\nStructural applications of stainless steel have generally used austenitic grades: types 1.4301\/1.4307 for rural, urban or lightly industrial environments and types 1.4401\/1.4404 for heavily industrial or marine environments. The use of duplex stainless steels is relatively new to the construction industry although they have a long track record in the petrochemical and pulp and paper industries. These alloys have high strength (design strengths around 450 N\/mm\u00b2) and ductility (>20%) with good weldability and formability. They have tremendous potential for expanding future structural design possibilities, enabling a reduction in section size and leading to lighter structures. A range of open and closed structural profiles are available in duplex grades. Design guidance can be found in the Design Manual for Structural Stainless Steel [1]<\/span> and updated guidance on fabrication has just been published [2]<\/span>.<\/p>\n
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<\/a>Figure 1: Cala Galdana Bridge, Menorca (Photo: Pedelta)<\/p><\/div>\n
<\/a>Figure 2: Ruffolo footbridge in Siena, Italy (Photo: Outokumpu)<\/p><\/div>\n
<\/a>Figure 3: Installation of stainless steel accommodation modules on the Armada Platform (Photo: Mech-Tool Engineering)<\/p><\/div>\n
Duplex grade 1.4362 was used for the arches of the Celtic Gateway footbridge in Holyhead, Wales, which was opened in 2006. This grade has a corrosion resistance similar to that of austenitic grade 1.4401 and twice the design strength. The more highly alloyed duplex grade 1.4462 was used for the arch of the Millennium Bridge in York and the first stainless steel road bridge, built in Menorca in 2005 (Figure 1). This grade displays superior corrosion resistance, especially to stress corrosion cracking.<\/p>\n
High nickel prices have more recently led to a demand for \u2018lean\u2019 duplexes with very low nickel content, such as grade 1.4162. The corrosion resistance of grade 1.4162 lies between that of 1.4301\/1.4307 and 1.4401\/1.4404. Due to its low nickel content of 1.5% (typical stainless steels have >3%), it is an economical alternative to other austenitic and duplex grades whilst also experiencing lower price volatility. One of the earliest structural applications of this grade was a footbridge with a 120 year design life in the suburb of Ruffolo, Siena in Tuscany. The cable stayed bridge spans 60m over a busy motorway and was completed in 2006 (Figure 2). Lean duplex 1.4162 was used for the pylons and load carrying longitudinal and transverse beams.<\/p>\n
Exploiting the distinctive properties of stainless steel<\/strong> The strength and stiffness retention characteristics of stainless steel at elevated temperatures also differ from carbon steel. Austenitic stainless steels show superior strength retention properties above temperatures of around 550\u00b0C and superior stiffness retention at all temperatures. Retention factors for the key grades of stainless steel are reported in the Design Manual for Structural Stainless Steel [1]<\/span>.<\/p>\n The combination of resistance to fire and blast coupled with exceptional durability has led to many structural applications in the oil and gas industry. Stainless steel grade 1.4401 was chosen for the fire and blast rated cladding for four new accommodation modules and two walkway modules installed earlier this year on BG Group\u2019s Armada Platform in the Central North Sea. The cladding, formed from corrugated sheet 80mm deep and 2mm thick, is designed to resist blast pressures of 110 millibar.<\/p>\n Hot rolled and welded stainless steel sections<\/strong> In 2003, SCI developed web-based design software for cold-formed stainless steel members such as hollow sections, channels and equal angles (available on www.steel-stainless.org\/software<\/a>). The software calculates section properties and member resistances for user-defined sections. It aligns to the Eurocode for structural stainless steel [3]<\/span> and includes a fire resistance design facility that estimates how quickly a section will heat up in standardised fire situations and its reduced resistance after exposure to the fire.<\/p>\n
\nThe structural performance of stainless steel differs from carbon steel because stainless steel has no definite yield point and shows an early departure from linear elastic behaviour with strong strain hardening. The shape of the stress-strain curve in the plastic range ensures higher plastic moment resistance than carbon steel of equivalent strength. Stainless steel is therefore an ideal material for explosion-resistant structures because it has high strength, good energy absorption characteristics and high ductility. An additional advantage when considering blast resistance is that the strain-rate sensitivity is more pronounced in stainless steels than in carbon steels with a proportionally greater strength realised at fast strain rates for stainless steel than for carbon steel, particularly in the region of the 0.2% proof strain.<\/p>\n
\nThere is no standardised family of sizes for stainless steel structural sections. Hollow sections are available in a wide range of sizes, with each manufacturer tending to stock its own range. Channels and angles tend to be produced by roll forming (rolling or bending) and are frequently made to order. The availability of hot rolled and laser welded I sections is increasing.<\/p>\n