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Heavy duty repairs planned for closed Hammersmith flyover ahead of Olympics

Transport for London is planning heavy duty repair work on the A4 Hammersmith flyover ahead of the Olympic Games, NCE has learned.

The decision to carry out the works comes as the Hammersmith flyover remains closed for at least another week as engineers assess the full extent of the corrosion damage.

TfL’s surface transport chief operating officer Garrett Emerson said that until engineers know the full extent of the corrosion on the pre-stressed steel tendons, TfL could not give a firm date on when the bridge will re-open.

“By the end of the week we hope to know a lot more about the condition of the bridge,” said Emerson.

Emerson added that in the short term the structure could potentially be re-opened to light traffic before major renovation work could take place. Although no details can be confirmed at this stage, Emerson said this could be enabled by the use of temporary props in the short term with the option of adding new post-tensioning to the bridge superstructure for the medium term.

The bridge is on the Olympic Route Network connecting key participants between Heathrow and central London and TfL is confident the bridge will be fully operational by then.

“[The post-tensioning] could be in place before the Olympics,” added Emerson. “It’s far from ideal [to be designing and constructing major works in such as short space of time] and it’s a major challenge.”

Engineers shut the bridge on 23 December after engineers discovered a serious structural defect. it has now been confirmed that this was the snapping of prestressed reinforced tendons, which hold the bridge together, around the bridge piers.

Reasons to close the flyover:

The sudden decision to shut the Hammersmith Flyover just days before Christmas is in part down to the structure’s unusual nature.

The 622.7m long precast segmental prestressed concrete structure is held together by prestressed steel tendons running along the bridge each carrying a force of between 30t and 40t.

Testing by TfL’s maintenance contractor Amey from September 2011 onwards revealed the number prestressed tendons snapping was higher than previously thought.

However, just before Christmas, engineers decided to break out two sections of the prestressed tendons close to the pier head from the surrounding grout, which revealed that they were in a far worse condition than previously thought.

“We discovered there was a steel duct full of water at the pier head and as a result some of the prestressed cables were completely severed,” explained Amey consulting managing director Andy Milner. As a result, the problems with the bridge now have to be dealt with much sooner than the model previously anticipated.

As 80% of the load carried by the bridge is self-weight, TfL ruled out restricted traffic measures and instead decided to shut the bridge until the extent of the damage could be ascertained.

The bridge relies on the prestressed tendons to squeeze the bridge together. However, until engineers have completely assessed each one, they cannot be sure of the current true strength of the bridge.

“We don’t believe the bridge will collapse,” added Emerson. “And as a result we are still allowing traffic to the run under the bridge.”

TfL had planned to extensively renovate the structure sometime in the next few years after the Olympics. Corrosion on the structure had been known by TfL since it took over the bridge from the Highways Agency in 2000.

Readers' comments (4)

  • Is this a prestressed or a post-tensioned structure?

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  • Jon - the flyover is a segmental prestressed structure using post-tensioning

  • Question - who designed and built the flyover?

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  • Barry Walton

    With reference to the technical observations above the bridge is still prestressed. Both pretensioning and post-tensioning exist under that general description.

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  • Paul - full details in the earlier article:
    http://www.nce.co.uk/serious-structural-defect-forces-closure-of-a4-hammersmith-flyover/8624312.article

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