Glass Act: Where Beauty and Engineering Clearly Meet

Seeing through today's spectacular bolted structural glazing systems to the precision engineering inside
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Sponsored by W & W Glass, LLC
Layne Evans

Nuts and Bolts

The choice and design of the stainless steel fittings in bolted structural glazing systems is critical to ensure that loads are transferred to the structural elements supporting the glass. Various load paths occur at every countersunk hole. Fittings are designed to prevent high stress concentration at the hole positions. (Stress analysis, including the strain gauge testing previously noted, is crucial.) But the glass will also expand and contract, deflect and rebound, constantly throughout its service life. Fittings have to cope with negative and positive wind loading, seismic loads, thermal movement, construction tolerances, live load and dead load movements. Correctly designed fittings incorporate movement diaphragms of stainless steel and durable flexible discs or rotules to allow for rotation behind the glass, at the building connection.

Ruins as old as the 12th century are protected by 21st century bolted glazing design and technology at Hamar Cathedral, Hamar, Norway.

Architect: Lund and Slaatto Arkitekter

Photo courtesy of W&W Glass, LLC

 

The choice of hardware is related to the specified glass, the design of the support system, the design and function of the building, the anticipated loads, and many other factors. All holes in the glass have to be drilled and polished with precision accuracy at the manufacturer, specifically for fittings specially designed for that application. The cutting is done with highly specialized drilling equipment, and the holes are then carefully polished. As noted earlier, this entire process must be completed before tempering. Thus the hardware must be matched with extremely tight tolerances, taking into account such critical details as the thickness of the glass, the loads the hardware must resist, the distance from the hole to the glass edge, and the maximum distance between hole connections.

The most common fitting in bolted glazing systems is the "spider" or star type. The highest quality are those made of austenitic Type 316 solid stainless steel lost wax investment castings, commonly two-point, four-point and sliding types, but many other forms and shapes are available or can be designed for specific applications. For example, a design with sliding arms to accommodate large racking displacement of glass under severe seismic events allows for movement of up to 1 inch in two directions at each glass joint. Specific fittings are also used with each of the basic types of supporting structures discussed below, such as pin joints for glass mullion systems.

In the continuing search for transparent expanses and minimum visible structure, one of the most recent innovations in fittings is concealed bolt structural glazing, where the bolt or fitting is recessed into the glazing. Concealed bolts are integrated into the interior laminate through the back lite of the glass, allowing completely flush facades or canopies with all fittings entirely concealed within the laminated panel and no visible exterior fasteners. This type of structural glazing is a proprietary system, but it has been fully tested for performance and safety standards.

Performance of the entire structural glazing system is a close inter-relationship between glass and hardware. For example, bomb resistance requires not only high strength laminates but also specially engineered connections.

Best Performanc e in Suporting Structures

In bolted structural glass systems, the glass is fixed to support systems, also often called back-up systems, which are in turn fastened to the building structure itself. There are three basic types of support systems but as can be seen in the examples below and in many other buildings all over the world an incredibly wide range of combinations and variations of these types are possible. Although these complex and dramatic glass facades, walls, skylights, canopies and other features are often associated with iconic signature buildings they are also increasingly being used in stores, office buildings, hotels and other smaller scale applications.

The supporting structures are the components of the glazing system where the building's designer has the most freedom. In the best cases, the design of the glass, the fittings and the supporting structures are integrated seamlessly into the design of the building.

Transparent glass fins (Mullions). Glass fins have been used continuously since the earliest suspended assembly systems in the 1960s and they are still the most common bolted structural glazing. Glass panels are bolted to the fins, which provide horizontal support against both negative and positive wind pressures. The weight of the glass is transferred back to the fins, which hang from the structure on the top of the building. Glass fins are available in a variety of different heights and widths, depending on the particular load criteria of the project, and are typically fully tempered and heat soaked. Glass fin suspended assemblies have functioned well in the Northridge (Los Angeles) and Kobe earthquakes.

Cable tension trusses help create the exceptional transparency of the façade of the University of Connecticut, Stamford, CT.

Architect: Perkins & Eastman

Photo courtesy of W&W Glass, LLC

 

In new construction, a common fin connection is the propped cantilever type. Movement is transferred to the building at the head of the wall, so more steel must be located there. A second type of mullion configuration is the pin-jointed fin, allowing absorption of live loads and thermal expansion through rotation around a steel pin. This type of connection requires fins to be deeper, but it is the most common type, especially in existing buildings where it would be too costly to add steel structure.

Glass fin systems are available for horizontal or vertical designs, and when executed correctly are extremely transparent.

Like any tall and narrow structural member, the glass fins used in glass façades could buckle if subjected to particularly high negative loading. In these cases, it would be the unsupported back edge of the glass fin that would typically buckle first. So in order to understand the stresses and ensure proper performance and safety every fin design should be subjected to specialized  buckling analysis. To support higher facades, taller fins, and higher wind loads, designers are using new anti-buckling fin technology. Stainless steel tension rods bolted to the back edges of the glass fins provide lateral support and prevent the fin from buckling or flipping out of plane during load.

At the River East Center in Chicago, IL, a structural glazing system with glass fin supports was used to create a 180' high glass entrance façade. The glass fins were spliced to achieve the required height; however, at 180', potential buckling still posed a serious problem. In order to prevent buckling of the spliced glass fins, the glass façade was designed with multiple steel support trusses every 30'.

For the 3 Times Square building in New York City, the architect did not want any steel to be visible within the 60-foot tall structural glass façade. Given a glass façade of this height, potential buckling of the glass fins was again a major concern. To avoid the use of horizontal steel supports, small anti-buckling cables were mounted across the back of the glass fins and tensioned wall to wall.

Steel support structures. Steel support structures can be used to support façades, canopies, screen walls, and even elevator enclosures designed with structural glass. These structures are extremely versatile, ranging from simple systems using steel tubes to elaborate four-sided glass boxes. The steel supports can be inside the system and virtually invisible, or exterior and highly visible.

A steel-supported structural glazing system was used to create a glass enclosure for the Hamar Cathedral historic site in Norway. Because it is located in such a cold climate, the Hamar Cathedral ruins required protection. Steel supports were combined with structural glazing to create this complex, sloping glass and steel structure.

Tension supported structures. Some of the most innovative recent designs in bolted structural glazing have utilized cable tension support structures of three basic types: the primary steel truss, self contained and imparting no tensile load to the boundary structure; the bow or bow string truss, with cables at the front and back of a center mast, also imparting no tensile load to the structure; and the cable net truss. The latter is the most expensive type since it requires field tensioning and heavier boundary structures.

Advanced structural glass canopy design as well as the critical importance of heat soaking tempered glass are both demonstrated at the Fox Plaza, Century City, CA.

Architect: Johnson, Fain & Pereira Associates

Photo courtesy of W&W Glass, LLC

 

One of the most famous examples of bolted structural glazing in the United States, the luminous Rose Center for Earth and Space in New York City, was the first large-scale cable-supported project in the United States. It is still regarded, as it was at its opening in 2000, as a dramatic demonstration of an American bolted structural glazing project every bit the equal of spectacular projects in Europe such as the Louvre Pyramid.

Other distinguished American examples quickly followed. At 300-feet long and 33-feet tall, the façade at the University of Connecticut is one of the largest, lenticular cable-truss supported structures in the United States. The insulated glass façade was built using a series of cable tension trusses. The absence of horizontal supports makes the façade visually very light.

A cable structure was also used to create an extremely transparent segmented curving glass façade for the NASDAQ market site in New York City. The NASDAQ project is an excellent example of the customized solutions available with tensionsupported structural glazing. In this case, the structure uses a series of horizontal loop cables tensioning against the vertical cables as a means to support the entire glass façade.

Cables were used to create a soaring 70' structural glass façade for a building at the Harvard Medical School in Cambridge, MA. In this case, a mid-truss allows for the use of two separate cable tension trusses, each approximately 35' high. Without the midtruss, the façade would have required trusses 8' deep and would have imparted massive loads on the boundary structure.

 

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Originally published in Architectural Record
Originally published in October 2008

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