By Darrell Smith
Today’s “big box” commercial and general public use buildings generally use glazing to make-up a significant portion of the exterior structure. This can be both an attractive business proposition, as well as a concern, in today’s rapidly changing social and climate environment.
Glass exteriors can be designed and installed to achieve the optimum benefits so as to obtain the maximum return on investment. For example, retail and fast food buildings will often have large glass walls where the public is able to both see in and be drawn in to visit and make purchases. Newer office buildings can be clad in glass walls on all sides of the structure to allow in natural light and for design impact to attract tenants.
Even older, existing structures built before today’s energy codes came into effect may have a significant amount of glazing since many of the windows were designed to be operable to allow in fresh air to help cool a building. In the U.S., a majority of buildings are air conditioned. In various parts of Europe, operable windows are often favored over central HVAC systems. In the United Kingdom, a 2012 study stated about 2/3 of offices had some AC and less than 1/3 of retail space was air conditioned.
The Benefits Of Exterior Glazing
Glass lets in natural light, which research shows improves human health and mental well being. Daylight reduces the costs associated with operating artificial lighting systems, which may also have negative health impacts for some people sensitive to UV or infrared lighting. So natural lighting can save operating costs, improve worker productivity, and offer health benefits as well.
From a design standpoint, glass may help to make a building reflect its surroundings so it blends in better with its environment. The exterior of the building is also easier to maintain and clean, as glass is designed to stand up to rain and general weather conditions. Another key benefit of glass is that it does not weather or rust, so environmental effects may not impact it. Glass used as cladding, not structurally incorporated but as a skin for the building, may reduce the overall weight of a structure when compared to the use of masonry or metals.
The Downside Of Glass Cladding
Glass is a brittle material, highly vulnerable to small defects and these factors compromise its tensile properties. In some cases, glass failure can occur suddenly and spectacularly, without warning. For example, when glass is subjected to a long-term tensile stress, tiny micro-cracks in the surface tend to grow longer, which makes them more intense, so they grow faster. This phenomenon of slow crack growth is what can lead to glass breaking unexpectedly, when nothing seems to have happened.
As part of the evaluation of glass it’s important to determine the type of glass that is on the building and how it behaves when broken. Most windows should have a code etched into the glass that designates the type of glazing present.
Here are three common types of glass and their properties.
Annealed glass is made by a process of slowly cooling the glass piece to relieve internal stresses after it is formed. This occurs by passing the material through a system that controls its cooling at the desired rate. While annealed glass is the least expensive treated type, it is the lowest in strength and will break in large, sharp shards.
Heat-strengthened glass involves the re-heating of the newly formed glass and then cooling it. This ‘quenching’ treatment hardens the outer surfaces of the glass which are compressed and strengthened, making it about twice as resistant to breakage as untreated glass. When broken, however, it also turns into large sharp pieces that may cause serious human harm.
Tempered glass goes through a similar process as heat strengthened glass, but the re-heating and cooling occurs even more rapidly, resulting in a product that is four times more resistant to breakage than glass that is not tempered. When it breaks it breaks up into bead-sized, granular pieces.
Unprotected Glass Cladding
With all of the above in mind, a glass cladded building is vulnerable to a variety of environmental impacts, both man-made and natural occurrences. Depending on the type of glass used in the cladding, the outcome could be catastrophic in terms of human tragedy and economic loss.
Natural impacts include such things as windstorms, tornados, hail, earthquakes, fire and uneven sunlight. High winds drive objects such as tree branches, road signs, metal awnings and even vehicles into the air and they can smash against the glass facade of building which may result in the loss of a building’s integrity and additional exposure to the elements once the glass wall has been breached. Thermal stress may occur from the sun’s solar heat gain and when glass surfaces heat unevenly, and unwanted breakage or stress cracking may be the result.