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How exposed structural steel’s strength and flexibility improves building design

As a building material, steel is as ubiquitous as it gets: This extremely strong, lightweight, and highly fire-resistant metal can be found throughout most buildings, from structural sections to reinforcing bars in concrete to sheet products and non-structural applications.

Indeed, the construction industry uses more than half of all steel produced around the world each year. That’s a lot of steel, considering the worldwide industry was worth US$2.5 trillion in 2017.

Rather than hide all that steel behind other materials, however, architects and designers are increasingly incorporating steel into their design choices. As we’ll show below, it’s a burgeoning architectural trend as practical as it is aesthetically pleasing.

Why steel as a building material?

Steel is so popular as a building material for several reasons: Along with being relatively easy to transport and assemble, it has the highest strength-to-weight ratio of any building material. That means steel can span longer distances, bear heavier loads, and provide greater design possibilities than other materials.

Steel is also 100 percent recyclable. Swiss architecture publisher Birkhäuser Verlag says the global recovery rate for steel in construction is 85 percent, and that exposed steel saves on other building materials typically used to hide the structural system.

What is exposed structural steel?

Architecturally exposed structural steel (AESS) typically consists of anchor rods, base plates, beams, bracing, canopy framing, columns, connection materials, crane stops, girders, lintels, posts, shear stud connectors, and trusses.

AESS must be strong enough to bear heavy loads of the building and other structures while also playing a huge role in the look and feel of the building. That’s possible because steel can be “fabricated in a nearly limitless range of shapes, tapered forms, curves, colors, and finishes,” according to the American Institute of Steel Construction (AISC).

“AESS components get the royal treatment during fabrication, shipment, and erection to avoid blemishes and unwanted surface appearance,” says the AISC. “Workers take extra care when handling the steel and removing temporary braces or fixtures. Oil, grease, dirt, and loose mill must also be removed from basic, unpainted steel for an attractive, striking finish.”

The AESS category system

AESS is fabricated and supplied under five industry-standard categories – primarily to determine its level of treatment and finish – depending on the intended use and the location of the exposed steel specified in relation to a building’s occupants:

AESS 1: Basic Elements
The minimum treatment of exposed steel – and lowest cost – other than standard structural steel.

AESS 2: Feature Elements Not in Close View
Meant for structural steel elements to be viewed at a distance of 20 feet or more.

AESS 3: Feature Elements in Close View
Structural steel located within a viewing distance of 20 feet or less.

AESS 4: Showcase Elements
Close-up structural steel for showcase elements, requiring smooth weld seams (designers can also try to hide seams by turning them out of sight, if possible).

AESS C: Custom Elements
Any deviation from the requirements of AESS 1, 2, 3, or 4.

If placed outside, AESS must also resist corrosion and damage from pollution or the elements.

While the AESS category system is comprehensive and wide-ranging, not every piece of exposed structural steel is defined as AESS. Other steel elements such as cables, castings, catwalks, and stairs aren’t typically specified under AESS.

What’s the correct AESS category to specify?

The standardized categories listed above are necessary partly because of the coordination required when adding AESS elements to any building. Everyone on a job needs to speak the same language.

Designers, after all, aren’t typically involved in structural design – that’s usually the domain of engineers – and as Birkhäuser Verlag notes, a ‘well-designed connection’ or ‘smooth surface’ has very different meanings for an architect, an engineer, or a fabricator.”

That’s why context, along with coordination, is vital when specifying the right AESS level for a building project. Several factors go into determining which AESS category is most appropriate for a piece of exposed structural steel, including member visibility, viewing distance, location, lighting, coatings, style, and adjacency. Since AESS is much more costly to fabricate than standard steel, it’s essential to strictly evaluate what you’re specifying using the following criteria:

1.Member visibility: Will the exposed steel be visible? If so, there’s a good chance it should be specified as AESS. To save costs, it’s possible to fabricate only one side or part of a piece of steel under AESS categories (it’s a waste of money to specify AESS for steel sections that aren’t visible).

2.Viewing distance: The distance from which steel is viewed is directly proportional to how much detail is seen – that’s why each AESS category is primarily determined by how far away most people will see the steel.

3.Location: Is your AESS for interior applications? Exterior? This question plays a huge role in the surface preparation, treatments, and coatings required, along with extra care at joints and connections to ensure no water penetration.

4.Lighting: AESS category levels are also influenced by the amount of light each piece of steel will receive – members in brightly-lit areas will likely require more finish than their counterparts in areas with low lighting.

5.Coatings: Coatings such as paint or corrosion and fire protection can also have an impact, with glossy coatings often showing more detail.

6.Style: Tectonic AESS has a rougher, more industrial appearance and often includes exposed seams and bolted construction. Plastic AESS is smoother and more uniform in nature.

7.Adjacency: AESS can typically achieve tighter tolerances than standard steel, thanks to its precise fabrication process. Structures requiring tighter tolerances may require different AESS categories, as defined in the 2016 AISC Code of Standard Practice for Steel Buildings and Bridges.

Case study: Montréal’s Préfontaine Metro Station

Unicel Architectural has extensive experience as a provider of highly engineered and elegant glass, timber, and aluminum building products for exposed steel structures and other buildings.

Préfontaine Metro Station in Montréal, for example – originally designed by architect Henri Brillon and inaugurated in 1976 – features bright red-orange exposed steel beams spanning the ceiling width of its large open cut area (overlooking the station’s main entrance and mezzanine) along with a broad expanse of glass and overhead skylights. This stunning design, however, was marred by leaky windows and skylights that couldn’t keep up with Montréal’s often punishing weather.

Unicel Architectural, during renovations initiated by Société de transport de Montréal (STM) in 2014, designed and fabricated a new skylight system to be integrated closely with the station’s exposed steel. Adding to the challenge was that the skylight system needed to fit seamlessly within the station’s dizzying array of multi-angle components including complex ¼” (6mm) brake metal.

Unicel Architectural’s prototype was selected after extensive testing for yield performance, water infiltration resistance, and shock absorption, and we worked with partners Provencher_Roy (architect) and MGB Associés (general contractor) to completely upgrade and replace the glass and skylights.

Today, the station still features its signature steel ceiling beams along with stunning natural light, but is buttressed by a rugged skylight system built to withstand extreme temperatures, moisture, and vibrations while seamlessly matching the station’s complex aesthetic elements.

Specify the right building products for your steel structure

Unicel Architectural offers a range of products that improve the look, feel, and functionality of any exposed steel structure or other building project. These include timber curtain walls (easily installed on any steel-framed building), glazed aluminum curtain walls, custom aluminum structures and solar shading, skylights, and fire-rated steel trim kits for doors, windows, transoms, and sidelights.

Contact us today to learn more about our skylight and curtain wall systems.

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