Unit 4. Beam bridge

 

Vocabulary.

Beam - балка

Compression – сжатие, компрессия

Tension - напряжение, натяжение, давление

Twisting – закручивание, скручивание

Bending – изгибание, кривизна

Tensile – прочность растяжения при изгибе

To span – соединять, протягиваться,

Dissipate – распределять, уменьшать

Piling – отсыпка, грунт

Daisy-chaining – последовательное соединение, гирляндная цепь

Lead-in

1. What is a bridge? What obstacles are usual to cross?

2. How long can bridges be? How do bridges differ?

3. What types of bridges according to their functions can you name?

4. Look at the illustrations. Which types of bridges do they match?

A.	E.
B.	F .
C.	G.
D.	H.

 

A beam bridge, sometimes called a girder bridge, is a rigid structure that consists of one horizontal beam supported at each end, usually by some kind of pillar or pier. In structural terms, it is the simplest type of bridge and is a popular selection because of its inexpensive construction costs. It began as a felled log supported by opposing river banks that was used to span a river or other body of water. Today, it is commonly made from reinforced concrete or steel beams for everything from pedestrian bridges to highway overpasses.

 

This type of bridge works on the principles of compression and tension, that’s why it needs a strong beam to resist twisting and bending under the weight it must support. When a load, for example a group of traveling cars, pushes down on the beam, the weight of the beam pushes down on the piers. The beam's top edge is pushed together as the result of compression, and tension causes the bottom edge to stretch and lengthen. The top reaches maximum compression while the bottom snaps under too much tension.

Many beam bridges for road construction are made from concrete and steel because these materials are strong enough to bear the forces of compression and tension. The distance a beam can span is directly related to its height, because higher beams offer more material to dissipate tension and need reinforcement.

In spite of reinforcements like concrete, steel, and trusses, this type of bridge is limited by its length, so they rarely span more than 250 feet (76.2 m). But daisy-chaining bridge sections allow to reach longer distances. One of the world's longest bridges is a continuous span beam bridge. It’s located in Louisiana as a pair of parallel bridges of the Lake Pontchartrain Causeway that measure almost 24 miles (38.5 km) long and are supported by 9, 500 concrete pilings.

Answer the following questions:

1. What does a beam bridge consist of?

2. What loads does a beam carry by bending?

3. What materials are frequently used in beam bridge construction? Why?

4. Where are beam bridges most commonly used?

5. What does the distance of a span related to? Why?

6. What the world's longest beam bridges do you know?

Exercises:

1) Decide whether the following statements are true or false according to the text:

1) A beam bridge consists of a rigid horizontal member.

2) Beam is supported at each end by some kind of pile.

3) Less beam bridges are made from concrete and steel to hold out against the forces of compression and tension.

4) Beam bridges are rarely used in highway construction.

5) The biggest limitation of this type of bridge is still its length.

6) The deck of beam bridge is composed of one or several beams, which are supported by piers across the space.

7) Beam bridges typically span the greatest distance.

8) Today beam bridges are not built as foot-bridges.

2) List advantages and disadvantages for the beam construction used. Divide them into two columns:

Limited span; inexpensive relative to other bridge types; does not allow large ships or heavy boat traffic to pass underneath; easy to build; design generally not considered very interesting or eye-catching; used widely in urban and rural settings.

Advantages:	Disadvantages:

 

3) Learn information in the text how tension and compression forces impact bridges. Look at the figure 1 and decide what sort of force is it? Write down the force in the box.

FORCE:	DESCRIPTION OF FORCE ACTING:
	As live loads, such as cars and trucks, travel across the bridge, this force acts on the top of the roadway and passes down into the piers.
	This force acts on the underside of the roadway, which is pulled apart by the live loads pressing down on the top of the roadway.

4) Fill in the missing words from the list:

Adjacent, roadway, vertical piers, span, design, horizontal beams, strength.

Beam bridges are the oldest known bridges and tend to be the simplest to …… and build. Roughly half of all bridges in the world are beam bridges. They consist of……and …… A beam bridge’s ……. depends on the strength of the ……… and can be increased by adding additional piers. While beam bridges can be quite long, the …………, or distance between …….. piers, is usually small.

5) Make up the word combinations using the derivatives from the words in brackets:

(strong), of the material, bridge (construct), (suspend) bridges, (available) of steel, to be (embed) into the banks, (addition) support, ropes (suspend) from rocks, elaborate (scaffold), period of railroad (expand), to cost (consider), one of the most (common) used, concrete (reinforce) with steel bars.

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References: http: //en.wikipedia.org/wiki/Beam_bridge