Two weeks ago I was back in my university city of Bristol, and had the chance to have a guided tour of the Clifton Suspension Bridge. I’d heard previously that one of the abutments was hollow, and I was secretly hoping that we would get to abseil into one of them, but it wasn’t to be. One thing did really strike me though during the guided tour.
The Clifton Suspension Bridge built in 1864 was famously designed by Brunel. It wasn’t built in his lifetime due to a lack of funding but was then erected in memorial to him after his death. It is the picture postcard advert for both the city of Bristol and engineering in general; however based on what the guide said, I think it could nearly have ended in disaster.
The bridge span is 214m and the width is 9.5m. Therefore the ratio of span/width is 22.7. As far as I’m aware in Brunel’s time that wasn’t a ratio that people considered but it’s become significant since 1940 when the Tacoma Narrows Bridge collapsed. That had a span/width ratio of 71, which for the technology at the time was the highest of any bridge in the world. It really pushed the boundaries of innovation, but unfortunately the designers weren’t aware of the impact this would have on the resonance of the bridge. And as you can see in the youtube link here it oscillated in even relatively light winds until it was torn apart only a few months after it was finished.
What the guide said that caught my attention, was that Brunel’s design had been for one horse and cart, but after his death, the bridge design was widened at the request of one of the donors, who wanted two horse and carts to be able to pass each other. The bridge also now has space for pedestrians either side.
Could it be that Brunel’s original design was only 3m wide ? If so then it would have had a span/width ratio of 70 and could have suffered the same fate as the Tacoma Narrows Bridge. Who knows but maybe that extra horse and cart was the saviour of Brunel’s reputation!
If you’d like to read more about engineering failures, you might like this book I reviewed.