Finally a few words on the glass itself. There are now many forms of glass and glazing available to provide environmental control and thermal performance.These include:

- variable-transmission glazing;

- liquid crystal laminates;

- electrochromatic glass;

- low-emissivity coatings;

- transparent insulation;

- screen-printed glazing;

- fixed shading systems.

These are described in detail by Brookes and Stacey (I 992), pp. 29-41. Also, Button and Pye (1995) summarize much useful technical information.

The weakening effects of nickel sulphide inclusions in glass have been the source of much debate. Annealed glass is unaffected, but in toughened glass spontaneous fracture may occur in use, because of the expansion of the unstable nickel sulphide as it changes phase over time. It is not practical to detect these inclusions by a non-destructive process. However; heat soaking can be carried out after toughening.This is a destructive quality control procedure, as panes with inclusions will shatter during the soaking process, but it is an essential specification requirement.

At present there is no standard method for heat soaking, although it is generally recommended that all toughened glass shall undergo a heat soaking regime to achieve a maximum residual risk of one nickel sulphide failure per 40 tonnes. There is no applied symbol to indicate that glass has been heat-soak treated. It is therefore important that the current Kitemark should be modified to identify when a toughened panel has been heat-soak treated.

In applications where thermal shock is considered a potential risk, but the strength of toughened glass is not required, it is possible to specify a heat-strengthened glass, as it has approximately twice the mechanical and thermal strength of annealed glass.

Research into improving the thermal performance of double-glazed units continues. It is now possible to specify a double-glazed unit which has low-E coatings on surfaces 2 and 3 with a gas-filled cavity between to achieve a U value of I.I W/m2oC. Pilkington and BASF have together been carrying out research into a granular aerogel, which provides a transparent insulation layer achieving U values between 0.5 and I W/m2oC.This is now supplied by Flachglas of Germany, a Pilkington subsidiary.

Photovoltaic glass, recently considered only an experimental technique, is rapidly becoming a cost-effective option, but it has yet to be seen integrated into a major building facade.

Finally, developments in laminated toughened glass have allowed engineers and architects such as engineer Robert Nijsse, working with architects Kraayvanger Urbis on their own offices in Rotterdam, to build an all-glass bridge using a combination of 10 mm clear toughened glass, PVB laminate and 6 mm Eco-plus low-E glass for the walls and two layers of I 5 mm toughened glass bonded with a PVB laminate to form the floor (project architect: Dirk Jan Postel).This is an interesting example of collaboration between engineer architect and fabricator Having produced this all-glass bridge, Robert Nijsse believes 'it would now be possible to make just a glass tube without the supporting beams' (Fig. 7.38). The challenges of the next generation of suspended and structural glazing are enormous.

7.38 Laminated toughened glass as used at the glass bridge in Rotterdam by engineer Robert Nijsse.

References and further reading

Anon (1996) Glass pavilion at Aachen and Thames Tower,

Barometer London. Glasforum, 19-21. Brookes, A. J. and Grech C. (1996) The Building Envelope and Connections, Architectural Press.

Brookes, A. J. and Stacey, M. (1992) Product Review - Glazing and Curtain Walling. AJ Focus, Architects'Journal, July.

Brookes, A.J. and Stacey, M. (1993) Product Review - Glazing and Curtain Walling. AJ Focus, Architects' Journal, September

Brookes, A.J. and Stacey, M. (1994) Product Review - Glazing and Curtain Walling. AJ Focus, Architects' Journal, November.

Button, D. and Pye, D. 'Glass in Architecture,' Butterworths, 1995.

Frohnsdorff, G. (198 I) Proceedings of the second international conference on the durability of building materials and components, 14—1 6.9.81, National Bureau of Standards, Gaithersburg, Maryland, USA.

Guedes, R (1979) Encyclopaedia of Architecture and Technological Change,'Glazing systems', Macmillan: London, p. 284.

Hix,J. (1995)The Glass House, MIT: Cambridge, Mass., USA.

Nijsse, R. (2003) 'Glass in Structures' Birkhauser

Schittich, Staib, Balkan, Schuler Sobek (1999) Glass Construction Manual Birkhauser

Stroud, Foster and Harington (1979) 'Cladding - curtain walling'. Mitchells Building Construction Structure and Fabric, 2nd edn, Part 2, Batsford: London, pp. I 67-175.

Wiggington, M. (1996) Glass in Architecture, London

Wurm,J. (2007) Glass Structure, Birkhauser.

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