Reading Test 03
The bridge that swayed
1. When the London Millennium footbridge was opened in June 2000, it swayed alarmingly. This generated huge public interest and the bridge became known as London’s “wobbly bridge”.
2. The Millennium Bridge is the first new bridge to access 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.
3. 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 organised walk for a noble cause,” one of the pedestrians recounted what he saw that day. “At first, it was still. Then it 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 on to the handrails 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 order to fully investigate and resolve the issue the decision was taken to close the bridge on 12th June 2000.
4. 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 centre 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 the exerting excessive force on the many large flags and banners, but it was rapidly concluded that wind buffering 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.
5. The unexpected motion was the result of a natural human reaction to small lateral movements. It is well known that a suspension bridge has a tendency to sway when troops march over it in lockstep, which is why troops are 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.
6. Professor Funjino 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 a small boat at the same time. As more pedestrian: 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.
7. 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.
8. 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 behaviour of the individual tested.
9. 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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Tasmanian Tiger
A. Although it was called tiger, it looked like a dog with black stripes on its hack and it was the largest known carnivorous marsupial of modem times. Yet, despite its fame for being one of the most fabled animals in the world, it is one of the least understood of Tasmania’s native animals. The scientific name for the Tasmanian tiger is Thylacine and it is believed that they have become extinct in the 20th century.
B. Fossils of thylacines dating from about almost 12 million years ago have been dug up at various places in Victoria, South Australia and Western Australia. They were widespread in Australia 7,000 years ago, but have probably been extinct on the continent for 2,000 years ago. This is believed to be because of the introduction of dingoes around 8,000 years ago. Because of disease, thylacine numbers may have been declining in Tasmania at the time of European settlement 200 years ago, but the decline was certainly accelerated by the new arrivals. The last known Titsmanijin Tiger died in I lobar! Zoo in 193fi and the animal is officially classified as extinct. Technically, this means that it has not been officially sighted in the wild or captivity for 50 years. However, there are still
unsubstantiated sightings.
C. Hans Naarding, whose study of animals had taken him around the world, was conducting a survey of a species of endangered migratory bird. The hat he saw that night is now regarded as the most credible sighting recorded of thylacine which many believe has been extinct for more than 70 years.
D. “I had to work at night.” Naarding takes up the story. “I was in the habit of intermittently shining a spotlight around. The beam fell on an animal in front of the vehicle, less than 10m away. Instead of risking movement by grabbing for a camera, I decided to register very carefully what I was seeing. The animal was about the size of a small shepherd dog, a very healthy male in prime condition. What set it apart from a dog, though, was a slightly sloping hindquarter, with a fairly thick tail being a straight continuation of the backline of the animal. It had 12 distinct stripes on its back, continuing
onto its butt. I knew perfectly well what I was seeing. As soon as I reached for the camera, it disappeared into the tea-tree undergrowth and scrub.”
E. The director of Tasmania’s National Parks at the time, Peter Morrow, decided in his wisdom to keep Naarding’s sighting of the thylacine secret for two years. When the news finally broke, it was accompanied by pandemonium. “I was besieged by television crews, including four to five from Japan, and others from the United Kingdom, Germany, New Zealand and South America,” said Naarding.
F. Government and private search parties combed the region, but no further sightings were made. The tiger, as always, had escaped to its lair, a place many insist exists only in our imagination. But since then, the thylacine has staged something of a comeback, becoming part of Australian mythology.
G. There have been more than 4,000 claimed sightings of the beast since it supposedly died out, and the average claims each year reported to authorities now number 150. Associate professor of zoology at the University of Tasmania, Randolph Rose, has said he dreams of seeing a thylacine. But Rose, who in his 35 years in Tasmanian academia has fielded countless reports of thylacine sightings, is now convinced that his dream will go unfulfilled. convinced that his dream will go unfulfilled.
I. Dr. David Pemberton, curator of zoology at the Tasmanian Museum and Art Gallery, whose PhD thesis was on the thylacine, says that despite scientific thinking that 500 animals are required to sustain a population, the Florida panther is down to a dozen or so animals and, while it does have
some inbreeding problems, is still ticking along. “I’ll take a punt and say that, if we manage to find a
thylacine in the scrub, it means that there are 50-plus animals out there.”
J. After all, animals can be notoriously elusive. The strange fish is known as the coelacanth’ with its “proto-legs”, was thought to have died out along with the dinosaurs 700 million years ago until a specimen was dragged to the surface in a shark net off the south-east coast of South Africa in 1938.
K. Wildlife biologist Nick Mooney has the unenviable task of investigating all “sightings” of the tiger totaling 4,000 since the mid-1980s, and averaging about 150 a year. It was Mooney who was first consulted late last month about the authenticity of digital photographic images purportedly taken by a German tourist while on a recent bushwalk in the state. On face value, Mooney says, the account of the sighting, and the two photographs submitted as the proof amount to one of the most convincing cases for the species’ survival he has seen.
L. And Mooney has seen it all – the mistakes, the hoaxes, the illusions and the plausible accounts of sightings. Hoaxers aside, most people who report sightings end up believing they have been a thylacine, and are themselves believable to the point they could pass a lie-detector test, according to Mooney. Others, having tabled a creditable report, then become utterly obsessed like the Tasmanian who has registered 99 thylacine sightings to date. Mooney has seen individuals bankrupted by the obsession, and families destroyed. “It is a blind optimism that something is, rather than a cynicism that something isn’t,” Mooney says. “If something crosses the road, it’s not a case of ‘I wonder what that was?’ Rather, it is a case of ‘that’s a thylacine!’ It is a bit like a gold prospector’s blind faith, ‘it has got to be there’.”
M. However, Mooney treats all reports on face value. “I never try to embarrass people or make fools of them. But the fact that I don’t pack the car immediately they ring can often be taken as ridicule.
Obsessive characters get irate that someone in my position is not out there when they think the thylacine is there.”
N. But Hans Naarding, whose sighting of a striped animal two decades ago was the highlight of “a life of animal spotting”, remains bemused by the time and money people waste on tiger searches. He says resources would be better applied to save the Tasmanian devil, and helping migratory bird
populations that are declining as a result of shrinking wetlands across Australia.
O. Could the thylacine still be out there? “Sure,” Naarding says. But he also says any discovery of surviving thylacines would be “rather pointless”. “How do you save a species from extinction? What
could you do with it? If there are thylacines out there, they are better off right where they are.”
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The Analysis of Fear
Researchers are investigating the processes in the brain that give rise to fear in animals. The results may lead to new ways to treat human anxiety.
Over the years, the majority of people acquire a range of skills for coping with frightening situations. They will attempt to placate a vexed teacher or boss and will shout and run when chased by a hostile stranger. But some individuals become overwhelmed in circumstances others would consider only minimally stressful: fear of ridicule might cause them to shake uncontrollably when called on to speak in a group, or terror of strangers might lead them to hide at home, unable to work or shop for groceries. Why do certain people fall prey to excessive fear?
Ned H. Kalin and Steven E. Shelton at the University of Wisconsin-Madison are addressing this problem by identifying specific brain processes that regulate fear and its associated behaviors. Despite the availability of non-invasive computer imaging techniques, such information is still extremely difficult to obtain in humans. Hence, they have turned their attention to another primate, the rhesus monkey. These animals undergo many of the same physiological and psychological developmental stages that humans do, but in a more compressed time span. As we gain more insight into the nature and operation of neural circuits that modulate fear in monkeys, it should be possible to pinpoint the brain processes that cause inordinate anxiety in people, and to devise new therapies to counteract it Effective interventions wouJd be particularly valuable if they were applied at an early age, as growing evidence suggests overly fearful youngsters are at high risk of later emotional distress.
When they began their studies two decades ago, Kalin and Shelton knew that they would first have to find cues that elicit fear and identify behaviors that reflect different types of anxiety. With such information in hand, they could then proceed to determine the age at which monkeys begin to match defensive behaviors selectively to specific cues. Finally, by determining the parts of the brain that reach maturity during the same time span, they could gain clues to the regions that underlie the regulation of fear and fear-related behavior.
The experiments were carried out at the University of Wisconsin-Madison. Kalin and Shelton discerned varied behaviors by exposing monkeys between six and 12 months old to three related situations. In the alone condition, an animal was separated from its mother and left by itself in a cage for ten minutes. In the no-eye-contact condition, a person stood motionless outside the cage and avoided looking at the solitary infant. In the stare condition, a person was again present and motionless but, assuming a neutral expression, peered directly at the animal. These positions are no more frightening than those that primates encounter frequently in the wild, or those that human infants encounter every time they are left at a daycare center.
In the alone condition, most monkeys became very active and emitted frequent gentle ‘coo’ calls made with pursed lips. More than 40 years ago it was deduced that when an infant monkey is separated from its mother, it yearns to regain the closeness and security provided by nearness to the parent. These responses help to draw the mother’s attention. In contrast, in the more frightening no-eye-contact situation, the monkeys reduced their.
Activity greatly and sometimes froze for extended periods of time. When an infant spots a potential predator, its goal shifts from attracting the mother to becoming inconspicuous. Inhibiting motion and freezing are common attempts to achieve this in many species. If the infant perceives that it has been detected, its aim shifts to warding off an attack. So the stare condition evoked a third set of responses. The monkeys made several hostile gestures: barking (forcing air from the abdomen through the vocal cords to emit a harsh, growl-like sound) and staring back. Sometimes the animals mixed the threatening displays with submissive ones, such as fear grimaces, which look something like wary grins, or grinding of teeth. Having identified three categories of defensive behaviors, Kalin and Shelton set about determining when infant monkeys first begin to apply them effectively. Several lines of work had led them to surmise that the ability to make such choices emerges when an infant is around two months old. To establish the critical period of development, they examined four groups of infant monkeys ranging in age up to 12 weeks old. The babies were separated from their mothers, left to acclimatize to a cage, and then exposed to the alone, no-eye-contact and stare conditions. All sessions were videotaped for analysis. They found that the infants in the youngest group (no more than two weeks old) engaged in defensive behaviors.But they lacked some motor control and seemed to act randomly, as if they had not noticed the human beings that were present. Babies in the two intermediate-age groups had good motor control, but their actions seemed unrelated to the test condition. Only animals in the oldest group (nine- to 12-week olds) conducted themselves differently in each situation, and their reactions were both appropriate and identical to those of mature monkeys. This finding meant motor control was not the prime determinant of selective responding and that nine to 12 weeks is the critical age for the appearance of a monkeys ability to adaptively modulate its defensive activity to meet changing demands.
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