People have been using bridges to cross water, valleys or ravines since time immemorial. There were natural spans, such as tree trunks that had fallen, which early man used to traverse canyons. Later man used stones to build arches, and lashed timber together to put across bodies of water. The Romans built spans, one of which still survives in Spain. It is the Alcantara Bridge, which crosses the Tagus River. The Romans used cement made of water, lime, sand and volcanic rock in the construction of their spans.
Bridges are used by people to cross over water, canyons or other obstacles, on foot, bicycle, bus or car. They can also be used for trains which will frequently share the bridge roadway with cars. There are many examples of single track train bridge construction throughout the Amtrak system. This type of bridge, dedicated to train use, is called a trestle. Some spans are used to convey water and are called aqueducts. Others are built for carrying cable or wiring.
The Chinese built wooden spans, the earliest known of which was built in 595-605 CE, during the Sui Dynasty. Incas used bridges made of rope in pre-conquest Peru. The first metal bridge was built in Coalbrookdale, England. It was made of cast iron, and had arches. It was built in 1779. This was an important precurser to the modern bridge.
The development of steel led to a rapid growth of designs for spans. Many designers were influenced by the works of Gustave Eiffel, whose work was pioneering. Steel had properties that cast iron did not that made it an ideal material for bridge building material.
There are six different types of modern bridge: beam, cantilever, truss, arch, suspension and cable-stayed. A beam bridge is a length of materiel which is held in place by downward pressure of abutments on either end of the span. The first log people placed across a stream to cross over is an early example of a beam bridge. These spans have some weaknesses when they are long, so most of them are 250 feet or shorter. There are piers that connect spans if there is more than one span to a bridge. Though they were made of wood in the past, the modern beam bridge is made of steel.
A cantilever bridge is made of two cantilevers which reach to the center from either side. A cantilever is a beam which is supported on only one side. Cantilever spans are also made of steel much as the beam bridge.
A truss bridge is constructed of connected beams or girders that are connected horizontally vertically and diagonally. Some of the earlier truss spans were made of wood. This design allows for double decker truss and movable truss spans. Bridge designs allow for spans to open vertically to allow high masted ships to pass. Over designs have a portion of the span which rotates to the side.
Arch bridges have an arched roadway that pushes into the abutments on either end. Arch spans were built by the ancient Greeks. This design is still being used today and the longest arch bridge should be completed in 2012 in Dubai. These are elegant soaring designs.
Suspension spans are kept up by cables which are connected to towers along its length. The towers are built on foundations that are buried deep below ground. The Golden Gate Bridge is a suspension bridge. Early suspension spans used rope or vines for cable. A cable stayed bridge is similar to a suspension bridge but there is less cable in the design and the supporting towers are shorter. This type of bridge has been around since the 1700′s.
Bridges are designed to function at peak condition. They rely on principles of physics and engineering to perform their functions. Bridge inspection is necessary to ensure that the strains a bridge is subjected to have not damaged its support structure. They also have to be kept up to date with modern safety standards. Older models are retrofitted to conform with earthquake safety standards. When a bridge can no longer be adapted to maintain minimal safety levels, it can no longer be used and must be replaced.
There have been bridge collapses and failures reported in the news over the years. A bridge that has not been retrofitted may buckle and fail in the case of a severe earthquake. Other bridge failures were found to have been caused by cracks in their support structures. Recent bridge failures have alerted governments to set in place improved maintenance and repair protocols. Bridges are essential for transport. They provide easy access over water and other obstacles. A bridge can soar up to the sky or be squat and solid. Their designs are quite varied, limited only by the human imagination but their function is basically the same.