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This article originally appeared in the April 2012 issue of USGlass Only Online.

Step by Step: Working with Steel Curtainwall Systems

By Chuck Knickerbocker

The phrase "competition breeds excellence" aptly describes the development of steel and aluminum for curtain wall framing systems. The two materials have been vying for performance dominance since aluminum was adapted for use in buildings in the mid-1900s and challenged steel with its lightweight, versatile and rust-resistant properties.

While aluminum has been popular in curtain wall assemblies for many years, significant advances in manufacturing processes have led to steel's resurgence in leading building designs. Modern steel frames offer exceptional strength – but unlike earlier systems – are slender, versatile and available in a wide range of complex mullion profiles. Building teams can use such frames in captured and non-captured curtain wall systems that can stretch across building exteriors or be used to separate interior spaces – all with larger glass areas and less visible frame members.

Since steel's ability to enhance curtain wall design is driven by its performance capabilities, following are key attributes of such systems for glaziers to keep in mind as they work with other members of the design and building team.


Steel frames can improve a curtain wall’s aesthetic by reducing the number and visibility of vertical and horizontal mullions.


Steel curtain wall frames can help increase interior daylight levels by supporting large expanses of glazing.


Installation Considerations

Steel and curtain walls? This may appear to be an oxymoron. Curtain walls have to shed water, and steel and water are not an exactly friendly "match made in heaven" duo. To deal with this, well-designed steel curtain wall systems must include a continuous gasket across the full width of the frames to help prevent water in the glazing cavity from coming into direct contact with the steel back members. Nevertheless, it is still important to seal the lapped joints at horizontal-to-vertical connections to fully prevent water intrusion. Perimeter seals are strategically located to prevent contact between the steel and water, also.

Steel framing installs similar to typical stick-built aluminum curtain wall. Basic system components include shear blocks, pressure plates, and cover caps for connection of the framing components. These 'plug-and-screw' systems allow the framing joints to be watertight and readily assembled without welding.

To preserve the curtain wall's appearance and protect against corrosion, consider selecting modern steel framing with double-sided pre-galvanization and factory-applied finishes. For instance, steel framing may be prefinished with liquid zinc and top-coated with a durable primer and finish color to match virtually any design scheme. A variety of finishes are available, including powder coating and liquid-applied finishes specifically formulated for steel applications. Typical finishes applied to aluminum, such as liquid applied fluoropolymers, are not suitable for steel. While field painting is an option, factory painting helps protect the steel during transportation and installation.

As with any specialty glass and framing system, product nuances vary by manufacturer. Consult the supplier's documentation for specific installation instructions, particularly for advanced steel systems, such as structural silicone glazed steel curtain walls.

The Strength Advantage

Steel is strong and able to support larger free spans of glazing than traditional aluminum assemblies. Historically, curtain wall framing systems were unable to capitalize on steel's strength due to manufacturing limitations that restricted frame profile length and shape. Manufacturers have overcome these design restraints by using a roll-forming technique in which continuous steel coils are forced through dies and laser-welded to create framing profiles in long lengths and various shapes.

Coupled with steel's strength, these production advances enable new-generation steel frames to accommodate greater free spans than traditional steel and aluminum systems. For example, given 5-foot mullion spacing at a 30-lbs/sf-wind load, an aluminum mullion of 2.5 by 7.5 inches, including the glass and exterior cap, can span 12.5 feet. A similarly sized steel frame profile of 2.4 x 7.6 inches would only deflect one-third as much under the same conditions. Due to steel's strength, it is possible to increase the length of a steel mullion in this instance to span approximately 16.3 feet – a 30 percent increase over its aluminum counterpart. The benefits of increasing the glazed span are twofold: 1) Large sections of glazing can improve a curtain wall's aesthetic by reducing the quantity of vertical and horizontal mullions, and 2) they also better support daylighting designs, allowing more light to reach interior spaces.

Steel curtain wall assemblies also provide the necessary support for heavy double- or triple-glazed units, which are increasingly desirable given their ability to help moderate heating and cooling energy consumption associated with increasing the glazed area. 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. As such, it deflects less under applied loads, allowing for a significant increase in the types and sizes of glazing it can support. Steel systems are available that can support glazing infills up to 3 inches thick.

Steel's stiffness allows for narrow frame profiles. The face frame for steel can be as narrow as 1.75 inches, whereas a typical aluminum face frame is 2.5 inches wide. From an aesthetic standpoint, reduced depth dimensions help shift the visual focus from the frames to the glazing. They also improve sightlines, providing building occupants with more unrestricted views to the outside.


Steel back mullions can be formed into virtually any style to meet various aesthetic requirements.


New-generation steel frames add elegance to building designs.


Improved Energy Performance

Many framing systems have high thermal conductivity compared to other elements of the building envelope, and are susceptible to heat gain/loss. This remains a challenge even for systems using low-E glass or other energy-efficient glazing, such as insulated glass units (IGUs). With a thermal conductivity approximately 74 percent less than aluminum (approximately 31 Btu/hour for steel versus approximately 118 Btu/hour for aluminum), steel frames can help mitigate heat exchange. Moreover, the unique design of some modern steel assemblies does not require a thermal break; such systems help lower the potential for heat transfer and interior condensation on the frames by providing a significantly smaller area of metal for heat to pass through.

Steel frames also have a relatively low thermal expansion rate, with a thermal coefficient of approximately 7.3 in/in. 0F x 10-6. This is comparable to glass and concrete, which have thermal expansion coefficients of 5.0 and 8.0. As such, steel can work in close conjunction with surrounding building materials to help ensure a sound envelope as the temperature changes. Because steel, glass and concrete expand and contract at similar rates, steel frames require smaller expansion and/or perimeter sealant joints – if any – than traditional aluminum assemblies. This helps create a continuous and uninterrupted glazing aesthetic across the frame face.

Enhanced Design Flexibility

Steel framing overcomes a key design flexibility limit of traditional curtain wall assemblies – namely, fixed back mullion shapes, such as tubular back members. Advanced steel systems can be manufactured with back mullions of virtually any style to meet various aesthetic requirements. Mullion design options range from hollow-, I-, T-, U-, or L-shaped profiles to custom profiles that can feature one-of-a-kind patterns or designs cut out of the frame, including specialty shapes (imagine cut-outs in the mullion shaped like a company logo, for example).

The benefits of steel's design flexibility are further realized in veneer curtain wall systems, where the unique configuration of the glazing adaptor—or 'steel veneer'—allows it to overlay nearly any modular back mullion system to receive glass or other glazing materials. In such applications, the steel veneer is welded or fastened through a 'plug-and-screw' connection to the structural back member, allowing attachment to virtually any structural component that can support the curtain wall system’s weight and imposed glazing loads (e.g. wind and snow loads). Examples include glued-laminated (glulam) beams, I-beams, or round steel tubes, among other structural members. In appropriately designed systems, the option to incorporate virtually any back member allows for an increase in free spans. For example, a curtain wall system incorporating long, continuous steel back mullions can handle up to 40-foot free spans in a single member without splicing.

Even greater design freedom is possible with structural silicone glazed steel curtain walls. Similar to veneer curtain wall systems, the structural silicone glazed lites overlay (are structurally attached with silicone to) the front of the steel back mullions. Silicone sealant blocks water and air infiltration into the assembly, and transfers wind loads to the supporting steel curtain wall. Such systems can accommodate free spans up to 40 feet (nominal) in single members, allowing for large stretches of glass that are smooth and uninterrupted in appearance. To further reduce the visual impact of frames on a glass-clad façade or wall, sightlines from back members can be as narrow as 1.75 inches. As with all curtain wall systems, the necessary size for the back member depends on glass size, design loads, and connection or anchor points.

Steel mullions can also function as the structural component in point-supported glazing systems. In these instances, the glass is supported by custom connectors, such as 'spiders,' which are in turn connected to the glass and structurally attached to the steel supporting members. Such assemblies are lower cost than point-supported glazed cable systems, and can help make a seamless curtain wall aesthetic cost effective.

Chuck Knickerbocker is the curtain wall manager for Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems, along with other specialty architectural glazing products. With more than 30 years of curtain wall experience, he has worked with many architects, building owners, and subcontractors from development of schematic design through installation. Knickerbocker chairs the GANA Building Envelope Contractors (BEC) Division Technical Committee. http://www.tgpamerica.com/

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