What type of bridge is Tower Bridge?
Is Tower Bridge just a bascule bridge? Look a little closer and you will see there is a lot more to Tower Bridge than meets the eye.
Cables and steel links
If you pay attention, when following the roadway across the Bridge, you will notice the two-steel links and the suspension cables between the Abutments and Towers, holding up the road deck.
This part of Tower Bridge looks dazzling, but it's reason to be is much more than decorative. These steel links and suspension cables are elements of a suspension bridge, with the hangers (or rods) anchored in the ground.
The bascules
Continuing through the main Towers, you arrive on one of the moving spans of the Bridge. They are known as ‘bascules’, which is a French word for seesaw.
These spans are balanced around a pivot shaft, built into the piers. Although the part of the span hidden inside the Towers is a lot shorter that the riverside part, a 422-ton counterweight ensures that the weight either side of the pivot is more or less equal, favouring the riverside by between five to seven tons. This gives the driver better control of the Bridge Lifting process.
A bascule bridge can sometimes be mistakenly called a drawbridge. Even though the term can be used for many types of moving bridges similar to Tower Bridge, it's most commonly associated with defensive structures like castles. So, it's used to restrict access rather than facilitating both land and water transport, which is the primary purpose of other moving bridges.
Designs galore
During the competition to build Tower Bridge held between 1876 and 1879, fifty different designs were proposed, including many types of moving bridges. These included swing bridges, roller bridges, lifting bridges, transporter bridges, duplex bridges and many others including a proposal for a bascule bridge from the City of London Architect, Sir Horace Jones.
A modified version of this design, produced in partnership with Sir John Wolfe Barry, was finally accepted by both the City of London and Parliament in 1884.
What is a cantilever?
Entering the Bridge and walking up the 206 steps to the top of the North Tower, you will discover that the box girder Walkways are supported by cantilevers. This would then make the walkway part of Tower Bridge a cantilever bridge.
A cantilever bridge is a bridge built using cantilevers, which are structures that project horizontally into space. These are weighted and supported on only one end, its two halves meeting in the middle.
Tower Bridge’s High-Level Walkways are supported by more than just the two cantilevers. In 1960, to reinforce the strength of the High-Level Walkways, new suspension cables were added effectively converting them from cantilever to suspension.
The conclusion
So, finally, what type of bridge is Tower Bridge?
- Suspension: two roadways from the shore to the piers, held by suspension cables and steels links.
- Bascule: between the piers, this is the lifting part of the roadway.
Cantilever: the two High-Level Walkways, subsequently converted to suspension bridges in 1960.
It became customary to call Tower Bridge a ‘bascule bridge’ because of its primary purpose – opening and closing for vessels to pass. However, the landmark is, overall, supported by the suspension method.
All in all, Tower Bridge is three bridges for the price of one! It’s an impressive feat of Victorian engineering, known worldwide for its intricate design, and cutting-edge technology.
