The Best Laid Plans: Risk management in building envelope design and construction

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Photos courtesy Halsall Associates

By Andrea Yee, B.Sc.E, and Nastassja Pearson, M.A.Sc., P.Eng.
Today, more and more architects are literally thinking outside the box. Modern buildings are taking on unique shapes and forms, and structures are reaching staggering new heights. This shift means the purpose of the building envelope is also expanding. With new materials and construction methods at their disposal, design professionals are increasingly looking to the building envelope to help supply buildings with a distinctive identity, simplify construction, and reduce costs.

Despite this trend, the industry needs to remember the building envelope’s primary purpose has not changed—it is still to provide shelter and protection from the outside elements. Therefore, it is important a building envelope’s design and construction continues to support, not compromise, performance.

In this article, three current design and construction trends for the building envelope will be discussed—use of glass, increasingly complex and unusual designs, and prefabricated cladding panels. Additionally, the potential impact each can have on building envelope performance will be examined. Finally, ways for design teams to manage the potentially negative (and often unintended) consequences of adopting these trends are explored.

Glass and the building envelope
Employing glass in buildings beyond conventional vision areas is a popular building envelope trend. In cities around the world, an increase in advanced glazing assemblies such as high-spanning curtain walls, glass canopies, and glass railings are being seen. Curved glass applications are also becoming increasingly popular, as is the use of structural glass for floors, stairs, and walls. The primary advantage of glass is it provides visible light transmission (VLT) and, if designed and used correctly, it can also be a strong and versatile material.

However, designing and installing glass without fully understanding its in-service behaviour may leave design teams dealing with unexpected consequences. The most obvious of these is glass is a brittle and fragile material, and it is much more likely to break in its everyday use than metal or concrete.

Glass can be sensitive to imperfections, such as surface and edge defects, and to solid inclusions and impurities. These compromise its strength and can lead to premature failure. Glass can also fail due to excessive buckling if it is incorrectly supported or excessively stressed. Stresses are derived from many sources, including direct impact, temperature differentials, concentrated loads at uneven or improperly designed supports, or material impurities.

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Complex structures create new challenges for designers and builders.

The first step in minimizing the potential for glass to break is to apply good design. An example of a best design practice is to ensure the glass is separated from other hard surfaces, for example by leaving a gap between the glass edge and frame. Another is to allow sufficient accommodation for movement. Restricting the glass from moving can potentially create concentrated stresses and lead to premature failure.

Post-breakage behaviour
Anticipating the post-breakage behaviour of glass is also critical to safeguard the public when there is a failure. Depending on the type of glass, it behaves in different ways when it breaks.

Both annealed and heat-strengthened types of glass have a tendency to crack and/or break into large shards when they fail. While this creates a hazard to people wherever accessible, it poses a risk of greater injury in an overhead application if it falls onto someone below.

Tempered glass, on the other hand, breaks into smaller pieces and is intended to cause less injury in similar applications.

Where there is an increased risk of glass falling, it is often preferred to have the glass stay within its original opening until there is an opportunity to vacate the surrounding area to allow building staff or installers to safely remove the broken glass.

Laminated glass has the highest probability of staying in place subsequent to failure. The interlayers bonding the lites together are what keep the unit intact. However, if all the laminated glass lites are fully tempered, the glass may have a tendency to sag out of its opening upon breakage. This deformation is commonly referred to as the ‘wet blanket effect.’ Sagging is less likely to occur when a stiffer interlayer material is used or at least one of the lites is annealed or heat-strengthened because those larger shards can potentially provide additional support.

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