The Stainless-Steel City

Article January 01, 2006
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Bridge design engineers at Arup Hong Kong had a stringent requirement for materials that would be used on a significant portion of their city’s new, nearly 1-mile-long Stonecutters Bridge: a 120-year maintenance-free service life.

The material selected to meet these rigorous structural and surface finish needs was duplex stainless steel, now being specified in bridge construction around the world. (This form of stainless steel is called “duplex” because it combines many of the beneficial properties of ferritic and austenitic steels, offering high strength and high resistance to stress corrosion cracking along with very good resistance to uniform corrosion.)

Over the past several years, conferences of NACE International (an association for corrosion engineers) and other engineering groups have featured an increasing number of papers and technical sessions relating to the use of stainless steel in bridge construction. As a result, “Stainless steel—particularly duplex stainless—is becoming the material of choice for pedestrian, road and highway bridge construction, especially in Europe and Asia,” reported Elisabeth Torsner, vice president of market development for Outokumpu in North America. “There is a wide range of duplex stainless-steel grades, so the selection of the most suitable type depends on factors such as environmental conditions, type of corrosion anticipated and required mechanical properties.”

Longest in feet and years

When completed in 2008, the pole towers of the Stonecutters Bridge will reach a height of nearly 1,000 ft above the entrance to Hong Kong’s Kwai Chung container port—soaring above a 250-ft-high deck for automobile traffic. The physical difficulty and cost of performing maintenance on a bridge of this height and length—it will be the longest bridge of its type in the world—was sufficiently daunting to warrant the maintenance-free, lower-cost life-cycle requirement.

Anders Finnas, Outokumpu’s bridge application manager, reported that for the Stonecutters Bridge, Arup Materials Consulting of London selected Duplex 2205 Code Plus Two hot-rolled plate (S32205) from Outokumpu to clad the top 120 meters of the towers with a stainless-steel skin. Outokumpu stainless pipe was selected for cable sheeting. In addition, an S30400 stainless-steel reinforcing bar from Cogne was specified for concrete piers and main-tower splash zones.

“Some carbon steels were determined to have the necessary structural strength for construction,” explained Finnas, “but none offered the nearly zero maintenance required.”

Duplex 2205 offered the material strength required for construction with the good forming and fabrication properties of hot-rolled plate. Further, a durability assessment of the marine environment in which the materials would need to perform indicated a C5M exposure—the worst possible exposure under the ISO environmental classification. Duplex 2205 exceeded that essential requirement, combining excellent resistance to pitting and crevice corrosion with a high resistance to chloride stress corrosion cracking.

The duplex stainless steel for the Stonecutters Bridge was delivered primarily as 3?4-in.-thick curved plate, which was then welded and bolted together and joined with 5-in.-wide stainless structural flanges top and bottom to allow adjacent sections to be bolted. When lifted into position, the structure formed a receptacle into which concrete could be poured. Some 40,000 11-in.-long, 2?3-in.-thick studs of Duplex 2205 were welded into the plates to provide the necessary bond to the concrete.

The beauty part of it

Although designers for the Stonecutters Bridge specified stainless steel for much of their bridge’s exterior to meet structural and zero-maintenance demands, other designers are selecting stainless steel for its aesthetic appeal. Over the past 20 years, stainless steel has been specified for a number of pedestrian bridges, particularly in Europe.

“In these applications aesthetics have often played a large role in material selection,” noted Torsner. “For example, the world’s first bridge to use heavy duplex stainless-steel plate as its main structural element was built in Bilbao, Spain.”

The 469-ft-long Padre Arrupe Bridge, which opened in June 2005, leads pedestrians over the Nervion River to Bilbao’s famous Frank Gehry-designed Guggenheim Museum. The duplex stainless selected for the bridge, SAF 2304 (S32304) from Outokumpu, is expected to provide the 26-ft-wide structure with a life span of more than 50 years, protecting against exposure from the corrosive brackish river water underneath it and from salt-laden air out of the nearby Bay of Biscay. The bridge was designed in individual sections with opposing ramps that form stable trestles enabling the bridge to span, unsupported, across more than 260 ft. Side barriers and handrails for the bridge feature stainless tube and bar materials.

Why not choose both?

Finnas was quick to note that some ingenious bridge designs have incorporated stainless steel for both aesthetic and mechanical reasons. A covered footbridge, the Helical Bridge, which spans a narrow canal in a business district in central London, has “an unusual support structure made entirely of stainless steel,” according to Finnas.

Steel tube, bent into a spiral shape, is braced by six square-section transoms welded along its length. These transoms also serve as fixing points for curved glass panes on the tube’s interior. Since the canal has steady water traffic, the bridge is designed to retract. Bridge designers mounted the deck on a motor-driven trolley chassis that moves linearly on four concealed rails. As the bridge moves, the stainless-steel spiral takes on a rotary movement that looks as if the bridge is being “corkscrewed” into or out of the opposite bank. The nose end of the structure locks into a ramp on the other side of the canal when closed. The opening and closing procedure takes about 21?2 minutes.

Other duplex doings

“For bridges spanning less-corrosive fresh water, a lower-alloyed—and therefore less expensive—type of duplex stainless is beginning to find use,” noted Torsner.

In Siena, Italy, a 200-ft-long, all-stainless-steel cable-stayed footbridge was constructed from Outokumpo LDX 2101. This lean duplex grade provided good pitting and crevice corrosion resistance and high tensile and yield strength—but with a nickel content of only 1.5%. As a result, Torsner explained, LDX 2101 has less price volatility than high-nickel stainless steels.

The grade is ideal for load-bearing structures where moderate corrosion resistance is adequate such as in Siena, an inland city with low pollution levels. The high strength of LDX 2101 enabled the design engineers to build a lighter-weight structure compared to traditional bridge designs. Like the Stonecutters Bridge project, the hot-rolled plates for the Siena bridge were cut and bent into customized bridge components, and the edges prepared for welding.

“Weight—or lack thereof—can be another important factor in selecting stainless steel,” suggested Torsner. LDX2101 was selected for the 79-ft-long Likholefossen Bridge in the Gaularfjellet Mountains near Forde, Norway, in large part because the terrain necessitated bridge materials light enough to be installed by helicopter. “Outokumpu’s LDX 2101 duplex has sufficient corrosion resistance to stand up to an almost continuous fresh-water spray along with sun and wind in the summer and exposure to ice and snow in winter,” noted Torsner. Although she added that the road over the Likholefossen Bridge is not treated with deicing salt during the winter.

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