Expert Talk

Designing bridges to last

Andy Backhouse

Technical Manager
Outokumpu

Andy Backhouse explains the thinking behind the current trend for bridge builders to specify duplex stainless steel as a high performance material that delivers long term value.

The governmental organizations responsible for building and maintaining our bridges always have to keep tight control of budgets. When you’re responsible for the wellbeing of a community (and for spending its tax), it’s important to choose infrastructure that offers good value as well as good looks.

That’s why government agencies are asking engineers to create bridges and structures that are built to last and minimize the Life Cycle Cost. This is the sum of all the costs associated with a bridge from the earliest feasibility study through engineering, construction and regular maintenance and repair, all the way to end-of-life decommissioning and recycling.

 

Construction and Life Cycle Costs – white paper

Understanding that this holistic approach is important to our customers, we have written a white paper based on work undertaken in cooperation with engineering consultancy Arup. Our objective was to provide a worked example to establish the costs associated with a typical road bridge design that is used widely over highway, railway and river crossings. We considered how the Life Cycle Costs of a composite steel beam – concrete bridge compare when four different grades of steel are used to build otherwise similar bridges:

  • Painted S355-grade carbon steel
  • Unpainted S355W-grade weathering steel
  • Lean duplex Forta LDX 2101 grade stainless steel
  • Lean duplex Forta LDX 2101 grade stainless steel with an optimized design to reduce weight and cost

While lean duplex stainless steel has been used in the final two of these designs, the first of these has been designed without optimization of the thickness of beams. In the last design, the beams have been downsized to take greater advantage of the high mechanical strength of the material. In turn, this has reduced the amount of material required in the structure – and as a result given a lower construction cost.

For this reason, the optimized design in lean duplex stainless steel is shown to have comparable construction costs to a carbon steel composite bridge, even if some extra effort is needed during design and construction to take advantage of the high material strength. However, because it has high durability through its resistance to corrosion, lean duplex steel does not need regular repainting or repair. As a result, a lean duplex stainless steel structure has the lowest cost for maintenance.

The purpose of this study was to demonstrate that stainless steel need not be perceived as only a premium material for signature aesthetic bridges, but that lean duplex can make economic sense for many functional bridges.

 

Södertälje bridge optimizes materials and costs

Lean duplex was also selected for a pedestrian and cycle bridge over Sweden’s E4 motorway at Södertälje, which lies around 40 km southwest of Stockholm. The bridge design has been patented by fabrication company Stål & Rörmontage, which engineered the design to optimize the use of the stainless steel at the same time as minimizing the structure’s Life Cycle Cost. The design includes two spans, which were assembled and fabricated in the factory. This meant they could be installed on site during a short highway closure that minimized the impact on traffic. The structure includes polycarbonate panels that protect pedestrians from the weather, as well as LED lighting for the long winter nights.

Peter Månsson, Quality Manager of Stål & Rörmontage, said: “The bridge is made of two pieces, one of them is 36 metres long and the other one 24 metres. Our special, protected design was developed to make the most of the material and also to cut the bridge’s life cycle cost. This bridge has a lifespan of 120 years, with no need for maintenance. In our thinking, the total life cycle costs have to be taken into account, not only the immediate investment costs. The initial cost of a stainless bridge is higher. But at first maintenance check, at approximately 30 years, when a carbon steel bridge has to be repainted and a wooden bridge replaced, stainless pays itself back.”

“Outokumpu’s Lean Duplex Forta LDX 2101 was our first hand choice for this project. Long lifespan of the bridge was very important for the municipality as well. Our calculations indicate that, over a reference period of 120 years, wooden bridges and steel bridge costs are significantly higher than a stainless steel bridge.”

The bridge also has a low carbon footprint. Duplex stainless steel releases virtually zero metal ions into the environment. In addition, because it does not need painting, it eliminates the need for hazardous repair blasting and coating materials. Safety is another consideration as the safest way to maintain a bridge is when the bridge actually needs virtually no maintenance.

A further benefit is that its high mechanical strength means that a more lightweight structure can carry the same load – reducing the raw material consumption, as well as energy for transportation and cranage. Structures made from duplex stainless steel can be as much as 30 percent lighter than carbon steel.

When purchasing steel from Outokumpu, customers benefit from a low carbon footprint as Outokumpu’s production uses a large percentage of recycled steel as raw material and sources its energy from low carbon sources. Finally, at the end of its life, the Södertälje bridge will itself be recyclable and will be reborn as new products that will also provide durability over a long life.

Söderström bridge – designed to last

An old suburban railway bridge between Stockholm’s Old Town and the trendy district of Södermalm needed a heavy duty overhaul to improve its safety and carry heavier traffic flows.

Read the case

Read more about bridges

Södertälje bridge – designed to last

The goal was to build a pedestrian bridge to last with low LCC and reduced carbon footprint.

Read the case

Read more about bridges

Conclusions

Duplex stainless steels enable architects and structural engineers to create bridges that

  • look good
  • last a lifetime
  • look after the public purse.