Structural Silicone Glazing

Structural silicone glazing—sometimes called flush glazing, stopless glazing, or just structural glazing—is a method of fixing glass to its supporting framework by an adhesive. This fixing method results in a smooth, uninterrupted surface of glass, which significantly improves the appearance of glass walls.

Successful application of this concept requires combining a highly engineered material with a highly engineered application. Buildings whose entire façade is glued in place demonstrate the success of the concept.

The concept of structural glazing began with the idea of using the sealant as structural glue and also letting the same bead serve as the weathering sealant. Structural glazing was introduced only a few years after silicone sealants had entered the market. Even in the early 1960s, silicone producers had already generated data showing little change in silicone sealants after accelerated weathering devices.

SOME BRIEF HISTORYOF STRUCTURAL GLAZING

• Glass-to-glass structural seals—PPG TVS system (1965)
• Two-sided structural support of monolithic glass (1970)
• Four-sided structural support of monolithic glass (1971)
• Two-sided structural support of insulating glass (1976)
• Four-sided structural support of insulating glass (1978)

In the 1960's, silicones began to be used on storefronts, on ground-level windows held mechanically at the top and bottom, with silicone glued to some support along the vertical edges. Engineers, architects, and the glass companies studied these installations along with the sealant manufacturers. In 1971, an imaginative architect, Chuck Parise of Smith, Hinchman & Grylls (SH&G), decided to expand the structural glazing concept to multi-floor buildings by gluing all four sides. There would be no external support for the glass, if testing proved the concept viable. This is the first known structural sealant glazing system to incorporate four-sided structural silicone. Gradually, use of the system began to grow, until by the mid-1980s, literally thousands of buildings were structurally glazed each year. The four-story experiment led to more daring structures, such as 50 stories in Hong Kong with four-sided structural glazing to withstand 120 pounds per square foot of wind load.

The first true high-rise building to use this new and innovating design concept of gluing the glass panels in place was the 21-story Barnett Bank building in Miami, Florida. Due to its startling all-glass appearance, with no metal visible, the concept immediately caught the attention of the architectural community.

Before 1980, structural silicone glazing had been confined to low-rise structures and single glazing. A mechanical fixing backup was usually supplied for added security. The backup was usually a conventional glazing on two sides of each lite of glass, i.e., using exterior metal stops to retain the glass along the top and bottom sides. Alternatively, metal patch plates were fixed at the corners of each lite.

Many people, however, were concerned about the safety aspects of the method of structural glazing. The industry responded with improved materials, improved quality control procedures, and improved designs.

There are three aspects to be considered in structural glazing:

1. What are the forces exerted on the adhesive (silicone) sealant?
2. Does data show that the sealant can withstand wind stresses for an extended period of time?
3. How is structural glazing done, from the point of view of the material supplier as well as from the applicator’s point of view?

The silicone sealant materials suitable for structural adhesives are presently supplied by four major manufactures in North America. These manufacturers take an active educational role by holding seminars throughout the country. Technical representatives outline the essential requirements. One of those requirements is using all materials on a specific project tested for adhesion and compatibility. Manufactures undertake these tests free of charge to the glazing contractor or framing manufacturer.

The materials manufacturers have also introduced new products displaying better characteristics for structural sealant glazing. The sealant material originally used was a high modulus acetoxy cure silicone developed in the late 1950s. This material exhibited excellent adhesion to glass with good cohesive strength. However, its adhesion to other surfaces was problematic and required additional adhesion promoters. The market responded with new neutral cure silicones that have improved adhesive characteristics.

Nevertheless, while adhesion reliability dictates the success of structural sealant glazing, this critical adhesion can be affected in both the short and long term by the type and quality of the sealant, its environment, and the quality of the workmanship.

There is concern that the use of structural glazing has grown so fast that not all installations are being performed with the technical expertise and meticulous care required. There have been reports of field problems encountered during installation because of inexperience and lack of knowledge of proper sealant installation procedures.

Although life expectancy of silicone is potentially high, quality control is critical. Pre-bid qualifications for applicators and postconstruction surveillance, maintenance, and replacement issues also need to be further addressed.

Re-visiting the SH&G building after 17 years of performance exhibited no detectable changes in sealant properties. Although, examination of the sealant, whenever a lite of glass was replaced because of vandalism or impact damage, revealed a lack of original adhesion in portions of two out of ten replaced lites. In one lite, lack of original adhesion in about 40% of the perimeter was attributed to frost on the metal, which was not visibly detectable at the time of application. In the other lite, lack of original adhesion on a portion of the metal at the head was attributed to some unexplainable oil contamination that was not removed before application of the sealant. This experience points to the importance of conservative safety factors in new designs and the value of education, training and quality control on the part of the sealant applicator.

Workmanship is always of interest when installing sealants, but especially with structural glazing. How well did the worker clean the surfaces? How pure were the solvents—was residue left? Did the installation produce an air-free bead with good surface contact? These are all factors in the longevity of the building. The human factors cannot be factored into an equation before the installation, but they can be measured after the fact.

Why educate and train the sealant applicator? Because there are many natural forces at work on a structural sealant bead. Some are constant, others are cyclic, and still others are occasional. Consider each of the forces we know: wind load, thermal expansion of glass and/or mullion, rotational deflection of the glass in the wind, dead-load weight of the glass, flattening of tempered glass, slab deflection, seismic load deflection, settling deflection, and miscellaneous.

For example, on the windward side of a building the wind pushes the glass against the million. The only function the adhesive/sealant must serve is as a resilient rubber cushion. On the lee side of a building the wind creates a zone of lesser pressure (negative pressure in relation to the ambient or inside pressure). Thus, one side of a building sees a higher pressure than ambient, the other a lower pressure relative to ambient. Thus, a force inside the building will try to push the glass out.

Structural Sealant Glazing relies on the adhesive properties of silicone sealants to adhere glass to the framing system. Silicone sealants are usually used to provide an exterior weatherseal in these systems as well. Using structural sealants in place of exterior metal retaining members allows designers to create the illusion of long vertical or horizontal bands of glass, or entire facades of glass unbroken by metal framing.

Structural sealant systems may also be considered wet sealed systems, as the adhesive silicone bead also acts as an effective seal against air and water infiltration.

Structural sealant glazing installations depend on the adhesive properties of silicone sealants. The tensile strength of silicone is limited, and each installation must be carefully engineered to ensure that sealant bead size is adequate for the glass sizes and structural loads of a given project. It is also critical that the formulation and self-life of the silicone be precisely controlled, and that the product is properly applied. Quality control protocols must be in place throughout the supply, fabrication and installation process. Insulating glass edge sealants may also be subject to additional stress in structural glazing applications, and the unit edge construction must be engineered as well. Structural sealant glazing systems require close collaboration between glass, metal, and sealant suppliers.

Like conventional glazing systems, structural sealant systems can be designed to be glazed from the inside or the outside. It is common for these systems to be factory glazed, where the sealant can be applied under controlled conditions.

Glazing Recommendations and Guidelines typically address:

• Frame design and weep systems
• Typical glazing methods and details
• Glazing clearances
• Setting blocks
• Glazing materials
• Structural and butt glazing
• Glass storage and handling
• Glass protection and cleaning

It is difficult to catalogue the wide variety of glazing materials in use today. The evolution of glazing method continues as well. Therefore, there is no single source document that catalogues all the practices, methods, materials and principles of glazing.

Glazing Practices and Principles ⎯ the correct handling, setting of glass, and the proper selection and application of glazing materials are primarily the responsibility of the glazing contractor. Specifications should also require that all mechanics, from glazier to caulker, be skilled and have adequate training and experience.

CURTAIN WALL: The term curtain wall has historically been used in a functional sense to describe any non load-bearing glazed wall system used to enclose a building. Early systems were essentially coupled windows assembled into face sealed cladding systems. Experience soon proved that more robust systems were needed. The boom in high-rise construction, which started in the 1950’s, spurred the demand for higher performance glazed wall systems that could accommodate building movements and structural tolerances, thermal movement within the system, and wind driven rain⎯for any size of building⎯year after year.

There followed several decades of experimentation and technological innovation. Steel, aluminum, and bronze have all been used to construct metal curtain walls. Metal and precast concrete panel systems have been tried. It was not uncommon for custom walls to be designed to suit the architectural requirements of a particular building. Over time several distinct types of wall designs had been developed: site assembled ‘stick’ systems, factory assembled ‘unit’ systems, combination mullion-and-unit systems, panel systems, and column cover-and-spandrel systems.

The aluminum-framed wall has emerged as the most common type of curtain wall built today. Aluminum can be extruded into complex shapes to address all design issues, it is corrosion resistant, accepts high performance finishes and the standardized components can be fabricated to close tolerances to accommodate a wide variety of architectural requirements. In addition to glass infill panels, many of these systems can support cladding panels of metal or stone, which conceal the aluminum-framing grid below.

Aluminum curtain wall systems have a long history of development, and represent the most widely tested type of wall construction on the market. Architectural metal manufacturers offer standard systems which can be used to enclose any kind of building. Today the term curtain wall is widely used to describe a product type: high performance aluminum framed wall systems designed to build modular, glazed cladding walls. In this manual the term curtain wall is used to describe this kind of product⎯whether it is used as a building enclosure, as a window system, or as a storefront.

Aluminum curtain wall systems are built from extruded aluminum members assembled into frames, with infill materials secured with removable caps or stops. In capless systems, the infill of cladding panels are secured with adhesives or concealed fastening methods which offer designers great freedom in designing exterior facades.

Curtain wall systems are widely used to clad commercial and institutional buildings. They can also be used to frame monumental windows, strip windows, and entrances. Because they offer improved performance capabilities, their initial cost may be higher than the cost of glazed walls assembled from window or storefront components.

Curtain wall systems are available in the widest variety of finishes: baked enamels, powder coats, two and three coat fluorocarbon coatings, anodized finishes and custom colors.

Types of Curtain Walls:

• Stick Curtain Walls
• Factory Assembled Curtain Wall Systems
• Unitized Curtain Walls
• Unit-Stack Systems
• Strip Windows
• Window Walls

As a master mechanic of sealant installations, the types I write about above, there is no debating that the caulking contractor is responsible for bringing a trained crew to the site and being sure that this crew follows the installation procedure recommended by the supplier. The applicator is also responsible for being sure that the specified sealant is used and no changes are made unless agreed to by the architect.

Why is this information so important to the caulker in the field?

Because, not all sealing technicians are truly literate or truly aware of the subtle differences in the sealants' appearance or performance. Thus every member of the team from architect, supplier, caulking contractor and especially the sealant applicator must know what he or she is doing and cooperate with the others to ensure a successful sealing job.

Just consider the numerous sealant details on this Seattle, WA. project below. From structural sealants to perimeters seals. From butt-joints to recessing the sealant to the water-line of the glazing system. With a variety of substrates that include, glass, concrete, brick, block, precast, aluminum, steel; each material requiring a different methodology for sealant application.

Every master craftsman's secret is that they personally care about learning and their workmanship. Many will enter into the sealant trade, but few will actually accomplish the skills in it's entirety. There is much to know. Trade professionals reach success in their skills only because they have persevered in the face of many obstacles along the way. They distinguish themselves by giving a great performance on every project.

“God is in the details” ―Ludwig Mies van der Rohe, Architect