Politicians, building regulators and other decision makers face the challenge of choosing sustainable bridge designs, for which the price of the construction is not the only variable, but one of many, such as construction time, social impact (during construction and operation), environmental impact, maintenance, etc. Current procurement practices tend to favour the most economical bridge design. Unfortunately, today's awarding practice often results in cheap bridge designs with low construction cost, disregarding expenses due to operation and maintenance, and ignoring the impact that longer construction times have on traffic conditions and the public.
Approximately 80% of the bridges that comprise the European road network can be classified as small or medium span structures of less than 50 m. When performing a very simple analysis, reinforced concrete and prestressed concrete appear to be the most cost-efficient solutions available. On the other hand, it is widely accepted among the construction industry that composite structures are the best solution for spans larger than 50 m, when height restrictions apply, or if shorter construction periods are required . In recent years, numerous studies and examples have demonstrated that composite bridges provide cost effective solutions for shorter span structures even if these conditions do not apply.
The right choice of bridge design
For two-span road bridges with an average span and traditional abutments, manufacture of the abutments accounts for approximately 40% of total manufacturing costs of the bridge without temporary pit walls required; this rises to roughly 55% when temporary retaining walls are required for the construction . In these situations, abutments with sheet piles are a smart alternative to traditional construction, as the sheet pile can take over both functions, serve as retaining walls, and as permanent load-bearing structural elements .
Steel sheet piles are always highly cost-effective whenever the construction of the structure requires additional temporary pit walls. This is the case when a change of ground level is to be accommodated during the construction stage, when building below the groundwater table and when the excavation pit needs to be protected against flooding. Further, use of steel sheet pile abutments instead of abutments with temporary buildings pit walls, can reduce total construction time by a minimum of 10% .
There are also significant advantages when building the bridge deck using composite sections that are manufactured based on hot-rolled, high strength steel sections. To reduce the space needed for the construction site, these composite elements can be partially prefabricated offsite and supplied to the construction site as load bearing elements during construction and the final stages. In addition, it is possible to install the prefabricated bridge girders during brief shut down periods of the crossed road, avoiding major disturbances to commuters, allowing traffic to circulate below the bridge while any subsequent work is safely carried out above the carriageway.
The hidden costs of traffic congestion
Not only the direct building costs must be considered, but also the economic costs generated by delays, increased fuel consumption and air pollution caused by traffic congestion during the bridge's life cycle. Congestion has a massive economic impact. According to data published in 2016, the average cost per hour that a European passenger spends stuck in traffic is estimated at 20€ .
Choosing a design with a reduced construction time, and with low maintenance requirements, becomes a priority when a global analysis contemplates the cost of the external effects. Especially as one of the main reasons for slow moving traffic, the construction sites, are going to grow in number in the coming years, as the infrastructure investment in Europe is foreseen to increase.
A case study, performed by the Karlsruhe Institute of Technology (KIT), assesses all associated costs and performance of a traditional concrete bridge versus a bridge whose abutments are constructed using steel sheet pile sections. This comparison is performed for a two-span superstructure with spans of 22,50 m and a superstructure width of 11,50 m accounting for a standard crossing of a national road over a 3-lane motorway with an average daily traffic (ADT) of 70'000 vehicles:
Option 1: Steel Sheet Pile abutment (SSP abutment)
- 16 m long piles on the front wall
- 13 m long piles on the lateral wings.
- 4 piles per abutment plus 2 per wing wall
- 10 m length concrete piles of 90 cm diameter.
- 7% reduction on construction costs
- 3.5% reduction on lifecycle costs over 100 years lifetime (based on a real discount rate of 2%)
- Up to15 % reduction in the economic impact of the external effects during the service life of the structure.
Composite steel bridges with steel sheet pile abutments are robust structures that require limited maintenance and repairs due to use of state-of-the-art corrosion protection techniques, which ensure 100-year design life without need to replace any component.
Steel sections offer the advantage of easy dismantling, recovery and recycling after the bridge service life, ensuring that no residues remain in the soil, and adding a valuable contribution to the circular economy. This helps to prevent abuse of natural resources.
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Hot-rolled sections and sheet piles produced by ArcelorMittal: our contribution to pioneering projects.