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The Millennium Bridge is the first new bridge across the river Thames
in London since Tower Bridge opened in 1894, and it is the first ever
designed for pedestrians only. The bridge links the City of London near St
Paul’s Cathedral with the Tate Modern art gallery on Bankside.
The bridge opened initially on Saturday 10th June 2000. For the
opening ceremony, a crowd of over 1,000 people had assembled on the south
half of the bridge with a band in front. When they started to walk across
with the band playing, there was immediately an unexpectedly pronounced
lateral movement of the bridge deck. “It was a fine day and the bridge was
on the route of a major charity walk,” one of the pedestrians recounted
what ho saw that day. “At first, it was still. Then if began to sway
sideways, just slightly. Then, almost from one moment to the next, when
large groups of people were crossing, the wobble intensified. Everyone had
to stop walking to retain balance and sometimes to hold onto the hand rails
for support.” Immediately it was decided to limit the number of people on
the bridge, and the bridge was dubbed the ‘wobbly’ bridge by the media who
declared it another high-profile British Millennium Project failure. In
older to fully investigate and resolve the issue the decision was taken to
close the bridge on 12th June 2000.
Arup, the leading member of the committee in charge of the
construction of the bridge, decided to tackle the issue head on. They
immediately undertook a fast-track research project to seek the cause and
the cure. The embarrassed engineers found the videotape that day which
showed the center span swaying about 3 inches sideways every second and the
south span 2 inches every 1.25 seconds. Because there was a significant
wind blowing on the opening days (force 3-4) and the bridge had been
decorated with large flags, the engineers first thought that winds might be
exerting excessive force on the many large flags and banners, but it was
rapidly concluded that wind buffeting had not contributed significantly to
vibration of the bridge. But after measurements were made in university
laboratories of the effects of people? walking on swaying platforms and
after large-scale experiments with crowds of pedestrians were conducted on
the bridge itself, a new understanding and a new theory were developed.
The unexpected motion was the result of a natural human reaction to
small lateral movements. It is well known that a suspension bridge has tendency
to sway when troops march over it in lockstep, which is why troops arc
required to break step when crossing such a bridge. “If we walk on a
swaying surface we tend to compensate and stabilise ourselves by spreading
our legs further apart but this increases the lateral push”. Pat Dallard,
the engineer at Arup, says that you change the way you walk to match what
the bridge is doing. It is an unconscious tendency for pedestrians to match
their footsteps to the sway, thereby exacerbating it even more. “It’s
rather like walking on a rolling ship deck you move one way and then the
other to compensate for the roll.” The way people walk doesn’t have to
match exactly the natural frequency of the bridge as in resonance the
interaction is more subtle. As the bridge moves, people adjust the way they
walk in their own manner. The problem is that when there are enough people
on the bridge the total sideways push can overcome the bridge’s ability to
absorb it. The movement becomes excessive and continues to increase until
people begin to have difficulty in walking they may even have to hold on to
the rails.
Professor Fujino Yozo of Tokyo University, who studied the
earth-resistant Toda Bridge in Japan, believes the horizontal forces caused
by walking, running or jumping could also in turn cause excessive dynamic
vibration in the lateral direction in the bridge. He explains that as the
structure began moving, pedestrians adjusted their gait to the same lateral
rhythm as the bridge; the adjusted footsteps magnified the motion just like
when four people all stand up in small boat at the same time. As more
pedestrians locked into the same rhythm, the increasing oscillation led to
the dramatic swaying captured on film until people stopped walking
altogether, because they could not even keep upright.
In order to design a method of reducing the movements, an immediate
research program was launched by the bridge’s engineering designer Arup. It
was decided that the force exerted by the pedestrians had to be quantified
and related to the motion of the bridge. Although there are some
descriptions of this phenomenon in existing literature, none of these
actually quantifies the force. So there was no quantitative analytical way
to design the bridge against this effect. The efforts to solve the problem
quickly got supported by a number of universities and research
organisations.
The tests at the University of Southampton involved a person walking
on the spot on a small shake table. The tests at Imperial College involved
persons walking along a specially built, 7.2m-long platform, which could be
driven laterally at different frequencies and amplitudes. These tests have
their own limitations. While the Imperial College test platform was too
short that only seven or eight steps could be measured at one time, the
“walking on the spot” test did not accurately replicate forward walking,
although many footsteps could be observed using this method. Neither test
could investigate any influence of other people in a crowd on the behavior
of the individual tested.
The results of the laboratory tests provided information which
enabled the initial design of a retrofit to be progressed. However, unless
the usage of the bridge was to be greatly restricted, only two generic
options to improve its performance were considered feasible. The first was
to increase the stiffness of the bridge to move all its lateral natural
frequencies out of the range that could be excited by the lateral footfall
forces, and the second was to increase the damping of the bridge to reduce
the resonant response.
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