Thermal variance
Another potential underlying factor contributing to breakage is thermal variance, which could be caused by interior and exterior shading. If the temperature difference across a lite of glass is great enough, the accompanying stresses can reach breakage-causing levels. This is particularly true if the nucleation point of the crack was caused by a damaged edge or an inclusion. In this case, the break origin is often located near the edge of the glass.
The characteristics of tinted and spandrel glasses (higher heat absorption) and glass glazed into large concrete or steel structures (edges stay cooler longer) may contribute to the intensity of thermal-variance stresses in these applications.
Additionally, temperature variations could enable an IGU to partially collapse or, in other words, the glass plies bend inward. Since an IGU is sealed around the edges, as the air inside the unit is cooled, it condenses, reducing the pressure possibly allowing the glass plies to become closer together. This IGU glass collapsing may produce conditions with potential impact on thermal-variance stresses. For instance, with a collapsed IGU, there is often contact between the glass and muntin bars inside the window or between the two pieces of glass. This can create an alternative path for heat conduction, thereby establishing localized temperature gradients. The combination of contact, surface damage, and localized gradients can greatly increase the likelihood of breakage.
Pressure differentials
When the assembly and installation of an IGU occurs in two different places, a variation of altitude—even as little as 1524 m (5000 ft)—that changes atmospheric pressure, can jeopardize the unit’s integrity, causing the seal to rapidly fail and potentially lead to breakage.
Studies show altitude changes of only 305 m (1000 ft) accompanied by large temperature differences between the installation and assembly sites can cause IGU failures after extended periods of time. This is exacerbated at higher temperatures—when the temperature differential exceeds 21 C (70 F), as each 11 C (20 F) differential increase is equivalent to an altitude increase of 305 m.
Glazing contractors can offset this risk by installing capillary tubes to regulate pressure changes in IGUs. The tubes must be properly terminated at the final location by crimping or sealing to prevent moisture ingression. Since capillary tubes allow for gases to enter and exit the IGU, gas filling must be done at the final installation site to maintain pressure equilibrium. Care should be taken if the IGU includes a magnetron sputter vapor deposition (MSVD) low-emissivity (low-e) coating because moisture ingress during transportation and storage when the tube is open can be detrimental to the coating.
It is important to research framing systems and IGU spacers when altitude and temperature changes are factors.
Conclusion
In its pristine state, glass, as a prominent element in architectural design, can define contemporary buildings. However, sometimes glass breaks for no obvious reason. Whether it is a one-off or part of a continuing pattern of incidents, glass breakage is inconvenient, potentially dangerous and costly for building owners, construction professionals, and other stakeholders.
Although architectural glass and fenestration manufacturers continue to develop advanced technologies to make their products as ‘break-proof’ as possible, absolute perfection is simply impossible due to flaws inherent in the glass-manufacturing process and the wide range of in-service environments in which the material must ultimately perform.
Conducting ‘postmortems’ on glass breaks can help investigators identify the general reasons for each incident, including the type of failure causing the break, and the potential source of damage.
By using the techniques outlined in this article, building owners, contractors, engineers, glass manufacturers, and other stakeholders may be able to accurately identify the likely origin of failures. Doing so would have the advantage of placing more emphasis on the opportunity to use the information in design. A really good outcome of this article would be for the design team to learn to prevent conditions increasing the risk of breakage.
Steven D. Marino is manager of technical services for Vitro Architectural Glass (formerly PPG Flat Glass). Marino has more than 30 years of experience in automotive and architectural glass. He has held positions in manufacturing, quality, and other technical roles. He holds a bachelor of engineering degree in mechanical engineering from Youngstown State (Ohio) University. Marino is an active participating member of the Insulating Glass Manufacturers Alliance (IGMA) and National Glass Association (NGA)/Glass Association of North America (GANA). He also sits on multiple committees and task groups for both organizations. Marino can be reached at smarino@vitro.com.
