A 50-Year Evolution of Structural Silicone Glazing
By Jordan Scott
In 1971, Apollo 14 landed on the moon, All in the Family debuted on CBS, Walt Disney World opened in Orlando, and The New York Times broke the news about the Pentagon Papers. It’s also the year that Smith, Hinchman and Grylls designed the world’s first four-sided silicone structural glazing (SSG) project. The Cass Building, also known as 455 West Fort Street in Detroit, was remodeled by the firm the following year for use as its new headquarters. Fifty years later, the façade is still performing as designed.
“We recently wrote a paper about the 50-year history, and so we got to go inspect the building,” says Jon Kimberlain, senior TS&D scientist at Dow, based in Midland, Mich. “What’s unique about that building is that it had these spider fittings that were supposed to give the appearance that something was holding the glass in place. Over the years, the little rubber fittings have loosened up, so you can even spin some of them. They’re not really doing anything. They probably held things in place as the glazing cured, but they were most likely there for aesthetics. It’s only the sealant holding that piece of glass in place.”
The 50-year old sealant has withheld heavy storms, precipitation and cold weather. Today’s products are even better. Modern sealants can support architects’ preference for large and heavy glass lites and also more practical needs such as resistance to bomb blasts and hurricanes.
“People are using more sophisticated engineering analysis to move beyond a big box with a roof. They can create unique shapes and manipulate the glass using cold bending. Designers are using sealants in ways that probably weren’t envisioned in the 1960s and 1970s when people first decided to try to glue glass together,” says Kimberlain.
50 Years and Counting
According to the Dow report, “50+ Years of Proven Silicone Performance,” the first one-part silicone construction sealants were released in the mid-1950s. Kimberlain explains that an early single-component silicone sealant was an acetoxy sealant, which is now sold in big-box stores to caulk bathtubs and kitchen counters. This sealant from Dow Corning was used in the 455 West Fort Street remodel that is still in service today.
“There’s nothing wrong with acetoxy. However, it’s a little bit of a stiffer connection. So, what people sought was a sealant with more flexibility and tear resistance,” says Kimberlain.
Seattle-based Green Facades principal and founder Richard Green explains that acetoxy cures had more material compatibility issues, particularly when paired with laminated glass.
“You’d tend to see things like delamination at the edges of laminated glass caused by structural silicone. You don’t see that anymore because now we’ve got the neutral cure chemistries which work very well, and the interlayers have improved also,” says Green.
A class of sealants called alkoxy was developed in the late 1970s. Alkoxy sealants are also single-component but have different elongation and tear-resistance than the acetoxy sealants.
The next sealant evolution occurred in the early to mid-1980s. Kimberlain recounts that two-part sealants were first used during that time. Two-part sealants sped up the curing time and shortened the supply chain for unitization, allowing a curtainwall assembly to be hung on a building after a 24-hour cure period. The two-part sealant was first used in a project in San Diego called Regents Square 1 (now La Jolla Square) in 1984. Kimberlain says the building has been through several earthquakes but is still in service nearly 40 years later.
Events such as Hurricane Andrew and the Oklahoma City bombing in the 1990s led the industry to create sealants with more tensile capabilities to further protect buildings from growing impact threats.
“People began to focus on protective glazing applications. Then, heading into the 2000s, you started to see the use of digital tools to create an enhanced understanding of material behavior,” says Kimberlain.
What glaziers once saw as a risky approach has since become a regular part of the façade toolkit, says Ryan Cogan, director of engineering and design for Harmon, headquartered in Bloomington, Minn.
“From an application standpoint, we have come a long way in understanding how to use it properly on a project,” he says. “The industry has moved from a place of caution and concern to SSG being an accepted part of our business over time.”
Graham Coult, technical director for London-based engineering design firm Eckersley O’Callaghan says as structural silicones have evolved, they’ve enabled the main load transfer system to work from one component to the next, which he says is different from what structural silicones were originally designed to do.
“We’re often using the structural silicone to bond glass panels to steel components, which means that we can transfer higher loads without drilling into the glass. That’s been fundamental to some of our projects,” says Coult.
He explains that when it comes to cold or warm bent glass, bending the glass into a curved position adds stress and drilling a hole into that glass magnifies it further. In those cases, Eckersley O’Callaghan has designed the transfer of load from the glass to a steel plate to provide a smoother transition. Other glues and adhesives are thin and can’t accommodate much of the fabrication tolerance, but structural silicone is thicker and is much better suited for that purpose.
Structural silicone also has enabled modern design aesthetics.
“The structural silicone is one of the primary components of the weather barrier. If structural silicone didn’t exist, we’d have a lot of limitations. We’d have to have metal on the outside of the building to retain the glass, which would have major thermal implications, limit what we could do from an aesthetic standpoint, and would probably limit the size of the glass we could accommodate,” says Cogan. “We can apply non-structural components because we’ve taken care of the main component using structural silicone, securing the glass to the building.”
Today’s Challenges
In the 1990s, when Green was early in his career, he conducted inspections of buildings with four-sided structural silicone. The buildings were up to ten years old, placing them early in the evolution of SSG. Green says at that time, four-sided SSG facades weren’t being built with insulating glass units (IGUs) but with monolithic glass, which meant he could visually inspect if the surface was wetted by silicone or if it was starting to debond.
Some jurisdictions even required periodic inspections, which are now almost unheard of, he adds.
Many of today’s modern constructions include IGUs. The spacer prevents glazing contractors and inspectors from being able to see the structural bite to the frame. However, Green says today’s compatibility testing programs are well organized, and it’s known what types of materials the structural silicone can bond to and which it can’t.
Green points out that with increasingly larger glass lites, the silicone bite is becoming deeper and more challenging to inspect. Despite that, it’s important to ensure that there is full fill of the glazing pocket.
“The quality control process usually means taking a knife and cutting it out, seeing if there are any air bubbles or voids and then telling the glazier to do it again,” says Green. “You either say, ‘Yes, it’s good enough and do it just the same way,’ or, ‘No, there was a problem and make improvemnets.’ Nobody likes undoing things. The fact that we’re working with IGUs where you can’t visually inspect the silicone bite through the glass means that we have to do destructive testing, and nobody likes it.”
This can lead to people wanting to do less testing as they get more confident, which Green points out could lead to something going wrong. He emphasizes the importance of maintaining quality through surface preparation and not letting up on the testing regimes. As the rated capacity of structural silicone moves beyond 20 psi, Green says it’s important to keep proper documentation on the strength rating to ensure the correct product
is used when a reglaze is needed.
“European manufacturers are starting to go to 30 psi rather than 20 psi for certain circumstances. It might seem like it’s only 10 psi more, but that’s a 50% increase. That can have a real, tangible effect on what frame width you need and the aesthetic of the building,” he says, adding that down the road, in the event of a reglaze, it could be dangerous for someone to see a smaller bite but use a standard silicone rather than the high-strength silicone intended.
A Second Life
While many existing buildings are starting to get façade updates either for aesthetic or performance reasons, Kimberlain hasn’t heard of any early SSG projects undergoing a retrofit rather than a full replacement. Green agrees that SSG facades have experienced less degradation over time and look fresher than other building stock from that time. Cogan says most of the related retrofit projects Harmon has worked on involved the complete removal of the curtainwall and early structural silicone since curtainwall technology has advanced tremendously. Kimberlain believes SSG could be important in updating older buildings, whether they include early structural silicone or not.
“SSG lends itself well to retrofits because we view it as a highly thermally insulating material because you’re isolating the glass away from the frame. It meets the definition of a thermal break for NFRC,” he says, adding that retrofitting a monolithic façade with SSG could have a tremendous impact on the energy performance of the building as well as its durability.
If a project team is approaching the remodel of an earlier SSG project, Kimberlain says a major consideration is the windload of the original design.
“If it was designed with a 40 psf windload, code now dictates it to be 60 psf. That means you probably have to have a bigger bite. You need to know whether that framing system will allow that. If it doesn’t, are there innovative ways you can add things to the frame to make that happen,” he says. “If you have a monolithic lite, are you going to try to adhere it back into a frame? Is that going to give you an aesthetic look as you’re adding that to the IGU or the sightline?”
New Horizons
While structural silicones have advanced over the past 50 years, Cogan says building codes don’t reflect today’s performance levels. “The structural calculations and allowances are based on very old silicone technology. I think we’re starting to make inroads there,” he says. “It’s not a bad thing to be cautious, but certainly, these products that we’re dealing with today are lightyears ahead of the products that the codes were written around from
a performance level.”
Dow plans to roll out a program that will allow higher design strengths to be used in SSG projects.
“We continue to use our proven sealants, but we’re going to allow higher design strength because, with enhanced digital tools, we know more about our materials and how they behave,” Kimberlain says.
Digital tools allow Dow to simplify the design process so that designers can continue to use the conventional design methods and methods of calculation established in the mid-1990s while still taking advantage of higher design strengths to open up the sightline.
Silicone analysis is also changing.
“We’re seeing people do some interesting things. Rather than using rectangular bites, they’re using trapezoidal bites and making it a little softer on the edges. The glass wants to dish in as it deflects over time, and we don’t want to cause stress concentrations of the curve shape by trying to fit it on a rectangular shape,” Green explains. “If you make it softer on the edges, that allows for a more uniform stress within the silicone which, in turn, allows us to be more efficient with the ways the silicone is designed in terms of its overall size and the way it supports the unit.”
Green also anticipates the transfer of low-density structural silicone from Europe to the U.S. This type of silicone has a low thermal transfer, so its ability to act as a thermal isolator is improved.
“I think thermal performance is one of the things that will drive toward smaller glass bites, because reduced contact to the glass means smaller thermal bridges between the framing and the glazing system,” he adds.
The architectural community is moving toward improved sustainability. Coult explains that architects, engineers and other stakeholders are starting to ask questions about contaminants in materials, how much carbon energy it takes to manufacture materials, where that energy comes from and how well the materials treat the environment.
“We’re trying to build an understanding of the total carbon intensity of our buildings and trying to benchmark that against the past and how well we can reduce that,” he says.
Considering what happens after a building’s service life ends is vital. Coult says more consideration needs to be given to how materials can be recovered and reused.
“With silicone, you get a tight bond and a good seal, but when we take the glass from the building in the future, we’ll want to recover the glass and put it back into the supply chain,” he says. “However, we’ve got all this sticky stuff, which bonds things well. How do we simply and quickly debond all the surfaces? They’ve designed the silicone so it sticks well together despite contaminants that may be present. How can we reverse that to return the glass to the supply chain?”
With sustainability in mind, Coult is seeing a growing desire to just replace the parts of the façade that need replacing rather than undergoing a complete façade change, which could impact design and manufacturing in the future.
Kimberlain expects to see an increased focus on 3D manufacturing as the industry explores how to manufacture glazing products with less waste. He says improving the ability to recycle glazing materials will be another major focus for manufacturers.
“We want things to last a long time, but we don’t want to make it so hard that you can’t tear a unit apart to its individual elements,” he says.
Jordan Scott is a contributing writer for USGlass magazine.
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