Architectural Record BE - Building Enclosure

Categorized by Design: Architecturally Exposed Structural Steel

Specifying architecturally exposed structural steel (AESS) with defined categories to meet project expectations for appearance, budget, and schedule
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Sponsored by American Institute of Steel Construction
By Jayshree Shah, AIA, NCARB, LEED AP BD+C

Style

There are two basic styles that govern the design intent with AESS: tectonic and plastic. A tectonic look is more expressive of the details that showcase the steel assembly and tends to emphasize bolted construction. A plastic aesthetic is uniform and smooth, using more welded or cast connections for a near-seamless appearance. The approach to the design style is an integral part of determining which AESS category is suitable based on the desired level of finish.

The Southwest University Park stadium represents a more tectonic style (left), emphasizing bolts and connections. The intersection of multiple AESS members in the close-up image of the Loyola’s Institute of Environmental Sustainability Ecodome (right) are much more plastic, seamless, and smooth.

Photo courtesy of Walter P. Moore/Brian Wancho (left); © Skidmore, Owings & Merrill LLP (right)

The Southwest University Park stadium represents a more tectonic style (left), emphasizing bolts and connections. The intersection of multiple AESS members in the close-up image of the Loyola’s Institute of Environmental Sustainability Ecodome (right) are much more plastic, seamless, and smooth.

Adjacency

Structural steel is an ideal material for achieving tight tolerances, as it is fabricated with great precision. The 2016 AISC Code of Standard Practice distinguishes between standard tolerances and tighter tolerances with different AESS categories. Depending on the composition of the structure, tighter tolerances may or may not be necessary.

Photo of 837 West Washington in New York City.

Photo courtesy of Gilsanz Murray Steficek

837 West Washington in New York City faced the challenge of designing to 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.

Several factors have a direct impact on the selection of an AESS category. Coordination during the design phase can be better facilitated by understanding the context that surrounds the expression of steel.


Choosing a Category

There are five categories which identify the level of AESS for a project:

  • 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

More requirements and therefore typically greater costs are inherent as the AESS category number increases. AESS C is Custom and could require fewer levels of finish than Basic Elements in AESS 1, or go even further beyond what is included in Showcase Elements of AESS 4. When choosing AESS C, more information should be included in the drawings and specifications to clarify the intent. It is required that one or more of these five AESS categories be annotated in the contract documents when AISC 303-16 is referenced as a standard.

“Section 10 of the 2016 AISC Code of Standard Practice provides specifiers with a much clearer roadmap to convey what they are looking for,” acknowledges Babette Freund with fabricator Universal Steel of North Carolina. Freund serves as chair of the Code of Standard Practice Committee to recognize and document standards within the U.S. structural steel industry to meet the expectations of design between fabricators and designers. Larry Kloiber, PE, who serves on the committee with Freund, encourages architects to consider the appearance they really need to achieve for the design intent prior to selecting an AESS category.

AESS 1: Basic Elements

By default, AESS 1 is the minimum treatment of exposed steel beyond standard fabrication of structural steel. This category is typically the lowest cost of the AESS categories and also serves as a prerequisite for AESS categories 2, 3, and 4. There are aspects of AESS 1 that meet the same requirements of standard fabrication. For example, the tolerances required for standard structural steel are the same for AESS 1 and can be referenced in the 2016 AISC Code of Standard Practice, available for free download at www.aisc.org/freepubs.

Surface preparation in AESS 1 is different from standard fabrication, as the exposed steel must receive commercial blast cleaning to meet the provisions of SSPC SP 6. The Society for Protective Coatings (SSPC) sets the standards for coatings and protection on structural steel. The SP 6 standard requires steel be cleaned of oil, grease, dust, oxides, rust, etc. In order to complete this surface preparation, weld spatter and similar surface discontinuities must be removed and sharp edges ground smooth. Specifications for paint and coatings should be thoroughly reviewed for use with the required surface preparation of AESS members.

Consistency of appearance between components is also a key strategy when specifying AESS. The AESS 1 Category requires that bolt heads be consistently located on the same side of a member as well as on adjacent steel members. The goal is to provide a uniform appearance beyond the standard requirements of structural steel.

When steel components are welded together, there may not always be a need for a continuous weld per the structural design. Visually, however, the appearance of a continuous weld is more desirable than intermittent welds. AESS 1 requires that all welds are to appear continuous and can be caulked, filled, or additionally welded to achieve this look. The projection of welds can be no higher than 1/16-inch above the surface.

 

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Originally published in Architectural Record

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