Lead-in

1. How long have people use such a structure as bridge?

2. What materials served as first bridges?

Logs or vines that extended across streams probably served as the first bridges. From this at a later stage, a bridge on a very simple bracket or cantilever principle was evolved. Timber beams were embedded into the banks on each side of the river with their ends extending over the water. These made simple supports for a central beam reaching across from one bracket to the other. Bridges of this type are still used in Japan, and in India. A simple bridge on the suspension principle was made by early man by means of ropes, and is still used in countries such as Tibet. Two parallel ropes suspended from rocks or trees on each bank of the river, with a platform of woven mats laid across them made a secure crossing. Further ropes as handrails were added. When the Spaniards reached South America, they found that the Incas of Peru used suspension bridges made of six strong cables, four of which supported a platform and two served as rails.

The first bridge known to historians was an arch bridge built in Babylon about 2200 B.C. The ancient Chinese, Egyptians, Greeks, and Romans also built arch bridges, using bricks and stone as building materials.

During the Middle Ages, moveable bridges called drawbridges were built across the moats of many cas­tles in Europe. Truss bridges were developed in the 1500’s. Most bridges were made of stone or wood until the late 1700's, when cast iron and wrought iron were first used for bridges. Many suspension bridges that hung from wrought iron chains were built in the early 1800's. Between 1830 and 1880, as railroad building expanded throughout the world, bridge design and construction were aimed to carry these heavy vehicles over new obstacles.

Designers experimented with a wide variety of bridge types and had to meet the demand for greater heights, spans, and strength. Locomotives were heavier and moved faster than anything requiring stronger bridges. The basic beam bridge was strengthened by adding support piers underneath and by reinforcing the structure with elaborate scaffolding called a truss. During the period of railroad expansion iron trusses replaced stone arches as the preferred design large bridges.

The first plate girder bridge was completed in 1847, and the modern cantilever bridge was introduced about 1870. In the late 1800s, steel became the chief material used in bridge construction.

In 1855 British inventor Sir Henry Bessemer developed a practical process for converting cast iron into steel. This process increased the availability of steel and lowered production costs considerably. The strength and lightness of steel revolutionized bridge building. In the late 19th century and the first half of the 20th century, many large-scale steel suspension bridges were constructed over major waterways in the late 19th century, engineers began to experiment with concrete reinforced with steel bars for added strength. More recently, reinforced concrete has been combined with steel girders, which are solid beams that extend across a span. When the Interstate Highway System in the United States and similar road systems in other countries were constructed in the mid- to late 20th century, the steel-and-concrete girder bridge was one of the most commonly used bridge designs. The last three decades of the 20th century saw a period of large-scale bridge building in Europe and Asia. Current research focuses on using computers, instrumentation, automation, and new materials to improve bridge design, construction, and maintenance.

Дата добавления: 2015-08-27