Holding It Together
Structural Sealant Glazing Proves Its Worth
Decades of successful use, plus ASTM International standards from Committee C24 on Building Seals and Sealants, have given structural sealant glazing its place as a system of choice for many construction applications.
In spite of more than 40 years of experience with structural sealant glazing systems for the exterior enclosure of buildings, there are still some in the construction industry who question its viability. In fact, SSG was recently referred to as “crazy glue” during a glazing system approval process for an installation on the West Coast.
But SSG has proved itself around the world, having been used for many building types from single family residences to multistory structures, many over 80 stories high. SSG is used for both new construction (see Figure 1) as well as the rehabilitation of existing buildings (see Figure 2).
What is structural sealant glazing?
Briefly, SSG is adhering glass, or other materials, to a framing system using a specifically formulated structural silicone sealant as both an adhesive and a sealant. No traditional mechanical fasteners such as screws, bolts or clamps are used to retain components such as glass and other materials to the framing system. These components are held to the framing system by the adhesion of the structural sealant; this adhesion is enabled by the sealant’s compatibility with various other accessory components of the system.
For a material to be considered a structural sealant it must meet the requirements of ASTM International standard C1184, Specification for Structural Silicone Sealants, which refers the user to ASTM C1135, Test Method for Determining Tensile Adhesion Properties of Structural Sealants. The latter document contains a rigorous series of tests that serve as part of the criteria for qualifying a structural sealant. At 56 pages, the most comprehensive document available that describes many of the aspects of SSG is ASTM C1401, Guide for Structural Sealant Glazing. All of these standards were developed and are maintained by ASTM Committee C24 on Building Seals and Sealants.
Some SSG and C24 History
Structural sealant glazing evolved with the silicone chemistry research of the 1930s, when industry and academic investigations were conducted into the possibility of developing nonmetallic materials with the properties of glass. Silicones were a logical avenue for investigation since they seemed to have the necessary potential properties. This research was followed by the development of silicone compounds during the 1940s, notably those that enabled the high altitude flying required for the aerial war effort during World War II. The first silicone adhesives were developed in the 1950s.
Figure 1 — Chrysler Corp. Headquarters Building, Auburn Hills, Mich., a Four-Side SSG System
With the continuing development of silicones as an adhesive, it wasn’t long before SSG was created in the mid-1960s, when PPG Industries marketed the all-glass Total Vision System, which used a silicone sealant to adhere vision glass to glass mullions. This innovation was interesting because none of the principal structural elements was metal. Subsequent to the TVS system’s successful implementation, continuing research resulted in the development of more and better silicone adhesive sealants that would change the glass and glazing industry.
In spite of being the new kid on the block — an unproven concept without a track record — the first multistory, four-side SSG application was constructed in 1971. The architectural engineering firm Smith, Hinchman & Grylls, now SmithGroup, was actively involved in ASTM Committee C24 and participated in early SSG standardization efforts. As a result of their early knowledge and faith in SSG they decided to rehabilitate a building, which they had originally designed in the early 1900s, and turn it into their Detroit headquarters office (see Figure 3). Two sides of the building were reclad with an aluminum framing system glazed with monolithic glass, which was retained on all four sides by a structural silicone sealant (see Figure 4). This building is the “granddaddy” of the industry, having performed successfully for more than 40 years. Because of the dearth of real-world experience at the time, many quality control procedures were instituted to ensure a successful installation. Many are still in use today, having been incorporated into the ASTM SSG standards.
Figure 2 — Des Moines Federal Office Building, Iowa
The use of SSG continued to grow, and during the early 1970s insulating glass with structural silicone edge seal systems was developed. These were structurally glazed to framing systems on two sides and then, in the mid-1970s, on four sides. Research during the 1980s enabled the use of other materials, such as certain types of stone and metal and composite panels that were structurally glazed to framing systems. During these decades, worldwide research and testing concerning adhesion, compatibility and accelerated weathering confirmed that the SSG concept would stand the test of time and Mother Nature’s environmental effects. Contributing to the viability of SSG was a growing number of buildings successfully employing the technology, including healthcare facilities, industrial complexes, condominiums and numerous office buildings around the world.
Figure 3 — The Former SmithGroup Headquarters Building, Detroit, Mich.
Adhesion and Compatibility
As mentioned, successful applications of SSG are all directly related to structural sealant adhesion and the sealant’s compatibility with other components of the glazing system.
An inherent chemical compatibility exists between a glass surface and a silicone sealant that results in very good adhesion. Through research and experience, structural sealant adhesion to other surfaces, such as anodized aluminum, painted metal and certain stones, was achieved through the development of different structural sealant formulations and, in some cases, the use of appropriate cleaners and primers. ASTM C794, Test Method for Adhesion-in-Peel of Elastomeric Joint Sealants, determines the adhesion characteristics of a structural sealant to a particular surface to verify that it is appropriate for SSG. Adhesion to any surface is predicated on proper and appropriate cleaning prior to a sealant’s application, which is achieved by the use of such uncontaminated cleaning materials as solvents and rags.
The compatibility of the various components of a glazing system that contact, or in some cases are in close proximity to, the structural sealant is as important as adhesion. Materials that can exude oils or other components can adversely affect the structural sealant and subsequently its adhesion. This has been borne out by laboratory testing on small scale samples and even during full-size mock-up testing. ASTM standard C1087, Test Method for Determining Compatibility of Liquid-Applied Sealants with Accessories Used in Structural Glazing Systems, focuses on evaluating accessories and has proved to be a very successful test method.
It should also be mentioned that any SSG system is only as good as the framing system it is used with and that framing system’s overall design concept and weatherproofing capability. Neither can be considered to the exclusion of the other.
SSG has been used with many types of cladding systems, from windows — both residential and commercial — to doors to entire building fenestration systems. Where all-glass facades are desired for aesthetic appeal, SSG is the system of choice. When energy efficiency is desired, SSG can also be the system of choice due to the way it isolates metal framing system components from the outdoor environment and reduces energy transfer. Overhead glazing for skylights and sloped glazing has also benefited from structural glazing by providing all-glass surfaces with no protuberances; this application has self-cleaning characteristics and picks up less dirt. SSG has also been found to be beneficial for cladding systems in areas prone to earthquakes where the system has demonstrated good resistance to seismic activity.
Figure 4 — Cladding System SSG Detail
Standards and Papers
ASTM Committee C24 pioneered the development of SSG standards, which today are used worldwide. The success of the committee’s efforts is directly related to the ASTM consensus method, which depends on the active input of all stakeholders in a particular industry. Committee C24 has benefited from the efforts of many scientists, architects, engineers, manufacturers, testing laboratories and installers of glazing systems all working together (sometimes rather loudly) but in the end developing very good consensus SSG standards.
The committee has developed nine standards that directly relate to SSG. A review of ASTM C1401, Guide for Structural Sealant Glazing, reveals a list of 59 ASTM International standards plus those from nine other organizations that in one way or another affect, influence or are influenced by SSG. Committee C24 has also issued a compilation of SSG information, ASTM Standards and Technical Articles Relating to Structural Glazing, published in 2007. The compilation is currently being updated with new and revised standards and technical papers that have been published since its debut. A quick literature search shows that there are currently many peer-reviewed technical papers and numerous magazine and other articles that have been written concerning various aspects of SSG.
In many cases, SSG is the system of choice for aesthetic and technical reasons when considering a glazing system for a building. It can provide a flush, all-glass facade with no exterior metal, which can be very clean and precise in appearance, and at the same time provide excellent thermal characteristics to control the building’s energy use. With the rapid advances occurring in the development of high performance insulating glass units, very high energy performance can be obtained for an all-glass system using SSG technology. There is the real potential of achieving a thermal resistance, or R-value, of at least 12, for SSG glazing systems in the very near future using vacuum insulating glass technology. This is a substantial improvement over a conventional 1-inch (2.54-cm) IGU, which can attain an R-value of 4.
Committee C24 continues to positively impact the industry by revising existing SSG standards as new research and real world experience provide new data to incorporate. This progress is evidenced by a revision under development to add the fatigue resistance of structural silicone sealants to ASTM C1184.
Structural sealant glazing has a 40-year proven track record and has evolved into a glazing system of choice for many applications. It is a worldwide recognized glazing system with an excellent track record. “Crazy glue” it is not, and it never was.
Thomas F. O’Connor, FAIA, FASTM, recently retired from SmithGroup where he was a vice president and director of the building technology studio. A past chairman of ASTM Committee C24 on Building Seals and Sealants, O’Connor is also a member of ASTM Committee E06 on Performance of Buildings, the editorial board of the ASTM Journal of Testing and Evaluation, the American Institute of Architects and AIA Michigan.