Bridge construction

The first bridges were all designed empirically, from the experience gained by previous failures and successes. The history of bridge construction knows many examples when the bridges were built without basic knowledge of Mathematics, which resulted in great tragedies. Engineering failures, although often very costly in life and money, taught extremely valuable lessons.

You must have heard of Tay Bridge disaster which took place at the end of the last century. The Tay Bridge was designed by Thomas Bouch and opened in June 1878. It was two miles long and spoken about as one of the wonders of the world. But unfortunately principles of aerodynamics which should have been used in designing the bridge were yet unknown. So, the bridge was built without any knowledge about the force a wind can exert on the bridge.

On Sunday afternoon, the 28^th of December, 1879, a storm broke out at that place. By evening the wind had reached the greatest force, and the people began to be nervous of what might happen to it in such weather. Some of them went to the end of the bridge to await the arrival of the evening train from Edinburgh. What they saw was a lighted train crossing the bridge. Suddenly the lights disappeared as the engine and coaches fell into the water. All aboard the train were lost and it was difficult to estimate the exact number of people, as some may have been carried out to sea.

The Tacoma Narrows Bridge in the USA, which was the third longest bridge in the world, was opened for traffic in July 1940. Owing to its behavior in wind, it was nicknamed «Galloping Gertie». The deck of the bridge not only swayed sideways but large vertical waves appeared, and the roadway oscillated up and down under the action of quite moderate winds. Drivers of cars reported that when they crossed the bridge vehicles ahead of them completely disappeared from view and reappeared several times, owing to the wavy motion of the roadway.

From the very beginning it was clear that the bridge structure ought to have been strengthened. So, before the bridge was opened several attempts had been made to damp out [уменьшать амплитуду] the oscillations of the main span. But they all were not a success. In November 1940, only four months after the bridge was constructed, it collapsed under the action of wind of only 42 mph, whereas generally bridges are designed to withstand winds of 120 mph.

Early bridges had to be built out of material close at hand. In tropical jungles suspension bridges were made of long bamboo poles. In the places where there were many forests it was wood. In northern areas pier bridges were built out of stone. In Middle Ages people constructed wooden beam type bridges. They were usually built on stone piers or wooden piles. Later, there appeared concrete and metal bridges.

Nowadays some people are experimenting with different unusual materials. One of them is paper. Everyone associated paper with weakness. But paper-makers consider it to be a very strong material. In order to prove it paper-makers built a paper bridge across a narrow canyon in Nevada.

The bridge covered a span of 32 feet. The designers calculated that it could safely span 80 feet. The structure is ten feet wide and four feet high. It took the engineers only two months to design the bridge, to test and construct it. For testing the engineers chose a truck which weighted 12,000 lb (pounds). This truck drove quite safely across the paper bridge. The engineers are sure that the bridge can take six times the truck load of 12,000 lb. The bridge itself weighs 9,000 lb. So, the structure is light enough to be laid into place by helicopter.

LESSON SEVEN