|Name||London Millennium Bridge|
|Owner||Bridge House Estates for City of London Corporation|
|Architect||Foster together with Sir Anthony Caro (sculptor)|
|Contractor||Monberg Thorsen, Sir Robert McAlpine|
|Latitude||N 51 39' 36"|
|Longitude||W 00 05' 55"|
|Why||Footbridge over River Thames that links St Paul's Cathedral with the Tate Modern Gallery|
|How to read the bridge||Read more.....|
|Overall type||Hanging - unusually shallow suspension bridge which makes it somewhat like a ribbon bridge - but with multiple spans.|
|Spans||Main span: 144m. Side spans: north 81 m., south 108 m.|
|Materials||Steel with aluminium deck.|
|How to read the bridge||Read more about the book metaphor...|
|Chapter 1||Suspension system|
'Y' shaped piers support the main cables. They are designed to resist impact by shipping.
Supporting cables: Two groups of cables are anchored at each bank and are fixed to the piers. The cables sag 2.3 m. in the main span approximately 6 times shallower than a conventional suspension bridge. The cables are as much as possible below the level of the bridge deck to allow good views.
The 'Y' shape is formed by steel 'V' brackets fixed to a tapering
elliptical reinforced concrete body.
Cables: Each group of cables has four 120 mm. diameter locked coil cables.
Each arm of the 'V' is a steel box section of steel plate and stiffeners.
The arms are fixed to the concrete body of the pier using pretensioned 75 mm. diameter
high-strength steel bars cast into the concrete.
Cables: Each cable is fixed to the piers by friction clamps.
|Letters||Steel: Iron, carbon with other additives such as chromium etc
Concrete: Cement, sand, aggregate and water
Reinforced concrete: Concrete is strong in compression but weak in tension where it has to be reinforced with steel bars
|Paragraphs||Transverse arms span across the bridge between the two cable groups. Two steel tubes, on each side of the walk way, span along the bridge between transverse arms. The deck on which people walk and the street furniture.|
|Sentences||The arms are spaced at 8 m. along the bridge. Each one is a fabricated steel box section girder. Each steel tube and each section of the deck. The deck is made up of extruded aluminium box sections spanning betwen the edge tubes The street furniture consists of lighting, handrails etc are fixed onto the edge tubes|
|Words||The steel and aluminium plate, stiffeneres and welds.|
Abutments: are founded on 3 m. reinforced concrete pilecap.
Piers: are founded on 6 m. diameter caissons.
|Sentences||Abutments: The reinforced concrete pilecap is anchored by
group of reinforced concrete piles.
Piers: The caissons were excavated to about 18 m. below river level and filled with reinforced concrete.
|Words||Abutments: 12 RC piles on the north bank and 16 on the south bank.
Piers: The caisson is made of lined precast concrete segments, grouted into the surrounding soil before filling with reinforced concrete.
|Grammar||Technically the bridge is a way of taking forces from up in the air down to the ground.
So imagine the flow of those forces through the structure.
Think of a truck standing on the bridge and how its weight is transmitted through the bridge to the ground.
The design was largely driven by the dramatic architectural vision for the bridge.
This is an unusual form of bridge in that the cables are very shallow and the deck is articulated with sliding joints at regular 16 m. intervals along the length. The stiffness of the bridge derives from the tension in the cables. The cables are set wide apart in plan well beyond the width of the deck to reduce the effects of assymetric live load across the width of the deck and to improve the aerodynamics. The main loads on the abutments are the horizontal forces from the cables and their overturning moments. The movement of the anchorage points had to be constrained.
The structural designers knew that the bridge had to be designed carefully for vibrations. They were aware of examples of dynamic excitation of some previous bridges such as at the Auckland Harbour Bridge, New Zealand during a protest march in 1975. They did extensive calculations and considered the well known vertical oscillations that can occur when people walk in step.
However on the opening day between 80,000 and 100,000 people crossed the bridge with a maximum of about 2,000 people at any one time (i.e. about 1.3 to 1.5 people per metre). Unexpected excessive lateral vibrations occurred mainly in the south span but the vertical vibrations were not excessive. The lateral vibrations did not occur continuously but built up and died down depending on the number of people on the bridge. Unfortunately many people had difficulty in walking so the bridge was closed after two days.
The unexpected motion was the result of a natural human reaction to small lateral movements. 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.
The solution to stop the wobble of the London Millennium Bridge was to install shock absorbers, rather like a car. Using the results of their quantitative research the engineers designed a system of 37 shock absorbers called 'viscous dampers' and 54 weights attached to the bridge by springs to dampen the vertical motion.
Although these kinds of wobbles can occur on all long bridges they are, and still are, very rare. This is largely because very few bridges have experienced large enough crowds. The design did not cause the wobble because of its innovative structural form. The wobble arose as a result of the large number of people and insufficient damping in the structure.
At first the wobble was attributed to people walking synchronously with the motion of the bridge - the phenomenon was called Synchronous Lateral Excitation. However simple models of the human gait as an inverted pendulum with instantaneous transfer of supprt from one foot to the other have shown that the wobbles can be explained with no synchronisation between the movement of the bridge and the walking frequency of pedestrians. (see Reference Macdonald JHG 2009).
Dallard P, Fitzpatrick A J, Flint A, Le Bourva S, Low A, Ridsdill R M, Willford M.
The London Millennium Footbridge, The Structural Engineer, Vol 79, No 22, Nov 2001.
Fitzpatrick T, Linking London:The Millennium Bridge, The Royal Academy of Engineering, June 2001
Fujino Y, Pacheco B M, Nakamura S, Warnitchai P, Synchronisation of Human Walking observed during Lateral Vibration of a Congested Pedestrian Bridge, Earthquake Eng'g Struct Dynamics, 1993, Vol 22, 741, 758
Macdonald, JHG. 'Lateral excitation of bridges by balancing pedestrians', Proc. Royal Society A, (pp. -), 2009