What does it feel like to be an arch? Very different from how it feels like to be a cantilever.
Cornwall's Tintagel Castle used to be connected by a long-gone land bridge. Now a new bridge has just opened, with a competition- winning design by Brussels-based engineering firm Ney & Partners with William Matthews, who used to work with Renzo Piano and worked on the Shard in London. According to English Heritage:
Spanning a 190-foot gorge and with a gasp-inducing gap in the middle, the bridge follows the line of the original route – a narrow strip of land, long lost to erosion – between the 13th- century gatehouse on the mainland and the courtyard on the jagged headland or island jutting into the sea. So significant was this historic crossing that it gave rise to the place’s name, the Cornish Din Tagell meaning “the Fortress of the Narrow Entrance”.
An interesting feature of the bridge (and the reason I am writing about it in TreeHugger) is the way it is engineered; it in fact is not a single bridge, but...
...two independent cantilevers of approximately 30 metres length each that reach out from either side to – almost – touch in the middle. At the centre of the bridge, a narrow gap (40mm) has been designed to represent the transition between the mainland and the island, the present and the past, history and legend.
According to Oliver Wainwright in the Guardian, the story is not so evocative.
In truth, it was a practical necessity, in order to avoid excessive forces meeting at the centre of the double-arched structure, but it makes for a poetic sight.
In truth, that seems odd to me. I immediately thought of a book I used to read to my kids, Forrest Wilson's classic What it feels like to be a building. I don't have it with me, but did find a bit of it in a review, as I tried to remember what it feels like to be an arch, which has a keystone up at the top, with the rest of it leaning in.
As Wilson noted, the arch never sleeps; it is up there working, dealing with those so-called "excessive forces" and has been doing that for thousands of years since arches were invented. You can see how well they worked, holding up Notre Dame Cathedral, even as the wood roof above the arches and domes burned away. The arch never sleeps.
However, stick your arm out straight and you will quickly learn how it feels to be a cantilever; it hurts. It has to work really hard just to stay up.
The cantilever, like your arm, wants to fall down. It's restrained by deep anchors into the rock holding the top and the structure of that half-arch levering on that big block of concrete supporting it from underneath.
I am an architect, not a structural engineer, and admit that it is marvelous how they built this, bringing in pieces and building out from each side. You can't do that with a conventional arch; it doesn't stay up on its own until you put in the keystone. But here, they are able to build each side out independently without any annoying and expensive falsework holding it up in the middle until the arch is completed.
But I have been trying to make arguments for sufficiency, the question of "how much do you need?" And simplicity, which asks, "What is the easiest, most logical way to solve this problem?" And I always thought that means that a bridge wants to be an arch.
I would love to hear from any structural engineers out there, but my gut tells me that this solution resulted in increased complexity, more materials and higher costs. Or is this just two separate arches, and the most efficient way to build a bridge today?