Bridges

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One of the things I really wanted to do in France was see the world’s tallest bridge in Millau. We got there but also saw quite a few other bridges so I will discuss them here more or less in the order from oldest to newest. I will break the post into two pieces.

Pont Julien

This was on day two of our walking trip in Provence.



Completed 3BC by the Romans. Like so much of what the Romans did it was built so that their armed forces and administrators could get around to their colonies quickly. Access to Gaul (France etc) from Rome was impeded by the Rhone river which was their major hurdle but all the smaller rivers also had to be dealt with. This bridge was still used by cars until 2003 so 2000 years of use says lots about the Romans design & construction techniques. In the background are the Le Grand Luberon hills we had to walk into this day (day 2).



This view gives the scale and the fantastic condition of the bridge after 2000 years. It really is just a small stream that it spans.



I don’t know what the holes on the inside are about. Limestone it relatively easy to cut; just look how well the Romans could shape it, but it hardens on exposure to air so it was a fantastic material for them to have back then when their tools were so limited.



Pont Julien is not very wide. One Roman carriage at a time originally. Bicycles are still allowed across.



Wikipedia http://en.wikipedia.org/wiki/Pont_Julien has very little.
This http://www.beyond.fr/sites/pontjulien.html has full details.


Pont du Gard

Antonia knew about this from her archaeology and it is close to the first night’s B&B for our road trip.



Built about 50AD so only 50 years younger than Pont Julien. It is really an aqueduct for water but has been adapted as a vehicle bridge also.



Here is an extract from Wikipedia:

Quote:

It was constructed entirely without the use of mortar. The aqueduct's stones – some of which weigh up to 6 tons – were precisely cut to fit perfectly together eliminating the need for mortar. The masonry was lifted into place by block and tackle with a massive human-powered treadmill providing the power for the winch. A complex scaffold was erected to support the aqueduct as it was being built. The face of the aqueduct still bears the mark of its construction, in the form of protruding scaffolding supports and ridges on the piers which supported the semicircular wooden frames on which the arches were constructed. It is believed to have taken about three years to build, employing between 800 and 1,000 workers.

Quote:

From the Middle Ages to the 18th century, the aqueduct was used as a conventional bridge to facilitate foot traffic across the river. The pillars of the second level were reduced in width to make more room for the traffic, but this jeopardized the stability of the structure. In 1702 the pillars were restored to their original width in order to safeguard the aqueduct. In 1743, a new bridge was built by the engineer Pitot next to the arches of the lower level, so that the road traffic could cross on a purpose-built bridge. The aqueduct was restored in the 18th century, by which time it had become a major tourist site, and was restored again in the reign of Napoleon III in the mid-19th century.

Wikipedia http://en.wikipedia.org/wiki/Pont_du_gard has plenty of info.

Pont d’Avignon

When I realised that we would be going to Avignon (I really did not know where Avignon is) I immediately wanted to see the bridge there. My step-father was English but was terribly fond of everything French. Included in that was singing the popular French song ‘Sur le pont d’Avignon’ so I had grown up being aware of this bridge but knew nothing about it at all. I had no idea it is as big as is.



This is in the fair sized city on the river Rhone. The Rhone is the huge river that runs down the eastern side of France all along the Alps that form the border with Switzerland. It separates the eastern side of France from the rest so bridges are required. The bridge had great strategic importance as the only fixed river crossing between Lyon and the Mediterranean Sea, a distance of 280km (Avignon is 80km from the sea).



The notes at the site state that the Romans built the first bridge here but it was destroyed . It took only four years to build this bridge because of what remained of that Roman bridge. Built 1171 to 1185 (according to Wikipedia which is more than the four years stated on the plaque at the bridge) but wrecked in 1660



Here you can see how long the bridge was; 900 metres. Only the first three arches remain now, the last pier is marked slightly larger in the diagram. The river has been changed since the bridge was built so the islands have been joined together; there are now two main streams to the river but only the closest one shows in my photos. Note on the photo also the city wall of Avignon.



Here a nice big barge is being pushed upstream by a tug. Most barges have their own engine but in Bangkok we saw barges just the same as these.



The bridge is not very wide. The man who looks like he is talking on a cordless phone is in fact listening to a guided tour of the bridge. We went through the building with the spire and tower on the skyline. When you pay your entrance fee they give you one of those in your language. In each room there was a discrete notice with a number, key in that number & you get the commentary for that part of the building. It really worked well. There was no entry fee for the bridge but you could hire the audio guide as that man has done. So much better than being in a group with a shouting guide.



Looking down from the public gardens at the top of the hill.

Wikipedia http://en.wikipedia.org/wiki/Pont_Saint-B%C3%A9nezet has plenty.

Bridge in Segre

On day five of our road trip.



I have no idea how old this bridge is. Segre is ancient, built where two small rivers join. Notice the flower boxes on the bridge. Flower boxes are everywhere in France, lots of lamp-posts have geraniums and petunias in them.



This is another bridge in Segre. The arches are flatter so I suspect it was built later than the previous one. In Roman times the arches were usually a semicircle (look at the first two bridges in this post) but that slowly changed to use less and less of the circle making for flatter arches.

Bridge near Cahors.



Michelin mark the scenic roads in green. There are several green roads around Cahors where you drive alongside this river – the Lot river. Nearby are some caves with paintings that are very famous which Antonia HAD to see. You can see this is a more modern design than the previous two and it is quite a bit wider. I find it particularly attractive.

Wikipedia on arch bridges http://en.wikipedia.org/wiki/Arch_bridges



I include this much later steel rail bridge because it is alongside the road of the previous bridge. This railway was built in the mid 1880s. This is a girder box bridge which we are very familiar with on our railways. Each section of a bridge like this would fail by bending between the supporting columns under the weight of the train. The bending stress is reduced by the cube (see note below *) of the distance between the upper and lower beams but it all works out elegantly here as the height to allow a locomotive through results in sufficient bending strength that very little steel is required. Just look how small the top beam is each side and how little steel is required to join the upper and lower beams. In the USA the railway bridges were quite similar but often built of wood because that was the local material.

• Cube = the number times itself three times so 4x4x4=64. So if the railway line was to run on top of a box beam the support columns would have to be very close together to prevent the bridge from bending. But if the box was made four times higher the train can run inside the box and, because the bridge is now 64 times as stiff the support columns can be 64 times as far apart. That is roughly of how the engineering works.



This is another bridge over the River Lot. Here steel is again in use, it is a suspension bridge. There is no obstruction in the river so it will not interfere with barges. The load carrying steel is above the bridge deck as against below with an arched bridge leaving more space clear underneath.



Same bridge. It was early in the morning and the mist had not cleared.



I think that would be slippery on a bike when it is wet. Notice the similar steel construction to the railway bridge but here it is much smaller (only up to handrail height) because the load is carried by the suspension cables above each side. In total even less steel is used – but a road bridge does not carry as big a load as a railway bridge in most cases.



Here is a more modern suspension bridge. In Cognac I think. Same idea as the previous one but everything is simplified – look at the cables now a single bunch instead of four in a spaced group previously. This is the trend in engineering; as more is understood about something we can reduce the amount of material used to make it by simplifying the design. I don’t mean just the cables have been simplified, all of it has been. Also the cost of labour has increased faster than the cost of materials (steel) so the structures are designed to use less labour even if it requires a bit more material. Photo just shot from the car as we drove through.



Just a shot out the window as we drove along. I suspect it is a railway bridge. Notice the forest on all the hills, must be lovely public hiking trails in there.



Another of the many bridges we saw. This was close to the previous one. Another railway bridge but much higher.

Millau



That’s the one I really wanted to see.



Zooming in a bit.



It is huge.



Notice the little blue bridge just behind. Notice too the truck in the middle of the picture.



Look how the structure has been simplified. Besides the slim columns the cables have been reduced to just one along the centerline of the roadway instead of one each side and the roadway is not made up of lots of pieces of steel joined together like the suspension bridge and railway bridge shown above. ‘Simplify and add lightness’ was the framed instruction on the wall of a famous aircraft designers drawing office (Curtiss I think but checking in Google I find Colin Chapman ). ). The design of the steel deck of the bridge is similar to current shipbuilding methods.

An extract from Wikipedia:

Quote:

The Millau Viaduct (French: le Viaduc de Millau) is a large cable-stayed road-bridge that spans the valley of the River Tarn near Millau in southern France. Designed by the structural engineer Michel Virlogeux and British architect Norman Foster, it is the tallest vehicular bridge in the world, with one mast's summit at 343 metres (1,125 ft) — slightly taller than the Eiffel Tower and only 38 m (125 ft) shorter than the Empire State Building. The viaduct is part of the A75-A71 autoroute axis from Paris to Béziers. It was formally dedicated on 14 December 2004, inaugurated the day after and opened to traffic two days later. The bridge won the 2006 IABSE Outstanding Structure Award.

Full article here: http://en.wikipedia.org/wiki/Millau_bridge






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