Tower Bridge, a brilliant bascule
This August I had the amazing opportunity to spend a week and a half in Europe with one of my best friends, Joanna, who studies Biomedical Science at King’s College in London. While we were in London we spent a good chunk of our time walking around, and saw a great amount of the city. One of my favorite pieces of history that we saw was Tower Bridge, I was amazed by its construction, and mechanical functionality.
I’d like to make a quick PSA: Tower Bridge and London Bridge are separate bridges, although one is frequently confused for the other.
London Bridge is tremendously historically important itself, as it was originally constructed near the end of the 12th century, and initially completed in 1209. Though it went through periods of frequent destruction and repair, it was London’s sole crossing until Westminster Bridge opened in 1750.
The 19th century saw an increase in traffic in the city, and Londoners were not willing to put up with the horrendous traffic both on foot and water, and created a petition for a solution that was finally addressed with the construction of Tower Bridge.
There are quite a few layers to unpack with this structure, beginning with the actual layers of the exterior. At first glance, it may appear that the bridge is made of stone. In actuality, Tower Bridge is a steel bridge that relies entirely upon columns and girders. The stone work was added as embellishment to allow for the bridge to blend in with the aesthetic of the area and the Tower of London, built in 1078, which is a near neighbor on the north bank of the River of Thames. The Cornish granite, Portland stone, and brickwork surrounding the structure are both beautiful and functional in that they are ornate and traditional in appearance, yet shelter the steel from extreme weather.
The construction of the bridge, particularly the beginning, was quite the feat itself. Twelve caissons on each pier were cemented into the river bed, and divers then excavated gravel and the upper layer of clay until the bottom edge of the caissons sank 5 to 10 feet deeper into the London clay. Water was eventually pumped out, the divers dug deeper, and temporary caissons were added and filled with concrete. While construction was underway, water traffic did not stop. There were two “blows” that occurred in this process, but there was the pressure of London calling, so the water traffic continued in tandem with the construction of the bridge.
The resulting structure is half a mile in length, and contains two suspension spans, but the real star of the show is the bascule component. A bascule bridge features a counterweight that balances the leaves of the bridge as they lift up.
The graphic to the right is actually part of graphical instructions I found on how to make a sectional model of Tower Bridge, created by a company called I Can Make, but I think it is a good visual to understand how the bascule mechanism functions. The hole is where the pivot lives, and you can imagine how the counterweights would rotate down into the chambers below as leaves of the bridge lift up. Though they are usually closed to visitors, Tower Bridge has been hosting concerts and art shows in the counterweight chambers on special occasions since 2015. Imagine the wild acoustics in that space! The leaves of the bascule can each be raised to an angle of 86 degrees to the horizontal and closed for traffic to resume again in just 5 minutes. The hydraulic system has its hands full, with each bascule weighing 1,200 tons.
Currently, the final pinions that engage the racks connected to the bascules are driven by modern hydraulic motors that use oil instead of water, which was the original fluid. Back in the day, water was fed into boilers, then through stationary steam engines that pumped the pressurized water into immense hydraulic accumulators. The pressurized water then powered the piston-driven engines for the bascules to operate. Today, the bridge is electrically powered, but it was steam powered until 1976.
Commercial water traffic is not as popular in the modern day as it was a century ago, but Tower Bridge is still fully functional, and vessels that have a mast or superstructure of 30 feet or more that want to pass to the Upper Pool of London can request a bridge lift. All upcoming lift dates are published online so you can go see the bridge do its thing in person, and maybe catch a bascule chamber concert while you’re there too!