As we have already discussed in recent articles, Amendment 16 to NZBC B1/VM1, 5.0 Steel, introduced the new Standard AS/NZS 5131: 2016, Structural Steelwork - Fabrication and Erection. AS/NZS 5131 Section 10, introduces Architecturally Exposed Structural Steelwork (AESS) and although not a totally new concept, in AS/NZS 5131 it has been formalised and is now better integrated and coordinated with Standard Structural Steelwork (SSS) requirements. This means it can be specified with a degree of consistency, which in turn has allowed Masterspec to develop and introduce the new section 3415 ARCHITECTURALLY EXPOSED STRUCTURAL STEEL based on the requirements of AS/NZS 5131. It is intended that this new section is used in conjunction with existing section 3410 Structural Steel or any of the 3411 Purlins and Girts sections.
What is Architecturally Exposed Structural Steelwork (AESS)
It is more than Standard Structural Steelwork (SSS), which is the common steelwork often seen exposed in warehouses or covered up in office buildings. Architecturally Exposed Structural Steelwork (AESS) takes SSS structural requirements and overlays higher appearance requirements. AESS is exposed steel that must be designed to be structurally sufficient while supporting the architectural aesthetic of the building.
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Background
In the 19th century cast iron was celebrated in the raw or adorned with decoration. However, during the early/mid 20th century the quality of appearance of structural steel became important to the Modernist Movement (particularly the International Style) such as Mies van der Rohe (Farnsworth House) and Philip Johnson (The Glass House), where unadorned exposed steel structures were equally celebrated as part of the architectural language. This has persisted with different styles and movements through the late 20th century and up to today such as Richard Rogers (Lloyd’s Building) and Norman Foster (Sage Gateshead, Chateau Margaux). The problem has always been the lack of a common coordinated foundation on which to design, specify and construct AESS. In the early 2000's the Canadians consolidated a system for AESS as we know it now. This has since spread to the USA, Australia and New Zealand, and indeed AS/NZS 5131 AESS requirements for Australia and New Zealand are virtually identical to those of Canada and the USA.
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AESS Categories
The fundamental requirements of AESS are based around viewing distance and understanding the relationship between workmanship and visibility. The AESS Categories formalise this approach by providing 4 Categories plus a custom category, this helps define a projects additional appearance requirements over SSS requirements in a standardised way. Categories AESS 1 to AESS 4 get progressively higher requirements, while the custom category AESS C deals with custom elements which are specifically created for a project.
Each project that involves AESS work will need the AESS Category or Categories to be defined, the AESS Categories are listed in AS/NZS 5131, 10.2, as follows:
AESS Category
|
Description |
| AESS 1 |
Basic Elements - which require enhanced workmanship. |
| AESS 2 |
Feature Elements viewed at a Distance > 6 m - good fabrication practices with enhanced treatment of weld, connection and fabrication detail, tolerances for gaps, and copes. |
| AESS 3 |
Feature Elements viewed at a Distance ≤ 6 m - welds are generally smooth but visible, some grind marks are acceptable. Tolerances are tighter than normal standards. |
| AESS 4 |
Showcase or Dominant Elements - used where the designer intends that the form is the only feature showing in an element. All welds are ground and filled edges are ground smooth and true. All surfaces are sanded/filled to be “glove” smooth. Tolerances of fabricated items are more stringent. |
| AESS C |
Custom Elements
|
AS/NZS 5131, Table 10.2 Architecturally Exposed Structural Steel Matrix, provides the relationship between AESS Categories and various set characteristics. Some of these are discussed later.
AESS Categories and cost
When selecting the AESS Category(s) for a project the budget must be considered. AESS requirements are likely to increase costs, in some cases considerably. Some reports say 25% to 100%+ increase over SSS work, subject to the AESS Category and the type of work. To help manage this consider the Category required and the extent carefully, even consider different Categories for different areas or elements.
AESS considerations (subject to budget):
- Is AESS required at all, would SSS levels be OK (all or some elements)?
- What level of refinement is actually required (be realistic)?
- Is the same level of refinement required for all elements (can Category vary)?
- Where are elements viewed from (can it be hidden on the other side)?
- At what distance are elements (normally) viewed from?
- Are the elements within touching distance (and does it matter)?
- What applied coatings are being used (are they coatings that cover imperfections)?
- Connection details (will they make AESS difficult and/or expensive?)
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Remember, distance is a key factor in deciding level of finish, if something is far away imperfections may not be seen, if you are close enough to touch it you can see and feel imperfections. Varying levels of refinement relative to distance may save a considerable amount of money, but high levels of refinement over every element might be a waste of time and money.
Remember, distance is a key factor in deciding level of finish, if something is far away imperfections may not be seen, if you are close enough to touch it you can see and feel imperfections. Varying levels of refinement relative to distance may save a considerable amount of money, but high levels of refinement over every element might be a waste of time and money.
AESS expectations of quality
Even with a good Standard like AS/NZS 5131 and a good specification, sometimes expectation of aesthetic finish may be difficult to manage. Are the Architects/Designers, Engineers, Fabricator and Erector all on the same page? What processes can be used to help resolve this?
Firstly the new Masterspec section recommends an early meeting with all the related parties to discuss expectations of form, fit and finish of the AESS steel. This may help to identify and/or sort out possible issues, as well as helping to confirm everybody is on the same page.
Secondly the Standard and the Masterspec section both mention samples, these help define and maintain the quality required. Samples can take a variety of forms and have number of purposes. The forms can be virtual 3D renders, physical portable samples or physical fixed samples (temporary or built into the works). The purpose can be review sample, control sample, prototype (for review or testing).
Steel creates obvious extreme sample issues with its large size, heavy weight and high cost, so all of these have to be taken into consideration when thinking about the practicality of steelwork samples. 3D virtual samples are usually considerably cheaper than physical samples and maybe enough for review (like a complex connection). They could also be a first stage before going to physical samples to narrow down the options. Portable samples would, by necessity, often be at a reduced scale so they could easily be handled and moved. They probably involve jointing, connections, end points, finishes etc.
Fixed (usually large) samples may be, temporary such as a full scale mock-up in the fabricators yard (either scrapped or transported to site and used as part of works), or permanent such as a full scale sample erected on site as part of the building.
Tolerances
Non-AESS work under SSS requirements use AS/NZS 5131 Section 12 and Appendix F which allows two Tolerance Classes, 1 & 2. Class 1 tolerances are the default requirement in AS/NZS 5131 and should be acceptable for most steelwork construction whereas Class 2 tolerances are tighter and might be considered for higher structural specification work. However the purpose of these classes is to define minimum tolerances for structural purposes rather than aesthetic purposes. AESS requirements sometimes tighten up these tolerances.
With AESS work (assuming Class 1) there are three types of tolerances - Manufacturing tolerances, Fabrication tolerances and Erection tolerances.
- Manufacturing tolerances mill or manufactured surface finish which is covered by SSS Standards. AS/NZS 5131, Section 10, says:
- For AESS Categories 1, 2 and 3, the quality surface as delivered by the mills should be acceptable.
- For AESS Category 4, the steel surface imperfections should be filled and sanded.
- Manufacturing tolerances rolled shapes which is covered by SSS Standards. AS/NZS 5131, Section 10, has the follow exceptions:
- For AESS 3 and 4, the matching of abutting cross-sections is required.
- For AESS 2, 3 and 4, the as-fabricated straightness tolerance of a member is one-half of the standard camber and sweep tolerance in AS/NZS 5131, Appendix F.
- Fabrication tolerances, AS/NZS 5131, Section 10, Table 10.2, says:
- For AESS 1 to AS/NZS 5131, Appendix F (to SSS requirements)
- For AESS 2, 3 and 4, to one-half of the tolerances in AS/NZS 5131, Appendix F
- Erection tolerances are SSS requirements, but can be modified:
- For AESS 1 and 2 to Appendix F
- For AESS 3 and 4, to Appendix F
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Keep in mind that the connection between AESS work and SSS work or even non-steel elements, is a connection between different tolerances. To overcome the difference at connections between AESS tighter tolerances and normal SSS (or other structure) tolerances, connections must be carefully considered and/or the provision of adjustable connections may be required.
Welds
- Generally to AS/NZS 5131, Table 10.2 and the welding requirements in AS/NZS 5131, 10.4, including:
- For corrosive environments, all joints to be seal welded.
- For AESS 1, 2 and 3, smooth uniform welds. For Category AESS 4, welds to be contoured and blended.
- For AESS 1, 2, 3 and 4, weld spatter to be avoided/removed where exposed to view.
- AESS 1 and 2, weld projection up to 2 mm is acceptable for butt and plug welded joints. For Categories AESS 3 and 4, welds will be ground smooth/filled.
- Weld show-through - there will always be a degree of weld show-through depending on the finish type, weld size and material thickness. Subject to general AESS weld requirements, weld show-through requirements are:
- For AESS 1, 2 and 3, the members or components are acceptable as produced.
- For AESS 4, the fabricator to minimize weld show-through.
- Steel hollow structural sections (RHS and CHS) seams (seams from manufacture), to AS/NZS 5131, Section 10.4 (q):
- For AESS 1 and 2, seams to be acceptable as produced.
- For AESS 3, seams to be oriented away from view or as indicated on the drawings.
- For AESS 4, seams to be treated so they are not apparent. (cheaper to re-orient if possible?).
Connections, joints and junctions
The design of exposed connections, joints and junctions is crucial to the look and cost of AESS steelwork. Architect/designer and Engineer need to consider the combined aesthetic, structural and cost requirements as well as a balance of AESS requirements, to develop the optimum connections, joints and junctions.
For instance, keep it simple is just about always true, bolts are relatively cheap compared to complex heavily worked site welds. A repetitive complex multi member welded connection may be cheaper and easier as a cast connection.
With bolted connections consider all bolt heads being on the same side of the connection and all connections being consistent.
Preparation for finishes
The quality of preparation plays a significant role in the quality of the finishes and the end AESS product. This of course is subject to the thickness, gloss level and texture of the finishes and the distance from which they are to be viewed.
AS/NZS 5131, Section 10.4 (l), for AESS 1 to 4 requires a minimum surface treatment grade of P3 (refer AS/NZS 5131 9.8.4) which is a "very thorough treatment" including edges rounded and welds dressed smooth..jpg)
AS/NZS 5131 Section 10.4 (p), for AESS 1 to 4 requires a minimum surface preparation of Sa2 (mechanical cleaning) to AS 1627.4. This is generally OK for hot-dip galvanizing, some fire rated coatings and some thick coatings, however it is generally recommended that a surface preparation Sa2.5 (blast cleaning) is used for most other high quality coatings.
Masterspec
The new Masterspec section 3415 ARCHITECTURALLY EXPOSED STRUCTURAL STEEL, does not stand alone - it is intended to be used in conjunction with the SSS sections 3410 STRUCTURAL STEEL or 3411 STRUCTURAL STEELWORK.
This new AESS section 3415 will be able to be used by either the Architect, Designer or Engineer to define the additional AESS appearance requirements, while SSS structural requirements are specified in 3410 STRUCTURAL STEEL or 3411 STRUCTURAL STEELWORK. For instance the Architect/Designer could specify the project AESS work in the new section 3415, and with limited coordination, the Engineer specifies the bulk of the project SSS structural requirements in 3411.
The temporary AESS clauses added to the SSS sections last month, have been amended to reference the new and more detailed 3415 ARCHITECTURALLY EXPOSED STRUCTURAL STEEL.
LINKS:
If you need to refer to AS/NZS 5131: 2016, Structural Steelwork - Fabrication and Erection, it is available through Masterspec for those with Standards Online.
Articles:
http://architecturenow.co.nz/articles/architecturally-exposed-structural-steel/
http://www.tboake.com/SSEF1/what.shtml
http://www.enclos.com/assets/docs/Insight_04_-_Chapter_10_-_Architecturally_Exposed_Structural_Steel_Facades.pdf
https://www.aisc.org/globalassets/modern-steel/archives/2011/12/2011v12_three-cs.pdf
New Zealand/Australia guide to AESS:
https://www.scnz.org/Design+Tools/Specifying+Architecturally+Exposed+Structural+Steel+AESS.html
Videos - highly recommended:
http://steel.org.au/key-issues/steel-in-architecture/aess-video-presentations/
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