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Hammersmith flyover cable tensioning works complete two days early

Amey and Transport for London engineers last night finished the installation and tensioning of new post tensioning cables inside London’s Hammersmith flyover, two days ahead of schedule.

New cables have ben installed above and below the bridge deck on the five weakest spans of the 16 span structure. Amey now has one month to complete works to allow the structure to be fully reopened to all traffic. This includes waterproofing of the bridge deck, replacing the road surface and other ancilliary works.

The works, which began in January, have seen around 200m of the central reservation along the flyover removed, a new structural slab and concrete barriers installed, as well as tailored anchorages for the new cables installed within the structure. The new cables supplement the load capacity of the existing cables.

The new cables will be surrounded by wax oil to prevent deterioration and allow inspection and replacement if required in the future.

Work to strengthen the remaining 11 spans will take place next year under a new contract. This work will not require further weight restrictions to be imposed.

Readers' comments (2)

  • Barry Walton

    NCE has become very muddled about prestressing cables. They are not post tensioning. They are tensioned causing compression in the concrete.

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  • OK, let's clear this up.

    The general term 'pre-stressed concrete' refers to concrete sections which have high tensile steel strands under tension producing compression in the concrete. This counteracts the poor strength of concrete in tension.

    There are basically two ways of achieving this:-

    1. Forming a concrete section, letting it harden and then tensioning the steel by jacking the strands against anchor blocks cast into the concrete. It is usual for the strands to be bundled together in 'tendons', although the term 'cable' can be used. This process is known as 'post-tensioning' because the tension is applied after the concrete has hardened.

    2. The other way of doing it, almost exclusively used for precast beams, is to cast concrete around strands which have already been tensioned. This is done by having heavy steel plates at the ends of the beams, with holes, through which the strands are threaded and then jacked against the plate. The plates are supported by either the formwork or cantilevered off a base. Concrete is then poured into the form, around the strands and allowed to cure. Once it has gained enough strength the strands are released so they transfer their load into the beam. The strands are anchored by their bond strength with the concrete. This system is (perhaps confusingly) commonly referred to as pre-stressing, simply because the strands are tensioned before the concrete has hardened. Another, more accurate term, is 'pre-tensioning'.

    Hammersmith is a post-tensioned system, with precast concrete segments stitched together by the tendons.

    Sorry to have gone on, but I thought it may be useful to someone out there to know the difference.

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