How Stainless Steel Became the Material Of Choice For Modern Bridges

Of all the materials used for bridges over the centuries, stainless steel will likely prove the most effective and durable. Europe led the way in using duplex grades, though some use was also made of austenitics. In the USA, a ferritic-martensitic dual-phase grade has found favor, but duplex grades are being assessed and more frequently applied. Among the reasons for this are the excellent mechanical properties of duplex, its corrosion resistance, and its favorable strength-to-weight ratio.

By James Chater

Beauty and Function

For centuries, bridges have been an essential element of civilization. Not only do they assure mobility and connectivity, but they also often serve as a rallying point or an expression of people’s aspirations for a better quality of life. At their best, bridges combine the beautiful and the practical.

Background

Materials used for bridges have included stone, wood, wrought or cast iron, steel, aluminum, reinforced concrete (often with stainless rebar) and stainless steel. Stone bridges can last a long time, but the material is heavy and unwieldy. Wood, iron, and steel require a great deal of maintenance, which is all too often neglected. Two steel bridge collapses in the U.S. (Silver Bridge, West Virginia, 1967; Mianus River Bridge, Connecticut, 1983) drew attention to the need for maintenance and helped usher in the current preference for stainless steel, which greatly reduces the need for maintenance and disruptive repair work. Corrosion played a role in both collapses, as it did in that of the Morandi Bridge in Genoa, Italy, in 2018.

Stainless Advantages

The reduced maintenance required when stainless steel is applied largely stems from the fact that it requires little or no anti-corrosion treatment, such as coating or painting. Over time, it has become evident that life-cycle costs for stainless steel are lower than for carbon steel, even if the initial cost is higher. The projected life of bridges made from duplex is about 100 to 120 years, while those made of austenitics can be expected to last about 70 years.

Additional reasons for using stainless steel include its aesthetic appeal (its ductility makes it easier to create attractive designs) and the fact that it is 100% recyclable.  A major cause of corrosion is salt, whether derived from sea air or seawater or from deicing salts. So maritime locations and cold winters are factors that influence the specification of stronger grades. Other factors include industrial or other pollution (especially from sulphur dioxide), a humid climate, and exposure to water (especially when the relevant bridge section is underwater and, therefore, difficult to inspect or maintain). Stainless steel use can be structural – girders, beams, bearings, reinforcing bars in concrete (rebars), anchors, arches, cables – or decorative (handrails, mesh). Structural elements often use plates welded into I-sections, whereas non-structural elements are normally made from sheets or hollows.

Austenitics

One of the earliest, if not the earliest, use of stainless steel on a bridge-like structure was the Progreso Pier in Yu-katán, Mexico (1941), where stainless rebar in 304 was applied; it is still in service. More recent examples include the Waldeck-Rousseau road bridge at Saint-Brieuc, France (1998), one of a series of bridges using austenitic grades (mostly 316L or 314) supplied by Arcelor, of which a more famous example is the Simone-de-Beauvoir pedestrian bridge in Paris (2006) spanning the river Seine. The use of austenitics in both structural and non-structural applications persisted even after duplex grades started to become popular in the first decade of the current century. They were applied on pedestrian bridges, where aesthetics rather than mechanical strength was the primary consideration, though one vehicular bridge that made extensive structural use of austenitics is the Story Bridge in Brisbane (2015; 316L applied in mesh, I-beams, rebar, angle bar, polished tube, and cable).

Duplex

The introduction of duplex stainless steel in bridge construction roughly coincides with the start of this century. Duplex trumps austenitics by offering greater mechanical strength for the same weight, for example, a favorable strength-to-weight ratio. Thinner gauges can be used, making off-site construction, transportation, and installation easier. As a structural, load-bearing material, duplex is unbeatable.

The first bridge in which duplex was applied was possibly the Suransuns pedestrian bridge in Switzerland (1999), but it was London’s Millennium Bridge (2001) that made people sit up and take notice. Designed by Foster and Partners, Anthony Caro, and Ove Arup and Partners, and using duplex 2205, this foot-bridge across the Thames drew much praise and has since become a tourist attraction. It ushered in two decades in which Europe took the lead in duplex stainless-steel bridges, with Spain, Italy (Siena), Sweden, and the UK being notable hotspots. Initially, only the standard type 2205 was applied, but from 2003 (Pedro Arrupe Bridge, Bilbao, Spain) lean grades such as 2304 and Forta LXD 2101 were used with growing frequency. The first projects were pedestrian and bicycle bridges, catering to the trend towards making city centers more accessible to non-vehicular traffic.

The first road bridge made of duplex (grade 2205), was located in Cala Galdana in Minorca, Spain, which became accessible in 2005. This was one of many examples in which outstanding beauty was allied to functionality. Dynamic, fluid designs became the order of the day, such as tilted arches and curved decks. The complexity that it is possible to achieve can be seen in the double-helix designs of the Helix (Singapore, 2009) and Arup’s Christopher Cassaniti Bridge (2020).

Colour and texture often play an important role, along with the play of light, changing weather, and water conditions. For instance, the Sölvesborg Bridge (2013), Europe’s longest pedestrian and bicycle bridge, has been widely praised for its graceful design (deck support, railings, and arches in Forta LDX 2101) and spectacular lighting.

Grade 410 and its Derivatives

Another important grade in bridge construction is 410 and its derivatives. Grade 410 is a martensitic-ferritic dual-phase metal known for its hardness and its resistance to mild corrosion. First developed in the 1970s by Columbus Steel, it was applied as rebar in two bridges built in Durban, South Africa, and, as SUS 410, in the Nou Road Bridge in Japan (2012). The patent was bought by ArcelorMittal in 1991, where it was known as Duracorr®. In the U.S., where Arcelor had a manufacturing base, it was specified as ASTM A1010 before being incorporated into the ASTM A709 specification in 2017.

Since then, it has been known by its current name, ASTM A709 Grade 50CR. End users started to specify 50CR for bridges where time-of-wetness and chloride exposure were too high to allow painted carbon steels or weathering steels to be used. In 2004, 50CR was applied to welded plate girders on the Fairview Road Bridge in California; it was the first time the grade had been used on U.S. bridges. Since then, about ten other bridges using 50CR have been built in the U.S. (including one at ArcelorMittal) and two in Canada. The alloy received a big boost after tests were carried out to see whether it was possible to decrease the amount of chromium to lower the material cost while still meeting A709 specifications. None of the alternatives proved satisfactory, confirming 50CR’s dominance.

Duplex Comes to The U.S.

Due to the 50CR’s limitations (it is a lean grade with low pitting resistance: see Table), and also perhaps because 50CR was never marketed as a construction grade (it started life as an alloy in salt truck liners, coal processing, power plants, and so on), end users started to consider duplex stainless steel.

According to Jason Provines, Senior Re-search Scientist at the Virginia Transportation Research Council, its advantages include “greater availability, a smoother supply chain, the existence of several standards and best practices… Overall, there was a feeling that we did not have to reinvent the wheel.” Several U.S. initiatives to assess duplex use in bridges were undertaken, including the establishment in 2022 of an AASTHO/NSBA Collaboration Task Group, TG18, with the intent to “develop a set of cohesive guide specifications that will allow for duplex stainless steel to be used for vehicular bridges in the United States”.

Perhaps the first time duplex was applied on a U.S. bridge was the Harbor Drive Pedestrian Bridge, San Diego (2011). It is one of the longest self-anchored pedestrian bridges in the world, in which the cables are hidden inside welded pipes. The suspender cables and safety mesh were made in austenitic type 316, while the connectors for the cable system are in duplex 2205. In 2013, on the West 7th Street Bridge in Fort Worth, Texas, type 2205 was applied to the hanger bars. In 2020-22, the Iowa-Illinois Memorial Bridge (I-74) bridge used 2205 rebar and – apparently for the first time – super duplex grade 2507 for anchor rods as protection against chloride deicing salts during the severe winters.

Duplex And 3D Printing

One final development concerning duplex bridges is the use of 3D printing. In 2021, a Dutch company, MX3D, completed the installation of a duplex stainless-steel bridge across a small canal in the heart of Amsterdam’s old center. It was made with robots, using the WAAM (wire arc additive manufacturing) process. However, it was intended only as a temporary replacement for an older bridge and was removed in late 2023. The future destination of the bridge is unknown, as is the extent to which 3D printing will be used to construct bridges.

Conclusion

A publication of the ISSF dating from 2023 (1) lists 61 bridges made in stainless steel. The same document notes that there are 600,000 bridges in the U.S. alone. A great number of bridges all over the world need to be repaired or replaced. The bridges taking their place will require durable and sustainable materials.