Glass Act: Where Beauty and Engineering Clearly Meet

Seeing through today's spectacular bolted structural glazing systems to the precision engineering inside
This course is no longer active
[ Page 2 of 5 ]  previous page Page 1 Page 2 Page 3 Page 4 Page 5 next page
Sponsored by W & W Glass, LLC
Layne Evans

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.

 

[ Page 2 of 5 ]  previous page Page 1 Page 2 Page 3 Page 4 Page 5 next page
Originally published in Architectural Record
Originally published in October 2008

Notice

Academies