Writing+1

BRIDGES A bridge is a structure built to cross physical obstacles such as a body of water, a valley, or a road, with the purpose of providing passage over that obstacle. The design of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed, the material used to make it and the funds available to build it. There are six main types of bridges: beam bridges, cantilever bridges, arch bridges, suspension bridges, cable-stayed bridges and truss bridges. Common categories include the type of structural elements used, by what they carry, whether they are fixed or movable, and by the materials used. Some of the materials used for bridge building are: wood, brick, stone, cast iron, wrought iron, mild steel, high-tensile steels, alloy steels, aluminium, steel-reinforced concrete, pre-stressed concrete, glass-reinforced plastic. And the stresses that bridge materials must withstand, in a big variety of combinations are: bending, compression, impact, oscillation, pressure, tension, torsion, vibration; contraction, corrosion, erosion, expansion, fatigue, friction, rain, river flow, sea-water, scouring, temperature changes, tidal flow, turbulence, waves, wind erosion, wind gusts, wind pressure. In the 20th century, the technology involved in bridge construction evolved and was fueled by the Industrial Revolution. As the century turned, steel bridges were riveted together, not bolted; concrete bridges were cast in place, not precast; and large bridge members were built from lacing bars and smaller sections, not rolled in one piece. Since plastic had not yet been invented, construction techniques such as post-tensioning, slurry walls, soil freezing, and reinforced earth walls had not yet been conceived. Also, surveying was performed mechanically since infrared, optical technology was still 75 years away. Today new construction techniques and new materials are emerging. Bridge construction is changing as the new millennium begins. There are also new issues facing the bridge building industry relative to the research needs associated with these new techniques and materials. In the 21st century, this new technology will meet changing needs and provide alternatives that will lead to new standards in engineering and construction worldwide. The future economic impact of bridge construction will revolve around ways of implementing simple design and construction solutions through innovative thinking. Improved interaction among bridge design, construction, maintenance, and field performance will be essential in providing the most economical infrastructure. The advantages and disadvantages in bridges depend on the bridge to be built. These can vary in economic, height, construction, flexibility, foundation. Bridges can influence the place where they are built because they bring places together, shorten long distances and roads, and help economic costs in transportation helping the economy of a nation. As their technology advance they have also become architectural beauties, famous for their aesthetics, helping in tourism as well. media type="file" key="writing 1.mp3"