Bridges are structures made by man, to cross obstacles and allow both people and goods to move from one place to another, shortening the distance that would have to be traveled, by surrounding the obstacle and the risks that could be taken when trying to go through it.
The obstacles for which they are built can be both natural (rivers, lakes, mountains, cliffs, or valleys), as well as man-made (roads, railways, and canals), so bridges are built to “save ” these sections.
They can be for the exclusive use of human beings, or also for various loads, allowing the passage of vehicles, such as cars, trucks and trains, to transport various goods and articles, etc.
The various bridges that exist can be classified either by the material they are made of, the type of obstacles they allow to “overcome”, the type of structure on which they are based, or the specific use they are given, among other classifications.
Types of bridges according to the material from which they are made:
Wooden bridges.- They are among the oldest known to man, perhaps they were initially simple logs that were used to cross streams or some sections of cliffs, taking from there the idea to manufacture them expressly for crossing difficult places, like rivers and so on.
Throughout history they have been used in almost the entire world, being made in various ways, either by joining ropes with planks (suspension bridges), mooring barges (pontoons), or in a more elaborate way for the passage of human beings. , animals, carts and merchandise.
At present, the use of these materials has declined due to their fragility and easy deterioration, but in many parts of the globe, bridges of this material are still preserved for daily use, as is the case of the “chapel bridge” in the city Lucerne in Switzerland or the U-bien bridge, 1.2 kilometers long in Myanmar (formerly Burma).
Stone bridges.- They are among the oldest known to man, they are structures made from arranged stones (Neolithic bridges), made to be able to cross some small ravines or cross small rivers.
Over time the structures were perfected until they began to be built of masonry, already with mortar that joined the various stones. Examples of these are the several that are preserved from the Roman and medieval European periods, as well as several relatively modern ones that are made of this type of stone material, as is the case of the Venetian Rialto Bridge or the Charles Bridge in Prague.
Rope bridges.- They are made of ropes of materials such as vines, leather, bamboo, and various similar materials. They consist of one or several ropes that are tied to the other end as a floor, generally intertwined with other ropes, branches or planks, having one or two ropes (usually two), on the sides to guide or hold on during the journey, as well such as rope fabrics on the sides for better protection. They are the antecedent of the modern suspension bridges, and have been used for centuries to cross rivers, cliffs, mountains, valleys and others, being used even today by various primitive cultures in Africa, Asia, Oceania and America, as well as in some places in Europe.
Metallic bridges.- The bridges of iron or steel structures, were emerging from the end of the 18th century, when industrialization began. They are easier to build in that the shapes of the materials can be better manipulated, allowing structures to be made much stronger than stone and wooden bridges, and lighter (compared to stone), as well as less “ coarse”.
Structures made of wrought iron and later steel emerged first, like the current ones where this material abounds. Examples made of these materials are the wrought iron Coalbrookdale Bridge (which is one of the oldest of this material), the George Bridge (US Virgin Islands), and the Lupo Bridge (in China).
Reinforced concrete bridges.- They are those that are made of concrete and steel, they are high resistance structures that can be made of great extension.
They are the bridges that currently predominate, thanks to their characteristics of resistance to wind, tremors, the weather and for being able to support greater weights. They are used for various purposes, such as joining distances separated by rivers, lakes and even the sea, for some sections of roads, as well as to join mountains and gorges. Some examples of this type are the Colorado River Bridge in the USA and the Rio-Niterói Bridge in Brazil.
Types of bridges according to the structure:
Arch.- They are made in the form of an arch which is constituted by a part that is curved upwards, whose ends are supported by pillars or supports, which cover an empty space (called light). The curved part of the bridge is subjected to pressure forces, as are the supports (pillars), which support the structure. These are often used to bridge variable distances on rivers and lakes. They have been made of materials such as wood, masonry, iron and mixed materials.
Beam.- They are made mainly of horizontal sections, supported by pillars at the ends of the beams. In these, the beams that make it up support large amounts of weight that exert pressure on the beams, so they tend to flex. Beams can be made of wood, steel or reinforced concrete, currently they also have wiring supports. They are used both for the transit of people and vehicles (cars, trucks and trains).
Pendants.- They are those whose board (where it is transited), does not have supports on pillars, but are held by means of steel cables that are tensed and joined in a structure. In ancient times they have been used to cross obstacles such as rivers and cliffs, being made at that time based on ropes and planks tied to the “floor” or bridge deck. Sometimes having ropes on the sides, to be able to hold on when transiting and sometimes “fabrics” of rope or planks on the sides, for greater protection, however these did not have a support that gave them stability, so the currents of air could easily make them wobble.
Current suspension bridges have high-strength steel cables, which link the bridge deck to posts or towers. These allow the saving of materials and work that would entail placing posts. They also allow them to be built to be used over long distances, such as rivers, lakes and to join islands, such as the bridge that joins the island of Manhattan to the mainland (Brooklyn Bridge). It should be noted that this type of structure loses stability with natural phenomena such as wind, its transit being dangerous, for example when there are hurricanes in the area.
Pontoon bridges.- They are those used exclusively to join the banks of rivers, lakes and coasts, that is, to overcome water obstacles. They are bridges that “rest” on floats. They have been known since ancient times, barges having been used as floats, and for example they have been used to transport troops, weapons and merchandise during the wars between the Greeks and the Persians. More recently, by the armies in conflict, during the Second World War, already with specialized flotation materials for its construction.
Other types of bridges:
Pedestrian bridges.- They are those designed exclusively for the passage of people excluding vehicles, they are structures that allow overcoming natural obstacles such as rivers, canyons, valleys, mountains, or artificial obstacles, such as railways, highways or streets heavily traveled by automobiles, being generally short (especially within the cities for the passage of certain busy streets and avenues), being designed to support relatively little weight (the transit of people and some merchandise).
Some of these have been adapted for the passage of people with motor disabilities allowing the use of wheelchairs and other devices, as well as for transit with bicycles.
They usually take various forms according to optimal deviations for moving from one place to another, for example in bus, train, subway and air terminals, sometimes joining adjacent buildings, providing faster access.
Drawbridges.- They are generally wooden structures reinforced with metal (iron), which served to allow access to enclosures protected by moats, being in the fortifications and castles where they were most used. They had various mechanisms for their elevation, being pulled by animal or human force.
Currently drawbridges have complicated hydraulic systems, designed to allow the passage of traffic and pedestrians through tributaries of water, rising to allow the passage of vessels of considerable size, being built of metal structures and reinforced concrete, in addition of the hydraulic structure that allows them to rise. An example of these bridges is the Tower Bridge (England), whose lower part is drawbridge, and the Albatross Bridge in Michoacán (Mexico), among many others.