Advantages and disadvantages of beam bridges
Bridges can be classified according to different criteria. The best variant is the typology according to shape and construction, material and weight.
Shape and construction
The external characteristic of the girder bridge is usually the visible separation of the superstructure (bridge girder) from the substructure (supports, abutments) by bearings. The bearings transfer the loads from the superstructure to the substructure and give the bridge girder the necessary positional security and freedom of movement. The cross-sectional shape in the longitudinal direction corresponds externally to a beam, mostly the beam height is constant. However, superstructures with variable superstructure heights are also possible. As a rule, the lower chord has a curvature in accordance with the moment load; it is haunched. The beam makes optimal use of the strength of the material and is used as a static system in conventional bridges with small to medium spans (approx. 80 m). The girder bridge is often found in bridges, primarily because of its comparatively simple manufacture. Beam bridges can be designed with various cross-sectional geometries in the transverse direction.
- Plate bridge
- In terms of the static system, the solid panel is a wide beam. The plate is often used for overpasses, especially in crooked structures, with a limited construction height and a maximum span of 30 m. The full plate is often provided with cantilever arms on both sides and is then similar to the single-web T-beam. The location of the drainage is particularly problematic with the solid panel.
- The T-beam combines properties of a plate with those of the beam. To achieve greater spans or to save material, one or more girders are arranged in the longitudinal direction of the bridge under a comparatively thin plate.
- Box girder
- If the T-beam is supplemented with a lower plate, the result is a closed cross-section, the hollow box. Box-girder cross-sections are used in particular for girder bridges with medium and large spans or with curved lines. These are characterized by a high level of flexural and torsional rigidity, which enables great slenderness and efficient construction methods, such as the incremental launching method. Carriageway slabs that protrude far are often supported on the box girder via inclined struts.
Frame bridges correspond to girder bridges with the difference that the superstructure is rigidly connected to the substructures (abutment walls and / or supports). As a result, the bending moments in the field of the bridge girder are reduced, and thus its structural height can be reduced, or a greater span can be achieved for a given structural height. This is often used for motorway flyovers in order to be able to do without central pillars. The elimination of bearings reduces maintenance costs and simplifies maintenance of the bridge. However, replacing the superstructure, for example after collision damage, is more time-consuming.
Bridges that have no joints or bearings at all, d. H. are clamped into the abutment walls and any supports, are also referred to as integral bridges.
Trusses are dissolved supporting structures. These have the advantage that they consume less material than comparable full-walled structures such as beams, and they have a correspondingly lower weight. The bars of the framework are mainly subjected to tension and pressure. The usually greater height of the construction is a disadvantage. Truss bridges are made primarily with steel, but also with wood. Due to the high traffic loads, they are often built for railroad overpasses, but are also used for road bridges with larger spans, especially in the USA. Trusses are usually also hidden under the cladding of covered wooden bridges.
Lattice bridges or lattice bridges are truss bridges with very close-meshed bars. The technology was used around 1860 because at that time there were not enough large sheets available for solid wall girders on the European mainland. 
Pendulum pier bridge
With pendulum pier bridges, the piers are articulated to the superstructure and the foundation. As a result, the pillars are only subjected to compressive forces, while the longitudinal forces, for example from braking or starting vehicles, are completely transferred from the superstructure to the abutments.
With its high compressive strength, the arch is the most suitable type of supporting structure for solid building materials such as stone or concrete, since the arch is almost exclusively exposed to pressure loads with an optimal geometry. That is why this type of construction can be found on many old bridges. However, the subsoil must be sufficiently solid to be able to absorb the vertical thrust of the arch. Today arched bridges made of steel or reinforced concrete with elevated, overhead roadways are built to overcome deep valleys or cuts in the terrain. With a steel arch spans of up to 500 meters are possible, with a reinforced concrete arch 300 meters are possible. Steel arch bridges with attached, underlying carriageway occur due to the low construction height of the carriageway table, especially in flatlands when crossing bodies of water. Arch bridges with a central carriageway, such as the Karmsundbrua, are another possible way of overcoming obstacles.
An arch bridge consists of one or more arches and the bridge panel or roadway. The steel arch bridge also has hangers or posts to which the bridge panel is attached.
The suspension bridge is a further development of the rope bridge. Simple suspension bridges with the roadway still sagging can already be found in Stone Age cultures. Today the suspension bridge is mainly built to cross wider navigable waters with spans above 800 m. As a rule, it is not used as a railway bridge because of the tendency towards larger deformations. It is statically similar to the arch bridge with the roadway below. On the suspension bridge, a suspension cable is suspended between pylons. Hangers, vertical ropes that carry the roadway, are attached to this support rope. However, they are very susceptible to wind vibrations with large spans, as shown by the collapse of the Tacoma Narrows Bridge in the USA on November 7, 1940.
A famous example of a suspension bridge is the Golden Gate Bridge in San Francisco, the Akashi Kaikyō Bridge in Japan has had the largest span since 1998 with 1991 meters.
The Cable-stayed bridge or Stay cable bridge has proven to be technically particularly suitable and also economical for bridging wider bodies of water or areas with spans between 200 m and 1,000 m. The bridge is usually built using a cantilever. The state of construction with the widely cantilevered bridge is decisive for the technically possible span widths due to the lateral wind load. Due to its high rigidity, it can also be used for rail transport. A cable-stayed bridge consists of the pylons, the roadway and the ropes. All vertical forces of the bridge are introduced into the pylon via the ropes, which then apply them vertically as pure compressive forces into the subsoil. The cable-stayed bridge corresponds to a cantilever bridge, the deck forms the pressure-loaded lower chord, the cables are boom tension belts which transfer the vertical loads to the pylons and are anchored back in the deck.
A well-known example of this type of bridge is the Hamburg Köhlbrand Bridge. The Rio-Andirrio Bridge, which leads over the Gulf of Corinth (Greece), is another well-known representative of this bridge shape and one of the longest of its kind. The Viaduc de Millau has been the longest cable-stayed bridge in the world since 2004 at 2,460 m.
An extradosed bridge is a new type of bridge construction (a bridge with external prestressing) with stay cables, which is a mixture of a cable stayed bridge and a prestressed girder bridge.
Tension band bridge
The load-bearing element of a tensioning strap bridge are one or more tensioning straps that carry the track and are connected to the end supports in a tensile manner. The concave sag of the tensioning strap in the fields is characteristic. The smaller the sag, the greater the tensile forces in the tensioning strap. Because of the sag, this type of bridge is mainly built as a pedestrian bridge. The tension band can also be guided over intermediate pillars. A well-known representative of this type of bridge is the wooden bridge at Essing over the Main-Danube Canal, which, in addition to the unusual use of glued laminated timber as a tensioning strap, was also the longest wooden bridge in Europe up to 2006 at 193 m. This status was transferred to the 225 m long Dragon Tail Bridge on the grounds of the Neue Landschaft Ronneburg. In the case of road bridges, the roadway is usually elevated above the tensioning strap, so that, despite the sagging of the tensioning strap, a roadway is created that corresponds to the alignment of the road.
Movable bridges are built when the local conditions mean that a fixed bridge is not economically or structurally possible. This can be the case if, for example, a ramp would be too expensive in the flat country and, without a ramp, the passage height for the traffic line below would remain too low. This type of bridge has the disadvantage that the crossing of traffic cannot take place independently of one another, but one of the traffic routes is always blocked.
These bridges are described in more detail by the type of construction. So there are drawbridge or drawbridge, in which the roadway is folded up, the Bascule bridgewhose mechanism has no pull cords (the most famous example is Tower Bridge in London), one Draw-in bridge like near Gouda in the Netherlands and, as a special variant, a three-field bascule bridge, like the Hörnbrücke in Kiel, a folding bridge. Other movable bridge types that cross shipping lanes and enable larger passage widths are the Swing bridge, which can be rotated completely around its vertical axis (the Malchow swing bridge), the Lift bridge, which is raised completely (the Kattwyk Bridge over Hamburg's Süderelbe) or in Haarlem, the Netherlands, as well as the Kippbrücke, the Senkbrücke and the Rolling Bridge.
Floating bridge (pontoon bridge, ship bridge)Instead of pillars, floating bridges have floating bodies lying on a body of water, which connect short superstructures with one another. Pontoons, ships, rubber boats, hollow plates or amphibious vehicles are used as floating bodies. The usability of swimming bridges is particularly influenced by the water level and the water flow. Depending on the water level, when driving on the bridge between the bank and the first floating body, a considerable upward or downward gradient has to be overcome, which makes it difficult or impossible to use, especially in the case of railway bridges. Often the bridge is guyed on the bank because otherwise it has only a low transverse rigidity.
Today floating bridges are mostly used as temporary bridges to replace destroyed infrastructure until they are restored. A typical application is in the military sector, where on the one hand the aim is to temporarily restore destroyed infrastructure, but on the other hand also to gain operational advantages through flexibility. In the past, floating bridges were also used as an inexpensive alternative to fixed bridges.
In Norway, floating bridges are still built today as permanent structures. One example is the 845 m long Bergsöysund Bridge near Kristiansund, which has an arched floor plan without anchoring. The 1246 m long pontoon bridge section of the 1614 m long Nordhordlandsbrua near Bergen is of the same type of construction. The 2019 m long, on the other hand, is straight in plan and held in transverse direction with anchors Lacey V. Murrow Memorial Bridge across Lake Washington near Seattle.
Wood is the oldest bridge building material in the form of a tree trunk over a ravine or a body of water. It is used for yoke bridges and truss bridges. In the 18th century, the construction of wooden bridges reached its first high point with the Schaffhausen Rhine Bridge by Hans Ulrich Grubenmann. This was 120 m long and had only one intermediate pillar. The further development took place in the first half of the 19th century in North America with the construction of the railway lines through the continent, among other things with the wooden trestle bridges, consisting of simple girder bridges with a fine-meshed arrangement of round timber.
Today, wood is used in particular for pedestrian bridges, footbridges or other subordinate bridges such as goods road bridges or house driveways. The low weight of the wood is particularly advantageous. Wood is rarely used today for larger bridges, such as near Mäntyharju in Finland. The longest wooden bridge in the world approved for road traffic at 168 m (maximum span 42 m) was opened there in 1999. The longest covered wooden bridge in Europe is the Bad Säckingen wooden bridge in southern Germany. The Chapel Bridge in Lucerne is one of the most famous. The longest covered wooden bridge in the world is the Hartland Bridge in New Brunswick, Canada, at 390 meters.
The rope bridge is one of the oldest bridge types, although there are several variants.
- The 1-rope bridge is the simplest variant and consists of an inclined tensioned rope that can be used hanging on a pulley.
- The 2-rope bridge consists of a support rope (below) and a tether (above). This is a very shaky affair because the carrying and holding ropes can move horizontally with respect to one another.
- The 3-rope bridge is an improvement of the 2-rope bridge with an additional holding rope and connections between the holding ropes and the supporting rope. This results in greater stability and makes using the bridge safer.
- Compared to the 3-rope bridge, the 4-rope bridge is supplemented by a further supporting rope. A covering (mostly made of wood) is attached between the two suspension ropes. This increases the convenience of use.
Pure rope bridges can still be found in Africa, Asia, and South America and Micronesia. The rope is often made of natural fibers, sometimes steel.
Stone bridges are bridges in which not only the bridge piers, as is the case with iron or wooden bridges, but also the superstructure are made of stone. Stone was used early on as a bridge building material, initially with unworked and later with processed stones. The first stone bridges were built with cantilever arches and then as real arched bridges as technology progressed. If the distance to be bridged is short, stone bridges can be built with a single arch stretched from bank to bank, and for longer distances with several arches vaulted between stone pillars on which the bridge board lies. Differences also arise here from the shape of the arc, which can be a semicircle, a flat arc (stitch arc / segment arc), a pressed or a raised arc. The semicircular arch bridges of the Romans had spans of up to about 28 meters (Bridge of Alcántara). A maximum clearance of around 45 meters is possible with this geometric shape with stone bridges, which was achieved with the Pont du Diable bridge in France. With the segment arch used from the Middle Ages, which is statically more favorable but more difficult to control due to the higher sideshift, spans of up to 72 meters could then be achieved with stone bridges. The Trezzo Bridge, with a segment arch of 72 meters clear width, was the largest stone arch bridge in the world for 39 years until it was destroyed. Today stone is only of secondary importance in bridge building, mostly in the form of cladding.
Concrete is an artificial stone made from a mixture of cement, aggregate (sand and gravel) and water that solidifies as a result of a chemical reaction. Concrete can also contain concrete additives and admixtures. This building material is ideal for building bridges because it can be poured liquid into any form (formwork) and, after hardening, results in an artificial stone that can withstand pressure. Concrete (like stone) is only able to absorb high compressive forces and low tensile forces, which is why it is mainly used for arched bridges.
Reinforced concrete bridge or prestressed concrete bridge
Reinforced concrete combines the advantages of concrete and steel. The concrete surrounds the steel and protects it from corrosion. The steel brings its tensile strength into this connection, which is only possible because both materials have a very similar coefficient of thermal expansion.
There are several types of reinforced concrete
- Reinforced concrete: The reinforcement is built into a formwork and then poured with concrete. This is also known as slack-reinforced.
- Prestressed concrete: The use of prestressed concrete bridges is the norm for spans of more than 10 m to 25 m. Part of the reinforcement, the prestressing steel, is preloaded (tensioned). This results in compressive stress in the concrete, which prevents the concrete from cracking and thus preventing greater deflections. This enables lower heights of the bridge girders. With regard to the position of the prestressing steel in the bridge cross-section, a distinction is made between internal preload and external preload. With internal pre-tensioning, the tensioning cables are arranged in the concrete cross-section and completely encased by the concrete. With external prestressing, the tendons are outside the concrete cross-section and can be exchanged. The clamping forces are only introduced into the concrete cross-section at deflection saddles or consoles. The external prestressing is mostly used in combination with the internal one for hollow box cross-sections.
Cast iron is an iron alloy with a lower melting point than steel and is therefore easier to process. Due to its lower stability, cast iron is no longer important in structural buildings and has been replaced by steel. Many bridges used to be made of cast iron, for example the cast iron bridge over the Severn. Most of the cast iron bridges were unable to cope with the increasing load and were therefore replaced by steel bridges. Only the pendulum supports could cast iron hold up until the beginning of the First World War. One example of this is Hartung's column, which was used in large numbers during the expansion of the railway system in the Berlin area from 1880 to 1910.
- Steel bridge
- Steel has a very high resistance to compressive and tensile forces. Steel is mainly used in bridge construction in the form of ropes, profiles or sheet metal. Today, parts made of cast steel are used to an increasing extent.
A decisive disadvantage, however, is rusting (corrosion), which usually requires anti-corrosion coatings and leads to high maintenance costs. Steel is mainly used in the superstructure of tied arch bridges, deck bridges, truss bridges and suspension bridges.
- Composite steel bridge
Composite structures have spatially separate cross-sections that consist of two or more building materials. In contrast to, for example, reinforced concrete, the bond with each other is created using special fasteners. For example, in the case of a composite steel bridge, the carriageway slab, which is made of reinforced concrete, lies on the steel bridge girder. The bond between the two building materials is ensured by head bolt dowels. This creates a force-fit connection and both cross-sections work together as one cross-section. When coupling pre-stressed concrete and steel composite construction parts, one also speaks of one hybrid bridge.
- Aluminum bridge
The first use of aluminum in bridge construction took place in 1933 with the renewal of the road construction of the Smithfield Street Bridge in Pittsburgh. In 1946, the single-lane railroad bridge Grasse River Bridge, made of newly developed aluminum alloys, was built near Massena (New York) with aluminum main girders. In later years, extrusion technology was further developed. This enabled the production of profiles specially tailored to the requirements of bridge construction. In the period between 1980 and 1990, the aluminum bridge, which is characterized by its low weight, corrosion resistance and low maintenance costs, became widespread in the Scandinavian countries.
Bridges have also been made of fiber-plastic composites since the 1990s. The technology is mostly used on smaller bridges for pedestrians and cyclists. One of the oldest designs is the footbridge built in 1990 on a golf course in Aberfeldy, Scotland. Boothbay, Maine has the longest plastic bridge, the 160-meter Knickerbocker Bridge. According to studies from the Netherlands, plastic bridges require less embodied energy than bridges made from conventional materials, and their construction emits less CO2 free.
Another way of dividing bridges is their function. Then you can distinguish between, among other things Road bridge, footbridge, Railway bridge, Canal bridge / water bridge (Trough bridge) and Wildlife bridge (Green Bridge). A railway bridge thus carries railways, a road bridge carries roadways. Often, however, a bridge has several purposes.
Other bridge types with function names include the Conveyor belt bridge (in mining) that Line bridge (e.g. in the chemical plant) but also the Pioneer bridge or the Temporary bridge. Auxiliary bridges on the other hand, bridges are made of prefabricated, mostly steel construction kit elements, which are used to bypass the construction site during repair work on existing bridges. An auxiliary bridge can also remain in use for a longer period of time. Bridges can also have no function for a longer period of time: These are then popularly called "soda bridges".
The topological location is also a possible criterion for assigning bridges. So, among other things, one can distinguish between Viaducts, City bridges, hillside bridges, River bridges or canal bridges and High bridges.
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