Corrosion damaged post-tensioning cables to be replaced at eastern end of London viaduct.
Work to permanently repair the damaged Hammersmith flyover in West London, was set to get underway this week.
Transport for London and its maintenance contractor Amey were last week in the final stages of designing the strengthening work. This involves adding steel cables to the 50-year old flyover allowing it to carry full traffic loading ahead of the Olympic Games in July.
The new steel cables will be post-tensioned to recreate internal forces lost as a result of the corrosion of the original post-tensioning cables. These were fed through the viaduct to add strength to the structure.
Replacing stressing forces
“Essentially we will be replacing the stressing forces lost by corrosion of the existing steel cables in the structure,” said Amey consulting managing director Andy Milner.
Site workers will install steel cables within the six of the flyover’s 16 spans which have the most corroded steel cables. The corroded cables will remain insitu but will be regarded as redundant.
The new cables will be installed on the eastern section of the viaduct. They will be fed through holes in pairs of newly installed concrete blocks installed above and below the deck’s central reservation. The blocks will be placedin pairs on either side of the five piers at the bridge’s eastern end.
Pairs of concrete blocks
The pairs of concrete blocks will be connected by steel bars, which anchor them in position (see diagram).
New steel cables will then be fed through holes within each of the blocks at each of the five piers. These will then then slowly post-tensioned.
The 622.7m long precast segmental structure carrying the A4 in West London was closed to traffic on 23 December. It has since opened to light vehicles, but restrictions will remain until the strengthening work is complete.
TfL confident other infrastructure will avoid emergency repairs
Transport for London (TfL) has said that it is confident that no other structures under its control are at risk of needing emergency repairs.
TfL has faced questions about whether vital pieces of infrastructure are being appropriately monitored after the shock discovery of post tension cable corrosion forced the closure of Hammersmith flyover in West London in December.
The transport body’s surface transport director Leon Daniels said at last month’s London Assembly Transport Committee meeting that its structures were in generally good condition, given that it inherited the Hammersmith flyover from the Highways Agency in 2000.
But TfL roads director Dana Skelley said no other structures were at risk and that Hammersmith Flyover was a unique case because of its mode of failure.
Skelley insisted all of TfL’s structures had been fully checked and it had a “robust regime in place” in assessing structures.
TfL looks after London’s 580km strategic route network carrying 33% of the capital’s traffic. These include major structures like the A40 Westway in west London and the Blackwall Tunnel in the east.
Skelley said since TfL took over London’s strategic road network structure in 2000 it had brought its knowledge of the condition of its structures up to a “uniform level”.
Skelley added that TfL had adopted a risk-based approach to managing its assets as opposed to using time-based inspections.
“Instead of prescribing a principal inspection every six years, and evaluation every two years to each structure we have now adopted a risk management approach,” said Skelley. She also said TfL had the budget to keep its structures in a “good state of repair”.