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Clad in glass Glass is one of the few areas of strong technical innovation in construction, showcased at a recent cladding conference

technical & practice

Project for Tower Place, London, on which James Carpenter Design Associates worked with Sir Norman Foster and Partners. Top: the struts were envisaged as glass tubes post-tensioned by 10mm stainless steel rods along their centres. Right: the idea of making the glass wall of a north-facing atrium partially reflective to retain some of the sunlight shining through the roof

This year's Bath cladding conference, Glass in Buildings*, focused mainly on innovation in the material rather than in systems. This was because, as Tim Macfarlane pointed out, glass is rarely used structurally. The glass beams and balustrades described by Philip Wilson of Dewhurst Macfarlane recently (aj 18.3.99), and Bath's own work on glued glass T- beams, remain the exceptions. Rather, strength comes from the metal framing. Moreover, where rod or cable systems are used they are much more 'live' than conventional structures. For example, the 30 x15m facade of som's glazed cable net entrance pavilion for General Motors' hq in Detroit can experience up to 60mm deflection due to wind loads. One trick is to use metal-to-glass connections, such as ball joints, that keep forces in the plane of the glass, minimising out-of-plane bending or torsion.

Mick Eekhout from the Netherlands queried whether it was better to avoid these large-deflection problems by using structural mullions. It is possible to pare these down to be relatively unobtrusive; they can also be strutted away from the glass fixings. And they are cheaper than the frequently one-off cable and rod systems. For a design practice that has projects around the world, using structural mullions saves the cost of visiting systems to check any loss of pre-tension and thus stiffness in the structure, with the risk of overloading the glass.

For those who do favour cables and rods, McCalls Special Products has come up with spring tensioners that seek to maintain structural stiffness (see illustration).

Glass technology

For glass itself there are many new developments.

Structural glass - The biggest coup de theatre was Mick Eekhout unveiling a prototype laminated glass rod. Such rods might provide glass struts for structural systems.

Already moving in this direction is James Carpenter Design Associates, working with Foster at Tower Place in London, a project now on hold. Luke Lowings from Carpenter's showed struts in the cable net as unlaminated glass tubes, held in compression by pretensioned rods within. Another neat idea for this project is to treat the inside glass surface of the north-facing atrium to be partially reflective of daylight. Sunlight would enter through the glazed roof and be reflected back into the interior (see sketch).

Laminated glass - Graham Dodd of Arup described using curved glass laminated with a solar control film for refurbishment of the Floral Hall at London's Royal Opera House. Normal solar coatings would not survive the bending process.

New pvb films around 1.5mm thick can be used to provide acoustic performance, as John Libby of Solaglas Saint-Gobain described. This is more compact than normal acoustic double glazing, with its air gap of several cm, and cheaper than a laminate incorporating cast resin. Acoustic pvb laminates are more expensive than normal pvbs, with some reduction in strength. A balance has to be struck. For example, at T2 Heathrow the performance menu for laminated glass was acoustics, solar control, optical quality and blast containment.

Photovoltaics - Thin-film photovoltaics were laminated into glass panels of 2 x 4 feet (0.6 x 1.2m) for a demonstration pavilion in New York last year. Andre Chaszar of ftl-Happold described how the panels were laser- etched to provide clear stripes covering 30 per cent of the glass area, providing some sense of the outside view. One of the problems is that the largely black panels heat up and then radiate inwards. A second layer of glazing or some other radiation shield could protect occupants.

Reducing U-values - In the course of a paper on global warming, Rick Wilberforce of Pilkington described a Swedish competition to achieve window U-values below 1.0W/m2K. Typically this is better than the frame. However, some people questioned the cost and payback of this direction of development.

In progressing toward such low U-values, other measures of performance are likely to be needed. Wilberforce referred to an eu (cen) committee developing an 'Effective U-value' measure which would indicate the year- round balance of heat gains and losses.

Developments in vacuum glazing by Richard Collins and others at Sydney University, sold by Nippon Sheet Glass Company, are achieving U-values approaching 0.6W/m2K. Sheet sizes are up to 2.4 x 1.35m. Double-glazed units of low melting point soda lime glass are edge-sealed with solder glass. The panes are held apart by 0.5mm-diameter metal pillars at, say, 20mm centres - surprisingly unobtrusive. Glasses can be solar-control coated.

Collins would like to be able to toughen the glass, but the heat of edge- fusing would destroy the toughening effect. If toughening could be retained, the stronger glass would allow pillar spacing to be increased, potentially producing mid-pane U-value to below 0.4W/m2K.

Photochromics - One problem with very low U-value glazing is that its high insulation is at some times more useful than at others. Variable glazings could offer a better match of climate control to required internal environment. Photochromics is a variable glazing technology common in sunglasses which responds to changes in incident light intensity. Corresponding light transmission can change from 75 to 25 per cent. The photochrome material is laminated between heat-strengthened glass.

Chris Hoar of Saint-Gobain predicts photochromic glazing will be available soon in sizes up to 1300 x 2500mm, in green or brown/amber. But we are back to some of the low U-value problem; while performance does vary, it is not controllable. For example, on a bright, cold day the glass can reduce solar gain if necessary.

Electrochromics - Here multi-layered thin film devices laminated into glass can provide sensor-controlled switching between transparency and translucency, though this is currently very expensive. In conversation, Norbert Wruk of Pilkington in Germany (nee Flachglas) said trial panels had reached 2.0 x 1.2m. The width of 1.2m is the controlling dimension. Make it larger and the charge in the glass, and thus the degree of obscuration, becomes non-uniform. Surprisingly, for comfort on bright days, light transmission may need to be reduced to 5-10 per cent.

It is not clear who will be interested in producing complete electrochromic systems for buildings. Glass producers tend to be focused on commodity sales, such as to window manufacturers or to the automotive industry. There, electrochromic glasses are being used in rear-view mirrors to reduce night-time headlight glare.

Fire resistance - Wruk also described how the size of fire-resistant glazing panels is increasing. The maximum sizes of double glazing with a water glass interlayer successfully tested - for various fire ratings - are 120-140cm x 200-230cm. High transparency can be maintained by using low-iron glasses; say 85 per cent for a 90-minute insulating glass panel. And the fire-resistant glazing can be combined with other glass panels to provide safety glazing, resist snow and wind loading, provide insulation, etc.

Surface quality - Checks need to be made on surface quality, such as roller marks as the glass cools in manufacture, uneven layering in coating giving coloured hues, and clamping marks and air bubbles in laminating.

Coming shortly - Chris Hoar of Saint-Gobain suggested that we can expect the following:

- general developments in coatings. These include enhanced thermal performance and optical clarity in low-E coatings

- low-reflective glazing in shopfront sizes

- pvb colouring and patterning and more acoustic pvb options

- self-cleaning coatings

- smart glass as message screens in street furniture, such as glass bus shelters telling you when a bus is due.

Translucency - A lot of work is going into avoiding unwanted translucency, seeking greatest clarity. But Wayne Forster of Cardiff Architecture School pointed out that this is not always the designer's objective. This Modernist approach sometimes gives way to a search for translucency - glass blocks, layered walls like Corbusier's mur neutrilisant, translucent materials such as glass fibre panels, applied finishes, and nets and foils as with Hasegawa and Ito.

What materials have been rejected by glass manufacturers' r&d departments for lack of transparency over the years that might today find architectural expression?

*Glass in Buildings. Bath, 31 March-1April. Stephen Ledbetter and Richard Harris (editors). Centre for Window & Cladding Technology, tel 01225 826541. 280pp. £80

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