Topic of the month August

MODERN GLASS ARCHITECTURE: SHAPING THE FUTURE (Part 2)

Bending coated glass

One of the key factors behind the increased use of glass in general is the development of coating techniques. These have helped to create glass properties with a huge impact on the range of potential applications; examples include improved thermal insulation and protection against solar heat radiation. Pyrolytically coated glasses or so-called hardcoat glasses can be bent and tempered almost like ordinary glass.

Soft-coated glasses usually offer better thermal insulation and solar heat protection properties than hardcoat glasses. The problem with these coatings is their sensitivity to treatment and their low heat resistance. Soft-coated glasses cannot be subjected to the kind of temperatures required by bending and tempering, and therefore they are always tempered before coating. With straight glass there are no problems because several suppliers have coating machines for flat glass.

No machines are as yet available for purposes of coating bent building glass. This means that normal soft-coated glass cannot be obtained in curved form. In Germany, for instance, Wärmeschutzverordnung requires of facade glass a K coefficient of 1.1 W/m²K, which can only be achieved with soft-coated glasses. If curved glass is nonetheless incorporated in the façade plan, there remain two options: either compromises will have to be made with regard to the colour, the light and thermal transfer properties, or solar heat radiation properties of curved glass, with the structure submitted for individual approval light, or else the curved surfaces are replaced by straight ones.

Given the difficulties involved in the treatment and particularly the post-processing of soft-coated glass, leading glass manufacturers have in recent years been working to produce more durable types of glass that also have sufficient thermal and solar heat radiation properties. There are two main trends; work is under way to develop:

 new pyrolytic methods (e.g. CVD, or chemical vapour deposition) such as Sunergy and PPG's Sungate which can offer properties superior to those of normal pyrolytic glass; and
 soft coatings that can withstand bending and tempering temperatures; e.g. Guardian's Sunguard.
 These efforts to develop high performance coated glasses that can be tempered and bended will indeed open up significant new markets for bent glass.

Curved glass offers load-bearing capacity

The same design parameters are usually applied to curved glass as are used with straight glass. This means that the load-bearing capacity offered by the curved shape is not taken into account - which in turn means that the final structure is heavier and more expensive than would have been necessary.

Yet the curved shape offers some significant advantages, provided the calculations are right: the thickness of the glass can be significantly reduced, which will obviously reduce the overall weight of the structure and brings savings in costs. The lighter weight of the glass is directly reflected in the design of the load-bearing structure - which again cuts costs.

A good example is provided by the Hannover EXPO Skywalk. The material used is 2*6 mm laminated float glass, sized 2*2.25 m. Had the design equations for straight glass been used, the structure should have been built with 2*12 mm laminated heat-strengthened glass. On the reverse side of the coin, this structure had to be submitted for separate approval because the necessary calculations could not be performed as per the norm. (Source: Tambest Oy, Finland)

The extra rigidity of curved glass allows for greater freedom in the process of architectural design, particularly in terms of larger free glass surfaces. This is a significant advantage especially in the design of skylights. Given the lighter weight of structures and frameworks, material costs will also be down.

The key here lies is the standardisation of design principles, testing and approval procedures: ultimately we will need to get rid of the expensive and time-consuming process of tailoring on a case-by-case basis.

Standardisation and building regulations for curved glass

There exists no comprehensive set of norms for the use of bent building glass, its properties and testing.
Building regulations now in force apply equally to flat and bent glass. As regards the pretreatment of glass (cutting margins, location of holes and notches, shapes, etc.), the norms compiled for flat glass are used in so far as they are applicable. Individual manfacturers may apply their own recommendations with respect to the positioning of holes, for instance.

As far as tempered glass is concerned, the norms for flat glass are applied so far as this is possible. For instance the definition of tempering degree (grain size, quantity, shape) can be applied as such. Testing practices vary. Breakage under pressure of a sharp object can be tested according to the norm. Impact resistance cannot be tested as such because the methods and equipment have been designed for testing straight glass. Therefore the corresponding flat structure is often tested, or a test is specially designed for each case.

As for special types of bent glass (impact, burglary and bullet resistant glasses), the norms for these specific categories are applied. The structure is tested with straight glasses and applied as such to bent glass.
Problems occur most typically in situations where the norms for flat glass cannot be applied as such to curved glass. These include:

a) The testing methods available (e.g. load-bearing tests and impact resistance) are designed for flat glass and cannot normally be applied as such to curved glass. This will usually mean that structures have to be separately approved, adding to costs.

b) Quality criteria, such as accuracy of shape and optical quality. In the absence of international standards either the manufacturer's norms or those introduced for each particular case will be applied, which may lead to differences of interpretation.

c) Design principles and static calculation. As long as there are no calculation standards, curved glass structures will have to be designed on a case-by-case basis. Standardisation would also have the benefit of allowing us to take into account the extra rigidity generated by the curved shape without having to resort to expensive one-off calculation and approval procedures.

A comprehensive norm would certainly support and bolster the use of bent glass in the building and construction industry by providing information about the use, the properties and the testing of glass, by harmonising practices and reducing the need for expensive, separate testing. The ASTM norm published in the United States for bent glass is an important step forward, but it does not address all the problems discussed.

Source : Glassrobots OY- www.glassrobots.fi/
OGIS GmbH www.glassglobal.com