Glass: An architectural and structural perspective

March 2012 » Columns » STRUCTURAL ENGINEER & ARCHITECT
Ciro Cuono, P.E. and Michael Wyetzner, AIA, LEED AP
The above photo shows a building with a floating curtain wall that includes low iron, ceramic frit, and honeycomb for privacy and safety. Designed by Michielli + Wyetzner Architects with Hage Engineering, the Greenpoint Emergency Medical Services Station for the New York Fire Department and the NYC Department of Design and Construction is currently under construction in Brooklyn.

Architect's perspective
By admitting natural light, fresh air and views, glass allows the architect to connect the interior of a building directly to the exterior. Originally only available in small panes, today, glass can be manufactured in huge sheets, insulated and curved, tempered or laminated, fritted and textured. It can not only act as the skin of a building but can structurally support the skin as well. When designing with glass, there are many factors to consider. Some of these include color, safety, transparency, controlling solar heat gain, and structural support.

Manufactured as float glass, and suspended in a state between liquid and solid, most building glass is composed of primarily a mixture of limestone, silica (sand) and calcium oxide (soda ash). Inherently hard and brittle, when broken, glass shatters into dangerous knife-like shards. For many years, thin wire mesh has been inserted into window and skylight glass to prevent injuries. To make glass tougher, it can be heat treated by a process known as annealing. Heat tempering makes glass even stronger. Known as safety glass, when shattered, laminated glass holds together with spider-like cracks.

The clear interlayer in laminated safety glass can also be translucent to provide privacy. Frit can also be used to control privacy by baking on a ceramic pattern of dots, lines etc. In addition, text and imagery can be employed on the interlayer film or the frit, creating a graphic façade. Glass can be ribbed and textured to reduce transparency or modified by sand blasting and acid etching the glass surface.

With large areas of glass, sun control is of paramount concern. Most exterior glass units are insulated, usually, with a one-half-inch air space between two layers of one-quarter-inch thick panes, but solar heat gain still poses a concern. Solar heat gain can be controlled by numerous methods, including the addition of brise-soleil and other sun shading devices. Low emissivity glass, or Low E as it is commonly called, can also reduce the amount of solar heat gain by the addition of a coating on the surface of the glass that reflects radiant infrared energy. In addition, the surface of the glass can be treated with film and honeycomb interlayers, as well as baked-on ceramic frit patterns, to limit the amount of sunlight. Colored tints and electronically programmed self-tinting glass can also reduce sun infiltration.

Hung off the building structure, insulated glass units in curtain walls are commonly glazed into steel or aluminum mullions with a structural support to account for wind loads. The amount of glass captured in the mullion, known as the bite, is determined by the frequency of mullions and the square footage of the glass units. Another method of attachment is structural glazing, which allows the glass to be glued directly to the mullion with a silicone sealant. In addition, point fixing allows glass to be held in place by a support that fits through a gasketed hole in the surface of the glass that is then tied back to a support system. The joints between the panes are filled with silicone to provide a water-tight enclosure. The support structure can vary from a system of steel members to a tensioned cable net. In addition, laminated glass fins can provide a backup support for the curtainwall and act as structural beams to support canopies and limited roof enclosures.

Structural engineer's perspective
Most structural engineers in a typical design office have probably not studied glass as an engineering material in school nor designed a glass curtainwall or glass beam. Most cases where glass is used structurally are relegated to specialty firms or manufacturers. Yet, it is important for the structural engineer of record on a building project to understand the basics of glass, including its strength, unique properties, and limitations.

Heavier than concrete, with a compressive strength greater than 140 kips per square inch and an elastic modulus of roughly 10,000 pounds per square inch; glass seems to have decent engineering properties. However, glass is not a forgiving material. While a material such as steel exhibits plastic deformation beyond its elastic range, giving ample notice before a failure, glass remains purely elastic and then has a sudden brittle failure. Basic knowledge of fracture mechanics is helpful as glass is extremely sensitive to surface cracks and defects. These microscopic cracks (Griffith flaws) can propagate until a sudden burst of energy is released (failure). Glass strength is also susceptible to load duration, humidity and its edge conditions.

Most modern glass in the building industry is float glass, a process developed in the 1950s. The basic types are annealed, fully tempered, and heat strengthened. Fully tempered glass is about three to five times as strong as annealed glass and heat strengthened glass is twice as strong as annealed glass.

These types of glass are heated and then quenched, usually with jets of cool air. This creates a temperature difference that allows the outer layers to cool more rapidly than the inner layers, forcing compressive stresses in the outer layers. Compression in the outer layers keeps surface defects and cracks closed, preventing crack propagation; hence a tremendous gain in strength is realized.

Strength, however, must be balanced with other considerations due to glass's susceptibility to brittle and sudden failure. The concept of post-failure behavior is important in glass design. As an example, when fully tempered glass cracks it breaks into many small pieces, while annealed glass breaks into larger pieces. Another important design concept is redundancy. Layers of glass panels are often laminated together with PVB (Polyvinyl butyral) or, in insulating units, two or more layers are separated by an inert gas. If, for example, a glass beam is designed with four laminated layers of fully tempered glass, a good engineering approach would be to consider a design case where the outer two layers have failed and the inner two layers have to take the load, or possibly some statistically reduced load.

Simple structural analysis approaches have limited accuracy due to the potential for fracture, lack of plastic flow, and load duration effects. A more accurate approach is to do a finite element analysis, taking into consideration holes and other causes of high stress concentrations. Another consideration to keep in mind is that linear structural analysis is based on small deflection theory, that is, the deflections are not more than the thickness of the member/plate. If deflections exceed thickness, then a more realistic approach would be a non-linear analysis. Load testing and other experimental testing is useful and in some cases may be dictated by specifications or codes.

One frustration for the designer is that, unlike other materials, there is no one definitive source like a design manual or code for designing glass. One would have to build up a library of information that includes several ASTM standards, industry publications, as well as international publications from the UK, Germany, Australia, and Canada.

Many designers are pushing the envelope in glass design with glass beams, full stair structures, vertical glass fins, and point supported and hung glass façades. These cases require specialized knowledge of glass, including their fittings and sealants. A detailed finite element analysis is required, particularly for point-supported elements. Another interesting area is structural silicones. These are usually limited to out-of-plane loads but are certainly an area where new research and innovations can be expected.

Ciro Cuono, P.E., LEED AP, is an associate at Hage Engineering. He can be reached at ccuono@hageengineering.com. Michael Wyetzner, AIA, LEED AP, is a partner at Michielli + Wyetzner Architects, New York. He can be reached at mwyetzner@mwarch.net.


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