Nima Balasubramanian, AIA, NOMA, AISC director of architecture
The first time I saw the International Spy Museum in Washington, I understood why getting the specification for architecturally exposed structural steel (AESS) right matters so much. Stand in front of the building and look up. The bold, raking red columns sloping at a dramatic 2.5:1 pitch are not a decorative skin applied over a structure. They are the building’s gravity-load carrying system—bold, expressive, and functional. That is AESS.
Unlike standard structural steel (SSS), which is typically concealed behind fire protection and finishes, AESS is fabricated and erected to a higher standard of craftsmanship, detailed and finished specifically for the location, viewing distance, and lighting conditions of each element. Although not all exposed steel must be designated as AESS, when specifying AESS, the geometry and aesthetic of the structure are highly considered.
At the Spy Museum, designed by RSHP with Hickok Cole as architect of record, AESS isn’t just the structure or an accent. It is the architecture. For instance, within the glass veil on the west façade, there is a monumental series of stairs and platforms constructed from custom AESS members with fully expressed connections. That feature creates a circulation sequence that is structurally honest and spatially dramatic. Every connection is visible and is part of the architectural language of the building.
The specification gap
Here is what I see too often in practice: An architect has a vision for a project where the structure is the expression; where the bones of the building are visible, honest, and elegant; and where exposed steel defines the character of a space rather than hiding behind a suspended ceiling or a layer of fire protection. The details are resolved on the drawings and ready for construction.
But when the steel arrives onsite, reality doesn’t live up to expectations. The surface reads differently than intended, the weld conditions aren’t what the team imagined, and the columns that were meant to feel refined look raw and industrial, leaving the entire team standing in a building staring at steel that is already erected, wondering what happened.
It got to this point because the conversation about whether to specify AESS or SSS, and what AESS actually means for this specific project, started too late or never happened at all. The divergence between design vision and delivered product is rarely a failure of craft. It is almost always a failure of early communication.
AESS is fundamentally a tiered category system differentiated by architectural detailing requirements and calibrated to technical and cost realities, where viewing distance, building type, and budget should drive every specification decision.
AESS defined across five categories
The American Institute of Steel Construction (AISC) Code of Standard Practice for Structural Steel Buildings and Bridges (ANSI/AISC 303-22) defines AESS across five categories. On this spectrum of craftsmanship, each step up represents greater surface refinement, tighter tolerances, more intensive labor, and a corresponding higher cost. A summary of each category is as follows:
AESS 1, Basic Elements, has workmanship requirements that exceed non-AESS construction. The steel is visible but distant, positioned more than 20 ft from viewers.
AESS 2, Feature Elements Not in Close View, exceeds the basic requirements and allows the viewer to see the art of metalworking through geometry without finish work. The category covers feature elements viewed at a distance greater than 20 ft.
AESS 3, Feature Elements in Close View, is achieved through geometry and basic finish work for elements viewed at 20 ft or less, and treats elements that visitors may touch. For acceptance, approval of a mockup is required.
AESS 4, Showcase Elements, applies to elements with special surface and edge treatment beyond fabrication; those for which the designer intends that the form is the only feature showing in an element. Like AESS 3, acceptance is based on an approved mockup.
AESS C, Custom Elements, allows for requirements that are more or less stringent than those in any other category, as described by the contract documents. Steel in the AESS C category may combine characteristics from other categories at the specifier’s discretion.
Wait! Does your project even need AESS?
Before you start selecting categories, it’s wise to ask yourself: Do I really need to specify AESS for my project? This is the question I find myself asking most often, and the answer is not always yes. There are many applications in which the as-produced appearance of fabricated and erected structural steel may be deemed sufficient without any special additional work. For example, standard structural steel can be appropriate and more cost-effective when a tectonic or industrial aesthetic is desired or the viewing distance simply doesn’t justify the investment.
At the CITYPARK soccer stadium in St. Louis, project specifications initially called for an AESS designation until mockups were built and the team stood in front of actual fabricated steel and asked honestly: Is AESS necessary to meet the design intent? Is this what we wanted? The answer revealed that the desired aesthetic could be achieved without AESS and at a lower cost.
At the Truist Leadership Institute in Greensboro, N.C., the answer was different but the process was the same. Working closely with the steel fabricator, the design team developed two custom AESS categories, a more refined finish where the steel would be most visible and a less refined finish where it would not, ensuring the tactile quality of every weld condition matched the design intent precisely where it mattered.
The conversation you cannot skip
For AESS 3 and 4, mockups are required under AISC’s Code. When a mockup is constructed, standard practice is for the project team—architect, structural engineer, fabricator, general contractor, and erector—to assemble, examine the mockup under approved conditions, and ask honestly: Does this meet the design intent? Is this what we wanted?
During the Spy Museum project, the team reviewed a full-scale mockup of the expressed column connection at the fabricator’s shop before a single production piece was cut, aligning everyone on surface articulation, coating quality, and finish tolerance. The result was a building where the steel arrived onsite as envisioned.
By engaging your fabricator early, building a mockup when necessary, and having vital conversations before fabrication begins, you can help ensure that the project team is in alignment, laying the groundwork for a successful outcome.
PS: If you can’t go to Washington to check out the Spy Museum for yourself, here’s the next best thing.
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Want to learn more?
- The latest version of AISC’s AESS Sample Specification, designed to make it easier to achieve your design intent, is now available for download.
- On July 21, the American Galvanizers Association is hosting the webinar AESS Uncovered: Design Excellence Meets Galvanized Protection.
- Terri Meyer Boake, the expert who literally wrote the book(s) on AESS, breaks down what you need for clear communications and better project outcomes. Just need more details about AESS? Head over to her YouTube channel.
- We have more AESS resources and project case studies available at aisc.org/aess.
