You might look at Tower Bridge and ask yourself: 'what type of bridge is Tower Bridge'?
Most will say that the Bridge is a bascule bridge. Essentially, this statement is true, particularly considering the landmark is known all over the world because of its Neo-Gothic Towers and moving spans.
However, if you look a little closer, you will see that there is a lot more to Tower Bridge than meets the eye.
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.
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.
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.
Pen-and-ink drawing by Sir Horace Jones, 1884 © London Metropolitan Archives, City of London
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 Walkways, new suspension cables were added effectively converting them from cantilever to suspension.
So, finally, what type of bridge is Tower Bridge?
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.
There is so much more to discover inside Tower Bridge, from the unique views of London to the history of the landmark.
