Glass at the cutting edge.

Glass is a material which has always inspired architects and engineers. Designers have used it to realize the seamless transparent skin and to create clear yet loadbearing walls and canopies. In future, glass may be multivalent - combining transparency with high performance enviornmental control.

Glass is a material of alchemical mystery; it flows, reflects, transforms; it encloses space without appearing to do so; its transparency 'de-materializes' a wall, yet when you look obliquely into its green depths it has substance, like the sea. The simile is appropriate; glass is a super-cooled liquid - solid, yet molecularly amorphous. And it combines transparency with rock-like hardness and resistance to the elements. Unlike traditional construction materials which slowly weather and acquire a patina of age, glass remains virtually pristine through time.

Glass has been used in building for 2000 years, but the technology required to produce large panes developed only in the nineteenth century. At the same time innovations in the use of cast and wrought iron, and later, steel, offered the possibilities of creating huge lightweight structural frames to form uncluttered spaces clad with glass and filled with natural light. This new architectural language generated new building types: conservatories - the Palm House, Kew, 1845-48 by Turner and Burton; exhibition halls - the Crystal Palace, London, 1851, by Paxton; and the great railway sheds of St Pancras and Paddington, London.

A new technology emerged, based on ingenious methods of exploiting iron and glass prefabrication (Paxton even devised a glazing trolley' running on wheels along the gutters of the Crystal Palace, which held four glaziers and their equipment). At the same time, glazed roof structures were being adapted for a new building type - the shopping arcade; like railway stations these were not building interiors but streets with glass roofs to protect customers from the weather. An early surviving example is Mengoni's glazed barrel-vaulted Galleria Vittorio Emanuele II in Milan, built in 1861. In the direct line of descent is Nicholas Grimshaw & Partners' Waterloo terminal, London, its sinuous roof glazed like the scales of a lizard's back, and Eva Jiricna's glass Orangery in Prague. In both cases, frameless panes of glass are suspended from a delicate steel structure.

Toughened glass is very strong, but has been known to fracture spontaneously due to nickel sulphide inclusions. When single panes of toughened glass are used overhead such failure can have dangerous consequences. The problem can be overcome by heat-soaking the glass after toughening to identify impurities, or by using an additional lower laminated layer of glass as a safety bearing layer, which also improves thermal insulation qualities and allows access onto the glass roof for cleaning.

Minimum structure, maximum transparency

In 1922 Mies van der Rohe designed a glass tower with a vast sinuous glass wall which exploited the possibilities of inter-reflection and the changing angles of light. Although this was never built, the concept of an almost seamless wall was a goal to which architects and engineers aspired.

A few years later, almost by accident, a new type of glass was invented which would eventually make this concept a reality. Sheet and plate glass manufacture had developed in the early years of the century by using a combination of drawing, rolling and polishing methods. The new invention, toughened glass, was made by heating a pane of glass and rapidly cooling it by blowing cool air onto both faces, the outer surfaces contract and are pulled into compression. The resulting pane is four times as strong as annealed glass and, if shattered, breaks into rounded non-sharp pieces.

Toughened glass can be used without edge framing and one of the first buildings to exploit this was the Willis Faber and Dumas (now Willis Corroon) office by Foster and Partners, completed in 1975. The facade is of large faceted panes of glass, following the curve of the street; 12mm toughened bronze anti-sun glass panels, 2m wide, are suspended from the roof by bolted clamping strips and stiffened by glass fins. The only visible connections are patch fittings at the corners of the glass panes, and the coated surface a reflector by day, changing the building into a luminous lantern by night.

A pane of toughened glass will maintain its integrity when holes are drilled quite close to the edge, enabling it to be suspended and carried on bolts. In 1980 the architect Adrien Fainsilber teamed up with structural engineers Martin Francis and Peter Rice, and architect Ian Ritchie to use this characteristic of toughened glass to create a series of large transparent enclosures for the Science Museum at the Parc de La Villette, Paris. The team, RFR, designed a four-way connector which penetrates the glass through holes drilled at the corners. A system of spherical bearings and string supports connects the glass wall to a minimal cable truss system. The glass connector has since been developed into patented systems known as structural glass, such as Pilkington's Planar.

These systems can be used in overhead glazing and can be faceted to suit curved forms. The headquarters of Cellular Operations, Swindon, UK, designed in 2000 by Richard Hywel Evans, is a 9m high glass vault which curves like a ribcage over two floors of open-plan office space and tapers at one end to a bull-nose. The facade is formed of glass panes faceted at different angles, some differing in angle at all four of their edges. They are fixed to specially designed castings with Planar adjustable bolts, a modification of the standard system.

Until recently, structural glass bolted systems were limited by the amount of deflection which was deemed acceptable by structural engineers. Schlaich Bergermann's Kempinsky Hotel [1] in Munich broke this spell. The glass entrance wall is composed of l.5m sq panes attached at their corners to a cablenet structure so delicate as to be almost invisible, yet flexible enough to move up to a metre under wind load. A new arts complex in Philadelphia, US, by Rafael Vinoly and structural engineer Dewhurst Macfarlane, takes this idea one stage further in the search for ultimate transparency; the centre has a glass roof enclosed at its ends by a glass curtain suspended only from vertical cables.

Glass as loadbearing structure

We think of glass as a delicate material which must be framed or bolted in place on a building. It is - in theory - as strong as steel but whereas steel, when stressed, will yield, glass contains flaws which cause it to fracture under stress. In the past the answer was to design glass to very low stresses, but as structural engineers and architects began to understand its characteristics more fully, and as methods of laminating became more sophisticated, the idea of a glass loadbearing wall became an irresistible temptation.

The entrance canopy to KP Foods at Billingham, Teeside (architect Studio BAAD, structural engineer Techniker), is a wing-like glass canopy supported at each end by three glass panels, arranged in a Y-shape on plan to give support and lateral stability. Each panel is 2.1 m high and consists of two panes of toughened glass laminated together; the canopy rests on stainless steel fins UV-bonded into the 3mm laminate between the panes.

A system of laminated glass beams and glass columns using mortice and tenon joints at the junctions of beam and column was developed by structural engineer Dewhurst Macfarlane and Design Antenna (AR August 1995) to create a glass entrance foyer to - appropriately - a Glass museum in Dudley, UK. The glass elements required an almost incredible precision in manufacture - joints between glass roof panels had to meet along a glass beam only 30mm wide.

In 1995 Macfarlane was asked to produce ideas for a 10m cantilevered glass canopy for a plaza in the Tokyo International Forum, Japan, designed by Rafael Vinoly. [2] His solution - a glass beam made from a series of glass blades connected at their ends and at their mid-points to form a rigid cantilever - relies on a bolted connection, the loads evenly distributed by a metal bezel fitted in the bolt-hole.

On a much smaller scale, Foster and Partners, working with Techniker, have investigated the combination of glass - providing in-plane stiffness - with flat steel mullions, in a small glass conservatory on the roof of a building in Belgrave Square. The solid roof, carried on laminated glass beams, appears to float on glass eaves and the space is opened out by sliding doors which draw away from unsupported corners.

The future

It may be that the pursuit of transparency has reached its own glass ceiling. Concerns to conserve natural energy and reduce the greenhouse effect in the earth's atmosphere have led to designs which combine seamless transparent skins with high performance environmental control. The walls of the Business Intelligence Centre at Duisburg, by Foster and Partners, are a double-glass skin which modifies light and heat transmission, and delivers natural ventilation.

New responsive glass facades use self-regulating thermal protection and solar control measures to adapt in a dynamic way to changing light and weather conditions. In this way they meet the needs of building users while reducing energy consumption levels. [3]

The facade of the future must demonstrate that it contributes to sustainable symbiosis. Environmental control will be provided at a molecular level, using sophisticated responsive coatings. Glass inert and transparent, is the ideal substrate for such coatings. Already a new responsive panel, Serraglaze, can be used with glass to reduce the need for artificial light. It enhances daylight penetration, redirecting light from the sky by optical refraction and reflection.

A self-cleaning glass, originally developed for aircraft windscreens, is now available for use in external windows, produced by Pilkington. The glass, coated with microscopic chemical coatings, has properties which repel moisture and dirt, allowing them to be washed away during normal rainy weather.

Michael Wigginton, [4] who chairs a 10-nation European research agenda to develop the interactive facade, believes the eventual outcome will be nanometric, chromogenic energy control, in which energy flow will be controlled by a switch or an automatic sensor. Michael Davis has provided a poetic image of this future. 'Look up at a spectrum-washed envelope whose surface is a map of its instantaneous performance, stealing energy from the air with an iridescent shrug, rippling its photogrids as a cloud across the sun; a wall which, as the night chill falls, fluffs up its feathers and turning white on its north face and blue on the south, closes its eyes but not without remembering to pump a little glow down to the night porter, clear a view patch for the lovers on the south side of level 22 and to turn 12 per cent silver just before dawn.

(1.) Structural Use of Glass in Buildings, The Institute of Structural Engineers "" 10 Dec. 1999.

(2.) Working Details "" Susan Dawson, Emap Construct, 1999.

(3.) Intelligent Glass Facades, Andrea Campagno, Buxuauser, Basel, 1995.

(4.) Glass in Architecture, Michael Wigginton, London, 1996.