Curious Questions: How does Tower Bridge work? And why was it built like that in the first place?

Tower Bridge celebrates its 125th birthday on June 30. But how does it work? And why was it built that way in the first place?

Building a bridge across a deep, fast-flowing river is a tricky proposition at the best of times. But building one which opens up to allow ships to pass through is an even more difficult engineering task. It’s something that you only do when there’s no option.

And that’s what happened in the case of Tower Bridge. In the late 18th century the Pool of London, between London Bridge and the Tower of London, was the centre of the capital’s trade, handling 10 ships a day and over 600,000 tons of cargo a year. The new West India docks built at the turn of the 19th century eased some of the pressure, but as volumes continued to grow the Pool of London was still crucial to London’s financial power.

Yet equally crucial was London’s need for new river crossings east of the City that would link north and south. The Thames Tunnel and Tower Subway had gone part of the way to filling the need, but neither were a proper substitute for a new bridge – and in 1877, the ‘Special Bridge or Subway Committee’ was formed under the chairmanship of Sir Albert Joseph Altman.


[READ MORE: Did a bus really jump over Tower Bridge?]


The committee advertised a competition to design a bridge that would offer clearance of at least 135ft, enough for the tallest ships of the era to pass through. Rather nicely they also specified that it should be a thing of beauty, specifying Gothic style to mirror the recently-built Palace of Westminster at the other end of London.

Funnily enough, the winner from the 50+ designs submitted himself had a link to the Palace of Westminster. The plan chosen was that of Sir John Wolfe Barry, son of Charles Barry, who was the man who had just finished building the new Houses of Parliament.

Barry’s plan was for a bascule bridge. This is not a drawbridge, but instead a bridge which opens via the use of counterweights. It was an ingenious idea, but not a new one: such bridges had been used since ancient times. But the genius of the plan was in how cleverly the design hides the weights out of sight beneath the roadway to create the illusion that the halves of the bridge are held up almost by magic.

In reality, it’s just physics: the ‘bascules’ are beautifully balanced so as to minimise the amount of power needed to open and close the bridge. That’s no small consideration even now, when the bridge still opens a couple of times a day. In its heyday, opening and closing dozens of times a day, it was a huge consideration.

Inside a bascule chamber at Tower Bridge - when the bridge is opened, the counterweights slide into this empty space.

Inside a bascule chamber at Tower Bridge – when the bridge is opened, the counterweights slide into this empty space.

Work began in 1886 and finished in 1894, at a total cost of £1.1 million — that’s equivalent to just over £130m in today’s terms, and seems like an astonishing bargain since postcard sales alone probably top that figure.

The bridge was officially opened on June 30, 1894, by the Prince of Wales (the future King Edward VII) and his enormously popular wife, Princess Alexandra.

The rest, as they say, is history – and a very colourful history it is too, taking in daredevil pilots, a quick-thinking bus driver, hundreds of thousands of marathon runners and lots more besides. The Premier Inn hotel chain have put together this nice retrospective, but here are some of the highlights over the years:

1894: The opening ceremony

The opening ceremony featured the Prince of Wales, later King Edward VII, on June 30, 1894.

A German visitor

The German airship the Graff Zeppelin flies over Tower Bridge, London, 1931.

Silver Jubilee: The bridge’s first big paint job

The Queen's 25th Jubilee in 1977 – until this date the bridge was painted brown; ever since, it's kept its colours of red, white and blue.

The Queen’s 25th Jubilee in 1977 – until this date the bridge was painted brown; ever since, it’s kept its colours of red, white and blue.

1981: The first London Marathon

2012: The London Olympics

Tower Bridge, London, with the Olympic Rings showing ready for the forthcoming 2012 games.

Tower Bridge, London, with the Olympic Rings showing ready for the 2012 games. Somehow the bridge has never looked prettier.

2014: Remembering the First World War

Planting The Ceramic Poppies, Blood Swept Lands and Seas of Red, Tower of London – Art work by Paul Cummins and Tom Piper.