Curtain call: steel & fire-rated curtainwall systems expand design options.

In popular usage, a curtain is typically considered to be a relatively light and thin material that hangs freely from a supporting structure. In this way, the name "curtainwall" for exterior non-load bearing facades of buildings is appropriate. However, in many instances, the types of materials available have limited the ability of architects and designers to achieve a truly light and open feel with curtainwalls incorporating glass. Designs using traditional aluminum frames have often forced the use of bulky framing with large numbers of intermediate mullions, limiting design freedom and reducing the potential for large expanses of glazing.

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Advances in material design and manufacturing are providing new options for glass curtainwalls, allowing for more slender frames and usage in specialized applications, such as walls requiring a fire rating. The result is an almost unlimited ability to use glass throughout a building--an especially important consideration for aesthetics and daylighting.

A Return to Steel Framing

Originally, curtainwalls were constructed with steel frames, such as those used in San Francisco's Hallidie Building and other early examples of the then-new form of design. Steel was strong, readily available and was a natural choice given its use throughout other areas of buildings. Later, aluminum became the material of choice for curtainwall framing, largely due to the ease with which it can be formed into numerous profiles.

Now, steel is emerging once again as a preferred material for glass curtainwall systems, given a number of superior performance characteristics it offers compared to traditional aluminum systems. Specifically, the larger spans of glass and minimal frame dimensions allowed by steel are increasingly desired by architects and designers engaged in curtainwall design. The strong and narrow steel frames are well adapted for applications ranging from large and open straight walls to curved and serpentine walls to vaulted ceilings and atriums.

The re-emergence of steel for curtainwall systems is made possible by advances in manufacturing processes. European manufacturers have developed methods for forming steel into a multitude of profiles, enabling it to match the profile choices of traditional aluminum systems, while offering a greater range of benefits.

In addition, advances in steel framing design include connections that allow components to be assembled without welding, eliminating weld beads or visible fasteners for crisp corner joints.

Architects and designers in Europe have used these advanced steel curtainwall systems for a number of years, and now such systems are also available in North America.

Advantages of Steel Over Aluminum

One of the primary benefits of steel over aluminum is its higher strength: steel has a modulus of elasticity three times that of aluminum, which provides for substantially greater wind load capacity. For a given framing profile cross section, architects and designers can create larger spans of glass with steel than with aluminum, with no additional reinforcements required. Or, for a given set of load and deflection requirements, a narrower steel frame profile can be used than is possible with aluminum. The bottom line is steel enables designs with less framing, providing for more open areas of glass--a highly desired trait for curtainwalls.

The difference in frame profile sizes can be dramatic. Depending on the specific design criteria, advanced steel curtainwall systems can sometimes utilize profiles only one-third the size of a typical aluminum system. Internal reinforcements can also be added to steel curtainwall systems to further reduce the required frame depth.

Another key advantage of steel over aluminum is its thermal properties. Steel has a lower thermal expansion than aluminum, which allows for greater design flexibility with fewer or no extension profiles. Steel also offers long-term durability. With back mullions and cover caps made of steel, such systems are much more resistant to scratches, dents, and dings over the life of the building. Steel also conducts much less heat than aluminum--about one-fourth the amount--providing a lower chance of heat loss and condensation on the frames.

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Advances in Specialty Curtainwall Applications

In addition to the evolution in steel design options for curtainwall systems, other technological developments have allowed for use of curtainwalls in areas previously not possible. One specific design application now available to architects and designers is curtainwalls for areas requiring a fire rating, such as large enclosed stairwells, elevator cores and some exterior walls. Before the advent of modern fire-rated glass and framing systems, areas critical for providing exit corridors or for dividing a building into fire-blocking compartments required masonry, gypsum or similar types of solid walls. Now, glazing can be included throughout a building, allowing in natural light and opening up the interiors of buildings.

During the last two decades, the glazing industry has witnessed dramatic changes in the specialty field of fire-rated glass and framing. At one time, wired glass was the only choice for achieving a fire rating. Now, newer wireless materials have surpassed wired glass with additional benefits and performance capabilities. Today's fire-rated glass looks like ordinary window glass, exceeds the fire performance of wired glass in fire testing, and can provide substantially better impact safety and thermal insulation. From an aesthetic perspective, fire-glass is also now available in large sizes, and a range of finishes such as beveled edges, etched surfaces and a variety of textures.

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Fire-rated framing has also improved considerably in recent years, advancing beyond the functional, yet bulky wrap-around style hollow metal frames of the past. While those frames perform well at blocking fire and smoke, they do not necessarily lend themselves well to many design considerations, especially not curtainwalls.

Fortunately, the situation is changing as fire-rated framing is entering a new era. European-style frames have now been tested to North American standards. These newer narrow profile frames offer a strikingly different and improved look than traditional hollow metal steel. The difference is easily recognizable and is due largely to the method of manufacturing. Typical hollow metal steel framing is formed by bending flat sheets of steel in a press, while the European style fire-rated framing is roll-formed into precise steel tubes that are given shape in an extrusion-like process. The end product is extremely strong, yet more slender than traditional fire-rated framing.

Design Options for Fire-Rated Curtainwall

Advances in fire-rated framing have enabled the development of fire-rated curtainwall systems, allowing architects and designers to incorporate large expanses of fire-rated glass stretching across multiple stories. Fire-rated curtainwall frames can be used in interior applications, as well as exterior applications since they have been tested for air and water infiltration. They install similarly to pressure plate curtainwall and come with a wide variety of face caps to match any design. Powder coating of the steel frames also offers a nearly endless variety of colors.

Lobbies and atriums are well suited to the use of fire-rated curtainwall to provide uninterrupted views. And floor-to-ceiling fire-rated curtainwalls are sometimes used to divide interior spaces while offering security and aesthetic appeal along with fire protection.

For exterior applications, most building codes do not typically require fire-rated materials for the skin of the building, unless the building is close to a property line. The focus on fire protection is usually on the contents of the building rather than the space around it. However, the outside of the building can play a key role in helping prevent the spread of fire within the building. For example, in a widely reported case from 1988, a fire broke out in the First Interstate Bank building in Los Angeles. The building envelope did not include fire-rated materials and the fire was able to spread to higher floors along the outside of the building. Fire-rated curtainwall, on the other hand, can help contain fire from spreading within a building, or to adjacent buildings.

With new steel curtainwall systems, the promise of achieving greater design flexibility with glass is coming true. New products are providing architects and designers with more choices than ever to expand their design concepts using large expanses of glass that allow in natural light, and that can provide for life and property safety. With modern curtainwall systems, form and function have come together.

About the Author: Jeff Razwick is the Director of Business Development for Technical Glass Products (TGP), a Kirkland, Washington-based supplier of fire-rated glass and framing systems, along with specialty architectural glass products. www.fireglass.com