As seen in Architectural West, May/June 2009
Steel Fenestration Frames Come Full Circle
By Chuck Knickerbocker
In the early 20th century, steel became a widely accepted and popular material for window framing. It was strong, integrated well with other building systems, and was readily available. These properties were also critical to the development of glass curtainwalls, including the pioneering installation in San Francisco’s Hallidie Building.
Yet, as often happens in architecture, time exposes weakness. Steel was heavy, and in early applications, prone to rust. By mid century, it fell out of favor for fenestration and was replaced by aluminum – a lightweight and versatile material that could be readily formed into a number of mullion profiles.
Today, because of significant advances in manufacturing processes, steel framing has come full circle for glazed windows and doors, as well as curtainwalls. Manufacturers can form it into a wide range of framing profiles, overcoming the limited choices inherent with earlier steel framing systems.
Modern steel framing offers a number of technical advantages over traditional aluminum assemblies. These include higher stiffness, lower thermal expansion, better energy performance and greater longevity.
European architects have used steel framing for decades to allow larger spans of uninterrupted glass and minimal frame dimensions – capturing more natural light and improving aesthetic design. Advanced steel systems are now available to design professionals across North America.
Stiffness
Steel is nearly three times stiffer than aluminum and allows for either larger spans of uninterrupted glass or narrower framing profiles. Its load capacity is greater than that of typical aluminum assemblies, deflecting less under wind loads and the weight of the curtainwall itself.
Thus, for a given frame width and depth, steel framing can support larger glass panes than can aluminum of the same shape. This is an important design consideration for many multi-story curtainwalls, where the glass, rather than the framing, is the primary aesthetic element.
Another advantage of steel’s stiffness is seen in narrower frame profiles. For example, in a typical two-story curtainwall, unreinforced steel frames can be 1 ¾” wide and 5 ¾” deep versus 2 ½” wide and 8” deep for aluminum. By reducing frame dimension profiles by approximately 25 percent, crisp sightlines allow the glazing to take center stage.
Thermal Expansion
Steel expands and contracts at a rate about 40 percent lower than aluminum. Because of this, steel framing requires fewer, or smaller, expansion joints compared to traditional aluminum assemblies.
By reducing the amount of expansion joints, architects can create a continuous and uninterrupted appearance across the frame face. This not only improves the framing aesthetics, but also creates assemblies that are easier to install.
With a thermal expansion coefficient comparable to glass and concrete, steel also works as a complementary material to help ensure a sound building envelope.
Energy Performance
For environmentally conscious building designs, steel frames also significantly reduce heat gain and loss. The thermal conductivity of steel is approximately 74 percent less than aluminum. The result is lower potential for both heat transfer and interior condensation on the frames.
To optimize overall energy performance in steel curtainwall assemblies, designers can specify insulated glass units (IGUs) incorporating clear, low-emissivity (low-e) glass. Steel systems with IGUs can provide U-values of approximately 0.31, which greatly surpass the thermal performance of many aluminum curtainwalls.
Longevity
Another inherent property of steel is its long-term durability. For steel framing systems there are two advantages. First, steel framing resists scratches and dents, for lower maintenance costs and improved long-term aesthetics. Second, its ability to resist sagging or joint failure makes it ideal not only for curtainwalls, but also for storefronts and door assemblies where high foot traffic can weaken the structure.
To overcome corrosion, modern steel framing is available with double-sided pre-galvanization and factory-applied finishes. For instance, steel framing may be prefinished with liquid zinc and then top-coated with a durable primer and finish color to match virtually any design. Also, the system design incorporates gaskets to isolate any water from contacting the steel components.
Steel Frames In Modern Glazed Design
Advanced roll forming in steel manufacturing enables sharp corners and crisp edges, without visible weld beads and fasteners.
To further expand design freedom, steel-framing systems can now be manufactured with back mullions of virtually any style, surpassing traditional aluminum curtainwall design. Modular systems are available with hollow-, I-, T-, U-, and L-shaped mullions or custom profiles. As a result, modern framing systems offer architects the versatility to use as a back mullion nearly any type of structural member, including glue-laminated beams, I-beams, and round steel tubes. Exterior caps or interior back mullions of steel systems can be made from stainless steel.
The clean, crisp lines of these steel profiles when used as the primary framing members provide a unique attribute in that the structural steel is not hid under aluminum or other metal cladding. Eliminating cladding simplifies the framing by reducing the number of fasteners and seams.
Modern steel framing systems offer a multitude of technical and aesthetic design advantages over traditional aluminum framing assemblies. They provide performance benefits that enable design professionals to push the limits of glazed curtainwall design, and to provide more durable and aesthetic window and door framing.
Chuck Knickerbocker is the curtainwall manager for Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems, along with specialty architectural glazing products. With over 25 years of curtainwall experience, Chuck has successfully worked with numerous architects, building owners and subcontractors from development of schematic design through installation. www.tgpamerica.com, (800) 426-0279
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