Hammersmith innovative post-tensioning system

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.

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.

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.