MILLAU BRIDGE

The Millau Viaduct is a cable-stayed road-bridge that

spans the valley of the river Tarn near Millau in

southern France.

In February 1995, on the basis of proposals of the architects and structural engineers, five general designs were identified.

Then, The solution of a cable-stayed bridge, presented by architects Norman Foster was declared the best.

In February 1995, on the basis of proposals of the architects and structural engineers, five general designs were identified.

Then, The solution of a cable-stayed bridge, presented by architects Norman Foster was declared the best.

Designed by the French structural engineer Michel

Virlogeux and British architect Norman Foster, it is

the tallest bridge in the world with one mast's summit at

343.0 metres above the base of the structure.

It is the 12th highest bridge deck in the world, being 270 metres

between the road deck and the ground below.

The viaduct is part of the A75-A71 autoroute axis

from Paris to Montpellier. Construction cost was approximately

€400 million. It was formally dedicated on 14 December 2004,

inaugurated on the 15th, and opened to traffic on the 16th.

Problems with traffic on the route from Paris to Spain

along the stretch passing through the valley near the

town of Millau, especially during the summer when the

roads became jammed with holiday traffic, necessitated

the building of a bridge across the valley.

Construction Records

The bridge’s construction broke several records:

•The highest pylons in the world: pylons P2 and P3, 244.96 metres and

221.05 metres in height respectively, broke the French record previously

held by the Tulle and Verrières Viaducts and the world record previously

held by the Kochertal Viaduct (Germany), which is 181 metres at its

highest;

•The highest bridge tower in the world: the mast atop pylon P2 peaks at

343 metres

Costs and Resources

The bridge's construction cost up to €394 million,with a toll plaza 6 km

north of the viaduct costing an additional €20 million.

The project required about 127,000 cubic metres of concrete, 19,000

tonnes of steel for the reinforced concrete and 5,000 tonnes of pre-stressed

steel for the cables and shrouds. The builder claims that the lifetime of the

bridge will be at least 120 years.

Heights of the piers

P1 P2 P3 P4 P5 P6 P7

94.501 m

244.96 m

221.05 m

144.21 m

136.42 m

111.94 m

77.56 m

Pylons and Abutments

Each pylon is supported by four deep

shafts, 15 m deep and 5 m in diameter.

The abutments are concrete structures

that provide anchorage for the deck to the

ground in the Causse du Larzac and

the Causse Rouge

Pylons and Abutments

Each pylon is supported by four deep

shafts, 15 m deep and 5 m in diameter.

The abutments are concrete structures

that provide anchorage for the deck to the

ground in the Causse du Larzac and

the Causse Rouge

Deck

The metallic deck, which appears very light despite its total

mass of around 36,000 tonnes, is 2,460 m long and 32 m wide. It

comprises eight spans. The six central spans measure 342 m, and

the two outer spans are 204 metres. These are composed of 173

central box beams, the spinal column of the construction, onto

which the lateral floors and the lateral box beams were welded.

The central box beams have a 4 m cross-section and a length of

15–22 m for a total weight of 90 metric tons. The deck has an

inverse Airfoil shape, providing negative lift in strong wind

conditions.

Masts

The seven masts, each 87 m high and weighing around 700 tonnes

are set on top of the pylons. Between each of them, eleven stays

(metal cables) are anchored, providing support for the road deck.

Each mast of the viaduct is equipped with a monoaxial layer of eleven

pairs of stays laid face to face. Depending on their length, the stays were

made of 55 to 91 high tensile steelcables, or strands, themselves formed

of seven strands of steel. Each strand has triple protection

against corrosion. The exterior envelope of the stays is itself coated

along its entire length with a double helical weatherstrip. The idea is to

avoid running water which, in high winds, could cause vibration in the

stays and compromise the stability of the viaduct.

Stays

Surface To allow for deformations of the metal deck under traffic,

a special surface of modified bitumen was installed by

research teams from Appia. The surface is somewhat flexible

to adapt to deformations in the steel deck without cracking,

but it must nevertheless have sufficient strength to

withstand motorway conditions. The "ideal formula" was

found only after two years of research.

Construction

Two weeks after the laying of the first stone on 14 December

2001, the workers started to dig the deep shafts. There were 4

per pylon; 15 m deep and 5 m in diameter, assuring the stability

of the pylons. At the bottom of each pylon, a tread of 3–5 m in

thickness was installed to reinforce the effect of the deep shafts.

The 2,000 m3 of concrete necessary for the treads was poured at

the same time.

Construction

Two weeks after the laying of the first stone on 14 December

2001, the workers started to dig the deep shafts. There were 4

per pylon; 15 m deep and 5 m in diameter, assuring the stability

of the pylons. At the bottom of each pylon, a tread of 3–5 m in

thickness was installed to reinforce the effect of the deep shafts.

The 2,000 m3 of concrete necessary for the treads was poured at

the same time.

In March 2002, the pylons emerged from the ground. The

speed of construction then rapidly increased. Every three days,

each pylon increased in height by 4 m (13 ft). This performance

was mainly due to sliding shuttering. Thanks to a system of

shoe anchorages and fixed rails in the heart of the pylons, a

new layer of concrete could be poured every 20 minutes

The bridge deck was constructed on land at the ends of the

viaduct and rolled lengthwise from one pylon to the next, with

eight temporary towers providing additional support. The

movement was accomplished by a computer-controlled system of

pairs of wedges under the deck; the upper and lower wedges of

each pair pointing in opposite directions. These were

hydraulically operated, and moved repeatedly in the following

sequence: The lower wedge slides under the upper wedge, raising

it to the roadway above and then forcing the upper wedge still

higher to lift the roadway.

Both wedges move forward together, advancing the roadway

a short distance. The lower wedge retracts from under the upper

wedge, lowering the roadway and allowing the upper wedge to

drop away from the roadway; the lower wedge then moves back

all the way to its starting position. There is now a linear distance

between the two wedges equal to the distance forward the

roadway has just moved. The upper wedge moves backward,

placing it further back along the roadway, adjacent to the front

tip of the lower wedge and ready to repeat the cycle and advance

the roadway by another increment. It worked at 600 mm per

cycle which was roughly four minutes long.

The mast pieces were driven over the new deck lying down

horizontally. The pieces were joined to form the one complete

mast, still lying horizontally. The mast was then tilted upwards,

as one piece, at one time in a tricky operation. In this way each

mast was erected on top of the corresponding pylon. The stays

connecting the masts and the deck were then installed, and the

bridge was tensioned overall and weight tested. After this, the

temporary pylons could be removed.