Engineered for strength

Engineered for strength

Featured Article
20 Jul 2020

For most applications where safety glass is required, it is usually a choice between toughened or laminated glass. However, we’re seeing the popularity of laminated toughened glass increase as it is used in applications where clients are looking to combine the benefits of both products. Here we describe the applications in which laminated toughened glass is being specified.

Pilkington Optilam™ Clear T

We love having glass in our buildings. It allows daylight to flood interior spaces creating pleasant environments in which to live and work, and can deliver fantastic views for occupants. However, while glass is often chosen first and foremost because of its transparency, there are many applications where safety, security and strength are also key factors.

For most applications, specifying safety glass is a simple choice between toughened and laminated glass. This is often dictated by regulatory requirements.

Understanding the difference

Toughened glass is produced by heating annealed glass to approximately 620 ºC, at which point it begins to soften, and the surfaces of this heated glass are then cooled rapidly. The process creates a state of high compression in the outer surfaces of the glass and, as a result, the bending strength is increased by a factor of up to five times that of annealed glass. When broken, the toughened glass fractures into small pieces, and as these particles do not have the sharp edges and dagger points of broken annealed glass, it is generally regarded as a safety glass. 

In contrast, laminated glass consists of two or more panes of glass sandwiching one or more interlayers, most commonly PVB (polyvinyl butyral). It is no stronger than annealed glass of a similar thickness, however, when laminated glass breaks the glass fragments tend to stick to the PVB. Although the glass itself may be annealed glass, upon breaking any sharp cutting edges are not generally exposed.

When to specify?

Toughened glass is often specified for its strength, safety and resistance to thermal fracture, whereas laminated glass tends to be specified for its safety, acoustic performance and resistance to penetration.

Monolithic toughened glass has been the material of choice for barriers for many years, with relatively few breakages. Barriers are designed to protect people from falling, for example where there are stairs, floors and balconies in buildings such as shopping centres, and there are clear regulations surrounding the use of glass in such situations. In some countries the regulations for barriers insist on handrails being used in conjunction with glass infill panels and free standing glass protective barriers, which are sometimes referred to as balustrades. 

However, in modern building design there is a growing trend for handrails to be omitted from barriers for aesthetic reasons, and this has led to a rise in the use of laminated toughened glass in such applications.

How does laminated toughened glass differ?

Laminated toughened glass typically comprises of two panes of toughened glass that are subsequently bonded together. In a barrier, should one of the toughened glass panes break, then there is still another pane intact. However, it is worth remembering the remaining pane may not be able to resist the full design loads by itself. This is particularly important if the handrail in a barrier has been designed out by the structural engineer.

Even when a handrail is incorporated, laminated toughened glass is increasingly preferred for its strength, combined with its ability to stay in place in the event of breakage.

As well as barriers, there is a growing trend for specifying laminated toughened glass in building facades. For the insulating glass units installed on large towers in city centres, there has been a move away from monolithic toughened glass as the outer pane. Although there are no building regulations requirements in this area, project risk assessments sometimes raise the fear that, in the event of breakage, fragments of toughened glass could fall from the frame and onto the ground below.

The obvious solution of laminated annealed glass then raises other issues. As well as needing to be strong enough to resists loads, such as wind, the glass may be at risk of thermal overstressing from solar radiation - particularly if incorporating a tinted or coated solar control glass. As with any solar control glass, a thermal safety check should be undertaken early in the process. If the result of the check is such that the glass is at risk, then it should be specified in heat-treated, for example toughened, form. 

Increased application

Laminated toughened glass such as Pilkington Optilam™ Clear T has been used for many years in specialist applications, such as bolted structural glazing systems, underwater windows and glass floors, combining the best of both products.

As architects’ use of glass in our built environment grows steadily more ambitious, laminated toughened glass will have a critical role to play in ensuring those buildings are not only beautiful but also safe, strong and secure.