Victoria Bridge: An Inefficient Landmark?

If you happen to walk along the River Avon, just off Bath’s town centre, you will surely notice what looks like an old bridge, but with a mess of steel parasitising through it:

By Konstantinos Voulpiotis

This is the almost 180 year old Victoria Bridge. It opened in December 1836, engineered by James Dredge. It used to be an important toll link for transporting goods across the river, however after 1946 it has only been carrying pedestrians. In 2011, an engineering assessment deemed the bridge unsafe for use. The bridge was closed and a temporary steel truss was erected to provide some support to the suffering deck whilst allowing pedestrians to keep using the crossing.

Before 2011 one could admire the bridge at its original state. A beautifully slender deck is suspended from iron chains via those aesthetically intriguing inclined hangers. The slightly tapered Bath stone towers still enhance the Georgian look, whilst the green paint blends the structure in the surrounding environment, particularly the green river. Unfortunately the prison-like parapets ruin this otherwise magnificent looking structure.

Now we are left with the intruding view of this temporary truss spanning over, and hiding, Dredge’s landmark. Why?

One needs to understand how this bridge works: it looks like a suspension bridge, however it is not. Victoria Bridge is a hybrid construction of a suspension and a cable stayed bridge; its inclined hanger pairs in their triangular arrangement have a much more special purpose other than being aesthetically pleasing: they create a double cantilever, which forms the bridge.

More notably, this Grade II listed structure is Dredge’s first ever bridge designed as such. He went on to build another 50 during his life, in the UK, India and Jamaica. Dredge was also the first engineer to realise that a suspended cable carried the least tension at its midspan and progressively more tension near the supports as it also carries its own weight; he therefore designed his bridges with cables which were thinning near the midspan, best known as the Taper Principle. This allowed Dredge to save a lot of material – and money – in his projects.

Structurally, the inclined hanger pairs put the deck into compression and enhance its stiffness; this is particularly important with wind loads, which is the greatest drawback of modern suspension bridges. Dredge’s design was proved best when Victoria Bridge was hit by a hurricane force storm before it was even completed and survived with no damage at all.

However, what Dredge did not understand was that his design suffered from a fundamental flaw: his inclined hanger pairs are prone to fatigue. More force is required to lift a vertical load with an inclined hanger than with a vertical one. On the other hand, when only one half of the bridge is loaded (for example a group of students start crossing the bridge), the inclined hangers on the other half go slack. Although the triangular arrangement is structurally stable, their continuous cyclic loading has caused their deterioration and ultimate closure of the bridge in 2011.