Development of a General Procedure to Evaluate the Probability of Breakage for Glass Plates in Insulating Glass Units Due to Thermal Stresses Induced by Solar Irradiance

Abstract

Over the past several years, there has been a significant increase in the number of glass plate breakages caused by thermally induced stress due to exposure to solar irradiance. Glass plate breakage that is caused by thermal stress has become one of the leading issues that the architectural glass industry faces. The increase in glass plate breakage is coupled with the expanded use of insulating glass (IG) units and the ever increasing demand for more energy-efficient windows. More often than not, the glass plates that break are part of windows that incorporate IG units.

This issue is nothing new to the architectural glass industry and an ASTM International (ASTM) standard practice has been developed to evaluate the probability of breakage (POB) for windows that incorporate monolithic glass plates that are subjected to solar irradiance. However, there is no similar standard practice to evaluate IG units subjected to solar irradiance. A current goal in the architectural glass industry is to develop a similar standard practice for IG units. The primary objective of the research presented herein was to develop procedures that can be used to evaluate the POB of glass plates in IG units that are subjected to solar irradiance. It is anticipated that these newly developed procedures will serve as the basis for the development of a new standard practice.

The research herein shows that the non-linear behavior of the heat exchange process that occurs between the inner and outer glass plates, through the gas space cavity, of an IG unit can be reasonably estimated using a single, linear combined energy exchange coefficient (CEEC). This greatly simplifies the thermal analysis procedure that is required to analyze IG units by removing the need for iteration. A formal test procedure (FTP) was developed herein that can be used to determine the CEEC. The FTP couples straightforward physical experiments with a parameter identification optimization procedure (PIOP) that uses finite element (FE) analyses to determine the optimum CEEC.

The CEEC was then coupled with a formal design procedure (FDP) that was developed to evaluate the POB for a glass plate in a specific IG unit that is subjected to a specific set of environmental conditions. The FDP is the most accurate method to evaluate IG units and requires building a detailed FE model of the insulating glass unit, measuring the CEEC for the gas space cavity, and determining the allowable stress for a given POB using the glass edge strength failure prediction model (ESFPM).

Finally, a simplified design procedure (SDP) was developed that can be used to evaluate the POB for glass plates in generic IG units for a general range of typical environmental conditions. This SDP presents the framework that is needed to develop general procedures that can be incorporated into a design standard, practice, or code and provides a tool for the design of IG units that are subjected to thermal load conditions.