Myth versus reality: Revealing the curtain wall capabilities of modern steel

by nithya_caleb | January 1, 2020 12:00 am

by Chuck Knickerbocker

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Architects and specifiers are starting to take note of how modern, cold roll-formed and laser-welded steel framing methodologies are resulting in previously unimagined esthetic possibilities for curtain walls. Both steel formation processes allow for the development of larger sections of thin-gauge carbon or stainless material (see “Steel Forming Processes: Then and Now”). When used as the primary framing member in curtain walls, they make tall, unobstructed free spans of glass feasible, checking high-design and daylighting boxes in one system (Figure 1). They also eliminate the need to remediate hot-rolled steel sections for compliance with architecturally exposed structural steel (AESS) requirements. For instance, when working with AESS, exposed welds must be ground smooth and obvious markings should be removed. With laser-welded profiles, there are no exposed welds or mill roll or identification marks to remedy.

Compared to traditional steel and aluminum assemblies, this new generation of steel frames provides certain basic design benefits such as:

• narrower frames;
• sharp edges versus rounded profiles;
• corner joints without any visible weld beads or fasteners; and
• flexibility to use back mullions of different shapes and sizes.

Since framing establishes the base for a curtain wall assembly’s performance, systems employing this new forming methodology can effectively satisfy demanding design and performance criteria.

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Moreover, cold roll-formed and laser-welded steel processes can capitalize on steel’s obvious strength over more conventional materials like aluminum. Steel has a modulus of elasticity nearly three times of aluminum (2.9 x 10⁷ psi versus 10 x 10⁷ psi, respectively). By applying new forming methodologies, this can translate into smaller system frame profile and component parts than aluminum without decreasing end-use performance.

Consider a 6-m (20-ft) long steel or extruded aluminum mullion with the same cross-sectional properties, using 1436 Pa (30 psf) in a 1.5-m (5-ft) module as the design load (discounting allowable deflection limits momentarily). In this scenario, an aluminum mullion deflects 112 mm (4 in.), whereas a steel mullion deflects 39 mm (1.5 in.). This allows the designer to do one, or all of the following, when using steel as the primary framing material:

• use smaller system shapes while meeting the same deflection criteria, thus translating to larger, uninterrupted views of the outside environment and greater illuminance for interior occupied spaces;
• support larger lites of glass (or other glazing infills) than their aluminum cousins; and/or
• span larger heights without the need for reinforcing or more frequent connections to the building structure.

Despite the many benefits associated with these new steel forming methodologies, myths continue to circulate around steel’s limitations in curtain wall systems. These steel framing processes are well understood and used in Europe. However, they are still new to the North American markets, accounting for much of this confusion. To help set the record straight, this article will address three relevant inaccuracies as they relate to Canada’s snow-laden winters, subarctic temperatures, and Leadership in Energy and Environmental Design (LEED) v4.1 goals.

 Myth 1: Steel frames are likely to corrode

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Among the most common misconceptions about steel is the carbon alloy is unsuitable for use as a primary framing material due to corrosion. Indeed, early steel curtain walls were vulnerable to rust, and the industry turned to more corrosion-resistant materials, such as aluminum, by the mid-1900s. Especially in a country largely subjected to freezing winters with heavy snowfall, those in the design-build sector are rightfully wary of building materials that cannot stand up to moisture and air, two of the main catalysts for corrosion.

The Europeans have led the way in developing systems that eliminate much of the water and steel contact in glazing systems. Water is typically present in the glazing pockets, where the glass is captured along its edges, to hold it onto the framing system. The means of capture employs gaskets, since metal to glass contact would not create the necessary air and water seals required by specification. Pioneering a solution, the Europeans took the gasket at the face of the primary framing member, which forms the interior portion of the glazing pocket, and entirely covered the face of that steel with the gasket, completely removing any possibility of water coming into contact with the steel. Water from other portions of the façade, such as the perimeter interface with surrounding construction or from the surrounding construction itself, is precluded by intelligent detailing and execution of those conditions. The governing principle is water should not come in contact with the steel, and the systems themselves address water isolation from steel internally.

This has allowed steel to become a more suitable material for consideration as a glazing system framing material. Notably, steel frames do not require cladding or reinforcement to support expansive captured and non-captured curtain wall systems. When properly sized, the primary frame members can serve as structural elements, although this would require co-ordination with the general contractor (GC), structural steel fabricator/erectors, and glazing subcontractors. A wide variety of finishes are available, from powder-coating to liquid-applied coatings. However, it is the glazing systems that typically isolate the carbon steel from coming into any contact with water.

Weather seals, moisture protection, and ASTM E331

The gasketing methodology employed by these systems has been the major contributor to steel’s comeback in curtain wall applications. With the addition of a continuous gasket covering the full width of the steel face, steel back members are now isolated from water within the glazing pocket (Figure 2).

Specifically, gaskets are typically ethylene propylene diene monomer (EPDM) rubber or silicone. While manufacturers handle sealing gasket joints at framing members in different ways, many overlap and seal gasket joints at framing member joints at intersections. This further prevents moisture intrusion. Any water collecting in the glazing pocket is redirected to the sill of the opening, or at intermediate locations in tall vertical members (greater than 6000 mm [236 in.]), where the water is then weeped out of the glazing systems. The glazing pocket is also free of metal, supporting condensation resistance.

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As for whether or not steel-framed curtain wall systems meet third party, independent laboratory testing, it is important to examine steel’s performance as per ASTM E331[5], Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference. With ASTM E331, water is applied to both the outdoor face of the curtain wall and opening perimeter joints to adjacent materials at the same time. The static air pressure is kept uniform, though the air pressure at the outdoor face is higher than the pressure at the indoor face. While all of this is occurring, the curtain wall’s level of resistance is measured. Under these requirements, steel glazing systems have met zero water penetration when tested at 718 to 1436 Pa (15 to 30 psf).

Air protection and ASTM E283

Equally important to corrosion prevention is resistance to air penetration. When oxygen comes into contact with water on a steel surface, the rusting process begins.

Fortunately, the continuous gasketing mentioned above in relation to water penetration also prevents air from entering through the steel curtain wall systems to the occupied, interior, conditioned air spaces. In essence, the gaskets form a tight, protective seal against air penetration. Installers commonly utilize pressure plate systems as a two-line resistance strategy for air and water penetration. Functionally, pressure plates with self-sealing fasteners keep the glass in place. They also help maintain adequate pressure on the glass, gaskets, and framing, creating the seals necessary to meet the air and moisture resistance mentioned above.

Steel can meet ASTM E283[6], Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen, requirements. According to the test method’s scope, “This test method covers a standard laboratory procedure for determining the air leakage rates of exterior windows, curtain walls, and doors under specified differential pressure conditions across the specimen. The test method described is for tests with constant temperature and humidity across the specimen.”  When tested with a pressure differential of 299 Pa (6.24 lb/sf), a steel curtain wall’s air leakage has been consistently measured at 0.005 L/s of wall area. 

These airtight framing capabilities also debunk another myth that is associated with steel products—poor energy efficiency.

 Myth 2: Steel structures are not energy efficient

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Many curtain wall framing materials have a reputation for high thermal conductivity compared to other elements of the building envelope. Design teams often pair framing systems with low-emissivity (low-e) glass or other energy-efficient glazing to help boost centre-of-glass (COG) thermal performance values. The problem is the system’s overall thermal efficiency remains significantly less effective where the captured or retained glass edge meets the supporting frames.

With Canada’s predominantly subarctic climate and long history of stringent standards for energy efficiency in new construction, many design teams therefore opt for thermally broken aluminum frames. By incorporating thermal breaks (i.e. separations between the inner and outer frames), they can help reduce the heat flow associated with the material’s high thermal conductivity (i.e. approximately 124,500 joule [118 Btus] per hour).

Contrary to popular belief, thermally broken aluminum frames are no longer the only high-performance alternative. Steel’s thermal conductivity is approximately 74 per cent less than that of aluminum (i.e. approximately 32,707 joule [31 Btus] per hour). This is equivalent to that of thermally broken aluminum frames. Even more importantly, due to the design of steel profiles, some advanced steel frames do not require a traditional thermal break. Steel frames without a thermal break need less metal to support the glazing than traditional aluminum frames, and therefore, reduce the pathway for heat transfer and offer greater resistance to heat transfer than aluminum systems.

U-value and AAMA 1503, NFRC 100 and 200

When combined with clear insulated glass units (IGUs) with low-e coatings, independent testing agencies have calculated steel’s modelled U-values to be between 0.36 to 0.39 U-value, or thermal transmittance tests, including American Architectural Manufacturers Association (AAMA) and United States-based National Fenestration Rating Council (NFRC’s) thermal modelling and testing regimens, 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, and NFRC 100, Procedure for Determining Fenestration Product U-factors, and/or NFRC 200, Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence.

With NFRC 100 and 200, simulations and/or physical testing are used to determine frame materials’ U-values. NFRC 200[8] additionally measures fenestration product solar heat gain coefficient (SHGC) and visible transmittance at normal incidences. During NFRC 100 computer simulations of steel frame materials, IGUs and low-e coatings, the system’s U-values were 0.31. With 25-mm (1-in.) IGUs comprising clear glass and non-gassed airspace as well as triple glazing, steel U-values have been recorded as low as 0.19. It is important to note these are approximate values only. Actual values will vary depending on specific product configuration per project requirements.

 Support for high-performance glazing

Steel can also provide the necessary support for heavy triple-glazed IGUs, and/or for a wide variety of glass thicknesses, whereas most aluminum glazing systems can only handle a single glazing infill thickness. This is paramount as traditional framing materials may not be able to support the required loads associated with high-performance glazing, simply due to its size, thickness, and/or weight.

This is not the case with steel. Due to the material’s strength, some steel curtain wall systems can support glazing infills up to 76 mm (3 in.) thick and weights up to 112 kg/m² (23 lb/sf). This far surpasses the typical thickness (i.e. 45 mm [1 ¾ in.]) and weight (i.e. 48.8 kg/m² [10 lb/sf]) of triple-glazed units. In sum, the material can support high-performance glazing with marginal effects on the design intent.

Myth 3: Steel curtain wall systems have little impact on LEED requirements

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Steel curtain wall systems can also help design teams meet LEED requirements, from optimizing energy performance to contributing toward daylight and views, further trumping misconceptions about its impact on energy inefficiency and daylighting (Figure 3).

The Canada Green Building Council[10] (CaGBC) explains, “Since 2005, LEED buildings have eliminated nearly 2.5 million [carbon dioxide] CO₂ tonnes of greenhouse gas (GHG) emissions annually, diverted nearly three million tonnes of waste from landfills, and saved 24 billion litres of water per year, benefiting all Canadians.”  Continuing with Canada’s legacy to advance green building, green professionals can use steel to help earn points in the following categories.

 Energy and atmosphere (EA)

Steel can help design teams optimize energy performance[11] in this category “beyond the prerequisite standard to reduce environmental and economic harms associated with excessive energy use.”  As discussed earlier, steel’s lower thermal conductivity means it functions as a better thermal insulator than aluminum, leading to decreased energy consumption.

 Indoor environmental quality (EQ)

To earn points for daylight, which seeks “to connect building occupants with the outdoors, reinforce circadian rhythms, and reduce the use of electrical lighting by introducing daylight into the space,” there are three options. The first requires annual computer simulations for spatial daylight autonomy. The average spatial daylight autonomy must have a value of at least 40 per cent. Minimum thresholds[12] for option 2 (computer simulations for illuminance take place at the equinox) and option 3 (measure illuminance) are 55 per cent for regularly occupied spaces.

Steel’s aforementioned tall free spans, made possible in part by steel back mullions, can help design teams meet these daylight requirements. For example, curtain walls utilizing these long, continuous steel elements can handle up to 12-m (40-ft) free spans in a single member without splicing. With steel curtain walls, natural light can penetrate deep into a building’s interior easily, increasing illuminance (Figure 4).

The steel mullions are further available in a variety of sizes and shapes, such as box, I-beams, and T-shapes. Compared with aluminum’s basic ‘box’ back counterpart, steel T-shapes are slimmer and allow for more unobstructed views. LEED[13] requires “a direct line of sight to the outdoors via vision glazing for 75 per cent of all regularly occupied floor area.”  Steel back mullions, along with tall free spans, can help achieve quality views.

 Before getting started

With these three common steel myths busted, architects and specifiers can rest assured that steel- framed window and curtain walls can resist air and water penetration, and eliminate the potential for corrosion, while earning LEED credits for energy efficiency and EQ. Taller free spans of glass, larger glass lites, sleeker profiles, and mullion versatility also make steel an attractive choice for curtain wall applications.

To set-up design teams for success, early involvement with steel framing manufacturers and glazing subcontractors is crucial. During these discussions, hammering out the fine details like the proper anchor/embed can lead to more precise fabrication and subsequent installation, saving time on the jobsite. Bigger picture topics such as schedule, budget, and project needs must also be addressed before construction begins. They are key to identifying and solving potential pitfalls before they arise.

For instance, take a steel curtain wall in a public library. The project consisted of two different, yet compatible steel systems to save costs (Figure 5). A laser-welded steel profile system was used for the vertical mullions, which provided expansive views of the library’s surrounding city hall and parks in line with the library board’s vision. The verticals were mated to a rolled profile system for the horizontals. The horizontals mirrored the look of the verticals, and resulted in lower project costs than if laser welding had been used throughout.

Given the move toward higher performance buildings, it is also advisable to consider how steel curtain walls fit within the overall building enclosure and the move to whole building commissioning. With decreased air loss, tighter water resistance, and lower U-values steel can help the design team meet tougher energy performance specifications.

STEEL FORMING PROCESSES: THEN AND NOW

The earliest known glass curtain walls used hot-rolled steel shapes. However, during World War II, advances in aluminum extrusion production allowed for its application in windows and curtain walls. In the years since, steel forming processes have progressed, allowing the material to outperform its aluminum cousin. Here is a look at how these methodologies have evolved with time.  Then Hot-rolled process Steel ingots are rolled into any shape imaginable (i.e. I-beams, Ts, channels, angles, flat bar, and sheet products). Controlling tolerances allows bow, twist, and sway in the final product. Not possible to precision roll the shape to the point where their use in curtain walls is practical.  Cold-drawn process Physically pull steel through a shaping die without heat. Ideal for smaller shapes, such as bars, wires, and general shapes with smaller cross sections. Sharper corners than possible with the hot-rolling process. Not ideal for exterior façades as their structural requirements dictate larger shapes.  Now Cold-rolling process Flat steel sheets or continuous coils are fed through a series of rollers to desired shapes for curtain walls and windows. Allows for the development of larger sections out of thin-gauge carbon or stainless material (material thickness ranges from 24 gauge 0.6 mm [0.024 in.] to 6 mm [1/4 in.]). Rollers can introduce complicated shapes into the material (gaskets, box screws).  Laser cut, laser-welded shapes Process takes long, flat carbon or stainless plates in lengths ranging from 12 to 15 m (38 to 49 ft), in thicknesses from 4 to 50 mm (5/32 to 1 15/16 in.), and laser cuts them into bars or strips in the size required for the desired shape. Once cut, the bars are assembled into the required shapes, with joints laser welded into the desired profile, such as rectangles, channels, Ts, angles, square tubes, and I-beams. A shape can meet the required width and depth, and also change the wall thicknesses to develop the necessary structural properties for the application.

 

[14]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 worked with numerous architects, building owners, and subcontractors from development of schematic design through installation. He can be contacted via e-mail at chuckk@tgpamerica.com[15].

Source URL: https://www.constructioncanada.net/myth-versus-reality-revealing-the-curtain-wall-capabilities-of-modern-steel/

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