What is another modern method of surveying?

26. Why are pictures in an aerial survey arranged to overlap in a mosaic? Where does the word mosaic come from?

27. What does geodetic surveying involve?                                                      ^

28. What is the shape of the earth?

29. What other kind of survey is often necessary for civil engineering projects? What does this involve?

30. What are some subsurface factors? What can they determine? Give an example of an area where these factors are critical.

31. How are borings obtained?

32. What does a gravimeter measure?

33. What does a magnetometer measure?

34. What does a seismograph measure? How is it used?

 

2.3. Review

Match the expression

1. Horizontal plane	a. Horizontal and vertical lines within a telescope that permit
2. Vertical plane	focusing.
3. Plane surveying	b. Lines parallel to the horizontal cross hair in a telescope that
4.Geodetic surveying	are used to measure distances.
5. Transit	c. In line with the direction of gravity.
6. Cross hairs	d. A point whose elevation has been determined so it can be
7. Stadia hairs	used as a basis for other measurements.
8. Contour lines	e. A tube of fluid with a bubble of air that is used by
9. Bubble level	surveyors and carpenters to determine a horizontal level.
10. Altimeter	f. Measuring the earth's surface as a flat plane.
11. Bench mark	g. Measuring the earth's surface to take into account its
12. Boring	curvature.
13. Gravimeter	h. A device for measuring the earth's gravitational pull.

 

14. Magnetometer	i. A device that indicates elevation by means of atmospheric pressure.
15. Seismograph	j. A device for measuring the strength of vibrations within the earth.
	k. A device for measuring the strength of the earth's magnetic field.
	l. Perpendicular to gravity.
	m. Lines on a map that enclose areas of equal elevations.
	n. A sample of subsurface soil and rock obtained by a hollow drill.
	o. A telescopic device to measure distances and horizontal and vertical angles.

UNIT THREE

MODERN BUILDINGS AND STRUCTURAL MATERIALS

Special Terms

Thrust: The pressure or force exerted by each part of a structure on the other parts. Post-and-lintel: A type of construction in which a horizontal beam rests on two vertical supports.

Shear: The tendency of a material to fracture or break along the lines of stress, the force that strains or deforms a structure or its parts.

Flying Buttress: A support for walls or pillars that absorbs their outward and downward thrust. They were used in building the Gothic cathedrals.

Dead Load: The total weight of all the materials in a structure; in other words, the weight of the structure itself.

Live Load: The weight that will be added to a structure as a result of its use- people, furniture, cars, machines, and so on.

Impact: The force at which the live load will be exerted on a structure. Cars have a greater impact than people, but airplanes have a greater impact than cars. Compression: The force that presses or pushes two or more materials Tension: The force that stretches or pulls apart a material.

Neutral Plane: The place in a material where there is neither tension nor compression. Masonry: Building materials such as rock, brick, or tile, often bound together by mortar or cement.

Pozzolana: A natural cement used by the Romans.

Bitumen: A binding agent made from natural tars.

Tensile Strength: The ability of a material to retain its strength under tension. Fatigue: The tendency of a material to weaken as a result of continual changes in stress.

Steel: An alloy or mixture of iron with a small amount of carbon or some other substance that increases its strength. Steel has great tensile strength.

Portland Cement: A mixture of limestone and clay, which is then heated and ground into a powder.

Aggregate: Small stones or gravel or crushed rock used as a building material. Concrete: A mixture of cement, aggregate, sand, and water. It is the most important material for masonry construction in modem times. It has great strength under compression.

Reinforced Concrete: Concrete with steel rods embedded in it to increase its tensile strength and resistance to shearing forces.

Bond: A union between two materials.

Prestressed Concrete: A variety of reinforced concrete in which the steel rods are pulled tight in advance before pouring concrete to give the material the greatest strength possible.

Grout: A thin mortar or cement used as a binding agent.

Curtain Wall: A wall supported by a steel or concrete structure of a building. A curtain wall does not have any support function itself, as does a bearing wall. Riveting: The process of fastening together pieces of metal with rivets that do not have threads.

Welding: The process of fastening together pieces of metal by melting a bonding metal between similar metals.

Polymers: Long chainlike compounds of elements, principally hydrogen and carbon. They are the basis for many plastics and are now being used to produce concrete with more strength and durability. Accordingly, they help cut concrete weight to some extent.

3.1. Vocabulary Practice

1. What is thrust?

2. What does post-and-lintel construction indicate?

3. What is shear?

4. What kind of buildings used flying buttresses? What was their purpose?

5. What is the difference between dead load and live load?

6. What is impact?

7. What is the difference between compression and tension?

8. What is the neutral plane of a material?

9. What is masonry made of?

10. What are pozzolana and bitumen?

11. What is tensile strength?

12. What is fatigue?

13. What is steel?

14. What is Portland cement?

15. What is aggregate?

16. What materials are combined to form concrete?

17. What is reinforced concrete?

18. What does bond mean?

19. How is the strength of prestressed concrete increased?

20. What is grout?

21. What is the difference between a curtain wall and a bearing watt?

22. What is the difference between riveting and welding?

23. What are polymers? Why are they now being mixed with concrete?

 

                            Modern Buildings and Structural Materials.

Many great buildings built in earlier ages are still in existence and in use. Among them are the Pantheon and the Colosseum in Rome, Hagia Sophia in Istanbul, the Gothic churches of France and England, and the Renaissance cathedrals, with their great domes, like the Duomo in Florence and St. Peter's in Rome. They are massive structures with thick stone walls that counteract the thrust of their great weight. Thrust is the pressure exerted by each part of a structure on its other parts.

All of these buildings and many others represent engineering solutions to challenging problems. The Romans made extensive use of the arch to distribute thrust more evenly, thus making larger openings possible. Architects and engineers before the Romans had used a post-and-lintel construction for the most part, with two vertical columns supporting a horizontal beam. If the beam is too long, or if it has to support too much weight, it is subject to shear, the tendency to fracture or break along the lines of stress. Stress is the force or pressure that tends to strain or deform a structure or its various parts.

In the Gothic cathedrals, stone pillars that had comparatively thin walls between them were raised to great heights. The cathedral at Beauvais, France, reached a height of 157 feet, about the same as a modem building of fifteen floors. The vault collapsed on the first attempt to raise it to such a height, but it was later rebuilt and still stands. The great stone ribs of the Gothic cathedrals were supported by flying buttresses that absorbed the outward and downward thrust. In great domed structures like Hagia Sophia or St. Peter's, the thrust was distributed by a series of arches or half-arches that were supported by enormous piers (vertical supports).

These great buildings were not the product of knowledge of mathematics and physics. They were constructed instead on the basis of experience and observation, often as the result of trial and error. One of the reasons they have survived is because of the great strength that was built into them-strength greater than necessary in most cases. But the engineers of earlier times also had their failures. In Rome, for example, most of the people lived in insulae, great tenement blocks that were often ten stories high. Many of them were poorly constructed and sometimes collapsed with considerable loss of life.

Today, however, the engineer has the advantage not only of empirical information, but also of scientific data that permit him to make careful calculations in advance. When a modem engineer plans a structure, he takes into account the total weight of all its component materials. This is known as the dead load, which is the weight of the structure itself. He must also consider the live load, the weight of all the people, cars, furniture, machines, and so on that the structure will support when it is in use. In structures such as bridges that will handle fast automobile traffic, he must consider the impact, the force at which the live load will be exerted on the structure. He must also determine the safety factor, that is, an additional capability to make the structure stronger than the combination of the three other factors.

The modem engineer must also understand the different stresses to which the materials in a structure are subject. These include the opposite forces of compression and tension. In compression the material is pressed or pushed together; in tension the material is pulled apart or stretched, like a rubber band. In the diagram alongside, the top surface is concave, or bent inward, and the material in it is in compression. The bottom surface is convex, or bent outward, and the material in it is in tension. When a saw cuts easily through a piece of wood, the wood is in tension, but when the saw begins to bind, the wood is in compression because the fibers in it are being pushed together.

In addition to tension and compression, another force is at work, namely shear, which we defined as the tendency of a material to fracture along the lines of stress. The shear might occur in a vertical plane, but it also might run along the horizontal axis of the beam, the neutral plane, where there is neither tension nor compression.

Altogether, three forces can act on a structure: vertical-those that act up or down; horizontal-those that act sideways; and those that act upon it with a rotating or turning motion. Forces that act at an angle are a combination of horizontal and vertical forces. Since the structures designed by civil engineers are intended to be stationary or stable, these forces must be kept in balance. The vertical forces, for example, must be equal to each other. If a beam supports a load above, the beam itself must have sufficient strength to counterbalance that weight. The horizontal forces must also equal each other so that there is not too much thrust either to the right or to the left. And forces that might pull the structure around must be countered with forces that pull in the opposite direction.

One of the most spectacular engineering failures of modem times, the collapse of the Tacoma Narrows Bridge in 1940, was the result of not considering the last of these factors carefully enough. When strong gusts of wind, up to sixty-five kilometers an hour, struck the bridge during a storm, they set up waves along the roadway of the bridge and also a lateral motion that caused the roadway to fall. Fortunately, engineers learn from mistakes, so it is now common practice to test scale models of bridges in wind tunnels for aerodynamic resistance.

The principal construction materials of earlier times were wood and masonry- brick, stone, or tile, and similar materials. The courses or layers were bound together with mortar or bitumen, a tarlike substance, or some other binding agent. The Greeks and Romans sometimes used iron rods or clamps to strengthen their buildings. The columns of the Parthenon in Athens, for example, have holes drilled in them for iron bars that have now rusted away. The Romans also used a natural cement called pozzolana, made from volcanic ash, that became as hard as stone under water.

Both steel and cement, the two most important construction materials of modem times, were introduced in the nineteenth century. Steel, basically an alloy of iron and a small amount of carbon, had been made up to that time by a laborious process that restricted it to such special uses as sword blades. After the invention of the Bessemer process in 1856, steel was available in large quantities at low prices. The enormous advantage of steel is its tensile strength; that is, it does not lose its strength when it is under a calculated degree of tension, a force which, as we have seen, tends to pull apart many materials. New alloys have further increased the strength of steel and eliminated some of its problems, such as fatigue, which is a tendency for it to weaken as a result of continual changes in stress.

Modem cement, called Portland cement, was invented in 1824. It is a mixture of limestone and clay, which is heated and then ground into a powder. It is mixed at or near the construction site with sand, aggregate (small stones, crushed rock, or gravel), and water to make concrete. Different proportions of the ingredients produce concrete with different strength and weight. Concrete is very versatile; it can be poured, pumped, or even sprayed into all kinds of shapes. And whereas steel has great tensile strength, concrete has great strength under compression. Thus, the two substances complement each other.

They also complement each other in another way: they have almost the same rate of contraction and expansion. They therefore can work together in situations where both compression and tension are factors. Steel rods are embedded in concrete to make reinforced concrete in concrete beams or structures where tension will develop. Concrete and steel also form such a strong bond - the force that unites them - that the steel cannot slip within the concrete. Still another advantage is that steel does not rust in concrete. Acid corrodes steel, whereas concrete has an alkaline chemical reaction, the opposite of acid.

Prestressed concrete is an improved form of reinforcement. Steel rods are bent into the shapes to give them the necessary degree of tensile strength. They are then used to prestress concrete, usually by one of two different methods. The first is to leave channels in a concrete beam that correspond to the shapes of the steel rods. When the rods are run through the channels, they are then bonded to the concrete by filling the channels with grout, a thin mortar or binding agent. In the other (and more common) method, the prestressed steel rods are placed in the lower part of a form that corresponds to the shape of the finished structure, and the concrete is poured around them. Prestressed concrete uses less steel and less concrete. Because it is so economical, it is a highly desirable material.

The availability of steel and concrete, together with the elevator, which was also developed in the nineteenth century, have made possible the most characteristic kind of modem structure: the steel or concrete frame building. Not only towering modem skyscrapers, but also many less gigantic and spectacular buildings have a skeleton of steel or concrete that bears the weight of the structure. Until this type of construction became possible, the exterior walls - called bearing walls - had to carry the weight of the building. This meant that the walls on the lower floors of a tall building had to be tremendously thick. The old Pulitzer Building in New York, for example, built about the same time as the first steel-frame building, the Home Insurance Building in Chicago, and about the same height, had walls that were nine and one-half feet thick at the base.

Since the weight of modem structures of this type is carried by the steel or concrete frame, the walls do not support the building. They have become curtain walls, which keep out the weather and let in light. In the earlier steel frame building, the curtain walls were generally made of masonry; they had the solid look of bearing walls. Today, however, curtain walls are often made of lightweight materials such as glass, aluminum, or plastic, in various combinations.

Another advance in steel construction is the method of fastening together the beams. For many years the standard method was riveting. A rivet is a bolt with a head that looks like a blunt screw without threads. It is heated, placed in holes through the pieces of steel, and a second head is formed at the other end by hammering it to hold it in place. Riveting has now largely been replaced by welding, the joining together of pieces of steel by melting a steel material between them under high heat.

Prestressed concrete has made it possible to develop buildings with unusual shapes, like some of the modem sports arenas, with large spaces unbroken by any obstructing supports. The uses for this relatively new structural method are constantly being developed.

The current tendency is to develop lighter materials. Aluminum, for example, weighs much less than steel but has many of the same properties. Aluminum beams have already been used for bridge construction and for the framework of a few buildings.

Attempts are also being made to produce concrete with more strength and durability, and with a lighter weight. One system that helps cut concrete weight to some extent uses polymers, which are long chain like compounds used in plastics, as part of the mixture.

3.2. Discussion

1. What are some of the buildings built in earlier ages that are still in existence and in use?

2. What is thrust? How is it counteracted in these great buildings of earlier times?

3. How did the Romans solve the problem of thrust?

4. What kind of stress can post-and-lintel construction be subject to?

5. How were the high pillars and walls of Gothic cathedrals often supported?

6. How was thrust distributed in great domed structures like Hagia Sophia or St. Peter’s?

7. Why have these buildings from earlier times survived? What is an example of engineering failure in ancient times?

8. What advantage do the engineers of modem times have over those of earlier days?

9. What four factors must an engineer take into account when he plans a structure?

10. What stresses on the materials that he uses must the engineer understand?

11. Where might a material fracture? What force causes this kind of fracture?

12. What are the three forces that can act on a structure?

13. How must these three forces work in relation to each other?

14. What was one of the most spectacular engineering failures of modem times? What happened?

15. What has been the result of this failure?

16. What were the principal construction materials of earlier times?

17. How did the Greeks and Romans use iron? Give an example.

18. What kind of cement did the Romans use?

19. When were steel and cement introduced?

20. Why had steel been restricted to special uses prior to 1856?

21. What enormous advantage does steel have as a construction material?

22. How have new alloys of steel increased its strength?

23. What is modem cement made from? How is it used to make concrete?

24. What are the advantages of concrete as a construction material?

25. In what ways do steel and concrete complement each other? How do these factors work together in reinforced concrete?

26. What are two methods for prestressing concrete? Why is prestressed concrete highly desirable?

27. What method of construction have steel and concrete made possible?

28. What is the difference between the walls in buildings built with and without steel or concrete frames? Give an example.

29. What is the function of curtain walls? What kind of materials do they often use today?

30. What method was used for fastening together steel beams for many years?

31. What method is now usually employed to fasten steel beams together?

32. What kind of structures has it been possible to build with prestressed concrete?

33. What kind of materials are currently being developed? Give examples.

3.3. Review

A. Match the expression on the left with the statement on the right.

1. Thrust	a. The force at which the live load will be exerted on a
2. Stress	structure.
3. Shear	b. A binding material made of limestone and clay, heated
4. Dead load	and ground to a powder.
5. Live load
6. Impact 7. Compression 8. Tension	c. The tendency of a material to weaken because of continual changes in stress. d. A masonry material made of a mixture of cement,
9. Masonry 10. Tensile strength 11. Fatigue 12. Steel	aggregate, sand, and water. e. A wall supported by a steel or concrete frame; it does not bear the weight of the structure. f. The force that presses or pushes a material together.
13. Portland cement	g. The tendency of a material to break along lines of
14. Concrete	stress.
15. Grout	h. The weight of all the materials in a structure.
16. Riveting 17. Welding 18. Curtain wall	i. The pressure or force exerted by each part of a structure on the other parts. j. Fastening pieces of metal together with bolts. k. A thin mortar or cement used as a binding agent. l. The weight that will be added to a structure as a result of its use. m. The different forces that can strain or deform a structure or its parts. n. The force that stretches or pulls apart a material. o. Fastening together pieces of metal under high heat.

 

	p. Materials such as stone, brick, or concrete used for
	construction.
	r. The ability of a material to retain its strength under
	tension.
	s. An alloy or mixture basically of iron and a small
	amount of carbon.

 

UNIT FOUR

TRANSPORTATION SYSTEMS

Special Terms

Fooling: The surface on which a foundation rests.

Wearing Surface: The top level of a road that receives the wear of traffic.

Base Course: The level of a road between the footing and the wearing surface; it is usually made of crushed rock.

Crown: A slight convex curve in the surface of a road to permit drainage. It is also called camber.

Compacting: Pressing down in order to make a material like sand or soil denser and firmer.

Traffic Engineering: The design of highways for high-speed, heavy-volume traffic, emphasizing the concept of maximum use at minimum cost.

Cloverleaf Interchange: An intersection between two roads designed like a four-leaf clover. It permits cars to change from one highway to another without interrupting the flow of traffic.

Cut; Fill: A cut is a land area from which earth is being removed. A fill is a land area where earth is being deposited.

Soil Mechanics: A new science that deals with the classification of different kinds of soils according to density, stability, and so on.

Bulldozer; Vibrating Roller: Earth-moving machines. The bulldozer pushes earth aside; the vibrating roller compacts earth.

Concrete Train: A group of machines that are used together to carry out the different steps in laying a concrete wearing surface for a highway.

Slipform Paver: A single machine which can perform almost all the same jobs that a concrete train can.

Runway: The paved strip on which airplanes land or take off at an airport.

Urban Rapid Transit Systems: Public transportation systems in cities. They include subways (underground railways), elevated railways, and bus and trolley lines. Each system is designed for and operated at higher speeds.

Superelevation: Raising the outer edge of a road or railroad line above the inner edge on a curve; in other words, banking the curve.

Ballast: Crushed rock used on top of the footing of a roadbed for a railway line. Sleepers: Pieces of timber, also called crossties or ties, that are laid perpendicular to the line on top of the ballast. The rails are usually spiked but sometimes are bolted to the sleepers.

Monorail: A railroad with only one rail, usually elevated.

Vacuum: A space from which most of the air and other substances have been removed.

4.1. Vocabulary Practice

1. What does footing mean?

2. What is the wearing surface of a road?

3. What is the base course of a road?

4. What is a crown in a road? What is another term for crown?

5. What does compacting the soil mean?

6. What is traffic engineering?

» '

7. What is a cloverleaf interchange?

8. What is the difference between a cut and a fill?

9. What does soil mechanics deal with? What are soil stabilization techniques used for?

10. What do bulldozers and vibrating rollers do?

11. What is the difference between a concrete train and a slip form paver?

12. What is the runway at an airport?

13. What do urban rapid transit systems include?

14. What does banking mean? What is a technical term for the same thing?

15. How is ballast used in railroad construction?

16. How are sleepers or crossties used in railroad construction?

17. What is a monorail?

18. What is a vacuum?

Transportation Systems

Transportation has always been one of the most important aspects of civil engineering. One of the great accomplishments of the Roman engineers was the highway system that made rapid communication possible between Rome and the provinces of the empire. The first school that offered training specifically in engineering was the Ecole des Ponts et Chaussees, the School of Bridges and Highways, established in France in 1747. And in England, Thomas Telford, a roadbuilder, became the First president of the Institution of Civil Engineers in 1820.

Modem highways are still built according to the principles laid down in the eighteenth and early nineteenth centuries by a Frenchman, Pierre Tresaguet, the Englishman Telford, and a Scot, John L. McAdam, whose name has passed into English in the words macadam, macadamize, and tarmac. These men designed the first modem roads that had a firm footing, the surface on which the foundation rested. Their roads also included good drainage and a wearing surface - the top level that directly receives the wear of traffic - -that could not be penetrated by water. Both Tresaguet and Telford used a heavy foundation of stones, on top of which a base course of lighter crushed stones and a wearing surface of still smaller stones were built up. Their roads were also slightly curved in a crown or camber so that the water would run off. McAdam realized that the soil itself could bear the weight of the road when it was compacted or pressed down, as long as it remained dry. He was able therefore to eliminate the heavy cost of the stone foundation by laying a base course of crushed stone on top of a compacted footing. The iron wheels of the carriages of his day ground the stones of the top level into a continually smoother and more watertight surface.

These roads were adequate during the nineteenth century when wagons and carriages had tires made of iron or steel. When the automobile appeared at the beginning of the twentieth century, however, its rubber tires broke up the smooth surfaces. Therefore, the top layer was bound together more firmly by mixing the crushed rock with tar or asphalt. Millions of kilometers of roads throughout the world today have this kind of surface.

Basically, roadbuilding has improved in only two ways in the twentieth century. The first improvement involves the use of concrete for the wearing surface. The other is traffic engineering, the design of highways for high-speed, heavy- volume traffic, highways that are economical to build and safe for vehicles and their passengers. Traffic engineering has produced the modern express highway, or freeway, that has only limited access and maximum safety controls. The angular intersections common on older roads have been eliminated in favor of cloverleaf interchanges or others with even more complicated designs. Modem freeways usually have special lanes where traffic can either slow down before exiting or speed up upon entering. Extreme curves or steep slopes are minimized so that the traffic can continue to move without slowing down. Since monotony has proved to be a safety hazard, traffic engineering even includes the landscaping of the borders of the road.

When construction on a new highway begins, huge earth-moving machines called bulldozers level the ground along the designated route. The amount of earth to be moved, both in leveling and filling, has been previously calculated. Wherever possible, the amount in a cut where earth is being removed should be equal to the amount needed for a nearby fill. Moving earth from a distant point is extremely expensive, and economy is a critical aspect of an engineer's work.

After the earth has been moved and shaped according to the design of the road, other machines prepare the footing. The most important of these is probably a vibrating roller, which compacts the earth until it can bear the weight of the base course and wearing surface that will rest on it. In many cases, however, the soil must be stabilized by mixing some other material with it. This may be bitumen or a grout of concrete or some other substance. The new and complex science of soil mechanics classifies soils and relates those classifications to their load-bearing capacity in a number of different ways.

The base course, which is made of either crushed stone or a layer of thinly- mixed concrete, comes next; the wearing surface, which may be a layer of asphalt or a series of reinforced concrete slabs, is then laid. A concrete surface must be laid in segments separated by joints to allow for expansion and contraction under differing weather conditions. One method of laying a reinforced concrete wearing surface is to put down the steel rods, usually in the form of a grill or mesh, after a certain proportion of the concrete has been poured. The top level must be poured within twenty minutes of the pouring of the bottom level to assure proper bonding. Another method is to pour the entire thickness of concrete and then force the steel mesh down into it to a predetermined level.

A group of machines collectively called a concrete train usually perform these operations at a rate of about three-fourths of a meter per minute. A single new machine called a slipform paver can perform all the different jobs of a concrete train except laying the reinforcing mesh. However, a technique has been worked out in which the mesh is held in place before the machine passes over it and pours concrete around it. The slipform paver can operate at a rate of about two meters per minute.

The construction of airport runways follows similar procedures, except that the slabs in a runway are much wider than those in a highway. Airport runways must also be designed to carry the heavy load of the big modem jumbo jets, as well as to withstand enormous blasts of heat from the engines of the aircraft. The impact factor, mentioned in the previous unit, is also greater with airplanes than with automobiles. Highways differ from airport runways, however, in size and strength rather than design and construction methods.

If huge concrete freeways are the transportation phenomenon of this century, then the railroads were the transportation phenomenon of the nineteenth century. Transportation by rail was not unknown before the invention of the steam locomotive in 1829, but rail lines had been limited to the short distances over which horses or other draft animals pulled the load. Most of the lines were built in conjunction with mining operations. After the steam locomotive became a practical machine, a surge of railroad construction rapidly spread a network of railroads across the face of the world.

By 1920 the great age of railroad building generally seemed to be over. As automobiles came into increasingly wider use, the emphasis shifted to highway construction. Except for a few improvements such as electrification, railroads were neglected. Now, however, there is a renewed interest in railroads. The Japanese, for exat»ple, have recently opened the Tokaido line between Tokyo and Osaka, with express trains that average 166 kilometers per hour and reach speeds as high as 210 kilometers per hour in normal operation. Even in the United States where many of the railroads are in deplorable condition, especially in the eastern part of the country, there is an increased interest in providing better service. There is also considerable interest - not only in the United States but in many other countries as well - in urban rapid transit systems. New subway (underground) lines have opened in Montreal, Mexico City, and San Francisco, and at this writing, lines are under construction in Washington, D.C.

The construction of railroad lines is undertaken only after an extensive survey of the route. Every phase of the project must be studied in minute detail. Steam locomotives did not operate efficiently at a grade of more than 2 percent. This fact resulted in a great deal of bridge and tunnel construction, as we shall see in the next two units, in order to keep the lines from rising at too steep an angle. The curves also had to have a longer radius than is necessary for an automobile road. Railroad curves are always banked, with the outer rail raised above the inner one. This is known as superelevation, and it is also a principle in road design.

In conventional railroad building methods, a load of crushed rock is laid after the footing has been prepared. The crushed rock is called ballast, and it serves both to distribute the load and to provide drainage. Sleepers, large pieces of timber also called crossties or ties, are placed at regular intervals on top of the ballast. The steel rails, I-shaped beams, are then bolted to the ties. When railroad lines run over swampy or unstable ground, they are sometimes supported by layers of timber and smaller pieces of wood that form a sort of raft on which the ballast and tracks are placed.

In the last few years, important technological advances in railroad building have taken place. Soil mechanics and soil stabilization techniques have been used in the construction of footings for railroad lines as well as for other kinds of construction. Prestressed concrete sleepers that have been designed to replace the conventional wooden ones are being tested, and welded rails have begun to replace jointed rails. All these developments permit higher speeds and a smoother ride. Experiments are being carried on with monorails and other new concepts. One revolutionary proposal is for a railroad between Washington and Boston in which the trains would be pushed through a vacuum tube by means of atmospheric pressure.

The automobile has many advantages, but it has also proved to have disadvantages-air pollution and a high energy consumption are probably the most serious. These two negative factors are behind the increased effort to find new and improved methods for mass transportation systems.

Discussion

1. Whose principles of road-building are still applied to the building of modem highways?

2. What elements were included in the roads that these men designed?

3. How did the roads designed by McAdam differ from those designed by Tresaguet and Telford?

4. Why were Me Adam's roads adequate in the nineteenth century but inadequate in the twentieth?

5. How was the surface of such roads first improved to handle automobile traffic?

6. In what two ways was road-building improved in the twentieth century?

7. What kind of roads has traffic engineering produced?

8. What is the first step in the construction of a new road?

9. Why should the civil engineer carefully calculate the amount of earth to be moved?

10. What kind of machine is probably used to prepare the footing for the road? What does it do?

11. How can the soil be stabilized? On what do soil stabilization techniques depend?

12. What are the base course and the wearing surface of modern highways usually made of?

13. Why must a concrete surface be laid in segments?

14. What two methods are used to lay a reinforced concrete surface?

15. What machines are used for laying the concrete slabs of a highway? How is the reinforcing mesh laid with the second machine? How quickly do they work?

16. How do airport runways differ from highways? What special factors must be considered in constructing runways?

17. When was the steam locomotive invented? What rail lines were in existence before that time?

18. Why did the great age of railroad building generally seem to be over by 1920?

19. What is one example of the renewed interest in railroad building?

20. In what other kind of transportation system is there considerable interest?

21. What must be done before construction of a railroad line can begin?

22. At what grade do steam locomotives stop operating efficiently? What has this resulted in?

23. What special considerations were necessary in designing curves on railroad lines?

24. What steps are followed in conventional railroad building methods?

25. What sort of base is laid down when a railroad line passes over swampy or unstable ground?

i

26. What technological advances have taken place in railroad construction in the last few years? What do these permit?

27. What are some experimental or proposed forms of rail transportation systems?

28. What disadvantages does the automobile have that have caused an increased interest in new mass transportation systems?

4.3. Review

A. Match each expression on the left with one of the statements on the right.

1. Footing	a. The paved strip on which aircraft land or take off at an
2. Base course	airport.
3. Wearing surface	b. A place where earth is deposited or dumped during
4. Crown	construction.
5. Compacting	c. A machine used to compact soils.
6. Cut	d. The surface on which a foundation rests.
7. Fill	e. A wood or prestressed concrete beam to which the rails
8. Soil mechanics	for a railway line are fastened.
9. Soil stabilization	f. A new science dealing with the classification of different
10. Vibrating roller	types of soils.
11. Concrete train	g. A layer of crushed rock that rests on a footing for a road.
12. Slipform paver	h. Crushed rock laid on top of a footing for a railroad

 

13. Runway 14. Ballast	roadbed. i. A single machine that lays a concrete wearing surface for
15. Sleeper (crosstie)	a highway. j. The top level of a road that directly receives wear from traffic.
	k. A convex curve in the surface of a road that permits drainage. l. Techniques for making soils denser and firmer so they can bear a heavier load. m. A place from which earth is removed during construction. n. A group of machines that work together to lay a concrete wearing surface for a highway. o. Pressing down soil to make it denser and firmer.

 

B. Recently, the world has been facing an energy crisis because of the increased consumption, short supply, and high prices of fuels, particularly petroleum products like gasoline. Discuss the possibilities for engineering new transportation systems or improving present ones so that they would consume less fuel and still provide adequate service.

 

UNIT FIVE BRIDGES

Special Terms

Span: The distance between two supports of a bridge. Span is also used as a verb, as in: The bridge spans a distance of 200 meters.

Masonry Arch Bridge: A bridge of masonry with arches between piers. Today the term usually refers to bridges made of stone.

Pontoon: A hollow drum that can float. Pontoons are used as supports for a pontoon bridge. Beam Bridge: The simplest kind of bridge. It consists of a rigid beam between two supports.

Suspension Bridge: A bridge supported by cables that are usually hung from towers.

Truss: A framework strengthened by diagonal beams that form triangles with horizontal and vertical beams. Trusses are used to strengthen beam bridges.

Cantilever: A type of structure in which a horizontal beam extends beyond its support. A cantilever bridge is a type of beam bridge.

Steel Arch Bridge: A bridge with an arch made with steel beams.

Deck: The roadway or trafTic-bearing surface of a bridge.

Stay: A cable that runs at a diagonal from the main supporting cable to the deck of a suspension bridge.

Concrete Arch Bridge: A bridge with an arch made of reinforced concrete. It is really a kind of masonry arch bridge, but it can span a much greater distance than a stonearch.

Lift Bridge: A movable bridge with a span that is raised by elevators.

Swing Bridge: A movable bridge with a span that swings open parallel to the channel. It is also known as a pivot bridge.

Bascule Bridge: A movable bridge with a span that is raised at an angle by means of a counterweight. There are two kinds: single-leaf with one section, and double-leaf with two sections.

Cofferdam: A watertight enclosure made of piles or steel sheets sunk into a water bed. It can be pumped dry so that construction work can be done inside it.

Pneumatic Caisson: A cylinder with a cutting edge that can be sunk into the water bed. Water is forced out by compressed air.

The Bends: A crippling or fatal condition, caused by excess nitrogen in the blood, that can result from working in compressed air. It is also known as caisson disease. Keying: Extending the piers of a bridge into bedrock instead of simply resting them on top of the rock.

Falsework: The temporary scaffolding used to support an arch while it is being constructed.

Crane: A device that moves and lifts heavy weights. It is also called a jack. Anchorage: The terminal support, usually a huge block of concrete, for the main cables of a suspension bridge.

5.1. Vocabulary Practice

1. What is a span? Use the word as a verb.

2. What does the term masonry arch bridge usually refer to today?

3. What is a pontoon? How is it used in connection with bridges?

4. What is a beam bridge?

5. What kind of bridge is a suspension bridge?

6. What is a truss? How are trusses used in bridges?

7. What is a cantilever?

8. What is a steel arch bridge?

9. What does deck refer to in connection with a bridge?

10. What is a stay?

11. How does a concrete arch bridge differ from other masonry arch bridges?

12. What is a lift bridge?

13. What is a silting bridge? What is another term for a swing bridge?

14. What is a bascule bridge? What two different kinds are there?

15. What is a cofferdam? What does it make possible?

16. What is a pneumatic caisson?

17. What are the bends? What is another name for this condition?

18. What does keying mean?

20. What is a crane?

21. What is the anchorage of a suspension bridge?

Bridges

Bridges are among the most important, and often the most spectacular, of all civil engineering works. The imposing bridges that have survived from ancient times are arched structures of heavy masonry, usually stone or brick. Herodotus, the Greek historian of the fifth century B.C., however, mentions a wooden bridge across the Euphrates River at Babylon. In Rome, the bridge of Fabricius, built in 62 B.C. and named for its engineer, still carries traffic across the Tiber River, as does the Sant'Angelo Bridge, built in about 136 A.D. Both of these bridges, and many other Roman bridges, have a series of arche supported by heavy piers that extend down to bedrock. Ancient sources also mention pontoon bridges, usually in connection with military operations. A pontoon is a hollow drum that can float; a series of pontoons anchored to a riverbed can support a roadway. The Incas of pre-Columbian Peru built remarkable suspension bridges, supported by cables of natural fibers, that crossed many of the deep gorges in their mountainous country.

The sudden expansion in transportation systems that began in the eighteenth century, and still continues in our own day, has enormously increased the need for bridge^ as a part of highways and railroads. Better understanding of the forces that are exerted on structures and the improved materials that became available in the nineteenth century have made it possible to build increasingly longer and stronger bridges. With the ability to span greater distances, the damlike effect of masonry arch bridges with several heavy piers that block the flow of a stream can be largely eliminated.

The simplest type of span is a beam bridge, consisting of a rigid beam between two supports. Today most simple beam bridges are strengthened by a truss, which is based on the triangle. Diagonal beams that extend between the horizontal and vertical beams give support against both compression and tension. Many early truss bridges were built of wood; one that was erected across the Susque-hanna River in Pennsylvania in 1815 had a span of 110 meters. Iron and then steel were later used in the construction of truss bridges, still further increasing their strength. Trusses are not only strong but also light, because all unnecessary material has been eliminated in their design.

Another type of beam bridge is the cantilever, in which a horizontal beam extends beyond its support. Cantilever bridges, like trusses, had also been built before iron and steel became available. Most cantilever bridges have two arms of truss structure that meet or support a section between them. Cantilevers enabled bridge builders to span longer distances than truss bridges. During the nineteenth century, cantilevers were frequently used to build railroad bridges. The Quebec Bridge, which crosses the St. Lawrence River in Canada, is the longest cantilever bridge in the world, with a span of 540 meters. It was completed in 1917, and until 1929 it was the longest bridge of any type in the world.

A third type of modern bridge is the steel arch bridge, which can carry a roadway either above or below its arch of steel beams. An arch exerts strong downward and diagonal thrusts, so the piers that support it must be especially strong. Probably the most famous steel arch bridge is the Sydney Harbor Bridge in Australia, with a span of 495 meters. The Bayonne Bridge between New Jersey and Staten Island in New York has a span one meter longer.

Suspension bridges span even longer distances than other types of bridges. The longest bridge of any type is the Verra-zano-Narrows Bridge in New York, with a span of 1,280 meters. The deck or roadway of a suspension bridge is suspended from steel cables that are supported by massive towers. The first modem suspension bridges used linked chains made of wrought iron. Some of them survived for many years, like one across the Danube River in Budapest, Hungary. It was completed in 1849 and destroyed during World War II, nearly a hundred years later.

When steel became available, cables of steel wires replaced chains of wrought iron. Several suspension bridges built in this manner collapsed, however, as a result of storms or the movement created by the rhythm of the loads moving across them. It was later discovered that these failures were caused by the lack of truss supports for the deck. The first major cable-type suspension bridge to overcome these faults was designed by John A. Roebling at Niagara Falls. Its span of 250 meters was strengthened by trusses between the two decks. Roebling also used stays, inclined cables that ran from the main supporting cables to the deck, to stabilize the bridge. Roebling went on to design the Brooklyn Bridge in New York, which was completed in 1883 by his son, George Washington Roebling. The Brooklyn Bridge, with a span of 486 meters, is one of the most important - and one of the most esthetically satisfying - bridges ever built. The method devised by the Roeblings for laying the component wires that make up the cables for the Brooklyn Bridge is essentially the same technique used today.

The development of reinforced and prestressed concrete has given engineers other important materials for bridge building. Concrete has been used particularly for relatively short-span bridges that are a part of freeway systems. These bridges often use precast concrete beams. Many arch bridges have also been constructed of concrete. Currently, the longest concrete arch bridge is the Gladesville Bridge in Sydney, Australia. It has a span of 305 meters, and its deck is above the arch. This is another example of an esthetically pleasing bridge.

Many bridges that pass over rivers or canals must be movable so that shipping can pass under them. One type is the lift bridge, with towers that can raise the entire span between them by means of counterbalances and electric motors. Another type is the swing or pivot bridge, which pivots the span on a pier so that the bridge can swing open parallel to the river or canal. A third type is the bascule bridge, which has one or two arms that can open upward at an angle by means of counterweights. A bascule with one arm is a singleleaf bridge, and with two arms it is a double-leaf bridge.

Bridge construction can present extraordinary difficulties. Usually the foundations for the piers must rest on bedrock, and often under water. One technique for working in these conditions is by means of a cofferdam. Piles usually made of interlocking steel plates are driven into the water bed. The water is then pumped out from within the area that has been enclosed.

Another technique is the use of the pneumatic caisson. The caisson is a huge cylinder with a bottom edge that can cut into the water bed. When compressed air is pumped into it, the water is forced out. Caissons must be used with extreme care. For one thing, workers can only stay in the compression chamber for short periods of time. For another, if they come up to normal atmospheric pressure too rapidly, they are subject to the bends, or caisson disease as it is also called, which is a crippling or even fatal condition caused by excess nitrogen in the blood. When the Eads Bridge across the Mississippi River at St. Louis was under construction between 1867 and 1874, at a time when the danger of working in compressed air was not fully understood, fourteen deaths were caused by the bends.

When extra strength is necessary in the piers, they are sometimes keyed into the bedrock - that is, they are extended down into the bedrock. This method was used to build the piers for the Golden Gate Bridge in San Francisco, which is subject to strong tides and high winds, and is located in an earthquake zone. The drilling was carried out under water by deep-sea divers.

Where bedrock cannot be reached, piles are driven into the water bed. Today, the piles in construction are usually made of prestressed concrete beams. One ingenious technique, used for the Tappan Zee Bridge across the Hudson River in New York, is to rest a hollow concrete box on top of a layer of piles. When the box is pumped dry, it becomes buoyant enough to support a large proportion of the weight of the bridge.

Each type of bridge, indeed each individual bridge, presents special construction problems. With some truss bridges, the span is floated into position after the piers have been erected and then raised into place by means of jacks or cranes. Arch bridge can be constructed over a falsework, or temporary scaffolding. This method is usually employed with reinforced concrete arch bridges. With steel arches, however, a technique has been developed whereby the finished sections are held in place by wires that supply a cantilever support. Cranes move along the top of the arch to place new sections of steel while the tension in the cables increases.

With suspension bridges, the foundations and the towers are built first. Then a cable is run from the anchorage - -a concrete block in which the cable is fastened-up to the tower and across to the opposite tower and anchorage. A wheel that unwinds wire from a reel runs along this cable. When the reel reaches the other side, another wire is placed on it, and the wheel returns to its original position. When all the wires have been put in place, another machine moves along the cable to compact and to bind them. Construction begins on the deck when the cables are in place, with work progressing toward the middle from each end of the structure.

5.2. Discussion

1. How were the bridges that have survived from ancient times constructed?

2. What other kinds of bridges do ancient sources mention?

3. What kind of bridges did the Incas of pre-Columbian Peru build?

4. What has enormously increased the need for bridges?

5. What has made it possible to build longer and stronger bridges?

What advantage does a longer span give?

6. What is the simplest type of span?

7. How are most beam bridges strengthened today? What advantage in addition to strength does this construction give to a bridge?

8. What is another type of beam bridge? Are all bridges of this type built from iron or steel?

9. What is the longest cantilever bridge in the world? When was it built?

10. What is another type of modem bridge? Why must the piers that support this type of bridge be especially strong? What are two examples of this type of bridge?

11. What kind of bridge can span longer distances than other types of bridges? What is the longest bridge in the world?

12. How is the deck of a suspension bridge supported? What other system was once used? Give an example.

13. What happened to some of the earlier suspension bridges that had steel cables? What caused these failures?

14. How did John A. Roebling overcome the difficulties in steel cable suspension bridges? What two bridges did he design?

15. What two types of bridges are often built today with reinforced concrete? What is the longest concrete arch bridge?

16. Why must some bridges be movable?

17. What are three types of movable bridges? How do they differ from each other?

18. Where do the foundations for a bridge usually rest?

19. What is one technique used in the construction of the foundations for the piers‘of a bridge?

20. What is another technique that is used for working on foundations below water?

21. Why must great care be taken when caissons are used? What happened during the construction of the Eads Bridge in St. Louis?

22. What is sometimes done when extra strength is necessary in the piers?

23. What technique is used when bedrock cannot be reached?

24. What special technique was used for the Tappan Zee Bridge in New York?

25. How is the span sometimes raised into place with truss bridges?

26. What is the usual construction method for concrete arch bridges?

27. What method has been developed for the construction of steel arch bridges?

28. What is built first for a suspension bridge?

29. How are the cables put in place on a suspension bridge?

30. What part of the bridge is constructed after the cables are in place?

5.3. Review A. Match each expression on the left with one of the statements on the right. 1. Span a. A stone or brick bridge with arches spanning the 2. Pontoon bridge distance between piers. 3. Beam bridge b. A movable bridge that opens by swinging on a pivot. 4. Suspension bridge c. A cylinder with a cutting edge that can be sunk into a 5. Truss water bed. Water is forced out by compressed air. 6. Cantilever d. The traffic-bearing surface c a bridge. 7.Masonry arch bridge e. The distance between two supports of a bridge. 8.Concrete arch bridge f. A bridge with an arch of steel. 9. Steel arch bridge g. A bridge consisting of a rigid beam between two 10. Deck supports. 11. Lift bridge h. Temporary scaffolding use-to support an arch under 12. Swing bridge construction. 13. Bascule bridge i. A movable bridge that swings upward at an angle by 14. Cofferdam means of a counterweight. 15. Pneumatic caisson j. An enclosure of piles sunk into a water bed. Water can 16. Keying be pumped out of the enclosed area.

 

	17. Falsework		k. A framework strengthened by diagonal beams between horizontal and vertical beams. l. A movable bridge with counterweights and electric motion to raise the span. m. Extending the piers of a bridge into the bedrock. n. A bridge supported by cables usually hung from towers. o. A structure in which a horizontal beam extends beyond its support. p. A bridge supported by hollow drums that can float. q. A bridge with an arch of reinforced concrete.

 

 

UNIT SIX TUNNELS

Special Terms

Tunneling Shield: A large cylinder with a cutting edge that can be moved forward by jacks. It is used when tunneling through clay or soft rock.

Tail: The back part of a shield. The lining for a tunnel is usually assembled in the tail. Pneumatic Drill: A compressed air machine that is used to make holes in rock. Heading: The point from which work progresses on a tunnel. Most tunnels are bored from two headings.

Pilot Tunnel: A small exploratory tunnel bored in advance of a tunnel project along the same route. It provides geological information, as well as ventilation.

Immersed Tube: A tunnel made of prefabricated sections that are sunk into position. Cut-and-cover: A tunneling technique. A trench is excavated and the tunnel lining assembled in it. The trench is then filled in.

Dredging: Pumping silt or sand usually from the bottom of a body of water such as a river or a harbor.

Duct: A tube or channel that carries something, as an aqueduct carries water. A system of ducts is used for ventilation in most tunnels.