Bylined article for Construction Executive (June 2010 issue)
Framing The Sun
The Role of Curtainwall and Window Framing in Daylighting
By Chuck Knickerbocker
Light passes through glass in windows, transoms, sidelites, curtainwalls, and skylights to illuminate building interiors. Given this, it is only natural that when discussing daylighting design and construction, much of the talk centers on the glazing material itself. Yet, this focus on glazing causes many to overlook a key contributor to effective daylighting design – the framing system.
Frame component strength determines the total allowable area of unobstructed glazing and in essence, defines the amount of light entering interior spaces. Profile shape impacts the quality of sightlines and depth of cast shadows projected on building occupants. Material makeup affects thermal expansion and energy performance, both critical elements of a properly balanced daylighting system.
As framing can establish the base for daylighting design, material selection is critical to the long-term performance and effectiveness of such applications. Over the last few decades, aluminum has been the popular framing material. It is lightweight, versatile and able to resist rust – a common problem in many early steel applications. However, advanced steel systems now offer a number of performance advantages over traditional aluminum assemblies.
Steel Framing For Modern Building Designs
Steel framing provides new options for curtainwall applications. Modern systems have sharper corners and crisper edges, without visible weld beads and fasteners to meet the aesthetic needs of sleek designs. More importantly, they overcome a key limit on design flexibility – fixed back mullions. Advanced steel systems can be manufactured with back mullions of virtually any style to meet various aesthetic requirements.
Modular systems are available with hollow-, I-, T-, U-, and L-shaped mullions or custom profiles. As a result, modern framing systems offer building professionals the versatility to use as a back mullion nearly any type of structural member, including glue-laminated beams, I-beams, and round steel tubes. This flexibility allows glazed curtainwalls to be incorporated into many different types of designs, from high-end urban storefronts and grand entrances, to rustic lodges.
Furthermore, exterior caps or interior back mullions of steel systems can be made from stainless steel, an increasingly popular material in modern design. Unlike traditional aluminum systems, this does not require cladding. Cladding increases the complexities of the framing system, including the need for additional fasteners and seams, along with increased bulk and increased labor and material costs.
Larger and Higher-Performance Glazing
One of the simplest methods for maximizing daylight is to increase the glazed area. This, of course, must also take into account potential heat gain and loss through the glass. High-performance triple- or quadruple-glazed units can provide needed energy efficiency, while capturing sunlight. Yet, due to the size and weight of such glazing applications, traditional aluminum framing systems may not be able to support the required loads.
Steel on the other hand, is well suited for large assemblies. It is nearly three times stiffer than aluminum with a substantially greater load capacity. When comparing steel to aluminum, for a given frame width and depth, steel framing can support larger glass panes than can aluminum of the same shape. In essence, steel allows for larger areas of uninterrupted glass with less framing to capture greater amounts of light.
This additional strength allows building teams to better achieve aesthetic daylighting designs where the glass, not the framing, takes center stage.
Reduced Shadows
Another advantage of steel’s stiffness is that it allows for much thinner frame profiles than traditional aluminum systems. 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. This reduces frame dimension size by approximately 25 percent.
The differences in frame dimensions can dramatically reduce the amount of shadows. Thinner frame profiles have a smaller surface area for the sun to move across during the day, and therefore result in the projection of smaller shadows. For retail or commercial spaces where such shadows may be a distraction to building users, narrower frames can help reduce this effect.
Crisp frame profiles can also increase internal viewing areas and accentuate the external appearance of modern curtainwall applications, which feature minimalist designs.
Improved Thermal Performance
An important part of any daylighting system is heat transfer. With extended exposure to large amounts of sunlight, poor daylighting design can lead to extensive cooling costs. Steel frames can help address this challenge, especially when combined with low-E glass or other energy efficient glazing.
The thermal conductivity of steel is approximately 74 percent less than aluminum. Steel also allows for narrower frame profiles, which provide a significantly smaller area of metal for heat to pass through. The result is both lower potential for heat gain/loss and interior condensation on the frames.
In addition, steel expands and contracts at a rate about 40 percent lower than aluminum. Its thermal expansion coefficient is comparable to glass and concrete, which allows steel to work with its surrounding materials to help ensure a sound building envelope as the temperature changes.
Because these complementary materials expand and contract closely together, fewer expansion joints are required in steel frames. On an aesthetic level, this helps create continuous and uninterrupted appearances across the frame face. Assemblies with fewer expansion joints are also easier for contractors to install.
Long-Term Performance
An inherent benefit of steel is its long-term durability. It resists scratches and dents to lower maintenance and new material costs, while its high strength helps prevent sagging or joint failure. This makes steel ideal not only for curtainwalls, but also for storefronts and door assemblies where heavy traffic can weaken the structure.
In previous years, these desirable benefits were offset by steel’s predisposition to rust. However, modern steel framing is available with double-sided pre-galvanization and factory-applied finishes to resist corrosion. 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. Many system designs also incorporate gaskets to isolate water from contacting the steel components. Material and environmental costs saved from system component replacements can be advantageous in the long term.
Other environmental factors for steel versus aluminum include the ability to use less material overall with a steel-framed system, as well as lower embodied energy. The amount of energy needed to acquire and process raw materials and manufacture steel is about 14 percent that required for aluminum.
Conclusion
Aluminum is a well-established material that will continue to be popular for framing a variety of glazing applications. However, for curtainwall and window assemblies designed to maximize capture of daylight, design and building teams may wish to consider the latest generation of steel framing. It has a variety of performance advantages, as well as high-end aesthetic offerings [see sidebar “Steel Framing for Modern Designs”].
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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