Millennium Bridge.

Millennium Bridge

Introduction

In June 2002 the Millennium Bridge was opened to the public. This was going to be an extraordinary day for engineers as new designs and structures had to be used to keep to the specifications of the job. Although with most bridge structures there is always a degree in movement, the Millennium Bridge had large groups crossing the bridge at one moment in time, this then caused a greater than expected sideways movement. This made people feel uncomfortable and even sick walking across the bridge. So on June 12th the bridge had to be shut for further investigations into why the sudden movements had occurred. For this assignment we have to investigate the construction of the Millennium Bridge, explain the causes of failure and the method used to stabilize the bridge.

Geography of the Area

The Millennium Bridge is situated on the Banks of the River Thames, London. Within London the Thames is one of the main tourist sites and business settlements, it is a very expensive and modern area of London. It is near the Tate Modern and the Globe Theatre which are high tourist based areas which get visited throughout the year. There are many other bridges built along the Thames but none of them specifically for foot and as this part of London is being regenerated a new bridge is needed. The areas foundations have clay found in the soil which means that when it dries out it goes very solid and brittle, but as the bridge is being built on a river bank the soil never dries out fully so is quite damp and slippy.

Reasons for the Bridge

As a new millennium is seen once in many hundreds of generations within a family,

England wanted to achieve many projects to celebrate the new millennium. The Millennium Bridge, Dome and Eye were all projects in which were happening around London. The area in which the Eye and Bridge were going to be situated were in an up and coming regenerated area of London, in which the Tate Modern and The Globe Theatre are situated. As these new tourist based businesses were being opened it was harder to get from one side of the Thames to the other by foot and transport is very expensive in the capital. The idea for a modern bridge came into speculation when in 1996 the Financial Times held an international competition in conjunction with the London Borough of Soutwark to design a footbridge crossing the Thames between Southwark and Blackfriars bridges. The bridge also was designed for creating an unrivalled view of London for pedestrians free from traffic and high above the Thames.

The need for a Specification

To be able to design and assemble such a huge project there was a need for a specification or else things will get out of control and the customer will not get what he/she wanted. Within a specification you need to take into account many aspects like:

Where the bridge is going to be situated?
The cost of labor, equipment and raw materials
Appearance
Load
Purpose of the bridge
Health and Safety
Support/ suspension
Base/ Area for support
Time period needed for the project to take place
Size of the bridge

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The need for a Specification

Where the bridge is going to be situated?
The cost of labor, equipment and raw materials
Appearance
Load
Purpose of the bridge
Health and Safety
Support/ suspension
Base/ Area for support
Time period needed for the project to take place
Size of the bridge
Materials going to be used
Strength

Also a specification is needed to show the customer what is going to happen and also question whether the customer has got his specifications right like purpose of the bridge, has ‘he’ got enough budget to pay workers and bills, has the customer got the design right? Why is the bridge needed? Type of traffic used? The volume of the traffic, physical appearance and size, whether it is going to effect the surrounding environment and the life of the bridge?

The Specification

The Millennium Bridge is a major project for London so specifications have to be correct and checked many times thoroughly or the project will get bad publicity.

Many of the reasons for the bridge appear in the specifications as well as others.

The bridge is going to be built on the banks of the river Thames connecting one side of London to the other. Also considering a link between Tate Modern and St. Paul’s Cathedral. It will be a quicker and cheaper way of traveling around London and a much more economical and environmentally friendly as there is no need for transport. The bridge has to be a long span bridge with many new engineering structures, have a visual impact which will link to the modern sense of the developing area- a piece of public architecture which represent the tradition of the many bridges that span the Thames in central London. The bridge has to create a minimal design that gives pedestrians unrivalled views of London, free from traffic and high above the Thames. The strength of the bridge has to be strong enough for pedestrians to walk over with ease but doesn’t have to be to the strength of many transport bridges.

Different types of Bridges

The shear design of the project represents today’s engineering techniques and knowledge so clear analysis was needed to which type of bridge the Millennium Bridge should be designed to look like

Beam Bridge

These bridges are the simplest of all bridge designs. It consists of a horizontal beam that is supported at both ends, either by natural land structure or by piers. Beam bridges are the most commonly used bridges.

Truss Bridge

Truss bridges are any that consist of a triangular framework. Plain beam bridges are made stronger by making them with trusses. Arch bridges also consist of triangles. The truss design distributes the stress throughout the bridge. Truss bridges can carry heavy loads and are usually cheap to build.

Cantilever Bridge

Cantilever bridges are a complex version of a beam bridge. In order to build one, two or more towers are built, and the bridge is built outwards from each tower. The arms are spaced so that a beam can be placed in between them. The cantilevered arms support the beam, and the towers absorb the towers. Cantilever bridges support themselves during construction.

Arch Bridge

Arch bridges are known for their stability and durability. In an arch, the force is sent out wards from the top to the end or bottom of the arch. Abutments or natural land keeps the bottom and sides of the arch in place. The arching shape spreads the load throughout the bridge. Arch bridges are very sturdy and efficient. An example of an arch bridge is the Roman aqueduct, which has lasted for thousands of years.

Suspension Bridge

Suspensions are made from two or more large towers with the main cables hanging in between them. The two large cables that are hung are also attached to smaller cables, which hold up the roadway. The ends of the main cables are held in place in large concrete blocks. The main cables support the weight of the bridge and transfer the load to the anchorages and the towers. Suspension bridges can be designed to span longer distances.

Is the customer getting what they want?

Already specified this project is very large and very important in London, so every precaution has to be met so no errors occur so the specifications have to be checked once more with all financial costs and time periods in which they can build the bridge, estimated. ‘Ove Arup’ check all the concepts of different designs of the bridge, they build mini models of the bridge and also show a computer analysis on how strong the bridge will be, what materials are needed etc. This is called a feasibility study.

Choice of the design

Specifications will then be looked at again to choose which design and which company ill build and manufacture the bridge. The choice of a suspension bridge came across when Ove Arup came up with idea of a a very shallow suspension bridge where the highly tensioned cables sag 2.3m over the 144m of the central span, a span to dip ratio of 63:1. which followed all specifications write in the early developments of the bridge.

Cost

Initial estimate was £16million for the whole cost of the bridge (this includes labor, equipment and raw materials).

Cost Breakdown

The Suspension Bridge (in detail)

A suspension bridge is where cables are strung across the river and the deck is suspended from these cables. Modern suspension bridges have two tall towers through which the cables are strung.

Forces

Compression; The force of compression pushes down on the suspension bride’s deck, but because it is a suspended roadway, the cables transfer the compression to the towers, which dissipate the compression directly into the earth where they are firmly entrenched.

Tension; The supporting cables, running between the two anchorages, are lucky recipients of the tension forces. The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage to anchorage. The anchorages are also under tension, but since they, like the towers, are held firmly to the earth, the tension they experience is dissipated.

Almost all suspension bridges have a supporting truss system beneath the bridge deck (a deck truss). This helps to stiffen the deck and reduce the tendency of the roadway to sway and ripple.

Scientific Information

Crystal Structures

When a substance is cooled from a liquid to a solid the atoms and molecules bond together to form crystals.
The shape of the crystals vary but any one substance with crystals produced in a particular way will have the same shape .

Cubic: salt/ diamonds (c) Hexagonal: Graphite (c)

The crystal shape determines many of the electrical and mechanical properties of solid. For example carbon can take two forms, properties are very different. Diamonds are very hard while graphite is soft, brakes easily and slides easily.

Atomic Models of Crystals

The simplest way to imagine a crystal is a cube with an atom at each corner and a spring linking each atom. We can consider the density of the structure by working out the PAVCKING DENSITY- how many atoms fit inside the single cube. For the simplest cube only ¼ of each atom is inside the cube. There are 8 atoms in each cube and so we effectively have 8x ¼ atoms packed in.

Dislocation

These are gaps where the crystal structure is not repeated:

We can stop the dislocation moving by introducing atoms of a different size. We produce and alloy.

Polymers- Plastics and Rubber

These are not crystalline. They are made up of long chains of carbon atoms-strings

When a force is applied the strings straighten.

From studying these crystal structures you can determine what type of metal you will need to use to make a strong sturdy bridge.