15 September 2022
Bold Bridges Series
“Impossible” Bridge Stands Tall in South of France
In this first installment of the “Bold Bridges” series, we’re looking at a bridge dubbed by naysayers as “impossible.” Yet, with the right architects and civil engineers driving the project, 4,500 vehicles per day now happily pay the toll to use the “impossible.” That’s in the winter. In the summer that figure increases to upwards of 50,000 vehicles per day. As Walt Disney famously said, “It’s kind of fun to do the impossible.” One look at the Millau Viaduct, and we couldn’t agree more.
Almost 3,000 years ago, a small village in the south of France relocated from the Granède hills to the left bank of the local river, the Tarn. Named Condatomagus (literally meaning “the market at the confluence”), the village later suffered repeated barbarian invasions and relocated to the opposite riverbank, changing its name to Millau. Perhaps the barbarians weren’t keen on crossing the water or were perplexed that the old village had seemingly disappeared, while a new one — with a different name — flourished on the right bank. Whatever the case, the subterfuge succeeded, and the little village of Millau grew to its modern proportions, with a population of around 22,000.
It’s kind of fun to do the impossible. — Walt Disney
Gridlock versus The Barbarians
By the 1980s, Millau was once again under pressure, but not from the Barbarians. This time, it was due to the massive congestion created by summertime travelers headed from France to Spain. Sick of the annual three-month gridlock nightmare, Millau decided it needed a solution to the traffic problem.
The ecological sensitivity of the river gorge valley, and the river itself, drew decades of debate and a multitude of submissions from architects and engineering firms. Seventeen years after the first sketches were drawn, a low-environmental-impact design was finally chosen.
Saying No to the Opposition
As with all great things, there was naturally some initial opposition to the viaduct concept. Numerous organizations attempted to put the kibosh on the project, with several arguments against construction. One argument was that because of the toll, drivers wouldn’t use the bridge, leaving Millau still faced with congestion issues. Clearly, thousands of drivers per day disagree.
Another argument was that the project would never break even. Toll income would never amortize the initial investment, leaving the contractor requiring subsidy. To date, this has not been the case, but more on that later.
One of the biggest problems the naysayers proffered was that the technical difficulties would be too great to overcome. The bridge would allegedly be dangerous and unsustainable, with the pylons sitting on the shale of the Tarn Valley unable to adequately support the structure. Eighteen years of steady traffic later, the evidence says otherwise.
Finally, and almost laughably, the opposition claimed that the viaduct represented a detour, reducing the number of visitors passing through Millau, and thereby slowing the local economy. (Clearly, these folks weren’t at the early meetings about the horrific congestion that prompted Millau residents to request the project in the first place.)
The solution to the congestion would come in the form of a multiple-span viaduct cable-stayed bridge, designed by French structural engineer and bridge specialist, Michel Virlogeux, and British architect and designer, Norman Foster.
Undeterred by the naysayers, the project was greenlighted, and the first stone was laid in December 2001. Three years later, the bridge was officially inaugurated and opened to traffic in December 2004, a month ahead of schedule.
The launching technique advanced the roadway at a rate of 24 inches (600 millimeters) per cycle, with each cycle taking approximately four minutes.
Vital Statistics
Built of concrete and steel, the Millau Viaduct stands impressively tall at just over 1,125 feet (343 meters), making it more than 62 feet (19 meters) taller than the Eiffel Tower.
Pylons P2 and P3 are the tallest pylons in the world at 803 ft, 8 inches (244.96 meters) and 725 ft, 3 inches (221.05 meters) respectively. The mast atop pylon P2 peaks at 1,125 feet (342.9 meters), making the Millau Viaduct the tallest bridge on the planet.
When opened in 2004, the viaduct sported the highest road bridge deck in Europe, at 890 feet (270 meters) above the Tarn River at its highest point, almost double the height of the previous record holders, the Europabrücke in Austria and the Italia Viaduct in Italy.
Each of the seven pylons is supported by a piling consisting of four shafts, measuring 49 feet (15 meters) deep and 16 feet (5 meters) in diameter. Concrete abutments provide anchorage for the road deck to the ground at either end of the bridge.
The metal road deck weighs in at 40,000 tons, and is 8,070 feet (2,460 meters) long and 105 feet (32 meters) wide. The deck has an inverse airfoil shape, which provides negative lift in strong wind conditions. Four lanes — two in either direction — allow traffic to easily cross the valley, alleviating the congestion that previously plagued Millau.
The seven masts, each measuring 285 feet (87 meters) in height, are set on the concrete pylons. Between each mast, eleven metal cables are anchored, providing support for the road deck. Depending on their length, the cable stays are made of 55 to 91 high-tensile steel cables, each formed of seven strands (one central strand surrounded by six intertwined strands). Each strand has triple protection against corrosion: galvanization, a coating of petroleum wax, and an extruded polyethylene sheath. The exterior is then coated in its entire length with a double helical weatherstrip.
Clever Construction
A large footing was poured on top of the massive pilings mentioned above. These were 10 to 16 feet (3 to 5 meters) thick, to reinforce the strength of the pilings. After work on the pilings commenced in December 2001, the pylons visibly rose above ground level in March of the following year.
At this point, the speed of construction increased rapidly, with each pylon increasing in height by 13 feet (4 meters) every three days. This was achieved largely due to sliding shuttering and a system of shoe anchorages and fixed rails in the heart of the pylons which meant that a new layer of concrete could be poured every twenty minutes.
Two years of research went into finding the ideal formula for the modified bitumen.
Linear Launch
Given the above-ground height of the pylons, there was only one feasible way to attach the road deck to the pylons. The deck itself was constructed on plateaus at either end of the viaduct and then pushed out onto the pylons using bridge-launching techniques. Each half of the assembled deck was pushed lengthwise from the plateaus to the pylons, passing from one pylon to the next. Eight temporary towers were installed to aid this process.
Hydraulic jacks pushed the road decks out over the pylons and each pylon was additionally topped with a deck-pushing mechanism. The computer-controlled mechanism (on either side of the bridge) consisted of a pair of wedges, installed under the road deck, and controlled by hydraulics. The upper and lower wedges of each pair pointed in opposite directions and were operated in a simple — yet effective — 4-step dance routine.
Step 1. The lower wedge slides under the upper wedge, raising it to the roadway above, and then slides in further, forcing the upper wedge to lift the roadway.
Step 2. Both wedges move forward together, advancing the roadway a short distance.
Step 3. The lower wedge retracts from under the upper wedge, lowering the roadway and allowing the upper wedge to drop away from the roadway overhead. The lower wedge returns to its starting position. The result is a linear distance between the two wedges equivalent to the distance forward the roadway has just moved.
Step 4. The upper wedge moves backward, placing it adjacent to the front tip of the lower wedge and ready to repeat the cycle, slowly advancing the roadway incrementally.
Multiple sensors on the pylons, road deck, masts, and cable-stays constantly monitor the viaduct’s structural health.
This launching technique advanced the roadway at a rate of 24 inches (600 millimeters) per cycle, with each cycle taking approximately four minutes.
Next came the road surface. The surface of the road deck needed to be flexible enough to withstand cracking in the event of deformation in the steel deck, but have sufficient strength, density, texture, adherence, anti-rutting properties, and so on, to withstand motorway conditions. Two years of research went into finding the ideal formula for the modified bitumen.
With the road deck in place and surfaced, the mast sections were driven onto the bridge, joined together to form one complete mast, and then raised from their horizontal position to full vertical on top of each corresponding pylon. Next, the cable stays could be installed, followed by tensioning and weight testing. Finally, the temporary pylons could be removed. The estimated lifespan of this remarkable bridge is 120 years.
Lights, Sensors, Action!
In proportion to the size of the bridge, the electrical installations seem disproportionately large, but there’s a reason for that. Multiple sensors on the pylons, road deck, masts, and cable-stays constantly monitor the viaduct’s structural health.
The builders financed the construction in return for the concession to collect the tolls for a period of 75 years.
Nineteen miles (30 km) of high-current cable, 12 miles (20 km) of fiber optics, 6.2 miles (10 km) of low-current cable, and 357 telephone sockets are just the start of this list. Add in anemometers, accelerometers, inclinometers, and fiber optic extensometers, and we’re starting to grasp the scope of the monitoring that occurs on the Millau Viaduct.
The Bottom Line
The construction of the Millau Viaduct cost around $394 million. The builders, Eiffage, financed the construction in return for the concession to collect the tolls for a period of 75 years, up until 2080. If, however, the bridge yields high enough revenues, the French government may assume control of the bridge by 2044. Toll fees are currently around €10.40 for light vehicles during the peak summer season and around €8.30 for the remainder of the year. Heavier vehicles cost more.
The bridge was officially inaugurated and opened to traffic a month ahead of schedule.
Per current estimates, in excess of 61 million vehicles have crossed the viaduct. By our calculations, this means the builders have fully recovered their costs, and then some.
Reduce Deadline Stress
If you’re an architect, engineer, or CAD designer and use MicroStation®, Revit®, or AutoCAD®, would you like to come in ahead of schedule on your own projects, much like the Millau Viaduct undertaking did? We can help you achieve that goal with software that enhances productivity and speeds up your process. MicroStation tools are here, Revit help is here, and AutoCAD assistance is here.
Alternatively, feel free to call 727-442-7774 to discuss your particular situation with a Service Consultant. Our goal is to get you home on time. Axiom: More time for me time.
