It is important to recognize that details are much less visible at 20 feet away as compared to an element 5 feet away. AESS 2 serves a level of fabrication and erection specific to structural steel elements viewed from a distance greater than 20 feet. The intent of creating such a distinction is to establish the same level of expectations for elements that are deemed as AESS but are located further away. The cost range is typically higher than AESS 1, as it builds on the characteristics of the basic elements in addition to meeting more requirements.
AESS 2 is more refined than AESS 1, as it mandates fabrication tolerances for straightness be half that of standard requirements. This is especially important when adjacent materials and components must closely integrate with steel members in a design.
Photos courtesy of American Institute of Steel Construction (AISC)
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.
During fabrication and erection, steel members are marked for inventory and tracking purposes. These numbers are visible on the steel even through certain coatings unless AESS 2 or a higher category is specified. The marks can either be ground out, filled, or simply turned away from view.
Continuous welds are already included with AESS 1, but AESS 2 goes a step further by requiring greater consistency in the welding process. The latter expects additional care and refinement during fabrication and erection. AESS 2 calls for uniform and smooth welds, which entails a consistent appearance on all welds in view range but does not necessarily deem welds are to be ground smooth.
AESS 3, 4, and C all require mock-ups unless providing one is impractical and other visual samples can suffice. “The key to a mock-up is to set expectations for shop work and fieldwork,” notes Steve Weiss of Weiss Architects. He goes on to say, “Mock-ups show the capability of worker craftsmanship in a particular shop. If [an assembly] is to be created in the field, then the mock-up should also take place in the field with the same contractors doing it under the same field conditions.” Lighting and placement impact the appearance of AESS, therefore similar conditions should be replicated when reviewing a mock-up for finish.
In general, as schedule and budget permit, mock-ups or visual samples are recommended for all categories. Shaina Saporta, PE, of ARUP, notes that majority of her projects involve AESS located in exterior conditions. Saporta encourages specifying physical mock-ups for larger, more complicated projects. She advises, “It will cost more in the long run if you do not do [a mock-up].”
Table 10.1 in the 2016 AISC Code of Standard Practice breaks down the requirements of each AESS category. An example of Table 10.1, AESS Category Matrix, can be found later in the course. The table denotes visual samples as optional for categories AESS 2, 3, and 4. Visual samples are a broad category inclusive of full-scale mock-ups, mock-ups of connections only, finish samples, 3-D renderings, photographs representing built AESS categories, and more. Fabricator Babette Freund of Universal Steel of North Carolina suggests tours of existing AESS projects and visual examples as methods to help communicate expectations for delivery of the desired finished product. There are multiple approaches to establish a basis of expectations during construction. To determine what level of visual sample is necessary, each project should take into consideration what ultimately needs to be achieved.
A higher level of fabrication and erection is provided as the number of AESS category increases. AESS 3 represents the next level of characteristics, specifically for components within a viewing distance of 20 feet or less. This category includes all the requirements for AESS 1 and 2 along with a more specific attention to detail as exposed elements are visibly closer.
Photo courtesy of American Institute of Steel Construction (AISC)
Mill marks from Steel Dynamics Inc. (SDI) are visible through the painted finish on this structural steel member.
Steel is marked with slightly raised characters, called mill marks, when it is delivered to a fabricator from the mill. Mill marks identify the steel mill from which the component was produced. A raised or depressed surface on structural steel members is also created with butt and plug welds. These marks and visible welds are deemed undesirable in AESS 3 and are required to be removed from view, typically by grinding them out or filling any depressions. Grind marks on exposed steel are easily visible, but applying a surface coating nearly or completely eliminates their appearance.
Photos courtesy of American Institute of Steel Construction (AISC)
Weld seams are to be less visible and consistently located in AESS Category 3. A series of intersecting hollow steel sections are visible along the pedestrian bridge (left) at Boston Logan International Airport. A close-up view (right) slightly shows the weld seams, which have been ground out consistently at each architecturally exposed member and coated with intumescent paint.
A weld seam is typically apparent when steel is joined together by welding. Welds are required to be continuous and consistent in appearance as noted in categories AESS 1 and 2. It is required that weld seams are to be less visible in AESS 3, as these elements are much closer in view range. Hollow structural steel (HSS) often includes weld seams as a typical method of connecting steel. An option to reduce visibility of seams is to simply orient them away from view. When such a strategy is not available, then aligning the seams in a consistent manner across all members is another solution. There are multiple methods of fulfilling this requirement, some which are less costly than others.
When two cross sections of steel come together, such as a column splice with connection plates, it is critical the sections align when in close view range. Lighting showcasing an AESS component can expose misaligned surfaces in an obvious and undesirable manner. AESS 3 specifically addresses components within range of touch or within closer view distances up to 20 feet away. Misalignments are not acceptable and require greater care in fabrication and erection to avoid these issues at cross-sectional surfaces that abut one another.
The tolerance for straightness must be tighter than the standard level, as noted under AESS 2. Tolerance requirements in AESS 3 go further to minimize the gaps between the components that have bolted connections. Greater precision during fabrication and erection is required so that these gaps are no greater than 1/8-inch and are uniform among all adjacent components.
The appearance of a bolted connection is not always desirable, especially when trying to achieve a seamless, “plastic” look. Determining the style of aesthetic is a prerequisite of choosing the appropriate AESS category. AESS 3 includes an option to utilize all-welded connections or reduce in the appearance of bolts. This can be achieved by coordinating the design intent with the structural engineer to use only welds for a particular connection. An alternate solution may be to hide the bolts by placing cover plates over them, a detail which should be clearly expressed in the contract documents.
It is commonplace to apply multiple AESS categories per project and per space. AESS 3 may be specified for elements within a close view range, whereas AESS 2 may be used for high ceiling elements. Provide clear notes on the contract documents to distinguish the location of different AESS categories. To reiterate, typically the architect will provide the design intent and location but should coordinate the documentation with the structural engineer’s drawings and specifications.
Photo courtesy of Walter P. Moore/Bob Perzel
Multilevel open concourses highlight the efficient use of multiple AESS categories in one space. The Sprint Center in Kansas City, Missouri, was completed prior to the implementation of the categories. Analysis of the space from eye level may imply AESS categories 1, 2, and 3 were used in the large concourse.
The sculptural nature of steel is meant to be the main focus when specifying AESS 4. This category draws inspiration from the expression of form as the featured aesthetic in a project. Making material connections appear seamless in a project can sometimes be the most challenging to design and construct. The latter is also true of structure, especially when architecturally exposed structural steel is to have a very smooth and sleek finished appearance. It is generally understood that AESS 4 components may entail the highest premium over the previous categories, not only per the desired “glove” smooth finish, but more often due to the complexity of structural geometry. The design approach should be discussed between the architect and structural engineer in advance of selecting the AESS category.
The characteristics of the previous categories, AESS 1, 2, and 3, are all included with the selection of AESS 4. In AESS 3, it is acceptable to reduce the visibility of weld seams. The “glove” smooth finish desired for showcase elements in AESS 4 necessitate that weld seams are no longer visible. Certain structural steel shapes and sections, such as pipes or some types of hollow structural shapes, lend themselves to have fewer or no seams. If at all possible, turn the seam away from view for the most cost-effective strategy.
The smooth and contoured appearance of welds enhances the style of the more plastic look of AESS 4 components. Majority of steel designs in this category focus less on bolted connections and implement more welded connections for a seamless aesthetic. There are cases where welds may show through the back face of an exposed steel element. Locations where welds show through to the other side of the AESS component are to be addressed to reduce their undesirable appearance. Open holes placed in the steel members, often for the welding process, are to be closed off and smoothed out for a clean, finished surface.
More labor and time is often necessary during fabrication and erection to achieve the quality of AESS 4 compared to the previous categories. Surfaces that are in view range are to be free of imperfections. Filling any deviations with a body filler as well as sanding textured surfaces are methods of fabrication used in this category. This higher level of care and detail allows for glossy coats on steel to be a truly successful aesthetic in close view range, as depicted in the Brookfield Place Entry Pavilion in New York City.
Any deviation from the requirements of AESS 1, 2, 3, and 4 falls under the Custom category AESS C. Occasionally, there are situations when sharp edges do not need to be ground smooth or erection and painted marks are not required to be removed from view. Allowing this flexibility in choosing characteristics provides designers with greater freedom but also notifies steel fabricators and erectors that there is a noteworthy difference from the typical category requirements. Custom elements should be clearly defined in the contract documents with AESS C located as needed.
The 2016 AISC Code of Standard Practice recommends using Table 10.1, similar to the AESS Category Matrix shown previously in the course, as a checklist for architects to coordinate or customize AESS requirements with structural engineers. The matrix should be used to specify special requirements for AESS C in the contract documents.
Image courtesy of American Institute of Steel Construction (AISC)
Note: I.D. numbers correspond to adjacent AESS image chart. Table 10.1 (similar) of the AISC Code of Standard Practice.
Images courtesy of American Institute of Steel Construction (AISC), except as noted
Note: Reference adjacent AESS Category Matrix for I.D. numbers and characteristics.
The casting of steel technically falls under “other steel” per the 2016 AISC Code of Standard Practice. Exposed elements that are cast can support structural loads and connect to the structural frame, but typically do not fall within the same category requirements as listed under AESS. Different levels of surface finish may be specified by the designer, as related to the project budget, to achieve the final aesthetic.
Photo courtesy of CastConnex/doublespace photography
The last portion of the AESS process is just as critical as a well-coordinated design phase. The 2016 AISC Code of Standard Practice addresses requirements for erection of AESS in Section 10. The higher-quality finish and treatment of AESS necessitates extra care and handling during transit and placement. The timeline for erection may be slightly longer when AESS is specified on a project. Erectors are tasked with assembling the steel with careful planning and methods to avoid damage to the finished product.
The decision to architecturally expose structural steel can add significant value to a building. Improvements in the 2016 AISC Code of Standard Practice satisfy the desire to create efficient and cost effective measures when implementing AESS on a project. The five categories (AESS 1, 2, 3, 4, and C) that distinguish architecturally exposed structural steel require sufficient evaluation and coordination in order to meet project expectations within budget and schedule. Establishing the same level of expectations between owners, architects, engineers, general contractors, fabricators, detailers, and erectors is critical to achieving the best end results for a project.
Left: Photo courtesy of American Institute of Steel Construction (AISC); Right: © Tim Griffith
Daylight penetrates through the interior of both the United Airlines Terminal at O’Hare International Airport in Chicago (left) and the Sacramento International Airport, Central Terminal and Concourse B Expansion (right).
Travel by commercial flight has grown considerably in the past few decades. This popular form of transportation has given designers the challenge of expressing lightness and airiness now commonly represented in airport design. Trends of integrating architecturally exposed steel in public spaces and airport terminals have grown since the design of the United Airlines Terminal at O’Hare International Airport in Chicago. Curved steel beams throughout the United Airlines atrium and passenger terminal support a sky-lit roof. The circular openings within the beams allow for light to filter through the space and create different patterns on the walls and floors throughout the day. Exposing the structure created the appearance of thinner and more slender elements, supporting the building and creating more space in the interior. O’Hare is a classic example of how AESS is integral to the design and serves as a prominent, visible feature.
More recently, the concourse and terminal at Sacramento International Airport was expanded to enhance the experience of travelers navigating through the airport. Large column-free spaces, sky-lit roofs, clerestory windows, and architecturally exposed structural steel create a world-class environment to greet passengers in Concourse B and the Central Terminal. It was critical for the project team to develop the aesthetic and structural requirements in tandem in order to achieve the design goals with efficient methods of construction. Due to the scale and complexity of the design, two steel fabricators were contracted for the project. Herrick Steel and Schuff Steel fabricators worked directly with the architects and engineers to achieve the higher-quality appearance of AESS with a framing system governed by high seismic requirements. The connections between steel members were critical to the seismic design, but exposing them architecturally added additional challenges. Custom solutions necessitated a peer review process to ensure code compliance in the structural design. The fabricators created custom equipment to assist in the welding process of the curved girders supporting the roof. Temporary bracing, which can often leave behind blemishes and unwanted surface appearance, was minimized as much as possible with careful planning for the erection process. Multiple levels of AESS were used throughout the project, though at the time of design and construction the category system was not in place.
Coordination between disciplines is critical to the success of any project utilizing AESS. Understanding the level of expectations in fabrication is much simplified by selecting categories to convey the design intent. Early involvement with steel fabricators and the design team can assist in project development and lead to efficient and cost-effective solutions.
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