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STAINLESS STEEL IS SELF-PROTECTING, SO MAINTENANCE IS KEPT TO A MINIMUM

STAINLESS STEEL

Stainless steel is a material that naturally appeals to architects.

A quintessentially modern material, it also works ideally with other materials. But popularity does not necessarily mean that it is understood.

In this supplement, sponsored by leading stainless steel manufacturer Ugine & Alz, we make matters clearer and investigate the potential of stainless steel in construction.

WHAT IS STAINLESS STEEL?

Stainless steel is steel containing a minimum of 10.5 per cent chromium along with other alloying materials, thus forming specific grades. Its unique resistance to corrosion is an intrinsic quality. This is thanks to the reaction of chromium with oxygen and the creation of a self-protecting passive layer on the surface.

If this layer is damaged, it reforms spontaneously, continuing the protection. Using other alloying components, such as molybdenum and nickel, will further enhance the properties of the metal.

Four main types of stainless steel exist. They are martensitic, ferritic, austenitic and duplex. The austenitics include the grades probably already best known to the architect as 304 grade and 316 grade. The 304 grade contains both chromium and nickel, and with the addition of molybdenum the 316 grade increases its corrosionary distance and allows stainless steel use in the more aggressive environments found in heavy-industrial or marine locations. Making the correct choice of grade is of key importance and depends on the type of environment to which the building will be exposed. All grades of stainless steel have different mechanical, physical and corrision-resistance properties.

It is important to seek professional advice when selecting the appropriate grade.

WILL IT RUST?

Stainless steel will not rust. One of the earliest architectural applications of stainless steel was for the cladding on the top of New York's Chrysler building, designed by William Van Allen and completed in 1929. At almost 80 years old, the durability of this cladding is a tribute to the properties of stainless steel.

The table opposite shows the corrosive resistance of two common grades of stainless steel in a variety of environments.

This shows that the 304 (1.4301) grade can be considered for most sites, except either heavily polluted industrial sites or marine locations. In these cases the 316 (1.4401) or 316L (1.4404) grades would be more appropriate.

As well as the overall environment, local conditions will have an effect. For example, natural rain washing is an advantage, as it reduces the corrosion risk from pollutants or condensates.

Because stainless steel is self-protecting, rather than relying on an applied protective finish or a patina to provide protection, maintenance is kept to a minimum.

IS STAINLESS STEEL EXPENSIVE?

Stainless steel may have a reputation as an expensive material, but if you look beyond the initial cost of the metal and interrogate the life-cycle cost, it becomes much more affordable. This is due, in part, to the longevity of the material and its low maintenance requirements. Another contributing factor often overlooked is the benefit to the overall build-up of the supporting structure and the fact that stainless steel is non reliant for ventilation to its underside.

Warm-roof design and construction is increasing and poses no threat to stainless steel.

Ugine & Alz calculates that it can provide stainless steel for roofing and cladding applications for as little as £8.50/m 2 (metal manufactured price only).

IS STAINLESS STEEL ENVIRONMENTALLY FRIENDLY?

Ugine & Alz is part of giant European steel company Arcelor, whose production sites are ISO 14001 certified. Arcelor's manufacturing process includes the recycling of large volumes of steel scrap, and at the end of its life all stainless steel is fully recycled.

One other main environmental advantage comes from the inert nature of stainless steel. Because stainless steel has no corrosion products forming on its surface there is no toxic run-off bi-product, making rainwater simple to harvest and reuse.

WHAT FINISHES ARE AVAILABLE?

Just as there is more than one type of stainless steel, so there are also numerous surface finishes available, including the possibility of colours.

Finishes offered by Ugine & Alz include:

Uginox AME, which is a stainless steel with a tin-coated surface that will gradually acquire a patina through weathering. Its appearance makes it suited to use in external applications where lead might have been the metal of traditional choice. Uginox AME offers the advantages of low weight, durability, minimal maintenance and no underside corrosion. The tin coating remains environmentally inert.

Ugibat, which is a light grey with a soft sheen and very good uniformity of finish and flatness, enabling it to be used in a variety of different applications.

Ugitop, which has a matt surface with a low-reflective finish.

Having a stable finish which can be simply maintained has made this stainless steel a favourite for surfaces where natural rain washing could be difficult.

Ugibright, which has a dramatic clean finish that can be further mirror polished. It is particularly valuable for use in reflecting the surroundings, giving a changing appearance throughout the night and day.

Ugisand, which has a very even surface finish and a textured appearance. It resists marking and is silky smooth to the touch.

Mecachim, which is a range of coloured and textured stainless steels, suitable for both internal and external applications. Coloured by either a patented Polispectral process or by titanium vapour, the steels come in a wide palette of colours and can also be stamped and patterned to a variety of finishes.

CASE STUDY:

The Public Architect: Alsop Architects

Subcontractor: Richardson Roofing Will Alsop is famed for his love of non-rectilinear shapes, and The Public in West Bromwich is no exception. Eight years in the making, this socially engaged community arts project is based on a box, but it is certainly a box with a difference. Clad in a black metal skin with a pink interior, it has irregularly shaped 'jellybean' windows. But this was not enough irregularity, so there are two additional shapes, a wall-clinger known familiarly as 'the rock' and a ground-hugging shape called 'the pebble'. In addition, there are faceted structures on the roof. All these 'extra' structures are clad in stainless steel from Ugine & Alz.

On 'the pebble' the material is 0.5mm mirror-finish Ugibright stainless-steel standingseam cladding. It is laid over a timber structure that has been built up rather in the same way as a boat, with a series of ribs and beams.

The same material was used to clad 'the rock', but in this case the cladding is faceted, rather than curved. Instinctively, one might think that this was a simpler solution, but Max Titchmarsh of Alsop Architects explained that it was actually more difficult. The shape is made up from prefabricated timber trusses fixed to steel cleats on the outer face of the cladding substructure. Triangular windows were fitted in, and then the cladding was applied. Because of the constantly changing angles, a lot of thought had to be given to the positioning of the seams, so that the water ran off in sensible ways. 'Sometimes we had to use flattened seams, ' explained Titchmarsh, 'so that each facet is articulated.'

The rooftop structures are also relatively complex, and again are clad in Ugine & Alz's Ugibright standing-seam system, 0.5mm thick.

The architect considered alternatives, such as composite-aluminium panels, but decided they would not be able to deal with the complexity of form.

CASE STUDY:

Doncaster Interchange Architect: Carey Jones Architects Frenchgate Interchange, Doncaster, is an £80 million project for Teesland Property that provides a fully integrated retail centre and transport interchange for this South Yorkshire city. It connects the existing Frenchgate Centre and the listed railway concourse buildings, reinforcing the excellent transport links in the area. Frenchgate Interchange provides a new 30stand, 'state-of-the-art' bus and coach facility, and a central multi-storey car park.

The development creates 30,000m 2 of top-quality retail space, which boasts high-profile tenants, such as Debenhams and Next.

It consists of six storeys plus mezzanines between ground, first and second floors. The primary structure consists of a steel frame acting compositely with precast-concrete floor units.

Because the building is both bulky and in a key position, the design was most important, since a lot was riding on it in terms of regeneration of the town centre. In order to break up the mass to some extent, a number of treatments are used. But most striking is a great curved wall that overlooks the railway line. Because of this and the functions contained within the building, this wall is almost entirely free of windows, making it dramatic but potentially forbidding.

The architect has dealt with this issue by adopting an approach that is both contemporary and yet links with some of the city's heritage.

This is the use of Uginox AME standing seam with vertical seams, which will weather to a lead-like finish. There is a slightly fluted effect in the form, like a gentle series of waves, which plays interesting games with the light. The flexibility of the metal makes it possible to achieve this form relatively easily.

In addition, the panel sizes give a more 'human' scale to the structure, and a feeling of something that has been handcrafted.

CASE STUDY:University of Hertfordshire

Architect: RMJM Subcontractor: Pace Roofing The University of Hertfordshire was one of the most ambitious single-phase educational development projects in the UK for 50 years. Set on the old de Havilland aircraft site, the campus offers a mix of academic, residential and sports facilities.

At its heart is a 500-seat auditorium clad in Ugitop stainless steel. Designed to be a flexible building, adjustable to suit different sizes of audience and with excellent acoustics, it has a beautifully considered stainless-steel roof, sat on top of it and bent over on either side.

Slightly anthropomorphic, it is also reminiscent of the roof of one of the three clustered pods on Renzo Piano's Casa de Musica in Rome. Like that building, the acoustics were taken seriously, and have a defining effect on the form.

The effect is of solidity and heaviness, but in fact the solution is relatively light, since the roof is covered with Ugine & Alz's stableappearance Ugitop 43 standing-seams and the walls in shingles in the same material.

In 2005, the building won a Stainless Steel Excellence Award. Colin Moses, managing director of RMJM, said the material 'suits the form of the building, is perceived as high quality, and provides a visually distinctive solution'.

CASE STUDY:Antwerp Law Courts

Architect: Richard Rogers Partnership/VK Studio Engineer: VK Engineering/Ove Arup The dramatic roof of Antwerp's new law courts is a landmark for the city, with its pointed roofs sparkling in the sun reminiscent of the cone hats, or hennins, that are familiar from the paintings of Memling. They are part of the strategy, by architect Richard Rogers Partnership, to create a legal building that is relatively transparent and pleasant to use, with plenty of natural light. One set of roof structures tops each of the eight main courts, composed of four doubly curved hyperbolic paraboloids. They are like a simple rectangular grid that has been pulled up (or down) at the corners to create the double curve.

On any roof two of these forms are much higher than the other two. The gap between is filled with vertical north-facing glazing to bring in natural light. Overhangs provide solar shading.

Held within a steel frame and then built up from a laminated timber structure that is exposed on the inside of the courts, this is a gridshell structure, so the entire load of the roofs is transferred to the perimeter and down to the building's structure. The roofs are clad with Ugine & Alz's 0.4mm-thick Ugibat stainless-steel roofing in strips 600mm wide. 'Because it is not malleable, it fioilcansfl which we quite like, ' said project architect Avtar Lotay, but the contractor laid it out carefully to get the least deflection.

Some of it has been cut to almost a banana shape to minimise oilcanning. The strips were folded over each other and welted, rather like a standing-seam roof. This approach is, says Lotay, 'very durable'.

The roofs were manufactured offsite and brought in by barge. Construction of the first one took four weeks, but once it was in full swing a complete unit was produced every week. As well as making site work simpler, this approach ensured quality control, since construction of the roofs took place under cover whatever the weather. Once the roofs were on site, they just had to be 'zipped together' and elements like the glazing added.

Lotay said that the architect did look at alternative materials, including zinc. 'But, ' he said, 'we would have had to do all sorts of protective details. And zinc corrodes. So we said to the client that stainless steel was best and what we should really go for.'

Judging by the result, the inhabitants of Antwerp will agree. It is hard to imagine another material that could brighten the scene in quite the same way. 2.

CASE STUDY:The Atomium, Brussels

Architect: Conix Architecten The Atomium in Brussels, constructed for the 1958 Brussels World Fair, is one of those temporary buildings like the Eiffel Tower that stayed on to become a well-loved icon. Based on an atomic structure magnified 165 billion times, it was in need of refurbishment and recladding. Most dramatic has been the replacement of the original failed aluminium with Ugibright stainless steel, chosen for its durability, robustness and lustre.

Because one of the main desires was to make the Atomium shine again, the 1.2mm-thick sheets were given a special electrolytic surface treatment to enhance their sheen.

The operation has been a major one.

The structure is 102m high, and each of the nine spheres, packed in a body-centred cubic configuration, has a diameter of 18m. There is public access, which means that the spheres have 'windows' set into them. They serve different functions, including exhibition spaces, a conference room and a children's sphere. The new cladding consists of 50,000 pieces of steel, with a total weight of 130 tonnes and a surface area of 9,000m 2, made in the Ugine & Alz plant at Genk. There are 2,000 different designs of cladding panel. Ugine & Alz worked closely with the client and architect to provide solutions for this complex project.

The cladding was fixed to a specially made steel frame, and access had to be carefully managed for both effectiveness and safety, with erectors in harnesses delivered to the surface in the most appropriate manner possible.

The recladding was completed in September 2005 and the Atomium reopened in February this year, looking as good as, or probably better, than it did when it first opened nearly 50 years ago.

This time, thanks to the use of stainless steel for cladding, it should not lose its lustre over time.

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