The turn of the millennium was a special event that many companies and countries commemorated. Millennium Edition became an almost inflationary term. In 1999, many thought that the world would end at the turn of the millennium.
Luckily, it's still standing and it would have been a shame if it hadn't. Without the new millennium, we would not have been able to enjoy London's Millennium Bridge. Opened in 2000, it soon offered an interesting spectacle: whenever many pedestrians passed it, it began to sway. Like a drunken sailor. You'd get seasick just looking at it!
But where were these vibrations coming from? What went wrong, and did the engineers involved manage to tame the bridge's unwanted behavior? One thing is for sure: the swaying bridge still bears its nickname and the videos of its first opening will immortalise it forever.
Millenium Bridge: Known Problem Not Considered
But why does a bridge start to sway when a certain number of people cross it? This problem is familiar to just about every engineer, because it is far from unknown. In fact, this behavior has caused serious disasters in the past, with many bridges being torn from their supports and falling to their deaths. We are talking about an initially invisible enemy of every bridge engineer: natural frequencies.
Natural Frequency as Natural Enemy of Suspension Bridges
The concept of natural frequencies is now a well-known and extensively researched phenomenon in the construction industry. It plays a particularly important role in bridge design. The explanation is quite simple. Every solid body has a natural frequency at which it vibrates. If these vibrations are increased, for example by applying a rhythmic force, the movements are further amplified.
If the rhythm remains the same, the vibrations increase until the material finally gives way. This has happened many times in the past, on a much larger scale than we have been able to observe with the swaying Millennium Bridge. We will briefly describe two of these events.
Tacoma Narrows Bridge in 1940
It was the third largest suspension bridge in the world: On July 1, 1940, the Tacoma-Narrows Bridge in the US state of Washington opened to traffic. It was a special light-weighted suspension bridge, of which its builders were particularly proud. But the bridge had a flaw. Because of its light weight, it swayed in the wind. This wind stimulated the natural frequency of the bridge, which then increased its vibrations. If you've seen pictures from that time, you might think it's a caricature or a clip from an old cartoon. But no – the swings up and down were up to 60 cm.
What at first seemed like a terribly inappropriate comedy movie quickly became serious. Many chose to avoid the bridge, while others travelled there just to ride the roller coaster. On August 7, 1940, the deformations of the Tacoma Narrows Bridge reached their peak.
As usual, it swayed up and down in moderate to strong winds. However, the simultaneous lateral air turbulence caused the bridge not only to sway up and down, but also to slightly rotate. The result was as frightening as it was definitive: the bridge gave way just four months after opening. In the end, the entire structure collapsed. Captured by chance by the camera of a professor who was there to study the vibrations.
Broughon Suspension Bridge Near Manchester
However, in the case of the Millenium Bridge, it was not wind that made the bridge sway. In this case, people were more or less to blame. And it was not the first time something like this had happened. But how does it happen, why does a bridge start to sway?
The Broughton Suspension Bridge in Manchester, one of the first suspension bridges in Europe, was a particularly impressive example. Opened in 1826, its builders were well aware of the risks involved. The chain bridge was not very strong.
At that time, as you had to pay a fee to cross the privately built bridge, there was also a bridge keeper who, among other things, checked that the bridge was not overloaded. There were no major incidents during the first five years.
On April 12, 1831, 74 soldiers, a regiment on their way to lunch after a training exercise, crossed the bridge. Nothing unusual so far. On long marches, the soldiers walked "without tread", as the jargon goes, without walking in step. That was far too strenuous for long distances. So no danger to the chain bridge?
Unfortunately, one of the officers up front started whistling a familiar marching song. And the human herd instinct kicked in. Some joined in or sang along, but everyone unconsciously began to match their steps to the beat of the song. And this song, as you might have guessed, coincided with the natural frequency of the bridge, again without meaning to. The whole thing lasted less than 60 seconds and there was a loud bang.
Shortly afterwards, one of the supporting chains gave way. The bridge began to lean and collapsed sideways. Fortunately, no one was killed. After a brief moment of shock, the remaining soldiers on the bridge reacted and launched a massive rescue operation. This was the first suspension bridge accident to be described in detail. The problem of natural frequencies was not known at the time, and was only discovered much later.
Millenium Bridge: Monument at Turn of Millenium
In the final spurt of the 20th century, plans were made to build another bridge over the Thames, but not for vehicles, just for pedestrians. It was hoped that the new bridge would provide a great view of the city's main attractions, such as St Paul's Cathedral. This would attract many tourists as a viewing and photographic point: a big plus for London tourism.
An architectural competition was held in 1996. After all, it was not easy to build anything in the historic city centre. Pickles, overflowing beer glasses, and broken glass come to mind: No, not a failed party, but real buildings! Because anyone who knows the London skyline and has ever been there knows that the buildings there are usually very oddly shaped.
The reason is a ban on obstructing certain views of the city's historic landmarks. People should be able to see buildings, such as St Paul's Cathedral, from key points in the City of London. We have already published a separate blog post on this subject. Take a look!
Gherkin, Tulip, and Beer Glass: London SkylineFacts and Figures on Design of Millenium Bridge in London
A design by Norman Foster won the competition. Many know him from his later work. His works include The Gherkin in London and the glass dome of the Reichstag in Berlin. So the Millennium Bridge was waiting in the wings.
As well as providing a perfect view of London's landmarks, the height had to be taken into account. If the cables were too high, they would interfere with the viewing corridors, so innovative solutions had to be found. In the end, it was decided to use cables that ran directly along the pavement. Not, as we know from suspension bridges, much higher up, similar to modern cable-stayed bridges. As a result, the Millennium Bridge looked impressively flat and in no way disturbed the cityscape – or tourists taking photographs.
The Millennium Bridge itself rested on two piers and was divided into three sections. It reached an impressive total length of 325 m. A special feature at the time were the eight suspension cables with a total load-bearing capacity of 2000 t. It could easily carry 5,000 people on top of its own weight without suffering any damage. It certainly sounds sturdy and probably brings beads of sweat to the foreheads of some claustrophobics. Nevertheless, two days after it was opened, it hit the headlines. Suddenly, the Millennium Bridge was swaying! How could that be?
Shaky Business: Opening of Millenium Bridge in London
The Millennium Bridge project was finally implemented on April 28, 1999. The long-awaited opening of the new footbridge took place on June 10, 2000, two months late. Talking of figures: Waht was the cost of the Millennium Bridge?
Over budget by £2.2 million, the final cost was £18.2 million. Not a small project, in other words. Above all, the bridge quickly attracted national and international press attention. Even if things didn't go quite as well as those responsible would have liked.
The celebrations did not last long. Just two days later, the bridge was closed again. The Millennium Bridge was swaying – and a lot. Movements of up to 6 cm to the left and right quickly made some unsuspecting pedestrians dizzy. But what was causing it? Why was the Millennium Bridge swaying so much?
Why did the Millennium Bridge sway?
As we learned from the previous examples: Natural frequencies can have strong effects. But unlike the events of the past, we were well aware of this in the early 2000s. The natural frequency of the bridge was initially calculated to be 1 Hertz, or one vibration per second.
For pedestrians, this is a perfectly possible, albeit unusual, frequency. But let's not forget that almost everyone would have to move in step to really excite this natural frequency. A few individuals would not be enough. Once again, engineers have failed to take into account human nature, which is to create problems where none should exist through unconscious behavior.
In fact, this problematic behavior makes perfect sense in other parts of our lives. We unconsciously move on surfaces that vibrate in a lateral direction in order to maintain our balance. People adapt their gait to the ground, whether it is uneven or moving. Like on a rocking ship. It happens all by itself.
In the case of the Wobbly Bridge, as the Londoners called it, pedestrians unconsciously adapted to the natural frequency of the bridge. Only when the vibrations of the Millennium Bridge became clearly noticeable did they stop and hold on to the railings. Then the vibrations stopped. If they continued to move, they quickly fell back into the old pattern and the game started all over again.
Could the Millenium Bridge be repaired?
An extensive renovation of the bridge followed to minimize horizontal vibrations in the future. It took two years for a specially designed damping system to give the Millennium Bridge new stability. The cost of the repair: another £5 million. Was it worth it in the end?
Diagonal dampers were installed under some of the bridge's spans. A total of 58 new external vibration dampers ensured that the Millennium Bridge would not sway noticeably in the future. The upgrade was successful, but it also showed that too little attention had previously been paid to horizontal vibrations in bridge structures. So engineering has learned something new. Sometimes, it is not the structures that behave unpredictably, but the people who use them.
Summary: Millenium Bridge Today
And today? Since February 2002, the Millennium Bridge has been carrying thousands of pedestrians across the Thames. Safely and without any significant fluctuations. Only in 2008, the bridge was closed to the public for two days. Were there any problems?
Actually not. At the time, the bridge was being used to film the sixth in a series of movies that everyone knows about. It starred a young wizard with an unusual scar on his face. In the film, the Millennium Bridge was destroyed – fortunately, it escaped this fate in real life.
Today, tourists and London lovers can still enjoy the view of the city's most famous landmarks from the Millennium Bridge. A true monument to the turn of the millennium, the Millennium Bridge is definitely a highlight for anyone who wants to see London at its best – without all the screeching buses and other vehicles. It's well worth a visit!