The Iron Bridge

The Iron Bridge in Shropshire was technologically ground-breaking when it was built in 1779, the first ever freestanding structure made entirely of metal. Following the recent reopening of the engineering icon and UNESCO World Heritage Site after a major conservation project to safeguard its future, Jo Caird talks to Morgan Cowles, Head of Conservation and Maintenance at English Heritage about the challenges of these ambitious works.

The Iron Bridge in Shropshire

What makes the Iron Bridge so special? 

It’s a World Heritage Site, so it’s not just important in terms of the history of England and Britain, it contributes to the development of engineering and design in the world. We refer to it as the great grand-parent of all subsequent metal designed structures. In the short term after the Iron Bridge came the railways and the great engine sheds, St Pancras and Euston, all those big arched metal spanning structures that followed this pivotal turning point in engineering and design. But then that also fed into the design of the high-rise structures that we see today. All of those things can trace their evolution back to this, what was a prototype structure at the time.

Ironmaster Abraham Darby and architect Thomas Farnolls Pritchard really made a statement about what they could achieve in this material. They could have done something simpler, quite frankly, and maybe it would have been cheaper but there’s a real statement here about their ambition and that’s their legacy. 

The Iron Bridge in Shropshire
The Iron Bridge in Shropshire

What was the impetus behind the conversation project?

Cast iron is a very brittle material and the bridge is located in a very steep gorge. We can see that the span of the bridge is now a foot smaller than it was when it was first built. That’s ground movement compressing the bridge that had given rise to wide-scale cracking around the radial arch. 

These days when we build a bridge, or any structure, we include things like expansion joints. You need to allow for these fractional movements that happen over years and years. They didn’t allow for that because that kind of knowledge wasn’t known at that time, so that’s given rise to the cracking. We had got to the point where some of the cast iron had become quite fractured and fragile and we had to bag and net some parts so that they didn’t fall into the river. We had to act.

The Iron Bridge in Shropshire

How did you work out where to focus your attention?

There was quite a lot of analysis involved to get to a really accurate understanding of the condition. We spent over a year doing 3D geometry modeling and a laser point cloud, which is essentially a laser scan of the whole skeletal structure that’s accurate to within 1 mm in any direction. We then brought that into a computer model where you’re able to put a grid mesh over the various beams and components and then start to apply load testing and stress testing to it. 

What that means is where we had previously visually looked at a part of the original historic structure and noted that it appeared to be cracking in a certain location, we weren’t able to make informed decisions about what really needed to be repaired in order to strengthen the structure and what are just the natural signs of aging. 

The Iron Bridge in Shropshire

Having completed that analysis, how did you decide how far to go with works to the bridge? 

As a conservation project not a restoration project there was a delicate balance to be struck. This project wasn’t about setting back the Iron Bridge to look like it was when it was first constructed. It was about conserving it in situ, stabilising it for the long-term. But it wasn’t about replacing everything that was damaged and putting new bits in. 

The Iron Bridge in Shropshire

Tell us about the costs of the project

It’s a free-to-visit site and the project cost £3.6m so we decided to launch a crowd-funding campaign – a first time for us – and we were really floored with the response. We set an initial target of £25,000 and we received £47,000 of pledges. We also had significant donors: one from a German arts and culture foundation who donated €1m to the project. It’s the first time that they’ve donated in the UK. We had a pledge of £15,000 from a trust fund who are descendants of the original builder of the bridge, Abraham Darby. 

The Iron Bridge in Shropshire

How would you like this project to be regarded in years to come?

Each generation has done their bit and carried out in different ways minor phases of work to repair the bridge, but this was a once-in-a-generation opportunity to secure this iconic symbol of the industrial revolution for generations to come. 

Words Jo Caird

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