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10 FAQs on Advanced Steel Framing Applications

According to the World Steel Association, the construction industry accounts for up to 50 percent of global steel production. In large part, this is due to steel’s high strength-to-weight ratio and reputation as a structurally sound building element. From structural framing to roofing, building teams rely on steel to support some of the most demanding loads without warping, splitting or cracking.

Given steel’s strength, it seems like the natural material of choice to support glazed curtain walls. However, aluminum has been dominant since the mid-1900s—outperforming steel with its lightweight, versatile and corrosion-resistant properties. 

Today, due to developments in manufacturing processes, steel framing is reemerging as a high-performance material in glazed curtain walls. It surpasses traditional aluminum framing systems in terms of free span size, narrow sightlines and design flexibility.

Following are answers to 10 frequently asked questions for construction professionals interested in using steel to advance modern building design.

1. What free span dimensions are possible with steel curtain walls?

Coupled with new manufacturing techniques that enable framing profiles in long lengths and various shapes, steel’s load carrying capacity can accommodate greater free spans than traditional steel and aluminum systems.

For example, given 5-foot mullion spacing at a 30 pounds-per-square-foot wind load, an aluminum mullion of 2.5 inches by 7.5 inches (including the glass and exterior cap) can span 12.5 feet. Due to steel’s strength, a similarly sized profile of 2.4 inches by 7.6 inches would only deflect one-third as much under the same conditions. As a result, it is possible to increase the length of the steel mullion in this instance to span approximately 16.3 feet—a 30 percent increase over its aluminum counterpart. The increase in free span size can better support daylighting designs and help reduce the quantity of vertical and horizontal mullions, lowering installation costs.

Today, appropriately designed curtain wall systems incorporating long, continuous steel back members can handle up to 40-foot free spans in a single member without splicing. As with any curtain wall system, variables such as center-to-center location of verticals, span length and structural loads have an impact on the size of the framing member required to support the project’s performance criteria. 

2. What types of glazing units can steel curtain walls support?
Approximately three times stiffer than aluminum, steel provides the necessary support for heavy double- or triple-glazed units. It has a Young’s modulus (E) of about 29 million pounds per square inch (psi), compared to 10 million psi for aluminum. As a result, steel frames deflect less and provide a substantially greater load capacity. This allows glass sizes to be significantly larger in steel frames versus aluminum frames of the same shape. 

Depending on product selection, some steel systems can support glazing infills up to 3 inches thick. As a result, steel framing can support high-performance glazing to help offset solar heat gain in large glazed assemblies with marginal effects on the design intent.

3. How do the frame profiles of steel curtain walls and aluminum systems compare?
Newer steel frames provide many aesthetic advantages compared to traditional steel and aluminum assemblies. They can be much narrower, have well-defined edges (rather than rounded profiles) and have corner joints with no visible weld beads or fasteners.

In addition, because steel is stronger than aluminum, it can support the necessary load and deflection requirements of glazing with less material. For example, in a typical two-story curtain wall, unreinforced steel frames can be 1.75 inches wide and 5.75 inches deep, versus 2.5 inches wide and 8 inches deep for aluminum. This reduces frame dimension size by approximately 25 percent to take the focus off the frames and onto the intended views.

4. What types of back mullions are available with steel curtain wall systems?
Because manufacturers can produce steel frames in long lengths and various complex shapes, design teams can select from hollow-, I-, T-, U-, and L-shaped mullions or custom profiles.

When using a “steel veneer” (or glazing adaptor), the flexibility in back mullions provides building professionals with exceptional design freedom. The glazing adaptor can overlay nearly any modular back mullion system, enabling it to receive glass or any other glazing material. The adaptor is welded or fastened to the structural back member. As a result, it can attach to virtually any structural component that can support the curtain wall system’s weight and imposed loads (e.g., wind and snow). This includes using the veneer connector to attach the curtain wall to glued-laminated (glulam) beams, I-beams or round steel tubes, among other structural members.

5. What is the maximum height allowed for a steel curtain wall installation?
The maximum allowable height for single- and multi-story steel curtain wall installations depends on many factors, including applied loads and thermal expansion and contraction. However, the primary factor influencing the maximum height is where the curtain wall’s dead load will bear on the structure. For example, no open-splice joints are required in curtain wall members if the entire dead load can be applied to one floor. By comparison, if multiple floors support the curtain wall, then the curtain wall system must accommodate the live load deflection of each floor, usually resulting in open-splice joints in vertical curtain wall members.

Because loads and other factors can affect steel curtain wall height, it is important to assess each situation individually. Input from the architect, glazing subcontractor and structural engineer can help the project team configure system components within the limits of what the structural frame can handle.

6. How do modern steel curtain wall systems protect against moisture intrusion?
To help prevent moisture intrusion, manufacturers design advanced steel curtain wall systems with a continuous gasket across the full width of the frames. This prevents water in the glazing cavity from coming into direct contact with the steel back members. The installer assists in eliminating any water intrusion by sealing the lapped gasket joints at the horizontal-to-vertical connections during the normal installation process.

7. To what degree is corrosion a concern with modern steel curtain wall systems?

Double-sided pre-galvanization and factory-applied finishes are available to help preserve the curtain wall’s appearance and protect against corrosion. 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. Advanced steel curtain wall systems with gaskets that completely isolate water from contacting the steel components provide supplemental protection against water intrusion and corrosion.

8. What other options are available for steel curtain wall systems?

Due to manufacturing advances that allow the glass to properly adhere to the back supporting member, steel is available for use in structural silicone-glazed curtain wall systems.

Similar to veneer curtain wall systems, the structural silicone-glazed lites overlay (structurally attached with silicone) to the front of the steel back mullions or to aluminum adaptors. With carbon steel frames, either an aluminum extrusion or stainless bar has to be structurally adhered or applied to the steel to receive the structural silicone. With stainless steel framing members (not carbon steel), it is possible to apply the silicone directly to the base.

9. Are steel mullions compatible with point-supported glazing systems?
Steel mullions can function as the structural component of point-supported glazing systems. The glass in such systems is supported by custom connectors (i.e., spiders), which in turn structurally attach to the steel-supporting member. This helps ensure the structural component has a narrow sightline and a smooth, monolithic exterior.

Compared to a glass mullion or cable-supported glazing system, point-supported systems with steel structural components can reduce construction costs. For example, in the case of a cable wall, the building structure has to be increased to support the loads imposed by a cable system, whereas point-supported glazing systems with steel mullions require no increase in the building structure.

10. What finishes are available for steel curtain wall systems?
Framing members can be produced from stainless or carbon steel. The exterior caps are available in a range of finishes and materials, including extruded aluminum and stainless steel.

To match aesthetic design schemes, manufacturers can powder-coat or liquid-coat carbon steel systems using any coating suitable for architecturally exposed structural steel. For stainless steel framing, applied finishes in brushed, angel hair, swirl finishes and other patterns are available. Design teams also can specify custom steel profiles that feature patterns or designs cut out of the frame, including specialty shapes such as company logos.

Chuck Knickerbocker is the curtain wall manager for Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems and other specialty architectural glazing products. For more information, email chuckk@tgpamerica.com.

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