The phrase European design is synonomous with high performance. Whether it is the precision engineering of a luxury German automobile or the exacting assembly of an Italian racing bicycle, several cutting-edge manufacturing techniques have emerged from Europe in recent years. These include techniques to create architectural products–especially metal building systems and components.

A case in point is curtainwall framing. Advanced steel curtainwall systems pioneered in Europe and used there for years now are fabricated in North America.

Using a roll forming process, steel framing can be produced in shapes and sizes not possible with older technologies used for hollow-metal steel and hot-rolled steel. In roll forming, continuous steel coils are processed through a series of dies and then are laser-welded. Long-length framing members thus can be created in a multitude of narrow profile shapes similar to those that previously only have been available with aluminum framing.

Such steel systems offer key design advantages over traditional aluminum curtainwall assemblies, including the ability to support larger areas of glazing, narrower framing sightlines and improved thermal performance. And, modern steel framing systems overcome the limitations of earlier steel frames—notably susceptible to corrosion.

Aluminum likely will continue to be a popular material for curtainwall framing as it is lightweight, relatively lowcost and widely available. However, for applications requiring higher performance— especially instances in which larger open areas of glass and narrower frame profiles are desired—architects and designers may wish to specify steel framing. Modern manufacturing processes now enable the beneficial qualities of steel to be incorporated into curtainwall framing of nearly any configuration.

When considering materials for curtainwall framing, the acronym S.T.E.E.L. provides an easy reminder of the metal’s primary performance characteristics: Stiffness, Thermal Expansion, Esthetics, Energy Performance and Longevity.

Stiffness and Expansion
Since a goal of many curtainwall designs is to maximize the area of open glass—for greater capture of natural light and a clean, unobstructed view— steel framing can help expand design flexibility.

Steel curtainwall framing has a windload capacity approximately three times greater than typical aluminum assemblies. As a metal, steel is much stiffer than aluminum, with a Young’s modulus (E) of about 30 million pounds per square inch (psi) compared to 10 million psi for aluminum. Steel frames deflect less under load, allowing designers to create curtainwalls with either larger spans of glass or narrower frame profiles. By comparison, aluminum curtainwall assemblies typically require shorter spans or smaller glass sizes to prevent sagging or bowing.

For a given frame width and depth, steel can hold glass lites about three times larger in surface area than can aluminum (see Fig. 1). Spans in excess of 20 feet are possible without additional reinforcements. The actual area and span that can be achieved depends on the specific application, and is affected by design factors such as the shape of the frame profile and wall thickness.

In cases where the profile of the framing is more important than the size of the glass panes, architects may also want to consider steel. Under a given set of load, deflection, and glass panel size requirements, steel framing can offer a narrower mullion profile—for sleek and clean sightlines (see Fig. 2). In a typical two-story curtainwall, steel framing can be specified with a 1 ¾-inch mullion face width compared to 2 ½ inches for aluminum. Framing depth is also less—5 ¾ inches for steel versus almost 8 inches for aluminum. In both dimensions, steel framing is more than 25 percent narrower. Some steel systems also allow for internal reinforcements to be added to reduce the frame depth further.

In addition to stiffness, another inherent property of steel is its minimal thermal expansion. Steel expands and contracts at a rate about 40 percent lower than aluminum. As a result, steel curtainwall systems require fewer, if any, expansion joints compared to typical aluminum assemblies. Because steel’s thermal expansion coefficient is comparable to that of both glass and concrete, it works well with those materials to help provide a sound building envelope.

The ability to design with fewer or no expansion joints enables architects to create a continuous appearance across the face of the curtainwall frames and to minimize joint size and expansion issues with surrounding construction materials. And, because the frames have fewer components, they are easier to install.

Aesthetics
A key advantage of aluminum curtainwall framing has been its aesthetics, with crisp frame profiles. Older hollow-metal steel systems, on the other hand, used manufacturing techniques that resulted in bulky wrap-around frames. As such, they were not well suited for curtainwalls. With advanced roll-forming, however, steel framing is much narrower now; it can have “sharp” edges, rather than rounded profiles, and corner joints without visible weld beads or fasteners.

Steel framing systems also allow greater design flexibility for a curtainwall’s back mullion. With a typical aluminum assembly, the back mullion is often limited to a square shape. Modular steel framing systems are available with back mullions of virtually any profile, allowing designers to use I-beams, round steel tubes, glulam beams and other types of structural member as a curtainwall back mullion. With I-, T-, U- and L-shapes, among others, modular steel curtainwall systems can attach to almost any structural component that can support their weight.

For building designs that include stainless steel, steel curtainwall can be incorporated seamlessly. The exterior cover caps or interior back mullions of steel systems can be made from stainless steel. Aluminum systems, by comparison, require cladding if the look of stainless steel is desired. The addition of cladding increases the complexities of the framing, including additional fasteners and seams, as well as increased bulk.

Energy Performance
To help reduce the heat gain and loss through a glazed curtainwall, steel framing offers two potential advantages. First, steel itself has about a 74 percent lower thermal conductivity than aluminum–31 British thermal units (Btu) per hour compared to 118 Btu per hour. This provides a significantly lower possibility of heat transfer through the framing, as well as reduced potential for interior condensation. Second, the ability to use narrower frame profiles reduces the surface area through which heat can pass.

For overall energy performance in the curtainwall, insulating glass units incorporating clear low-emissivity (low-E) glass can be used. Such assemblies can provide U-values of approximately 0.31, which greatly surpasses the thermal performance of many aluminum curtainwalls.

Longevity
Another inherent property of steel is its long-term durability. It resists scratches and dents, reducing maintenance costs. Steel also can withstand hard use better than aluminum, with less risk of sagging or joint failure. These attributes make steel a good choice not only for curtainwalls, but also for storefronts and door assemblies where high foot traffic can impact the framing.

Modern steel framing systems also overcome corrosion–a key reason steel window framing fell out of favor in the mid-20th century. Steel framing can be pre-galvanized with liquid zinc and then top-coated with a primer and finish color to match virtually any design. And, watertight seals designed as part of the framing make steel assemblies suitable for either interior or exterior use.

Jeff Razwick is vice president of business development for Technical Glass Products (TGP), a Kirkland, Wash.-based supplier of fire-rated glass and framing systems and specialty architectural glass products. Mr. Razwick’s opinions are solely his own and not necessarily those of this magazine.

Architects' Guide to Glass & Metal
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