Bridges_notes

===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.===

The bridges have many benefits, one of the most important is the ease of getting to places in the shortest time possible.
===If there were no bridges the transfer of goods would be more tedious because the food is usually found in far away places that need the presence of a bridge, then the market begins to take place more easily and quickly. Bridges have joined cities, islands, and many places that had always been separated by many hours of travel, with the bridges many ways life has become easier.===

Types of bridges:
=== This is the earliest and simplest kind of bridge: a fallen tree lying over a stream, perhaps, or a log lodged across a narrow chasm? We still come across these when walking in the hills, and there is no way of dating the first time some hominid straightened or stabilised such a 'beam' and thereby became the world's first bridge engineer. ===
 * ===The beam bridge:===

=== The design is as simple as a single rigid 'beam', resting on supports at either end and unsupported in the middle. The weight of the beam, and of any traffic on it, is carried directly to the ground by the supports, often called 'piers' in the trade. ===

=== Cantilever bridges are built using cantilevers—horizontal beams that are supported on only one end. Most cantilever bridges use a pair of continuous spans extending from opposite sides of the supporting piers, meeting at the center of the obstacle to be crossed. Cantilever bridges are constructed using much the same materials & techniques as beam bridges. ===
 * === Cantilever bridges ===


 * === Arch bridges ===

**[[image:arch_bridge.jpg width="207" height="173" align="right"]]Arch bridges have abutments at each end. The weight of the bridge is thrust into the abutments at either side.**
===Suspension bridges are suspended from cables. The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo. In modern bridges, the cables hang from towers that are attached to caissons or cofferdams. The caissons or cofferdams are implanted deep into the floor of a lake or river.===
 * ===Suspension bridges===

===** 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. **===
 * === ** Cable-stayed bridges ** ===

===Movable bridges are designed to move out of the way of boats or other kinds of traffic, which would otherwise be too tall to fit. These are generally electrically powered.===
 * ===Movable bridges===
 * ===Movable bridges===
 * ===Movable bridges===
 * ===Movable bridges===
 * ===Movable bridges===

Suitable materials for a beam:
===Stone is very strong under compression and is thus ideal as a material for the piers, but it is not strong under tension, cracking easily if given a sudden blow. Therefore, it is not ideal for use as a beam despite its apparent strength. In addition, stone slabs are very heavy and therefore difficult to manoeuvre into position.===

===Wooden tree trunks or cut planks form a very strong beam when laid across a stream, despite their comparative lightness. Wood is able to carry both compression and tension equally well – wood is organic, after all, and heavy horizontal branches often carry similar loads to bridges themselves. Indeed, the longitudinal fibres within wood are designed to spread stresses within a tree without breaking.===

Of course, wood is not as hard wearing as stone and needs replacing relatively often.
===These days, bridges carrying the huge weights of trains across small roads are almost always beam bridges made from steel. They are very robust and stable, and often do not require any intermediate pier.===