With seven dramatic towers and a slender steel deck, the landmark bridge carries the A75 motorway over the Tarn River valley, providing a direct link between Paris and the Mediterranean.

From the moment of conception to its official opening, the design and construction of the Millau Viaduct represented an unprecedented step forward in cable-stayed bridge technology; even today its achievements remain largely unsurpassed.

  • Owner
    Compagnie Eiffage du Viaduc de Millau
  • Client
    Eiffage Construction Métallique
  • Delivery date
    December 2004
  • Partners
    Main contractor: Eiffage
    Engineer: SETEC
    Construction engineer: Bureau Greisch
    Designer: Michel Virlogeux
    Architect: Lord Norman Foster

Key figures of Millau Viaduct project

Since the bridge opened to traffic in 2004, vehicles have soared across the Tarn River at 268 m above ground level on its gently-curving, orthotropic steel deck that extends nearly 2.5 km from one side of the valley to the other.

The aerodynamic, slimline superstructure is supported by a total of 154 cables from the seven steel towers that rise to a height of 87 m above the road deck. The six main spans are each 342 m long, with 204 m-long side spans at each end – all eight of which are continuous.

  • 2,460 m
    Length of the bridge (342m span)
  • 154
    Number of cables
  • 180 m
    Longest cable length
Millau Viaduct Worldwide Landmark

Worldwide landmark

While the final form of the bridge is now an enduring landmark that is recognised across the world, the skills and technology used to design and build it showcased the state-of-the-art in structural engineering at the time.

The structure was built as part of a toll road for concessionaire Compagnie Eiffage du Viaduc de Millau and had to be completed in just 39 months – an incredibly short programme for such a ground-breaking design. With such a structure, many of the key elements were on the critical path, leaving little room for error.

An innovative construction method using cantilevered spans

In order to meet this deadline, the engineering team proposed an innovative and daring construction method for the superstructure. It was assembled in two sections, one on each side of the valley, and gradually launched out over the tops of the concrete piers and a series of temporary intermediate piers to finally meet in the middle of the span over the River Tarn.

The cantilevered spans at the leading edge of each deck section were supported by half of the cable set from the steel tower tops of the river span, which were installed on each deck section before the launching operation began.

Millau viaduct - cantilever spans
Millau Viaduct Stay cables - Maintenance of stay cables

New design allowing permanent cables to be used in the launching phase

For efficiency and speed, Freyssinet proposed that the permanent cables be designed so that they could also be used in the launching procedure, rather than a set of temporary stays having to be installed and subsequently replaced.

This had important and challenging implications for the work, since the cables had to be erected with a very low-tension force for the construction phase. Standard installation uses around 30% of the final tension force; here the initial plan was to install them with just 5% tension – little more than self-weight of the cables – to accommodate the change in profile of the very flexible deck as the launching sequence started.

However it proved difficult to accurately install strands at such a low tension force; instead they were erected with a nominal tension of around 20% before being de-stressed to the required level.

Millau viaduct - anchors

Freyssinet also designed and installed special deviation devices at both the tower top and deck anchors, to support and protect the integrity of the cables during the launch phase.

Once the first deck launch had taken place the stays had to be adjusted overnight for the subsequent move, during a very short time-frame of just six hours.

On completion of the deck launch, the remaining tower tops were transported onto the deck and rotated into position before the main cable erection phase began. This involved the mobilisation of a huge crew of skilled staff into multiple teams working to successfully complete this critical-path activity in just two months.

A durable structure

Cable anchorage durability was a key concern for the concessionnaire, with dehumidification systems fitted inside both the towers and the bridge deck to eliminate corrosion. The owner’s focus on durability and desire to track tension forces in the cable stays led to the installation of an instrumentation system that could record and analyse the necessary data.

Millau-viaduct-cables

Construction of the Millau Viaduct was a really exciting challenge for all those involved. It was an opportunity for Freyssinet teams to demonstrate their ability to offer a versatile solution that was well-suited to the project constraints.

The flexibility of Freyssinet stay cables was the appropriate response to such a demanding construction method, while the concern for sustainability was also a major component of the project, to meet increased durability requirements for stay cables.

Finally, it is worth mentioning the very close and constructive collaboration between the Freyssinet and Eiffel teams during the cable installation process.

Manuel  
Chairman and CEO of Soletanche Freyssinet (Project manager on Millau Viaduct for Freyssinet, 2002-2004)

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