Millau Viaduct

The Millau Bridge and the Rion-Antirion Bridge are the most modern and most important examples for series of cable-stayed bridges. They answer the main questions for series individually: – How can the inner towers be stabilized to avoid too large beam deflections due to spanwise live loads? – How can the changes in beam lengths of these long bridges, e. g. due to temperature, be taken into account, together with longitudinal forces, e.g due to breaking, together with the forces from an earthquake?

In 2004 the large viaduct of Millau was opened for traffic, thereby closing the last gap in the second north–south highway A 75. The bridge has earned ist title „Record Bridge“ not only because it is the highest in the world, crossing the Tarn River at a height of 270 m, but also the construction period of only 38 months and the costs of about € 400 million are outstanding. For the second north–south highway the main obstacle was the wide valley of the Tarn River near the city of Millau, well-known for ist seemingly endless queues during the summer. It was decided to cross the valley by a 2 460 m long viaduct. In 1996 a series of cable-stayed bridges with two side spans of 204 m, and six main spans of 342 m was selected. For the financial success of the concession company, it was important to keep the construction period to a minimum in order to receive tolls as early as possible. For this reason, the French contractor Eiffel Construction Métallique proposed an alternate with a steel beam and steel towers above deck against the tender design of a post-tensioned concrete bridge. This alternate was selected in March 2001 and construction started in October 2001. The advantages of the steel design against the original concrete design are:

• light-weight and slenderness of the beam (36 t against 120 t)
• reduction of the depth of the beam to 4.2 m, meaning smaller wind loads
• improved safety: less work at great height due to pre-assembly on ground and incremental launching construction
• minimizing the number of stay cables and the size of foundations
• reduction in total costs – the over-riding advantage.

Within two and a half years nearly 43 t of steel were fabricated for the beam, the towers and the auxiliary piers. For the complete design phase Michel Virlogeux was the engineer- in-charge for the French Highway Authority SETRA, and during the construction phase he was an important advisor.

Design

The Millau Bridge has a total length of 2 460 m and comprises eight spans: two side spans with 204 m and six inner spans with 342 m. The cross-section consists of a steel box with orthotropic deck, two vertical inner webs and two inclined outer plates. The vertical webs are required for construction by incremental launching, and the triangular outside boxes create a streamlined cross-section which reduces the wind load on the bridge. On the outside of the beam, wind shields are installed which prevent the overturning of high-sided vehicles. The rounded edges on both outsides improve the aerodynamic behavior and the appearance. Due to the great height above the valley a central girder, with one cable plane only, was the most effective design for avoiding twin piers. The required torsional stiffness is provided by the box girder. Governing the design of the bridge was the condition that the up to 230 m high piers are stiff enough to carry unsymmetrical loads in the longitudinal direction. Also they have to be flexible enough to follow the beam’s longitudinal changes due to temperature. The solution is a strong concrete box girder for the piers with a vertical slot underneath the beam. This bisectioning of the box provides for the required flexibility. The 87 m high towers above deck form stiff A-frames in the longitudinal direction. The spread legs of the towers meet at the beam the spread upper pier halves. In this way, an overall system from pier and tower is created which converts the moment from loads acting at the tower tips into a couple of tension and compression and thus restrains the tower head. The 90 m long pier shafts are vertically post-tensioned to counteract the tensile stresses due to wind in the inner piers and due to temperature changes in the outer piers. Bearings are located between the steel girder and the concrete piers, which are stressed down against uplift. Unsymmetrical loads and extreme winds cause support reactions on each pier of up to 100 MN. A new type of spherical bearing was used. The towers above deck are made from steel to make them as light and slender as possible. The central cable plane is anchored at the tower heads between longitudinal steel plates. Due to architectural reasons the towers extend beyond the upper cables.

Construction

Piers

The piers have varying cross-sections, but dimensions were selected in such a way that they could be formed easily. Four sides have constant dimensions, and the others vary uniformly in each construction section. This permits construction with outer jumping forms and inner forms which are stepwise lifted by the tower crane. Sliding forms could have made the correct positioning of the built-in items difficult.

Beam

About 2100 stiffened panels, four per work day, were fabricated from plates and stiffeners in the shops of Eiffel Construction Métallique in Lauterbourg (Alsace). After transportation to site they were welded in the two 170 m long pre-assembly yards. The central boxes were pre-welded ahead in Fos-sur-Mer. Up to 75 welders were working in each pre-assembly yard.

Launching the beam

The detailed design and construction engineering of the steel parts – beam, towers and auxiliary piers – was provided by the consultant Greisch from Liège, Belgium. The engineer-in-charge was Jean- Marie Crémer. During launching from both abutments several special measures were introduced to control the large cantilever moments. The spans were reduced by half with temporary telescopic piers, a launching nose was used and the front tower with some of the final stay cables was launched together with the beam. These cables were not ad- justed during launching! After joint closure on 28 May 2004, the remaining towers and stay cables were installed and the auxiliary piers removed. The steel beams were launched from both ends at the same time with a closure joint above the Tarn River. In the center of the large span trussed auxiliary truss piers were installed with the exception of the center span which was bridged by cantilevering from both sides. The launching bearings on top of the piers with a distance of 20 m reduced the beam moments quite significantly in the ratio of span lengths (151 / 171)² = 0.78.

The auxiliary piers are built from prefabricated sections with 12 m heights. They are designed like a crane. The elements prefabricated on the ground from steel sections are lifted with an inside lifting device in such way that the next sections could be installed from underneath and also lifted until these telescoping auxiliary piers reached a height of up to 175 m. Their highest load came to 7 000 t roughly equal to the total weight of the Eifel Tower. The final piers were only provided with an auxiliary scaffolding for launching. A unique characteristic of this launching was that, for cost rea- sons, the pre-assembly yard was located at the level of the future road gradient, 4.8 m above the final bridge deck elevation. A shortened view during launching shows the elasticity of the steel beam by overcoming the gradient offset. Each of the two beam halves was launched over an auxiliary pier and a launching nose across the outer side spans. After that the final tower was installed at the front end as auxiliary support, but with a reduced height of 70 m instead of 87 m to minimize the transverse wind loads during launching. The highest wind speed permitted during launching was 3 km/h. Due to the extreme heights of the piers the friction forces during launching had to be equalized, so on top of each pier two active launching bearings were installed in each support axis. Horizontal hydraulic jacks acted between the beam and the piers in such a way that the pier tips remained in place during launching, centrally controlled by sensors. Finally the center span was bridged from each side with cantilevers, supported by towers. The design and construction on site was an extraordinary engineering achievement.

Towers

After the steel beam was in place, the remaining steel towers were pre-assembled behind the abutments. Each tower was then moved with crawler cranes over the bridge beam to ist final position. The total weight of such a convoy came to 8 MN, thus acting as a test loading. The towers were lifted from their horizontal position with the help of a temporary guyed tower. Finally, they were connected with the beam, and the stay cables were installed.

Completed Bridge

The Millau Bridge is an important example for a series of cablestayed bridges. A structure with impressive elegance has been built in an impressive landscape, where it gracefully crosses the valley at great height.

Excerpt from: Svensson, Holger; Cable-Stayed Bridges, Wilhelm Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH, Berlin (Germany), ISBN 343302992X; pp. 411-417. References to figures and literature were omitted.