The June article expresses an outline of the many elements that contribute to the condition of thermal stresses that occur in insulating glass units. Here, I will highlight some of the conditions that contribute to thermal stress when shadows occur on the glass in a commercial or residential building. The IGMA bulletin that is intended to be released this fall after the final review by the IGMA technical policy committee and board of directors has addressed this item in the Building Design section of the bulletin.
The architect or building designer will create a building façade or residence with specific architectural features that include overhangs, extended vertical and horizontal framing members, punched openings, special shade devices and other items that will cast shadows on the glass at certain times of day on certain elevations of the building. These must be considered in the thermal stress review. One of the most common examples of thermal stress breakage can occur
from conditions arising from a clear, cold winter night when the glass cools to an ambient condition overnight and then is exposed to solar exposure in the morning with shading conditions that allow portions of the glass to stay cold while other portions of the glass are being heated by the sun. The heated areas of the glass tend to expand, being forced into a state of compression, while the cool zones that are encapsulated by the framing system and/or shaded are forced into a state of tension.
The phenomenon resulting from the heated area entering a state of compression and the cooler areas maintaining a tension state can lead to a thermal stress condition that is in excess of the strength of the glass. This is especially important for annealed glass and will be dependent on the solar properties of the glass with regard to the solar absorbance, reflectance, and transmittance. The edge tension in combination with inherent edge flaws, damage and cutting conditions may create a situation where the glass fracture is initiated at the edge of the glass. Edge flaws and conditions are found to be more predominant for thermal stress breakage than surface conditions. Surface conditions resulting from scratches and surface damage in critical stress regions have been found to cause fractures, but this is not as frequent as the edge condition.
Experience and examination of fracture origins has verified that the conditions described can and will occur from thermally induced glass fracture and can occur in summer or winter conditions. This is because the thermal gradient between the heated area of the glass and cooler edges is the critical condition to evaluate. The shadowing of the glass enhances the thermal stress issue and certain types of shadows have more of an impact than others. The shadows that cast an angular shadow or a V shape on the glass have been demonstrated to develop larger thermal stresses that linear shadows.
Bill Lingnell has over 46 years of experience in the technical field of glass and architectural products. He holds three Masters of Science degrees in engineering: civil, mechanical and engineering science. Lingnell is the technical consultant for the Insulating Glass Manufacturers Alliance.