Steel re-emerges for use in demanding curtain wall designs
In Albuquerque, N.M., the world’s longest passenger tramway extends across nearly three
miles to the top of Sandia Peak. Four steel cables 100,000times longer than they are wide
carry up to 55 people in 9-ton loaded gondolas up the mountain. As in other structures--from
suspension bridges to skyscrapers--steel allows for slender, high-strength components that can support heavy loads.
For these same reasons, architects and designers are increasingly using steel for glazed curtainwalls, as well as window and door framing. Advanced manufacturing techniques pioneered in Europe also enhance design possibilities by forming steel into virtually any profile.
With these advances, steel framing now offers a number of performance benefits over traditional aluminum systems. These include the ability to support larger spans of glass, design with narrower framing sightlines, and provide longer-term durability, while overcoming previous limits such as bulk and risk of corrosion.
Steel frames can be used to meet stringent design challenges including tall, multistory
entrances or designs that incorporate large lites of glass. As such, they work especially well in daylighting designs for maximizing natural light in interior spaces.
Manufacturing technology expands potential for steel frames
Many glaziers might be familiar with older style steel framing, including hollow-metal steel and hot-rolled steel. These manufacturing methods can create bulky frames, or designs that are usually more functional than aesthetic.
A primary advantage of steel over aluminum is that it is about three times stiffer. Because it deflects much less under windload and the weight of the curtain wall itself, steel allows for larger areas of uninterrupted glass with less framing.
For example, depending on the shape of the frame profile, span and other factors, steel
framing can support individual panes approximately three times larger than typical aluminum
systems. The difference for curtain wall designs is dramatic, with the glazing becoming more
prominent than the framing.
Steel’s stiffness also allows for narrower frame profiles and larger free spans. For a given set of load and deflection criteria, steel frames can be more slender than aluminum. In a typical twostory curtain wall, for example, unreinforced steel frames can be 1 ¾-inch wide and 5 ¾-inch deep versus 2 ½-inch wide and 8-inch deep for aluminum. With 30 percent slimmer framing in this case, the sightlines of the curtain wall are much crisper, again, allowing the glazing to take center stage.
For applications with large spans, such as a two- or three-story entrance, steel curtain wall
provides the ability to span the required 20- to-30-foot distance without intermediate support.
This performance provides a preferable alternative to cumbersome and bulky reinforcement of traditional aluminum systems.
A second advantage of steel is its thermal performance. Steel conducts heat at about onefourth the rate of aluminum. Combined with narrower frame profiles, the result is lower heat gain/loss and less risk of interior condensation on the frames. National Fenestration Rating Council 100 computer simulations on thermally broken steel curtain wall systems combined with 1-inch insulating glass units (IGUs) incorporating clear low-E glass provided U-values of 0.31, far exceeding the thermal performance of many aluminum curtain walls.
A third aspect of steel as a framing material is its long-term durability. It can better resist the
potential for sagging or joint failure. In hightraffic areas, including storefronts and door assemblies, this can be an important benefit for reducing maintenance costs and ensuring a quality appearance over time.
A primary reason steel originally fell out of use for fenestration was its susceptibility to
rust. Modern steel framing, though, is available with double-sided pre-galvanization and factory applied finishes to resist corrosion. Coupled with watertight, full-width gaskets, and proper installation, steel framing is suitable for demanding interior and exterior applications. Designers and glaziers also can incorporate stainless steel to meet anti-corrosion needs, as well as aesthetics.
In traditional aluminum curtain walls, the back mullions are square-shaped. This configuration limits the system performance, as well as aesthetic flexibility demanded by architects.
To overcome this design limitation, steel framing systems can be manufactured with back
mullions of virtually any profile. For example, such modular systems are available with I-, T-,
U- and L-shaped mullions, as well as custom profiles. Almost any type of structural member
can thus be used as a curtain wall back mullion.
These include glue-laminated beams, I-beams, and round steel tubes. As a result, glazed curtain walls can be used in many modern and classic designs, including high-end storefronts, grand entrances and even rustic lodges.
For building designs that incorporate stainless steel, the exterior cover caps and interior back
mullions can be made from this material. By comparison, achieving the look and durability of
stainless steel with traditional aluminum curtain wall systems requires cladding. This makes the framing system more complex and time consuming to install.
Steel framing installs similar to typical stickbuilt aluminum curtain wall. Systems generally
include shear blocks, pressure plates and cover caps for connection of the framing omponents.
This allows the framing to be readily assembled without welding. As a result, glaziers can provide crisp corner joints with no visible fasteners or weld beads.
The framing is made watertight by traditional pressure plates and a full-width interior gasket,
which completely covers the interior frame profile and is secured in grooves. Steel framing
systems have met testing requirements for dynamic water penetration (AAMA 501.1) and
static water penetration (ASTM E331), as well as for air performance (ASTM E283) and structural performance (ASTM E330).
For unique and high-end designs, steel can provide the technical capabilities not possible
with aluminum framing. While designers will likely specify aluminum for many common applications, when designs call for large glass and wide free spans, glaziers can anticipate a growing call for steel.