Glass Act: Where Beauty and Engineering Clearly Meet  

From storefronts and lobbies to the most stunning signature buildings in the world, the expanses of glass continue to get bigger, taller, more complex and ever more transparent. The metaphors vary - disappearing walls, invisible structure, bringing the outside in, dissolving the boundaries between the building and the street and sky - but the basic desire for the most transparent structure possible has driven the development of glass architectural material for about two thousand years.

The current state of the art is the bolted structural glazing system, also referred to as point supported glazing: glass, stainless steel fittings and increasingly imaginative support or back-up structures conceived and executed as an integrated unit. As glass structures get clearer and buildings more see-through, the science and skill required to implement them gets increasingly complex. This article will outline the still continuing evolution of bolted structural glazing systems, the precise engineering, stringent testing and technological innovations in each of their major elements, particularly the glass itself, and the critical importance of bringing together the design, engineering, manufacturing and fabrication of these interacting elements into a single system, a sum very much greater than its component parts.

Frame Free

Throughout the colorful history of glass in buildings, from the first not-very-clear windows in first century Rome through the stained glass of medieval cathedrals, the Sun King's mirrors in Versailles and the 293,635 panes of glass in London's famous Crystal Palace in 1851, glass was always captured in a frame. Individual panes of glass became larger in the 19th century, with the inventions that led to mass production, but frames of lead, steel or aluminum were still required. Only in the 1960s, predominantly in Europe, came the invention of the patch plate hardware fittings that could connect individual glass lites into a matrix without frames.

These early systems were a remarkable advance in transparency. They typically were suspended assemblies, consisting of panels of face glass connected to vertically oriented glass fins by sliding knuckle hinges and corner patches. The face glass hung from adjustable steel rods along the top of the structure, and was stiffened by the fins, which hung independently from the face glass and were designed to take lateral loads. The vertical and horizontal joints separating the individual lites of glass were typically sealed with two-part epoxy.

One of the most celebrated and spectacular examples of bolted structural glazing systems in the world is the Rose Center for Earth and Space, American Museum of Natural History, New York City, NY. Architect: Polshek Partnership Architects LLP Photo courtesy of W&W Glass, LLC

 

This basic system was prevalent for over 25 years, but in the 1980s, technical advances in two different areas led to the next big step forward in transparency. Structural silicone systems began to emerge, with properties superior to previous epoxies, more weathertight and more able to withstand the flexing and stresses of large glazing systems. Around the same time, in England in 1982, the use of a countersunk hole in glass was invented and quickly recognized as a key breakthrough. A hole about the size of a quarter was drilled very close to the glass edge, and much smaller fittings, flush with the exterior face, could be used. The innovation of the countersunk hole, instead of the heavy corner patch plates used in the original suspended systems, allowed structural glazing to be used as an entire cladding system, in any plane, not just the vertically suspended facades. Each lite could be fastened back to the glass fin, making each lite independent of those adjacent. The true point supported, bolted glass façade emerged.

The Time Warner Center

The Time Warner Center located in New York City utilizes state-of-the-art cable tension design at its main entrance façade and "Prow" structure. The Prow is a three-sided transparent glass structure specifically designed to house large electronic signage. The large horizontal steel elements visible in this image are in place to support signage and do not provide any structural support for glazing. The Prow was constructed using a combination of low-iron glass, vertically-hung double steel cables, and horizontal glass fins. The horizontal laminated glass fins shown below are designed to take up the lateral wind loads. The vertically-hung double steel cables are designed to support the dead load suspended weight of the glass wall. The 80' wide x 180' tall entrance façade is supported by a cable net, which consists of a series of cables tensioned horizontally and vertically. The boundary structure is critical in this type of design due to the large loads imparted by the cable net. Cables are tensioned at every vertical and horizontal joint, allowing the façade to move a full 23" in and out at its center (46" total), under full wind load conditions.

The famous transparent glass "Prow" at the Time Warner Center, New York City, NY. Architect: Skidmore, Owings & Merrill, LLP Photo courtesy of W&W Glass, LLC

 

Impressive buildings all over the world began to incorporate these systems from the late 1980s on. The basic components - glass, fittings and support structures - remain the same today. However, as in every other technology, the pace of innovation in each of these components has accelerated in recent years. Today's glass is available in forms that are lighter, flatter, clearer, and highly engineered to meet more stringent energy and building codes. The design of fittings has been refined and expanded to include applications for the most extreme conditions. And today's glazing is often bolted to sophisticated steel supporting structures, including trusses and tensioned cable riggings, which make architectural statements of their own.

But perhaps the most important overall lesson learned has been the recognition of how closely the performance of each element is connected, and how carefully these relationships have to be managed to create a single, sole source, tightly controlled, precisely engineered, integrated system.

It's All About the Glass

The clear heart of the structural glazing system is the glass itself. As crucial as the right fittings are to the system, steel fittings are far simpler to engineer than glass. And even the most sophisticated design for a support structure will only perform as well as the glass performs.

On the top, standard roller wave distortion is clearly visible. On the bottom, the perfect reflection made possible in glass controlled for roller wave, in the extraordinary bolted structural glazing system on the facade of the Brain and Cognitive Sciences Complex, Massachusetts Institute of Technology, Cambridge, Mass. Photo courtesy of W&W Glass, LLC

Lead Designer: Charles Correa. Design of laboratories and research spaces: Goody Clancy and Associates. Photo © Anton Grassl/Esto

 

Glass is one of the most mysterious substances known, the most liquid of solids and the most solid of liquids. It is technically "perfectly elastic," which means if deflected (moved), it will return to its original shape. But it is also technically "brittle," meaning that it cannot bend very far without fracturing. Theoretically glass has higher tensile strength than steel, but it does not behave in a "linear" way. Doubling the load will not necessarily double the deflection of glass. Compare, for example, the stress and load relationships of metal and glass. A metal coat hanger will reach its yield point (it will bend) long before it breaks. In glass, however, the yield point and the breakage point are exactly the same. That point is reached with no visible warning, and not necessarily at the point where stress is highest. A small crack from an infinitesimal imperfection or impurity will propagate at very high speed throughout the glass, causing total failure.

In engineering glass facades, the two essential design criteria are stress - the structural strength of the glass when subjected to various loads; and deflection - how much the glass will move when subjected to forces such as wind. Glass in an architectural application will be subject to multiple and constantly changing load factors: weather, positive and negative wind effects, temperature effects, snow loads, seismic factors, possibly live loads from the supporting structure, and in the case of canopies and skylights, possible falling objects. The dynamic and static loads acting on glass will cause it to deflect. The amount and shape of the deflection will depend on the glass size and thickness, and the glass edge support conditions, as well as the loads. The glass and glazing system must be designed not only to have the strength necessary to withstand the design load, but also to limit deflection.

ASTM E1300 "Determining Load Resistance of Glass in Buildings" is basically a failure prediction model taking into account the random nature of the kind of flaws and damage that can cause fractures in glass. It is the industry standard used for determining the load resistance of glass in buildings. It also includes information for calculating the deflection of glass based on its size and thickness. (ASTM E1300 specifically excludes glass with holes and notches, so they have to be accounted for by other analyses.)

Many other (often proprietary) complex and exhaustive computer models have been developed to analyze the performance, strengths and tolerance of glass. Design panel charts, for example, allow engineers to accurately predict how a given panel of structural glazing will perform under various loads. With this data the engineer can design specific panel geometries and specify glazing systems of the appropriate thickness. A typical analysis by a glass engineer would require determining the size of the panel in square meters (vertical axis) and the design wind load in Newtons per meter (horizontal axis). The cross section of these two elements will determine the appropriate thickness of glass for a given load.

So the properties of a simple pane of glass are already far from simple to quantify and predict, but drilling countersunk holes also creates areas of stress concentration that have to be taken into account. The loads on glass are normally transferred at the corners of the glass panels. Specially designed fittings that allow for movement, as talked about more below, are critical for exactly this reason, but toughened glass is still necessary to accommodate the high stresses at connections.

In consequence, the glass in structural glazing systems must be engineered and manufactured with extreme accuracy and quality control measures. Testing and analysis must be stringent, continuous and based on actual empirical data from the glass, the assemblies and the existing façades and completed projects. New building codes with higher wind and seismic requirements mean testing and analysis are even more important for compliance. The eventual beauty, performance and safety of glass in any building, but particularly in bolted structural glazing systems, are directly determined by the level of its engineering way before the system reaches the manufacturer, and at every precise step after that.

Performance in the Making

The basic process of manufacturing very high quality float glass begins with melting about 70 percent silica sand, 13 percent dolomite and limestone, 12 percent soda ash and small amounts of other materials. Often some percentage of the batch is in the form of cullet, or cleaned and crushed glass recovered from previous glassmaking, which lowers the melting temperature required. About 50 other elements are available to add in precise formulas to affect performance, depending on the application. The melted mixture produces a continuously rolling 12-foot wide glass ribbon. The molten glass flows from the furnace and "floats" over a bed of molten tin. It is then "annealed", a carefully controlled cooling process to minimize internal stresses and maximize potential mechanical resistance. For structural glazing systems, the glass is also "tempered", to make it four times stronger than annealed glass. (The term "heat strengthened" actually refers to a slightly different process. Heatstrengthened glass, while twice as strong as annealed, will break into large jagged fragments, unlike tempered glass, which breaks into small, much less dangerous fragments.)

Transparency and performance are both enhanced with an ionoplast interlayer in the glass roof of Yorkdale Mall, Yorkdale, Ontario, Canada. Architect: MMM International Architects Photo courtesy of W&W Glass, LLC

 

Tempering involves reheating the glass to the point where it starts to soften followed by rapid controlled cooling or "quenching." The outer surface of the glass cools faster than the inner layer. As the inner layer cools it contracts and compresses the outer layer, increasing the flexural strength of the glass by up to four times.

All fabrication of the glass is completed before tempering. All holes are drilled, and the other carefully controlled polishing, edging, notching and finishing processes completed, before tempering, because nothing can be altered afterwards.

Not all tempering is created equal. One measure is compressive strength. Typical furnaces average approximately 11,500 psi (pounds per square inch, a unit of pressure), and some of the most advanced modern furnaces achieve minimum compressive strength of 16,000 psi. The added strength can be especially important in structural glazing for the added safety at countersunk hole locations where maximum stress occurs.

Another measure is the occurrence of edge dip, roller wave and bow. These may sound like garage bands, but they are actually visual distortions that, although inherent to the tempering process, can be minimized by stringently controlled manufacturing processes. Edge dip and roller wave are caused when the ribbon of semi-molten glass sags even a small amount on the continuous casting roller during the tempering process. Bowing is caused when the two sides of the glass are cooled at even slightly different rates. These distortions are difficult to control for and are quite visible in the wavy trees and runny clouds reflected in many glass facades. The peak-to-valley "waves" in glass can be measured, and the published norm is 0.05".

However, it is possible to specify a minimum 0.0008" peak to valley wave, which virtually eliminates visual waves and results in high clarity and the perfect reflection of the surrounding environment.

The following are some of the most important additional analyses, tests and measurements that should be specified to ensure safety and performance in bolted structural glazing systems:

Heat soak. Many experts consider the heat soak to be one of the most important safety tests for tempered glass, to be specified and performed on all structural glazing systems before they are shipped. The heat soak process is a destructive test developed to find and eliminate the tiny, invisible impurity nickel sulfide, naturally present in the silica in float glass. Even a single inclusion of nickel sulfide can cause spontaneous breakage in a piece of glass.

In the most stringent heat soak tests, glass is exposed to a temperature of 290°C (+/-) for a period of 8 hours. In these tests 99.9 percent of the nickel sulfide impurities will be destroyed. Only the fully heat soaked glass that has survived this process should be specified for bolted structural glazing systems.

Strain gauge. Drilling the countersunk holes into glass creates areas of additional stress. These effects have to be factored into the system's overall performance parameters. The strain gauge is a mechanical test to assess a variety of loads applied to glass panels in horizontal or vertical orientations. Sensors translate pressure forces into measurable electrical resistance.

Uniform load test. Tempered laminated glass is loaded past the breaking point, and the laminate layer must support the weight of the broken glass.

Finite element analysis. a numerical analysis to define how a structure or material will react to loading conditions depending on the anticipated stress levels at various points and under various conditions. In the early 1970s finite element analysis was limited to the most expensive mainframe computers such as those belonging to aviation, defense and the nuclear industry. In our age, of course, increased computer power makes it possible for 3-D computer models to predict accurate results for all kinds of parameters and variables, such as mass, volume, temperature, strain energy, force, displacement and many others.

Tests for specific applications. Glass and assemblies destined for bolted structural glazing systems typically undergo many additional tests for specific conditions, including wind load resistance, hurricane performance, air and water penetration, seismic performance, impact resistance and bomb blast loading.

Layer Forward

One of the newest technological advances in glazing for structural systems is between the panes: a new interlayer for laminated glass. Laminated, or safety glass, was one of the key breakthroughs in glass technology for widespread use. Although it had been invented as early as 1910, the roots of the thin interlayer used in today's laminated glass go back to the development of windshields for automobiles. Clear polyvinyl butyral (PVB) was sandwiched between two panes of glass. If the glass broke, most of the glass fragments adhered to the plastic layer. PVB in various formulas is still widely used today.

Most laminated glass is used within frames, like your windshield, and safety glass is very good at keeping broken glass within the frame. But in bolted structural glazing systems, the framelessness is the point. Lamination, however, is still essential in large glazed areas, sometimes at considerable heights and in horizontal roofs or canopies. For these applications, PVB has limitations. It doesn't react well to moisture, so it can discolor or break down if edges are exposed. It also doesn't react well to the silicone sealants used for weatherproofing at the butt joints between panes in structural glazing. And there is a safety concern. With PVB, in a frameless panel, if both pieces of glass are broken, the glass will fall out.

The new ionoplast interlayers were originally developed for hurricane prone areas but are now increasingly specified for structural glazing systems. Not only is the substance not reactive with silicone or with moisture, but it also bonds better to the glass, is more rigid, and allows the glass to act as a composite monolithic unit. The interlayer's structural strength allows the use of much thinner, lighter panes, and is clearer even than clear PVBs. Sealing the deal is its innate structural strength and stiffness. If the glass breaks, the stiff interlayer tends to stay in place, making it much safer when considering post-breakage performance. The strength and stiffness also allows architects to design large, relatively thin glass panels with minimal support systems that can handle massive loads -whether impact from hurricane or blast, or snow and ice loads. Loads like that normally would require extremely thick laminated tempered glass.

The ionoplastic interlayers are being used in assemblies that look exceptionally transparent and light, particularly when used with low-iron ultra-clear glass, but are actually composites that solve multiple problems. For instance, in the Yorkdale Shopping Centre in Yorkdale, Ontario, Canada, each 7-foot by 4 -1/2 foot panel of glass in the huge barrel-vaulted glass roof consisted of fully tempered heat soaked clear insulated glass, a high-performance ceramic frit for solar control, an ionoplast interlayer for structural strength and argon gas in the air gap for insulation.

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.

Clear out to Here: Skylights, Roofs and Canopies

Structural glass is ideal in horizontal applications because it can be engineered to accept very high and complex loads and can be very efficient in terms of heat loss and solar gain. The supports for skylights, roofs, and canopies can be positioned either above or below the steel structure. All overhead glazing must be laminated in order to meet building safety codes, and some codes may require nets in certain applications. To prevent ponding, a 3 degree minimum slope is required. But within these minimal constraints, the sheer transparency of bolted glazing is tailor made for skylights and other designs where maximum openness and clarity are at a premium.

In the Yorkdale project in Canada mentioned earlier, the architects wanted a structureless system for a large span skylight addition, but they also needed the overhead glazing to be strong and safe for the public in the shopping mall below in the face of frequent heavy snow and ice loads.

The 60-foot high, 300-foot long barrel-vaulted atrium was added to the center of the original building built in 1964. The skylight incorporates insulated laminated double glazing that is hung from an exterior structure with gussets supporting seismic fittings. Additionally, it is a showcase for other high technology, as discussed earlier, including ionoplast interlayers and argon gas insulation.

The curved skylight created for the Brooklyn Museum of Art consists of an extremely complex geometric, stepped design. The curved skylight angles downward, with intermediate steps interrupting the roof plain. These vertical glass steps are supported from the interior by an integrated system of steel cables and glass trusses.

At Orlando International Airport the skylights and glass roofs were designed to combine two different types of backup structures. Rather than use steel as a backup, the architects chose to lighten the structure as much as possible. This was accomplished by using laminated glass fins in conjunction with cable trusses located at 11' on center and perpendicular to the fins.

The Imperial Bank Tower Segerstrom located in Costa Mesa, CA involved the design of a 130' long and 20' wide barrel-vault atrium. The biggest concern of the structural glass engineers working on the project was that the glass barrel-vault had to be designed to act as a seismic joint between two very different existing buildings. As a result, the steel and cable tension system needed to be able to support the weight of the glass and allow for movement during a seismic event. New fittings were developed to allow for the extreme movements required by the California Building Code. These custom fittings have since been used on canopies, skylights, and vertical façades in seismically-active areas throughout the world.

The structural glass canopy at the Fox Plaza in Los Angeles was specifically designed to catch glass breaking because of nickel sulfide inclusions and falling from the existing tower above. The canopy was tested for structural and seismic performance, and was designed to withstand the impact of falling glass from 150' in the air. This project serves as a clear reminder of the importance of heat soak testing all tempered glass in point supported structural glazing systems.

Worst Case Scenarios

Structural glazing has been successfully tested and approved for resistance to bomb blast loading. Bomb-resistant structural glazing is typically constructed of insulated laminated double-glazed units. When designing bomb-resistant structural glazing systems, it is important to introduce additional controlled flexibility at the fittings. By understanding the severe deflections that will be induced in the glass during a blast event, the design of the support fittings can be enhanced to achieve significantly greater blast resistance, while from the outside, the fitting can look identical to a conventional support system. Blast-resistant glazing with special seismic fittings was used to create the glass façade of the BBC Television Center in London. Unfortunately, the system had a real-life test when the building was bombed in 2001. Ninety percent of the glazing remained undamaged. In contrast, a large glass curtain wall located about 500' away from the BBC building was severely damaged by the blast.

The Future is Clear

The most advanced current bolted structural glazing systems are lighter, more transparent, more energy efficient, stronger, more resistant to extreme stresses from explosions, hurricanes and earthquakes, and able to fit into more imaginative designs than ever before. Structural glass, when made correctly under tightly controlled conditions, is flatter and clearer than ordinary tempered glass, and options are proliferating in coatings, colors, decorative patterns, high performance, energy efficiency, acoustical, low-iron (for enhanced clarity), solar and even self cleaning glazing. Design innovation has also accelerated in fittings and in supporting structures.

But as will be clear by now, the most important component of the system is knowledge. With the benefits of brilliantly designed and executed structural glazing systems come considerable risks if testing, manufacturing, fabrication and installation are not of the highest order. Liability issues can be especially complex when so many professional teams are involved - project architects and engineers, manufacturers, glass engineers, system designers, testing specialists, skilled fabricators and installers, and many others. The use of a single experienced source bringing together as many services as possible, and offering a single long term system warranty, is one way to effectively manage these risks, and to achieve the integrated approach that marks the most successful - and spectacular - projects to date. With the fundamental factors necessary for long-term safety, performance and beauty ensured, the next generation of bolted structural glazing systems should be even more extraordinary, and even closer to that long-held vision of perfect transparency.

W & W Glass, LLC, is the NY metropolitan area's largest architectural glass and metal contractor. The company is also the largest supplier of structural glass systems throughout the United States. The company exclusively supplies the Pilkington Planarâ„¢ structural glass system. www.wwglass.com