The Millennium Bridge - Case StudyOn the 10th of June 2000 a brand new iconic bridge across the Thames river in London was opened to celebrate and mark out the millennium year. The bridge called ‘The Millennium Bridge’ and was the result of world class architects Norman Fosters and Partners, civil engineering consultancy Ove Arup and sculptor Sir Anthony Caro winning a competition run by the Financial Times and London Borough of Southwark to design a new foot bridge. The now twelve years old ‘Millennium Bridge’ serves as a functional link over the Thames as well as an iconic structure in the heart of London (Fitzpatrick, 2001).

The bridge is a shallow suspension bridge supported by two piers with two sets of four 120mm diameter locked coil cables spanning from end to end supporting a 133m long deck. Steel box sections span between the two sets of coiled cables supporting the deck structure which comprises of two steel edge tubes and extruded aluminium box sections. The pier body is a tapering ellipse cast in C60 reinforced concrete and on top of these rests a steel v bracket. The foundations for the north and south abutments consist of a series of cast in situ concrete piles made of C40 concrete and 44 x T50 steel bars. The pier foundations are designed 6m diameter caissons within a sheet pile cofferdam. The probable design life for the structure is 120 years as although there is not any specific information released it meets the 120 year category as marked by the BS 7543, The Design Manual for Roads and Bridges volume 2 BD 29/04 ‘The design manual for footbridges’ as well as the Design standards for urban infrastructure volume 7 ‘Bridges and related Structures’ (Fitzpatrick, 2001).

The structure is situated on the Thames river in the United Kingdom and thus exposed to some harsh conditions. The exposure conditions for reinforced concrete in the design would be XD3, XC2 and XF4 in accordance with table A.5 (British Standards Institute, 2006). The form of exposure this structure is subject to encompasses long-term water contact and parts permanently submerged in water, an array of chemicals including chlorides which have ability to induce corrosion on concrete and steel, de-icing agents and cyclic wet and dry conditions.

Figure 1. Satellite view of the Millennium Bridge, London Figure 2. Street map view of the Millennium bridge (UK Grid Reference Finder (2012).

The exact location of the Millennium Bridge in UK is TQ 32049 80677 (UK grid reference finder).

Structural Components

Reinforced concrete piles were used for the foundations of the north and south abutments. The foundations are subject to a 30MN combined live and dead load acting horizontally due to the bridge cables producing a large overturning moment. (Fitzpatrick, 2001) The use of reinforced concrete is a common selection for foundations as its composition of concrete and ribbed steel bars allows it to be both good in compression and tension. For this structure it is essential as it must withstand both horizontal and vertical loading. Reinforced concrete has very good durability qualities and will have been designed to the correct specification according to the ‘Eurocode 2:Design of Concrete Structures’ meaning that an appropriate cover and finish will have been selected to ensure the structure withstands the corrosive environment and meets the required design life. Failures may occur in the reinforced concrete through poor design or bad construction. The design may have underestimated the effects of sulphate attack or reactive aggregates and thus weaken the concrete and expose the steel bars or be constructed poorly so as not to meet the specification and thus negatively affect the material properties (Mulheron, 2012).

The manufacture of the piles should be done in accordance with the regulation laid out by FPS (Federation of Piling Specialists) in association with HSE (Health and Safety Executive). The documentation highlights all the possible health hazards from materials and manufacture of piles (HSE, 2010).

Concrete is highly alkaline and it can cause serious burns if it comes in contact with skin and eyes. There is also the possibility of impaling oneself on the steel rebar on site. Appropriate clothing and PPE should be worn.

An alternative material that could be used instead of reinforced concrete is steel piles either hollow or ‘H’ sections. Steel piles are robust, light to handle, capable of carrying high compressive loads and create a high frictional resistance in the ground (Tomlinson, Woodward, 2008). Although there are metallic coatings such as paints and organic polymer films, steel is still subject to chemical corrosion and direct oxidation which limits its long-term function and increases costs in maintenance (Mulheron, 2012).

The piers for the bridge are manufactured from C60 reinforced concrete and a mild steel V bracket. The piers are subject to being submerged in water and must be capable of withstanding ship impacts. They are also subject to high compressive and axial loads (Fitzpatrick, 2001). Reinforced concrete with its good compressive and tensile character as well as its mass make it a suitable material. Reinforced concrete is also durable having good resistance against chemical corrosion and if designed according to correct standards will protect the steel reinforcement. The mild steel used in the V bracket has a strong yet malleable form that allows it to be easily machined, shaped and welded. It can also be tailored for its use by using different finishes which enhance certain characteristics such as cold rolling which produces internal stresses in the material increasing its strength (Mulheron, 2012).

The health and safety issues involving the steel bracket encompass unguarded machinery, exposure to controlled and uncontrolled energy sources, skin contact with chemicals and hot metal and extreme temperatures (International Labour Organisation 2005).

A suitable alternative to steel in the bracket would be titanium. Titanium is lighter and stiffer than steel and also has a high level of corrosion resistance without requiring any expensive finishes. Titanium is however still extremely expensive and difficult to shape (Mulheron 2012).

The ‘Millennium Bridge’ deck is manufactured from two steel edged tubes which support extruded aluminium box sections. The deck is designed to withstand an imposed pedestrian load in accordance with BD 37/88 (Fitzpatrick, 2001) and wind load and the environmental exposure including rain water runoff and de-icing agents. Steel is a appropriate material for the edged tubes because of its strength, ease of manufacture and its ability to act well under both compressive and tensile forces (Mulheron 2012). It does however require finishes to protect it against oxidisation and chemical corrosion. Polymer paints can be used to protect the steel but require maintenance throughout its service.

Aluminium has a reasonable strength and stiffness so it is able to carry the imposed pedestrian load. It is highly reactive and forms a colourless oxide layer on its surface which gives it a high resistance to corrosion without damaging its aesthetic appearance. Aluminium is also quite a light material lowering the overall weight of the structure (Mulheron 2012).

Austenitic stainless steel also possesses a high corrosion resistance allowing it to withstand harsh wet conditions. This kind of stainless steel can also be hardened by cold working which can increase its load carrying capacity. In comparison stainless steel is more expensive than aluminium and does not deliver the same aesthetical appeal (Mulheron 2012).

Hazards and risks should be highlighted during design and appropriate information provided to workers. Checks on CSCS and CPCS tests of workers should be carried out and appropriate welfare facilities provided throughout the construction period. Any obvious risks should be immediately reported. The contractors responsible for the work should carry out risk assessments and abide by the ‘Health and Safety Executive’ regulations HSE (2007).

Life Cycle

The life cycle of the ‘Millennium Bridge’ was initiated in September 1996 when the Financial Times and the London Borough of Southwark organised a competition to design a new pedestrian foot bridge in London. The winning teams design was taken forward and given financial backing (Fitzpatrick, 2001).

Following this the preliminary design stage begins with research, sculpting and redesigning. Then detailed design takes over and a series of professional drawings clearly showing every aspect of the bridge are produced and sent to the client, architect and project manager. If no amendments were required the project moves to the tender stage. Balfour Beatty won the contract for the enabling works and joint venture Monberg Thorsen and Sir Robert McAlpine the main contract (Fitzpatrick, 2001).

Because of the sensitivity of the area an archaeological excavation started In late 1998 and the first pile went into the ground in April 1999. The main superstructure was underway in

the beginning of 2000 and the bridge officially opened to the public on the 10th June 2000 (Fitzpatrick, 2001).

Once opened the bridge experienced a large lateral loading and began oscillating at concerning levels. A research program was undertaken to find a solution at Imperial College London and Ove Arup over saw the design work. Cleveland Bridge UK won the contract for the amendments and the bridge was opened again at the end of 2001 (Fitzpatrick, 2001).

The ‘Millennium Bridge’ enters service for its designed life time which is likely to be 120 years . However during this period it will require maintenance checks and repair works. The bridge will need to be repainted frequently throughout its life time and the vicious dampers replaced several times (Fitzpatrick, 2001).

Finally the bridge will be taken apart much of it recycled with a small amount demolished and disposed of in landfill.

End-of-Life Options

The structure as a whole at the end of its life is likely to be decommissioned after a structural inspection finds it unsafe for service and taken down. It could be taken down and rebuilt in a different location and refitted. Another alternative could be modifying/expanding it for a continual service in London.

There are specific components of this structure which could be reused in other buildings. The cables used to support the deck could be taken apart and employed on another suspension bridge. Suitable testing and checks would need to be carried out beforehand but this is certainly a possibility. The main deck could also be dismantled and installed on a new foot bridge as could the hand rails. The foundations could be reused for a new structures in the same location.

The particular materials also have different end of life options available. The extruded aluminium deck could be recycled or reused on another footbridge depending on its condition. Aluminium scrap is in high demand with a good price for it as it melts at a much lower temperature than the virgin material so it is more efficient and cost effective to recycle (BMRA). The steel edged tubes, box sections and V bracket can be reused or recycled. The reinforced concrete piles and caissons could also be recycled. The steel must be removed from concrete which is achieved by sawing and breaking the section. This releases the steel which can then be processed as scrap metal and the concrete broken up further and recycled as an aggregate base.

A Different Design Approach

The ‘Millennium Bridge’ is a piece of architectural magnificence and engineering achievement. This being so it is an extremely complex structure which requires specialist components, materials and construction. A far more simple structure could have been designed using minimal amounts of new materials and feature a simple dismantling process so recycling the structure could be made extremely efficient.

Approaching the concept design with an aim of dematerialisation, trans-materialisation and durability would limit the amount of materials needing to be processed and ensure that the

materials used were the most sustainable without compromising on long term performance. This would have a positive impact on end of life options as there is less material to work and what there is can be done so in a sustainable manner.

References

1. Fitzpatrick, A.T. (2001) ‘Linking London: The Millennium Bridge’ Royal Academy of Engineering, pp. 4-28

2. Fitzpatrick, A.T.(2001) ‘The London Millennium Footbridge’ 79/22 pp. 17-33 The Structural Engineer [online] Available at http://www.londonmillenniumbridge.com/ (Accessed:10 December 2012)

3. BRITISH STANDARDS INSTITUTE. (2006) BS85000-1 : 2006. Required Cover related to Exposure Conditions. Milton Keynes: BSI

4. The Design Standards for Urban Infrastructure (2012) ‘Bridges and Related Structures’ Urban Services [online] Available at: http://www.tams.act.gov.au/__data/assets/pdf_file/0003/12576/ds07_bridges.pdf (Accessed 10 December 2012)

5. DMRB (2004) ‘ Design Criteria for Foot Bridges’ Available at: http://www.dft.gov.uk/ha/standards/dmrb/vol2/section2/bd2904.pdf (Accessed 10 December 2012)

6. UK grid reference finder (2012) Available at: http://gridreferencefinder.com/# (Accessed 10 December 2012)

7. ARUP. (2001). ‘General Arrangement and Damper Layout’ : Ove Arup and Partners.8. ARUP. (2001). ‘North and South Piers General Arrangement’ : Ove Arup and Partners.9. Mulheron, M. (2012) Construction Materials, ENG2100 [Lecture notes] Construction

Materials, Metals and there Alloys, Concrete Technology, Reinforced Concrete. Construction Materials. University of Surrey, Faculty of Engineering and Physical Sciences, Guildford.

10. HSE (2007) ‘Contractors Roles and Responsibilities’ Available at: http://www.hse.gov.uk/construction/cdm/contractors.htm (Accessed 10 December 2012)

11. Tomlinson. M, Woodward. J (2008) Pile Design and Construction Practice. Ebrary [online] Available at: http://www.irssg.com/civil/files/library/structure/pile.pdf (Accessed 10 December 2012)

12. International Labour Organisation (2005) ‘Code of Practice on the Safety and Health in the Iron and Steel Industry’ [online] Available at: http://www.ilo.org/wcmsp5/groups/public/@ed_protect/@protrav/@safework/documents/normativeinstrument/wcms_112443.pdf (Accessed on 10th December 2012)

13. BMRA (2012) ‘About Metal Recycling’ Available at: http://www.recyclemetals.org/about_metal_recycling (Accessed 10 December 2012)

14. The concrete Society (2012) ‘Design Working Life’ Available at: http://www.concrete.org.uk/fingertips_nuggets.asp?cmd=display&id=750 (Accessed 10 December 2012)

15. Keoleian, A, G. (2005) ‘Life Cycle Modelling of Concrete Bridge Design’ [online] Available at:

http://deepblue.lib.umich.edu/bitstream/2027.42/84889/1/KeoleianJInfraSystems2005.pdf (Accessed 10 December 2012)

16.FPS (2012) ‘Notes for Guidance on Puwer’ [online] Available at:

http://www.fps.org.uk/fps/safety/PUWER%20Guidance%20Ed2%20-%20Mar

%202010.pdf (Accessed on 10th December 2012)

By Benjamin J Chase10th December 2012