Above: From stairs and platforms to built up building columns, the International Spy Museum uses AESS prominently. Photo: Nic Lehoux
March, 2025- Ageode, with its round profile and rough exterior, looks like an ordinary rock at first glance. Collectors and geologists like to call them “nature’s surprise packages.” Once the stone is cracked open, its cavity reveals nature’s artwork in the shape of crystalline structures in vibrant colors and complex patterns.
Steel’s high tensile strength and lower cost make it one of the most commonly manufactured materials in the world. But the visual appeal of clean lines and the options for smooth or rough surfaces makes the alloy a dynamic choice for contemporary architecture and commercial spaces. Like a geode, a growing number of designers and engineers are opening up the exteriors of new buildings with the help of architecturally exposed structural steel (AESS). The system is a unique combination of art, science and know-how.
According to the AISC Code of Standard Practice (ANSI/AISC, 303-22), AESS is typically applied to fabricated structural steel elements such as beams, bracing, canopy framing, columns, connection materials, girders, posts and trusses. Unfinished, reused, galvanized or weathering steel members can also be fabricated with AESS requirements. Its use allows people to see the complex details and functional integrity of the buildings around them.
Designing with AESS requires an architect to balance form with symmetry and economy. Member visibility, viewing distance, location, lighting, coatings and style must be taken into account. Requirements for AESS are also significantly greater than the standards for common structural steel frames and connections hidden beneath a building’s facade.
ORIGINS
“AESS first appeared in 1976,” says Lawrence Kruth, owner of Kruth Engineering LLC. “It was talked about in general but there were some misconceptions. Architects would specify AESS and expect a car fi nish. It’s structural steel. You are going to get an industrial look, not a car finish. When AISC reworked Section 10 of the Code of Standard Practice for Steel Buildings and Bridges in 2016, the market for AESS really took off because the text provided architects, engineers and fabricators with clear, detailed instructions on its use and treatment.”
Brightly painted exposed steel visible from inside and out is a key characteristic of the International Spy Museum.
Kruth, a consultant for steel construction including fabrication, erection, safety, quality and connection design, is one of the country’s foremost experts on AESS and has monitored its evolution closely. The former owner of a large steel fabrication and erection company for more than three decades, he served as AISC vice president of engineering and research until 2023. That same year, Kruth was made an Honorary Member of AISC by the organization’s board of directors. Only 13 other individuals have received the award since 1921. Kruth is a member of multiple AISC committees and chairs the Research Council on Structural Connections Specifications Committee. He is a registered professional engineer in Michigan and Pennsylvania and is the recipient of a number of awards.
AESS is typically applied to beams, bracing, canopy framing, columns, connection materials, girders, posts and trusses.
“Architects love the look of exposed steel and the revised Code of Standard Practice makes it much easier to use,” Kruth says. “As a result, demand for AESS has been growing. The advantage of the revised standard for both designers and fabricators, is that it helps them pinpoint exactly what the requirements are for the look they want to achieve. And that saves labor and costs as well.”
BLUEPRINT As part of its modifications, AISC divided AESS into five categories.
• AESS 1 refers to basic elements with specifications that call for higher fabrication tolerances and surface quality than that of standard structural steel.
Stairs and platforms for the museum were constructed from AESS members of different shapes and profiles. Photo: Dan Cocciardi.
• AESS 2 identifies feature elements viewed at a distance greater than 20 ft. and defines stricter requirements regarding surface finish, welding quality, connection details and overall aesthetic consistency.
• AESS 3 refers to specifications for features viewed at a distance of less than 20 ft. Similar to category 2, AESS 3 dictates that welds and mill marks be ground smooth and joint gaps minimized. Appearance is prioritized with function.
• AESS 4 strives to achieve a flawless aesthetic appearance with minimal imperfections. Mockups are required for AESS 3 and 4.
• AESS C describes custom elements that could require very tight tolerances, meticulous surface finishing or extra care during fabrication, handling and erection to achieve a near-flawless aesthetic.
“It [C] can also refer to a finish less than AESS 1,” says Kruth. “AESS C lets the designer choose the level of finish to meet specific needs that are not reflected in AESS 1, 2, 3 or 4.”
The versatility of steel coupled with the blueprint provided by the revised standard allows architects and designers to shape and manipulate the material to fit their vision. “There is also a requirement that if an architect or engineer specifies AESS, they must also specify the category,” he says. “The standard has really streamlined the process. I’m seeing architects push more and more for AESS in a wide range of structures from office buildings and hotels to shopping malls, hospitals and airports.”
A sweeping canopy, featuring guitar shapes, anchors the lobby and international arrivals facility at Nashville International Airport. Photo: Jordan Powers.
APPLICATION
One project the Nashville International Airport Terminal Lobby and International Arrivals Facility Addition won the Innovative Design in Engineering and Architecture with Structural Steel (IDEAS2) Award presented by the American Institute of Steel Construction in 2024.
The 151,285-sq.-ft. structure weighs 1,750 tons. Twin 685-ft.-long contiguous spine trusses resemble radio waves. The grand entry lobby and airwave roof reshape the terminal’s form and volume, providing dramatic architectural expression that also incorporated design elements from a guitar. AESS was used for the roof trusses, columns and girders.
The team was very thoughtful with the form of the building itself and the roof,” says Hannah Valentine, structural steel specialist and AISC 2024 IDEAS² awards judge. “Passengers are going to be thinking about engineering as they walk through because it’s exposed and visible to them. It means a lot to engineers to have their structures not just seen, but seen elegantly.”
Steel was the only material choice able to provide the long spans needed to support the new roof trusses over the existing roadway and terminal yet accommodate the wavy geometric form needed to realize the project’s architectural aesthetic.
In 2022, the International Spy Museum in Washington received an IDEAS2 Merit Award–$75 million to $200 million. The 141,000-sq.-ft., eight-story, $162 million facility houses three floors of exhibits that sit on a base of retail, education and lobby spaces and are topped by offices and additional space for education and dramatic events. The building uses AESS prominently. The building has a five-story glass atrium called the Veil. Behind the glass, an intricate series of stairs and platforms were constructed from AESS members of different shapes and profiles. Each of these members have expressed connections to the built-up AESS building columns. Unlike some projects that use AESS as a highlight or accent, the system was used as a key portion of the building’s architecture. AESS features can be seen by museum patrons, tourists and commuters on nearby I395.
The location of the museum’s steel columns had to meet AESS tolerances. Elements produced with AESS needed different finish requirements, fabrication and erection details to accommodate varying coating types and thicknesses.
Workers erect the Nashville airport’s wavy roofline.
The terminal lobby and international arrivals facility are designed to bring in lots of natural light.
DETAILS
Fabricators that choose to take on an AESS project are advised to understand the ramifications. “A manufacturer can use normal fabrication procedures to meet AESS tolerances but they need to be aware of what they may have to do,” says Kruth. “For example, mills supply you with steel that contains a mill mark.
To meet AESS 3 or 4 tolerances, you will have to do some remedial work such as using metal shot blast to clean steel and eliminate the mark.” Mill marks are required by ASTM A6 and are used for traceability and to ensure that the steel meets specific quality standards by providing information about the heat number, grade and manufacturing details. The data is crucial for quality control and safety in construction projects where the steel is used. Steel mills began using the identifying marks during the late 19th century.
“I was in Las Vegas recently visiting an exhibit at the Luxor that has a 15- ton section of the Titanic’s starboard hull,” Kruth says. “It was 20 ft. high by 26 ft. wide. I walked around the backside of the exhibit to see if I could find the mill mark. A security guard asked me what I was doing. I explained I was looking for the mill mark on one of the channels and I found it. The mark belonged to British Steel, an old company that has been bought out and renamed many times. But the mark was still visible.”
EXPERIENCE
Kruth has experience processing AESS both before the code of standard practice was revised and after the modifications were put in place. “For AESS 3 and 4, mill marks have to be removed,” he continues. “Welds have to be ground to make them invisible. And because steel can be prefabricated off site and delivered, you have to make sure you adhere to the instructions of the category you are processing the steel for in terms of handling. Prior to the standard, I used to wrap my steel in carpet samples to prevent scratching. It’s one of the reasons AESS is more expensive. The higher the category, the more labor there is involved for the fabricator therefore, it is more costly.”
The standard not only makes the task of working with AESS easier but was also established to protect the entire steel industry. “It’s an ANSI document,” Kruth says. “That means engineers, general contractors, architects, fabricators and quality personnel were involved in the rewrites. As a result, each member of a construction project’s team knows their responsibilities.”
Though Kruth has retired twice, industry interest in AESS has him crisscrossing the country giving presentations.
“I feel with the experience I’ve gained over the years, I need to share it so other people can learn,” he says. “My position now is to help the steel industry any way I can.”
American Institute for Steel Construction, 312/670-2400, http://aisc.org/
Kruth Engineering LLC, 517/282-5058, http://kruthengineering.com/
