The Clifton Bridge, Part 1

A couple of weeks ago, I was lucky enough to have Bill Harvey give me a tour of the Clifton Suspension Bridge in Bristol, England. I had been to the bridge once before, in 1994, when I spent almost a week hunting down any structure I could find with a connection to I.K. Brunel. The bridge’s Brunel connection is a bit weak, as I’ll discuss below, but since I went to Bristol at that time to see Brunel’s SS Great Britain, I went to the bridge as well. It turns out, unsurprisingly, there’s a big difference between seeing the bridge yourself with a guidebook in hand and seeing it with an expert. Regarding the picture above, southwestern England really is that green in January, which looks quite odd if you’re used to colder weather.

First, my host, standing on the deck. Bill’s facing west, towards the Leigh Woods end of the bridge; I’m facing east towards the center of Bristol:

Before the trip, Bill mentioned how the bridge intersects his engineering career: “Starting with with a view age 7 and actually working on it at 70.” So it’s hard for me to imagine a better guide.

That’s the west tower behind Bill, and the west side span between me and the tower. Note that the suspension chains are almost straight and have no hangers until past the tower: the side spans are not suspended. That configuration makes sense for suspension bridges over gorges where the side spans are above land at roadbed grade. As you can see in the 1890s image below, the towers are more or less at the top of the gorge side slopes, so there really is no place to put suspended side spans.

Brunel’s design and the towers date from the 1830s, but funding problems led to the work stopping in the early 1840s, with the towers complete but the suspension structure unbuilt. After Brunel’s death, the bridge was finished by William Barlow and John Hawkshaw, who modified Brunel’s design to make it somewhat heavier. One of the changes was to replace Brunel’s proposed double chains of eyebars with triple chains, which can be seen next to and behind Bill. The staggering of the joints in the three chains is necessary to prevent geometric interference, but it’s one of those necessary details that looks good. Here’s another view of the side span, with flagstones as the footpath paving on grade, up to the tower:

Eyebar chains were reasonably common in the nineteenth century for suspension bridges, even ones with relatively long spans, like Clifton (702 feet). Quality control was generally better on forged wrought iron than on woven rope; the modern practice of laying the cables in place one wire strand at a time had not yet caught on. The best thing about eyebar chains is that it is possible to replace individual eyebars and pins without disassembling the whole suspension system. Whether the chains are better or worse looking than cables is a matter of personal taste, but cables are here to stay in suspension bridges.

More tomorrow…