From the small southern
French town of Millau, the 2.5-kilometer-long deck of the worlds
largest multispan, cable-stayed bridge seems to be sliding imperceptibly
over the horizon. On piers as tall as skyscrapers, unseen hydraulic
"translateurs" are feeding the steel decks from assembly
lines on both sides of the valley toward an expected rendezvous
some 270 m over the Tarn River this June.
Even with a short 39-month contract
to design in detail and build the $410-million bridge, the
private-sector sponsor has brought completion forward a few
weeks to this December. Contractors have found their pace,
so the early opening should happen "if the weather stays
favorable," says Marc Legrand, director general of Compagnie
Eiffage du Viaduc du Millau (CEVM).
Using GPS produced formwork's 4-mm accuracy. (Photo by
With pylons reaching higher than
the spire of New Yorks Chrysler Building, the viaduct
is the last major link in the A75 highway between Bézier
and Claremont-Ferrand. Its construction will relieve seasonal
congestion around the town of Millau and was ordered by the
transportation ministry, partly because of the difficult hilly
terrain it must cross.
To avoid the road snaking down
the valley sides, the viaduct will carry the highway on eight,
generally 342-m-long spans, each supported by a single, central
plane of stays. The deck is a 27.75-m-wide aerodynamic steel
box, widened by curved nosings supporting wind barriers. It
is 4.2 m deep with two central vertical stiffening plates
set 4 m apart over the launching mechanism positions.
Seven equally spaced concrete piers
rise between 75 m and 245 m above the valley floor to support
the steel box deck. The piers split in two near their tops,
along the line of the deck, to merge with the inclined legs
of 87-m-high steelwork pylons.
The slenderness of the piers and
length of deck dictated the construction process, devised
by Paris-based contractor Eiffage S.A. The groups CEVM
division has a build-operate-transfer contract for the viaduct,
with 75 years to run. Its subsidiary, Eiffel Construction
Métallique S.A., is handling steelwork, which accounts
for just over half the total cost, says director Marc Buonomo.
Eiffage T.P., another division, is doing the rest.
Flexibility also was an issue for
designers of the structure. That is because the viaduct has
six continuous stayed deck sections and only two distant anchor
spans, says Michel Virlogeux, CEVM's adviser (8/5/96 p. 13).
His cure was to split the pier tops into two arms ending some
15 m apart. They provide a wide base to counter the pylons
rotational tendency. Their reduced flexibility allows them
to flex as the deck expands in the sun, he explains.
AND LONG Millau Bridge will allow motorists to
avoid congestion on narrow Tarn River valley roads. (Photo
by Peter Reina for ENR)
Virlogeux promoted the multispan
concept for the Millau site over a decade ago, when he ran
the government's large-bridge design organization. Even after
quitting the civil service, he stuck with the project through
its eventful evolution and retained his design influence.
Seeds of the Millau project took
root nearly 20 years ago and were nurtured by the governments
Service dEtudes Techniques des Routes et Autoroutes,
where Virlogeux headed the big bridge division until the mid-1990s.
Because of the viaducts prominence, the government ordered
a design competition, with an emphasis on architecture.
After leaving SETRA, Virlogeux
teamed with the British architectural firm Foster & Partners
to win the competition in 1996. The team also included three
Paris-based firms, now among Eiffages designers. Europe
Etude Gecti S.A. leads concrete design with Thales Group and
Société DEtudes R. Foucault et Associes.
Later, Belgiums Bureau Greisch S.A., Liège, joined
to confirm the governments calculations and design the
steelwork along with its erection method.
On winning the competition, Fosters
then project director, Tim Quick, conceded the design was
"very much an engineering generated form" and the
product of "collaboration with a set of good engineers."
The architects knew what they wanted aesthetically, but also
understood engineering demands, says Virlogeux.
Curving the long crossing for better
sight lines, specifying multifaceted pier profiles and the
sculptured wind barrier supports are among architectural features
dictated by the Foster team. Their design, with an embargo
on all but the smallest visible deviations, became the basis
of the governments subsequent procurement.
Though planned as a public-sector
project, the viaduct came to market just as the French government
was approaching build-operate-transfer procurement as an alternative
to using its traditional highway network franchises. In 2000,
the government prequalified four potential bidders for the
Along with two largely foreign
groups, including Swedens Skanska A.B. and Spains
former Dragados S.A., was a formidable multi-firm, Paris-based
consortium. It included the giant contractors Bouygues S.A.
and what now is Vinci Group. Facing that galaxy of rivals,
Eiffage stood alone.
By bidding singly, Eiffage avoided
having to make decisions by committee, inherent in joint ventures,
and eliminated potential conflicts of interest, says CEVMs
Legrand. And by taking the unusual step of financing the project
with its own balance sheet, Eiffage cut out delays and costs
of more conventionally raised bank loans secured by the projects
predicted revenue, he adds.
Eiffage is a relatively new group
formed by the merger during the last decade of some medium-size
French contractors. The group now is substantially owned by
its staff and reports annual sales of around $8.5 billion,
mainly in France. Its most recognizable subsidiary is Eiffel,
with a lineage reaching back to the Victorian builder of the
eponymous Paris tower, notes Buonomo.
Translators eliminate friction to push deck sections.
(Photo by Peter Reina for ENR)
During 2000, Eiffages bidding
team was under strict instruction to eliminate all important
uncertainties and ensure the bid price would be accurate,
says Buonomo. Trawling through every engineering detail raised
Eiffages engineering cost to nearly $4 million, he adds.
Legrand puts the total bid cost nearer to $6.5 million, including
commercial and legal issues. "If we had lost the bid,
we would have lost [it all]," he says.
In their calculations, Eiffage
engineers investigated both steel and concrete possibilities
for the bridge, finding the costs to be close, says Buonomo.
But the steel option was more slender and allowed faster construction.
A concrete deck would have been greatly heavier, over 0.5
m deeper and needing three times as many cables.
With the government contract secured
in early October 2001, Eiffage immediately began earthmoving.
Altogether, 360,000 cu m of terrace excavation was needed
to create foundation space for the piers, says Thomas Tieberghien,
Eiffage T.Ps director of works. Four excavated piles,
up to 5 m dia and reaching around 15 m down, underpin each
hollow pier. The piles are capped with rafts, which are about
5 m deep and contain around 2,000 cu m of concrete.
The piers are all identical above
a common level. Their lower plan shape resembles a diamond
with points squared off. At the base, the biggest piers
footprint is 25 m by 17 m. Viewed from the side, the piers
taper slightly and split into two prongs 90 m below the deck
level. The taper of the other elevation is more pronounced
and follows a gentle parabola, with the width reducing to
11 m at the top.
here to view chart
HIGH Temporary steel support structures reduce
distance between piers for placing deck sections with
specialty designed pushing device. (Photo by Daniel Jamme)
With the constantly variable geometry
and demands for high-quality finishes, the formwork performance
has been "fantastic," says Virlogeux. Eiffage is
casting piers in 4-m lifts with self-climbing external formwork
and using cranes to raise internal shutters. The contractor
ruled out slipforming to ensure a surface good enough for
the viaducts 120-year design life, says Tieberghien.
For the same reason, Eiffage ordered
steel rather than wood shutters from its German-based supplier
Peri GmbH, Weissenhorn. They produce a good finish and reduce
the number of disfiguring bolt holes, says Tieberghien. But,
being made of 6-mm steel, they are twice the load for the
270-m-high tower cranes. Workers climb the tall piers on rack
and pinion hoists in trips lasting up to eight minutes.
To speed up formwork assembly,
Eiffage used satellite-based global positioning, says Tiberghien,
who is delighted with the 4-mm accuracy recorded. "I
didnt believe in GPS [for this] at the beginning"
he says. "I think its the beginning of this kind
of precision. Ten years ago, it was 10 cm," he adds.
His surveyors also developed software to predict pier distortions
under the suns heat, avoiding the need for slower site
measurements in making formwork adjustments.
A snag in the seemingly smooth
concrete casting was the increase of prestressing at the pier
tops, says Tiberghien. Acting under Virlogeuxs advice,
the owner ordered more vertical stressing to bolster ties
between the deck and piers. The result was haut couture
rebar detailing, he adds. "We had to do drawings at scale
one-to-one to prepare the reinforcing."
Those pier top modifications cost
around $2 million, demanding another set of formwork to compensate
for longer casting. But at least Eiffages short command
chain made the decision a fast one, says Tiberghien.
Altogether, the top 5 m of pier
arms contain 60% more steel than the rest because of high
forces from deck translators, cantilevering from their sides.
Four of these 12-tonne devices per pier lift and advance the
deck, supporting loads of up to 2,400 tonnes each in the windiest
conditions, says Jean-Yves Belforno, designer Greischs
This translation mechanism was
devised by Eiffel and Greisch from the start because pushing
the deck from each end was ruled out. With frictional resistance,
each pier top would have deflected excessively. Instead, the
four 4-m-long pushing devices on each support eliminate friction
between the sliding deck and piers.
Each translator is mounted on four
to six vertical hydraulic cylinders on the support to balance
the deck. Within each device, a single horizontal cylinder
powers a sliding wedge that lifts the deck 2 cm off the pier
bearing, transferring its weight to low-friction skids on
the translator. Two other horizontal cylinders then act, pushing
the deck 60 cm forward, with the whole cycle repeating many
MOTION A permanent pylon with some of its 11 cables
supports cantilevered deck section. Tension varies, sometimes
dissipating as decks flex.
(Photo by Peter Reina)
During deck launching, translators
are finely synchronized to avoid unbalanced forces bending
the piers. Each device is operated by a computer connected
by wire to a central mobile controller. Glendale, Wis.-based
Enerpac Inc. supplied controls and hydraulic equipment, all
assembled into translators by Germanys Maurer Söhner
Altogether Eiffage paid about $9
million for the 64 translators now at work, says Buonomo.
They are operating at the abutments, atop each pier and on
seven temporary steelwork props at all but one of the mid-spans,
to halve the launch lengths. Like tower cranes, the 12-m-wide
props are raised in pre-assembled sections by hydraulic jacks
in the supporting frame.
The tallest temporary prop already
reaches over 170 m above the valley floor. A prop 100 m taller
would have been needed near the river, three spans from the
north abutment. But constructing it was not feasible. Instead,
Eiffel is timing the launching for the approaching deck sections
to meet near the highest point, cantilevering around half
a span from either side.
Launches lasting about three days
happen roughly every five weeks, as enough deck is built at
the abutments. Steel erectors are assembling the decks on
production lines reaching back several hundred meters. From
near Marseilles, they receive 4-m-wide by 4.2-m-deep boxes
forming the decks spine. They then weld on sections
made in Lauterbourg, near Germany, to complete the full profile.
Painters then do their work, before wind barrier arms are
attached at the head of the next launch.
When a half span of 171 m is built,
the deck is launched, curving tightly down into the first
void. To reduce the extent of temporary excavations behind
the abutments, the contractor assembled the production and
jacking equipment at the final road level, making the deck
nearly 5 m too high.
At the front of each deck, a permanent
pylon with some of its 11 cables holds up the cantilevering
end. Tension in these cables vary widely, sometimes dissipating
completely as decks flex in motion, says Virlogeux.
Projecting from the end of each
deck is a steel frame cantilever that reaches out some 50
m toward the awaiting pier. The front 36 m of the cantilever
is a light structure designed to steady the deck horizontally
as it makes contact. The rear 13 m is much more substantial
and is equipped with vertical jacks. Its job is to raise the
decks dipping end to land at the pier top.
Checking deck safety in windy conditions
occupied much of Virlogeuxs time in reviewing the contractors
launching proposals. When the deck is parked and temporarily
tied down if needed, it is almost as safe as the final structure,
For launching to proceed, Eiffel
must have wind speed forecasts of under 72 km per hour for
three successive days. If winds exceed 85 km per hour in mid-launch,
the operation must stop, Virlogeux says.
With steelwork fabrication
starting in April 2002, and site assembly following the next
September, the deck is due to complete its journey over the
valley this June. Then, the remaining 670-tonne pylons will
be assembled from box sections on site for installation. Each
pylon will be transported on its side to a lifting frame over
its allotted pier. There, it will be hooked at its center
of gravity to be raised, becoming vertical at the same time.
With the last pylon due for placing
this August, workers from Paris-based Freyssinet International
are scheduled this summer to return to Millau. In one of the
projects last operations, they will install the remaining
Spanish-made cables and stress them to bear the weight of
the viaducts first traffic, avoiding Millau en route
to Christmas by the Mediterranean Sea.
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