Hammersmith Flyover, London, UK - Post Tension Anchorage

The Hammersmith flyover is an essential element of the London road network and its renovation created concerns related to its structural performance deficiency, its location in the heart of a busy downtown area and its crucial role in the smooth flow of traffic – especially given the context of the 2012 London Olympics. When London’s “Hammersmith Flyover” bridge gets innovative Ductal® UHPC renovation...

 

A downtown bridge located in West London, the Hammersmith Flyover is one of the first prefabricated structures built using post-tensioned concrete and connects downtown London with Heathrow Airport and western England. Comprising 16 spans and measuring 626 m (2,052 ft) in length (863 m or 2,831 ft with its two ramps), the bridge carries the express lane of the A4 highway as it crosses over the Hammersmith roundabout. 

A bridge under constant supervision

Built in 1961, the bridge presented an innovative structure for its time. However, wear and tear has taken its toll over the years, especially as a result of water damage. Inspections by the Highways Agency in the early 2000s revealed corrosion in the post-tension cables supporting the prefabricated concrete structure. For that reason, Transport for London (TfL) decided to put in place one of the largest structural monitoring programs in Europe, installing more than 400 acoustic sensors. The monitoring quickly indicated a higher rate of wire breaks than other similar programs in the United Kingdom. A new inspection at the end of 2011 confirmed the structure’s deterioration, leading to its emergency closure to complete a first round of renovation work ahead of the London Olympics, scheduled to begin six months later. A second renovation phase was also planned for 2013. 

 

A reinforcement solution built on innovation

The final idea consisted of reinforcing the bridge with an additional post-tensioning technique which added blocks to the structure in order to anchor the new rebar. The system as a whole represents an innovation, since the post-tensioning technique is a hybrid solution that includes bracing stays, making installation faster, safer and more efficient. It was also necessary to design machines and develop alignment procedures and topographic processes in order to adapt to a structure that never fully conformed to its original design. 

The renovation also capitalized on the strengths of Ductal® UHPC, which had rarely been used with post-tensioning prior to this project. Thanks to its specific properties (low volume for high performance), Ductal® UHPC helped to solve the challenge of supporting new external cables installed on a structure that was not originally designed to support these cables. Using Ductal® as a key element in the new system also represents an innovation in post-tensioning and a first in the United Kingdom. It made it possible to create anchoring blocks that are compact, aesthetically pleasing and safer to install. Certain blocks were prefabricated at a plant, while others were poured directly on site.

Their main advantage consisted in reducing the size and weight of the anchors. And the Hammersmith Flyover renovation required two types of anchors:

  • Prefabricated anchors for the bridge exterior
  • Anchors poured on site for the bridge interior

In addition, two separate production methods were needed to meet the challenges met by using UHPC. 

 

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