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VII. Make the precis of the text



Text 2. TYPES OF BRIDGES

Introduction

Bridge designs differ in the way they support loads. These loads include the weight of the bridges themselves, the weight of the material used to build the bridges, and the weight and stresses of the vehicles crossing them. There are basically eight common bridge designs: beam, cantilever, arch, truss, suspension, cable stayed, movable, and floating bridges. The types of bridges vary in total length, the length of their spans, and the weight they can support. Before deciding which kind to build at a particular place, engi­neers determine the length of the structure and of each span. They also must consider the maximum load the bridge will carry and the materials available to construct the bridge.

Combination bridges may incorporate two or more of the above designs into a bridge. Each design differs in appearance, construction methods and materials used, and overall expense. Some designs are better for long spans. Beam bridges typically span the shortest distances, while suspension and cable-stayed bridges span the greatest distances.

Beam Bridges

Beam bridges represent the simplest of all bridge designs. A beam bridge consists of a rigid horizontal member called a beam that is supported at both ends, either by a natural land structure, such as the banks of a river, or by vertical posts called piers. Beam bridges are the most commonly used bridges in highway construction. Single-piece, rolled-steel beams can support spans of 15 to 30 m (50 to 100 ft). Heavier, reinforced beams and girders are used for longer spans.

Girder Bridges

Girder bridges, which include many highway bridges, are made of beams called girders whose ends simply rest on piers or abutments. These bridges may be used to cross most areas. The span length of girder bridges ranges up to about 1.000 feet (300 meters).

There are two main types of girder bridges. In one type, called a box girder bridge, each girder looks like a long box that lies between the piers or abutments. The top surface of the bridge is the roadway. Box girder bridges are built of steel or concrete. In the other type of girder bridge, the end view of each girder looks like an I or a T. Two or more girders support the roadway. This type of bridge is called a plate girder bridge whenmade of steel, a reinforced or prestressed concrete girder bridge when made of concrete, and a wood girder bridge when made of wood.

COMPREHENSION CHECK

I. Decide whether the following statements are true or false according to the text:

1. These loads include the weight and the stresses of the vehicles crossing them.

2. The kinds of bridges vary in the length of their spans.

3. It is very important to consider the maximum load the bridge will carry.

4. Each bridge incorporates two or more of the designs into a bridge.

5. Beam bridges typically span the greatest distance.

6. A beam bridge consists of a rigid horizontal member.

7. Beam bridges are rarely used in highway construction.

8. Girders are supported at both ends by natural land structure.

9. The span length of girder bridges ranges up to about 500 metres.

10. There are two types of girder bridges: a box girder bridge and a plate girder bridge.

II. Answer the following questions:

1. How do bridge designs differ?

2. What do these loads include?

3. What should engineers determine before deciding which kind of bridge to build?

4. What is a combination bridge?

5. What is the difference between beam bridges and suspension bridges?

6. What does a beam bridge consist of?

7. Where are beam bridges most commonly used?

8. What are the two main types of girder bridges?

9. Why is a box girder bridge called so?

10. When is a bridge called a plate girder bridge and when a reinforced?

LANGUAGE FOCUS

III. Choose the contextual meaning of the word:

load 1)груз 2) вагон 3) нагрузка 4) бремя

stress 1) нажим2) напряжение3) давление4) ударение

span 1) период времени 2) длина моста 3)перегон 4) пролет

rigid 1) жесткий 2) твердый 3) неподвижный 4) стойкий

abutment 1) межа 2) береговой устой 3) контрфорс 4) опора

IV. Match the meanings of these terms with their definition:

span, design, girder, load, pier, roadway;

1. The weight or force supported by a structure or any part of it.

2. Each arch or part of a bridge between piers or supports.

3. A drawing, plan or sketch made to serve as a pattern from which to work.

4. One of the solid supports on which the arch of a bridge rest.

5. A main supporting beam made of steel, concrete or wood.

6. The part of a road used by wheeled vehicles.

V. Fill in the correct prepositions:

to be made…, to vary…sth, to be better…sth, to consist…,

to be supported… sth, to be used…sth, to differ…sth

VI. Fill in the missing words from the list:

bridge design, supports, to carry, short, loads, to hold, long, public’s, spans, goal

A bridge is a structure that … horizontally between supports and its function is to carry vertical … .The prototypical bridge is quite simple – two … holding up a beam. The supports must be strong enough … the structure up and the span must be strong enough … loads. Spans are generally made as … as possible; … spans are justified where good foundations are limited. All major bridges are build with the … money. Therefore … that best serves the public interest has a threefold … efficiency, economy, elegance.

VII. Make the precis of the text

Text 3. TRUSS BRIDGES

Truss bridgesare supported by frameworks called trusses. The parts of the trusses are arranged in the form of triangles. Such bridges are built over canyons, rivers, and other areas. A truss bridge may have a main span that extends more than 1,000 feet (300 meters).

Each truss consists of steel or wood parts that are connected to form one or more triangles. The simplest truss consists of three parts fastened together at their ends to form a triangle.

Most truss bridges have one set of trusses on each side of the roadway. The majority of modern truss bridges have the roadway on top of the trusses and are called deck truss bridges. The roadway of a through truss bridge runs between the trusses.

In a simple span truss bridge, each truss extends between two abutments or piers. In a continuous truss bridge, each truss has three or more such supports.

Some locations are suitable for either a truss bridge or a girder bridge. In such cases, some engineers choose to build a truss bridge because it requires less construction material than the girder type. However, many engineers prefer a girder bridge because it is more attractive and easier to construct and maintain.

Arch bridges

Arch bridges are structures in which each span forms an arch. The spans range up to about 1,700 feet (518 meters) long. The arch bridge is one of the oldest types of bridges. Early arch bridges consisted of large stone blocks wedged together to form an arch. Today, the majority of arch bridges that have short spans are made of concrete or wood. Arch bridges with long spans are built of concrete or steel.

Engineers must design arch bridges so that the sides of the arch do not spread apart and collapse the bridge. The roadway of some arch bridges lies on top of the arch and is supported by vertical columns called spandrel columns. These columns transfer the load of the roadway to the arch, which bears the weight of the bridge. The roadway of a tied arch bridge is below the curve of the arch. The roadway is supported by girders or other types of beams that hang from the arch. The girders or beams also connect to the ends of the arch to prevent the ends from spreading out. The abutments support the weight of the bridge.

COMPREHENSION CHECK






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