Steel curtain walls that get the curtain call

by Katie Daniel | March 10, 2016 3:54 pm

by Chuck Knickerbocker
Modern curtain wall systems require structural supports as strong as they are versatile to keep pace with today’s increasingly large free spans, challenging angles, and sophisticated glass-clad aesthetics. While steel curtain wall frames have long met strength criteria, they have only recently provided the necessary design flexibility.

Steel’s reputation as the workhorse of the modern building industry is well-earned. From soaring bridges to skyscrapers, it is able to withstand some of the most demanding structural loads without deforming, splitting, or cracking. Despite its exceptional performance, manufacturing limitations have prevented its widespread use as the primary framing material in glazed curtain wall assemblies. However, in recent years, advanced processing methods have overcome this challenge.

Using cold-roll forming techniques developed in Europe, manufacturers feed continuous steel coils through dies, forming basic, closed profiles that can frame the curtain wall. Another option is to laser-cut long (i.e. up to 14.9 m [49 ft]) steel plates (3.8 to 38 mm [0.15 to 1.5 in.]), which are then laser-welded together into a range of different shapes, from C, T, I, H, and L profiles, to custom shapes. While the welds on flat surfaces are ground smooth, the laser welds on the inside corners (of Ts or I-profiles, for example) are continuous and smaller (radius < 1.7 mm [0.07 in.]) than conventional fabrication welds (e.g. hand-done fabrication welds). The process of welding bars into a single shape allows project teams to analyze and use the shapes as composites, rather than assembled members.

Additionally, steel curtain wall suppliers have developed all component parts to the point where a complete system is often available, including:

• connection detailing and hardware;
• gasketing;
• exterior pressure plates and cover caps; and
• complementary door and entry systems, as well as detailing.

Complete curtain wall systems help simplify and standardize fabrication and installation methodologies, while still meeting the higher performance criteria required of modern curtain wall constructions—regardless of the framing material selected. For example, water resistance can be as much as 25 percent greater in an off-the-shelf steel curtain wall system than that of a conventional extruded aluminum curtain wall system. Also, air penetration is almost non-existent in steel curtain walls.

For design teams, these developments have led to strong, slender, and versatile steel framing members and component parts with a significant load-carrying capacity and a sleek aesthetic. With appropriate design and specification, they can help building teams push the limits of curtain wall design while streamlining time and cost (Figure 1). To aid in this process, this article offers considerations for using steel to its full capacity in seven complex curtain wall applications.

Steel curtain walls with expansive free spans
Steel is strong and has a high load-carrying capacity with a Young’s modulus – of approximately 207 million kPa (30 million psi), compared to aluminum, at approximately 69 million kPa (10 million psi). This allows design professionals to specify steel curtain wall systems with greater free spans (be it vertical height and/or horizontal module width) and reduced frame dimensions than conventional aluminum curtain walls with similar dimensions and applied loads.

Generally, a steel profile can be two-thirds the size of a comparable aluminum profile while meeting the same curtain wall performance criteria. Steel’s inherent strength allows it to be used in non-rectangular grids, where the length of the frame member might be longer than is typically required in conventional, rectangular horizontal/vertical curtain wall grids.

Assemblies using laser-cut or welded composite members with free spans greater than 6 m (20 ft) typically require use of a glazing adapter or ‘glazing veneer’(Figure 2). The glazing veneer is versatile, and can be applied not only to these steel profiles, but also to virtually any structural component that will adequately support the curtain wall’s weight and imposed glazing loads (e.g. wind and snow loads).

For example, the glazing veneer can attach to a range of different structural materials, including glued-laminated timber (glulam) beams, steel, stainless steel, and aluminum. Additionally, due to advanced steel processing methods, it can attach to steel mullions of different shapes, including hollow-, I-, T-, U-, or L-channels, and custom mullions.

The flexibility to select from a wide range of back members not only expands the design team’s aesthetic freedom, but it can also provide internal profile reinforcement—as opposed to conventionally built up reinforcing—to increase the allowed free spans. For example, appropriately designed curtain walls incorporating long, continuous steel back members can handle up to 12-m (40-ft) free spans in a single member without splicing or additional internal reinforcing. Single spans are typically limited to a maximum of about 12 m (39.4 ft) due to shipping and finishing constraints. However, up to 12.3-m (40.3-ft) steel lengths are available at a premium. If longer length members are desired, it is possible to splice individual members together to form continuous frame members. If splices are permitted in these longer, multiple span runs, the total length of the framing member(s) is unlimited.

When working with curtain walls featuring these expansive free spans, the loading will be higher because the contributory areas are larger. For example, a 1.5 x 3-m (5 x 10-ft) window weighing 75 kg/m² (15 lb/sf), subject to a 241-kPa (35-psf) wind load, applies an approximate gravity load of only 340 kg (750 lb) at the base of the vertical, and 400 kg (875 lb) of lateral load (due to wind load) to each end of the vertical.

By comparison, a 1.5- x 11-m (5- x 35-ft) curtain wall contributory area will introduce approximately 1190 kg (2625 lb) of gravity load at the base of the vertical, and 1389 kg (3063 lb) of lateral load into the anchors at each end of the vertical. The building structure must adequately bear these loads, and the design team must size the supporting structure accordingly, as well as the curtain wall anchors tying the wall to the structure.

Steel curtain walls with large glazed lites
Due to steel’s strength, it can support larger glazed lites than aluminum frames of the same shape. For design professionals working to increase the free span or glass module size, this means it is possible to specify double- or triple-glazed insulated glass units (IGUs) up to 76 mm (3 in.) thick without revising the system to support the intended glazing. This advantage makes advanced steel curtain walls a powerful alternative to fully utilize the massive glass units being introduced by today’s glass manufacturers. It is also an effective way to help increase the natural admission of light, and balance it with the energy costs associated with artificial light.

Additionally, because steel has a thermal conductivity of approximately 32,700 joule (31 Btu) per hour, compared to aluminum at approximately 124,500 joule (118 Btu) per hour, it can help reduce potential for heat gain and loss through the glass and frames. This is a critical performance benefit as a system’s overall thermal efficiency is often substantially less effective where the captured or retained glass edge meets the supporting frame. Where frames with low thermal conductivity are critical to project goals, specifiers can select from advanced steel frames without a traditional thermal break. Such frames use less metal to support the glazing than traditional aluminum frames—further reducing the pathway for heat transfer.

When incorporating large glass lites, specifiers can also work with the manufacturer or supplier to determine the minimum required frame width. Steel can meet curtain wall load and deflection criteria requirements with less material than aluminum, enabling design teams to preserve thin frame dimensions and narrow sightlines, even as the glass size or unit thickness is increased.

Multi-story steel curtain walls
Steel’s strength and versatility also enables multi-story curtain walls (Figure 3). As is the case with any curtain wall, the maximum allowable height depends on factors such as applied loads, and thermal expansion and contraction. However, the primary factor influencing the allowed height is where the curtain wall’s dead load will bear on the structure. This determines the length of individual pieces of vertical framing, the number of splices (if required), and how the glass accommodates the movement within the curtain wall system at splice locations.

For example, rather than have a six-story curtain wall’s full weight (about 1.1 kPa [25 psf] due to thick glass) forced on the lowest level of the structure, the project engineer required each floor carry the imposed curtain wall dead load. To accommodate the load, the steel mullion was sized at 60 x 180 x 2.5 mm (2.3 x 7.1 x 0.1 in.) thick, and required a 12.7-mm (1/2-in.) dynamic splice joint at every floor. Additionally, to ensure the live load deflection did not exceed 0.95 mm (0.375 in.) at these conditions, it was necessary to change the sizing of the structural steel frame to limit it. Exact conditions often vary between projects; this allows larger splice joints and/or greater structure flexibility. It is important to communicate these requirements in the construction documents for review by the steel system detailers.

Another important consideration during the specification process is the impact of the structure’s long-term potential movement at the points where the curtain wall anchors to the structure. column or structure foreshortening (shrinkage) will generally not be included in the curtain wall specifications. However, this information is critical for the curtain wall designer to know in order to properly design and specify anchorages, splice connections, and sealant perimeter joints.

Segmented steel curtain wall systems
In buildings where it is desirable to install a curtain wall with a curved aesthetic, design professionals can use steel mullions to create a segmented steel curtain wall system (Figure 4). For off-the-shelf standard steel systems, the degree of turn is typically limited to a maximum of 10 to 12 degrees at any single mullion. Custom steel systems can have a greater degree of turn to help design teams achieve a tighter radius. However, manufacturing order minimums can prove cost-prohibitive in small- and mid-sized curtain wall projects.

When working with segmented curtain walls with a tighter radius, the mullions are typically spaced more closely for standard steel systems. For a curtain wall with a 4.5-m (15-ft) radius that goes around a 90-degree corner, the turn is typically limited to 10 degrees for standard steel systems. In this instance, the curtain wall would require approximately nine mullions spaced 0.76 m (30 in.) apart on center (oc). If the radius was opened up to 9.4 m (30 ft), the nine mullions would be spaced 1.5 m (62 in.) apart.

The degree of turn at individual mullions, as well as glass thickness (specifically, the ability to maintain the minimum glass bite dimensions at glass edges), will also determine whether it is possible to use standard pressure plates and covers. For example, if the glass thickness or degree of turn exceeds the capacity of the standard pressure plates and covers, manufacturers can furnish custom shapes to meet project demands.

While steel and glass can be easily bent or curved (albeit at a high cost), the following issues commonly preclude their use and result in segmented steel curtain wall systems. Firstly, the way cold-roll formed steel is processed does not curve in a similar fashion to hot-rolled or standard steel shapes. Secondly, other curtain wall components—such as the glazing veneer—will not hold their shape when rolled, compromising the system’s functionality. Lastly, each component has its own set of bending or forming tolerances and many are not compatible with each other. For example, one component may go to the ‘positive’ side of its allowable tolerance when curved, while an adjoining component may be manufactured at the ‘negative’ side of its tolerance.

To date, coordination of tolerances to eliminate differences between the various material suppliers has not been possible. However, when a curved curtain wall or window is assembled, the system generally relies on a sealant to account for any such differences and to ensure air and watertightness when in service.

Structural silicone glazed steel curtain wall systems
Where a frame-free, glass-clad aesthetic when viewed from the exterior is desired, a popular option is to install structural silicone glazed (SSG) steel curtain wall systems. In such systems, structural silicone attaches silicone glazing-compatible glass lites to modular steel-back mullions. This setup utilizes steel’s strength while eliminating the need for exterior pressure plates and caps, which typically retain the glass to the frame. For design professionals, this opens the door to curtain walls with soaring free spans (up to 12 m [40 ft] nominal free spans in single members) and an uninterrupted exterior glass surface.

As is the case with most curtain wall systems, the size of the back member depends on glass size, design loads, and connection or anchor points. One additional factor specific to SSG systems is the amount of sealant required to hold the glass to the frame. The size of a glass lite and its anticipated design wind load determine the amount of silicone that must be in contact with the glass and framing. This dimension is called the bond line width. In addition to the amount of gasket or glazing tape and the exterior weather joint between lites of glass, the bond line width generally sets the width of the framing member.

Depending on the type of SSG system, the waterproofing methodology may also be different than conventional captured curtain walls. For example, if water is not allowed within the glazing pocket past the exterior face of the glass, it is possible to employ a barrier system where the sealant joint between the lites of glass is responsible for keeping water out (Figure 5).

Where it is necessary to have SSG curtain walls with ‘two lines of defense’ for air and water resistance, it is possible to use a cassette or toggle system. In such systems, a small aluminum extrusion is structurally silicone glazed to the glass. Toggles are then inserted into the channel and fastened to a fully gasketed steel curtain wall framing system to retain the glass. The gasketing provides additional air- and water-resistance within the system. From an aesthetic perspective, these weather joints look the same as a conventional barrier system.

Point supported steel curtain wall systems
Design professionals desiring glass façades and walls with narrow sightlines can also use steel mullions as the structural component in point-supported glazing systems. In such systems, custom structural fittings (i.e. ‘spiders’) connect the glass to the steel supporting members (Figure 6). This design mechanism concentrates loads at the connector locations, rather than applying a uniform, consistent load to the frame members. As a result, it is important to work with the curtain wall supplier’s structural engineer to ensure the frame member size is sufficient.

Additionally, it is crucial to verify the glass in point supported systems is able to accommodate any loads transferred between it and the structural connector without fracturing. Many connectors, including popular spider brackets, require the manufacturer to drill holes in the glass. These holes concentrate the wind load and dead load (i.e. weight of the glass) transfer from the glass at the connector before transferring their loads to the curtain wall framing member. This makes it paramount for the glass to be constructed in such a way as to resist these concentrated point loads. This challenge becomes even more complex when point-supported curtain walls incorporate IGUs or laminated glass constructions. The glass fabricator must seal and align the holes between the IGU lites at the spider penetrations, driving up the tolerance constraints of fabrication. Given these challenges, manufacturers are developing alternative methods of connecting glass panels to the structural supports, including a spider bracket connected to the glass via structural silicone, thereby eliminating the need for holes through the IGU.

When the challenges with glass in point support curtain walls are properly addressed, systems with steel back members can serve as a cost-efficient curtain wall option. Due to steel’s strength, framing members are typically only required in one direction to reduce material costs. This is often the verticals because they handle gravity better. While horizontal framing members may be required to resist buckling loads in I- or T-beam vertical members, it is possible to reduce the size and frequency of horizontal framing members. Fewer framing members can help put the glass and the resulting, less-obstructed views to the exterior on center stage, while also furthering cost savings.

Fire-rated curtain wall systems
Modern steel frames are advantageous for non-rated curtain wall systems, and also for fire-rated systems. One pronounced benefit is their ability to help design teams match the aesthetic of fire-rated systems to neighboring non-rated curtain wall, window, and door systems.

For years, fire-rated ‘hollow-metal’ steel frames have been viewed as bulky wrap around affairs that, although functional, are at odds with today’s slender, monolithic glass façades and walls. Design professionals can now achieve a uniform look by using steel fire-rated frames formed from tubes and shaped in a rolled process similar to new generation steel frames. The frames have narrow profiles, well-defined corners, and crisp edges. They can also be custom painted or powder-coated to match or complement non-fire-rated frames, and can incorporate custom cover caps to provide a wide range of design schemes. Together, these benefits make it possible for design teams to install fire-rated frames that integrate well with surrounding curtain wall, window, and door applications.

New York’s Fulton Center achieved this. The building’s retail and transit interior spaces are flooded with light from a 16.2-m (53-ft) diameter glass oculus, and thus a crucial part of the design aesthetic was ensuring the grand atrium’s interior curtain walls had clean sightlines—even in areas required to provide fire protection. To successfully achieve this design, the project team used non-rated and rated custom captured horizontal steel mullions that fit the atrium’s distinct shape and created a matching look across the upper (the fire-rated portion) and lower (non-rated) curtain wall assemblies (Figure 7).

While new generation steel frames make it possible to match the aesthetic of non-rated systems, it is important to note fire-rated glass requires stiffer frame deflection criteria (under wind load) than non-rated glass and can weigh between 100 to 350 percent more than non-rated glass. To limit deflection, design teams may need to increase the framing member size, or reinforce smaller profiles. While reinforcing the profiles is a viable option in instances where a larger profile may hinder the design intent, it can significantly increase the cost of the framing.

An alternative to captured fire-rated curtain walls is glazed assemblies that emulate the smooth appearance of silicone glazed (SG) non-rated systems. They are one of the most recent fire-rated glass systems on the market, and can help design professionals achieve a sleek, frame-free exterior aesthetic through a toggle retention system that becomes hidden once installed, eliminating the exterior pressure plate and cover cap of a conventionally captured system.

Conclusion
Today, architects continue to seek curtain wall assemblies that soar higher, hold larger glass lites, and minimize the visual impact of frames. Once an obstacle to these goals, steel frames are now a catalyst for innovation.

Chuck Knickerbocker is the curtain wall manager for Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems, along with specialty architectural glazing products. With more than 35 years of curtain wall experience, he has successfully worked with numerous architects, building owners, and subcontractors from development of schematic design through installation. Knickerbocker chairs the Glass Association of North America (GANA) Building Envelope Contractors (BEC) Division Technical Committee. He can be contacted via e-mail at chuckk@tgpamerica.com[1].

Source URL: https://www.constructionspecifier.com/steel-curtain-walls-that-get-the-curtain-call/

---