Facts About Thermal Stress

Glass is a wonderful building material! It is beautiful and enduring. It keeps the elements out, but lets the light in. And it gives building occupants a link to the outdoor environment that has proven psychological benefits. Unfortunately, glass also breaksThere are a number of common reasons that lead to glass breakage: excessive wind and/or snow loads, impact, building or glazing system movement, the dreaded inclusion caused “spontaneous breakage,” and thermal stress. In more than 40 years in the glass business I have never personally seen a lite of glass that broke due to excessive wind or snow loads. I know it can happen and I’m sure it has happened, but I have never personally seen it. Even in hurricanes and tornados, the vast majority of glass breakage is due to impact by flying debris, not by the force of the wind itself. I have seen glass breakage caused by the glazing system or building movement; and, I have seen breakage due to inclusions. However, the most consistent and frequent source of glass breakage that I have dealt with in my long glass life has been due to thermal stress. And, amazingly, thermal stress breakage is the one type that is most preventable.

Thermal stress in glass (or any material for that matter) is caused by temperature gradients. The center of the glass is heated by the sun and expands, while the edges that are captured in the glazing system remain relatively cold and do not expand as much as the center. The result is stress, typically at or near the edge of the glass, which happens to be the weakest part of a glass lite. The absolute temperatures are not the issue; it is the gradient – the difference between the edge and the center of the glass – that causes the stress that leads to breakage. A general “rule of thumb” is that a gradient of 40° F is sufficient to result in a probability of glass breakage of 8 lites per thousand. With the popular high performance tinted and coated glass products in use today, temperature gradients of 60° or more are easily possible.

While the sun and the solar absorption of the glass are the main drivers creating the thermal stress, there are a number of design factors that exacerbate the situation, including:

  •  Outdoor shading caused by the glazing system, overhangs, fins, adjacent buildings, etc.;
  • Interior shading devices such as drapes and Venetian blinds;
  • Interior structural glazing pockets;
  •  Heating registers;
  • Type of glazing system, i.e., thin, thick, massive;
  • Color of the glazing stops; and
  • Minimum outdoor design temperature

Some design conditions, such as solar intensity and design temperature cannot be controlled; others, such as the glazing system and overhangs are controllable. It is also important to anticipate design elements that can and very often will change during the life of the building. The most common change that occurs is in the type of interior shades or, most critical, the addition of interior shades where none previously were installed. When a building has been happily performing without thermal stress breakage suddenly begins to experience such breakage, it is very often caused by a change in or the addition of interior shades.

Procedures are available for performing thermal stress analysis for glass products and predicting the probability of glass breakage. The procedures require that someone knowledgeable about the design of the building and glazing system furnish the appropriate design conditions. Remember the old adage: “garbage in – garbage out.” If the designer says there will be no outdoor shading, and there is; or that there will be no interior shades, and there are, the end result may very well be thermal stress breakage when the thermal stress analysis says it should not occur.

More to follow on this important subject.