|Designer||G. Maunsell & Partners|
|Total length||622 m|
|Longest span||42.7 m|
|Number of spans||16|
|Constructed by||Marples, Ridgway and Partners|
|Construction begin||January 1960|
|Construction end||16 November 1961|
|Construction cost||£1.2–1.3 million|
|Opened||16 November 1961|
|Daily traffic||85,549 (2011)|
The Hammersmith flyover is an elevated roadway in west London which carries the A4 arterial road over and to one side of the central Hammersmith gyratory system, and it links together the Cromwell Road extension (Talgarth Road) with the start of the Great West Road. It is one of the first examples of an elevated road using reinforced concrete.
An elevated road employing reinforced concrete balanced cantilever beam supports with a single central column. The deck spine and wings are of hollow prestressed concrete design, with each span being tensioned by longitudinal tendons (four clusters, each of sixteen 29mm steel cables). The flyover was designed by G. Maunsell & Partners, Consulting Engineers, led by Peter Wroth and is 622 metres long. When built the flyover included heating cables to "eliminate ice formation." The system was initially successful, though a £4800 bill for the 1962–63 winter "so shocked Hammersmith Borough Council that, as a protest, it cut off the electricity." London County Council paid the bill after this.
The heating system became defective at some point a "long time" before 2000.
Completed in 1961, it was one of the first examples of its design. Marples, Ridgway and Partners, a Westminster-based civil engineering contractor, built the flyover at a cost of £1.3 million. The then Conservative Transport Minister Ernest Marples had been a Marples, Ridgway shareholder. To avoid a conflict of interest Marples undertook to sell his controlling shareholder interest in the company as soon as he became Minister of Transport in October 1959, although there was a purchaser's requirement that he buy back the shares after he ceased to hold office, at the price paid, should the purchaser so require.
In December 2011 Transport for London (TfL) issued a statement that "damage to the ageing 1960's structure has been caused by water ingress, including salt water due to grit laid during the winter months, which has corroded and weakened the cables which help support the flyover." TfL stated there was "a very remote possibility that Hammersmith Flyover [would] collapse."
The Fulham & Hammersmith Chronicle claim that they were contacted on 14 December 2011 by a whistleblower who revealed that problems with the structure were far more severe than was being made public. The flyover was closed to all traffic from 23 December 2011 to 12 January 2012 after structural defects were discovered. From 13 January to 27 May the bridge was only open to a single lane of light traffic in each direction for repair work. TfL plans to strengthen six of the sixteen arches before the London 2012 Olympic Games and the remaining ten arches after the games. New post-tensioned steel cables "are being installed in concrete blocks above and below the deck’s central reservation". The repairs will extend the life of the bridge by 15 years. The cost of repairs is estimated to be around £10 million.
In June 2013 it was announced further repairs were required, a project estimated to cost £60m.
Transport for London has announced that "The 18-month project will see the flyover strengthened, the bearings underneath the structure replaced, a new carriageway drainage system [fitted] and the entire flyover waterproofed and resurfaced."  TfL has said verbally that the flyover is otherwise sound, and that its life after repair is expected to be another 60 years.
It has been proposed that the flyover be replaced with a tunnel as a longer term solution. A group of architects, including Assael Architecture ran an event as part of London Festival of Architecture in 2012 to introduce the idea and gauge public interest.  Following a positive response the West London Link was set up to explore the option more seriously.
Similar projects have been planned in other cities for example the Alaskan Way Viaduct replacement tunnel in Seattle. The Mayor of London, has suggested that the bridge may be replaced by a tunnel in the future.
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- McCarthy, Michael (Mon 9 January 2012). "Race against clock to fix vital transport artery for Olympics". The Independent.
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- Engineering Timelines: Hammersmith Flyover
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- The Surveyor and municipal and county engineer 122. "The road heating of the Hammersmith flyover has been particularly successful during this prolonged cold spell"
- Snow on the Roads 214 (1). The Economist. p. 55. "The bill, at £4800, for heating the Hammersmith flyover in the winter of 1962-63 so shocked Hammersmith Borough Council that, as a protest, it cut off the electricity. (The LCC now pays.)"
- "Freedom of Information Request: Hammersmith Flyover Heating Correspondence". WhatDoTheyKnow.com. 17 January 2012. Retrieved 25 January 2012.
- Personal Statement - The Minister of Transport (Mr. Ernest Marples), Hansard, House of Commons, 28 January 1960
- "TfL report warned of Hammersmith Flyover collapse risk". BBC. 3 December 2012. Retrieved 4 December 2012.
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- Hammersmith Flyover 'unsafe' claims TfL whistleblower
- Hammersmith Flyover Is 'Unsafe'
- "Hammersmith flyover closed due to structural defect". BBC News website. 23 December 2011. Retrieved 23 December 2011.
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- "Hammersmith Flyover fully opens after repairs". BBC News. 28 May 2012. Retrieved 28 May 2012.
- "Hammersmith flyover to reopen for cars and light traffic as vital repair works continue" (Press release). TfL. 11 January 2012. Retrieved 13 January 2012.
- Lynch, Declan (9 February 2012). "Hammersmith flyover cable replacement starts this week". New Civil Engineer. Retrieved 13 February 2012.
- Dominiczak, Peter (26 January 2012). "Hammersmith flyover? The solution could be to dig a tunnel, says Boris". London Evening Standard. Retrieved 26 January 2012.
- Lynch, Declan (23 February 2012). "Hammersmith flyover: returning to full strength". New Civil Engineer. Retrieved 25 February 2012.
- Prynn, Jonathan (13 June 2013). "Hammersmith flyover needs £60m repair job after £10m emergency work two years ago". London Evening Standard. Retrieved 13 June 2013.
- "Hammersmith flyover strengthening and refurbishment". Transport for London. 22 October 2013. Retrieved 22 October 2013.