London Millennium Footbridge - Essay Example

Millennium Bridge Table 1 Facts and Figures Official Name London Millennium Footbridge Carries Pedestrian Crosses River Thames Locale London, England Maintained By Bridge House Estates and City of London Corporation Design Suspension Bridge Total Length 370 meters (1214 feet) Width 4 meters (13 feet) Opened 10 June 2000 Introduction The London Millennium Footbridge is a steel suspension bridge which has been constructed to be used by pedestrians for crossing the River Thames in London. Owned and maintained by the Bridge House Estates, this bridge located between Blackfriars, which is on upstream side, and Southwark Bridge, which is on the downstream side. The construction of the bridge started in 1998 and the bridge was opened in 2000. The initial design of the bridge was flawed and the pedestrians experienced swaying motion while crossing the river through the bridge, consequently the bridge was closed just two days after its first opening and was reopened in 2002 with some design modifications which eliminated the vibrations. The bridge is a natural expression of structural engineering and architecture but a city center footbridge is equally about people and the environment; in short a piece of public architecture. The bridge gives the pedestrians unique views of London, free from traffic and high above the Thames. In September 1996, a competition was organized by a London-based newspaper the Financial Times and London Borough of Southwark to design a new Footbridge across the River Thames. The idea behind the competition was to get the best design in every aspect therefore the teams participating in the design were structured to have an engineer, an architect and an artist. More than 200 teams participated and the competition was won by Arup (engineer), Foster (architect) and Sir Anthony Caro (Sculptor). The height restrictions and the view behind the bridge required an innovative design which was provided in the form of a design which included some unusual practices; the suspension design had supporting cables below the deck level. This innovative design was given the name ‘blade of light’ by its designers. The structure of the Millennium Bridge is innovative and complex but it has been designed to achieve an apparent simple form. The design of the Millennium Bridge is based on the following considerations: An evolved support system in which the majority of the bridge stiffness is created by shallow cable profile derived tension. This allows a light bridge deck structure. A modular design in which several structural members and components can be repeated in the structure, thus reducing the fabrication costs. This can also simplify the maintenance after the construction and allows easy execution of the construction phase. The distribution of the forces on the foundations should be such that they do not disturb the existing structures on the north bank and also the foundations of the existing bridges. The aesthetics of the bridge at night were also a big concern and a light pipe system illumination was proposed to create a ‘blade of light’ across the river at night. The river traffic analysis is also important while designing a bridge at such a location. A major issue was the provision of planning advice for gaining the necessary approvals to construct a new river crossing in the heart of London. Moreover ways of generating the finances for the bridge were also to be considered while designing the bridge. Superstructure Design The bridge design is a shallow suspension bridge in which the view behind the bridge is facilitated by keeping the suspension cables below the bridge deck. The bridge is supported on two river piers through two groups of four 120mm diameter locked coil cables which span from one bank to the other. The three spans of the bridge have different lengths. The middle span which lies between the two piers is 144m long. The north span is 81m long while the south span is 108m in length. Fabricated steel box sections which are known as the transverse arms span between the two cable groups every 8m. These transverse arms hold the steel tubes in place which support the 4m wide deck of the bridge. The deck of the bridge is made up of aluminum box sections which span between the edge tubes. The edge tubes not only support the aluminum sections of the bridge span but also the railing and the lights. The river piers comprise of a steel bracket which is V shaped fixed to a tapering elliptical reinforced concrete body. The cables supporting the aluminum deck are of stainless steel. The bridge is a very shallow suspension bridge as the highly tensioned cable which is the middle span cable, sags only 2.3m over 144m span giving a span to dip ratio of 63:1. This gives a structure which is almost 6.5 times shallower than a conventional suspension bridge. The four cables on each side of the deck are anchored at each abutment and propped by two river supports. Cable and Pier Design The primary structure on which the whole bridge is supported is the cables and the structure has a very shallow cable profile. The complexity of the cable design is caused by the multiple spans of the bridge which makes the Millennium Bridge slightly different from a typical ribbon bridge. The cables are designed keeping in consideration the abutment and the pier stiffness. The parametric analysis revealed that the 80% stiffness is given by the abutment while the piers give 20% of the required stiffness. This is the reason for using shallow cable profile. The total dead load cable tension is 22.5 MN. The Millennium Bridge is different from the ordinary suspension bridge in many ways as it has the same structural system to resist the lateral wind loads and the normal dead and live loads. The deck of the bridge is not braced and derives its strength from the tension in the cables which makes them stiff enough to resist the lateral loads. To increase the torsional stiffness of the bridge the cables are set wide apart in plan and are set well beyond the width of the deck. This has also enhanced the aerodynamic performance of the bridge as the increased torsional stiffness helps separate the torsional and translational frequencies. Parametric studies made it possible to use typical spacing or 16m making the deck structure very simple and the use of prefabricated aluminum panels for the deck, facilitating the deck construction process. The design of the piers served two major objectives; minimizing the river hydraulic forces and withstanding the ship impact. The ship impact forces were calculated by careful round the clock survey of the river traffic. The cast in C60 reinforced concrete piers have tapering elliptical cross section. The construction of the piers was done very systematically by fabricating the formwork offsite. The formwork was fabricated in three 5m sections which were brought to the site by barge. To make the whole structure effectively monolithic the V bracket was connected to concrete by using the 75 mm steel bars. North and South Abutments The anchoring of the cable on the north abutment has been provided by vertical shear walls on each side of the steps leading up Peter’s Hill. A horizontal deep reinforced concrete beam is connected to these shear walls to resist the large horizontal forces in the cables. The shear walls transfer the forces to 3m pilecap. Another 3m pilecap is used on the southern abutment which is connected to steel ‘strut and tie’ which in turn transfers the load from the cables. The steel ‘strut and tie’ adjust the variation in the orientation of loading caused by live loads. The design of the foundations of the abutments is based on the borehole observations which revealed Made ground overlying a sequence of Terrace gravels, London clay and Lambeth beds. After considering the geology, the next parameter for designing the abutment foundations is the loading on the abutment. The major load on the abutments is the horizontal load of 30MN caused by the tension in the cables due to the combined effect of dead and live loads. The foundation piles resist the horizontal loading by lateral loading of the soil. Frame action and vertical push-pull loading of the piles. The major considerations while designing the piers was to limit the horizontal movements and also the effect of piles on the adjacent buildings. The Vibrations and the Modifications The bridge was closed just two days after its first opening due to unexpected vibrations and was reopened after two years with some modifications. The movements were caused by Synchronous Lateral Excitation; small sideways oscillation caused by natural sway motion of people walking on the bridge. This problem can occur with bridges with lateral frequency modes of less than 1.3 Hz and low mass. The solution of the problem was achieved by damping the vibrations by using 37 fluid-viscous dampers to control horizontal movements and 52 tuned mass dampers to control vertical movements. Word Count: 1495 References Saunders C. (2002), London: The definitive walking guide, Cicerone Press Ltd. Jones N. R. (2005), Architecture of London, Scotland and Wales, Greenwood Publishing Group Fitzpatrick T. (2001), Linking London: The Millennium Bridge, Royal Academy of Engineering Kawada T. (2010), History of the Modern Suspension Bridge: solving the dilemma between economy and stiffness, ASCE Publications Ayre J., Wroe-Brown R. (2002), The London Millennium Bridge: excavation of the medieval and later waterfronts at Peter’s Hill, City of London, and Bankside, Southwark, Museum of London Archaeology Service Read More