Dos and Don’ts for Structural Glass

By Rebecca J. Barnabi

Designing is all about timing, and the timing is perfect for structural glass. These applications most commonly involve projects in which the glass itself acts as the support structure, and the use of structural glass has increased in popularity, as it offers a clean, crisp, transparent aesthetic.

“This is a great time to start making the public aware that the document is processing,” says Richard Green. Green is the technical chair for the American Society of Testing Materials International (ASTM) work group on Structural Use of Glass in Buildings, and the document to which he refers is a new standard for structural glass.

Developing New Guidance

At press time, ASTM was in the process of balloting a new standard for structural glazing that was 10 years in the making, according to Green, a façade specialist, who has worked in design and facades for 30 years, written glass codes in Australia since 2003 and been part of ASTM since 2009.

“There have been little bits of it here and there, but this is the first time it’s been aggregated in a single document,” he says of the standard now being balloted. “I always thought we’d get there. The question was how long till we got there and what would be in it.”

And the ballot process is only the beginning of the adoption process which will include a public comment period before final establishment of the standard.

The fact that these principles are being considered, Green says, is a step in the right direction toward providing guidance to architects and engineers interested in structural glass.

The standard will set broad design principles, get everyone in the industry on the same page, and, hopefully, give architects something official to reference with building departments and project committees when pitching a design concept. “They will be able to say, ‘I’m following a standard.’”

“It makes their designs defensible,” says Green.

He points out, however, that some architects and engineers do not understand structural glass and are hesitant to utilize the design concept.

“At the moment, [structural] glass design is specialized [and done] by very few people,” Green says.

ASTM International’s standard will provide guidance for architects and engineers to take risks with designs that they previously would not have understood were possible.

“It will enable them, it will give them the equations … to design more confidently,” Green says. It will provide a higher level of understanding as no design professionals “want to take responsibility for things they do not understand.”

Understanding Structural Glass

Architects have two misconceptions about structural glass: either the glass breaks easily or they apply a “heroic” quality to the glass “that you can do anything with it.” Green says that if glass is treated properly, it can be strong, but detailing is important in design, especially having a failure mechanism properly in place if the glass breaks.

“Glass is unusual. Damage and breakage rarely happens” with overload. What usually happens when glass is impaired is that a chip was in the glass during installation or something was dropped onto the glass which affected its strength after installation. Green says that glass should be detailed so that if it is chipped or anything abnormal happens, risk will not be a factor.

He says the standard will recommend the overall failure for overhead glazing to be 1:1000 instead of 8:1000, which is the recommendation for windows. The International Building Code sets requirements that if overhead glass falls it will not break.

Standards vary depending on where the glass is and how it is used, or what Green refers to as “circumstance and consequence.”

The ASTM standard will be available for use by architects, designers, specifiers and others in the United States. Green says that the European standards organization is developing its own guide for structural glass.

According to Green, there have been several advancements in structural glass applications, including a change in the quality of interlayers and the size of manufacturing equipment. Interlayers have advanced so they have the ability to prevent a crack in the second lite of glass in the event that the first lite is cracked. The structural glass will then “continue to do useful work even in a broken state.”

“And the ambition of architects, owners and engineers continues to advance the field of glass engineering,” he says. Research continues in the strength and application of glass.

Green says architects should take into consideration the size, position of glass in a structure, and the possible occupancy near the glass which “could force you to design the glass differently. So glass is unique in that respect.”

Risk Assessment

Glass has four risk categories, according to Green, which will be included in ASTM’s standard. The first category is glass that’s installed in areas near few people. The second category is glass installed in locations not intended for large crowds of people. In the third category, lots of people will be around, so it is important that the glass cannot fall. The glass does not need to function, but cannot fall on people walking by or standing nearby. The fourth category is for glass, such as hurricane-resistant glass, that must perform even if it breaks.

Architects and engineers design glass depending on how, where and when it will be used, they must consider retention and redundancy in design. Retention is the ability of glass to stay in place and not partially or totally exit its design space, if an event happens such as a breakage or impact. Redundancy refers to the capacity of the glass to take a load after an event has happened. The question with redundancy becomes if the glass breaks, does it stay in place and can it support the design load?

To review the standard, visit

https://www.astm.org/DATABASE.CART/WORKITEMS/WK37764.htm.

Rebecca Barnabi is a contributing writer for the Architects Guide to Glass & Metal. She can be reached at rbarnabi@glass.com.

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