Different shapes resist forces in different ways, so as every material withstands forces differently. To design a good and safe bridge, an engineer must know the forces in every member of the bridge. In turn, he must choose the appropriate material for that member, or for all members. He must know the characteristic of every material under various forces which may occur on the bridge. For example, should he choose concrete or stone for the pier and abutment? Is it steel instead of concrete which is the best material for a particular bridge? In the ancient times, stone, wood, earth, and brick were used. In the mid 19’th century iron, both cast iron and wrought iron were used.
The advent of steel replace those material. Nowadays, steel and concrete (reinforced concrete and prestressed concrete) are the most popular material. Why choose one over another? Basically, there are four reasons behind that: Characteristics Cost Technological Level Availability (1) Characteristics Every material reacts differently to different forces. For example, concrete is strong to withstand compression, but it breaks easily when tension force is applied. Another example is brick. It is a good material to support compression force.
You can pile couple feet of bricks without affecting the lowest layer bricks. However, when you try to break it even with your bare hand, it will snap into two. Hence, it is said that brick has weakness in shear. What about the cast iron? This material is strong. You can apply tension and compression as well. So why engineers do not use it anymore? It is brittle! Until a certain load, for instance you load it with a car, this case iron looks great.
The Term Paper on Metallic Construction Materials Building Concrete
Besides masonry, a brick work, any engineer must know about heating and ventilation. They are two branches of engineering which are very closely connected. Both they are treated (1) as a dual subject. Heating is to prevent (2) too rapid loss of heat from the body. The rate of heat lost from the body is controlled. Some old concepts of heating have been gradually (3) changed since engineers ...
However, when you put some other load, the iron breaks without any warning. It does not show noticeable cracks before starting to rupture. Steel has better characteristic about the abrupt rupture. (2) Cost Bridges are expensive! It is so expensive so that cost is always a major concern. Hence, a designer will not choose a more expensive material if cheaper materials available. (3) Technological Level The materials to be used are depend on the technology we have.
When our ancestor did not know how to cast the iron, they used stones, bricks, or wood. At that time, those materials were the best thing they could make. Today people use steel instead of cast iron. The technology to make steel were devised quite recently. People in mid 19 th century did not know about steel, and iron was the best material. Innovations are always found.
Perhaps in the next millennium people build bridges with plastics. Who knows? (4) Availability Well, even though you know how to make steel, but you do not have enough ore, or nobody produce enough steel for your bridge, you will not design a steel bridge. Another example is that some places do not have good stones for a concrete mixture. To import stones from another places sometimes too expensive. In the opposite, that place has abundant of woods. A good engineer will adapt to this condition and use woods instead of stones.
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