Challenges of Glass Design

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Most materials used for building construction utilize well-established design standards which are adopted by the building code and have evolved over time. Steel, concrete, wood, and aluminum all have codified documents which offer thorough and robust guidance to structural designers. Unfortunately, no such standard exists for glass when it is used in a structural capacity. In the absence of adopted standards, structural glass engineers rely on a collection of industry best-practices and alternative design standards to fill in the gaps. The design of glass presents unique challenges which require special consideration. The best practices for structural glass design can be classified into 6 Rs, resistance, redundancy, residual capacity, retention, replacement, and regulation:

Glass has unique properties in comparison to most modern construction materials. Perhaps the most challenging from a structural designer’s perspective is that it is brittle, not ductile. Material ductility is desirable because it provides a predictable onset of yielding which gradually transitions to an ultimate failure. Most traditional building materials either inherently exhibit ductility (e.g. steel) in their load resistance behavior or have incorporated construction techniques which impart ductile qualities to a composite (e.g. concrete rebar).

As a brittle material, glass strength is controlled by the presence of microscopic surface flaws which when loaded will propagate to full fracture without a yielding transition. The unpredictable nature of surface flaws necessitates that a statistical model is used to determine the load resistance of in-service glass. The industry standard for acceptable probabilities of breakage will vary depending on the application, with common probabilities of breakage being 1/1000 and 8/1000.

Tempering is a common thermal treatment which effectively strengthens a glass surface against crack propagation. The tempering process increases load resistance by imparting a residual compressive surface stress to the glass. A conceptually similar method of strengthening is observed in post-tensioned concrete slabs, another brittle construction material.

Redundancy is an important concept for all structural design. A redundant structure ensures there are multiple ways for a structure to distribute and transfer any applied loads. This is crucial to ensure that if any individual component fails, the applied loads may be redistributed to other components without a progressive collapse. One of the primary methods by which redundancy is imparted to glass is via lamination of two or more lites of glass with a binding interlayer.

aminated glass is used in a multitude of applications in which glass failure could result in severe injury or fatalities. For example, lamination is used for modern automobile windshields for the added safety and redundancy it provides to passengers. In buildings, glass redundancy via lamination can be observed in modern guard and handrail design and sloped overhead applications, such as canopies.

Glass breakage can occur during service even with proper design. Accidental or unintended loads from building occupants could stress glass in unanticipated ways. Spontaneous breakage may occur due to virtually indetectable imperfections, such as nickel-sulfide inclusions, created during the manufacturing process. A glass designer must consider glass’ remaining load capacity requirements following a fracture. It may be acceptable for window glass to have little to no residual capacity, but sloped glass or walkways which may be supporting occupants at the time of fracture must be able to accommodate the breakage and still carry the load until occupants can safely relocate.

Accepting that glass will fracture during service also means considering what will happen to the glass afterwards. What would happen if glass fractured? Would the glass remain entirely secured to the building structure or is falling debris likely? Would falling debris create a significant hazard for occupants? Each glass component’s retention must be considered during design. For example, the designer may decide that it is acceptable for a monolithic window to fracture if the hazard created by falling glass fragments is minimal. Conversely, a laminated canopy (which could fall on occupants directly below) has a relatively higher risk of great bodily harm if not retained to the main building structure. Chapter 24 of the International Building Code (IBC) requires designers to consider the effects of building movement on intact vertical glazing to ensure glass panels in curtain walls and storefronts will not disengage and become a falling hazard.

When glass fractures it must be remedied for both temporary and permanent occupation of the building. Lead times for glass production can vary widely depending on the complexity of the application. A simple insulated glass window panel may be able to be ordered and replaced within a week. More complex, custom applications such as point-supported glass, or oversized units which are produced overseas, may require months of lead time to replace. These delays can significantly hinder the building’s use. Glass connection details can and should be designed to ensure replacement is achievable without compromising other building finishes whenever possible. Careful planning is an important design step to ensure an efficient and swift replacement plan is in place if glass were to be compromised during service.

The IBC devotes Chapter 24 to glass and glazing. Topics addressed include a wide range of applications including sloped glazing, safety glazing, and specific use cases like handrails, guards, walkways, and elevators. However, many glass applications in modern building design, such as point-supported glazing and beam-columns (glass fins) are not addressed by the IBC. The IBC has historically updated language in Chapter 24 with each iteration, but additional guidance from outside references remains essential. The IBC also references documents in Chapter 35 which are relevant to glass design, including notable ASTM standards E1300, E2751, and E2358.

Existing documents from other agencies are also very helpful tools to a modern designer. NGA’s GANA Glazing Manual, NCSEA’s Engineering Structural Glass Design Guide, and Eurocode’s BS EN 16612 are excellent resources for understanding and designing modern glass structures. While the IBC does not adopt these documents, they are widely recognized and referenced within the structural glass community. Additional anticipated documents from ASTM and other groups aim to increase the available guidance for structural glass designers moving forward.

Glass design is a complex discipline which requires nuanced thought and care to safeguard building occupants. Until the adopted building code offers more specific guidance, glass designers and engineers must rely upon best-practices and industry standards to do their work. Determining the level of safety required for a particular application requires careful consideration of the risks associated with fracture. It is the responsibility of a glass designer to work with a project’s design team to communicate risks and discuss design decisions associated with these risks as they are made. A knowledgeable and experienced glass design professional will be able to facilitate a smooth and efficient design process. ■

About the Author

Ted Kraemer, PE, is an Associate at KPFF in Roseville, CA. His engineering specialization focuses on structural glass and building facade.
(ted.kraemer@kpff.com)