Volume 47, Issue 4 - April2012

Architects Guide to Glass
A special section of USGlass magazine

The Weakest Link?
Structural Silicone Glazing and Design Performance
by Ellen Rogers

Any building project is only as strong as its weakest link and, when it comes to structural silicone glazing (SSG), the strength of the adhesive that holds the glass in place—especially in a four-sided SSG—is paramount. The first four-sided SSG projects were built in the early 1970s and demand, interest and even acceptance of this type of construction have continued to evolve.

In an SSG system a structural silicone adhesive is used to attach the glass to the building’s frame. The resulting project can be two-sided, where only the vertical joints are structurally glazed with silicone, or four-sided, where both the vertical and the horizontal joints are structurally glazed with silicone. It’s four-sided SSG that offers the often sought-after uninterrupted glass wall appearance.

Today, such an aesthetic is in high demand. As a result architects, general contractors and contract glaziers must work in close collaboration to ensure a successful, sound and sealed building.

From the Start
According to Ned Kirschbaum, technical design director and principal with Fentress Architects in Denver, while there’s much to consider when beginning an SSG project, working with an experienced, knowledgeable contract glazier is critical.

“What’s most important is to write a specification that requires you have an experienced glazing contractor who is familiar with SSG as it is a highly specialized field,” says Kirschbaum. “Though it’s now reasonably common, your everyday contract glazier is still not likely experienced [enough] with all the preparations and procedures you need to assure the design performs correctly.”

For a best-case scenario in these projects, Kirschbaum says the contract glazier is providing, at least, design-assist services, allowing them to work collaboratively. “This way we’re able to understand their concerns and can collaborate on the design

Passing the Test
Whether field-installed or shop-fabricated, the quality control procedures surrounding the glazing sealants are critical to the success and performance of the building. Typically, field-construction employs a one-part sealant, while two-part sealants are mainly used in the shop, as they require mechanical mixing/pumping machinery. The Glass Association of North America’s Sealant Manual provides quality control checklists for both application types.

One-Part Sealant/Field-Applied Structural Glazing:
• Shop drawing review;
• Compatibility and adhesion testing for each substrate and glazing component(s) in contact with structural silicone;
• Field testing documentation, including date, type of testing, location of tests, recommendations and results; and
• Recording sealant batch/manufacturer’s control numbers and where applied to what part of building for both sealant and primer.

Two-Part Sealant/Shop-Applied Structural Glazing:
• Shop drawing review;
• Compatibility and adhesion testing for each substrate and glazing component(s) in contact with structural silicone;
• Testing documentation, including date, type of testing, location of tests, and results; and
• Recording sealant batch/manufacturer’s control numbers and where applied to what part of building for both sealant and primer.

Specific to Two-Part Sealants:
• Butterfly tests, on pump start up. Here, the mixed sealant is gunned on to a sheet of paper, the paper is folded and compressed, spreading the sealant inside the paper, and then reopened. The sealant is observed for any streaking in the sealants, as the two parts are typically two different colors, and any streaking indicates the material has not been properly mixed. If homogenous in color, the sealant is mixed correctly, and application can proceed.
• Tracking mix ratios: on pump start up of sealant application, mix ratios are documented, along with batch numbers of sealant base and catalyst, temperature, and humidity.
• Snap time tests: usually at pump start up on a daily basis, this test relates to mix ratio, and indicates how the sealant will cure based on temperature, humidity, and mix ratios.
• Pump snake tests: cylinder seal wears in the pump can allow improper mix ratios, and soft spots may occur in the sealant bead. Pump technicians are consulted to make necessary repairs, and sealant application should not proceed.
Source: GANA Sealant Manual, 2008 Edition

together,” says Kirschbaum. “I always encourage open dialog. There is a lot of knowledge in the glazing industry we can call on and have great rapport when we have the glazier on board.”

Jeff Benson, vice president of project management at Haley-Greer in Dallas, says that depending on the project, his company is very involved with architects up front and says there have even been instances where his company was awarded the job before the general contractor.

“It’s not unusual for us to get a call from the architect to talk about the design work and then [the results of that conversation] are reflected in the drawings,” says Benson. “From our experience, many architects express concerns regarding compatibility of various substrates and proper documentation of shop and field testing to ensure quality control throughout the project.” And, when reviewing architectural drawings, he says they always look for proper water-proofing details as that’s an important beginning of a successful project.

George Chrisman, vice president with Walters & Wolf in Fremont, Calif., points out that SSG is really about choosing the right system and going through the necessary testing and regulatory requirements. He says his company also spends a great deal of time working and communicating with architects about these projects.

“The biggest myth is that structural glazing is always more expensive than a conventional curtainwall system. This can be true if it’s field-applied, but not if it’s unitized,” says Chrisman. “Field-applied systems are a lot more costly and that’s where the premium comes in.”

ccording to Chrisman an SSG project is tougher to do field-glazed than in-shop. It’s his company’s policy to avoid four-sided SSG in the field and recommend a unitized system/shop glazing.

According to the Glass Association of North America’s Sealant Manual, 2008 Edition, under shop conditions, the application of structural silicone is much easier to control compared to in the field. As the manual points out, “temperature ranges, dry conditions, and absence of field dust dirt are typically more consistent, and do not vary in the more controlled environmental conditions of the assembly/glazing facility as they do in the field from hour to hour or day to day.”

Taking Control
Quality control is also important. Benson says at his company, for example, they monitor the process by hiring a consultant and/or relying on the sealant manufacturer to provide reviews and feedback.

“There are also several tests we do when shop glazing. These tests include, for example, the butterfly test and snap test, both of which are done in the shop-glazing process. In the field we conduct field adhesion testing, which consists of a series of pull tests,” he says. “We will invite our sealant manufacturer to witness our field testing and inspect the installation of the joint sealants on every project as this provides another level of quality assurance for our clients.” (See information in above box for a closer look at testing.)

Kirschbaum says his company requires compatibility, adhesion and stain testing for all of its projects. Likewise, the firm also requires the silicone itself tested to make sure it complies with ASTM C 1401, Standard Guide for Structural Sealant Glazing.

Where’s the Gloom and Doom Now?

When structural silicone glazing systems came onto the scene in the 1970s and 1980s there was a lot of doubt as to whether such a construction project could stand the test of time.

According to Jeff Benson, vice president of project management with Haley-Greer, while the early- to mid-1970s brought the introduction of silicone sealants to the industry by the 1980s there was much skepticism about the capabilities of silicone being used in a structural silicone application.

“The continued use of silicone sealants in the 1980s paved the way that launched many of the next-generation glazing systems that we see engineered and installed in building envelops today,” says Benson. “Since that time the industry has moved away from butyl and urethane sealants and has exclusively relied on the technology of silicones, not only in a structural application, but as a weather seal, too. Because of this, new high-performance glazing systems such as unitized and four-side structural silicone curtainwalls have become a common architectural element for the building envelope.”

Ned S. Kirschbaum, technical design director and principal with Fentress Architects, agrees there is always skepticism by some about any new technology that has not stood the test of time.

“Professionally, we like to be on the leading edge but not the bleeding edge of technology,” says Kirschbaum. “There was and still is an ‘old school’ wisdom that says all materials should be mechanically fastened for reliability; adhesives of whatever kind might (and probably will) fail over time. One major failure can put a company out of business. Consequently, the building industry is conservative and slow to adopt change.”

He points out, though, that in the case of structural silicone, success can be attributed to extensive research, testing, and support of the product and its application by large, sophisticated material manufacturers and glazing contractors.

“The successful use of all products is highly dependent on workmanship,” says Kirschbaum. “Perhaps the high level of concern about the product encouraged a higher than normal level of quality control measures, which has helped ensure success.”

Benson points out with adhesives there are a number of considerations for which they must account and that makes compatibility testing critical. He says it is important to consider the other water-proofing materials that are being supplied as it’s not unusual to have compatibility issues with those other materials if proper review of the water-proofing details is overlooked during the planning stage.

“There are times we may not have total control of all the water-proof detailing on a project,” says Benson. “Those times we have to make sure our sealants will be compatible with those other materials; if it’s not we may sometimes have to use a different product so that we can ensure all the substrates will properly adhere and be compatible with each other.”

Depending on where the project is located, there may also be geographic considerations. For instance, Chrisman says in California seismic issues must be taken into account from the start.

“You have to check the seismic racking and make sure the sealants are holding up as they should,” he says. “If it’s four-sided SSG then the next question [an architect may have] concerns how much building movement the system can accommodate. You have to design for that movement.”

Problems Afterwards
While those interviewed for this article say their companies have been fortunate enough to not have been involved on any jobs that experienced problems or failures after installation was completed, the fact is sealant failures can and do happen. So, ow would such a case be handled?

“That’s when our company will help facilitate a meeting with the general contractor and all parties involved so that a solution will be worked out. It’s so important that all the upfront work is done correctly to avoid such issues,” says Benson. “It is imperative to have the correct waterproofing details on the drawings that are right for the project.” He says in some cases they will even hire a water-proofing consultant to review and comment on the drawings.

Then and Now
SSG construction has seen a lot of changes over the past 40 years. Kirschbaum says his firm was involved with one of the first SSG projects in the Denver area, 116 Inverness, built in the early 1980s.

“At the time this was still rare and there was skepticism over the longevity of the adhesives’ capability and it took a great deal of convincing the client to go with this look,” he says. “But its acceptance within the industry as a reliable glazing method has since grown.”

Kirschbaum says aside from the aesthetics, SSG construction also provides a number of benefits. These include an enhanced overall U-value given there is no exterior metal exposed to the environment, as well as increasing focus on use in protective glazing.

“There has been a big focus on blast resistance and keeping the glazing in the unit is almost always achieved with SSG,” says Kirschbaum.

Chrisman says he, too, has seen the evolution of SSG.

“I think unitized systems have become more cost effective so that’s leading to the use of more SSG,” he says. “You can use it on smaller jobs now because you have the trade-off of using the less expensive shop labor as the unitized systems are more cost effective.”

For Benson, much of this evolution ties into quality control and assurance.

“Early on [the industry] was perhaps not as sophisticated as it is today. The people in our industry are more technical and our glazing equipment is more advanced, not to mention we all have learned the value of good practices and implementing good procedures both in the shop and field. There are tremendous liabilities if a four-sided SSG is not built and installed correctly.”

He continues, “The large building envelope contractors have become better educated and intelligent about unitized and four-sided SSG systems. We have to continue taking ground in technology and quality to ensure our industry installs systems that will perform the expected life of the building.”

Ellen Rogers is the editor of the Architects’ Guide to Glass & Metal magazine, a sister publication of USGlass magazine. She can be reached at erogers@glass.com or follow her on Twitter @AGGmagazine and like AGG magazine on Facebook to receive updates.

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