Architecturally Exposed Structural Steelwork (AESS)

Architecturally Exposed Structural Steelwork (AESS)

Architecturally exposed structural steel (AESS) is a mixture of art, science and know-how. It can express the structural integrity of a building in a striking way and put the structural system at the forefront aesthetically. AESS can also combine form and function, expressing a playful intent while at the same time functioning as a primary load carrying system.

Increasingly, buildings are showcasing AESS to provide a signature identity, whether that is up close and personal in the entrance lobby or externally as a statement to establish the architectural presence and strength of the building. In these multifarious roles, steel can be viewed from a range of distances and must respond accordingly to level-of-detail considerations, balancing the degree of finish and detailing against cost and practicality.

The ASI has developed and provided a range of support material for architects to enable fit-for-purpose specification of the appropriate AESS requirements.

Communication is key

Delivering a successful AESS structure is not difficult, provided that clear communication about intent is maintained among the architect, engineer and fabricator. AESS has significantly higher requirements for the form, fit and finish for fabrication, erection and coatings than regular structural steelwork. These must be taken into account throughout the development process. Architects therefore become part of a triangle of communication in their documentation with the engineer and the fabricator, which will affect the way engineers design connections and the way fabricators finish connections, as well as the cost.

This interactive design process is very different from the more linear process associated with ‘standard’ structural steel that is hidden from view and where the architect can often step back from detailed involvement.

Communication can be improved by considering the following points:

  • Ensure a section on AESS is included in the Structural Steelwork Specification: the ASI National Structural Steelwork Specification includes appropriate clauses and is a free download
  • Adopt a category approach: as implemented in the NSSS and AS/NZS 5131 Structural steelwork – Fabrication and erection. The four-category system (AESS1 through AESS4) differentiates AESS as a function of building type, distance of view, surface treatment and budget
  • Facilitate early engagement: of the architect, engineer and fabricator with face-to-face meetings to discuss expectations, which may include meetings at the fabrication shop to view real-world samples
  • Provide clarity regarding acceptable substitution: substitution of different member types or shapes may need to be limited to ensure architectural intent.

Connection considerations

Connections that are to be exposed must be designed differently than those that are concealed. Decisions relating to the use of welded versus bolted connections must be informed by the overall requirements for the design of the structural system.

Integrating connections with the architectural intent can be improved by considering the following points:

  • Make stitch welds continuous: for longer viewing distances, the same visual effect may be achieved by caulking
  • Reduce the number of pieces used in connections, including stiffeners: for example, making webs thicker to avoid the use of stiffeners, or apply a stiffener only to the less visible side
  • Soften the edges of elements with rounded, tapered curved edges: in particular, near or at connections
  • Minimise gaps: between plates, and be consistent. Large gaps and inconsistency can make for a messy appearance
  • Redesign connections to be unobtrusive: large bolted splices between CHS/RHS, for example, can be redesigned with in-line connection (perhaps cover plated) rather than transverse end plates
  • Hide and/or streamline large connections: for example, by matching SHS/CHS member sizing at connections to avoid steps and/or stiffeners
  • Utilise castings: purpose-made castings for connections can be streamlined and elegant. Can be cost-effective for larger connections with high loads or complex member orientations.

Minimising cost

Cost estimates from fabricators can vary markedly depending both on the type and extent of AESS treatment and also on their understanding and comfort level. Be specific and communicate early. Understand the impact of viewing distance and use as this may allow simpler detailing and other finishing options.

The following considerations will help minimise unnecessary costs:

  • Specify minimum surface preparation of Sa2 to AS 1627.4: insufficient surface preparation can result in costly rework when final product finish is impaired by show-through
  • Take into account the final corrosion protection or fire protection system: refined details may not be necessary if thicker fire protection is required, for example
  • Avoid excessive grinding or other surface treatment in remote or less visible locations: if you cannot see the detail, it does not need detail treatment
  • Be judicious about specifying samples: creating samples can be expensive, so be reasonable and detailed in the kind of sample truly needed. There are various types of samples possible, including 3D renderings, a steel coupon with applied finish, a scaled down mock-up or full-scale mock-up, all with commensurate costs
  • Check for material availability: particularly important for larger round tubes. Delays can add cost if not managed properly
  • Do not over-specify: use AESS requirements judiciously and appropriately. Many projects can work well with more common standard steel member shapes, with aesthetically cleaner shapes only where necessary.