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BRIDGE HISTORY --A General History of Bridges

Bridge technology was modernized during the first half of the twentieth century. Older highway bridges were commonly steel truss structures, but with improvements in concrete and composite steel and concrete design, the massive truss structures began to disappear. Concrete proved to be an adaptable material and was suitable in arched bridge designs or in post and beam construction over short spans. Prior to 1920, most bridges accomodated only one lane of traffic. Motorists traveling at high speeds did not wish to stop and wait for other vehicles to clear a bridge. Consequently, as the old truss bridges were taken down, bridges that could accomodate two lanes of traffic took their place.

Societal Impact of Bridges:

In an age of rapid transportation the importance and impact of bridges is often grossly under estimated. They allow for roads and railroads to cross over otherwise impassable obstacles such as rivers or other roads thus enabling expansion and development of society.

By the turn of the century there were three main types of bridges being used around the world:

the arch bridge,
the bascule bridge (draw bridge)
and the cantilever bridge.
These three types of bridges reflect the needs of those who built them and the availability of materials. Bridges have been widely used since ancient times when humankind began migratory movements and trade expansion. Colonisation of various parts of the world led to further construction of roads and, with them, bridges. The bridges constructed by early colonists were made chiefly of wood. By the late 1800's bridges were being built mainly with steel, iron and concrete. As demand for railroad bridges grew bridges increasingly had to be built in demanding and difficult places. Some examples are:

In 1867 James Eads was comissioned to build a bridge across the Mississippi river. Because the bridge supports had to be built in the bedrock bellow the sandy river bottom he had men digging in airtight capusules at depths of up to a hundred feet.
In 1898 an arch bridge spanning 840 feet was constructed below Niagara Falls. The bridge was constructed mainly of steel. Towards the end of the century they also became combined with concrete to create a concrete reinforced bridge. The first concrete reinforced bridge was completed in 1898.
One of the largest concrete reinforced bridges in the United States is the Tunkasnnock Creek Viaduct in Pennsylvania which spans 2,375 feet.
In the early years of WWII, a railway bridge was constructed across the Hawkesbury River in NSW. This bridge required the huge steel spans to be floated on barges up the river where they were then lifted into position up onto huge sandstone piers.
With the continued expansion of the railroad, however, another type of bridge became widely used. The cantilever bridge's complex design allowed it to support an incredible amount of weight, ideal for the trains that weighed hundreds of tonnes. This engineering advancement enabled more rapid expansion of the railroads.

The last fifty years has seen the development and wide use of the suspension bridge. These bridges are constructed so that they can be build over longer distances. There are several bridges that span almost two kilometres. Bridge design in the 1990s and early 21st century are completed using computer-aided drawing and design technology with architectural software before the first stone is laid.

Clearly, the 21st century will witness a growth of technology with subsequent advances in construction techniques and the use of new materials.

最好再具体一点，如20世纪美国桥梁。