Vacation Experiences

 

Emil Bizon

Two years ago my wife, Andrée, and I spent two weeks in the south of France. No, not in Provence, but in Laguedoc-Rousillon which is on the west side of the Rhone River and bordering the Mediterranean. A write-up of the tourist sites we saw would not be so interesting but, at the time, I was struck by the quality and intelligence behind various engineering works in the country so the thought was to describe some of this. The subject is large. It  would include:
 
>The urban transportation system for Paris
>The design and execution of their new highways, called Autoroutes, some with tolls and others free, and including the ring road system around Paris
>The high speed passenger train system- Train Grand Vitesse
>A note about the numerous hydro-electric installations, some with a head of a thousand metres
 
 
I think the most remarkable item we encountered was the new bridge across the Tarn River, near the town of Millau and known as Le Viaduc de Millau.  The first glimpse of this structure simply takes your breath  away because of its daring design and fragile appearance. So I will not describe this bridge because there are excellent websites for this but I will give a preamble as to its purpose and the geography of this area. I have been there several times including a bicycle camping trip with some friends in 1991 when we were twenty five miles north of the site.  
 
The central part of France has a roughly circular plateau of elevation a thousand metres or so and about two hundred km in diameter. The southern portion is mainly carbonaceous rock with many deep ravines created by the effect of slightly acidic rainwater (due to absorption of carbon dioxide) dissolving the rock. The area has the appearance of a desert because vegetation is scarce as the rain quickly penetrates the formation and exits at a lower elevation into any available channel. These terrains are called  "Karst" by geologists and are common in France and on the Dalmatian coast, for example. These arid plains are known as Causses,  from the word for chalk or lime-chaux.
 
The Tarn River rises near the eastern edge of the plateau and has developed a spectacular canyon about 500 metres deep which can be nicely viewed from a location named Point Sublime, about 30 km northeast of the bridge. Here one can see, from the edge of the canyon,  the river approaching from the east and turning south in a ninety degree bend. We took in the site and then descended into the valley and followed the river upstream for about fifty km till the canyon was reduce to only a few metres.
 
The Tarn flows generally east-west and empties into the Garonne below Toulouse. Travel to the south  then meant going through the Rhone valley to the east, which is very heavily congested, or via the Autoroute A 20 which skirts the western edge of the Massif.  These are 250 km apart and there was no direct route to the important cities of Montpellier, Béziers, Narbonne, Perpignan and, ultimately, Barcelona, just over the Pyrenees. A new Autoroute, A 75 has been built over the years but was missing the link over the Tarn.
 
The sites for the bridge are:
 
> http://www.ViaducdeMillau.com
 
> http://bridgepros.com/projects/Millau_Viaduct
 
> http://en.structurae.de?structures/stype/index/cfm
 
The last one is a massive listing of structures of all types. One should look under cable stayed bridges on page seven of the listing. The first site is in French and easy to follow. Hit "entrer" and you will get the home page and there is a listing of years of the project with a mass of photos of the activities for that year. Hit "accueil" to return to the home page. The second listing is in English. All have links to articles and other sites.  I did not see a listing for a technical article from, for example, the American Society of Civil Engineers, which would, likely, have a good technical description of the design, at least, and  maybe of the method of construction.  
 
The architect for this was the Brit Norman Foster and I could not find any criticism of this choice and no recriminations because a non-French firm was engaged for this. The success of the design appears to have been the main desire of the authorities.
 
A brief description of the orientation of the site. The river flows from the east to the west and the town is east of the bridge. There is a slope of 2.5% as you go south and the bridge is on a 20 km radius centred upstream.
 
There are two columns on the north side of the river and five on the south. The two nearest the river are 245 metres tall. All the columns are hollow concrete structures with post tensioning and slip-formed with a gradual change of cross section as the column advanced. At a point below the deck the column splits in two and these legs diverge slightly. The deck expansion is taken up by elastic deformation of these two legs; there are no sliding bearings.
 
The spine of the bridge is a square tube, they call it a caisson, which looks about five metres square with internal stiffeners and anchors for the cable stays. The plates look at least 25 mm on the sides and top and about 40 mm on the bottom.  These were shop manufactured in lengths of about fifteen to twenty metres and field welded. There are many photos of the shops and the welding process. The deck is cantilevered off this spine and has three traffic lanes and a sidewalk on each side but I do not know if a shoulder is also provided. So the intriguing question, at least for this non-structural engineer is: how is the traffic load absorbed? Most cable-stayed bridges have two rows of stays, one on each side,  so the structure is simple. Here, the load must be taken up as torsion of the central tube.
 
An interesting detail is the windscreen which was the result of wind-tunnel tests, because it does get breezy here. You can see through it but not very well so there is not too much distraction to a driver. Also, because the bridge describes an arc, the cable-stays coming off a mast are not in the same plane. The method of getting the deck in place is well described in the links. The Spanish subsidiary of the US firm Enerpac supplied the hydraulic equipment for nudging and lifting the deck and this was a sophisticated technique which seems not to have produced any tremors. The wavy appearance of the deck looks alarming but, evidently, no deflections beyond the yield point took place.
 
This magnificent piece of engineering cost 300 million Euros or about two F-22 fighters. It will be of service to the people of France for many generations, whereas the fighters will be ballasting the US desert in a dozen years or so.   
 
I could add that the fabricator for the caissons was a firm named Eiffel and the contractor and operator is Eiffage, both descendents of the famous man. About a hundred km to the north is a railway bridge, designed and built by him in the late nineteenth century, the Viaduc de Garabit, visible from the A 75. It crosses the Truyère River just upstream of a major power dam.  This is painted a reddish orange color, and is a graceful arched bridge assembled, like the tower, from a vast number of small pieces with lots of rivets. It is still in use and still pleasing to the eye! 
 
I hope this will interest a few of our classmates.