When bridge building actually started is unknown. One of the first documented bridges is a relief showing Seti I (1,291 to 1,279 BCE) crossing a bridge, while returning victorious from fighting in Canaan, in the first year of his reign. The beginning of bridge building engineering, to cross longer distances than just a few feet, is generally credited to the Romans. Their construction of permanent roads with bridges were created for the movement of troupes. A few of these structures still stand today.
The beam type of bridge is the easiest construction to understand; these include when a tree that has fallen, or been cut to cross a small creek. The stacking of stones on each end to support a slab of stone is a beam type or span bridge. Multiple upright beams supporting a single or multiple decks allows this type of bridge to span distances more than just a few feet.
The Roman's primary bridges were arch type constructed from heavy stone blocks. These bridges were engineered by wedging stones against another forming an arch. The last stone to be put into place, completing the arch, is called the keystone. Some of these bridges contained many arches to transverse further distances. Many of the Roman built bridges still stand today. Some are still carrying pedestrians and other traffic, while some were constructed to carry water.
The rigidity of a triangle that is anchored at each joint is the basic configuration used in the construction of truss bridges. One triangle attached to another, making a straight line in single or multiple layers can be constructed to span long distances. A king post truss bridge is two right triangles attached at the 90 degree angles, with the one end of the hypotenuse attached at a beam, and the other attached to the post in the middle of the bridge. A queen post adds a horizontal top span, and can transverse even longer distances.
Suspension type bridges are the longest in the world. They are similar with cable-stayed wire type bridges, and are capable of spanning over a mile. Cables are hung from suspenders to support the deck from one pier to another, and are anchored at the end pillar or post. Multiple combinations of these engineering techniques are utilized to construct some of the most famous bridges in the world.
The cantilever bridge is engineered where the deck projects farther than its support and is anchored only at one end. These bridges can be designed in conjunction with arch, truss and suspension engineering for a multitude of construction arrangements. The terrain and materials used will dictate the construction array.
Movable bridges are made up of two types: the draw bridge, and the swing bridge. The draw bridge is hinged on one end and cables or tresses hold the other end in place when the bridge is down. Raising and lowering the bridge is performed by either gears cranking the bridge up, or the use of cable pulleys. The other type of movable bridge is a swing bridge. It can be moved from a center pivot or pivoted to the side with the aid of floating pontoons.
All pontoon bridges are temporary bridges supported by boats, barges or pontoons. The rigid deck spans from one pontoon to another so that no one floating support the traffic load. Pontoon bridges are primarily used by the military, and for temporary traffic while the main bridge is repaired or a new bridge is constructed.