XIII. Describe the engineering works proposed for the Mangla dam

Unit 9

Bridges

I. Memorize the following words and expressions:

deck - покрытие

winch - лебедка

span - пролет, перекрывать пролет

silt - илистый грунт, наносы

beam - балка

truss - ферма

cantilever - консольный

bridge abutment - устой моста

abutment - примыкание, сопряжение

anchorage – крепление

II. Read the text .

Bridging the Waters

The modern bridge is a mark of our civilization. Today, we build double-deck bridges, bridges that lift, and bridges that swing. We take old bridges down and put new ones in their place without interrupting traffic. We raise or lower them bodily and we move them sideways. We pick up and set heavy bridge sections with winches and mammoth cranes. We float bridges into position, and we roll them in. We jack and we pull and we push bridges weighing thousands of tons. We build beautiful bridges and ugly bridges. We can build a concrete arch of 1000-foot span, and a steel suspension bridge spanning 4260 feet.

Tomorrow, our theory tells us, with our present materials we will be able to build a concrete arch spanning one mile, and a steel suspension bridge spanning two miles. With new materials and subtler theories, our scope will reach almost beyond imagining. But though man's strategy in his struggle with nature is growing bolder, it is never reckless; everything is carefully pondered and calculated beforehand.

The first thing the engineer must consider is the foundation: on this depends the stability of his bridge. Severe problems can arise with bridge foundations, for rivers naturally tend to carve their way through the weaker and softer strata making up the river bed. The waters often deposit thick layers of silt. The engineer must then dig through the silt in order to strike firm ground beneath.

There are three basic types of bridges: beam or girder bridges; arch bridges; and suspension bridges. In a simply supported beam, the two ends rest on their supports. A continuous beam is joined at one or more intermediate supports. A cantilever beam has no support at one end, and therefore must be anchored down at the other. A truss is a type of beam built from a number of pieces or members to form a series of connected triangles, giving strength combined with lightness.

Beam bridges exert a simple downward pressure on their supports. Because of its shape, an arch bridge thrusts or pushes hard against its ends; its stability therefore requires ground firm enough to take these inclined thrusts. A suspension bridge is basically a rope from which the bridge deck is hung. The rope passes over two towers and then to anchors on each side of the river. Their inclined pull is in contrast with the inclined push of an arch against its abutments.

The great bridges of today have grown from these basic types of structures. They stand as tributes to the vision of the men who first conceived such projects, to the imagination of the designers who prepared the plans and to the art of the builders.

Some bridges have spans so large that no other material but steel would do. But for small and medium spans steel is being ousted by prestressed concrete. The stresses in a bridge arise from dead and live loads, wind forces, temperature changes, and so on. They produce either compression or tension; steel stands up well to both. Ordinary concrete, on the other hand, can resist only moderate compressive forces, and it is weak in tension. To overcome this, steel rods are buried in the concrete, which is then called reinforced concrete. A reinforced concrete beam shares its work between the two materials, the concrete taking the compression and the steel rods the tension.

In prestressed concrete, the concrete is artificially put into compression before any load from the structure comes upon it. The concrete can then resist tension up to the extent of the compression that was put into it beforehand: an applied tension partly undoes the precom-pression. The principle is similar to the trick of picking up several bricks or blocks of wood at a time by pressing them hard together. The compressive prestress is put into concrete by high tensile steel rods or wire. These are drawn taut by hydraulic jacks and then fastened to the ends of the concrete member. The tension on the wire pulls the two ends of the concrete together, thus compressing it.

Since concrete in its initial stages is a plastic material, concrete bridges have normally to be manufactured and built as a unit in their final position. This is called casting in situ and requires a temporary system of formwork "centering," "stagings," and props to hold the concrete until it "has set hard and become self- supporting.

III. Answer the questions:

1. Why do people build bridges?

2. What techniques are used in bridge construction?

3. How long is a normal bridge span?

4. Why is a foundation of primary importance?

5. What are the structural differences of various types of bridges?

6. What is a truss?

7. What materials are mainly used in bridge construction?

8. What makes combination of concrete and steel beneficial?

9. How is prestressed concrete made?

10. What is called "cast in situ"?

IV. Complete the sentences: