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SentryGlas® Plus: engineered glass for the 21st century

Acclaimed structural engineer Tim Macfarlane compares the introduction of laminated glass incorporating SentryGlas® Plus interlayer and Secure™ technology to the invention of reinforced concrete at the beginning of the last century

Tim Macfarlane Dewhurst Macfarlane & Partners, London & New York

Our architectural clients are increasingly asking us to engineer laminated glass stairs, floors and bridges. It’s all about managing light; making structural elements like these out of a transparent or translucent material – glass – means that more natural light is brought down into the building. Glass bridges, for example, generate a sense of lightness, of floating, of poetry even. For these reasons, laminated glass balustrades hold an obvious attraction for architects who want to keep the line of the bridge pure, and for whom the balustrade can be an ugly distraction. There have been some lovely bridges using laminated glass for the deck, too. This really minimizes the bridge structure. It’s especially attractive if the glass deck is under-lit to give the appearance of a diffused, lit surface on top.

However, engineers of my generation still have this initial knee-jerk reaction to the concept of glass bridges, stairs and floors. That is, that they are completely alarming from a safety point of view! Glass is a fragile material that you can cut yourself with, isn’t it? What is the collapse mechanism? How much damage could it cause to people below if it got broken? It’s still incredible to me that high rise buildings in some parts of the world can have glass facades of heat-strengthened or toughened glass, not laminated glass! This is definitely not a safe structural solution. What about if the glass falls out? However, I believe that due to new interlayers like SentryGlas® Plus and new safety codes worldwide, the next generation of architects and engineers will have a completely different set of reflexes when it comes to designing structural elements of glass. Designing safely with glass will already be a more natural concept for them.

The industry has progressed an incredible distance over the past 10 years when it comes to engineering glass floors, stairs and bridges in glass. The challenges remain the same – you need to create a surface for walking on with good traction resistance, that can’t be scratched and that can be cleaned easily. The material can be a composite but has to look like glass. With the introduction of SentryGlas® Plus and Secure™ technology, we have made a huge leap forward in solving these challenges.

I remember the first glass stairway our firm engineered, for the architect Eva Jiricna about 10 years ago. It was made of 19 mm thick annealed glass with a 15 mm thick sheet of acrylic. That was the only way I knew of at the time to get the necessary failsafe guarantee into a glass staircase. The disadvantage was that dust got into the gap between the acrylic and glass; acrylic is also easy to scratch and degrades through time. So there were major disadvantages from the aesthetic point of view. The whole construction, although sufficiently strong, was diametrically opposed to the purity of form that most architects want to achieve.

At the other end of the scale our New York office worked with the California office of architects Bohlin Cywinski Jackson and laminator Depp Glass of New York to complete the staircase in the Apple store in SoHo, New York in 2002. For me, the Apple staircase is the zenith of state-of-the-art structural design with laminated glass. It was made possible through the use of DuPont™ SentryGlas® Plus ionoplast interlayer and Secure™ technology. First, the design fulfils all the practical requirements that an engineer could want from a glass stair. The wear surface on the top glass is laminated to the remainder of the tread. This provides traction resistance because it is an acid-etched surface. It is also smooth and easy to clean, and it’s translucent so you can’t look up through the treads. Most exciting, though, is the fact that, together with BCJ and DuPont, we succeeded in designing a staircase where the treads are spanning from side to side across 240 cm as a two-way span without edge support.

This means, for the first time that I know of, that the interlayer has become an integral part of the structural makeup of the stair, meaning that it contributes in a significant way after failure. The treads are just pure, solid plates of glass. The aesthetic effect is therefore stunning. We were able to achieve this innovation because we had a client with sufficient interest and budget to really push the design team to the absolute degree in order to break new architectural and engineering boundaries and achieve a more startling aesthetic effect than had been dreamed of before. Apple is now using similar staircases with SentryGlas® Plus in stores in Los Angeles and Chicago. They seem to have become part of the Apple aesthetic. We could certainly not have confidently proceeded without the backup of specialists from DuPont Glass Laminating Products’ central research laboratories who helped us develop the maths and physics behind the stairway.

The construction of the Apple staircase treads is 8 + 15 + 15 + 8 mm, all annealed, with a 1.52 mm layer of SentryGlas® Plus between each glass layer. Because of the enhanced stiffness of the ionoplast interlayer, the stair really performs beautifully. PVB would have tended to give a more flexible tread but the ionoplast interlayer really participates in the composite action of the tread. It’s comparable to reinforced concrete, where the steel and concrete contribute continuously and powerfully to each other’s existence.

If all three layers of annealed glass making up a stair tread crack, the SentryGlas® Plus, because of its enhanced stiffness and overall terrific properties, acts with even greater strength than the acrylic in Eva Jiricna Architects’ stairway of 10 years ago. The strength of the ionoplast interlayer over a very wide temperature range gives even the most cautious engineer enough margin to feel absolutely safe about using it for stairs, bridges and walkways. This means more design freedom for architects.

The ionoplast interlayer is nothing short of revolutionary but it requires handling in a different way, laminators and glaziers need to be trained to work with the product. SentryGlas® Plus is malleable too so there is a lot of potential to explore bending the laminated glass using heat and pressure in ways we could not do before to make extended shell structures, utilizing the stiffness of the ionoplast interlayer.

There’s so much more we could be doing with this interlayer. When reinforced concrete was introduced in the early 20th century it was hard to imagine how flexible and universal it would become in the construction industry. I believe the next few years will see its widespread take-up in horizontal floor surfaces and that we will be seeing even more applications for the laminated glass with this incredible interlayer that we cannot even imagine yet. For me laminated glass incorporating SentryGlas® Plus interlayer and Secure™ technology looks certainly set to become THE ‘engineered glass’ material of the 21st century.