Bridges in ports and estuaries as we know them today have not always existed. Many other techniques had to be used to move large numbers of pedestrians and motorists when the means were lacking or the environment is unfavorable to bridges.  

Ferries (BAC in french) have often been used to allow motorists and pedestrians to travel to opposite islands or shores. Today this solution is set up for long-distance crossings. We will take as an example the ferry going from Royan to Soulac which crosses the Garonne. This ferry can accommodate about 600 passengers and 150 cars and avoids more than 50 km of detour along the highway.

When a bridge of considerable height was needed, transporter bridges were used. Often used in the ’50s and ’60s the majority of transporter bridges are no longer running because of their supplementation by modern bridges. This is due to several reasons, including the fact that this type of bridge can’t move large amounts of motorists and requires a significant cost for maintenance. The transporter bridge is based on the principle of sliding : hooks and cables are attached to the deck and slide by on rails. This allows this bridge to move forward. We will use the example of the transporter bridge of Rochefort which passes over the Charente. It is the only transporter bridge still used today in France. It consists of four tall metal pylons of 66 meters, a deck 50 meters high and covers a distance of 175 meters long.

The science behind the bridges is not stagnant: in a few years new innovative technical solutions may be used to replace the bridges. In the city center of some cities and urban centers cable cars are used. This is the case in Brest, which houses the first urban cable car in France. Costing more than 20 million euros, the crossing lasts three minutes, rising at speeds of 120 km / h. It travels a distance of 460 meters and is able to carry about 1,200 passengers every hour. This is just one of dozens of proposed solutions that can be put in place in the future.

Suspension bridges have existed for almost as long as arched bridges : they date back to the Middle Ages using rope instead of steel cables. It is thanks to these that we can build suspension bridges of important length. Today the technology and materials have advanced, but the principle remains the same: it is necessary to maintain the weight of the deck with steel cables firmly secured to the banks. These cables are supported by towers on each side of a span.

This type of bridge makes it possible to connect long distances, but it has many drawbacks: Firstly, you need four linking points to be able to withstand the force exerted by the cables. Because of this, a suspension bridge can’t be built in a lot of situations. In addition, if someone needs to replace or repair one of the cables, the entire bridge has to be shut down due to safety concerns. Finally, it is necessary to study the effect of wind on the bridge during construction, because this type of bridge easily balks at strong winds.

Case Study : The Golden Gate Bridge

The “Golden Gate Bridge” is probably the most famous bridge in the worl. Easily recognizable by its “international orange” color, this bridge remains an icon not only of San Francisco but also of the United States.

Officially inaugurated on May 27, 1937, it was built in four years, between 1933 and 1937. Despite many complications during its construction, which costed $ 35 million, it managed to beat a great number of world records : the tallest towers, the longest cables and the widest foundations. In addition, the Golden Gate Bridge was also the longest suspension bridge in the world until 1964, measuring 1966 meters long and 30 meters wide. It has six traffic lanes open to car traffic and two pedestrian lanes. To withstand winds of 160 km / h, the bridge can swing two meters on each side. Finally, its two towers rise to some 230 meters above the waters of the Golden Gate Strait. Each tower has approximately 600,000 rivets. Bridge maintenance involves maintaining the iconic coat of paint to protect it from the sea salt in the air.

An arch bridge is a bridge featuring the titular arc in it’s design. At each end there are butresses in place to support the arches, which are attached to the decks. As previously mentioned, the bridge draws it’s name and unique shape from these arches. The origins of this type of construction take root in ancient times, starting with the Greeks building primitive arced bridges before being improved upon by the Romans. At the time they were built with cement or stone. Today, modern materials such as steel or concrete are used (for aesthetic reasons, cost, and utility reasons)

As the arches are blocked between the stops, they are blocked in palce and can therefore support the bridge. The center of mass on these bridges is located in the central part of the vault, called the “key of the vault. The heavier the weight of the bridge, the more force is exerted on the butresses and thus the better the bridge would stand.

A classic example of an arch bridge is the Harvour Bridge, the most famous bridge in Sidney. It is a metal arch bridge, used to cross the Sidney Bay. It was built by Georges Imbault and inaugurated in 1932. Measuring 134 meters above sea level, the bridge was the tallest monument of the city until 1967. It is the sixth longest arch bridge in the world measuring 503 meters long, and the widest bridge in the city measuring 48 meters wide. On average, the bridge carries 204 trains, 160,435 vehicles and 1,650 bicycles a day.

A beam bridge is one of the simplest types of bridges to put in place. They are often used to cover great distances, connect islands to the continent (for example the Island of Ré), or when an area requires a low hanging bridge

These bridges can be made of a multitude of materials, be it metal or concrete. In addition, the beams can be under different forms: straight or curved. Thus, this style of bridge can adapt to any type of ground. The girder bridge may have several bays: spaces between two beams that allow vessels to pass. The height of these can be constant or variable. The beams are generally parallel and secured by means of spacers. They then welcome a slab that will support the roadway. All these elements form the deck of the bridge

The beam bridge presents many advantages. Its foundations are simple to manufacture, unlike some bridges. The materials that make up these bridges are durable and require very little maintenance if the construction is well done. The bridge can be pre-fabricated so it is built faster, therefore the bridge can be even cheaper. Beam bridges remain almost invulnerable to erosion. That being said, this type of bridge is not very aesthetic.

Case Study Beam Bridge: La Seudre Bridge

The bridge of La Seudre is a work of art connecting the towns of Marennes and La Tremblade

It was commissioned in 1972, measuring 957 meters long and 10.92 meters wide. It is thus wide enough to support 2 traffic lanes. It is composed of 12 stacks 79 meters apart each, it has a height of 20 meters but at the high tide, only 15.10 meters remain for the ships to pass. There are 2 navigable passes. The builders used 9,680 m³ of concrete and 535 tons of steel. There are 24 piles based on soles and 8 on piles barrettes, it costed around          23 400 000 francs or about 3.55 million euros.

Moveable bridges are characterized by a mobile span. Depending on the bridge, the deck can be moved through different ways, the two most common being bascule bridges and vertical-lift bridges. This type of bridge is often used when there is a limited amount of space but a considerable amount of traffic from large vessels such as barges or trade ships. That being said, these types of bridges aren’t used very often due to their exorbitant cost and the need to regularly upkeep the powerful engines. Therefore these bridges require a permanent staff of qualified personnel to allow it’s regular functions to continue. To lift the deck, a complex system of pullies and tressed steel cables are used.

Case Study : The Bridge of Chaban Delmas

Unlike the Tower Bridge of London, the Chaban Delmas bridge is a vertical-lift bridge. To acheve this four 77 meter-high pylons were erected. At their feet are reinforced concrete bases of the following dimensions : 44 m long, 18 m wide and 16.5 m high for a total weight of 5750 tons. Each base supports two pylons : hollow elliptical pillars, accommodating the lifting mechanisms of the bridge (counterweight, cables, pulleys 4 m in diameter, etc.) but also elevators and fire escapes. The counterweight weighs 2400 tons in total, 100 less than the deck to be lifted. Thus, when lowered, the span exerts this residual weight on the supporting beams.

The metal deck of the bridge is composed of 5 spans: two for each side of the bridge and one central span, the mobile took place to four others which are fixed. The central span is 117 m in length. The decks are built in 5 sections: for the fixed spans, the long section corresponds to larger span, increased by about fifteen meters, this to realize the construction joint between the two bays and outside the support zone strongly sought. The central isostatic bay is made of a single tenant.