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Hammersmith flyover: returning to full strength

Since the Hammersmith Flyover was dramatically closed two months ago, engineers have been busy working out how to enable its reopening to full traffic loading before this summer’s Olympics.

Closing a major piece of infrastructure is rare, and it is not an easy decision to make. So when Transport for London (TfL) decided to close the Hammersmith Flyover last December, it was a major shock to the travelling public, and a potential nightmare for the Olympic planners.

“It was not a decision taken lightly,” says TfL director of roads Dana Skelley. “The decision making went right up to [transport commissioner] Peter Hendy.”

Main artery

Hammersmith Flyover carries the A4 in west London, and is a main artery for traffic from Heathrow Airport and the west. This year it is also a crucial part of the Olympic Route Network. As such it is vital for transporting athletes, delegates and foreign government dignitaries from Heathrow airport to Olympic sites in central London and to the Olympic Park in east London. So, after structural defects were found in the flyover, the need to find a repair solution was more than urgent.

After investigating a range of options, engineers are installing new high-strength cables on the six most damaged spans of the flyover to bring the bridge up to full traffic load capacity before the start of the Games in July (NCE 9-16 February). “We went through a number of options but this was the most suitable one,” says Skelley.

The 622.7m long, 16-span flyover carries the A4 over several road and rail lines. The pre-stressed post-tensioned structure was built between 1959 and 1962, by constructing a series of 2.6m long precast concrete segments that were squeezed together with longitudinal post-tensioned cables.

“Placing one or two props would have been fine, but placing them under six spans would have been impractical”

Andy Milner, Amey

On 23 December 2011 serious corrosion was discovered on the steel prestressing cables running through the bridge, and TfL took the unprecedented step of closing the bridge until the strength of the flyover could be ascertained.

Detailed analysis, including endoscope and radiographic surveys of the reinforcement, together with computer modelling, gave TfL the confidence to allow one lane to reopen to light traffic in each direction in early January.
Since then, TfL, its maintenance contractor Amey and other stakeholders, have been developing a strengthening
system to allow full reopening.

The team considered several options, including installing temporary props. Later, working with specialist sub-contractor Freyssinet, they opted to design a system to install new cables on the affected spans of the bridge that would enable them to reintroduce the prestressing forces and squeeze the bridge sections together again.

“Placing one or two props would have been fine,” says Amey Consulting managing director Andy Milner. “But placing them under six spans would have been impractical.”

Permanent solution

Installing new cables meant there was no need for headroom restrictions and no need for new foundations. There was also the potential for it to be developed into a permanent solution.

“The new system has the capacity to completely replace the existing reinforcement,” says Milner.

Although Hammersmith Flyover’s corrosion has been known about for many years, recent testing in 2011 revealed it to be far worse than previously estimated.

TfL’s key concern was the possible failure mode as prestressed post-tensioned structures rarely give warnings - for example by displaying severe cracking - before they fail.

Sudden collapse

The cables are encased in grouting - similar to other structures in this area - meaning they are difficult to inspect. Furthermore, 80% of the load carried by the bridge is self-weight meaning a collapse could be sudden.

TfL took control of the structure in 2000, and in 2009 carried out a major investigation, which found that the structure was adequate to carry normal highway loading. However, Milner says: “We expected the flyover to require intervention, but we thought within 10 years.”

The longitudinal cables that run through the concrete deck segments are key to the structure’s integrity, because they act by squeezing the bridge segments together. There are four clusters of 16 cables, with each cable made up of 19, 6.75mm diameter wires.

“Every section will have to be scanned to ensure there are no clashes with existing reinforcement”

Andy Milner, Amey

When acoustic monitoring in 2011 revealed that wire breaks were occurring on a daily basis, there was grave concern that, as the cables corroded further, the likelihood of a sudden and catastrophic collapse of the flyover would increase. As a result, the deterioration model put in place in 2009 had to be adjusted downwards.

“The problem with these types of structures is that the cables may be fine in one place, but 100mm away they may have deteriorated to critical levels,” says Milner.

The new post-tensioning system will replace the prestress forces lost as a result of the corrosion. It takes advantage of the good condition of the concrete, which in part was down to the pioneering offsite construction process used to create the precast segments during construction.

The new cables will be fed through specially installed concrete blocks anchored on the deck and below it inside the void underneath and below the central reservation of the structure. Once in position, the cables will be tightened up to squeeze the segments together again, re-introducing the prestress force lost by the corroded cables.

Computer models

The team will be using computer models to ensure the right prestress force has been loaded into the structure.

“We must be careful not to overstress the concrete,” explains Milner.

To install the new cables, engineers will first hydro-demolish the existing central reservation surface down to the top of the precast concrete segments around the five most easterly piers, where corrosion in the reinforcement is at its worst. Then they will cast a 200mm thick reinforced concrete slab to provide a uniform surface for the concrete blocks.

At each concrete block location, engineers will then drill 12 or 24 vertical holes, depending on the location, through the surface. These will house vertical steel bars which will hold the blocks in position (see diagram). In elevation, the pairs of concrete blocks will be placed at each side of the piers about 2.6m, 7.2m and 13m from their centre lines, sitting on alternative precast segments adjacent to the pier (see diagram).

“The cables may be fine in one place, but 100mm away they may have deteriorated to critical levels”

Andy Milner, Amey

The 2.6m long concrete blocks on the surface and within the structure will be cast using pre-formed formwork. It will take seven days for the 60N/mm2 strength concrete to cure.

After the concrete has cured, engineers will then insert 12 or 24, 50mm thick steel bars through each of the concrete blocks, depending on the location. The bars will have bolts at each end and will connect the concrete blocks on the top and the underside of the reservation. The bolts will be tightened to anchor the blocks to the structure.

“Every section will have to be scanned to ensure there are no clashes with existing reinforcement,” explains Milner.
Amey project manager Darren Bearwish adds: “It will be a delicate process to install the blocks,” explains.

“There’s very little room in the reservation.”

High tensile cables

With blocks fully anchored, engineers can then install the high tensile cables that will add the strength back to the structure. In all 10 longitudinal cables will be fed through anchorage blocks across each of the piers - five below and five on top.

The cable will be fed through the blocks, and then mono-jacks will be used to tension them
to slowly introduce the new prestress force.

Engineers will be monitoring the structure before, during and after cable installation to ensure the remediation works has the desired effect on the structure.

Installation is due to be completed in June, ahead of the London 2012 opening ceremony on 27 July. The cost of the repair is expected to be around £10M, but that has not yet been confirmed.

A second phase is due to commence in summer 2013 to strengthen the remaining spans.

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