Мосты. Тоннели

II. Answer the questions

1.What are the most ingenious parts of bridge building?

2.Why is the construction work a grave hindrance to normal life in the area?

3.What is done before any construction work?

4.How can computer simulations supplement the tests?

5.How can the stresses of the structure differ?

6.When must jacking be provided?

7.What are the difficulties in excavation for foundations?

8.How can the pillars of the towers of big suspension bridges be stabilized?

9.What is a keystone?

LANGUAGE FOCUS

III. Match the words with their synonyms.

IV. Match the words with their opposites

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V. Insert the prepositions.

To subject … sth, … the form of sth, in conjunction … sth, in case … sth, a great variety … sth, an opposition … sth, to differ … sth, a set … sth, to prevent … sth, the origin … sth, the value … sth, to deal … sth.

VI. Insert the words from the list

spans, cantilevers, jack, rams, completion

1.The four legs of the Tour Eiffel were provided with … which were adjusted from time to time as the structure grew. Even after … a concrete structure may be subject to creep and the ground may settle.

2.Above ground, until … are joined wind, can be a great hazard.

3.Before the Pont de Normandie was completed? There was serious discussion about the use of active stabilizers to keep the long thin … in place.

4.Large weight would have been moved by … .

VII. Make the precis of the text

Text 2. Bridge Construction (According to the Types).

There is nothing especially complicated about the lifting process, and the stresses in the beam are more or less as they will be in the final position. Nevertheless, a heavy object suspended in space is potentially dangerous, and accidents do happen. During the lifting of the suspended span of the Quebec bridge, something broke, and the span fell into the river and was destroyed. During the lifting of one span of the Britannia bridge, a jack burst, and the end of the span fell. Fortunately, Stephenson had given strict orders to insert packing after every few inches of lifting. Nevertheless, a slight distortion of the beam did occur. The incident shows the importance, not only of correct instructions, but of good communications. Accidents have happened because instructions were not received, or if received, modified or ignored. An exception of the method of lifting may be adopted in the case of the suspended span of a cantilever bridge, as in the case of the Forth railway bridge. Here, the beams were built out from the ends of the cantilever arms until they could be joined in the middle. Only then were the temporary rigid connections removed. Such an operation has to be done with great care, because large stresses are released and created. The bending moment at the join completely disappears, for example, while the stress

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in the top chord of the beam changes from tension to compression. Some continuous box girder beams and concrete beams are constructed as cantilevers until they are joined up. New parts may be taken along the existing part of the bridge, or they may be lifted up from below. Sections of a segmented prestressed concrete bridge may be lifted by a crane which rests on previously attached segments, or they may be manipulated by means of a launching girder which is slowly advanced along the already built part of the span. Another method is to use moving formwork which progresses with the construction. Because the cantilevered part span is subject to stresses for which the complete span is not designed, temporary stays are sometimes attached between the segments and a temporary tower, turning the span into a cable-stayed one until closure is complete.

Yet another technique, when the lower surface of the span is horizontal, is to rest the formwork on the steel girder which reaches to the next pier. The casting of each segment is often done using the previous segment as a part of the formwork. This technique, called match-casting, ensures an almost perfect fit at assembly time, and greatly reduces the tendency for misalignments to occur during erection. A film of epoxy may be spread between segments. After each segment is placed, steel pre-stressing wires, or tendons, are fitted to hold it in position. The pre-stressing tension prevents the concrete ever reaching a tensile state with any foreseeable load. This method of pre-stressing is called posttensioning, because the stress is added after placement. When the wires are tensioned before the concrete is poured, the process is called pre-tensioning. The tendons are equivalent to the ties in the lattice truss, while the concrete is roughly equivalent to the struts. Thus the concrete and the steel are both used in the roles for which they are best suited.

The joints between the concrete segments are subject to vertical shear forces, which can be resisted by the use of matching corrugated surfaces.

COMPREHENSION CHECK

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

1 The lifting process is the most complicated in bridge construction. 2 Care should be taken during the lifting of the suspended span.

3 Accidents may happen in spite of correct instructions.

4 The temporary rigid connections were removed when the cantilever arms were constructed.

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5 The stress in the top chord of the beam completely disappears.

6 Cranes are widely used to lift sections of. prestressed concrete bridge.

7 One of the most popular methods in bridge construction is to turn the span into an arch until closure is complete.

8 The casting of each segment is done on the construction site.

9 Each segment is placed after steel prestressing wires are fitted to hold it in position.

10 The post-tensioning method is called so because the stress is added before placement.

II. Answer the questions:

1 Why are heavy objects suspended in space dangerous?

2 What do incidents show?

3 How was the Forth railway bridge constructed?

4 When were temporary rigid connections removed?

5 When does the bending moment at the join completely disappear? 6 How are some continuous box girder beams constructed?

7 What are the methods of lifting sections of a segmented prestressed concrete bridge?

8 How is the casting of each segment done?

LANGUAGE FOCUS

III. Match the words with their synonyms:

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V. Fill in the prepositions:

To be … the final position, the lifting … sth., the end … sth., an exception … sth., … the case of, … the middle, to do … great care, to change … sth. … sth., to rest … sth., to turn … sth., to add … sth., to be equivalent … sth.

VI. Make the precis of the text

Text 3: CAISSON

Caisson in building, is a watertight chamber used in the construction of building foundations, bridges, tunnels, and other structures. Caissons provide an area in which construction crews can safely work underground or underwater. Caissons also may be filled with concrete and become part of the finished structure.

Most caissons have the shape of a cylinder or a box. The walls may be made of steel, concrete, or timber. All caissons are open at the bottom, where digging takes place. But they may be open or closed on top. The two main types of caissons are open and pneumatic. An open caisson has an open top and bottom. The bottom edges, called cutting edges, are constructed so they can cut into the ground. The caisson sinks deeper into the ground as earth is removed beneath it.

A pneumatic caisson is closed at the top. It uses compressed air to keep water out of the working chamber and to provide oxygen for the workers. This type of caisson consists of two sections. The lower section, where the workers are, has cutting edges like those of open caissons. Concrete is poured into the upper section. Its weight helps drive the caisson deeper into the ground. Workers and materials move in and out of the lower section through a shaft. Pneumatic caissons are usually used to support bridge piers that are located in deep water.

Before workers enter a pneumatic caisson, they must step into a large airtight chamber called an air lock. The outer door is closed, and the air pressure in the air lock is gradually increased until it is the same as the pressure in the working chamber. When workers prepare to leave the caisson, they again pass

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through the air lock and the pressure is gradually reduced. If workers go through a change of pressure too quickly, they may develop bends, a painful condition also known as caisson disease. Bends can be fatal in some cases.

COMPREHENSION CHECK

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

1.Construction crews can safely work underwater using a watertight chamber.

2.Most caissons have the shape of a triangle.

3.The walls are usually made of cardboard.

4.Caissons are filled with water after the construction is over.

5.The three main types of caissons are open, pneumatic and closed.

6.An open caisson has an open top and bottom.

7.The top edges are called cutting edges.

8.A pneumatic caisson is closed at the bottom.

9.It uses compressed air to keep water out of the working chamber.

10.This type of caisson consists of two sections.

11.Concrete is poured into the lower section.

12.Workers and materials move in and out of the lower section through bottom edges.

13.When workers enter or leave a pneumatic caisson they must pass through an airtight chamber.

14.If workers stay in a caisson for a long time they may develop bends.

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II. Answer the questions

1.What is caisson in building?

2.What is the shape of caissons?

3.What are the kinds of caissons?

4.What is the difference between an open and pneumatic caisson?

5.What are the sections of a pneumatic caisson?

6.Why is concrete poured into the upper section?

7.What is an air lock?

8.How does an air lock operate?

9.Why is the pressure changed in an air lock?

10.What can happen if workers go through a change of pressure too quickly?

11.Are caissons used nowadays?

LANGUAGE FOCUS

III. Complete the sentence with the best answer (a, b or c) according to the information in the text

1. Caisson in building is a watertight chamber used

a)in the construction of roofs

b)in the construction of buildings

c)in the construction of bridges and tunnels

2. All caissons are open … where digging takes place

a)at the bottom

b)on the top

c)at the cutting edges

3. As earth is removed the caisson sinks

a)under the weight of concrete in its working chamber

b)under the weight of an entrance shaft

c)under the weight of concrete in its upper section

4. Workers and materials move in and out of the lower section

a)through a shaft

b)through a working chamber

c)through a top section

IV. Put the words in the right order

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1.If, may, of with, caissons, are filled, they, become, concrete, part, structure, the, finished.

2.What, made, are, the, of, walls, of caisson?

3.The, sink, caisson, under, can, the, concrete, of, weight.

4.Is, to, keep, air, water, compressed, used, out, of, chamber, the working.

5.Caissons, piers, pneumatic, bridge, support, deep, in, water, located.

6.An, is, to change, air lock, used, the air pressure.

7.Quick, may, change, cause, of, disease, pressure, caisson.

V. Fill in the correct prepositions

VI. Translate the passage and answer the question: What was done to facilitate the setting of a foundation.

The Romans surmounted the problem of building bridge foundation in water by using cofferdams, which were temporary enclosures made of timber that were driven into the river bottom, made watertight with clay and then pumped dry to facilitate the setting of a foundation. This method was long used and only significantly improved with the development of the pneumatic caisson in the 19-th and 20-th centuries. In the early days of caisson use there were many casualties among workers who made a rapid transition from the compressed chamber to normal atmospheric pressure and developed caisson disease.

VII. Make the precis of the text

Text 4: BRIDGE CONSTRUCTION

Beam bridges. All bridges need to be secure at the foundations and abutments. In the case of a typical overpass beam bridge with one support in the middle, construction begins with the casting of concrete footings for the pier and abutments. Where the soil is especially weak, wooden or steel piles are driven to support the footings. After the concrete piers and abutments have hardened sufficiently, the erection of a concrete or steel superstructure begins. Steel beams are generally made in a factory, shipped to the site, and set in place by cranes. For short spans, steel beams are usually formed as a single unit. At the

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site, they are placed parallel to each other, with temporary forms between them so that a concrete deck can be cast on top. The beams usually have metal pieces welded on their top flanges, around which the concrete is poured. These pieces provide a connection between beam and slab, thus producing a composite structure.

For longer spans, steel beams are made in the form of plate girders. A plate girder is an I beam consisting of separate top and bottom flanges welded or bolted to a vertical web. While beams for short spans are usually of a constant depth.

Arch bridges. Arches are normally fabricated on-site. After the building of abutments (and piers, if the bridge is multiarch), a falsework is constructed. For a concrete arch, metal or wooden falsework and forms hold the cast concrete and are later removed. For steel arches, a cantilevering method is standard. Each side of an arch is built out toward the other, supported by temporary cables above or by falsework below until the ends meet. At this point the arch becomes selfsupporting, and the cables or falsework are removed.

Suspension bridges. When bridges requiring piers are built over a body of water, foundations are made by sinking caissons into the riverbed and filling them with concrete. Cassions are large boxes or cylinders that have been made from wood, metal, or concrete. In the case of suspension bridges, towers are built atop the caissons. The first suspension-bridge towers were stone, but now they are either steel or concrete. Next, the anchorages are built on both ends, usually of reinforced concrete with embedded steel eyebars to which the cables will be fastened. An eyebar is a length of metal with a hole (or "eye") at the ends. Cables for the first suspension bridges were made of linked wrought-iron eyebars; now, however, cables are generally made of thousands of steel wires spun together at the construction site. Spinning is done by rope pulleys that carry each wire across the top of the towers to the opposite anchorage and back. The wires are then bundled and covered to prevent corrosion. When the cables are complete, suspenders are hung, and finally the deck is erected—usually by floating deck sections out on ships, hoisting them with cranes, and securing them to the suspenders.

Cantilever bridges. Like suspension bridges, steel cantilever bridges generally carry heavy loads over water, so their construction begins with the sinking of caissons and the erection of towers and anchorages. For steel cantilever bridges, the steel frame is built out from the towers toward the centre

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and the abutments. When a shorter central span is required, it is usually floated out and raised into place. The deck is added last.

The cantilever method for erecting prestressed concrete bridges consists of building a concrete cantilever in short segments, prestressing each succeeding segment onto the earlier ones. Each new segment is supported by the previous segment while it is being cast, thus avoiding the need for falsework.

Cable-stayed bridges. Construction of cable-stayed bridges usually follows the cantilever method (see Figure 3). After the tower is built, one cable and a section of the deck are constructed in each direction. Each section of the deck is prestressed before continuing. The process is repeated until the deck sections meet in the middle, where they are connected. The ends are anchored at the abutments.

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