47. Architectually Exposed structural Steel and Galvanizing

Architecturally Exposed Structural Steel (AESS) not only fulfils its structural function, but is left purposely exposed as part of the architectural aesthetic. Therefore, more attention is needed regarding detail, finish and handling than is normal with standard structural steel – and cost is generally higher.

The idea of AESS is not new; it could be said that early cast iron structures such as Iron Bridge (above), and Crystal Palace exhibited the aspects that define AESS:-

  • Predominantly bolted or welded connections;
  • Tolerances required at fabrication and erection;
  • Access to detail to perform the required finish.

The introduction of steel hollow sections in the 1970’s allowed architects and engineers increased scope in design and construction. Architects are increasingly using transparency in their design, as popularised by Helmut Jahn’s United Airlines Terminal at O’Hare Airport in Chicago. Once considered just for high profile projects, use of AESS is now widespread in structures including small retail stores, shopping centres, entertainment arenas, casinos and office lobbies.

However, it is possible to add considerable expense to a project by specifying requirements that are not needed. Not all AESS has to be created equal, most projects can neither afford – nor need – the same level of detailing.

AESS Specification

AESS design detailing can approach industrial design standards when creating joints between steel sections: structural requirements must be accommodated, along with tighter dimensional tolerances, form, symmetry and economy. The method of preparing and finishing connections using specialized welds and ground or filled finishes can become prohibitively expensive. Exacting (and expensive) detailing is only warranted in areas where the steelwork is close enough to be seen or even touched.

Increased use of AESS means that architects are becoming more involved in the connection issues and detailed structural specification that was once the domain of the engineer or fabricator. Similarly, the coating finish requires significant forethought regarding access for application, surface condition and preparation, and even – in the case of galvanising – the grade of steel required to consistently achieve the desired lustre on the finished product.

There is a need for increased communication between the architect, engineer, fabricator and coating specialist; and a shared meaning is required that helps standardise terms like ‘smooth finish’ and define where they are necessary when using AESS.

The result is the development of a Guide to Specifying Architecturally Exposed Structural Steel, summarised in the table below:-

The AESS specification adopts a categories approach, with each of the five categories specifying a unique level of finish. They reflect the key attributes of form, fit and finish but focus on three areas of concern:-

The viewing distance.

The ability to see the steel at close range defines the level of workmanship and finish. A distance of 6 metres was selected as a basic determinant; it is the distance at which a viewer can closely scrutinise or even touch the steel surface.

This distance should be considered as a 360o measurement. For example, in a multi-storey building the steel of the walls may be 40m in the air when viewed from the ground, but only 5m away when viewed (horizontally) from a higher floor or mezzanine.

Care must be taken to eliminate build-up of dust, dirt or rubbish on exposed steel members that can be viewed from above.

Some of the blemishes left on steel from manufacturing, fabrication or coating will not be visible at a distance, and can be accepted without the need for expensive and remediation.

Type or function of the building

This will have an impact on the aesthetic and finish requirements. The requirements for a hospital and an arena will differ and distinct AESS categories may be specified for spaces within the project. For example, roof trusses may be AESS 1, but the bases of supporting columns AESS 3. Such differentiation can produce substantial savings on a project.

Potential cost increase above standard structural steel

Using standard structural steel as a base, the matrix indicates a range of cost increases for fabrication and erection, depending on the AESS category.

Higher AESS categories call for better fabrication techniques, more attention to weld quality and surface finish. Erection costs increase depending on whether complex shapes can be fabricated in a shop or on site. The surface finish chosen will also impact on the cost.

Paint finishes will almost always require some degree of touch-up to repair scratches from transport or erection damage. This is less a problem with galvanized sections; cosmetically due to the tough nature of the galvanized coating; and from a corrosion prevention perspective because of its ‘self-healing’ galvanic protection.

The categories of AESS

Standard Structural Steel (SSS) is the baseline for technical reference. Its use is well understood and specified under Australian Standards such as AS 4100-1998, Steel structures and AS/NZS 4600:2005, Cold-formed steels used for light engineering purposes. Generally, the strength considerations of SSS take priority over its appearance, though in many cases it will be galvanized or painted for corrosion protection of aesthetics.

AESS 1 (Basic Elements)

is the first level above SSS. Enhanced workmanship is required, along with a superior finish. Examples of use include roof trusses in arenas; warehouses and canopies. Typically viewed at a distance >6 m, or out of normal eye-line, the price premium is relatively low at 20 – 60%.

AESS 2 (Feature Elements – viewed at distance >6 m)

Suitable for feature elements and requires good fabrication technique, enhanced weld treatment, connection and fabrication details, tolerances for gaps and copes. Applications include high-level roofs or ceilings, retail and architectural applications that can command the 40-100% cost premium.

AESS 3 (Feature Elements – viewed at distance ≤6 m).

In this category the architect begins to showcase the art of metalworking. Welds are generally smooth but visible and minor blemishes are acceptable. Tolerances are tighter than normal standards and – as the structure is likely to be touched by the public – it requires a smooth, uniform finish and appearance. AESS 3 may be found in airports, shopping centres, hospitals and lobbies where it attracts a cost premium ranging from 60-150% above SSS.

AESS 4 (Showcase Elements).

Used where the form will be the only feature showing in an element. Welds are ground and filled, with edges ground square and true. All surfaces are sanded and filled. Tolerances are more stringent at half those allowed for SSS. Welding is used extensively for joints, which are shop-prepared to maintain high quality. The coating finish is usually shop-applied to achieve the best quality outcome. Protection of the structure during transport and erection becomes a major issue and it is no surprise the cost premium is high at 100-250%.

AESS C (Custom Elements).

This category allows flexibility in the selection of characteristics, which can be drawn from any of the other categories. For example, custom elements may include 1.1 Surface Preparation; 2.2 Half standard fabrication tolerances; 3.1 Mill marks removed; and 4.2 Welds Contoured and Blended. The final surface finish may be specified as hot dip galvanized ‘Special Surface Quality’. In that case an additional Custom Element may be required to specify the grade of steel needed to achieve a consistent coating. The range of premium from a low 20 % to a high 250% is consistent with the range of Custom Elements that may be selected.

Galvanizing of AESS structures.

Galvanizing is increasing popular as a finish for AESS structures. Whilst the patina and texture can provide a striking finish, the primary role of a hot dip galvanized coating is to provide corrosion protection. Galvanized coatings will vary in colour depending on the grade of steel, its shape and thickness, the application technique and the skill of the operator.

Industrial Galvanizers has been involved in many architectural projects where consistency of surface finish is of paramount importance. It is vitally important that there is communication between architect, engineer, fabricator and galvanizer for a successful and cost-effective outcome. The earlier such communication begins the better will be the result. The galvanizer can provide advice on the size of sections that can be effectively dipped as a ‘single dip’ as well as recommending suitable schedules for the venting and draining of hollow sections. Similarly, thinner steel sections are susceptible to distortion when immersed in molten zinc, but Industrial Galvanizers representatives can provide advice on how this can be avoided or minimised.

Hot dip galvanizing can be applied to complex shapes of many sizes, with a zinc coating applied in hours to all internal and external surfaces.

A premium galvanized coating depends on surface preparation. For a Special Surface Quality finish, removal of weld spatter and proper attention to welds is required, prior to grit blasting of the steel. On delivery to the galvanizer, the steel must be ‘jigged’ (hung on moveable frames) in such a way to allow for proper drainage and – if hollow – venting of the work. Failure to do this with skill and attention to detail will result in uneven coating, inconsistent colour, damage to the product and increased cost.

The hot dip galvanising process ensures the removal of grease, dirt and scale by processing through a series of tanks containing caustic or acid solutions and rinse water.

The clean steel components are immersed in a preflux solution and then in a bath of molten zinc at 465oC. At this time there is a metallurgical reaction between the steel and the zinc, with a series of zinc-iron alloy layers being formed. This tough alloy coating is highly durable and provides threeway protection to the underlying steel:-

1. An external zinc patina is formed that prevents corrosion;
2. The zinc coating provides galvanic protection and is preferentially sacrificed to protect the underlying steel;
3. The alloy coating is tough and resists scratches that would render a thin paint coating vulnerable to attack by corrosion elements.

It is not necessary to further treat hot dip galvanized steel, though it can be successfully painted or powder coated to enhance aesthetics or increase longevity in specific environments.

It should be recognized that galvanised steel changes colour over time as the zinc patina develops. What comes out of the galvanizing bath as a bright, shiny finish will change to a dull grey surface over a period of time under normal atmospheric conditions. This is a natural and desirable outcome of galvanizing, but often a surprise to those unfamiliar with the process.

Care should be taken when specifying hot dip galvanizing for AESS. If cosmetic (aesthetic) appearance is of concern, the architect or specifier should select an experienced galvanizer – such as Industrial Galvanizers - and discuss any concerns about aesthetics (surface appearance) prior to any galvanizing. In most cases, Industrial Galvanizers can provide Visual Samples of the dipped components, as per the AESS Category Matrix.


Use of AESS is increasing and it is no longer just used for high profile projects. AESS is appearing in shopping centres, hospitals and super-store warehouses.

There has been a growing need to categorise and standardise the requirements and terminology around the use of AESS. The recently developed Guide for Specifying Architecturally Exposed Structural Steelwork (Boake) provides a consistent framework for architects, engineers, fabricators and coating specialists. It provides a categorised framework for communication between the parties before and during construction.

Further, it shows that AESS standards do not need to be equally applied across a project: AESS 2 may be applicable in some spaces, and AESS 4 prescribed in more visible areas. One outcome of this is in controlling the cost of the project, which could be prohibitive if the same high standard was maintained in areas where it is not required.

The finish coating of AESS is a key element. Where galvanizing is specified it is important that the architect, engineer and fabricator begin a dialogue with the galvanizer as early as possible. This will ensure that the correct steel chemistry is specified and that design considerations are included for the proper handling and processing of galvanized AESS.

In this way, the superior finish required of an AESS structure can be delivered, resulting in long-lasting and iconic infrastructure.


Australia/New Zealand guide for specifying architecturally exposed structural steelwork, Boake, T.M., Published by ASI / SCNZ, October 2012

Architecturally Exposed Structural Steel, Modern Steel Construction, May 2003

Architecturally Exposed Structural Steel (AESS): Sample Specification (for Engineers), Australian Steel Institute, 2012

Architecturally Exposed Structural Steelwork (AESS): Code of Practice (for Fabricators), Australian Steel Institute, 2012

Iron Bridge, completed in 1779, was built using concepts found in carpentry – including mortice & tenon and blind dovetail joints.
Federation Square, Melboune: galvanised interior AESS. Given the height of the building, most of the sections and joints are more than 6m away from the viewer. A different AESS standard can be applied compared to steelwork at ground level, which is highly visible.
Star City Casino, Sydney: AESS 2 standard preparation required as galvanized steel to be sandwiched between interior and exterior glass. Provides a visible framework for the building but will not be subject to close inspection by patrons.
Tempus Two Vinyard, Hunter Valley: Galvanized steel at AESS 3 standard was used in this outdoor undercover seating area. Highly visible and accessible to the public, surfaces needed to be smooth with a consistent and visually appealing finish.
Olympic Tennis Centre, Homebush: Galvanized steel at AESS 3 standard was used in this outdoor arena. High volume public pedestrian use called for smooth steel surfaces. Industrial Galvanizers provided a Special Surface Quality finish for this high profile venue.