Viaduct in Millau

The viaduct near the town of Millau is the longest multi-span suspension bridge with the highest pillars in the world. The progressive design required solutions to many technical problems. The design and construction of the concrete pillars and steel bearing structure demonstrate the current possibilities of construction.

Description of the structure
The bridge structure is a continuous steel beam supported by abutments and seven intermediate pillars. The total length of the bridge is 2460 m. The end spans, each 204 m long, are followed by 6 central spans of constant length, 342 m each. The height of the concrete pillars from the ground to the roadway level varies from 78 to 245 m. Above the horizontal bridge structure, steel pylons rise up to 343 m above ground level at the location of the tallest pillar - the bridge has the tallest pillars in the world. In the horizontal direction, the bridge curves with a radius of 20,000 m, and it rises from north to south at a slope of 3.025%. The bridge carries highway traffic, with two traffic lanes of 3.5 m width and one lane 3 m wide in each direction. Including inspection sidewalks and the central divider, the steel structure is 32.05 m wide. Massive guardrails and a 3 m high wall protecting vehicles from lateral wind effects are positioned at the edge of the bridge. The height of the steel box section with an orthotropic deck is 4.2 m. The height of the pylons above the deck is nearly 90 m. The hangers are arranged in one plane in the area of the central divider. This structure is the longest multi-span suspension bridge in the world.
The foundation conditions are determined by the limestone character of the area. The bridge pillars were founded on 4 shaft pillars with diameters of 4.5 to 5 m and lengths of 9 to 16 m. The actual pillar has a lower part consisting of a hollow concrete column shaped approximately like a rhomboid with profiled ends in a direction perpendicular to the longitudinal axis of the bridge. At a height of 90 m below the deck, the pillars split into two also hollow shafts, each topped with two dome bearings. The division of the pillars into two shafts has several reasons:
- Doubling the support points for the deck increases its stiffness in the longitudinal direction,
- Reducing the bending stiffness of the pillars reduces the stress on the bridge from temperature,
- The doubled pillars provide a stable base and sufficient stiffness for the inverted Y-shaped steel pylons.
The pillars are highly stressed due to the height and dimensions of the bridge. Particularly adverse effects arise from wind and temperature. The design acknowledged the influence of nonlinear behavior and the effect of cracks in the concrete. Due to the exceptional height and stress, the pillars are made of high-value concrete with a strength of 60 MPa, pre-stressed longitudinally at the upper part with 8 cables in each of the doubled shafts.
The horizontal load-bearing structure of the bridge is a steel box. Its maximum height in the middle reaches 4.2 m and decreases towards the edges. In the central part under the hangers, a central box girder 4 m wide is located, formed by two internal walls, an upper and a lower slab. On both sides, the upper and lower slabs extend, both reinforced with longitudinal closed stiffeners and interconnected by truss bracing. The thickness of the used plates ranges between 12 to 16 mm in the spans. Only the central box girder has plates (25 to 80 mm) and walls (20 to 40 mm) reinforced, particularly in the area of the pylons. Steel from grade S 355 and S 460 was used.
The steel pylons have a total height of 89 m above the bridge. Of this, 38 m is the pylon straddling in the longitudinal direction on two shafts of the concrete pillar. An additional 51 m is partially used for anchoring the hangers, and the remaining 17 m serves solely as an architectural enhancement that improves the aesthetic impression of the bridge. A total of 11 pairs of hangers are anchored to the pylon, arranged in a half-harp system. The hangers consist of T15 cables of class 1860 MPa, which are galvanized, sheathed, and waxed. The outer sheath of the hangers is made of non-injected PEHD tubes that ensure an aerodynamically favorable shape, provide UV protection, and have non-continuous screw threads on their surface that limit vibrations from the effects of wind and rain. The number of cables varies from 45 in the hangers at the pylons to 91 in the hangers at the center of the span.

Static system of the bridge in the longitudinal direction
Classic suspended structures have pylons fixed into the foundations or into the horizontal structure of the bridge, but also anchored to end firm blocks behind the abutment, or to end spans sometimes stiffened with auxiliary supports. In the case of the Millau Viaduct, such a system could not be realized. When one span is loaded, the top of the pylon tilts towards the loaded span. The stiffness of the pylons and pillars contributes to the stiffness of the bridge and together forms resistance against deformation. One can choose between two extreme alternatives:
in the case of non-stiff pylons and pillars, a relatively stiff bridge structure must be designed,
in the case of stiff pylons and pillars, the stiffness and thus the thickness of the bridge structure can be limited.
Simultaneously, the impact of temperature effects manifests, causing longitudinal displacements, which reach up to 0.6 m in the bridge described. In the case of the Millau Viaduct, the problem was solved by ensuring that the bridge structure is supported on each pillar at two points approximately 16 m apart on the split pillars. This ensures the possibility of horizontal movement with reasonable resistance and at the same time the stability and stiffness of the pylons sufficient to limit excessive deformations of the bridge spans under uneven loading. The bridge structure can then be relatively light and flexible.

Bridge construction
A project of such a large scale also placed extraordinary demands on the construction of the bridge. The general contractor was Eiffage TP - a member of the Eiffage group and the supplier of the steel structure was the company Eiffel from the same group. The construction site was divided into four areas with a total area of about 8 ha, and about 3500 m2 of area was occupied by construction at each pillar. In addition to the viaduct itself, two smaller bridges were built as part of the project - over the Tarn River and over the Millau-Albi road, which served for construction traffic alongside the viaduct during the construction period. Thus, an entirely independent system of construction communications was created, separated from the regular road network. The steel elements of the bridge were manufactured elsewhere and were only brought to the construction site for assembly, thus limiting further land occupations for the construction site.

Substructure
The foundation base cast on a quartet of shaft pillars had a thickness of 3 to 5 m. The concrete pouring volume of 1100 to 2100 m3 took 30 hours for the largest base. Concrete of class B35 with a cement content of 280 kg/m³ was used.
The pillars were cast in increments of 4 m high. The external formwork moved automatically, while the internal formwork was moved using a crane. Inside the pillar, intermediate horizontal slabs were designed that would disrupt the use of automatically sliding formwork, so moving the formwork with a crane was more economical. The construction speed was around 3 days per increment in the lower parts with one shaft and approximately 3 to 4 days in the upper split parts of the pillars. The reinforcement for the concrete was prefabricated and placed in the formwork as a reinforcement basket. Concrete of class B60 with a cement content of 400 kg/m³ was produced in two concrete plants located in the construction site area.
The pre-stressing of the pillars in the upper split section is a unique feature. Particularly, the injection of vertical cables nearing 100m in length was untested until now. Tests to verify the composition of the injection mortar, the duration of the injection, the functionality of the devices, and the technological process were carried out on the pillar of the Verrieres viaduct, several kilometers north of the bridge at Millau. The supports of the bridge are only 13m wide and are complemented by side cantilevers that visually extend the bridge to the point where the surface level meets the terrain.
For the construction of the bridge, a sliding method was designed. Since overcoming a span of 342m is a significant problem even for a steel structure, it was necessary to build temporary supports in the center of most spans. A total of seven temporary supports had a steel truss structure with plan dimensions of 12 x 12m. The corner columns of the tubular structure had a diameter of 1.016m. Five temporary supports were telescopic, automatically extending up to a height of 173m at the highest temporary support. The speed of construction reached 12m per day. Two supports in the end spans were built using a crane, as their height did not exceed 30m.

Construction of the horizontal structure
The horizontal steel load-bearing structure of the bridge was slid out from both abutments. Assembly halls were built behind the abutments for assembling the bridge structure from prefabricated steel parts brought to the construction site. There, the parts were welded and connected to the already constructed structure of the bridge.
Each workstation behind the abutment had three sections 171m long. In the furthest part from the abutment, the central box girder was assembled. In the central part, the other parts of the section were assembled and connected to the central girder. In the part closest to the abutment, the entire steel section was provided with anti-corrosion coating systems, and additional facilities such as windbreak posts were installed. About 75 welders worked at each abutment. The time required to construct a 171m long section of the bridge was about four weeks after the work processes were established.

Sliding of the steel structure
The sliding of the load-bearing structure occurred in sections of 171m length, i.e., half the length of a typical bridge span. In seven of the eight spans, temporary supports were placed, while in the eighth highest span, where it was not possible to place temporary support due to extreme height and the crossing of the Tarn River, it was planned to connect both sections slid from the southern and northern abutments. This ensured that the cantilever during sliding would not exceed a length of 171 m anywhere. However, a 171 m long cantilever is still long. Therefore, to limit its deflection, permanent pylons were used. A pylon located 171 m from the front end of the sliding part of the bridge was erected before the actual sliding. In addition, the front end of the bridge was equipped with a short truss nose, allowing for a smooth transition to permanent or temporary supports. During the sliding of the structure, only the pylon at the front end was assembled. The other pylons were mounted only after the completion of the sliding and the connection of the two parts in the highest span, approximately 270 m above the Tarn River level.
During the sliding using the classical method, horizontal forces act on the pillars bending them. In the case of such tall pillars, it would be difficult to ensure their stability and stiffness. Therefore, a unique sliding system was designed that allowed the horizontal forces to be eliminated. A special hydraulic device was placed on each (both permanent and temporary) pillar, which lifted the structure, moved it forward, and lowered it again. The device then returned to its initial position, and the process was repeated. Thus, a stepwise movement was created with a step length of 600 mm. A sliding speed of about 10 m/hour (16 steps/hour) was achieved. The computer-controlled system utilizing hydraulics from Enerpac was very complex, but at the cost of eliminating horizontal forces on the structure, it certainly contributed to the economy of the project.
During the sliding in the regular spans, the steel structure was not suspended. The steel section had to be dimensioned for the span between the temporary supports, i.e., half the length of the permanent span without hangers. Such stresses are not negligible; significant deflections of the steel structure of the bridge occurred during construction.
After the sliding was completed, the assembly of the pylons and hangers was carried out. Pylon elements up to 12m long and weighing up to 75 t were transported to the construction site and assembled behind the abutments. The pylons above the permanent pillars P2 and P3, which were also used during the sliding, weigh 850 t. The other five pylons differ in construction and weigh only 650 t. After assembly, they were loaded onto multi-axle trailers and transported across the bridge to their places above the permanent pillars. There they were lifted and attached using two steel lateral towers near the center of gravity and rotated to a vertical position. After being welded to the bridge, the installation and tensioning of the hangers could commence.

Eva Kosíková, Matěj Kosík