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;
- 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.