Volume 50, Issue 5 - May 2015

NewsAnalysis:COATINGS

On the Frits: Spandrel Breakage and Thermal Stress Become Focus of Debate

Spandrel glass breakage has made its way to the forefront of recent architectural glass industry discussions, and the question of whether ceramic frits weaken glass has been at the center of the debate.

Industry consultant Chris Barry says over the past six years, he’s become aware of more than 20 buildings that have had some sort of spandrel glass breakage.

Barry brought up the issue at ASTM, where a ballot was put out on a proposal to add ceramic frit to the types of glass excluded from ASTM E1300, “Standard Practice for Determining Load Resistance of Glass in Buildings.”

That has raised some debate among the industry, and according to Glass Association of North America (GANA) technical director Urmilla Sowell, the most recent ASTM E1300 meeting had 31 people in the room for this topic alone—more than usual. In that meeting, there was a motion to form a group to look into the issue.

“We need to figure out what kind of testing we need to do so we can get the real data and make a determination,” she says.

Barry notes that for some time, the German and European standards have stated that ceramic frit can reduce the load-bearing ability by 36-38 percent, though he has not been able to locate the root of the raw data used to determine that statistic. This is another reason some say more testing is required. He did, however, find a Journal of the American Ceramic Society article published in 2002, which concludes “Enameling was found to significantly degrade the strength of the float glass” based on testing at Pennsylvania State University.


The question of whether ceramic frits weaken glass and can cause breakage, such as this, has been a recent topic of industry discussion.


Barry gave a presentation in February at the Insulating Glass Manufacturers Alliance Winter Conference on the topic, citing results from recent testing done by Texas Tech University professor Dr. Scott Norville. Norville determined reduction in strength based on finite-difference method calculations of thermal loads, which were provided by Barry. In his presentation, Barry also gave recommendations that can keep the spandrel “more than strong enough,” including “simply using fully-tempered” glass.

“Ceramic frit products are used for a variety of applications and can exhibit a wide range of properties,” says Jeff Nixon, general manager of Glass Coatings & Concepts. “It is indeed possible that an improperly formulated frit would weaken a glass lite. For example, our company has frits that are meant for other glass products which, if inappropriately used for spandrel glass, could weaken such glass significantly.

“But that doesn’t mean that any and all frits would. Other factors also come into play, such as processing at the fabricator [level], design, film thickness, etc.”

Nixon says he thinks the “real issue” has been ignored.

“The root cause of the problem is that some glazing designs/installations are turning insulating glass units into highly efficient solar heat collectors. As a result, the inner lite can reach temperatures exceeding 250 degrees Fahrenheit. In such a scenario, a center-to-edge temperature gradient of more than 130 degrees Fahrenheit can occur. This will induce a tensile stress exceeding 6500 psi at the edge of the lite. Heat-strengthened glass under this tensile stress will be more susceptible to breakage regardless of whether it is fritted.”

Bill Lingnell of Lingnell Consulting Services addressed the GANA tempering division during its meetings in March and spoke about the topic of thermal stress conditions in heat-strengthened spandrel glass.

In looking at cases that have occurred in the field, Lingnell showed how thermal stress incidents can be different in different buildings and climate zones. Other considerations included the different characteristics of the glass, such as coatings, whether the glass is an insulting unit or monolithic, etc.

“There are design practices that can and do address thermal stress buildup and companies can avoid problems,” adds Nixon. “Sufficient space should be left between any insulation and the glazing unit. Ventilation can be incorporated into the spandrel backup construction. A little convection goes a long way to preventing extreme temperature gradients and the resulting thermal stresses. Framing systems can either exacerbate or mitigate temperature gradients. It is best to use those most suitable for a particular climate and elevation.”

The topic of ceramic frit and thermal stress, including the E1300 proposal, will continue to be discussed at an upcoming ASTM meeting in June.

—Nick St. Denis



USG
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