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SentryGlas® - A Wealth of Innovative Applications

“Engineering Solutions for the Impossible”

Arup is a global leader in producing innovative structural engineering solutions for architects worldwide. In this interview, Arup's New York façade engineering team leader, Neil McClelland, talks about the challenges facing architects today and explains how DuPont™ SentryGlas® structural interlayer for laminated glass can provide solutions for a wealth of applications beyond the hurricane-resistant glass for which it was originally designed some years ago.

Long gone are the days when glass was used by architects only in windows. Now, architects often come to us wanting to build whole structures from glass because of the transparency and beauty of the material - at the same time asking Arup to provide 'engineering solutions for the impossible'. As engineers, we have to ensure the soundness and safety of architectural structures and the obvious problem with glass is that it can break. In some instances it can break for no apparent reason - for example the 'spontaneous' breakage sometimes experienced with tempered glass due to nickel sulfide inclusions.

Laminated glass gives the post-breakage strength needed for a range of architectural applications, particularly laminated glass with DuPont™ SentryGlas® structural interlayer. For structural glass assemblies, laminated glass with its post-breakage strength is essential to minimize the risk of progressive collapse. The improved post-breakage behavior of SentryGlas® over other interlayer materials is ideally suited for these applications.

Because of these advances in materials performance, laminated glass is now being used by design professionals in more and more innovative ways, in projects that would have been inconceivable for structural engineers to approve five or ten years ago. However, it is not just advances in materials that allow innovative designs - it is also the willingness of the various parties to work together in an innovative way. This includes the architect who has the original design concept, the structural engineer who has the design tools to predict behavior, research companies like DuPont who share their knowledge with us and finally the manufacturer, if custom assemblies are required.

SentryGlas® was originally developed by DuPont for hurricane glass applications but it is now used by architects and structural engineers for a wealth of applications beyond the large and small missile impact resistance needed for hurricane glass.

Performance features of SentryGlas® can help engineers

First, laminated glass with SentryGlas® provides better clarity than traditional laminated glass; stunning transparency can be achieved by architects, particularly if the interlayer is used in conjunction with low iron glass.

Third, traditional laminated glass positioned off the vertical can sag or creep if exposed for long periods to the moderate-to-high temperatures typically generated by solar radiation. While this is not usually a structural problem, it is often an aesthetic or serviceability problem. SentryGlas® has a much lower loss of stiffness at temperatures likely to be experienced by architectural glass, so creep and sag are reduced or even effectively eliminated in many applications. This benefit is of particular relevance to architects wanting to use large expanses of horizontal glass, for example in extensive flat overhead glazing, sloped overhead glazing or glass walkways. As structural engineers, we would advise architects that laminated glass with SentryGlas® is a better choice than traditional laminated glass for such applications. Laminated glass with SentryGlas® would be an elegant and structurally sound solution for a long-span flat glass bridge, for example.

The glass make-up is four layers of 19- mm tempered, low-iron glass with three SentryGlas® Plus interlayers and a stainless steel insert molded into the structural interlayer. The model was created by Arup New York with input from DuPont and laminator Bent Glass Designs of Hatboro, Pennsylvania. © ARUP.

Fourth, Arup has recommended laminated glass with SentryGlas® in "wet" applications such as around water features and in yachts because of the interlayer's outstanding moisture resistance and edge stability. We would not recommend continuous water immersion. However, in the context of architecture, the moisture resistance of the interlayer means that architects can design in polished, exposed edges to their laminated glass applications with more confidence.

Fifth, SentryGlas® flows during the lamination process. This allows architects and engineers to cast metal inserts directly into the laminated glass. For structural glass applications, this allows an alternative connection method to end-bearing or bolted patch fitting connections. The increased strength, stiffness and temperature performance of SentryGlas®, as well as its good adhesion to many metals, allows higher load capacities than most of the more traditional bolted patch fitting connections - an essential feature in larger structural glass applications. These metal fixings can either be positioned at the edges of the laminated glass, or at the face of the glass.

Examples of where these fixings can be used include on the edge of stair treads or glass bridges, or into the face of laminated glass for retail store fittings. This is a much more advanced and elegant way of securing attachments to laminated glass than the traditional way of introducing fixings by patch fittings, which are bolted through the glass, or countersunk fittings. These give a flush finish at the back but still show the metal fitting.

Architects come to Arup with their design dreams. Working with research/ science companies like DuPont we are able to provide a fundamental understanding of the material (in this case, laminated glass) and suggest new structural engineering solutions to help the architect's dream become reality.

For me, discovering the many properties of SentryGlas® structural interlayer has involved blending the knowledge base of a cutting-edge science company (DuPont) and a cutting-edge engineering firm (Arup), working often in 'triplicate' with an outstanding laminator. This is how architectural dreams can become reality.

By Neil McClelland, Associate Principal, ARUP, New York