The history of bridge construction

The history of bridge construction in Belarus

Unit 2: KINDS OF BRIDGES

Lead in

What parts does a bridge consist of?

How do bridges differ?

Find the following terms and memorize their meaning.

truss bridge suspension bridge cable stayed bridge girder bridge reinforced framework roadway deck truss bridge through truss bridge cantilever bridge stiffening girder radiating pattern fan pattern star pattern viaduct aqueduct

spandrel columns tied arch bridge continuous truss bridge simple span truss bridge anchor arm cantilever arm suspended span side span anchorage cable main cable suspension cable bascule bridge swing bridge vertical lift bridge

Text 1. BASIC FORMS

There are six basic bridge forms: the beam, the truss, the arch, the suspension, the cantilever, and the cable-stay. Figure 11 illustrates these forms and indicates the way loads are carried by showing the approximate location of compression (where material is squeezed together) and tension (where material is stretched).

Beam. The beam bridge is the most common bridge form. A beam carries vertical loads by bending. As the beam bridge bends, it undergoes horizontal compression on the top. At the same time, the bottom of the beam is subjected to horizontal tension. The supports carry the loads from the beam by compression vertically to the foundations.

When a bridge is made up of beams spanning between only two supports, it is called a simply supported beam bridge. If two or more beams are joined rigidly together over supports, the bridge becomes continuous.

Truss. A single-span truss bridge is like a simply supported beam because it carries vertical loads by bending. Bending leads to compression in the top chords (or horizontal members), tension in the bottom chords, and either tension or compression in the vertical and diagonal members, depending on their orientation. Trusses are popular because they use a relatively small amount of material to carry relatively large loads.

Arch. The arch bridge carries loads primarily by compression, which exerts on the foundation both vertical and horizontal forces. Arch foundations must therefore prevent both vertical settling and horizontal sliding. In spite of the more complicated foundation design, the structure itself normally requires less material than a beam bridge of the same span.

Suspension. A suspension bridge carries vertical loads through curved cables in tension. These loads are transferred both to the towers, which carry them by vertical compression to the ground, and to the anchorages, which must resist the inward and sometimes vertical pull of the cables. The suspension bridge can be viewed as an upside-down arch in tension with only the towers in compression. Because the deck is hung in the air, care must be taken to ensure that it does not move excessively under loading. The deck therefore must be either heavy or stiff or both.

Cantilever. A beam is said to be cantilevered when it projects outward, supported only at one end. A cantilever bridge is generally made with three spans, of which the outer spans are both anchored down at the shore and cantilever out over the channel to be crossed. The central span rests on the cantilevered arms extending from the outer spans; it carries vertical loads like in the supported beam or a truss -that is, by tension forces in the lower chords and compression in the upper chords. The cantilevers carry their loads by tension in the upper chords and compression in the lower ones. Inner towers carry those forces by compression to the foundation, and outer towers carry the forces by tension to the far foundations.

Cable-stay. Cable-stayed bridges carry the vertical main-span loads by nearly straight diagonal cables in tension. The towers transfer the cable forces to the foundations through vertical compression. The tensile forces in the cables also put the deck into horizontal compression.

COMPREHENSION CHECK

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

1. There are four basic forms: the beam, the arch, the truss and the suspension.

2. A beam carries horizontal loads by bending.

3. The bottom of the beam is subjected to vertical tension.

4. If two or more beams are joined rigidly together over supports, the bridge is called a simply supported beam bridge.

5. A single – span truss bridge carries vertical loads by bending.

6. Trusses are popular because they use a great amount of material to carry large loads.

7. The arch bridge requires more material than a beam bridge of the same span as it has the more complicated foundation design.

8. The deck of the suspension bridge must be either heavy or stiff or both.

9. A cantilever bridge is generally made with two spans.

10. The tensile forces in the cables also put the deck into horizontal compression.

II. Answer the following questions:

1. What are basic bridge forms?

2. What loads does a beam carry by bending?

3. What is the bottom of the beam subject to?

4. When is a bridge called a simply supported beam bridge?

5. When does the bridge become continuous?

6. Why are trusses popular?

7. How does the arch bridge carry loads?

8. What parts of the suspension bridge are subject to compression?

9. Where does the central span rest in a cantilever bridge?

LANGUAGE FOCUS

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

beam, truss, arch, suspension, cantilever

1. A support on which something is suspended.

2. A large projecting bracket or beam that is supported at one end only.

3. A large long piece of timber ready for use in building or any of the main horizontal supports of a building.

4. A framework of beams or other supports usually connected in a series of triangles and used to form a support for a bridge.

5. A curved structure capable of bearing the weight of the material above it.

IV. Match the words with their synonyms:

1. common 5. exert a) base e) hard

2. carry 6. curved b) strain f) rope

3. foundation 7. cable c) bent g) bear

4. rigid d) usual

V. Fill in the correct prepositions:

1) the location …sth 2) the bottom …sth 3) … the same time

4) to depend … sth 5) a small amount …sth

6) to exert … sth 7) to be transferred …sth 4) to rest …sth

VI. Read the sentences and translate the words in brackets:

1. Engineers (проектируют) bridges of great length and strength to cross the widest rivers.

2. Arches often (образуют) the tops of doors, windows and gateways.

3. The Romans used the semicircular (арки) in bridges, aqueducts and large-scale architecture.

4. Large (балки) carrying the ends of other beams perpendicular to them are usually called girders.

5. For weight reduction beams of metal are formed as an I or other shape having a thin vertical web and thicker horizontal flanges where most of the (нагрузка) appears.

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