Global warming impact and life cycle assessment (LCA) of footbridges

We have dedicated this page to capturing and recording our work preparing Life Cycle Assessment (LCA) of footbridges. Within these posts we will also consider and discuss the various LCA methods and LCA tools and how they relate to bridges and footbridges.

Tracking environmental impact of projects – and particularly their global warming potential – will become more and more important for Councils and devlopers as all countries advance toward meeting their Paris Agreement target for emissions reductions. The Paris Agreement is a legally binding international treaty on climate change. It was adopted by 196 Parties at COP 21 in Paris, on 12 December 2015 and entered into force on 4 November 2016. Its goal is to limit global warming to well below 2, preferably to 1.5 degrees Celsius, compared to pre-industrial levels.

Life Cycle Assessment (LCA) for all projects is expected to start forming part of legislation for consenting in 2024 here in NZ (according to BRANZ).

If you are a client organisation, or fellow designer, with an interest in understanding how you can perform Life Cycle Assessment of bridges and/or LCA of footbridges feel free to get in touch to see how we can help you.

Our aim is to eventually create a table of comparitive life cycle assessment data for bridges of various types. In preparing this data, we will baseline all LCA bridge data into a per m_² basis and assess all bridges over a 100 life so that better apples-with-apples environmental impact comparisons can be made accross the various bridge forms and material choices. Watch this space for more data to come!

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LCA of Manganui Gorge Suspension Footbridge

20 January 2023

Stage of LCA: Completion of detailed design

We are keen to understand more about Life cycle assessment of footbridges so we can better help inform our clients decisions moving forward. As part of learning the processes and tools involved in these assessments we recently (Jan 2023) applied it to our completed Manganui Gorge Suspension Bridge design. This is a 100m long x 1.2m wide lightweight suspension footbridge. At this stage we were keen to understand how the decisions we have made during the design process have influenced the global warming impact of our design outcomes.

In the near future we will apply similar methods to other bridges in our portfolio so we can better understand and determine credible baselines for footbridges in NZ when using different bridge forms and materials. It is hoped that this retrospective review of our work will provide a more fundemental in-depth understanding of the consequences of our previous decisions. We can then apply this understanding and more accurately consider such choices moving forward during the intial concept phase of future bridges (where bridge form and construction materials are often selected). This is just the start of the journey….

The estimations and observations listed below were completed using “One Click LCA“. We found this tool the most comprehensive in terms of containing all of the bespoke materials that make up a suspension bridge. Other tools such as BRANZ’s “LCA Quick” (we tested v3.5) were great tools for building focussed projects but at this stage lacked the depth of material choice suspension cables and composite flooringneeded to assess complex structures like suspension (e.g. cables and composite flooring) that were required for a suspension bridge life cycle assessment. In the future we will experiment with manually adding new materials to the raw .XLS database contained in LCA Quick to see if this resolves the problem (noting that LCA Quick is a free tool).

Below is a summary of our LCA Footbridge findings. Further down you will find the corresponding tables and graphs for the Manganui Gorge Suspension Bridge.

Overview of global warming impact of footbridge

1. Total carbon dioxide equivalent impact for full life cycle is 123 tonnes of carbon dioxide equivalent (kg CO₂ e)
2. At this early stage we do not have other NZ bridge data to compare these results.
3. This is equivalent to the materials, construction, maintenance, and demolition of 2.3 times a “single story detached residential homes” (=54kg from BRANZ where we ignore their operational energy and water requirements).
4. This is equivalent to the full life cycle of 0.81 times a “single story detached residential homes” (=151kg from BRANZ when considering their ongoing operational energy and water requirements).
5. Action will be to:
   • Check for international LCA bridge assessments which we can benchmark our bridges against.
   • Build our own NZ LCA bridge assessment record of previously assessed NZ bridges.

Key Observations of our Footbridge LCA

1. Graphs below show the key impacts in relation to stages of life and materials. Since there is minimal operation energy required for a bridge, the largest impact is from “production” of the raw building materials such as steel, concrete, cables, and FRP = 62% of total impact (transport is 12%, construction processes which include helicopters is 4%).
2. For this bridge the replacement and refurbishment assumes a 100 year life and includes replacement of FRP decking at circa 50 years.
3. Largest material impact is from the structural steel which accounts for 43% (30% for FRP decking, 7% for concrete footings).
4. Worth noting that the FRP may last the full 100 years which would reduce its impact by not needing replacement.
5. Also, worth noting the impact data I am using is not derived from FRP flooring or Treadwell (I used the closest product I could find). I will ask Treadwell if they have any actual product specific and verified numbers we can use.

Early LCA comparisons with other bridge forms & materials:

1. We note that a timber bridge with timber towers will have a lower overall carbon footprint. However, since a timber bridge will need full deconstruction/replacement in year 50 to achieve a “comparable impact” with our 100 year steel structure. This will heavily handicap its net CO₂ benefit.
2. Designing for 100 years when using steel structures is a key global warming reduction method in the decision process and we recommend this is adopted for lightweight suspension bridges when favoured over timber equivalents (i.e. all steel mast structures to be designed for 100 years).
3. Although a timber deck will be much better performing than an FRP deck we would also need to consider the typically applied plastic mesh overlay and associated steel fixings used to prevent slip when using timber.
4. If we assume the FRP will last the full 100 years without replacement it will improve in its comparison with timber decks. This could be a sensitivity check on future LCA’s.
5. Future LCA analyses should more carefully consider the impact of re-coating steel structures insitu at year 40 (depending upon system). We would need to look more closely at the background data in these tools to see/understand/modify the assumptions made for maintenance of painted steels. This will be important for a true comparison with timber structures (which require replacement but no re-coating).

Likely cost to offset global warming impact of bridge with carbon credits:

1. As an example (noting the client might progress toward zero carbon projects in the future) the Manganui Gorge Bridge would require 123 carbon credits to offset the global warming impact of the bridge (1 credit = 1 tonne of CO2).
2. A carbon credit has a current cost of circa $80/credit for 2023 (refer https://www.commtrade.co.nz/).
3. It would cost circa $10,000 (in 2023) to offset the estimated global warming impact of Manganui Gorge Bridge.
4. The cost to offset the impact is 1% of the estimated capital cost to build (but we note that reduction at concept stage is preferred to offsetting).

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Actions for future LCA work

• Plan is to estimate and then tablulate the impact of each of the footbridges designed by DC Structures Studio moving forward (and retrospectively). This will include concept design phases of future bridges.
• LCA methods will form a further comparison emthod for concept evaluation of bridges.