, since the light has to pass through the stained glass to give life and animate them [86].

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The beauty of stained glass windows is subject to optical laws. The difference between the window and the painting is of two types: the colours of the glass are juxtaposed and not mixed and the stained glass window, made of single-layer glass, gets its effects with the translucency, unlike painting, the window is illuminated by the refraction and not by reflection. The shades have a relative value compared to the shades of opaque surfaces nearby. This relativity of tone depends on three factors: the light transmission value, the influence of colour and intensity of external light.

TRANSMISSION OF LIGHT: for light colours, the light transmission range is from 53 to 85%,

while it is between 5-10% for dark colours, hence the phenomenon of flooding of dark colours from light colours.

RADIATION OF LIGHT: is different depending on the colour of the glasses. Blue is the most

intense and radiant colour, and it is active, while the red colour is passive and less bright. Therefore the light of a stained glass window shall be adjusted with the separation of the blue and red colours to prevent the mixing and the effect of a purple glass. This separation can be made by lead joints, grisaille or white glass.

INTENSITY OF LIGHT: a visual balance in a given range of colour shall be found to make a

window readable. In the dark of night, the glow intensifies blue, while red becomes black. At midday, blue stands out and red becomes bright. This phenomenon must be taken into account in the composition of a window: more blue glass is required in North façades while and red glass is preferred in the South. The black does not exist in stained glass as colour, and it is included as a side lead or a grisaille. White can have a negative value when a window is broken or a positive value when working on.

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It was not until the 19th century that glass in architecture took its next significant step forward. Before this time, the manufacturing process itself restricted the use of glass to only small sheets, which is illustrated in the prominent use of cottage pane glass and intricately

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divided windows in 18th century architecture [6]. The introduction of iron and other materials during this time meant that glass could take on a whole new role in architecture. Thanks to the materials now existing to hold it in place, coupled with the new ability to produce large sheets, the possibilities for the use of glass in construction became nearly limitless.

Cast-iron architecture spread in Europe between the end of the 18th century and the first half of the 19th century. Points of the expansion of this technology applied to architecture were France and England but had good spread also in Italy. The development of engineering is essentially bound to the developments of the Industrial Revolution, which new processes started up in England in the late 18th century and spread rapidly in many other nations. An important step for the new role of engineers was in 1794 with the founding of the École Polytechnique in Paris and the establishment, at the same University , of a course in Construction Science [27].

The major technological innovations in the Industrial Revolution led to a significant increase in the production of steel and cast iron, with a significant reduction in costs; such materials once used in architecture for the realization of accessories, then found a greater application also in the building, where they were mainly used for the construction of iron bridges, buildings with metal frame, transparent envelopes made of steel and glass. Engineers began to experiment with things like conservatories and entire walls of glass that were held together by high trussed steel arches and finger fixings. Therefore, the most spectacular and important applications of this new technology are bridges, green houses, buildings for Universal Exhibitions, industrial buildings, railway stations, covered markets, theatres and galleries (Fig. 4 a, b; 5; 6; 7; 8; 9; 10; 11; 12 a, b).

However, the application of these new building materials did not lead to the formation of an architectural style, independent from the various 19th-century revivals, but it was often limited to the construction of roofs of flooded neoclassical, Gothic and neo-Renaissance. Even works made entirely of iron never achieved a real independence from tastes and shapes of eclectic architecture.

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BOTANICAL GREEN HOUSES

Fig. 4 a, b. Palmhouse, Kew Gardens, London, 1844-48.

GALLERIES

Fig. 5 Galleria Vittorio Emanuele, Milan, 1865. Fig. 6 Galleria Umberto I, Naples, 1890.

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EXHIBITION PAVILIONS

Fig. 9 The Crystal Palace, Hyde Park, London, 1851.

DEPARTMENT STORES

Fig. 10 Mercato Centrale, Florence, 1870-74. Fig. 11 Les Grand Halles, Paris, 1853.

THEATRES

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From a technological point of view, glass curtain walls can be traced back to the new different kind of buildings developed in the 19th century. Factually they were not viewed as architecture but rather as functional buildings: that is probably why a predominantly structural innovative construction could be developed even if it was far away the architectural design ideas of the time [27].

Beside the economic and industrial development, the transformation of social behaviours and of cultural interests provoked the introduction of new building types [32]. As an example, the new interest in foreign countries and plants that need as much daylight as possible brought out the need to have façades with a maximum of transparent area. Therefore, green houses were built with cast-iron columns and joists to form the primary load-bearing structure, while glass panes was filled with a secondary role: such construction method was not so different from later curtain walls.

But it was Joseph Paxton's building for the Great Exhibition of 1851, the astonishing Crystal Palace (Fig. 13 a, b), which reveals to the millions the potential of the new architecture. The Crystal Palace represented the most ambitious glass architectural projects of its time, a construction made up of 300 000 sheets of glass. Built in Hyde Park (London) to house the Great Exhibition in 1851, the Crystal Palace is commonly considered as a significant turning point in architectural history [32]. Even more significant is the famous speed of its design (one week of detailed drawing, after a preliminary jotting by Paxton on a piece of blotting paper) and of its construction (six months). Then, it was dismantled in 1852 and moved to another site at Sydenham, where it stood until its contents catch fire in 1936.

The modular steel-frame tradition of late 20th century architecture has in this building its most distinguished ancestor. This structure built from steel and glass paved the way for further exploration of glass as an architectural element: Von Gerkan and Marg's vast, barrel-vaulted Exhibition Hall in Leipzig, designed in collaboration with Ian Ritchie (March 1996), is clearly a late twentieth-century reinterpretation of the Crystal Palace, using contemporary structural and material technologies of trussed steel arches and silicone jointed glass sheets held in place by cast steel finger fixings.

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Fig. 13 a, b The Crystal Palace

At the end of the 19th century steel construction spread in the United States of America and especially in Chicago, where the first skyscrapers in the world were built. The development of such frame construction was driven by different reasons: in 1853 Elisha Otis invented the crash-safe elevator. In 1871 a huge fire destroyed almost completely the centre of Chicago, then engineers and architects evolved a new building type as an alternative to the massive construction method with load-bearing exterior walls. The frame construction method allowed for large unobstructed spaces, and a high degree of pre-manufacturing enabled fast construction [35].

Soon this construction technique spread rapidly throughout the country, especially in New York, where, with the new century, tall buildings rose even more than three hundred meters. The Chicago School also promoted the use of the new technologies in the construction of commercial buildings, and also developed new aesthetic, especially in the design of repetitive modules of façades, also influenced by the corresponding evolution of the architectural avant-garde in Europe prior to the Modern Movement.

Architects whose names are associated with the Chicago School include Henry Hobson Richardson, Dankmar Adler, William Holabird, William LeBaron Jenney (Fig. 14), Martin Roche, Daniel Burnham, John Root (Fig. 15), Solon S. Beman, and Louis Sullivan. Frank Lloyd Wright started in the firm of Adler and Sullivan but created his own Prairie Style of architecture.

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Some of distinctive features of the Chicago School were the use of the steel structure as the main element of the buildings with masonry “terracotta” cladding, large windowed areas and repetitive and limited use of external decoration. Neoclassical elements, however, are found in the skyscrapers of the Chicago School many of which are summarized in a re-design of columns. The first floor functions as a base, the intermediate planes as a tree of vertical columns, while the top of the building is surmounted by a semi-traditional frame. The typical "Chicago window" was divided into three parts: a single large central fixed glass panel, while the two surrounding panes were operable. Even if a deep study of such developments exceed the scope of the present research, it is useful to give a brief inside into the development of the use of glass areas in the curtain wall construction. A Second Chicago School later emerged in the 1940s and 1970s which pioneered new building technologies and structural systems such as the tube-frame structure [84].

Fig. 14 Home Insurance Building, William LeBaron Jenney, Chicago, 1885.

Fig. 15 Reliance Building, Burnham and Root, Chicago, 1895.

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In the early 20th century avant-garde movements formed in many European countries: they were different in style but based on a strong national tradition and known as Art Nouveau [27]. In this time, glass façades generally assumed a decorative function and stained glass windows animate facades and elegant wavy and discontinuous roofing: examples are the Hotel Tassel, 1893 (Fig. 16) and the Hotel Horta,1901 (Fig. 17) designed by Victor Horta in Brussels.

Fig. 16 Hotel Tassel Fig. 17 Hotel Horta

The Expressionist Movement gave the glass the symbolic meaning of a utopian attitude towards the future. Founded by the collaboration between the architect Bruno Taut and the anarcho-socialist writer Paul Scheerbart in the immediate post-war period, the Gläserne Kette (Chain of Glass) elected as symbols of a utopian social democratic rebirth the "glass cathedrals" and alpine architectures, where the crystal and its transparency created endless prospectives with colours and light. Western culture and its industrial progress were renegades from Taut in favour of the Eastern traditions, hailed as the bearers of a new light (ex Oriente lux) capable of giving a new breath of life to arts and architecture.

Confined to a pictorial and experimental dimension, the dreamlike visions of the Gläserne Kette experienced a realization of Glaspavilion (Glass Pavilion, Fig. 18 a, b, c) that Taut built as a temporary structure in 1914 for the first exhibition of the Deutscher Werkbund in Cologne: this construction can show the religious attitude towards glass as a material, depositary of the Expressionist utopias of a new society to rebuild [82].

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An early drawing of the Glass Pavilion by Taut says he made it in the spirit of a Gothic cathedrals and of Solomon’s Temple. The pavilion was set on a circular base of reinforced concrete, which gave the pavilion a temple-like quality, and had a polygonal plan bounded by walls: the concrete structure had inlaid coloured glass plates on the façade that acted as mirrors. It had a fourteen-sided base constructed of thick glass bricks used for the exterior walls devoid of rectangles. The structure of the prismatic glass dome drew an exotic profile and was composed of a mesh of glass plates: each part of the cupola was designed to recall the complex geometry of nature. The natural light spread inside the pavilion and the floor-to-ceiling coloured glass walls were mosaic, therefore the effect was of a large crystal producing a huge variety of colours. The composition of the interior was inspired by ritualistic Scheerbart’s ideas and included the presence of glass-treaded metal staircases and waterfalls, to emphasize the multiplicity of light effects [82].

Fig. 18 a, b, c. Glas Pavilion, 1914.

Taut described his little temple of beauty as [82]:

“..reflections of light whose colours began at the base with a dark blue and rose up through moss green and golden yellow to culminate at the top in a luminous pale yellow”

The purpose of the building was to demonstrate the potential of different types of glass for architecture. It also indicated how the material might be used to orchestrate human emotions and assist in the construction of a spiritual utopia.

The bond between the glass and the architectural visions of the early 20th century is particularly important because it shows how some insights into the potential of glass were already conceived, although the technologies and materials to implement them were not yet available. In this regard, it is interesting to note that the first conception of a fully glass

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curtain wall, hanging from the main structure of the building, dates back to two projects of skyscrapers made by Ludwig Mies Van der Rohe in the early 1920s: these projects were revolutionary, but not yet technically feasible .

Fig. 19 a, b. Skyscraper on Friedrichstrasse, 1921. Fig. 20 a, b. Project for a glass skyscraper, 1922.

The design for the skyscraper on Friedrichstrasse, 1921 (Fig. 19 a, b) expressed the constructive principle of a framed steel structure “covered” with glass panes. Mies refused the style of the Chicago School skyscrapers, where the steel was hidden and inserted in masonry walls. He used to say that:

“Steel structures in their essence are frame structures. Neither trouble nor armoured towers. In the construction to support reinforcement, a non-load bearing wall. Then they are skin and bones buildings" (Ludwig Mies van der Rohe, Bürohaus, in" G ", n. 1, 1923)”.

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In the project for a glass skyscraper in 1922 (Fig. 20 a, b), the plan of the skyscraper took the freedom of an organic form around two circular pillars. The building is investigated in the changing iridescent surface, transparent or reflective, as Mies stated:

"My studies on a glass model showed me the right way and I soon realized that the glass

does not create effects of light and shadow, but a rich play of light reflections" (Ludwig Mies van der Rohe)

From a technological viewpoint, the quest for transparency of the building envelope was progressively developed, until the structure of the façade was completely separated from the load-bearing structures of buildings.

The Fagus Factory in Alfeld an der Leine (Fig. 21 a, b) was one of the first buildings to employ this technique. This urban shoe factory was designed by Walter Gropius and Adolf Meyer in 1911-14 and used a thin steel structure to hold up a full glass façade to meet the client's brief of an attractive outlook [16]. Regarding Fagus Factory, Pevsner stated that:

“For the first time a complete facade is conceived in glass. The supporting piers are reduced to narrow mullions of brick. The corners are left without any support... The expression of the flat roof has also changed. Only in the building [the Steiner House, Vienna] by Adolf Loos which was done one year before the Fagus Factory, have we seen the same feeling for the pure cube. Another exceedingly important quality of Gropius's building is that, thanks to the large expanses of clear glass, the usual hard separation of exterior and interior is annihilated.” (Nikolaus Pevsner, Pioneers of Modern Design, 1949)

The use of floor-to-ceiling glass windows on steel frames that went around the corners of the buildings without a visible (most of the time without any) structural support. The other unifying element was the use of brick.

The combined effect is a feeling of lightness or as Gropius called it “etherealization”. In order to enhance this feeling of lightness, Gropius and Meyer used a series of optical refinements like greater horizontal than vertical elements on the windows, longer windows on the corners and taller windows on the last floor.

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Fig. 21 a, b Fagus Factory

Another milestone in glass curtain wall architecture is the construction of Bauhaus facilities in Dessau by Walter Gropius, 1926 (Fig. 22 a, b, c). In his design, Gropius refined architectonic ideas he first put into practice before WWI in the construction of the Fagus Factory in Alfeld. The three to five storey high building included classrooms, ateliers, dormitories, following what is the determinant principle of the machine to create space and forms of living. Each façade responded to the demands of the activity taking place inside: the front of the classroom block is composed of horizontal windows, whose function was to ensure adequate lighting; the dormitories, however, had individual openings to increase privacy [90].

Ateliers had multi-storey glass curtain walls, allowing maximum light and view. On this façade, as in Fagus Factory, the glass curtain wall is suspended in front of the load-bearing framework and openly showed the constructive elements. Gropius, rather than visually amplifying the corners of the cubic body of the building, allowed the glass surface to overlap the edges, thereby creating the impression of lightness [84]. The concrete floor slabs are visible and any sort of ornament is purposefully avoided. The front façade is where the first level is set back to produce the levitation of a higher volume consists of a curtain wall tension obtaining access to the product of the contrast of the opaque background volumes.

Fagus Factory and Bauhaus facilities are nowadays recognized as masterpieces of Rationalism and are protected by UNESCO, since they represent the concept of curtain wall

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structure in its purest form. However, lacking insulation and sun protection, this kinds of façades were definitely questionable in today’ building physical terms.

Fig. 22 a, b, c Bauhaus in Dessau

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Glass surfaces, combined with steel, took on a new and important function in the Modern Movement, whose ideas conceived the decorative art is not an end in itself, but full of meaning semantic and functional, not lacking in quality morphology. The Modern Movement, for which the glass is light, space, optimism, took hold in Germany and in France in the early 1930s and then spread all over the world [90]. La Maison Dalsace in Paris (1928-32, Fig. 23 a, b) by Pierre Chareau and Bernard Bijvoet, also known as “Maison de Verre”, is striking example of a building designed starting from the needs of individual environments and that makes visible the structure from the outside. The primary materials used were steel, glass, and glass block. The problem of illuminating this narrow building set back in the

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courtyard, led to use a steel structure with large areas of glass. The light came from the side of the garden and from that of the courtyard with huge walls on three floors, mainly in glass blocks. The steel structure was buffered with glass blocks (for each surface 4 "glass blocks Nevada"6 x 20 x 20 x 4 cm, produced by Saint Gobain Glass) and with a few openings and fixed panels made of transparent glass [84]. Therefore, the external form was defined by translucent glass block walls, with select areas of clear glazing for transparency. In 1921 Le Corbusier coined the concept of “a machine for living” in an article in the magazine "Esprit Nouveau". He meant a product to be carried out with precast series. The design and construction of Chareau’ s “Maison de Verre” follow these dictates.

Fig. 23 a, b Maison del Verre, Paris, 1932.

On a conceptual level, several architects of the Modern Movement theorized the importance of creating a supporting building “skeleton” to achieve the maximum freedom of expression in design both plans and façades [35]. Le Corbusier's canonical description of architecture as “the masterly, correct and magnificent play of masses brought together in

light” affirmed a new set of values for modern buildings, i.e. transparency and

dematerialization, achieved through material lightness and spatial interpenetration. Le Corbusier and his followers tried to apply in their projects the Le Corbusier's “Five Points of Architecture”, a set of architectural principles exposed in his book Vers une architecture (1923) and become the key criteria for the architectural design after the World War II.

Regarding the development of the curtain wall,the most significant concepts relate the design of the Plan libre (free plan), made possible by the creation of a framed structure,

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which eliminates the function of the load-bearing walls that forced the plan of the building. The idea of Façade libre (free façades) also derives from the introduction of frame structure, consisting of a series of horizontal and vertical bearing elements with infill opaque or transparent cladding. Then, the Fenêtre en longueur (or strip window) means that the façade can now be cut along its entire length by a window that occupies the desired surface, allowing an extraordinary illumination of the interior and a more direct contact with the outside world .The symbolism of glass and metal gradually found new expression in the form of a glass skin, as opposed to glazed openings in a skeletal structure.

From a technological viewpoint, some industrial innovations drove to the construction of new building systems and represented a substantial step forward in the technology of glass in façade. The aluminium extrusion process was developed and allowed to more economical sections and more detailed profile patterns, perfect for curtain walls. Another technological development occurred in 1959, with the Pilkington industrialized flat glass manufacturing. It provided large quantities of high-quality glass panes [6].

At the end of World War II, thanks to such major technological advances in the glass industry, to fundamental changes in attitude and art, and, not at least, to the need to rebuild Europe post-war, glass was hugely used for the formation of almost all the external "skin" of the buildings. In the United States Modern Movement, that will be then called International Style, spread with the arrival from Germany by Ludwig Mies van der Rohe and the construction of buildings of glass from steel structures.

In the 1940s and 1950s, the most radical use of glass took place in some private houses. Due to their relatively small size, and the possibility to entirely control the design and construction process, these types represented the ideal possibility to experiment at that time [84]. One of the most significant masterpiece in the purist architecture of International Style can be recognized in the 140 m2 Farnsworth House in Plano, Illinois, that was designed and constructed by Ludwig Mies van der Rohe between 1945-51 (Fig. 24 a, b). The steel and glass house is a one-room weekend retreat in a rural setting, commissioned by Dr. Edith Farnsworth, a prominent Chicago nephrologist, as a place where she could engage in her hobbies. The basic design idea reminds of the Barcelona Pavilion, with its spatial links fluids.

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The extensive use of clear floor-to-ceiling glass opens the interior to its natural surroundings to an extreme degree, allowing the continuity between interior and exterior space. The surrounding landscape, near a small river, recalls associations to the traditional Japanese house. Supported by eight steel pillars, two distinctly horizontal slabs, which form the roof and the floor, sandwich an open space for living. The structure is painted pure white, to enhance the minimalist configuration and the idea of an idealized space, exclusively defined by horizontal levels: the house is elevated 1.60 m above a flood plain by eight wide flange steel columns which are attached to the sides of the floor and ceiling slabs. The slabs’ ends extend beyond the column supports, creating cantilevers. A third floating slab, an attached terrace, acts as a transition between the living area and the ground.

Fig. 24 a, b. The Farnsworth House, 1945-51.

Fig. 25 a, b. The Glass House, 1949.

The Farnsworth House is significant as a first complete realization of Mies’s ideal, a prototype for his vision of what modern architecture in an era of technology should be. At the same time, however, the building has shown since its construction several weak points: the minimalist configuration of details is paid with huge structural damage, which emerged immediately after purchase. It is also a product of the limited energy and environmental

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awareness and subtle knowledge of building physics of his time, and hence the level of internal comfort is very low, especially in winter and in summertime, and the energy efficiency of the façade is extremely poor.

The Glass House (Fig. 25 a, b), built in 1949 in New Canaan, Connecticut, was designed by Philip Johnson as his own residence. Heavily influenced by the Bauhaus ideals, the building is designed along minimalist lines with the aim of achieving transparency in a standing structure: its severe minimalism is often compared to Farnsworth House as the two are very similar in form and principal. The first difference from the Farnsworth House is that this glass cube is not surrounded by wild nature, but by an artificial park. The key differences are, however, in the conformation of technological details. Johnson used façade profiles for windows: from the outside, with the exception of the four corner supports, such profiles mask the structure, then glass façades are a sort of “shell” [84].

Both buildings by Mies and by Johnson are an essay in minimal structure, geometry, proportion, and the effects of transparency. The houses are examples of early use of industrial materials such as glass and steel in home design.

Fig. 26 Lake Shore Drive apartments. Fig. 27 The Lever House. Fig. 28 The Seagram Building.

A number of outstanding curtain wall clad buildings were erected; amongst them the Lake Shore Drive apartments in Chicago by Mies van der Rohe (1948-51, Fig. 26), the Lever House in New York by Skidmore Owings and Merrill (1952, Fig. 27), the Seagram Building in

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New York by Mies and Philip Johnson (1954-1958, Fig. 28), which was a sort of “prototype” model for this kind of tall buildings.

The Seagram Building (Fig. 29 a, b, c) is a 38-story structure and, like virtually all large buildings of the time, it was built of a steel frame, from which glass curtain walls were hung to form an uninterrupted envelope around the building. Mies would have preferred the steel frame to be visible to all; however, U.S. building codes required that all structural steel be covered in a fireproof material, usually concrete, because improperly protected steel columns or beams may soften and fail in confined fires. Concrete hid the structure of the building, therefore Mies used non-structural bronze-toned I-beams to suggest structure instead. Another interesting feature of the Seagram Building is the window blinds. As was common with International style architects, Mies wanted the building to have a uniform appearance: the façade is fully glazed with coloured spandrels to hidden the underlying parapets. The windows are not operable and the building rely on HVAC systems [84].

Fig. 29 a, b, c the Seagram Building, details.

Nevertheless, at the beginning, the extensive use of glass in architecture was conditioned by its low thermal performances and by the related problem of the internal comfort conditions. This is the reason why the new lightweight building structure and skin could not have been possible without the invention of Willis Carrier in 1902, of the first fan coil dehumidifying system for the Sackets-Wilhelms printing company in New York, that heralded the beginning of modern air conditioning [90]. All the glass skyscrapers built in that period, with the support of those mechanical services, could ensure good internal environment systems, but also high levels of energy consumption.

The introduction of Structural Sealant Glazing in late 1960s allowed to “clad” the building envelope with a flat and uniform surface. The idea of transparency and

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dematerialisation was dominant during this time and architects the world over tried to use glass to create buildings that focused on a sense of light and space. All geometric shapes could be cladded, as an answer to the growing criticism to the rectangular shapes of the International Style architectures [32].

Such development was driven also by architecture: Postmodernism evolved as a countermovement against the predominant modern formal vocabulary. An example of this attitude was the Pacific Design Center in Los Angeles (Fig. 30 a, b), whose first section was made in 1976 by Cesar Pelli with Victor Gruen. The building resulted as a horizontal high-rise building, which resembled a huge moulding accidentally cut at some point. Roofing and façade are covered with a glass curtain wall in sparkling blue.

Fig. 30 a, b. Pacific Design Center.

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Architects use of glass continued to evolve throughout the 20th century although most of the larger, ambitious projects were confined to large office buildings with massive budgets. The first energy crisis of 1973 made people reflect on the use of glass in architecture and the necessity of exploiting renewable energy sources for heating, cooling, lighting and ventilating a building [84]. From that moment and for the following 20 years, the glass industry has been developing many new products to protect against excessive heat gain in summer and unwanted heat losses in winter and to optimise the use of daylight. Examples of such products are: multiple glazed units, glass with selective coatings, light diffusing glazing, etc. [6]. Designs with energy-saving double façades were introduced in experimental buildings and at the same time many architects, mainly belonging to high-tech

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trend in architecture, tried to create buildings that can improve the use of solar energy, natural ventilation and daylight [76].

One of the greatest feats in glass architecture in this field is the fifty-three-storey Commerzbank Headquarters building (Fig. 31, form a to f), the world's first ecological office tower and the tallest building in Europe at that time. The project was developed and realized in Frankfurt by Norman Foster (1991-97) explores the nature of the office environment, developing new ideas for the technological aspects, with a deep reliance on natural systems of lighting and ventilation. Every office is daylit and has operable windows, allowing the occupants to control their own environment, even if they are normally controlled by the Building Management System (BMS). The exterior façade, made by Gartner Company, is a double-skin façade with an airflow gap of 20 cm. By German code requirements, rooms more than six meters deep receive daylight from the exterior façade or through one of the 13 separate gardens. These gardens feature tall façades with fairly large windows (11 per garden). Due to high wind pressure, the garden windows, which are opened and closed approximately ten to twelve times a day, fail regular basis. Climate conditions, such as wind speed, wind direction, temperature, and humidity, are sent continuously to the BMS from data taken at 10 separate weather stations [90].

There is at least one station per every eight floors. Natural ventilation is suitable for maintaining interior comfort for as much as two-thirds of the year. The plan of the building is triangular and winter gardens spiral up around the atrium to become the visual and social focus for office clusters. The perimeter offices are ventilated through the double-skin, while the inner offices looking onto the atrium are supplied indirectly with fresh air. As noted before, the atrium spaces can be ventilated significantly, both to supply the necessary fresh air for the offices and to control the microclimate of the gardens and the atrium.

Based on information from the building weather stations, the BMS makes a decision whether or not to allow the operation of the natural ventilation system. Occupants are kept informed of operation conditions by a light located on control panels in each room. When the light is red, the mechanical ventilation is in operation and the windows are locked. When the light is green, the system is in the natural ventilation mode and the office workers are

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free to open the windows. Typically, when the outdoor temperature is above 25° C or below 3° C, mechanical ventilation is used. Users control lighting, shading, and window operation using a standard design panel located in each room. During the winter, the sun contributes controlled solar heat gain. A perimeter heating system is controlled by thermostat. Heat is purchased from the city of Frankfurt, which has a district heating system, using steam as a by-product from the large-scale production of electricity.

The result is energy consumption levels equivalent to half those of conventional office towers: the original energy consumption estimate was 13 million kWh per year, while actual energy consumption has been around 10 million kWh per year [83, 87].

Fig. 31 a, b, c, d, e, f. Commerzbank in Frankfurt, 1991-96.

The use of glass façades as a sustainable material and compatible with the functionality of everyday life has been demonstrated through the application of these technologies in the residential market, with the construction of fully glazed housing facilities [88]. Following the

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example of the glass houses proposed by Mies Van der Rohe and Philip Johnson in 1940s-1950s, some architects and engineers have experienced the construction of residential buildings with large glass surfaces, which also allow a high interior comfort. This rather novel concept in residential architecture seems to have broken through all conventions and set a new, if not higher, standard in innovative construction.

Fig. 32 a, b. R128 in Stuttgart, 2001.

One of the most representative example of this trend is R128 house (Fig. 32 a, b), a four-storey house designed by Werner Sobek for himself in Stuttgart, Germany (2000). R128 is devoid of a basement and has a glass façades with high quality triple-glazed glass with inert gas filling is in use. The use of a modular design, complete with glass panels and steel frames ensured easy assembly and disassembly of the construction. The insulated glass panels prevent overheating of the interior during summer and loss of energy and warmth during winter. The supporting steel frame comprises of 10 tons of steel. The ceiling of the Werner Sobek glass house consists of prefabricated panels overlaid by plastic. Beneath the unscrewed floor, aluminium ceiling panels are affixed by clip connections. Lighting, heating and cooling systems are fitted into that layer and this acts as an acoustic absorber pattern. Sensor controlled doors have been installed on the upper and lower levels of the house. All appliances and environmental systems are also controlled by motion sensors and voice commands [88].

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Windows are controlled by touch screen technology. Every floor has two folding windows each, which allow natural daylight and fresh air to enter the house. During summer, cool water running through the floor elements removes excess heat from the entire house with the help of a heat exchanger. Thus surplus energy is stored for use in winter. This ensures minimal energy consumption. 48 solar powered modules with a total capacity of 6.2 KW are installed on the rooftop, which are responsible for supplying all the power required by the pump system. This green show-house promotes energy-efficient architecture using glass (data from www.wernersobek.de).

Fig. 33 a, b. H16 in Tieringen, 2006.

Werner Sobek’ exploration in environmentally sustainable and self-sufficient prototype houses is developed in H16 House (Fig. 33 a, b), completed for a young familyin the village of Tieringen, not far from Stuttgart (2006). The house presents a glass and steel volume (7x17 m) for living and dining spaces perches serenely atop a black-concrete-panelled base. The two are linked to a third volume, a garage, by a roof deck and are backed by a limestone retaining wall. The glass cube is built with highly insulating triple glazing (individual pane dimensions: 2,36 x 3,63 m) to facilitate a pleasant room climate and the greatest possible transparency. A specially adapted climate concept facilitates emission-free heating and air conditioning. The utilization of ground heat (geothermal heating), in connection with a heat pump system, mechanical prime ventilation and a photovoltaic system ensures that the edifice can do without fossil fuels altogether: in energy terms, the building is entirely self-sufficient (data from www.wernersobek.de).

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Another of the biggest changes during the last three decades was the move away from seeing glass as only the material for the openings within a structure, but rather as the material for the structure itself. Glass skins became the challenge to tackle whereby a thin steel structure literally supported skyscrapers of full glass walls.

3.6 G

LASS AS A

S

TRUCTURAL

M

ATERIAL

:

H

IGH

-

PROFILE

E

XAMPLES

Glass curtain walls consist of non-load bearing cladding, commonly comprising a relatively light metal carrier framework and transparent infill panels. While the term embraces many different construction methods and systems, the predominant similarity is that the frameworks are hung from the building structure and are located exterior to the floor slabs. The anchors are fixed to the supporting main structure of the building, or, alternatively, to an independent substructure, rigidly connected with the main frame. Chronologically, curtain walls were the first system to build continuous glass envelopes [89].

In the last few decades, glass in architecture has increasingly gained new expressive values, and glass panels nowadays are not supposed as secondary elements, supported by an existing structure, such as façades, but they can create themselves the façade. Transparency, resistance and performance must be satisfied by the architectural element, and glass must enhance all its physical and mechanical properties. Structural glass can be then defined as the common glass, enriched with everything that confers superior mechanicalperformance and allow to use this material in novel applications [84, 85].

The pursuit of transparency in the building envelope has been a primary driver of structural glass façade technology [39]. The result has been an ongoing dematerialization of structure, achieved largely through an increasing predominance of tensile bracing elements in closed, freestanding systems and the extensive use of open systems comprised mostly or completely of tensile elements. More and more, however, the control of transparency as a mean to manipulate daylight and view is becoming the predominant concern, and not simply the maximisation of transparency with no regard to issues of thermal performance, daylighting and glare.

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Nonetheless, the consideration of façade transparency remains relevant as a design intent in many projects. The structural system plays a defining role in the perceived transparency of any long-span glass façade design [42]. The primary strategy in achieving dematerialization of the structural system involves the use of tension elements as part of the design vocabulary. This is consistent with a strategy of efficiency and sustainability doing more with less material; a methodology for the progressive dematerialization of structure can be describes as follows:

 Minimize the length of compression members.

 Minimize the number of compression members even if the number of tension members must be increased.

 Increase the depth of the truss as much as is practical, to reduce the axial forces.

 Explore the possibility of using more than one material in the truss, one for compression and another for tension.

Transparency in this context is primarily a matter of reducing the structural profile of a façade design. A structural system designed such that certain elements encounter only axial tension forces allows those elements to be significantly reduced in section area from elements designed to accommodate compression and bending loads. There are several theoretical reasons for such behaviour, but the simplest is that buckling disappears as a phenomenon of structural failure. The steps outlined above thus become a strategy for optimizing structural transparency: the cable supported systems abandon compression elements entirely.

Transparency becomes an effect of the interplay of light, structure and membrane. A long-span glass façade is perceived in layers of membrane and structure. The glass skin, even a skin if the most optically clear glass, is not transparent after all, but a reflective, semi-transparent luminescent membrane involved in a complex interaction with the surrounding environment, such as sky, buildings, streetscape and people. This is the dynamic beauty of the material: the glass does not disappear but it is perceived as a membrane. Most structural glass façades register visually as a combination of structure and membrane. A deep truss system with dominant and expressive truss elements is perceived as a layered depth of

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structure. Even glass fin walls reveal a layered depth that speaks to both membrane and structure. Cable-supported supported structures, in contrast, become much more about membrane as the structure disappears into the surface [41].

A chronological development of such technologies can be outlined through the analysis of high-profile buildings.

W

ILLIS

F

ABER

&

D

UMAS

H

EADQUARTERS

IPSWICH,UNITED KINGDOM;FOSTER ARCHITECTS, DESIGNED 1971-72, COMPLETED 1975.

Fig. 34 a, b. Willis Faber & Dumas Headquarters.

Fig. 35 a , b. Interior views.

 BRIEF DESCRIPTION:

The suspended panel façade is the earliest form of structural glass façade system dating back to the 1950s and the French Hahn system used at the Maison de la Radio in Paris in 1953. Here 2-story glass plates were suspended and laterally stiffened by the use of glass fins set perpendicular to the plates at the vertical joints between them.

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This structural scheme was applied and popularized in the suspended façade of Willis Faber building (Fig. 34 a, b), built in Ipswich by sir Norman Foster in 1974: the three-storey building was devoted to office headquarters (Fig. 35 a, b), and the architect was inspired by two famous projects designed by Mies van der Rohe around the early twenties.

This project inspired a diffusion of glass fin technology in numerous applications throughout Europe and America starting in the 1970s and continuing today. Glass fin-supported facades still represent one of the most transparent forms of structural glass facades and an economical solution, especially at lower spans.

 STRUCTURAL BEHAVIOUR:

In this kind of envelope, each single glass pane is hung by the upper and, at the same time, it supports the lower ones; the glass pane at the top is directly connected to the bearing structure (Fig. 36 a). The Ipswich façade consists of six rows of tempered glass, bronze colour, 12mm thick, hung at the top; a system of bolted plate and counter-plate (Fig. 36 b) assures the transmission of bearing vertical loads, while the waterproofing is ensured by to silicone joints. The horizontal movements are contrasted by vertical glass shelves, which are supposed to tensile stresses and are fixed at floor slabs.

 INNOVATIVE FEATURES AND COMPONENTS:

The entire project aims at defining the possible and alternative load paths and the role that, hierarchically, each structural element plays. Furthermore, for the first time, the engineering project refers to the concept of redundancy, according to which the structural elements are doubled. This technology was then developed and refined by Peter Rice and Hugh Dutton for the façades of the Museum of Science and Technology at Parc de la Villette in Paris, in 1986.

Fig. 36 a. Suspended glass. facade

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L

ES

S

ERRES

,

P

ARC DE LA

V

ILLETTE

PARIS;ADRIEN FAINSILBER WITH RICE FRANCIS RITCHIE (RFR), DESIGNED 1981-83, CONSTRUCTED 1984-86.

Fig. 37 a, b. Les Serres Parc De La Villette.

Les Serres were designed to be an element of mediation between the building and the surrounding park, so they should be extremely light and transparent (Fig. 37 a, b). RFR’s investigations in 1981 were inspired by defining transparency, and eliminating bending in the glass pane in response to the exceptional load demands resulting from the safety requirements imposed by the checking authority, Socotec [78].

Fig. 38 a, b, c. Novel technologies.

The glass façades are supported by a Vierendeel structure of steel cables, very light and deformable (Fig. 38 a, b). It therefore requires that the glass panes have a greater degree of freedom in the relative displacements, to avoid imposed deformation which will provoke

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breakage. Such constraint has led to the design of an innovative system of suspension (Fig. 38 c). Each 8mx8m composite panel of glass is 1920 kg, without including the fixings.

The glass panes consists of tempered glass 12mm thick; sixteen panes make up a panel, sixteen panels constitute the square façade, 32 meters high, facing the park that surrounds the building [78]. Three novel technologies can be identified and classified as follows:

1. Articulated connecting rods(spiders): the connection of rotules between themselves and

with the main structure is obtained through the use of articulated connecting rods or spiders. The spiders allow the relative displacement between the glass panes, avoiding that they are opposed to the deformation due to bending of the individual cables.

2. Suspension springs: the suspension springs ensure a homogeneous distribution of the

loads between the vertical rows of plates hung; springs also act as shock absorbers in case of breakage of a glass pane.

3. Rotules: the introduction of rotules avoids the creation of undesired bending moments,

since they maintain the glass at the central plane of the plate through a spherical joint. At the same time they ensure the transmission of the out-of-plane forces to the horizontal bracing cables.

Fig. 39 a, b. Plate and counter plate system.

Fig. 40 a, b. Planar Fixing (Pilkington, 1982).

Fig. 41 a, b. RFR Fixing (Dutton & Rice, 1986).

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Thus, an evolution in different systems of connections can be identified. In addition to the plate and counter plate system (Fig. 39 a, b), new point fixed systems are developed, fixed or articulated: the Planar™ fixing, invented in 1982 by Pilkington (Fig. 40 a, b), and the rotule fixing, developed by Dutton & Rice (RFR) in 1986 for Les Serres (Fig. 41 a, b).

The main differences between fixed and articulated fixings, which represents the “progenitors” of all the connections for the modern glass facades, can be briefly outlined as follows:

FIXED ARTICULATED

 Smaller size  Larger size

 Articulated at support  Articulated at glass  Smaller sizes requires better

understanding of stresses

 Larger sizes mean also larger pullout strengths

 Reduced size can mean less site tolerance

 Articulation needed by design often misused to provide added site tolerance  Rotation stiffness is difficult to model  More straightforward to model

One of the main innovations introduced in this building is the replacement of traditional glass fins with horizontal stiffeners made of steel cables, a technology derived from the shipping industry.

Another important novel technology concerns the system of hung glass panes, with the introduction of springs in the suspension points, and with the replacement of the connections to the plate and counter-plate bolted with an articulated system formed by rotules and connecting rods suitably connected [78].

The choice of using horizontal cables derives from the observation that the glass fins, when they are perpendicular to the plane of the façade, is not completely transparent, because of the reflected light. The traditional fins also fall within the visual field, while the cables, being horizontally, provide the viewer a continuous view from the inside towards the outside [84].

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G

LASS

F

AÇADE OF THE

K

EMPISKI

H

OTEL

MUNICH;MURPHY/JAHN ARCHITECT WITH SCHLAICH BERGERMANN PARTNERS, COMPLETED 1993.

Fig. 42 a, b. View of the glass façade.

Frei Otto developed and popularized cable nets as a structural system in the 1960s and 1970s. Architect Helmut Jahn with engineering firm Schlaich Bergermann applied the technology in a most innovative manner in 1992 as a flat cable net supported glass façade for the Kempinski Hotel in Munich, developing a widespread interest in this structural form in glass façade applications (Fig. 42 a, b).

The glass façade is 25 meters high and 40 long. The structure consists of a quadrangular mesh of 1.5x1.5 meters composed of stainless steel cables of 22 mm diameter which support the laminated and tempered glass, 10 mm thick. The glass panes are point-fixed to the cables, and inserted inside the joints of silicone; thus, each panel, grabbedat the four vertices withthe interposition ofa layer of silicone, must not bedrilled for theinsertion of rotules or other anchorages (Fig. 43).

The vertical steel cables have the almost exclusive function of supporting the weight of the laminated safety glasses. Such cables are weakly pre-stressed, to avoid overfill the roof structure to which they are connected.

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The horizontal steel cables are instead connected to the two wings of the building, and they are strongly pre-stressed (85 kN), so as to limit the deformations orthogonal to the glass surface (however high, in order to exploit the effects of the second order).

The horizontal cables replace the double parabolic cables introduced in La Villette’s façade and, since they lie very close to that the glass plane, they blend visually with the joints of silicone: the effect is an almost total dematerialization of the façade [41].

This system allows to avoid the drilling of the glass panes and ensures at the same time a sufficient tolerance for deformation out of plane. Due the high stresses at the corners, however, the use of tempered glass is required, which also ensures a good sound insulation, in addition to the advantage of the structural redundancy (Fig. 44 a, b, c, d).

Fig. 44 a, b, c, d. Details. Fig. 45 a, b. Clables structure.

In some applications, steel cables are used both to suspend the glass sheets and to work as cross-braced frame, with a structural function similar to that of a tennis racket, which ensures a good behaviour against deformation (Fig. 45 a, b).

The point-fastening system mainly consists of stainless steel friction terminals, connected to the cables by bolting.

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B

ROADFIELD

H

OUSE

G

LASS

M

USEUM

KINGSWINFORD;DESIGN ANTENNA +TIM MAC FARLANE &PARTNERS,COMPLETED 1994.

Fig. 46 Entrance. Fig. 47 View of the glass addition from the inside.

In the 1990s, after a long period of decline, Dudley Council decided to open a museum at Broadfield House to celebrate this portion of the Borough's history. Brent Richards of Design Antenna, with Dewhurst Macfarlane as structural engineers, won the limited competition to design the new entrance pavilion. This structure features glass beams and columns as well as glass walls and roof. The structure serves as the entrance (Fig. 46 and 47), ticket hall and shop to the museum, whose large collection of crystal glass

illustrates the history of its design and development from the seventeenth Century up to the present day. The choice of using glass meets the demands of the institution for the protection of monuments, aimed at the greatest possible visibility of the existing building. Full transparency do not alter the old building [84].

The linear structure of this volume, 3.50 m long and 11 m high, is simple and without visible links of metal. Supports and glass beams are realised with a triple-layer laminated glass and

are arranged at distances of 1.10 m. The main beams for the entire length of 5.70 me in the rear area rest on steel elements inserted in the existing masonry. On the façade glass beams

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are related to the mullions with a male and female joint and form rigid corners of the frame (Fig. 48). The same solution is adopted for the lintel above the entrance.

At the time it was believed to be the largest all-glass structure in the world (11m x 5.7m x 3.5m high). The pavilion faces South-West, and the glass is heavily tinted to limit solar gain (Fig. 49). The roofing system, with a slope of 1°, is realized with IGU (Insulated Glass Unit) 1.10 m thick.

It can withstand a snow load of 0.75 kN/m2 and can be practicable for cleaning. Being a glass-roof , the inner pane consists of a laminated safety glass. On the top, a ceramic grid functions as sun control device. The outer pane has a solar control glass, neutral coloured. These two devices allowed to reduce the passage of solar energy to 37%. The façade is covered with a solar control glazing. Here the passage of energy is set back to 59%, with an exploitation of daylight of 61%.

A

PPLE

C

UBE

,

N

EW

Y

ORK

(U.S.A.)

BOHLIN CYWINSKI JACKSON +ECKERSLEY O’CALLAGHAN.GLASS CONTRACTORS:SEELE GMBH.2006

Fig. 50 The glass cube. Noctural view. Fig. 51 Roof detail. Fig. 52 Glass fins detail.

The glass cube is part of the Apple worldwide retail expansion plan. These kinds of stores reflect the innovative design and the aesthetic being at the core of Apple’s product

Fig. 49 The old building with the new glass addition.

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development. The initial concept was to create a structure that allows maximum transparency through the space and not detract customers’ views of the products. At the same time the importance of design to Apple led them have not only functional but also “magical” structures, with highly refined architectural style and palette of contemporary materials.

The design for the retail store in New York site on Fifth Avenue (Fig. 50) required the development of a totally clear structure glass cube to create an imposing vestibule while creating a landmark for the square. The cube formed over a square opening in the plaza leads to a spiral glass stair descending to the lower store level and encapsulating a circular glass lift. The cube measures 10 m on edge, with the door opening centrally placed .

The glass cube structure is simply formed from vertical glass fins on all four sides and on the roof (Fig. 51 and 52). The roof structure is based on a lamellar principle whereby each beam section across the grid is supported via a pin connection to another glass beam section. These fins support a grid of intersecting glass beams on a grid arrangement (Fig. 53, 54 and 55).

Fig. 53 Connection between fin and façade

Fig. 54 Exploided view of roof beam connection and roof panel support

Fig. 55 Façade panel shear connection

The wall panels are a three-ply laminated heat-strengthened glass panels that are 3.35 m high circa, splitting the façade height into thirds (Fig. 56).

The fitting from the fin to the wall panels allows restraint to the fin and transfer direct loads such as wind. The fitting also provides shear transfer within the plane of the façade so

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that the walls act as a shear wall to give lateral stability. Fittings on the horizontal joint of the façade panels complete the shear transfer action.

To transfer the wind load, the façade transfers the force to the fins which moves into the base fitting and up to the roof plane (Fig. 57). At the roof plane it is transferred through the beams and roof panels into the adjacent wall. The analysis of the cube and the stairs was carried out using Strand software package (data from www.eckersleyocallaghan.com).

Fig. 56 Glass panes used for the fins. Fig. 57 Glass fins.

The key-developments in the design and fabrication of the glass cube come down to a drive towards the refinements of the fittings, their interaction with the glass and the development of large scale monolithic structural laminates. The fins at 10 m tall are the largest monolithic glass structural elements ever created in the world at that time. These have led to the idea of creating even larger members and even façade panels (data from www.eckersleyocallaghan.com).

The techniques used in laminating material into glass in such a manner as to allow direct connection of adjacent structural glass elements was a first on such large scale commercial project.

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MAS

-

M

USEUM AAN DE

S

TROOM

ANTWERP,BELGIUM;NEUTELINGS RIEDIJK ARCHITECTS.2011

Fig. 58 Noctural view. Fig. 59 The glass and stone cladding panels.

The MAS was designed as a sixty-metre high tower (Fig. 58). Ten gigantic natural stone trunks are piled up as a physical demonstration of the heaviness of history, full of historical objects that are the legacy of our ancestors. It is a storehouse of stacked history in the middle of the old harbour docks. Every storey of the tower has been rotated a quarter turn, creating a gigantic spiral staircase. This spiral space, in which a façade of corrugated glass is inserted, forms a public city gallery.

Façades, floors, walls and ceilings of the tower are entirely covered with large panels of hand-cut red Indian sandstone and glass curved panels, evoking the image of a monumental stone sculpture (Fig. 59). The four-colour variation of the natural stone panels has been distributed over the façade on the basis of a computer-generated pattern. The spiral gallery is finished with a gigantic curtain of corrugated glass. Its play of light and shadow, of transparency and translucence turns this corrugated glass facade into a light counterweight to the heavy stone sculpture.