Christopher Cassaniti Bridge uses duplex stainless steel for increased durability and lowered lifecycle costs

Photo by: Arup/Alex Bogdanova

Case story

The striking Christopher Cassaniti bridge in Sydney, Australia showcases the use of duplex stainless steel plates from Outokumpu that promise a lighter structure, durability, and reduced maintenance costs over its 100-year lifespan.

The Christopher Cassaniti Bridge is a one-of-a-kind double-helix bridge in Australia. The vibrant electric blue structure snakes across one of Sydney’s busiest motorway intersections, blending aesthetics and functionality and adding a dramatic touch to the urban landscape. 

Originally named the Lachlan Line Bridge, the 170-meter-long pedestrian and cyclist bridge was built to reduce the reliance on cars in the busy area. It was designed and engineered by Arup, a global engineering design consultancy dedicated to sustainable development.

The bridge used 200 tons of carbon steel and 80 tons of bead-blasted duplex stainless steel plates. To ensure minimal maintenance throughout the bridge's projected 100-year lifespan, the primary deck structure was crafted using Duplex 2205 (UNS S32205) stainless steel provided by Outokumpu. Duplex stainless steel is known for its remarkable corrosion resistance, strength, and impact toughness.

A durable and low-maintenance solution that can withstand the test of time

The design and material selection of the bridge are some of the most crucial elements of bridge projects, considering the increasing discussion around lifecycle assessment costs in bridge projects. Xavier Nuttall, a structural engineer at Arup explains how stainless steel emerged as the material of choice for the bridge’s deck.  

“Since it would be very difficult to access the underside of the bridge’s deck for inspection and painting, we wanted to provide an option that wouldn’t require a complex maintenance regime to our clients, the Roads and Maritime Services (RMS) of New South Wales. Stainless steel was an ideal choice for its durability and corrosion-resistant properties.”

By selecting stainless steel, particularly duplex stainless steel, the team aimed to reduce the need for regular upkeep and minimize ongoing maintenance costs over the bridge's lifespan.  Avoidance of maintenance closures allows the busy and crucial road intersection to remain open and keep traffic flowing. This decision also aligns with the evolving focus on long-term cost considerations in infrastructure projects 

“The Christopher Cassaniti bridge has been used by RMS as a benchmark project on how to use this material since it was the first time duplex stainless steel had been used to such a large extent on an infrastructure bridge in New South Wales,” Nuttall says. 

Duplex stainless offers higher strength and improved corrosion resistance

During the early design and concept stages of the bridge, Outokumpu played a vital role in advocating the use of 2205 duplex grade, which is known for its superior mechanical properties, strength-to-weight ratio, and its ability to deliver high durability performance. 

While mild steel was retained for the helical outer structure of the bridge to meet the design requirement of a blue-painted finish, duplex stainless steel was used for the deck structure and the wearing surface within the helix.  

“Duplex stainless steel has a significantly higher strength — approximately 30% to 50% stronger than mild steel —inherent durability and corrosion resistance in pretty much any environment. By using this material for the bridge deck, we could achieve a lighter assembly and overall bridge structure compared to conventional steel,” explains Nuttall.  

Reducing the weight of the bridge also has knock-on effects on the foundations, transport, craneage, and installation, according to Nuttall. Unlike mild steel components, stainless steel retains its condition over time, even after decades of use outdoors.  

Throughout the project design, development, and fabrication stages Outokumpu’s Australian team provided ongoing assistance in optimizing the material selection, and Outokumpu’s technical departments provided advice on product properties, welding, and fabrication techniques.  Outokumpu is a member of the Australian Stainless Steel Development Association (ASSDA). 

Bridging the gap between cost premium and long-term benefits of using stainless steel

Often, design and construction contracts prioritize upfront costs, disregarding the long-term implications and sustainability aspects, especially when the parties building the bridge are not necessarily the asset owners who will inherit the long-term maintenance costs.  

“However, when considering the embodied carbon of a structure, the cost of construction is relatively smaller when compared to the cost of maintaining and operating it throughout its lifespan. The right-minded client— the Christopher Cassaniti Bridge is the perfect example — is very much interested in the long-term benefits,” Nuttall adds.