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Unit 5. Timber and Masonry Bridges.
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Vocabulary.
masonry — каменный
limit — ограничивать
durable — долговечный
weather proof— погодоустойчивый
thawing — оттепель
limestone — известняк
adorn — украшать
elimination— сокращение
density — плотность
pile — свая
cribs — сруб
pine — сосна
larch — лиственница
arid — высушенный
dowel — шпонка
log — бревно
square sawn timber - брус
plywood — фанера
The early bridges were made of stone and timber because these building materials could be easily found everywhere. The earliest type of stone bridge is one that requires no designing. When large flat stones could be found it was a simple enough matter to build piers of square stones in the stream and lay the large flat slabs on the tops of the piers. Obviously, such a construction was very limited in application, for to find a flat stone large enough to span a reasonable distance was hard enough to start with, and when found would be difficult to handle with primitive tools owing to its weight. 1 The local name for these is clapper bridges, and the Postbridge clapper bridge has three spans of 15 feet each. Such bridges are examples of beam or girder bridges in stone.
Stone used for bridge construction must be durable, weather proof and resistant to freezing and thawing. 2 Sometimes the builders use the artificial stone i.e. concrete made of cement, crushed rock or pebbles, sand and water.
As the stones were “dressed”, that is cut, shaped and finished by stone masons, buildings in dressed stone are called masonry. The greater the skill of the masons the longer does the building last, and this applies to bridges as well as houses and castles. Of course, wars and weather play their part too, but other things being equal, a bridge built of carefully wrought masonry will last longer than one stuck together, as it were, with cement or mortar. The Romans knew this, although they were experts at making concrete. 3 But the Romans frequently depended entirely on a good fit between the stones for many of their greatest works.
Today it would require quite careful design by engineers to construct a bridge by up-to-date methods, using modern materials. But the Romans had nothing to guide them but common sense and experience.
The masonry bridges offer the following advantages:
1. Long durability. Some Roman bridges survive to our days. The only reasons for their destruction are wars and disasters.
2. Aesthetic values of these bridges adorn many cities.
3. Greater rigidity under the extra heavy super load.
4. Considerable elimination of maintenance cost.
Possible disadvantages of masonry bridges are:
1. Greater dead weight as a stone density is between 2 and 2.7 t/m3
2. Only the arch structure may be used. It produces the horizontal force – the thrust, which requires powerful foundations and solid ground to rest on.
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3. Masonry bridge construction is difficult to be mechanized. It requires much handwork. So it takes the builders much more time to erect a masonry bridge in comparison with other bridge types.
Timber bridges are used as temporary structures during 10 – 15 years. Piles and cribs made of wood are often applied as bridge foundations. 4 But metal spans save much time during bridge construction because they may be much longer than those made of wood.
The best timber for bridge building is pine, fur-tree and other soft wood as well as larch, arid cedar.
5. The expensive timber species such as oak, hornbeam, and beech are used only for the most important elements – the caps and dowels.
To increase its waterproof, timber is impregnated with antiseptics. It results in the service life prolongation up to 25 – 30 years. Plywood structures are widely used abroad and the spans made of this material are more durable, rigid and lighter than those made of logs and square sawn timber.
A. Examples of bridges of this type are found in Cornwall and Devon, owing to the prevalence of flat granite slabs on the moors, and a good example is still to be seen at Postbridge on Dartmoor.
B. Some of their bridge piers had, in fact, to be demolished by dynamite when the bed of the River Humber was deepened some 1500 years after their construction.
C. This sound wood is easily treated and does not decay.
D. Most favorable rock for this purpose is granite, basalt, dolomite and widespread and rather cheap sandstone and limestone.
E. Wood is also used for bridge supports and spans.
Answer the questions:
1. What were the earliest building materials for bridges?
2. What is the most durable material for bridge building?
3. What building materials are used for stone bridges?
4. Why were the bridges the Romans built very solid and durable?
5. What bridges are difficult to mechanize?
Exercises:
1) Find the equivalents:
каменная кладка
таяние; оттепель
известняк
украшать
исключение
плотность
свая
сруб; клеть; ряж
сосна
лиственница
высушенный
насадка
шпонка
бревно
брус
2) Match the synonyms:
| 1. timber construct temporary eliminate adorn sound durable | 2. solid momentary build wood strong cancel decorate |
3) Describe the following terms:
1. Timber bridge foundations.
2. Timber bridge supports.
3. The basic types of timber bridge spans.
4) Complete the sentences:
1. Possible advantages of timber bridges are minimal construction cost, minimal weight of its elements for transportation and erection, etc.
2. One of the disadvantages of timber bridges is …
3. The best timber species are …
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4. Durability of a building depends on …
5. Most favorable rock for masonry bridges is …
6. While bridge building the Romans were good at …
References:
http://www.wisegeek.org/what-is-a-beam-bridge.htm
http://www.pbs.org/wgbh/nova/tech/build-bridge-p1.html
http://thesaurus.com/browse/applicable
http://dictionary.reference.com/browse/chord?&o=100074&s=t
http://www.bris.ac.uk/civilengineering/bridges/Pages.html
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