Modern buildings and structural materials

Текст C

Many great buildings which were built centuries ago can still be seen in Greece and Italy, France and England. All of these buildings were construction solutions to difficult construction problems. These great buildings were not the result of scientific knowledge. They were constructed on the basis of experience, often as the result of trial and error. They have survived because of the great strength that was built into them - strength greater than necessary in most cases.

Today, however, the engineer has the advantage not only of empirical information, but also of scientific information that allows him to make careful design. When a modern engineer plans a structure, he considers the total weight of all its component materials. This is known as the dead load, which is the weight of the structure itself. He has also to consider the live load, the weight of all the people, cars, furniture, machines, etc. that the structure is to support when it is in use. In structures such as bridges that are to support fast traffic, he has to consider the impact, the force at which the live load will be exerted on the structure. He must also determine the safety factor, that is, an additional capacity to make the structure stronger than the combination of the three other factors.

The modern engineer should also understand the different stresses to which the materials in a structure are subject. They include the opposite forces of compression and tension. In compression the material is pushed together; in tension the material is pulled apart or stretched, like a rubber band. In the figure below, the top surface is bent inwards, and the material in it is in compres­sion. The bottom surface is bent outward, and the material in it is in tension.

In addition to tension and compression, another force which is called shear should be considered. Shear is the tendency of a material to crack and break along the lines of stress. The shear may occur in a vertical plane, but it may also run along the horizontal axis of the beam, the neutral plane, where there is neither tension nor compression.

Altogether, three forces can act on a structure: vertical - those that act up or down; horizontal - those that act in horizontal plane; and those that act upon it with a turning motion. Forces that act at an angle are a combination of horizontal and vertical forces. Since the buildings that are designed by engineers are to be stationary or stable, these forces must be balanced. The vertical forces, for example, ought to be equal to each other. If a beam supports a load above, the beam itself has to have sufficient strength to counterbalance that weight. The horizontal forces must also equal each other so that there is not too much thrust either to the right or to the left. And forces that may pull the structure around must be equal to the forces that pull in the opposite direction.

Notes:

altogether adv. в целом, всего

empirical information n. эмпирическая информация

furniture n. мебель

machine n. оборудование, станок, устройство

push (~ together) v. зд. сжимается

scientific adj. научный

stationary adj. устойчивый, неподвижный

survive v. уцелеть

today adv. в наше время, теперь

trial and error (метод) проб и ошибок

turning motion вращательное движение

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