New technical developments facilitate the use of large curved functional glass panes in facades. This produces glass sculptures with organic forms and lifelike building shells with changing appearances depending on the lighting and weather conditions. The transparency of a building can also be increased by highly-transparent and large-sized facade elements with convex or concave curved glass panes.
Cover Image: Musée des Confluences, Lyon. Photography: Copyright by Karin Jobst
Cold and hot bent panes can be used to form homogeneous and seamless curved surfaces, which expand the design vocabulary of the architecture. However, when installed in the building the curved panes must fulfil high structural and physical properties and must be able to withstand wind loads. For example, there are specific properties that must be taken into account with the construction and installation of curved facades to ensure that the panes do not break.
Curved facades open up many new possibilities in terms of architecture as they avoid disruptive corners, edges and offsets. Turns and twists in the building can be elegantly illustrated on the building envelope and create new horizontal reflections. Whilst horizontal and cylindrical curves can be planned and designed two-dimensionally, large, curved, free-form surfaces can only be designed and engineered with the help of expensive 3D programmes. As a general rule they require more effort in terms of planning.
Today, three by fifteen metre, highly-transparent glass panes, such as those used for the new Apple headquarters in Cupertino, can be cold bent during lamination and installed into a two-dimensional facade construction. With the 462 m-high Lakhta Center in St. Petersburg the panes were cold bent in the 2.7 x 4.2 m parallelogram-shaped facade elements by up to 40 mm. Thermal bending allows for even greater curvatures to be created, as can be seen with the Elbphilharmonie Hamburg, where up to 5 m-high and 3.-m-wide multi-functional insulating glass panes with a rise of 350 mm have been built into the facade. In the 88 m-high Soho-Tower in New York the corners of the building are clad with a construction made from hot bent insulating glass panes with a radius of one metre and a height of up to five metres.
Incidentally, all of the facades mentioned in this article were made by Josef Gartner GmbH. Before presenting individual projects with cold and hot bent facades, the common and the new techniques for the construction of bent facades and glass roofs will first be outlined.
1. TECHNIQUES USED FOR MANUFACTURING CURVED GLASS FACADES
1.1. Curved facades and glass roofs with planar, segmented surfaces
In the 1920’s, curved, single-glazed window elements were already being used in buildings to round off corners, for example. With modern coated and printed functional glass panes this was only possible to a limited extent in facade construction until around twenty years ago, as the technical know-how about the bending of glass panes like this was lacking, and with hot bending most coatings that had been applied to the glass surface were damaged. To create free shapes, planar insulating glass units which were then built over triangular or scaled facades needed to be used.
Elements arranged like shingles are an option to create vaults or shaped glass roofs in case the glass panes are not triangular but quadrangular. The curved surfaces are segmented and made with planar elements; the framing members surrounding the glass panes need to address the up and down at the transition from one glass pane to the other. This can be seen with the wavy glass roof construction of the Robert and Arlene Kogod Courtyard in Washington D.C. The 39 x 84 m elegant glass canopy of the Smithsonian American Art Museum is made up of elements with insulating glass units in an aluminium frame which lie on twisting steel girders.
The rounding of the cylindrical tower of the 112 m-high Westhafen Tower in Frankfurt has been emulated with triangular facade elements. The double-cone of BMW world in Munich, which is used as the main support for the cloud-like roof structure, is made up of triangular fields. 900 different glass elements were made for this construction which has a diameter of 35 m at its base, tapers in to 12 m in the middle and then tapers back out to 35 m. However, curved constructions such as this do not allow for a homogeneous-looking bending, they limit the architectural possibilities and often require massive, visible metal constructions.
1.2. Free-form shapes with curved functional glass panes
The real challenge with cold or hot bending is being able to bend the panes and fulfil high structural and also physical properties such as heat and sun protection at the same time. In order to do this, insulating glass must be coated and printed. Each bending also generates tension which is difficult to calculate and handle. As threedimensional panes are stiffer than flat panes, they can break more easily. Stiffness attracts load.
Fundamentally, the facade structure must always take the specific properties of the bent panes into consideration. The design of the frame of the facade required to prevent the breakage of glass can only be determined on an individual basis. Curved, three-dimensional framework requires more effort than a planar facade element. The more three-dimensional the elements are, the more difficult they are to construct and handle in order to get them to reach the desired radius and to absorb the operating forces, for example.
Once installed, facade elements must also be able to withstand high wind loads and other stresses. To fulfil requirements such as these, an extensive series of tests is required for each object with free-form facades. Above everything else, it is crucial that both the glass benders and the facade builders have experience and know-how as these are not standard components.
Curved glass facades can only be made in single-piece production and not on a production line. The highest level of care must be taken with the manual assembly of the glass and frames. As a general rule, rubber seals cannot be used when the curved panes are installed in the frames and therefore most of them are sealed with silicone. For the entire process, from the manufacturing, to the logistics and the assembly, there are increased requirements applicable so that damage is avoided.
1.2.1. Cold-formed functional glass panes
With cold bending, the facade builders use the elasticity of the glass. During the assembly of the elements, the planar, coated and printed glass panes are bent or rather formed into the three-dimensional manufactured frames. This way, homogeneous, curved panes are formed without unevenness and without any coating and printing limitations. The mechanical support of the pane is crucial. However, the degree of bending that can be achieved is lower than that which can be achieved with hot bent panes. The movement of the edges relative to one another can also influence the tightness of the edge seal.
A special type of cold bending is lamination bending. In this case, the glass manufacturer bends the individual panes of a laminated glass unit in the autoclave during the lamination process. The coatings applied to the glass cannot burn or oxidise in the autoclaves. In contrast to cold forming in the frames, where the bent glass returns to its original planar form when it is not in frames, the glass bent in autoclaves retains its stable shape.
1.2.2. Thermally bent functional glass panes
In contrast to the cold bent panes, panes that have been thermally bent can have a stronger curvature. If a high rise is planned, it must be checked in advance that the tension from the cold forming and the additional wind loads and other stresses do not exceed the permissible stresses in the panes and necessitate a hot bending.
With the hot bending of glass, the panes are first coated with a low-e and/or sun control coating and may also be printed whilst they are in a planar state. Only after this has been done the multi-functional insulating glass panes are “softened” at temperatures between 500 and 600 degrees Celsius, and the elastic material is bent into a shape precisely according to the design. The advantages of this technique are the high degree of bending, the ability to change the shape at different points on the pane and the stability of the shape. The coating options are limited and the manufacturing tolerances are more difficult to adhere to. With thermal gravity bending, the sputtered coatings can burn due to the long dwell spent in the bending furnace. Unevenness may also appear on the surfaces of hot bent glass. The panes are installed in the facade profiles after the bending has taken place.
2. PROJECTS WITH CURVED GLASS FACADES
2.1. Cold-formed functional glass panes in the facades of the Evolution Tower, Lakhta Center, Apple Campus 2 and The Circle
Like a helical strand of DNA, the new Evolution Tower in Moscow reaches almost 250m up into the sky. Each of the 52 floors of the office skyscraper, designed by the Moscow-based architects ZAO “Gorproject”, is rotated by 3 degrees. In order to integrate the rotation of the individual floors into the facade elements and to avoid any offset of the glass level from element to element, insulating panes, parapet sheets and corner plate structures on the corners of the building were three-dimensionally cold formed. The cold forming allowed for a homogeneous rotation which gave the facade the appearance of a vast, free-form surface and produced horizontal reflections of the surrounding skyscrapers. In total, the tower rotates 156 degrees. Twenty seven geometrically different facade elements measuring 1.5 x 4.3 m were built in and twisted on each floor, then installed at an angle between -15 and +15°.
The architecture firm ZAO “Gorproject” also designed the 462m high Lakhta Center in St. Petersburg which evenly rotates up into the sky, like a needle or a flame. From a relatively narrow base area on the ground, the highest skyscraper in Europe reaches its largest diameter in the lower quarter of the building and then narrows to a point as it continues upwards. Therefore, the facade is tilted outwards on the lower floors and is then tilted inwards on the upper floors. The floor plan is based on five wings around a circle in the middle which is used as a space for lifts, sanitary facilities and technical rooms. There are offices and conference rooms in the five corners, or wings, of the building. Each floor is rotated 0.82 degrees in relation to the floor below so that the skyscraper rotates by 89 degrees in total and creates a special dynamic. Initially, the architects planned to use small, planar facade elements as this would make it easier to create the rotation in the tower. However, disruptive edges and a high number of frames would have restricted the view from the building. Therefore, together with the architects, Gartner developed large, parallelogram-shaped elements made with cold formed glass which would facilitate the dynamic rotation of the tower without any disruptive corners and edges in an elegant way whilst providing a high level of transparency.
The curved, 4.2 x 2.7m facade elements are made from a three-dimensional frame with four straight profiles which are put together to create a non-planar element. The aluminium constructions were connected to each other with specially, spatially twisted mitres, in order to achieve a smooth transition between the individual facade elements without any disruptive offsets. During manufacturing, the panes were cold bent on a corner by up to 40mm and were pressed and sealed in this position. This resulted in homogeneously curved panes without any unevenness or coating and printing limitations. The mechanical support of the pane is crucial. The individual elements blend smoothly into one another and create a homogeneous reflection without any disruptive offsets.
When presented with the plans for the new Apple headquarters, the company’s founder Steve Jobs compared them to a landed spaceship: “It’s a ring with curved facades the entire way around.” This ring in Cupertino, California, designed by the architect Lord Norman Foster, has an external circumference of 1.5 kilometres and provides space for up to 13,000 employees. The curved glass facade plays an important role in the appearance of Apple Campus 2 requested by Steve Jobs. The 3 x 15m lamination bent glass panes join together in an almost seamless and delicate way to form the curved ring structure. Even the curved canopy used to provide shade accentuates this structure. The high transparency of the facade elements provides outstanding views of the interior park and the exterior structures, creating an intensive connection between the surrounding nature and the office spaces. The building is naturally ventilated by the automated ventilation slats on the upper edge of the glass facade which allow fresh air to flow into the building. In order to assemble the largest facade elements in the world quickly and safely, Gartner has developed their own aids, such as an assembly robot.
At Zurich Airport, the company has just assembled the first curved closed cavity facades at the service centre known as The Circle. The largest construction project in Switzerland, with a usable area of 180,000 m2, follows the ring shape of the motorway. In order to construct a homogeneous, curved outer layer, around 1,600 panes of glass on the ring facade are cold-formed on a corner by up to 50 mm. It is the first particularly sustainable closed cavity facade (CCF) in the world with cold-formed exterior impact panes. Thanks to the high sound insulation of the CCF of up to Rw = 50 dB, a new city without noise will be created.
The ring-shaped facade almost tilts over the motorway, as the parapet wall protrudes over the base by approx. 15m at the point of the greatest external inclination. The decision to use cold formed glass was taken after it was directly compared with thermally bent glass in a visual mock-up. It was found that the cold forming provided a surface without unevenness and with it, the facades appeared even more homogeneously bent. The elements are up to 2,700 mm x 5,500 mm in size and weigh 1,660 kg.
2.2. Thermally bent functional glass panes in then Prada, Elbphilharmonie, Musée des Confluences and Soho Tower facades
With the Prada Aoyama Epicenter in Tokyo, the thermally bent functional glass panes emphasise the branding of the fashion house. The building, which was opened in 2004 and designed by Herzog & de Meuron, has become an iconic piece of architecture and was partially vitrified outside and inside with spherically curved insulating glass panes with a rise of 150mm and interior panes made from laminated safety glass. The entire outer shell of the building is made from diamond-shaped glass panes and makes the building, with its irregular pentagonal ground plan and unusual sloping roof, stand out from its neighbours. The diamond-shaped facade grid, with convex, concave and planar insulating panes of glass, creates a multitude of reflections and distortions. So that the movements of the shell with the aluminium construction of the facade are not transferred to the glass, the glass was only fastened at certain points with bolts which slide into the edge bonding of the insulating glass unit.
For the Elbphilharmonie in Hamburg, the Swiss architects Herzog & de Meuron wanted to “develop a living facade which constantly changes under different lighting and weather conditions”, according to Project Manager Stefan Goeddertz. Through the geometric forming and the printing of the panes, the facade should produce surface reflections depending on the location and the weather, and should also produce different images of the outer shell when observed from different viewpoints. With the outwardly tilted panes of glass, the visitors to the concert house, which has been open since 2017, should be able to see almost directly vertically downwards – as if they were looking through a porthole of a ship.
The convex panes in the Prada building had a rise of 150mm, but with the Elbphilharmonie, there is a rise of 350mm. Never before have glass panes been printed, coated and bent at a temperature between 500 and 600 degrees Celsius precisely according to plans, one after the other. Together with internal and external experts and partners from the industry and technical universities, Gartner has developed new procedures and products. The planning and development lasted for several years, as did the load capacity and durability tests of the spherically curved panes, this work was done together with the University of Applied Sciences Munich.
The two standard types of elements are each bi-axial and are either 4.30m wide and 3.33m high or 5.00m wide and 3.33m high and weigh approx. 1,500 kg. Out of the 2,200 glass panes used, 600 pieces are spherically curved and the regular planar panes are made from particularly clear, low-iron oxide glass. Both sizes of elements can be bent outwards and inwards. An oval pivot vent was built into the surfaces orthogonal to the curvature to allow natural ventilation of the rooms.
A crystal sculpture, designed by the Viennese architectural firm Coop Himmelb(l)au, flooded with natural light dominates the entrance hall of the Musée des Confluences in Lyon. This 30m-high, funnel-shaped sculpture was formed from panes of glass which were extremely curved in some places. The thermally bent panes were curved on two axes, manufactured in radii of under 500mm and are only kept level through structural glazing at certain points. In addition, the oversized panes, which are up to 4.5m long, had to be manufactured, formed and thermally bent several times. Only then could the edges be sanded and cut down to exactly the right size.
Strong curvatures and extremely small radii were also needed in the steel construction. Together with the architects, Gartner particularly optimised the intersecting points of the steel construction of the funnel. Each of the approx. 160 tricky intersections were individually designed on a 3D computer model. At each intersection, six steel braces made from rectangular hollow constructions joined together. For each intersection, the orientation and axis of the kick plate was determined in order to make the intersection of hollow constructions possible and to ensure that the intersections have a more elegant design.
For the 88m-high Soho-Tower in New York, which is made up of two towers containing exclusive apartments which are connected with a steel bridge, Renzo Piano Building Workshop designed a structured and curved facade. This large, transparent and curved glass facade with elements that can be opened for natural ventilation makes the building appear open and soft. The large thermally bent insulating panes of glass on the corners of the building are unique. These 337 thermally bent insulating panes of glass have a radius of 1,153mm, and are between 3,350mm and 4,875mm high.