2383

II. Change the following sentences according to the model.

Model: It is necessary to examine the accuracy of these results.

1.The progress of the work depends on how efficient the organization is (efficiency).

2.The type of pump used will vary according to how much liquid is being pumped (amount).

3.It is necessary to test how much concrete will be consumed (quantity).

4.It is doubtful how accurate the results are (accuracy).

5.It is not clear whether these calculations are correct (accuracy).

Text 27 

I. Look through the text and cross out the items which do not fit in the content.

Bridge location. Bridge construction. Bridge structure. Way of traffic passing.

Replacing the structure. Decision to replace the structure.

II. Read the text and check your answers.

A UNIQUE TRANSPORTER HIGHWAY BRIDGE

The city of Duluth, Minnesota, which is situated at the head of Lake Superior, owns and operates a most unique type of highway bridge. The highway connecting Duluth with a strip of land behind which the harbour is located is crossed by the harbour entrance through which a large number of boats passes during the period of the year when the lake is open to navigation.

At the time that the bridge was constructed the highway traffic was relatively small and a type of structure was desired so that the shipping would be interfered with as little as possible. Such a requirement was found to be met by the transporter type of structure, this type of bridge consisting of an overhead frame of steelwork upon which a carriage moves back and forth.

Shipping is therefore free to pass at all times without it being necessary to open the bridge as would be the case if a movable span structure had been used. Highway traffic consisting of foot passengers or vehicles is loaded on the carriage on one side of the Channel and carriages propelled by electric motors moves to the other side and the vehicles unloaded.

For a number of years this bridge has served very satisfactorily but recently the highway traffic has increased so much that vehicles are delayed and have to wait in line for their turn to be carried across. So the city has deemed it necessary to replace this structure with one of the ordinary type of movable spans so that vehicles traffic will be served with less inconvenience. This transporter type of bridge is used rather extensively in Europe but the structure in Duluth holds the distinction of being the only one of its kind serving highway traffic in the United States.

III. What notion is described?

-an area of water protected from the open sea by land or walls, in which ships can shelter;

-a large vehicle used for carrying heavy objects;

-to change position;

-a time when each member of a group must or may do something;

-movement of ships over water

IV. Complete the sentences.

1.A unique type of highway bridge is situated …

2.The transporter type of structure was chosen because…

3.Passengers and vehicles were carried by…

4.Because of highway traffic increase…

5.This transporter type of bridge is the only in…

V. Match the town and the bridge situated in it. Can you describe each of

Text 28 

I. Match the Russian and the English equivalents. Use the English words in the sentences of your own.

повторение

место

II. Read the text and express the main idea.

bridgING the channel

The type of the bridge proposed for the Channel crossing with accommodation for road and rail traffic would consist of a long series of standard repetition spans of about 740 feet, each on concrete supporting piers and with such longer spans, at intervals, as may be required for navigation of larger craft. Some 800.000 tons of structural steel would be necessary for the superstructure of such a bridge.

The construction of a bridge of such a size and type presents problems and opportunities quite different from those encountered in normal structures. Its construction requires the fullest adoption of mass production methods at all stages of work.

The principle construction hazard of a Channel bridge lies in the prevailing weather conditions. These invite a scheme with the highest possible degree of preassembly on shore and the minimum of work on the actual site of the bridge.

The concrete piers on which the bridge spans would rest must be prefabricated on shore in large hollow sections, launched and floated to their permanent sites where they can be sunk into position with the aid of equipment on mobile working platforms.

III. Complete the sentences using the text.

1.The bridge for the Channel would consist of…

2.For the superstructure of such a bridge it would be necessary…

3.The construction of the bridge requires…

4.The concrete piers must be…

IV. Answer the questions.

1.What is the type of the bridge proposed for the Channel?

2.For what purpose do we need long spans?

3.For what part of the bridge would some 800.000 tons of structural steel be

necessary?

4.What methods are necessary for the construction of this bridge?

5.Where must the concrete piers be prefabricated?

Home Exercises

I. Copy and complete the table. Can you find the infinitives and participles below in the text? Be attentive answering the question.

II. Find the corresponding Participles to the nouns and make up sentences

III. Write the summary of the text.

Text 29 

II. What is the best way to preserve a bridge in its original condition? Read the text and make a list of words connected with bridge maintenance.

BRIDGE MAINTENANCE

After a bridge has been built it is put into a permanent operation. The State Acceptance Committee provides the final accept and studies the construction documents, examines the bridge involving the geodetic devices for more thoroughly inspection. Every structure is tested to locate defects before they become serious.

The testing is carried out under static and dynamic overloading of building components in order to find out how the bridge is supposed to accommodate the design load on different elements and joints, which can change their initial positions.

In case the structure meets the standard requirements and the acceptance documents are signed, the bridge is transferred to the possession of the railway and motorway maintenance sections.

The Building Code containing the rules and demands for current maintenance of the constructional works must be observed to provide safe and reliable operation during the design life for 80 – 100 years. All necessary surveillance and in-depth bridge inspection for long bridges is carried out by a bridge foreman. Short bridges and culverts are inspected by a road lengthman. They head the gangs which permanently provide structural inspection of the bridges and culverts to locate the damages and defects; repair small damages and defects; clean the bridge from snow, slush and mud; determine the wear- and-tear stage of the bridge elements draw up the service forms and records for bridge inspection and testing; execute records in case the bridge needs reconditioning or overhaul.

There are four assessment stages of a bridge state: «zero stage» for the normal bridge state,

«the first stage» in case of small faults and troubles which could be repaired while reconditioning,

«the second stage» when the bridge needs an overhaul,

«the third stage» when the bridge is subject to reconstruction or must be replaced.

The bridge carrying capacity is of great importance for the maintenance of the structure. The fact is that at present many railway and motorway bridges in Russia are designed and erected according to the Building Code and engineering specifications issued between 1884 and 1985.

On the other hand the modern Building Code issued in 1985 allows the live load, which might be applied in 80 or 100 years. But at present the value of the acting live load is considerable less. That is why specialists assess the bridge carrying capacity according to the classification of the superstructure elements and to the classification of the live load. When compared, these two values allow to make a conclusion on the scope of the bridge operation efficiency under the modern load.

Every bridge type has its own peculiarities and due attention must be given to them from the point of view of operation and maintenance.

For timber bridges one of the most urgent problems to be solved is the debacle and high-flood in case of short spans bridges (between 2 and 6 m).

The ground hollowed out by the rushing waters may cause the support slip or displacement. In addition timber can deteriorate and decay and as a result some bridge elements may be worn out. And besides, timber structures suffer from fire.

Metal structures also demand the appropriate care because their elements and joints fail by corrosion. That is why metal bridges need painting and stainless steel is too expensive. The high-strength bolts and rivets slackening call for permanent and qualified inspection as well as the breakage of the welded seams.

The reinforced concrete bridges do not require heavy maintenance cost when they are erected without any technological violation. But fractures and cracks, concrete chips and reinforcement corrosion, holes and shrinkage cavities might frequently occur and present most dangerous defects especially for the bridge supports of reinforced concrete. The bug holes may appear when concrete hardens. In addition the displacements and shifts of the supports occur if the soil is not hard enough.

Culverts need careful inspection and cleaning from mud before every high flood. Some culvert sections might be displaced by the uneven settlement of the embankment.

III. Choose the Russian translation for the English word(s). Give your arguments.

a)a building – строящийся; здание; строительство

b)to subject – к предмету; подвергать; субъект

c)according – аккордеон; созвучный; согласно

d)value – ценность; оценщик; ценный

e)care – каре; забота; держатель

f)geodetic – геодезический; геодетический; географический

IV. Make up beginnings of sentences and try to complete them.

V. Complete the chart. What process is described?

static and dynamic ____________

transferring to railway and motorway _____________sections

bridge foreman or road lenghman inspection

small defects ___________

cleaning

VI. Retell the text using the list of words you have made up according to Ex. II.

Home Exercises

I.Memorize the words from Ex. I page 108.

II. Change the Voice of the sentence.

1.The State Acceptance Committee studies the construction documents.

2.Bridge foremen carry out all necessary surveillance.

3.Culverts and short bridges are inspected by a road lengthman.

4.Builders must pay due attention to peculiarities of every bridge type.

5.Specialists test bridges to locate defects.

III. Copy the table and fill it in.

sectional resistance. The object of repairing such damage is to restore the original cross-sectional strength. Another possibility is misdesign of a bridge or parts of it. This includes all cases where the cross-sectional strength at crucial points is too low so that either the cross-sectional safety or the overall safety of the respective structure or structural element fails to comply with existing codes. Poor construction workmanship may mean that the cross-sectional strengths originally calculated are not achieved. For instance, the as-built cross-sectional dimensions may be smaller than those planned. Or it can happen that individual rebars or tensioning cables are incorrectly set, interchanged or even missing, which reduces the cross-sectional strength substantially.

Another severe problem is stress corrosion of prestressing steel. Even today it is not possible to rule out damage of this kind notwithstanding the great improvements made in the properties of prestressing steel. Many hundreds of thousands of bridges worldwide will need to be repaired within the next few years for one or other of these reasons.

There are a number of ways to repair a particular bridge or parts of one, depending on the type of construction and the given situation. Adhesively bonded external steel reinforcement is one possible way of achieving structural strengthening. This method was originally invented in France in the mid sixties; in the early seventies it was further developed in Switzerland, Germany and England and is nowadays state-of-the-art in Western Europe. Advanced composite strips or sheets can replace steel plates (which were used for poststrengthening). They do not corrode; they are easy to handle on the construction site and can be lifted onto the structure with a scissors-lift or similar device without expensive scaffolding; they are simply rolled on like "wallpaper"; the strips are available on endless reels, so no joints are necessary; they increase flexural and shear strength and reduce deflections and cracking; they cause minimal disruption to the bridge function; they require less time and labour to install; costs are lower than for other methods such as external post-tensioning.

From 1982, carbon fibre reinforced epoxy resin composites have been successfully employed for the post-strengthening of reinforced concrete beams. Loading tests were performed on more than 90 flexural beams having spans of between 2 and 7 metres. The research work shows the validity of the strain compatibility method in the analysis of various cross-sections. This implies that the calculation of flexure in reinforced concrete elements which are poststrengthened with carbon fibre reinforced epoxy resin composites can be performed in a similar way to that for conventional reinforced concrete elements. The work also shows that the possible occurrence of shear cracks may lead to peeling of the strengthening composite. Thus, the shear crack development represents a design criterion. Flexural cracks are spanned by the CFRP strip and do not influence the loading capacity. In comparison to the unstrengthened beams, the strengthening strips lead to a much finer cracking

distribution. A calculation model developed from the CFRP (carbon fibrereinforced plastic) composite agrees well with the experimental results.

When a change of temperature takes place, the differences in the coefficient of thermal expansion of concrete and the carbon fibre reinforced epoxy resin composites result in thermal stresses at the joints between the two components. No negative influence on the loading capacity of the three post-strengthened beams was found after 100 frost cycles ranging from +20-deg C to —25-deg C.

Highly filled epoxy resin is the classic adhesive for bonding. The adhesive must be applied to the CFRP strips in a roof shape so that the extra adhesive is squeezed out when the strip is pressed to the concrete structure. For a strip width of 140 mm, for example, the peak height in the middle of the strips is about 7 mm. Uniform pressing of the CFRP strips and evacuation of entrapped air can be achieved through the use of a hard-rubber roller. Excess adhesive can then be removed and the CFRP cleaned, if necessary. If required for aesthetic reasons, the outer face of the CFRP strips can be coated with epoxy paint. Cement mortars can also be applied after the reinforcement has been primed with a suitable bonding agent. Post-strengthening with strips is best suited for more or less flat girders and slabs. For a 1 mm thick strip, a minimum radius of curvature of approx. 300 mm is required. This method, therefore, cannot be used for wrapping of columns with a rectangular cross section.

Another important method for rehabilitation is external post-tensioning. This method whereby the steel tendons are not embedded in the concrete, but are placed instead “externally” to the structural elements, offers the advantage that the tendons can be inspected and replaced. However, this normally rules out grouting of the steel tendons, thus making them susceptible to corrosion. Stress corrosion is another problem for the highly tensioned steel cables.

So, it was decided to use composite materials as an alternative to steel. Advanced fibrous composites offer the engineer in the construction industry an outstanding combination of properties not available from other materials. Fibres such as glass or carbon can be introduced in a certain position, volume fraction, and direction in the matrix to obtain maximum efficiency. Other advantages offered by advanced composites are lightness and resistance to corrosion and stress corrosion. Some also offer outstanding fatigue performance and greater efficiency in construction compared with more conventional materials.

The question of which fibre is most suitable is still the subject of lengthy discussions. A careful evaluation showed that, in most cases, carbon fibre is the material best suited for bridge repair. This fibre is alkaline-resistant and does not suffer stress corrosion. These are very important arguments for such applications.

Single FRP wires or strands have been used for external post-tensioning for some years. Larger units of parallel wire or strand bundles have been rarely used in the past but two such applications were realised in 1998.