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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.
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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.”
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Considerations
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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