Bridge

A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath.

[1][2]: bridge1 The Oxford English Dictionary also notes that there is some suggestion that the word can be traced directly back to Proto-Indo-European *bʰrēw-.

Dating to the Greek Bronze Age (13th century BC), it is one of the oldest arch bridges still in existence and use.

[12] One type of cement, called pozzolana, consisted of water, lime, sand, and volcanic rock.

Brick and mortar bridges were built after the Roman era, as the technology for cement was lost (then later rediscovered).

With the advent of steel, which has a high tensile strength, much larger bridges were built, many using the ideas of Gustave Eiffel.

Common categories include the type of structural elements used, by what they carry, whether they are fixed or movable, and by the materials used.

Bridges may be classified by how the actions of tension, compression, bending, torsion and shear are distributed through their structure.

The world's longest beam bridge is Lake Pontchartrain Causeway in southern Louisiana in the United States, at 23.83 miles (38.35 km), with individual spans of 56 feet (17 m).

The arch of the Friedensbrücke, which was built in the same year, has the span of 90 m (295 ft) and crosses the valley of the Syrabach River.

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.

This follows from the importance of the engineering requirements; namely spanning the obstacle and having the durability to survive, with minimal maintenance, in an aggressive outdoor environment.

[43] Bridges are first analysed; the bending moment and shear force distributions are calculated due to the applied loads.

Many bridges are made of prestressed concrete which has good durability properties, either by pre-tensioning of beams prior to installation or post-tensioning on site.

Other garden bridges may cross only a dry bed of stream-washed pebbles, intended only to convey an impression of a stream.

Often in palaces, a bridge will be built over an artificial waterway as symbolic of a passage to an important place or state of mind.

A set of five bridges cross a sinuous waterway in an important courtyard of the Forbidden City in Beijing, China.

Before factoring to allow for uncertainty, the load effect is generally considered to be the maximum characteristic value in a specified return period.

In the past, these load models were agreed by standard drafting committees of experts but today, this situation is changing.

With extensive WIM databases, it is possible to calculate the maximum expected load effect in the specified return period.

It is also generally assumed that short spans are governed by a small number of trucks traveling at high speed, with an allowance for dynamics.

Weigh-in-Motion (WIM) systems provide data on inter-vehicle gaps but only operate well in free flowing traffic conditions.

One of the most famous examples is the Tacoma Narrows Bridge that collapsed shortly after being constructed due to excessive vibration.

More recently, the Millennium Bridge in London vibrated excessively under pedestrian loading and was closed and retrofitted with a system of dampers.

For smaller bridges, dynamics is not catastrophic but can contribute an added amplification to the stresses due to static effects.

For example, the Eurocode for bridge loading specifies amplifications of between 10% and 70%, depending on the span, the number of traffic lanes and the type of stress (bending moment or shear force).

[67] Given the wide variety of heavy vehicles on road bridges, a statistical approach has been suggested, with VBI analyses carried out for many statically extreme loading events.

The failure of bridges first assumed national interest in Britain during the Victorian era when many new designs were being built, often using new materials, with some of them failing catastrophically.

Many long-span bridges are now routinely monitored with a range of sensors, including strain transducers, accelerometers,[91] tiltmeters, and GPS.

There is considerable research interest in the challenge of smaller bridges as they are often remote and do not have electrical power on site.