Bent glass has been used for building purposes since the early nineteenth century, yet this is still a modern and dynamic product that has far from exhausted its potential. Today the use of bent glass is steadily increasing along with other uses of glass in modern architecture. Safety glass now accounts for a growing proportion of all production. With the continuing advances in glass technology, the improving safety properties and energy economy of glass and the new methods available for producing bent glass, the future market prospects for bent glass are promising indeed. This article looks at some of the new uses and applications that are open to bent glass and explores its development prospects.

Use of glass increasing in building and construction

The use of glass in public buildings and office complexes has steadily increased over the past few decades, and the trend looks set to continue. Glass is an inexpensive material which offers many superior properties in different applications. It is environmentally friendly and fully recyclable, an increasingly important consideration with the growing emphasis on life-span thinking.

There is a strong trend in modern architecture towards transparent structures, which allow natural light to enter buildings and on other hand open up the natural landscape to end-users inside. Glass creates an airiness, provides a sense of space that can only be achieved with larger glass sizes and in lighter support structures.

Modern glass technology has done away with the need for any compromises in terms of energy economy. Different types of coating and insulating glass structures mean that modern glazing applications can be so designed that they meet even the most stringent thermal insulation requirements (which in glass structures are typically expressed in the K-value). Ideally, glass surfaces and glass walls will allow a suitable amount of warmth in, but keep the hottest sunshine out the building.

Bent glass is a powerful tool of aesthetic design. Curved surfaces help effectively to tone down the coldness and hardness that is usually associated with glass. They also bring large glass surfaces alive and help the building stand out of the mass, a value widely acknowledged among architects and end-users. Bent glass can be used as part of the building facade, or to make up the whole facade. In the case of offices and business premises, facades are an integral part of the corporate image.

Tempered bent glass now available

A growing proportion of all bent glass is processed to make safety glass. Bent laminated glass has the same safety features as flat safety glass. Shattered safety glass remains firmly in place: it does not drop out its frame, no sharp pieces are left lying around and there is no gaping hole in the surface. Bent laminated glass has long been available in the marketplace. The basic unit is made by laminating two sheets of glass together with a PVB film. Bent glass can also be laminated to different impact, burglary and bullet resistant categories. Laminated bent glass is ideally suited for different kinds of railing applications in staircases and elevated walkways, skylights, elevator doors, revolving doors and display windows, particularly in cases where both safety and aesthetic considerations matter.

The mechanical and thermal resistance of tempered glass is four to five times greater than that of ordinary glass. Upon breaking it shatters into small pebble-like pieces. Availability of tempered bent glass used to be limited, as was the size in which it could be produced, but modern technology means that it is now possible to bend and temper glass surfaces up to four metres in length. This has increased the use of tempered bent glass in facades and in interior architecture, for instance.

Lamination of bent tempered glass allows us to combine different safety properties. Skylights, for example, are traditionally made by using insulating glass elements, with a load-bearing tempered glass sheet on top and a laminated sheet that does not shatter underneath. In applications where an insulating glass element is not needed or where it is not feasible, this structure has been replaced by laminating two tempered or heat-strengthened glasses together. A typical application might be a glass ceiling or a railing glass.

More and more applications

Bent glass has been used primarily in public buildings, office complexes and in the facades of corporate facilities. Typical building projects where bent glass is used include airports, exhibition areas, museums, concert halls and shopping arcades. There are two main categories of application, i.e. interior and exterior architecture:

• Exterior architecture:
  • facades and display windows
  • skylights and cupolas
  • skywalks
  • entrances, revolving doors, canopies
  • winter gardens and conservatories
• Interior architecture:
  • railings for staircases and elevated walkways
  • elevator glass panels
  • partitions

The specific type of glass required for different applications is usually determined by building regulations. Work to develop these regulations and safety glass norms is continuing to progress, which will certainly facilitate the task of making the right choice. It is indeed crucial that for each application a type of glass is chosen that best meets the requirements for that particular application. Although the choice of structure is ultimately the responsibility of the authority concerned, the glass professionals may nonetheless suggest a better alternative. For instance, the norm for staircase applications is usually tempered glass, even though it is known it will drop out of its frame upon breakage and leave a gaping hole in the railing. For such applications laminated glass, or for bolt-secured railings tempered laminated glass, might well be a more sensible choice: this would protect users from falling pieces of glass and keep the railing intact even upon breakage.

Removing the obstacles

Perhaps the main obstacle to the wider use of bent glass is that both architects and building contractors are not well enough informed. The design and manufacture of a bent glass structure requires special expertise, which remains quite rare in these professions. On the other hand a bent glass structure is always an individual, unique product. Standardised solutions are mainly found in applications that are produced in longer runs, such as revolving doors and elevators. In facades, each structure has to be designed and usually submitted for approval separately, which not only requires special expertise and experience but also raises the cost of building. However it has to be said that most glass benders have done an excellent job in providing architects with the information they need. This is indeed the best address for users who need to know about bent glass and its uses: glass benders can always provide a full and comprehensive information package.

Previously the price of glass structures was also pushed up by the fact that the frames and in many cases the load-bearing structures had to be separately designed for curved glass. For instance, the bending of aluminium profiles is a slow manual process, which obviously adds to the cost of the final product. However new stretch forming methods have very much alleviated this problem. At the same time new bolt-secured glass structures, for instance, have allowed for more flexible adaptation to curved glass.

The image of curved glass has long been tainted by quality problems that have now been largely eliminated with new advanced furnaces and mould technologies; this is true at least of laminated glass. Most glass benders today are capable of producing bent laminated glass with superior optical and design properties. Combined moulds and optimal temperature control in electric furnaces have been among the key factors in this development.

Architects, however, are not always content with the optical properties of tempered curved glass. Curved building glass used to be manufactured with roller hearth furnaces, where the end product tends to suffer from waviness caused by the use of rollers and from the overheating that was necessary to compensate for the cooling required during bending. We are now getting rid of this problem, too, with the introduction of new furnaces in which the bending is done inside the furnace.

In relative terms the production costs of curved tempered glass are lower than those for curved laminated glass. The tempering process requires heating and cooling in any case; the extra costs come mainly from the time required by bending and bender amortisations. For curved laminated glass, the whole bending process is added on top of this and the processing times are considerably longer. In other words the recent technological advances in production methods and the increased production capacity are set to open up whole new prospects for the use of curved glass with cheaper prices, improved technical properties and better availability.

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.