By Terri Meyer Boake, BES, B.Arch, M.Arch, LEED AP
As its name suggests, architecturally exposed structural steel (AESS) must be designed to not only support the primary structural needs of the building (or canopies, ancillary structures, or pedestrian-scale bridges), but also achieve the esthetic qualities required by being exposed to view. Since it is part of a project’s architectural language, the material’s design, detailing, and finish requirements typically exceed that of standard structural steel normally concealed by other finishes. AESS needs to be durable and maintainable; it must be able to resist corrosion if placed in a hostile environment, and the design and finishes must hold up to urban pollution and general wear.
The Canadian Institute of Steel Construction (CISC) has developed a new suite of documents to assist the design and construction industry with using the material.1 Before the development of these tools, there was no clear standard or specification for AESS, and a lack of recognition for the need to differentiate the qualities of projects as a function of their use, finish, and the ability of people to see or interact with the building or structure—namely, viewing distance.
At the core of the approach was the recognition not all AESS need be equal. This is not to suggest it should not be well-crafted, but just the level of finish must suit its end use. For example, high-profile exposed steel for airports, atriums, or museums have fabrication and finish requirements excessively expensive if applied to mid- to lower-end commercial or industrial projects.
Further, the architect, engineer, and fabricator had no way of communicating what was meant by “nice details”—each had different views (Figure 1). The architect wanted fully ground, smooth-welded connections, the engineer wanted the load paths straightforward and chose welded versus bolted connections to suit, and the fabricator was happiest with bolts as these made erection much simpler.
The CISC tools are designed to provide clear and definitive communication amongst the architect, engineer, and fabricator to make aspects of the decision-making process routine and allow the team to concentrate on more critical issues. Each document speaks to the specific interests and role of the parties.
What is in each document?
CISC’s National AESS Ad-hoc Committee created three documents for, respectively, the engineer, the fabricator, and the architect:
- “Sample AESS Specification;”
- “Appendix 1 to CISC Code of Standard Practice;” and
- CISC Guide for Specifying Architecturally Exposed Structural Steel, ed. 2.
These three resources are tied together by the AESS Matrix to create a complete and coherent means for the team to communicate.
The “Sample AESS Specification” is the legally binding document in the contract set. This text-based document incorporates the AESS Matrix, which references the association’s ‘categories’ and ‘characteristics’ discussed later in this article. The fabricator will refer to the specific choices in the specification when applying the requirements for fabrication of various AESS categories selected for the project. The designated AESS categories become contractually binding as they are noted in the drawings and/or additionally in a schedule. For clarity, a schedule of AESS types is suggested for larger or more complex projects.
The AESS categories are noted on the engineer’s drawings as these are the documents to which the fabricator refers when creating his or her own detailing and shop drawings. The specification also addresses care in handling and requirements for bolted and welded connections. It is intended to be used in conjunction with MasterFormat Section 05 12 00–Structural Steel Framing, as this addresses all the structural requirements for the AESS.
Section 01 45 00–Quality Control, Section 05 30 00–Metal Decking, Section 05 20 00–Metal Joists, and Section 09 91 00–Painting are noted in the “Sample AESS Specification” as having related requirements. It is meant to be clear that AESS is not covered under “Miscellaneous Metals” as the material has to fulfill structural functions.
The “Appendix 1 to the Code of Standard Practice”contains all the details regarding the AESS categories. This means when the categories are noted in the drawings, or in a separate schedule, the fabricator looks at the appendix to fully understand the role of the characteristics associated with each. These are very clearly outlined in the matrix to which the fabricator will also refer.
The CISC Guide for Specifying Architecturally Exposed Structural Steel forms a key reference for all parties involved in the project, including the client. It includes extensive descriptions of all the AESS categories and characteristics, and is illustrated with close to 200 colour images to help the team more fully understand the nature of the details and requirements of each. The guide includes sections on connections, finishing, and use of steel with glass and timber, as well as some additional information on curved steel and castings. The applications of AESS can be wide-ranging, and the central idea behind the illustrated guide was to provide inspiration to encourage use of AESS in diverse projects.
It is highly recommended team members be familiar with all the documents to better understand the details of the AESS categories and requirements. Clients should also be brought into the discussion to more completely understand the cost and timeframe implications of the suggestions they might be innocently making on the project. Many of the finer details pertaining to the characteristics in the AESS system have been set based on aspects that are absolutely necessary to obtain a good quality product, and those that should only be applied to situations requiring finer detailing.
Categories and characteristics
The AESS categories, and their respective characteristics, were created to define the treatment required during steel fabrication to properly prepare for its final use. Separate notes and specifications address handling procedures during erection and the application of finishes, including paint, corrosion protection, and fire-protecting coating application. These are implied by virtue of the choice of AESS category and clarified by the finishing-related specifications.
There are four primary categories, and one additional custom one, that recognize the use, distance of view, and finishes’ impact on a project. These are further differentiated by characteristics of fabrication that are additive as the categories ascend.
Although finishes might be applied last, they must be acknowledged at the outset of a project as the use of a heavier fire retardant or galvanized corrosion protection would permit a less-refined approach to fabrication and detailing that could afford some cost savings. The choice to use bolted or welded connections will also impact the choice of AESS type.
Depending on the project, it may be likely two adjacent categories will be used due to varied viewing distances, with ceiling elements often employing a lower category. For example, a project might use AESS 2 on the ceiling elements and AESS 3 on the column elements. High-profile projects could use AESS 3 for the ceiling elements and AESS 4 for the lower elements.
AESS 1 is the first step above standard structural steel. This type of application would be suitable for basic elements that require enhanced workmanship (Figure 2). This type of exposed structure could be found in roof trusses for arenas, warehouses, big box stores, and canopies; it should only require a low-cost premium in the range of 20 to 60 per cent due to its relatively large viewing distance, as well as the lower profile nature of the architectural spaces in which it is used.
AESS 1 characteristics are important as they form the basis for the detailing of all AESS categories. These include:
- commercial blast cleaning (to ensure the finishes adhere properly);
- sharp edges ground smooth;
- continuous weld appearance;
- use of hex-head or TC bolts with the bolt-head side specified for a clean and orderly appearance; and
- removal of all weld spatter.
AESS 2 refers to structures intended to be viewed at a distance of greater than 6 m (19.7 ft) (Figure 3). The process requires good fabrication practices with enhanced treatment of welds, connection and fabrication details, tolerances for gaps, and copes. This type of AESS might be found in retail and architectural applications where a low to moderate cost premium in the range of 40 to 100 per cent over the cost of standard structural steel would be expected.
AESS 2 characteristics include:
- one-half standard fabrication tolerances to acknowledge the increased precision of these types of structures’ geometries;
- uniform and smooth welds; and
- no apparent fabrication marks.
AESS 2 introduces the option of a visual mockup. If required, these might be as simple as the use of a 3-D rendering accompanied by a visit to a fabricator’s previous job to ascertain the approach to detailing. For major projects, it is possible to request the fabricator create prototypical “first pieces.” After shop inspection by the architect, engineer, and client, the subsequent pieces may be modified, but these first pieces will be incorporated into the final project. It is important to remember the lighting, orientation, and viewing distance for these members will not be the same as in the final project.
AESS 3 refers to structures that will be viewed at a distance of 6 m or less (Figure 4). The category would be suitable for ‘feature’ elements—where the designer is comfortable allowing the viewer to see the art of metalworking. The welds should be generally smooth, but visible; some grind marks would be acceptable. Tolerances must be tighter than normal standards.
As this structure is normally viewed closer than 6 m, it might also frequently be subject to touch by the public, warranting a smoother and more uniform finish and appearance. This type of structure could be found in airports, shopping centres, hospitals, or lobbies. It could be expected to incur a moderate cost premium that could range from 60 to 150 per cent over standard structural steel as a function of the complexity and level of final finish desired.
AESS 3 characteristics include:
- removing the mill marks (by grinding);
- butt and plug welds ground smooth and filled;
- orienting the weld seam on hollow structural steel (HSS) members for reduced visibility;
- cross-sectional abutting surfaces aligned; and
- minimized joint gap tolerance.
These aspects will create a cleaner, tighter-looking steel structure that is warranted given the close viewing distance. All welded connections are noted as optionally included. It is suggested this requirement be discussed amongst team members as it is possible to provide discreet or hidden bolted connections that will satisfy the esthetic intention while making the erection process simpler, faster, and less expensive.
AESS 4 is used for showcase elements where the designer intends the form to be the primary focus of the design (Figure 5). All welds are ground, and filled edges are ground square and true. All surfaces are sanded and filled. Tolerances of these fabricated forms are more stringent, generally to half of the usual tolerance for standard structural steel. All the surfaces would be ‘glove-smooth.’ The cost premium of these elements would be high and could range from 100 to 250 per cent over the cost of standard structural steel—completely as a function of the nature of the details, complexity of construction, and selected finishes.
AESS 4 characteristics include:
- weld seam not apparent;
- surfaces filled and sanded;
- welds contoured and blended; and
- minimized weld show-through.
AESS C (“Custom Elements”) is to be used on projects that do not cleanly fit into the previous four categories, which are not to be modified, except for the noted optional characteristics. As the CISC AESS category system becomes an integral part of all exposed steel design, a clear understanding of the categories and characteristics should become automatic, therefore any modifications to the defined categories will be problematic and muddy the establishment of accepted norms.
AESS C was created to allow for clearly noted modifications to the categories where the team might wish to add or remove a characteristic. For highly unusual projects, the AESS Matrix can be used to create customized, à-la-carte requirements. This would be suggested in sustainably minded projects where existing (historic) steel may be reused, or for projects that use weathering steel.
Although the AESS documents have only recently been released, many projects in Canada are making effective use of them, as illustrated by the photos throughout this article. The result has been smoother communication and a more satisfying project experience. The question often asked is, “Who is supposed to suggest either the use of the system or the appropriate categories?” The answer is, “anyone on the team.”
It is preferable to have the AESS categories selected and incorporated into the contract documents used for bidding. This would suggest categories are selected by the architect in consultation with the structural engineer. However, if this is not the case, the category system can be introduced at the time of bidding. This was the case for a recent tender situation for a large institutional project. The contract document set did not employ the AESS category system, but the bidding fabricators were all familiar with the benefits of the system and proposed it be used to clarify the requirements. The process was so successful the AESS category system has been incorporated into the project’s second phase.
The CISC system does not completely eliminate all problems associated with the design, fabrication, and erection of architecturally exposed structural steel. It does, however, make routine many basic requirements for the preparation and fabrication of the steel so the team can focus discussions on more important detailing issues.
Terri Meyer Boake, BES, B.Arch, M.Arch, LEED AP, is a professor at the School of Architecture at the University of Waterloo. She is the author of the Canadian Institute of Steel Construction’s CISC Guide for Specifying Architecturally Exposed Structural Steel. For Birkhäuser, Boake has authored Understanding Steel Design: An Architectural Design Manual, with two more books on the way: Diagrid Structures: Systems, Connections, Details and Architecturally Exposed Structural Steel: Systems, Connections, Details. She can be contacted via e-mail at firstname.lastname@example.org.