According to Mike Clarke’s 1775 bridge list, there were two stone arch bridges on this stretch of canal when it was first cut;- Swift’s bridge, and what came to be called “Knight’s” bridge. All other bridges were wooden swing or draw bridges at that time. But the road traffic soon became too heavy, and the busiest swing bridges were soon replaced by stone arches, which in turn were mostly replaced by iron, reinforced concrete and steel “beam” bridges as metallurgical technology advanced. These are four types of bridge to be found on this stretch of canal:
- Stone Arch Bridge. This includes bridge 8 (Blue Anchor), Bridge 5 (Swift’s) and middle section of bridge F (Boundary). Swift’s bridge is the oldest, dating from 1774 when the canal was built.
- Swing Bridge. This includes bridges 9 (Hancock’s) and 6 (Netherton), both of which are electrically operated by CRT staff only. Hancock’s bridge is operated by CRT because of an agreement with the local council regarding traffic. Netherton bridge is locked to prevent vandalism, and only CRT personnel have the key. Bridge 2I is currently being replaced by a hand operated swing bridge.
- Beam Bridge. Most of the bridges on this section are beam bridges, but they can be divided into sub categories.
- Bridges at Vauxhall (B), Atholl St (D), Sandhills Lane (F), Merton Road (O), and footbridges at Carolina Street (1) and Stanley Road (2A) are made of iron and have an arch shape. However, they react to loading in the same way as normal beam bridges.
- Bridges at Bedford Place (L), Pennington Road (2G), Field View (3A), and Kirkstone Road (4) are made of steel and have a slight arch. However, they react to loading in the same way as normal beam bridges.
- The bridges at Ceres Street (K), Pauldings Lane (3) and all of the rail bridges along this stretch are box beams. That is, the deck, sides and (sometimes) top connect together to form a box beam.
- The bridge at Dunnings bridge road is made of reinforced concrete, and is shaped as an arch. However, it reacts to loading in the same way as a normal beam bridge.
- All other bridges are simple iron or reinforced concrete beam. Most of them are higher on the off side. In effect, the two bridge ramps meet above the off side bank rather than being separated by the width of the canal, reducing the length and building cost of the bridge.
A bridge is comprised of a deck (The bit you walk or drive on) and something to support the deck. The bridge is classified by the type of support used.
In engineering, supports come in three types:
- A strut is something that is compressed (squashed). A stone arch bridge is a set of struts, arranged to transmit the weight of anything on top of the bridge down to the ground equally on both sides.
- A tie is something that is tensioned (stretched). The cables of a suspension bridge are ties arranged on either side of towers so that the weight on each side balances.
- A beam is something that flexes (bends) in the middle. Unlike arch and suspension bridges, a beam bridge doesn’t need to be balanced.