Follow-up on Economics and Metal Fabrication

Bill Harvey sent me this close-up after our sight-seeing trip to the Clifton Bridge.* Putting aside the beauty of the photo in itself, it has something to say about changes in metal (in this case wrought iron) technology over the years.

That’s a connection between two parts of an iron tie in the vaults at base of the west tower of the bridge.¬†Masonry arches and vault produce outward thrust under vertical gravity loads, and depending on the geometry of the masonry** those thrusts may require ties to keep the masonry in place. I don’t know if this tie was meant for construction loading (when the gravity load is lesser and may be more unbalanced than the permanent condition) or for permanent loading. It’s there and it’s not coming out now.

The connection consists of (on the right) a plain bar end with a vertically-oriented hole in it, (on the left) a forked end of the other bar with a hole in each branch of the fork, and (in the center) three wedges passing through all of the holes. This is a very primitive connection – a few years later, this would have been riveted or bolted together, or the tie ends threaded and joined by a sleeve nut or turnbuckle.

At the time the towers were built, in the 1840s, wrought iron was being used in a lot of engineering structures in Britain. It was not as difficult to get relatively large pieces of iron as it was in the US at that time. But mass production of standardized shapes was still in the future.*** Under the circumstances, it was cheaper and easier for Brunel to have that connection forged than to build it from more standard iron. Given that the suspension elements of the bridge are wrought-iron eyeball chains with pinned connections, less crude details were obviously available for the vault tie. I don’t know why this detail was used, but I’d guess it was a combination of it being cheap, easy, and located where it would never be seen.

* It took all of my self-control to not title this post “The Clifton Bridge, Part 4 of 3.”

** Vaults in the middle of a row of identical vaults don’t need ties as their thrust is balanced by their neighbors’ thrust. Vaults at the end of a row or with neighbors of differing geometry don’t have that balance and often need to be tied.

*** For example, the standardization of bolt treads in the UK began in 1841 and obviously took some time to complete.