By far and away the most common type of bridge that you will come across in the British Isles is an old stone arch bridge across a River, Stream, Ditch or other watercourse. In the last 1000 years there must have been many thousands constructed, often on the same site after a flood washed the previous one away. However, despite this enormous quantity, they all adhere to the same constructional principles and, ornamentation apart, look the same!
A Basic Arch Bridge
The most basic type of arch bridge uses one semi-circular stone arch to cross the river. If it is narrow and hump-backed with low parapets, it will probably be commonly called a Packhorse Bridge. Most single-arch bridges are across very small rivers or streams, often in rural areas, although the same basic principle can be used to cross much wider channels by using a flatter arch.
The principle seems to have developed because it was possible to span a wider channel with an arch than with a Log or Clapper Bridge. Whilst an Arch bridge would take a lot more effort to construct, and indeed a lot more material, it should, in theory be able to last longer as there were no intrusions into the water and it was unlikely to roll away or break like a log bridge might. Permanency was its main benefit.
For wide channels, it was more common to use more than once arch, such as at Exebridge on Exmoor (right). The biggest obvious difference between a single arch and a multi-arch bridge was the need to put piers into the river channel to support the road. This therefore made it a much more dangerous operation, as constructing masonry in a fast flowing river was never easy. However, the vast quantity of such bridges across the British Isles are testament to the fact that medieval masons were able to overcome the problems and construct just such a bridge.
The earliest surviving Arch Bridges, as a rule, use semi-circular arches. It is a shape that can be traced back to Greek and Egyptian times, although not necessarily as a bridge. It is also commonly used in Church building, supporting vaults and forming arcades and windows. However, while churches rapidly moved to various styles of pointed arch, bridges stuck more rigidly to the semi-circle.
Pointed arches, however, did make an appearance in various places around the country. They seem to have been particularly favoured on the long causeway-like bridges across wide river channels in rich Medieval towns. Indeed, in such places the whole bridge might show an ecclesiastical tendency with stautary, gargoyles and perhaps even a bridge chapel, as at Bradford-on-Avon (left).
More common than the pointed arch was the Segmental Arch, where the curve of the arch is produced from more than one centre point below the arch base. Also referred to as flat arches, they became particularly popular in the later Victorian era after Brunel proved their strengths with his bridge on the GWR at Maidenhead (?).
How an Arch stays up
At first glance, there is perhaps no obvious reason why an arch should stay in place, and not collapse as a pile of stones in the river below. It is all down to the skill of the mason (or engineer) who designed/built it. Essentially, an arch is formed from a number of wedge-shaped stones (or bricks, etc.) stacked one on top of another from each side until they meet in the middle. For the vast majority of bridges, it is necessary to construct formwork underneath the arch until the Keystone has been lowered into place to lock the arch together. Because all of the stones are wider at the top than the bottom, none of them can fall out, and with the weight of the rest of the bridge and the roadway all pressing down the stones are unable to move about.
The exceptions to this type of arch are corbelled arches, only occasionally known in bridge construction, where stones are laid one on top of another, each projecting a little further across the void until they meet in the middle. Due to the loads and stresses seen on bridges such a construction method does not form a particularly stable bridge arch. The other exception to the standard arch is that some very old, and necessarily small bridges are believed to have been constructed without formwork, with the stones forming the arch ring being large enough, and accurately cut to create a hybrid corbel arch, that is still held together by a Keystone.
The most obvious point to add decoration to an arch bridge is on the arch itself. Projecting arch-rings or just Keystones are common features, with decorative piers to match. Moving up, the bridge spandrels may be pierced either as pure decoration or, as at Kenmore Bridge (right) to allow exceptionally high floodwater to pass through the bridge more easily. Above the arches, the level of the roadway may be marked by a cornice below the parapet, or for more austentation the whole parapet may become a decorative feature, with balustrades running between the piers.
At either end of the parapets, the termination may be formed by square pillars, used on toll bridges to hang the toll gates. On grander bridges, such as North Parade Bridge in Bath (although not actually a stone arch!), they may become small pavilions or even cottages. And, while we are talking about Bath, you can't get much more decoration on a bridge than Pultney Bridge, which is lined with parades of shops on either side of the road!
As can be seen from the diagram below, there aren't that many technical terms to get to grips with in Arch Bridges:
- Abutment: The piece of the river bank, or constructed masonry from which the Arch springs.
- Arch: The curved face of the whole that lets the river flow under the bridge, but you knew that one!
- Cutwater: The triangular masonry on the front of the pier which spreads the water flow away from the masonry.
- Flood Arches: (not marked) Additional arches on either side of the river channel, often quite small, to allow fllodwater to pass under the bridge approaches.
- Key Stone: The trapezoidal stone, usually slightly larger than the others, found at the highest point on the arch and which locks the other stones in a stone arch into position.
- Parapet: The wall on either side of the roadway which prevents traffic falling off the bridge!
- Pier: (not marked) The Masonry between the arches which is used to take multiple arches across a wide channel.
- Refuge: A Triangular or curved 'bulge' in the Bridges roadway above a pier to allow pedestrians to escape the passing traffic.
- Spandrel: The Masonry between the abutment and arch, or between the pier and arch. On some bridges this is pierced to reduce the amount of stone used.
- Packhorse Bridges - a specific form of Stone Arch Bridge.
- Iron Arch Bridges - the first major development in bridge design.
- Concrete Bridges - many employ Arched forms.
- Bowstring Bridges - a specific type of arch bridge.