The bridge in question, St Margaret Street Bridge, is in what Network Rail described as a “very poor condition”, and has reached the end of its operating life.
A proposal paper outlining why a bridge in Bradford on Avon in Wiltshire must be replaced has highlighted the debate about the exact point when bridge repairs are no longer an economically viable approach and the alternative is a complete replacement.
The bridge in question, St Margaret Street Bridge, is in what Network Rail described as a “very poor condition”, and has reached the end of its operating life.
The images and description in its report highlighted the scale of the issue, with considerable corrosion damage to many of its structural elements, to the point that there was very little in the way of corrosion control or repair that can be undertaken to save the original bridge.
Many of those structural elements would have to be replaced to avoid involuntary weight restrictions or even a full road closure, with extra patching or less substantial repairs either ineffective or not economically viable.
Most residents of Bradford on Avon suspected the works were coming, and the Wiltshire councillor for the town noted that the replacement was set to happen in 2020 but was delayed by circumstances. Many concerns that were raised focused on road closures and traffic disruption rather than the necessity of the works.
This leads to the question of the criteria for replacing a bridge rather than repairing it.
Three primary criteria that can affect the need to replace rather than replace a bridge are condition, suitability and purpose, alongside other factors such as heritage and preservation for particularly historic structures.
Many bridge repair projects are based on the first factor, typically as a cost-benefit analysis; if repairing the bridge would cost more than replacing it and there are no other factors that make it worth keeping, it will likely be replaced with a bridge that will require far fewer repairs for years to come.
Suitability can become an issue for bridges depending on the traffic density of the nearby area. If a historic bridge is frequently troubled by congestion, it may need to be expanded or replaced with a more expanded bridge in a more suitable location.
Finally, there is the purpose of the bridge. If the bridge is no longer widely used then it may be closed and replaced with one that is more valuable to the community.