Architectural Glass Breakage
Glass is potentially very strong; however, in sheet form the strength is reduced by the presence of invisibly small imperfections, known as Griffith cracks, which cause stress concentrations allowing cracks to propagate. Strength may be reduced further by larger visible imperfections. Most glass breakage is caused by one or more of the following conditions:
- Surface or edge damage
- Deep scratches or gouges
- Severe weld splatter
- Missile/windborne debris impact
- Glass to metal contact
- Wind/thermal loading
Generally, thermal loads on glass occur as a result of the glass being exposed to sunlight and/or interior heating. If the glass is heated nonuniformly, temperature gradients occur within the glass, creating tensile stresses. The amount of tensile stress is a function of the extent of temperature differences within the glass. Thermal breakage occurs when the tensile stresses exceed the glass edge strength.
Reducing the Risk of Thermal Breakage
Glass is vulnerable to thermal breakage under several circumstances. One common example occurs when glass is partly shaded by building overhangs or extensions. In this situation, heat causes the center of the glass to expand, while the edges remain cool, which can result in stress and thermal breakage.The other situation occurs when glass is installed before heat is turned on in the building. Here again, edges will remain cool in the frames, while the center of the glass may be heated by the sun. The resulting temperature difference between center and edge can cause breakage. Generally speaking, the greater the area of the edge, the higher the risk of thermal breakage. But other factors can also come into play, both during construction and after the building is occupied.
- Putting the glass frame in direct contact with concrete or other materials that may increase cooling of the edge
- Excessive coverage of the edge by the frame
- Installing glass in an unheated building
- Attaching heat-absorbing films after the glass is installed
After the building is occupied:
- Curtains, shades or blinds that are placed too close to the glass. Heated or cooled air trapped too near the glass can cause thermal stress. Air must have sufficient space to circulate
- Airflow from heating or cooling vents that is not directed away from the glass
How can you reduce the risk of breakage?
Ask your Guardian Architectural Sales Manager or Guardian's Technical Advisory Center for a computer-modeled estimate of potential thermal stresses when you’re selecting glass for your project.