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stainless steel pipeStainless steel in construction

Stainless steel has unique properties which can be taken advantage of in a wide variety of applications in the construction industry. This paper reviews how research activities over the last 20 years have impacted the use of stainless steel in construction. Significant technological advances in materials processing have led to the development of duplex stainless steel pipe with excellent mechanical properties; important progress has also been made in the improvement of surface finishes for architectural applications Structural research programmes across the world have laid the ground for the development of national and international specifications, codes and standards spanning both the design, fabrication and erection processes. Recommendations are made on research activities aimed at overcoming obstacles to the wider use of stainless steel in construction. New opportunities for stainless steel arising from the shift towards sustainable development are reviewed, including its use in nuclear containment structures, thin-walled cladding and composite floor systems.

Stainless steel has many desirable characteristics which can be exploited in a wide range of construction applications. It is corrosion-resistant and long-lasting, making thinner and more durable structures possible. It presents architects with many possibilities of shape, colour and form, whilst at the same time being tough, hygienic, adaptable and recyclable.

The annual consumption of stainless steel has increased at a compound growth rate of 5% over the last 20 years, surpassing the growth rate of other materials. The rate of growth of stainless steel used in construction has been even faster, not least due to rapid development in China. It is estimated that in 2006, approximately 4 million tones of stainless steel went into construction applications worldwide, 14% of the total quantity consumed.

Stainless steel has traditionally been used for facades and roofing since the 1920s. There are also early examples of it being used structurally, for example in 1925 a reinforcing chain was installed to stabilize the dome of St Paul’s Cathedral, London. Nowadays, stainless steel is used in a very wide range of structural and architectural elements, from small but intricate glazing castings to load-bearing girders and arches in bridges.

This paper seeks to summarise the recent technological advances in the stainless steel sheet which have had an impact on usage of stainless steel in construction. New applications which have emerged over the last 20 years are described. Areas of research needed to respond to current market and procurement challenges are discussed. Finally, new opportunities arising from the shift towards sustainable development are described.

Stainless steel producers are continually developing their manufacturing processes with the aim of reducing costs, lowering emissions, shortening lead times and improving quality. These improvements have helped to control the cost of stainless steels, within the constraints set by the dependence on raw materials.

Perhaps the most significant recent advance impacting the construction sector has been the use of duplex grades for structural applications, which offer a combination of higher strength than the austenitics (and also the great majority of carbon steels) with similar or superior corrosion resistance. Table 1 compares the composition and mechanical properties of the two widely used austenitic stainless steel coil, 1.4301 and 1.4401, with those of three duplex stainless steels. (The ferritics in the table are discussed in Sections 3 Expansion of construction applications over the last 20 years, 4 Research in response to market and procurement challenges.) Duplexes have tremendous potential for expanding future structural design possibilities, enabling a reduction in section sizes leading to lighter structures. It is worth noting that although they have good ductility, their higher strength results in more restricted formability compared to the austenitics.

The corrosion resistance of duplex grade 1.4362 is similar to that of 1.4401. The more highly alloyed 1.4462 displays superior corrosion resistance, especially to stress corrosion cracking. High nickel prices have more recently led to a demand for lean duplexes with low nickel content, such as grade 1.4162 shown in the table. The corrosion resistance of grade 1.4162 lies between that of 1.4301 and 1.4401; it currently costs slightly less than grade 1.4301.

Although usually used internally in buildings, some ferritic grades have been developed which are suitable for building envelope and structural products. For example, over the last 10 years, grade 1.4510 has been used widely in France in a tin-coated roofing system. This tin-coated finish weathers over time, gradually developing into a matt-grey patina.

Over the last 20 years, significant developments have occurred in materials processing and finishing technology, often driven by exacting architectural requirements for specific projects. The range in surface finishes has extended, ranging from matt to shiny, smooth to very rough, with combinations possible by juxtaposing finishes, adding colour etc. More finishes have become available—involving metallic and organic coatings, electrolytic and PVD (Physical Vapour Deposition) coating processes or skin passing operations. They have improved the competitive position of stainless steel compared to other high volume metallic roofing materials such as zinc, aluminium, copper and even carbon steel. The performance of the stainless finishes has also been improved in order to meet strict hygiene and cleaning requirements. Improved manufacturing processes have resulted in greater consistency of surface finish, both across a sheet and from batch to batch. Products are also now able to meet tighter dimensional tolerances.

Traditionally stainless steel welded tubes were produced by tungsten inert gas (TIG) welding. However, with the advent of reliable, high-power laser power sources, the laser beam welding (LBW) process has moved quickly into the production of stainless steel longitudinally welded tubes. The energy concentration reached in the focused spot of a laser beam is very intense and is capable of producing deep penetration welds in thick section stainless steel, with minimal component distortion. The process originally employed high capital cost equipment and its use was reserved for mass production manufacturing. However, now that more compact equipment has been developed, the use of laser welding is becoming more widespread. In addition to hollow sections, laser welded stainless steel I sections, angles and other shapes are now available (Fig. 2).

In recent years there has also been a dramatic increase in the use of stainless steel profiles in which a focused laser beam is used to melt material in a localised area. A co-axial gas jet is used to eject the molten material from the cut and leave a clean edge with a continuous cut produced by moving the laser beam or workpiece under CNC control. There is no tooling cost, prototyping is rapid and turn around quick. The improvements in accuracy, edge squareness and heat input control mean that other profiling techniques such as plasma cutting and oxy-fuel cutting are being replaced by laser cutting.

2.2. Design

The development of codes, standards and specifications for stainless steel as a result of research studies carried out by industry and academics has played a significant role in enabling the wider use of stainless steel in construction.

The structural performance of stainless steel differs from that of carbon steel because stainless steel has no definite yield point and shows an early departure from linear elastic behaviour with strong strain hardening. There can also be significant differences between the stress–strain curves for tension and compression. This has implications on the buckling behaviour of members and the deflection of beams. Designers require guidance on grade selection and the use of stainless steel in contact with other materials (e.g. carbon steel, reinforced concrete, masonry, timber and aluminium) in order to avoid corrosion between the dissimilar materials. Methods of connection also require specific guidance, particularly where welding is concerned, to maintain surface finish and corrosion resistance.

Prior to the development of design standards for structural stainless steel, designers were forced to conduct their own investigations or abandon stainless steel in favour of alternative materials which have proven track records and design guidance. They were required to work from first principles with an unfamiliar and costly material with unusual mechanical properties. This was an unsatisfactory situation; at best it was wasteful of the designer’s time, at worst it led to misconceived design practice, misuse and either unserviceability or failure.

The Gateway Arch in St Louis, Missouri, inspired a great amount of research into the structural performance of stainless steel in the US in the early 1960s. The first American specification dealing with the design of structural stainless steel members was published in 1968 by the AISI [1]. Following an extensive research project at Cornell University, in 1974 the specification was revised and published as the Specification for the Design of Stainless Steel Cold-Formed Structural Members[2], and this has subsequently been extended and updated in 1991 and 2002. Australia, New Zealand and South Africa have published approximately equivalent standards largely based on the American standard 3., 4..

In 1995, the Design and Construction Standards of Stainless Steel Buildings were published by the Stainless Steel Building Association of Japan [5]. These specifications cover the design of welded, fabricated sections from relatively thick plate. A recent Japanese research programme studied the behaviour of lightweight stainless steel members and the Design Manual of Light-Weight Stainless Steel Structures was subsequently published in 2005 [6].

Between 1989 and 1992, SCI carried out a research project to develop European guidance in the areas of material selection, design, fabrication and maintenance to ensure the safe and proper application of steel in construction. The project included forming a properties database, materials tests, member and connections tests, analysis of results, design recommendations and worked examples. The resulting guidance was published by Euro Inox in 1994 as the Design Manual for Structural Stainless Steel[7]. Subsequently the draft pre-standard Eurocode 3 Part 1.4, giving rules for the design of structural stainless steel pipe fittings, was published in 1996, closely based on the Design Manual.

A European research project between 1997 and 2000 carried out a further programme of tests and analyses into the performance of structural stainless steel [8]. The results of the project were incorporated into the Second Edition of the Design Manual, published in 2002, with an extended scope including circular hollow sections and fire resistant design. A further European research project studied the behaviour of high strength structural members made from cold worked stainless steel through further tests and analyses between 2000 and 2003 [9]. The results were included in the Third Edition of the Design Manual, published in 2006. The same year, Eurocode 3: Part 1.4 (EN 1993-1-4) was issued as a full European Standard [10]. Its contents are aligned with the Design Manual, with the exception of the guidance on fire resistance where the Design Manual presents a less conservative approach. A Commentary to the Design Manual has also been prepared as a separate document which explains the basis of the recommendations and presents the results of relevant test programmes [11].

In a fire, austenitic stainless steel columns and beams generally retain their load-carrying capacity for a longer time than carbon steel structural members. This is due to their superior strength and stiffness retention characteristics at temperatures above 500 ∘C (Fig. 3). SCI has coordinated a year research project studying the behaviour in fire of a range of structural stainless steel solutions through testing and numerical studies. The project included fire tests on stainless steel and concrete composite columns and beams, separating structures and load-bearing systems designed to retard the temperature rise. Slender hollow sections were also studied. The final report is due to be published in 2008 [12].