The Historic Iron Bridge Restoration

The project that was constructed over the River Severn at Coalbrookdale, England, in 1779 is located in an area full of minerals, coal and iron ore.

Iron Bridge is one of the most influential constructions in modern civil engineering and is considered the ancestor of railways and skyscrapers. It was the world’s first single-span arch bridge to be made of cast iron.

After its erection, it was observed that its abutments were being pushed together due to ground movement. In 1801, the board of directors considered holding the abutments apart using a timber support bar but the idea was rejected. However, the issue was addressed over 150 years later in a period when the bridge came very close to collapse as the abutments were displaced by about 60 cm. Therefore, a concrete support bar was constructed to hold them apart.

Recent investigation showed that severe cracking poses a threat to the construction. Cracking was caused by stresses in the ironwork dating from the original construction but it has been amplified since the addition of the aforementioned concrete support bar that caused the structure to be under compression and due to a powerful earthquake that struck the region in the 19th century. Moreover, corrosion of the iron is also an issue that threatens the structure’s stability.

After carrying out a detailed investigation, Consultant Ramboll created a finite element model to simulate the behavior of the structure. The results showed that the bridge’s framework does not have to be reinforced but its elements have to be examined and replaced if needed. “We haven’t had to strengthen the structure. The Iron Bridge was in vehicular use for a long time, so there is quite a high degree of redundancy, which means there is no problem with its load-bearing capability. What we’re doing now is a case of really trying to make sure that, where elements are damaged or have failed, they won’t fall off in the future, and we can retain them for as long as possible,” Morton Partnership, the company that conducted the design of the works using Ramboll’s report, stated.

One of the most challenging tasks that had to be tackled during the restoration was the replacement of over 250 corroded wedges in the deck. For each replacement, an individual plywood template was made before crafting a new cast-iron wedge. “It was quite an intensive job,” Matt Greenhalgh, technical and estimating engineering director, commented.