Lightweight, versatile and rust resistant, aluminum has surpassed steel as the curtainwall framing material of choice for decades. Now, steel is making a comeback. Significant manufacturing advances have enabled building and design teams to overcome the limited choices inherent with earlier steel framing systems, including susceptibility to corrosion and fixed back mullions.
Using a rollforming technique in which continuous steel coils are forced through dies and then laser-welded, manufacturers can produce steel window frame members in long lengths and various complex shapes. Compared to traditional steel and aluminum assemblies, the new generation of steel frames are much narrower, can have sharp edges rather than rounded profiles, and can have corner joints with no visible weld beads or fasteners. The result is slender, strong frame components that can stretch across building exteriors or be used to separate interior spaces-all with larger glass areas and less visible members. Following are key design considerations for utilizing new generation steel framing in curtainwall systems.
Utilize steel’s strength
Modern manufacturing processes enable steel frames to have an aesthetic and system design similar to aluminum. However, since steel is inherently stronger than aluminum, it can support greater free spans than an aluminum system of similar dimensions and applied loads. (Steel is approximately three times stiffer than aluminum, with a Young’s modulus [E] of about 30 million psi, compared to 10 million psi for aluminum.)
Consider the following scenario: Given 5-foot mullion spacing and a 30 psf wind load, an aluminum mullion of 2.5 by 7.5 inches, including the glass and exterior cap, can span a total of 12.5 feet. By comparison, if that aluminum mullion was steel, sized at 2.4 by 7.6 inches, it would only deflect one-third as much under the same conditions. As a result, the length of the steel mullions can be increased to span almost 16.3 feet-a 30 percent increase over their aluminum counterparts. In some instances, appropriately designed curtainwall systems incorporating long, continuous steel back mullions can handle up to 40-foot free spans in a single member without splicing. Steel also provides the necessary support for heavy double- or triple-glazed units (up to 3 inches thick). Incorporating high-performance glazing, such as triple-glazed units, can help offset the energy costs associated with increasing the glazed area.
Another benefit of steel is its ability to meet necessary load and deflection requirements with less material than aluminum. For example, in a typical two-story curtainwall, un-reinforced steel frames can be 1.75-inch-wide and 5.75-inch deep, versus 2.5-inch-wide and 8-inch-deep for aluminum. This reduces frame dimension size by approximately 25 percent. Aesthetically, reducing frame profile size helps accentuate the minimalist appearance of modern curtainwalls by shifting the visual focus off the frames and onto the glazing.
Go modular
Advanced steel curtainwall systems overcome the challenge of fixed back mullions through a ‘steel veneer,’ or glazing adaptor. The glazing adaptor can overlay onto nearly any modular back mullion system, enabling it to receive glass or any other glazing material. Since the steel veneer is fastened through a ‘plugand- screw’ connection to the structural back member, it can attach to virtually any structural component that can support the curtainwall system’s weight and imposed glazing loads (e.g. wind and snow loads).
The flexibility to use back mullions of different shapes and sizes provides building professionals with tremendous design freedom. For example, design teams can select from hollow-, I-, T-, U-, and L-shaped mullions or custom profiles. Another option is to use the veneer connector to attach the curtainwall to glued-laminated (glulam) beams, I-beams, or round steel tubes, among other structural members.
Additional design freedom is possible with structural silicone glazed-steel curtainwalls. Similar to veneer curtainwall systems, the structural silicone glazed lites are structurally attached with silicone to the front of the stainless steel back mullions or to aluminum adaptors. Steel mullions can also function as the structural component in point-supported glazing systems using custom connectors such as “spiders,” which in turn structurally attach to the steel supporting member.
Design for durability
Some steel curtainwall systems offer high anti-corrosion protection-such as a doublesided pre-galvanization with a factory-applied curable primer and finish color-to further protect against corrosion. After the framing components are fabricated and before they are installed, they can be either powder- or wet-coated to match any desired color scheme. In addition, numerous stainless steel options are available, which is a common desire for high-end storefronts or applications in coastal areas.
Advanced steel curtainwall systems include a continuous gasket across the full width of the frames to help prevent water from coming into direct contact with the steel back members. Back mullions are typically made of carbon or stainless steel, and cover caps made of rolled stainless steel or aluminum extrusions. The installer can further prevent water intrusion for a long-lasting system by sealing the lapped joints at the horizontal-to-vertical connections.
Chuck Knickerbocker is the curtainwall manager for Snoqualmie, Wash.-based Technical Glass Products (TGP). Knickerbocker also chairs the Glass Association of North America
(GANA) Building Envelope Contractors (BEC) Division Technical Committee. For more information, visit www.tgpamerica.com.