IGMA: June 2013
By Margaret Webb
Understanding thermal stress
By Margaret Webb
The U.S. Department of Energy is looking to develop a rating program for such fenestration product attachments as awnings.
The U.S. Department of Energy is looking to develop a rating program for such fenestration product attachments as awnings. It is extremely likely that both the U.S. and Canadian Energy Star programs will adopt these ratings once completed. One of the potential challenges for the fenestration industry will be to manufacture windows that can withstand temperature differentials across the insulating glass unit, which can lead to an increase in thermal stress breakages. The majority of thermal stress breaks occur in winter when the exterior versus interior temperature differentials are the greatest.
One of the common factors that can lead to breakage of annealed glass used in insulating glass units is thermal stress. Breakage due to thermal stress involves the occurrence of a thermal gradient induced by uneven heating of glass by either solar irradiance or other heat sources. When sunlight impinges on a glass plate, some energy is reflected from the surface of the glass, the glass absorbs some energy, and some energy is transmitted through the glass. Energy absorbed by the glass increases the temperature of the glass above the previously existing equilibrium condition. When glass is uniformly heated and if the support system can accommodate the thermally induced expansion of glass, no major stresses will be induced by the uniform temperature increase. However, the part of the glass surface that is shielded from direct sunlight by the edge supporting system or by shadow conditions will be heated unevenly. The uneven heating of glass will give rise to thermally induced in-plane tensile and compressive stresses. When the thermally induced tensile stresses become high enough and interact with certain edge conditions, a thermal breakage can result.
The IGMA Technical Services Committee has developed a bulletin, TB-1300-13, Guidelines to Reduce Instances of Thermal Stress. This Technical Bulletin provides guidelines relating to thermal stress considerations for window glass products used in residential and commercial building envelope projects. Its purpose is to give the user specific guidelines and design assistance toward avoiding glass problems that arise from breakage caused by thermal stress conditions. The Technical Bulletin is intended to consider, but not be limited to, conditions known to be important to thermal stress in glass. It also includes a knowledge base of reference materials, available industry information, do’s and don’ts guidelines for thermal stress, and practical design considerations for the review and analysis of thermal stress.
This bulletin addresses the following conditions as a single design element. Users of the document must be aware that it is rare that only one of these conditions exists as a single item: altitude, building design, design winter conditions, elevation (orientation), glass edge conditions, post installation films, risk conditions associated with framing considerations, glass kind and type, heat traps (indoor conditions), number of glazing lites, solar radiation, spandrel and on-site storage conditions.
Once you starting combining conditions, the potential can increase exponentially. IGMA recommends that each manufacturer work diligently with their glass supplier. Most glass suppliers provide a software tool to calculate thermal stress and when to fabricate with heat-strengthened, fully tempered or even laminated glass to reduce possible consequences of broken glass.
ASTM 2431-12, Standard Practice for Determining the Resistance of Single Glazed Annealed Architectural Flat Glass to Thermal Loadings is available for the industry however it only addresses monolithic glass. IGMA is providing the majority of funding for the development of standard to determine thermal stress for insulating glass units.