One of the main functions of road bridges is certainly that of allowing vehicles moving on wheels to cross waterways. Sometimes we speak of bridges that rise very high above narrow canyons, perhaps containing small streams; at other times we are dealing with bridges that connect the banks of rivers that are wide and deep, to the point of being navigable. And it is here that a problem arises: often the very presence of the bridge, with its piers and deck, can end up preventing the passage of boats, and especially that of taller or larger vessels. Hence the great benefit of movable bridges. But what are they, and how do they work?
What is a movable bridge and how does it work?
A movable bridge is defined as a bridge that moves, meaning an elevated crossing that is designed to move so as to clear the way for boats. Some constructions of the past, such as the drawbridges protecting mediaeval castles, can also be considered as movable bridges. In this case, the function was certainly not that of allowing boats to cross the waterway below, but rather that of preventing enemies from crossing from one side of the moat to the other.
The oldest movable bridge was probably built as far back as Ancient Egypt, and other examples were found in the Middle East, dating back to the 6th century B.C. In the past, drawbridges were operated by means of wheel winches, moved by humans or animals, with the possible help of counterweights; modern movable road bridges, instead, are generally operated by an electric engine.
Movable bridge types
There are many types of movable bridge, each one of which is designed differently so that the structure can open or move to allow the passage of boats. The type of bridge is chosen each time based on different criteria, from specific needs to its functionality and the shape of the site where it will be built. Here is a list of the main types of movable bridges that exist:
– Drawbridge: this movable bridge is distinguished by the fact that it is hinged at one end;
– Tilt bridge: a special drawbridge with counterweights;
– Folding bridge: a drawbridge with multiple folding sections;
– Curling bridge: a movable bridge with transverse divisions that close to form a wheel;
– Vertical-lift bridge: the bridge is lifted by counterweights mounted on side towers;
– Retractable bridge: the opening and closing mechanism is enabled by the possibility of laterally “retracting” the bridge;
– Swing bridge: the movement in this case occurs around a central fixed point.
The 5 most famous movable road bridges in the world
After considering the functioning and types of movable bridges, we can now suggest a list of the five most famous movable road bridges in the world, because of their architectural peculiarities.
1- Tower Bridge, London
One of the most famous movable bridges in the world is undoubtedly the Tower Bridge in London, inaugurated in 1894 and a symbol of Victorian engineering and the English capital ever since. In this case, it is a movable road bridge with the central part that can be raised along the two side towers. The original mechanism that allowed the movement was hydraulic, while since 1974 an electro-hydraulic drive system has been operating.
2- Slauerhoff Bridge, Leeuwarden
In the Netherlands there is a famous swing road bridge with a double carriageway, one for cars and the other for cyclists and pedestrians. The bridge makes it possible to cross the Harlingervaart canal, thanks to a truly original mechanism: it consists of a 15 metre by 15 metre, square metal plate, which is lifted by two powerful mechanical arms; unlike classical drawbridges, here the structure moves like a balance, thanks to a counterweight positioned at the side of the deck. The result is a movable road bridge that can be operated very quickly.
3- Jacques-Chaban-Delmas Bridge, Bordeaux
The third of the world’s most famous movable road bridges is this movable vertical-lift bridge crossing the Garonne River in Bordeaux. Inaugurated in 2012, it is 5,757 metres long and has 4 carriageways. It is worth noting that, at the time of its construction, it was assumed that about 60 openings per year would be possible for the passage of cruise ships; in reality, it has reached well over 100 openings per year.
4- Vizcaya Bridge, Portugalete
Here we are in the Spanish province of Vizcaya, along the Nervion river. Vizcaya Bridge, in Basque Bizkaiko Zubia, in Spanish Puente de Vizcaya and for the residents Puente Colgante, in fact is not a real movable bridge: instead, it is a transporter bridge, and more precisely the oldest transporter bridge in the world. Indeed, it was built in 1893, and since then it has only been shut down during the Spanish Civil War, only to resume service in 1942. The bridge is 164 metres long in total, while the swing section can carry up to 6 cars at a time. The crossing takes about 90 seconds, and takes place every 8 minutes, 24 hours a day, every day.
5- Trinity Bridge, Saint Petersburg
One city in the world has a great number of movable road bridges: it is Saint Petersburg, which boasts a total of 13 movable bridges. One of the most interesting ones is certainly the Trinity Bridge, inaugurated in 1903, at the time of Nicolas II, thanks to the genius of French developers. 582 metres long and decorated in the Art Nouveau style, it has swing section, which was completely rebuilt between 1965 and 1967.
Other movable bridges of international renown
Movable bridges are not only used for motor vehicle traffic. On the contrary, a large part of movable bridges built in recent years are intended only for the traffic of cyclists and pedestrians. Because of the lighter weight the structure must bear, it is not surprising to find some of the most original and extravagant constructions within this category. Take, for example, the Rolling Bridge in London, the spiral movable bridge that “curls” by folding its 8 triangular, hinged sections back on themselves. Or the spectacular movable tilt Gateshead Millennium Bridge in Newcastle, or the famous Ponte de la Mujer in Buenos Aires. The latter was designed by Santiago Calatrava, and it is 160 metres long: two sections are fixed, while one is movable, and turns on a cone-shaped white concrete pier, allowing vessels to pass through in less than two minutes.
