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The Carbon footprint of bridges

How much carbon is emitted in the construction of bridges and viaducts today can be used to reduce the amounts in future structures?

The construction industry accounts for approximately 40% of worldwide carbon emissions, with steel and cement production, each accounting for approximately 8%. Due to the use of steel and cement, bridges and viaducts require high levels of carbon input compared to other types of infrastructure, such as roads and railways. Consequently, bridges with environmentally conscious designs can help reduce carbon emissions in the construction industry.

Video Short Link: Can BRIDGES Reduce Our Carbon Footprint? #shorts

To achieve our goal of limiting and ultimately reducing carbon emissions, it’s crucial that we have a clear understanding of our current usage. As part of a continuous research effort, the data collect presented in this research calculates the carbon footprint of various infrastructures, buildings, and, most recently, bridges. The following article will create a narrative based on the data.

Size is Important

There is a correlation between increased carbon emissions and the general structure of bridges (length, span and area). The bridges in this study vary from 2m wide footbridges to 50m wide highway suspension bridges. To eliminate the impact of varying widths, the best metric to analyze is the bridge’s area. As seen in Figure 1, the carbon footprint is measured in tonnes of carbon emissions. The data reveals that the 10 largest bridges account for a staggering 75% of the total carbon emissions. This highlights the importance of bridge sizes in measuring carbon emission and as such, we need to consider minimising carbon emission in our largest bridges.

Figure 1. Bridge area (square meters) to carbon footprint (tonnes of carbon emissions) with good (green) and poor (red) data highlighted. Credit: David Collings

Our data reveals several interesting trends. Footbridges (orange dots) are grouped at the lower end of the data with lower carbon. Repair and refurbishment projects also have lower carbon footprints; keeping existing bridges going is good carbon! Bridges above the general trend tend to have larger foundations or underground structures. By giving more consideration to the foundations and substructures under our bridges, we can make a significant impact on reducing the carbon footprint of our bridges.

Figure 2. Bridge cost (Million Euro) to carbon footprint (tonnes of carbon emissions) with UK/Europe GDP-Carbon shown as a pink line. Credit: David Collings

Costs are a crucial aspect of bridge construction and are carefully reviewed by designers, clients, and builders. Data reliability is a key issue when analysing carbon databases. Due to reliability concerns from public databases on building carbon, 60% of the data was rejected as part of the study. The main issue with bridges is the lack of available data rather than the unreliable nature of the data.

Cost of Carbon

The correlation between a country’s GDP and its total carbon emissions is well established. Our research shows that a similar correlation exists between the cost of a bridge and its carbon footprint (as seen in Figure 2). The data clearly illustrates that as the cost of a bridge increases, so does its carbon content. This trend aligns with previous findings on buildings and GDP data. The costs in Figure 2 are presented in Euros as the majority of bridges in the data are from the UK and Europe.

Costs are a crucial aspect of bridge construction and are carefully reviewed by designers, clients, and builders. Data reliability is a key issue when analysing carbon databases. Due to reliability concerns with public databases on building carbon, 60% of the data was rejected as part of the study. The main issue with bridges is the lack of available data rather than the unreliable nature of the data.

Working the Data

The basic data presented above provides a broad indicator of the trends in anomalous data. A common method of data analysis is normalising data, allowing researchers to compare different data in a standardised format. A useful method for normalising bridges and buildings is dividing the carbon data by the area such that the smallest (50 m2) footbridge can be compared directly with the largest (135,000 m2) suspension bridge. Normalising the data can give a different perspective and assist with benchmarking new bridges. Figure 3 gives the normalised carbon footprint with the bridge span and material.

Figure 3. Normalised carbon (tonnes per square metre) with bridge span (metres) and material; steel (red) or concrete/cement (black). Credit: David Collings

The average normalised carbon for the bridges is 2.4 tonnes per square meter of the bridge. There was little difference in the average values for bridges constructed primarily with steel or cement and concrete. Our research anticipated a trend in bridge span, but this was not observed in bridges with a span of less than 200m. Another key conclusion from this data is the importance of reducing the variation in material usage and carbon emissions, with the goal of all bridges being towards the lower end of the normalized range (as indicated by the green in Figure 3).

Future Implications

A lot of the research on carbon emissions talks of the need to reduce carbon. This research puts some clear numbers to the problem. This will allow engineers and architects to benchmark future designs in carbon terms.

The variation in the data indicates that not all architects or engineers are considering carbon in current designs. In future, this needs to change with a clear focus on all bridges to achieve low-carbon designs.

Engineers and architects put a lot of effort into the parts of the bridge we see. The research shows we are putting a lot of steel and concrete into the ground. In future, we need to give more consideration to this unseen, buried carbon.

The numbers show clear trends that should influence future designs, and size is the most important. Tackling the carbon emissions of our largest structures will go a long way to achieving our carbon reduction targets.

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Journal reference

David, C. (2022) The Carbon Footprint of Bridges, Structural Engineering International, 32(4), pp. 501-506. https://doi.org/10.1080/10168664.2021.1917326

Dr. David Collings is a bridge designer, author, researcher, lecturer, and Engineering Director with the international consultancy firm, Arcadis. He holds a PhD from the University of Surrey and is a Fellow of the Institution of Civil Engineers. He is involved in designing new bridges and maintaining existing ones. Additionally, he is engaged in various research topics that aim to foster closer links between industry and academia.