Alan Harrison, technical advisor for the British Stainless Steel Association, explains why the use of stainless steel is so varied in architectural applications
Since its invention in 1913, stainless steel has been used in a wide variety of architectural applications, stretching back to New York’s famous Chrysler Building in 1929 and taking in iconic projects such as London’s Lloyd’s Building and Canary Wharf. Some of these applications are obvious such as cladding, roofing, handrails and balustrading, and rainwater goods, whereas others, such as stainless steel building support products and reinforcing bars, are not.
In all of these applications the dominant properties are durability, aesthetic appearance, mechanical strength and ease of fabrication. The first two of these derive from the self-healing, passive, chromium-rich oxide film, which protects the steel surface from all but the most aggressive environments. The second two derive from the metallurgical structure, which arises from the carefully controlled chemical composition and manufacturing process route.
The choice of grade and surface finish, plus good design and fabrication practice, ensure that stainless steel meets the demanding expectations of architects.
What is stainless steel?
Essentially, stainless steel is a steel with a minimum of 10.5 per cent chromium. In practice, for architectural applications, the chromium content is usually higher at about 18 per cent and often contains elements such as nickel and molybdenum for improved fabrication and corrosion resistance. The two most common grades in the architectural sector are 304 (EN 1.4301) and 316 (EN 1.4401). However, newer grades are finding a role. For example, the increased strength of duplex grades leads to a benefit in thickness and weight reduction and some ferritic grades can match the corrosion resistance of 304 and 316 while being somewhat cheaper due to the absence of nickel.
The grade of stainless steel can therefore be matched to the structural properties required and the environmental conditions it will have to withstand. These range from benign internal and rural atmospheres to severe marine atmospheric or urban environments.
Stainless steel can be supplied in a wide range of surface finishes, depending on the effect that the architect wants to achieve. At one end of the spectrum is a mirror polished or bright annealed surface: an approach exemplified by the Bristol Planetarium, which reflects the sky and streetscape in its spherical structure.
For where the architect requires a more subdued approach, brushed, bead blasted or patterned stainless steel surfaces can be chosen. It is even possible to mimic a bead blasted surface by rolling it on at the steel mill, as in the award winning BDP Building in Manchester. The insulated stainless steel panels on the south side of the building help to achieve the Excellent BREEAM environmental rating.
The addition of a wide range of coloured stainless steel is also providing architects with greater design options. Colouring can be achieved using chemical, electrochemical and physical vapour deposition (PVD) processes. A striking example is the Wales Millennium Centre in Cardiff.
In many cases, stainless steel works behind the scenes where specialised brick, masonry and concrete supporting products are typical of this sector. Few people outside the industry would recognise the contribution of the 1,000 special stainless steel wall ties to the striking brick facade at St Pancras Station.
However, sometimes aesthetic and structural properties come together in buildings, such as the recently extended Whitworth Gallery in Manchester. In this case, bright polished 304L stainless steel columns were used as internal structural elements in the gallery space. They act both as essential structural elements and to reflect the surrounding park environment. Even before the official reopening in October 2014, the project has drawn favourable comments from many sources.
The use of stainless steel for reinforcing bars is a small but vital part of this sector. This is particularly true for coastal applications or where run off from road salt produces an aggressive environment where normal carbon steel reinforcement is vulnerable. Stainless steel reinforcement is also used in specialised buildings such as an MRI scanner suite where the non-magnetic nature of austenitic stainless steel is required for non-interference with the MRI technology.
Like all materials, good design principles need to be followed to ensure optimum performance. For stainless steel this includes clean flowing lines avoiding places where rainwater can accumulate, maximum natural rainwater washing especially at coastal locations, access allowance for maintenance and cleaning, and understanding the technical requirements of specific environments, for example, swimming pools.
Occasionally, very little needs to be said because the building says it all. An example of this is the King Abdulaziz Cultural Centre at Dhahran Saudi Arabia, due for completion in 2015. This uses stainless steel to produce a magnificent structure which is also capable of meeting the LEED environmental standard.
Stainless steel provides a versatile material to meet the most demanding environments. It is a material well able to fulfil the dreams and aspirations of the most visionary of architects.