Light passes through glass in windows, transoms, sidelites, curtainwalls and skylights to illuminate building interiors, making choices in glazing materials a hot topic. Yet, architects and contractors shouldn’t overlook a key contributor to effective daylighting design: the framing system.
Frame component strength determines the total allowable area of unobstructed glazing and defines the amount of light entering interior spaces. Profile shape impacts the quality of sightlines and the depth of shadows cast on building occupants. Material makeup affects thermal expansion and energy performance, both of which are critical elements of a properly balanced daylighting system.
While framing establishes the base for daylighting design, material selection affects the long-term performance and effectiveness of such applications. During the last few decades, aluminum was the most popular framing material. It is lightweight and versatile, and it resists rust—a common problem in many early steel applications. However, advanced steel systems now offer a number of performance advantages over traditional aluminum assemblies.
Large, High-Performance Glazing
One of the simplest methods for maximizing daylight is to increase the glazed area. Of course, this also must 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 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 aluminum. 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 achieve better aesthetic daylighting designs in which the glass, not the framing, takes center stage.
Fewer Shadows
Steel’s stiffness allows for much thinner frame profiles than traditional aluminum systems. For example, in a typical two-story curtainwall, unreinforced steel frames can be 1¾ inches wide and 5¾ inches deep versus 2½ inches wide and 8 inches deep for aluminum. This reduces frame dimension size by approximately 25 percent.
Thinner frame profiles have a smaller surface area for the sun to move across during the day, which results in the projection of smaller shadows—a benefit for retail or commercial spaces where shadows may distract customers.
Crisp frame profiles also can accentuate the aesthetics of modern curtainwall applications that 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 higher 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, and steel’s narrower frame profiles provide a significantly smaller area of metal through which heat can pass. The result is lower potential for heat gain/loss and less interior condensation on the frames.
In addition, steel expands and contracts at a 40 percent lower rate than aluminum. Its thermal expansion coefficient is comparable to glass and concrete, which allows steel to work with surrounding materials to ensure a sound building envelope as the temperature changes.
Because these complementary materials expand and contract together, fewer expansion joints are required in steel frames. Aesthetically, this helps create continuous and uninterrupted appearances across the frame face. Assemblies with fewer expansion joints also are easier for contractors to install.
Long-Term Performance
An inherent benefit of steel is its long-term durability. It resists scratches and dents, lowering maintenance and new material costs, and 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 benefits were offset by steel’s predisposition to rust. However, modern steel framing with double-sided pre-galvanization and factory-applied finishes resists corrosion. For instance, steel framing may be prefinished with liquid zinc and then top-coated with a durable primer and finish color to match almost any design. Many system designs also incorporate gaskets to isolate water from contacting the steel components.
Other environmental factors make steel a smart choice. Steel-framed systems use less material overall, and the amount of energy needed to acquire and process raw materials and manufacture steel is about 14 percent lower than for aluminum.
For curtainwall and window assemblies designed to maximize daylight, design and building teams should consider the latest generation of steel framing. It offers a variety of performance advantages, as well as high-end aesthetics.