The unplanned closure of Hammersmith Flyover, which carries a busy highway in west London, marked the start of a major project to bring the structure back to full strength and extend its life expectancy. Concerns over the condition of existing post-tensioning cables in the 50 year-old concrete segmental box girder bridge led to it being closed to all traffic in 2011.

  • Name of the owner
    Transport for London
  • Name of the client
  • Delivery date of the project
    October 2015
  • Partners of the project
    Main contractor: Costain
    Consulting engineers: Ramboll & Parsons Brinckerhoff
    Checking engineer: Cowi (formerly Flint & Neill)
Hammersmith flyover - London

An urban worksite

An emergency repair contract enabled the critical structure, which is on the main highway route from Heathrow Airport to central London, to be reopened in time for the 2012 Olympic Games. But a more extensive intervention was subsequently required to secure its long-term future.

The standard approach to such an operation is relatively straightforward, but the particular circumstances and environment of Hammersmith Flyover demanded a rethink by specialist subcontractor Freyssinet’s team. The viaduct carries the highway over a congested interchange where local roads, bus routes and footpaths compete for space around commercial buildings, subways and a tube station. Maintaining headroom for vehicle access below the viaduct was just as crucial as the safety of traffic on top of the flyover.

Post-tensioning system

The project included the design and installation of a series of new post-tensioning cables. Six conventional tendons each consisting of 37 strands up to 385 m long were added inside the box girder, providing around two-thirds of the capacity required. These were supplemented by shorter strands which, due to lack of space within the structure, had to be attached to the underside of the bridge.


  • 156
    External post-tensioning cables
  • 192
    Precast fibre-reinforced concrete blisters
  • 54
    Internal deviators
Precast blisters using ultra-high strength fibre-reinforced concrete

Precast blisters using ultra-high strength fibre-reinforced concrete

The use of conventional steel or concrete blisters to secure new post-tensioning cables to the outside of the box girders was deemed inappropriate; the size required would have interfered with traffic headroom; created unreasonable maintenance demands; or affected the aesthetics of the structure.

The solution was to precast the blisters using ultra-high strength fibre-reinforced concrete instead of the standard materials. This meant they could be reduced in size, making them easier to handle on site and reducing any impact on clearance below the structure.


Plug and play post-tensioning system

But even with smaller elements, installation was challenging. Blisters had to be secured along the length of the viaduct underside, and cables erected and tensioned. One of Transport for London’s key project objectives was to limit disruption to traffic, residents and local businesses. All the works had to be carried out either from below, or inside, the box girder structure, and road closures were only permitted for a few hours each night.

With that in mind Freyssinet proposed a ‘plug and play’ system that kept access requirements to the absolute minimum. Rather than the conventional grouted duct solution, individually-sheathed strands were installed one by one and stressed as they were erected. This eliminated grouting operations over live traffic, and also meant there was no pressure to complete the whole cable installation by the end of the night-time closure.

Hammersmith - Plug and play post-tensioning system
Hammersmith - Custom-designed blister handlers

Custom-designed blister handlers

Four custom-designed ‘blister handlers’ were developed for use on the site; they could be mounted on a scissor lift or telehandler and had multi-axis control for manoeuvring the blisters into position. The substantial investment in the design and procurement of these units paid off; they cut blister installation time from 15 days to just three hours.

In situ injection

Another tricky operation was the creation of the corresponding backing slabs inside the box girder, to which the blisters were secured. There was no space to manoeuvre precast units, so these slabs had to be cast in situ using ultra high-performance fibre-reinforced concrete. As this material cannot be pumped without affecting its structural properties, Freyssinet developed specialist equipment that acted like an hydraulic syringe, injecting the concrete into position up to 9 m above ground level.

Hammersmith-Internal-post-tensioning system

Freyssinet brought considerable technical expertise to the project team with tailored and innovative solutions in methodology, materials and equipment that significantly reduced the requirement for onsite work, reducing construction defect and programme risk and ensuring the successful refurbishment of this critical piece of London infrastructure.

Programme Director, Costain

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