Glass at the cutting edge - Glass used in building design

Architectural Review, The, August, 2001 by Susan Dawson

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.