Bridges_notes


 * Bridges Notes **



** What is the meaning of the word //bridge//? ** A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed, the material used to make it and the funds available to build it.

** Are all bridges alike? **   No  ,   they vary by material and construction form.

** What are they made of? **   Bridges   are made of steel, wood, concrete , stone, cables, among others.

** What are they connecting? ** Bridges were made to connect two places separated by geographical obstacles.

** How is the span between their piers or towers? ** Most of the bridges have the towers in the extreme sides of the bridge, depending of the length of it.

** What social and/or political benefits can bridges provide to places? ** ** Why were the bridges built? **   The   bridges have their origin in the same prehistory. Possibly the first bridge of the story was a tree that prehistoric man used to connect the two banks of a river. They also used stone slabs to small streams where there were no trees nearby. The following bridges were bows made of logs or planks and eventually stones, using a simple bracket and placing cross beams.

** Did they provide benefits? To whom? Why? **   Yes,   the bridges provide benefits to society where they are built because it facilitates communication, thus increasing the local economy. It is also tourist source.

** Benefits ** Supplies of food and traded goods can get across an obstacle or through difficult terrain in a shorter time.

** General Information ** ** Types of Bridges **

**__ Beam bridges: __** are horizontal beams supported at each end by abutments, hence their structural name of simply supported. The world's longest beam bridge is Lake Pontchartrain Causeway in southern Louisiana in the United States, at 23.83 miles, with individual spans of 56 feet. __ Advantage: __ Good for short spans, wide distances can be spanned by resting beams on  piers. __ Disadvantage: __ Can be expensive, even for fairly short spans, because of the need to use steel. A cheap material for beam bridges is concrete, but concrete on its own, is not strong enough to stand up to high forces of tension (pulling), so concrete needs to be reinforced with steel mesh; very expensive for long spans, because of need for piers to support the long beams; it is not always possible to build support piers.



**__ Cantilever bridges __** : are built using cantilevers—horizontal beams that are supported on only one end. The largest cantilever bridge is the 549-metre Quebec Bridge in Quebec, Canada. __ Advantage: __ Building out from each end enables construction  to be done with little disruption to navigation below. The span can be greater than that of a simple beam, because a beam can be added to the cantilever arms. Cantilever bridges are very common over roads. Because the beam is resting simply on the arms, thermal expansion and ground movement are fairly simple to sustain. Cantilever arms are very rigid, because of their depth. __ Disadvantage __ : Like beams, they maintain their shape by the opposition of large tensile and compressive forces, as well as shear, and are therefore relatively massive. Truss construction is used in the larger examples to reduce the weight. **__ Arch bridges: __** have abutments at each end. The earliest known arch bridges were built by the Greeks and include the Arkadiko Bridge. Dubai in the United Arab Emirates is currently building the largest arch bridge in the world. __ Advantage: __ It is an economical mom to traverse small distances and is able to cope with bending forces. __Disadvantage:__ An arch bridge requires a very stable foundation. The bridge diverts the tension exerted on it onto the foundations on either side of the bridge.





**__ Suspension bridges: __** are suspended from cables. In modern bridges, the cables hang from towers that are attached to caissons or cofferdams. The longest suspension bridge in the world is the Akashi Kaikyo Bridge in Japan. __ Advantage: __ Can cover very long spans; The steel cable used is cheaper than steel girders. __ Disadvantage: __ A very flexible bridge, so not used for railways. As trains are heavier than cars or lorries, their weight is less spread out across the bridge.



**__ Cable-stayed bridges: __** like suspension bridges, are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately shorter. The longest cable-stayed bridge is the Sutong Bridge over the Yangtze River in China. __ Advantage: __ The two halves may be cantilevered out from each side. There is no need for anchorage's to sustain strong horizontal forces, because the spans are self-anchoring. They can be cheaper than suspension bridges for a given span. Many asymmetrical designs are possible. __ Disadvantage __ : In the longer sizes, the cantilevered halves are very susceptible to wind induced oscillation during construction. The cables require careful treatment to protect them from corrosion.

** Glossary **

**__ Abutments: __** the side support at either end of an arch bridge, necessary to withstand the horizontal forces generated by the arch's shape.

**__ Anchorages: __** the points at which the main cables on a suspension bridge are 'anchored' to the ground at either end of the bridge.

**__ Buttress (es): __** a structure, usually brick or stone, built against a wall for support or reinforcement to resist the sideways pressure of the weight of the building.

**__ Corbelled: __** an arch constructed by successive layers of brick or stone projecting further towards each other from either side of the arch, until the gap is spanned.

**__ Struts: __** a part of a framework that is carrying compressive forces, or keeping two elements of a structure apart.

**__ Suspended deck: __** a roadway or railway that hangs from cables slung over towers or from the underside of an arch

**__ Girders: __** a beam, usually made from concrete or steel, that is designed to strengthen another structural element. Concrete beam bridges often have steel girders beneath the road deck in order to help bear the tensile forces.

**__ Hangers: __** The vertical rods or cables that are directly attached to the road deck of a suspension bridge, and 'hang' from the two main suspension cables that pass over the towers.

**__ Span: __** The unsupported length of a bridge, between its towers or piers.

**__ Ties: __** A part of a framework that is carrying tensile forces, or keeping two elements of a structure together