Hammersmith Flyover, London, UK - Post Tension Anchorage

08.09.2016

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.

Prefabricated external anchors: reduced size and weight with UHPC
Anchors poured on site for the bridge interior: efficient load bearing with minimized volumes

The mechanical properties of UHPC made it possible to design anchorage blocks that offer high performance and reduced size, while helping to reduce the effort of cable tensioning. The 192 anchor blocks were prefabricated, with weights varying between 1.2 and 2.5 metric tons (1.32-2.75 tons). Installing prefabricated elements significantly lowered the amount of work at height required over the course of the project, thereby limiting the inherent working risks and greatly enhancing worksite safety. Prefabricated elements also offer better overall quality than elements poured on site, both in terms of dimensions and specifications, in addition to anchor block positioning and aesthetics. The prefabrication process involved a fire retardant formula (M0 for 1 hour) that was used to comply with the bridge’s fire safety constraints. The prefabricated blocks were thermally treated to achieve maximum performance from the material. The blocks were then prepared for post-tensioning and joined to the bridge’s different segments.

It was also necessary to install cables on the bridge’s interior, located within compartments offering only limited access. The solution of pouring UHPC on site made it possible to minimize interior anchor volumes within the confined area, while ensuring a substantial load-bearing capacity. Once again, safety constraints called for a concrete that teams could pour rapidly and with ease.

Find out more about the other Ductal® innovative solutions for bridges

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