Current usage of the London Millennium Footbridge - Essay Example

Study Skills STUDY SKILLS GRACE SARKAR Order No. 440265 11 May Table of Contents Introduction … 3 Current usage … 4 Survey of opinions … 5 Survey of literature on the project … 5 Costs, benefits and risks … 11 Conclusion … 12 Sources … 13 STUDY SKILLS Introduction The London Millennium footbridge was opened on 10 June 2000 across the river Thames in London. The inauguration took place at the hands of the Queen. It was a popular bridge and people in London looked forward to its completion with high expectations, although it quickly turned out to be a disappointment. The bridge had to be closed two days later due to its abnormal swaying once people started walking on it in large numbers. The bridge soon became known as London’s “wobbly bridge”. The bridge was designed by Sir Norman Foster and cost £18.2 million. It spanned 325m across the river linking St Paul’s Cathedral on the north side of the river to Tate Modern gallery on the other side. 100,000 people turned up to cross the river on this bridge before it was closed to public beset with problems. Excessive swaying occurred as the number of people crossing the bridge grew. It was then decided to limit the number of people crossing it at a time. The problem of swaying, however, persisted. The swaying was severe enough for people on it to stop walking and hold on to the rails for support (Newland, David E). While the number of people on the bridge swelled, the bridge began to sway and twist in regular oscillations and the worst movement occurred on the central span where the deck was moving by up to 70mm. The engineers insisted the bridge would not fall down but people were left unnerved. Finally, the engineers closed the bridge completely when limiting the number of people failed to make any difference (Millennium Bridge (c), 2000). It was closed to public on 12 June 2000 for re-examination and remedial work. It was later re-opened on 27 February 2002 and now forms part of London’s many architectural marvels (Millennium Bridge (a)). The solution to this problem “involved installing dampers under the deck and between the deck and the river piers. This has provided an excellent solution as it does not detract from the aesthetic impact of the bridge as originally designed” (Millennium Bridge (e), London, 2007). Current usage The bridge is now used by thousands of people and cyclists every day. It is a key pedestrian link and is a simple concept that has achieved a simple form via a complex and innovative design. The bridge is accessible throughout the day. The nearest underground stations are Blackfriars or Mansion House on the Circle line as well as the District line. The bridge affords breathtaking view of panoramic London. The view of St. Paul’s Cathedral majestically towering over other structures is the major attraction. There is also the fresh, cool breeze that wafts onto all those walking across. One cannot help feeling a bit of elation at the crossover without any fear of bumping into some vehicle. Survey of opinions regarding current usage of the bridge Some hundred people in the age group of 18-65 were surveyed at random and most of them felt the bridge served a useful purpose. While 80% felt the bridge served a useful purpose 15% had some reservation about the use of bridge exclusively for pedestrians and cyclists and felt the cause would have been better served had the bridge been made bigger to have it used by vehicles as well. 5% were neutral and said they like the bridge as it was or even if it had been built with facilities for vehicles. On the whole, the bridge is a welcome development of the millennium. Those interviewed overwhelmingly felt (86%) that this structure will prove to be a forerunner to many other such structures around the globe. It has certainly proved beneficial even to the green movement. Survey of literature on the project now and in the future The bridge was developed in close collaboration between Foster and Partners’ as architects with Sir Anthony Caro and Arup engineers. “The concept was to create a structure of minimum intervention; it was to be a ‘ribbon of steel’ across the river. This has been achieved with a very shallow suspension bridge consisting of two ‘Y’ frames and eight cables, four each side. The lightweight deck, four meters wide, passes between these two sets of cables and is supported by structural arms which connect onto the cables at eight meters intervals. The cables dip below the deck level at the mid-span point, so that the bridge does not impede views of London from further up and down the river. The lighting has been incorporated into the structure and is activated by photo-cells at dusk, transforming the structure into a ‘blade of light’” (Millennium Bridge (e), London, 2007). “The bridge has three spans, the longest being the centre span of 144 meters. In order to meet its artistic requirements, the bridge’s suspension cables sag only 2.3 meters which is a fraction of the traditional suspension bridge of the same span. As a result, the cables carry a very high tension force for a bridge of this size, totaling some 2000 tons” (Newland, David E). “Video pictures showed later that the south span had been moving through an amplitude of about 50 mm at 0.8 Hz and the centre span about 75 mm at 1 Hz approximately. Probably higher amplitudes occurred periodically and several modes were involved. There was a significant wind blowing on the opening days (force 3-4) and the bridge had been decorated with large flags, but it was rapidly concluded that wind buffeting had not contributed significantly to vibration of the bridge. Another possible explanation was that coupling between lateral and torsional deck movements was allowing vertical footfall excitation to excite lateral modes, but this was not found to be a significant factor. Evidence in support of this conclusion was that the 1 Hz mode of the centre span was the span’s second lateral mode; with nodes at its centre and at the two bridge piers, this mode had practically no torsional movement. “It was realized very quickly that the problem was one of lateral excitation and although allowance had been made for lateral forces it had not been expected that pedestrians would so easily fall into step or that the lateral force per person would be as great as was apparently proving to be the case. “An immediate research program was launched by the bridges engineering designers Ove Arup, supported by a number of universities and research organizations. “It was found that some similar experiences had been recorded in the literature, although these were not well-known and had not yet been incorporated into the relevant bridge building codes. A German report in 1972 quoted by Bachmann and Ammann in their IABSE book (1987), described how a new steel footbridge had experienced strong lateral vibration during an opening ceremony with 300-400 people.  They explained how the lateral sway of a person’s centre of gravity occurs at half the walking pace. Since the footbridge had a lowest lateral mode of about 1.1 Hz, the frequency of excitation was very close to the mean pacing rate of walking of about 2 Hz. Thus in this case an almost resonating vibration occurred. Moreover it could be supposed that in this case the pedestrians synchronized their step with the bridge vibration, thereby enhancing the vibration considerably. The problem is said to have been solved by the installation of horizontal tuned vibration absorbers” (Newland, David E). “The concept of synchronization turned out to be very important, and a later paper by Fujino et al. (1993) was discovered which described observations of pedestrian-induced lateral vibration of a cable-stayed steel box girder bridge of similar size to the Millennium Bridge. It was found that when a large number of people were crossing the bridge (2,000 people on the bridge), lateral vibration of the bridge deck at 0.9 Hz could build up to an amplitude of 10 mm with some of the supporting cables whose natural frequencies were close to 0.9 Hz vibrating with an amplitude of up to 300 mm. By analyzing video recordings of pedestrians’ head movement, Fujino concluded that lateral deck movement encourages pedestrians to walk in step and that synchronization increases the human force and makes it resonant with the bridge deck. He summarized his findings as follows: "The growth process of the lateral vibration of the girder under the congested pedestrians can be explained as follows. First a small lateral motion is induced by the random lateral human walking forces, and walking of some pedestrians is synchronized to the girder motion. Then resonant force acts on the girder, consequently the girder motion is increased. Walking of more pedestrians are synchronized, increasing the lateral girder motion. In this sense, this vibration was a self-excited nature. Of course, because of adaptive nature of human being, the girder amplitude will not go to infinity and will reach a steady state. “Although Fujino records the damping ratio of the 0.9Hz lateral mode as , he found that only 20% of the pedestrians on the main span of the bridge were completely synchronized to the girder vibration and the amplitude of vibration was only 10 mm (compared with 75 mm for the Millennium Bridge). Impressions from video clips of the Millennium bridge are that a good deal more than 20% of walkers had synchronized their step. Also in Fujino’s example, the very large movement of the suspension cables (300 mm amplitude) may have made these act as dynamic vibration absorbers and so limit the extent and consequences of synchronization. “It was clear that data specific to the Millennium Bridge was urgently required and Arup undertook an extensive program of testing to obtain this. In addition to commissioning tests on human gait and how this is affected by movement of the walking surface, the main tests were carried out on the bridge itself. These included artificially shaking the bridge to confirm mode shapes and damping and a comprehensive series of crowd tests. Detailed vibration measurements and video records were made with pedestrians walking at different speeds and densities on each span. These allowed reliable quantitative data on the synchronous lateral excitation phenomenon to be established and a self-excitation model to be developed which could give a reliable prediction of structural response” (Newland, David E). “Arups loading model is described in Fitzpatrick et al (2001).  Using experimental data from controlled pedestrian loading tests, with an approximately constant density of pedestrians walking at steady speed, Arup found that there was strong correlation between the amplitude of the pedestrians’ (modal) excitation force and the amplitude of the bridge deck’s (modal) lateral velocity. Measurement of deck velocity is straightforward, but the excitation force was calculated from a power flow analysis based on the concept that the work done by the net excitation force (foot-fall force less damping force) is equal to the gain of kinetic energy per cycle. This led to the conclusion that, when synchronization has occurred, the amplitude of energy-transferring force per pedestrian is linearly proportional to velocity and acts as a negative damping force. These results can be reached by a completely different approach using a feedback model of synchronous lateral excitation. It is based on the idea that people will naturally fall into step with each other and that they will unconsciously adjust their group phasing so that the bridge vibration increases to a maximum. This is important. As the assumed phase of people’s walking and the bridge’s movement changes, bridge amplitude changes enormously. From this feedback model, the phase to give peak response can be calculated theoretically. It turns out that the answer agrees with the phase found by Arup from their experimental studies. ”The physical mechanism of synchronous lateral excitation is well described by Arup (Dallard et al, 2001) in the following way: "Chance footfall correlation, combined with the synchronization that occurs naturally within a crowd, may cause the bridge to start to sway horizontally. If the sway is perceptible, a further effect can start to take hold. It becomes more comfortable for the pedestrians to walk in synchronization with the swaying of the bridge. The pedestrians find this makes their interaction with the bridge more predicable and helps them maintain their lateral balance. This instinctive behavior ensures that the footfall forces are applied at a resonant frequency of the bridge, and with a phase such as to increase the motion of the bridge. As the amplitude of the motion increases, the lateral force imparted by individuals increase, as does the degree of correlation between individuals. The frequency "lock-in" and positive force feedback caused the excessive motions observed at the Millennium Bridge" (Newland, David E). The lifespan of the bridge is not mentioned anywhere. However, with regular checks it could last a few centuries. The construction is quite robust to last a few centuries. The London Municipal Council can always consider constructing another bridge should demand increase. Costs, benefits and risks associated with the project It is difficult to calculate the costs, benefits and risks associated with this bridge. Of course, there is tremendous saving by way costs to those residing in the vicinity of the bridge. They save on travel costs. Also, people are more willing to save on fuel costs and walk across the bridge. The benefits accruing out of savings in terms of fuel costs is immense considering the large number of people who prefer walking up and down the bridge. The beautiful structure adds to the picturesque skyline. Risks arising from the bridge are minimal. There is the threat from extremists who could find the bridge a good target if security is lax. Conclusion The Millennium Bridge is a landmark structure, although initially it created some consternation due to technical faults. The bridge may have been an object of dismay for many initially. However, it now occupies a place of pride in the annals of architecture in London. The bridge is a road to the past as well as the future. It is a road to the past because people have once again found the pleasant exercise of walking preferable to using vehicles. It is a road to the future because it is certainly a harbinger of many more such structures to come up around the globe in the not-so-distant future. Sources: Millennium Bridge (a), http://www.londontown.com/LondonInformation/Attraction/The_Millennium_Bridge/7d5d/ Millennium Bridge (b), 2000, http://www.fosterandpartners.com/Projects/0953/Default.aspx Millennium Bridge (c), 10.06.2000, http://news.bbc.co.uk/hi/english/static/in_depth/uk/2000/millennium_bridge/default.stm Millennium Bridge (d), London, http://www.urban75.org/london/millennium.html Millennium Bridge (e), London, 2007, http://www.galinsky.com/buildings/millenniumbridge/index.htm Newland, David E; Vibration of the London Millennium Footbridge: Part I – Cause, http://www2.eng.cam.ac.uk/~den/ICSV9_06.htm The Millennium Bridge, http://info.arup.com/millenniumbridge// Read More