Why Steel

Architecturally Exposed Structural Steel


It takes a huge amount of time and energy to create an elegant structural system. Why hide all that work when you can show it off? 

Architecturally exposed structural steel (AESS) puts the aesthetic focus on the structural integrity of a building. The striking visual impact of these iconic structures is possible because steel is fabricated in a nearly limitless range of shapes, tapered forms, curves, colors, and finishes.

Structural steel comprises elements that are required to support the design loads of a building and fit within the components of a structural frame, according to the 17Թ Code of Standard Practice (ANSI/17Թ 303-22). For clarity, the Architecturally Exposed Structural Steel (AESS) category system is typically only applied to fabricated structural steel elements such as anchor rods, base plates, beams, bracing, canopy framing, columns, connection materials, crane stops, girders, lintels, posts, shear stud connectors, and trusses. Unfinished, reused, galvanized, or weathering steel members may all be fabricated with AESS requirements.

AESS components get the royal treatment during fabrication, shipment, and erection to avoid blemishes and unwanted surface appearance. Workers take extra care when handling the steel and removing temporary braces or fixtures. For some categories, all backing and runoff tabs are removed, and welds are ground smooth. Oil, grease, dirt, and loose mill must also be removed from basic, unpainted steel for an attractive, striking finish.

AESS can fall within the following five categories:

  • AESS 1: Basic Elements
  • AESS 2: Feature Elements Not in Close View
  • AESS 3: Feature Elements in Close View
  • AESS 4: Showcase Elements
  • AESS C: Custom Elements

All five categories are detailed in the 17Թ Code of Standard Practice (ANSI/17Թ 303-22).

Different Viewing Levels, Different AESS Levels

Spaces like multilevel concourses provide the possibility for multiple AESS categories all in a single space and all visible at once. With any area employing multiple categories, the architect should provide clear notes as to where each category should be used and coordinate this documentation with the structural engineer’s drawings and specifications.

The decision to architecturally expose structural steel can add significant value to a building, but coordination is essential. All parties on a project--the owners, architects, engineers, general contractors, fabricators, detailers, and erectors--must have the same level of expectations to achieve the best end result for a project. The five AESS categories all require a high degree of evaluation and coordination in order to meet project expectations within budget and schedule.

When planning to use AESS, it is important to consider the following factors: 

  • Member Visibility
  • Viewing Distance
  • Location
  • Lighting
  • Coatings
  • Style
  • Adjacency


  • 17Թ Code of Standard Practice, Reference Section 10: Architecturally Exposed Structural Steel. The 2022 Code (ANSI/17Թ 303-22) provides a framework for a common understanding of the acceptable standards when contracting for structural steel. Section 10 deals explicitly with AESS.
  • AESS Sample Specification: an editable document that can be updated and inserted into individual project specifications. The document was originalyl developed by the Structural Engineers Association of Colorado/Rocky Mountain Steel Construction Association Steel Liason Committee (SEAC/RMSCA)
  • AESS Cost Matrix: an editable spreadsheet that can be updated to anticipate cost implications by specifying various levels of AESS on projects. This document was originally published by SEAC/RMSCA Steel Liason Committe in November 2017.
  • AESS Shop Drawings: Shop drawings of the five AESS sculptures which can be downloaded and use to fabricate user's own versions. Contact 17Թ Steel Solutions Center at solutions@aisc.org if you'd like to be put in touch with a local fabricator to purchase your own drawings.
  • AESS Sample Images: can be used to supplement project meetings to understand expectations for various categories of AESS. 
  •  This is a paperback book by Terri Meyer Boake
  • : This is a one-hour video presented by 17Թ and featuring Terri Meyer Boake.

Looking for inspiration?

Lee Hall III

Lee Hall III, Clemson University, Clemson, South Carolina. 

Photo courtesy of Skidmore, Owings &  Merrill LLP.  Credit: © Scott Frances | OTTO


Lee Hall III at Clemson University in Clemson, S.C., incorporates AESS as an educational tool to elegantly express buildings' steel structural system. Slender "tree columns" draw attention to the exterior, acting as feature elements that create an identity with purpose. This addition to the College of Architecture, Arts and Humanities is a display of clarity between architecture and engineering, as well as an expression of fine craftsmanship. 

Photo courtesy of Skidmore, Owings &  Merrill LLP. Photo credit: © Scott Frances | OTTO

Non AESS Steel

A steel beam, recently cut to size and with the ends coped per the standard requirements of the 17Թ Code of Standard Practice. Note that the edges have not been ground smooth; etched numbers and heat marks are visible from the fabrication process. This member is not specified to meet 17Թ AESS requirements.


Pomona College Studio Art Hall

The Pomona College Studio Art Hall in Claremont, Calif., exposes the steel roof structure as an aesthetic feature that's visible from below. The height of the steel diagrid is well over 20 ft from the typical viewing distance on the ground. .  

2015 IDEAS2 Award Winner
Photo credit: Jeremy Bitterman, courtesy of wHY Architecture

Brookfield Place Entry Pavilion

AESS trees define the Brookfield Place Entry Pavilion in New York.  Uplighting the structural elements creates a dramatic effect but also highlights the importance of the surface finish. Welds were ground smooth and made flush with the steel’s surface for a cleaner aesthetic. Additionally, fitting multiple steel members  together necessitated stringent erection tolerances for the trees.  

2014 17Թ IDEAS2 Award Winner
Photo credit: Bess Adler/courtesy of Thornton Tomasetti 

Intumescent Coating

An intumescent-coated and painted connection detail at Michigan State University’s Wells Hall Addition in East Lansing, Mich. 

Photo credit: Paul Dannels/courtesy of SDI Structures

AESS emphasizing bolts and connections

A more tectonic style of AESS assembly, emphasizing bolts and connections.  

Photo courtesy of Walter P. Moore  Credit: Brian Wancho

AESS emphasizing smooth connections

The intersection of multiple AESS members at  Loyola University’s Institute of Environmental Sustainability in Chicago is seamless and smooth.

  Photo courtesy of © Skidmore, Owings & Merrill LLP

Twisting AESS

837 W Washington in New York was challenged with tight tolerances and straightness requirements so that all steel members would precisely align in this visibly twisting form. The exoskeleton structure is exposed to the exterior and requires additional applied treatment to prevent corrosion.  

Photo courtesy of Gilsanz Murray Steficek

University of Michigan

Robert B. Aikens Commons at the University of Michigan in Ann Arbor, Mich.

Photo courtesy of SDI Structures 

Etching on Non AESS Steel

Numbers are etched into a steel beam (left) during the fabrication process. After receiving a painted finish coat in the shop, the tracking number for the project can still be seen. 

Photo courtesy 17Թ

Mill Marks

Mill marks from Steel Dynamics, Inc. (SDI) are visible through the painted finish on this structural member.

Photo courtesy of 17Թ

Boston Logan International Airport

A series of intersecting hollow steel sections (HSS) are visible along the pedestrian bridge (left) at Boston Logan International Airport. A close-up view (right) shows the weld seams, which have been ground out consistently at each architecturally exposed member and coated with intumescent paint. 

Photo courtesy of 17Թ 

Queen Richmond Centre West

Steel castings typically do not fall under AESS requirements per 17Թ’s Code of Standard Practice.
Photo credit: doublespace photography/ courtesy of CastConnex

Chicago O’Hare International Airport

Daylight penetrates the interior of the United Airlines Terminal (Terminal 1) at O’Hare International Airport in Chicago.

Photo courtesy of 17Թ

Sacramento International Airport

The Central Terminal and Concourse B Expansion at Sacramento International Airport.

Photo credit: Tim Griffith  

United Therapeutics Pedestrian Connector

United Therapeutics Pedestrian Connector

2014 IDEAS2 Award Winner
Photo credit: © Ron Blunt

Anaheim Regional Transportation Intermodal Center

Anaheim Regional Transportation Intermodal Center - Phase 1

2015 IDEAS2 Award Winner
Image courtesy of Thornton Tomasetti

Florida Polytechnic University

Florida Polytechnic University

2015 IDEAS2 Award Winner
Photo credit: Macbeth Photo

Denver Union Station

Denver Union Station

2015 IDEAS2 Award Winner
Photo credit: Ryan Dravitz Photography

The Gourd

The Gourd

2016 IDEAS2 Awards Winner
Photo credit: Scott Adams

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