Anne Coleman, photography Jonathan Primmer – April 2010
We are all familiar with London’s sprawling skyline, or rather river-line. Dotted with structures such as the London Eye, the Millennium Bridge and Canary Wharf, which are all household names, it is hard to picture a London without them. They represent our culture, our heritage, and our message to the world that we are a buzzing city with much to offer. It is almost inevitable that every few years another iconic structure will pop up on the banks of the Thames and before we know it, it will be as if it had always been there. But how do these buildings stand the test of time, how are they built to support us safely and what happens when things go wrong?
The Millennium Bridge
All structures are exposed to the forces of nature. Be it the battling winds, torrential rain, or just the lengthy process of decay, engineers are forced to think about how to defy the elements. But it is not only these that have to be taken into consideration. The structure itself has to compensate for the force of the many people who seek to enjoy it.
The Millennium Bridge, a 320-metre long suspension bridge, was built as the result of a competition in 1996. It opened to the public in 2000 to showcase London’s design and engineering capabilities, and was dubbed the ‘blade of light’. Through its carefully measured construction, the bridge was able to support the downward forces of the 80,000 people who crossed it during its opening day. However, what engineers had failed to anticipate was the magnitude of the horizontal, or lateral, forces exerted by those people.
When we walk, the rise and fall of our bodies create a repeating pattern of forces. “One of these is a sideways force, known as a lateral force.” This is caused by the sway of our mass as we walk with our legs slightly apart. Our weight shifts from left to right, as we place our feet on the ground. This is only about one-tenth of the vertical force we create, so one would not think it enough to be a serious consideration. However, other factors creep into play. With any slight movement in a bridge, people tend to move their legs outwards to balance it. A greater force is exerted as a result. A person’s walking pattern echoes that of the swaying bridge, and soon every footstep of every person on the bridge is in unison. This synchronized lateral force causes the bridge to sway even more and the cycle becomes unbreakable.
The bridge moved 7cm from side to side before it was deemed unsafe and was closed for modifications.
Why was the Millennium Bridge in particular affected?
In a paper published in Nature in 2005, Strogatz, a scientist who studies collective behavior of biological oscillators, tried to explain why: ‘It was by design, a flexible structure, and its natural frequency is close to that of human walking.’ What this means is that the time that it takes for the bridge to resonate from side to side matched that of the pace of the walkers. Just like when you push a child on a swing from its highest height you give the swing more energy, so the people on the bridge gave it more and more momentum.
Subsequent tests on the bridge revealed startling results. Only 160 people, walking in unison with the same natural frequency of the bridge, would be needed to cause the same effect as on the opening day. There were actually 2000 people at any one time crossing the Millennium Bridge the day that it was closed.
“What the Millennium Bridge needed, and what many other structures in London depend upon to support us, was dampers.”
What are dampers and why are they so crucial?
Just as the Millennium Bridge vibrated its way into a quandary, other structures, such as the London Eye and Canary Wharf, can be nudged into swaying uncontrollably. What stops this from happening and keeps us nice and stable is a principle known as ‘damping’. Damping can happen on its own. A violin string will vibrate when plucked, but if left to oscillate alone will slowly die down until it is stationary again. This is due to natural damping, as the energy of the string dissipates. When excessive force is continually applied, natural damping is no longer effective. This is why such structures need added dampers. The London Eye, a 2,100 ton structure on the South Bank, has a total of sixty-four dampers situated round the wheel that help to absorb motion created by the wind. Each damper consists of a spring attached to a mass that has the same natural frequency as the wheel. However, the dampers vibrate in opposition to the direction of the wheel to counteract unwanted swaying.
And it’s not just bridges and wheels that need a helping hand. Buildings with over forty floors have the tendency to move quite dramatically in response to the wind. A remarkable form of damping in place at Canary Wharf, known as ‘slosh’ damping, uses a pool of water to balance out the natural swaying. The water, placed at the top of the building, is forced to slosh in opposition to the movement. “Ideally, one could combine the damper with a swimming pool so that the businessmen could relax at the end of a hard day.” Sadly, the damping efficiency relies heavily on the shape, size and depth of the pool, meaning that the end result isn’t much fun to swim in!
‘The Shard’ – the future?
Come 2012, ‘The Shard’ will be the newest landmark to grace the banks of the Thames. Once completed, it will be the tallest skyscraper in Western Europe, covered from base to tip in tiny glass mirrors reflecting the mood of the sky. With exciting designs come new engineering challenges, and the slender nature of ‘The Shard’ means that it is easily susceptible to wind. “Skyscrapers are built to withstand a deflection that could occur if the worst winds in a fifty-year period were to strike.” Engineers calculate this deflection by a general rule of thumb: the height of the building divided by 500. Towering at 310 meters means ‘The Shard’ could potentially sway 0.6 meters to each side – not particularly comfortable for the residents in the luxurious top floors of the 72-storey building.
Incredibly, the material make-up of the building can be used to dampen its own motion, caused by complex forces produced by the wind flow. A central concrete core and a steel framework with enhanced concrete floors have damping characteristics. Most importantly, the weight at the top keeps ‘The Shard’ firmly stable. The energy from the wind is absorbed into the framework, through the post-tensioned (enhanced) concrete floors and core, and carried down into the foundations of the building.
The threat of Terrorism
The potential threat of terrorism cannot be ignored when cities are erecting sky high buildings; these buildings are statements of western culture, society and economic prowess. With the collapse of the World Trade Center’s Twin Towers in 2001, it would be naïve to presume that architects and engineers alike do not take these events into consideration when creating iconic structures. Evacuation routes and robust construction are tantamount to design, with engineers having to consider how a substantial impact on a building would affect its subsequent collapse. “‘The Shard’ only gained planning permission by the skin of its teeth, having been proposed a few months after the fall of the Twin Towers.”
It is clear that London is a canvas full of engineering triumphs and lessons learnt. As structures dare to be more bold and ambitious than their predecessors, we can only hope that the science behind them stands the tests of time, nature and human impact.