What Are The Classification Of Surveying?
Surveying may be classified on the following basis:
(i) Nature of the survey field
(ii) Object of survey
(iii) Instruments used and
(iv) The methods employed.
Classification Based on Nature of Survey Field
On this basis survey may be classified as land survey, marine or hydraulic survey and astronomical survey.
Land Survey. It involves measurement of various objects on land. This type of survey may be further classified as given below:
(a) Topographic Survey: It is meant for plotting natural features like rivers, lakes, forests and hills as well as man made features like roads, railways, towns, villages and canals.
(b) Cadestal Survey: It is for marking the boundaries of municipalities, villages, talukas, districts, states etc. The survey made to mark properties of individuals also come under this category.
(c) City Survey: The survey made in connection with the construction of streets, water supply and sewage lines fall under this category.
Marine or Hydrographic Survey. Survey conducted to find depth of water at various points in bodies of water like sea, river and lakes fall under this category. Finding depth of water at specified points is known as sounding.
Astronomical Survey. Observations made to heavenly bodies like sun, stars etc., to locate absolute positions of points on the earth and for the purpose of calculating local time is known as astronomical survey.
Classification Based on Object of Survey
On the basis of object of survey the classification can be as engineering survey, military survey, mines survey, geological survey and archeological survey.
(a) Engineering Survey: The objective of this type of survey is to collect data for designing civil engineering projects like roads, railways, irrigation, water supply and sewage disposals. These surveys are further sub-divided into:
Reconnaissance Survey for determining feasibility and estimation of the scheme.
Preliminary Survey for collecting more information to estimate the cost of the project, and
Location Survey to set the work on the ground.
(b) Military Survey: This survey is meant for working out plans of strategic importance.
(c) Mines Survey: This is used for exploring mineral wealth.
(d) Geological Survey: This survey is for finding different strata in the earth’s crust.
(e) Archeological Survey: This survey is for unearthing relics of antiquity.
Classification Based on Instruments Used
Based on the instruments used, surveying may be classified as:
(i) Chain survey
(ii) Compass survey
(iii) Plane table survey
(iv) Theodolite survey
(v) Tacheometric survey
(vi) Modern survey using electronic distance meters and total station
(vii) Photographic and Aerial survey
The survey is taught to students mainly based on this classification.
Classification Based on Methods Employed
On this basis surveying is classified as triangulation and traversing.
(i) Triangulation: In this method control points are established through a network of triangles.
(ii) Traversing: In this scheme of establishing control points consists of a series of connected points established through linear and angular measurements. If the last line meets the starting point it is called as closed traverse. If it does not meet, it is known as open traverse.
What Is Surveying and What Are The Objects And Uses Of Surveying?
Surveying is the art of making measurements of objects on, above or beneath the ground to show their relative positions on paper. The relative position required is either horizontal, or vertical, or both.
Less precisely the term Surveying is used to the measurement of objects in their horizontal positions. Measurements to deteremine their relative vertical positions is known as levelling.
OBJECT AND USES OF SURVEYING
As stated in the definition, object of surveying is to show relative positions of various objects of an area on paper and produce plan or map of that area. Various uses of surveying are listed below:
(i) Plans prepared to record property lines of private, public and government lands help in avoiding unnecessary controversies.
(ii) Maps prepared for marking boundaries of countries, states, districts etc., avoid disputes.
(iii) Locality plans help in identifying location of houses and offices in the area.
(iv) Road maps help travellers and tourist.
(v) Topographic maps showing natural features like rivers, streams, hills, forests help in planning irrigation projects and flood control measures.
(vi) For planning and estimating project works like roads, bridges, railways, airports, water supply and waste water disposal surveying is required.
(vii) Marine and hydrographic survey helps in planning navigation routes and harbours.
(viii) Military survey is required for strategic planning.
(ix) Mine surveys are required for exploring minearl wealth.
(x) Geological surveys are necessary for determining different strata in the earth crust so that proper location is found for reservoirs.
(xi) Archeological surveys are useful for unearthing relics of antiquity.
(xii) Astronomical survey helps in the study of movements of planets and for calculating local and standard times.
What Are Cambers?
Camber is a curvature built into a member or structure so that when it is loaded, it deflects to a desired shape. Camber, when required, might be for dead load only, dead load and partial live load, or dead load and full live load. The decision to camber and how much to camber is one made by the designer.
Rolled beams are generally cambered cold in a machine designed for the purpose, in a large press, known as a bulldozer or gag press, through the use of heat, or a combination of mechanically applied stress and heat.
In a cambering machine, the beam is run through a multiple set of hydraulically controlled rollers and the curvature is induced in a continuous operation. In a gag press, the beam is inched along and given an incremental bend at many points.
There are a variety of specific techniques used to heat-camber beams but in all of them, the side to be shortened is heated with an oxygen-fed torch.
As the part is heated, it tries to elongate. But because it is restrained by unheated material, the heated part with reduced yield stress is forced to upset (increase inelastically in thickness) to relieve its compressive stress.
Since the increase in thickness is inelastic, the part will not return to its original thickness on cooling. When the part is allowed to cool, therefore, it must shorten to return to its original volume. The heated flange therefore experiences a net shortening that produces the camber.
Heat cambering is generally slow and expensive and is typically used in sections larger than the capacity of available equipment. Heat can also be used to straighten or eliminate warping from parts. Some of these procedures are quite complex and intuitive, demanding experience on the part of the operator.
Experience has shown that the residual stresses remaining in a beam after cambering are little different from those due to differential cooling rates of the elements of the shape after it has been produced by hot rolling. Note that allowable design stresses are based to some extent on the fact that residual stresses virtually always exist.
Plate girders usually are cambered by cutting the web plate to the cambered shape before the flanges are attached.
Large bridge and roof trusses are cambered by fabricating the members to lengths that will yield the desired camber when the trusses are assembled. For example, each compression member is fabricated to its geometric (loaded) length plus the calculated axial deformation under load. Similarly, each tension member is fabricated to its geometric length minus the axial deformation.
What Is Electroslag Welding? What is Electrogas Welding?
Electroslag welding (ESW) produces fusion with a molten slag that melts filler metal and the surfaces of the base metal. The weld pool is shielded by this molten slag, which moves along the entire cross section of the joint as welding progresses.
The electrically conductive slag is maintained in a molten condition by its resistance to an electric current that flows between the electrode and the base metal. The process is started much like the submerged-arc process by striking an electric arc beneath a layer of granular flux.
When a sufficiently thick layer of hot molten slag is formed, arc action stops. The current then passes from the electrode to the base metal through the conductive slag. At this point, the process ceases to be an arc welding process and becomes the electroslag process.
Heat generated by resistance to flow of current through the molten slag and weld puddle is sufficient to melt the edges at the joint and the tip of the welding electrode. The temperature of the molten metal is in the range of 3500 deg F.
The liquid metal coming from the filler wire and the molten base metal collect in a pool beneath the slag and slowly solidify to form the weld. During welding, since no arc exists, no spattering or intense arc flash occurs.
Because of the large volume of molten slag and weld metal produced in electroslag welding, the process is generally used for welding in the vertical position. The parts to be welded are assembled with a gap 1 to 1 1⁄4 in wide. Edges of the joint need only be cut squarely, by either machine or flame.
Water-cooled copper shoes are attached on each side of the joint to retain the molten metal and slag pool and to act as a mold to cool and shape the weld surfaces. The copper shoes automatically slide upward on the base-metal surfaces as welding progresses.
Preheating of the base metal is usually not necessary in the ordinary sense. Since the major portion of the heat of welding is transferred into the joint base metal, preheating is accomplished without additional effort.
Electrogas welding (EGW) is similar to electroslag welding in that both are automatic processes suitable only for welding in the vertical position. Both utilize vertically traveling, water-cooled shoes to contain and shape the weld surface. The electrogas process differs in that once an arc is established between the electrode and the base metal, it is continuously maintained.
The shielding function is performed by helium, argon, carbon dioxide, or mixtures of these gases continuously fed into the weld area. The flux core of the electrode provides deoxidizing and slagging materials for cleansing the weld metal.
The surfaces to be joined, preheated by the shielding gas, are brought to the proper temperature for complete fusion by contact with the molten slag. The molten slag flows toward the copper shoes and forms a protective coating between the shoes and the faces of the weld. As weld metal is deposited, the copper shoes, forming a weld pocket of uniform depth, are carried continuously upward.
The electrogas process can be used for joining material from 1⁄2 to more than 2 in thick. The process cannot be used on heat-treated material without subsequent heat treatment. AWS and other specifications prohibit the use of EGW for welding quenched-and-tempered steel or for welding dynamically loaded structural members subject to tensile stresses or to reversal of stress.
Shielded metal arc welding (SMAW) produces coalescence, or fusion, by the heat of an electric arc struck between a coated metal electrode and the material being joined, or base metal. The electrode supplies filler metal for making the weld, gas for shielding the molten metal, and flux for refining this metal.
This process is commonly known also as manual, hand, or stick welding. Pressure is not used on the parts to be joined. When an arc is struck between the electrode and the base metal, the intense heat forms a small molten pool on the surface of the base metal.
The arc also decomposes the electrode coating and melts the metal at the tip of the electrode. The electron stream carries this metal in the form of fine globules across the gap and deposits and mixes it into the molten pool on the surface of the base metal. (Since deposition of electrode material does not depend on gravity, arc welding is feasible in various positions, including overhead.)
The decomposed coating of the electrode forms a gas shield around the molten metal that prevents contact with the air and absorption of impurities. In addition, the electrode coating promotes electrical conduction across the arc, helps stabilize the arc, adds flux, slag-forming materials, to the molten pool to refine the metal, and provides materials for controlling the shape of the weld.
In some cases, the coating also adds alloying elements. As the arc moves along, the molten metal left behind solidifies in a homogeneous deposit, or weld. The electric power used with shielded metal arc welding may be direct or alternating current. With direct current, either straight or reverse polarity may be used.
For straight polarity, the base metal is the positive pole and the electrode is the negative pole of the welding arc. For reverse polarity, the base metal is the negative pole and the electrode is the positive\ pole.
Electrical equipment with a welding-current rating of 400 to 500 A is usually used for structural steel fabrication. The power source may be portable, but the need for moving it is minimized by connecting it to the electrode holder with relatively long cables.
The size of electrode (core wire diameter) depends primarily on joint detail and welding position. Electrode sizes of 1⁄8, 5⁄32, 3⁄16, 7⁄32, 1⁄4, and 5⁄16 in are commonly used. Small-size electrodes are 14 in long, and the larger sizes are 18 in long.
Deposition rate of the weld metal depends primarily on welding current. Hence use of the largest electrode and welding current consistent with good practice is advantageous.
About 57 to 68% of the gross weight of the welding electrodes results in weld metal. The remainder is attributed to spatter, coating, and stub-end losses.
Shielded metal arc welding is widely used for manual welding of low-carbon steels, such as A36, and HSLA steels, such as A572 and A588. Though stainless steels, high-alloy steels, and nonferrous metals can be welded with this process, they are more readily welded with the gas metal arc process.
What Is Plane Surveying? How Plane Surveying Works?
In the past, most survey work depended on triangulation from known fixed points using a theodolite and this may still be a suitable method for smaller sites. Again it is necessary to ensure the instrument is in good condition and that its base is truly horizontal.
Readings taken on both faces of the instrument may reduce residual errors. Setting out by taping along a line given by the theodolite may also still be the clearest way of providing centre lines or points, particularly for regular structure layouts such as building columns.
The appropriate time for this is when blinding concrete has been placed to column and wall foundations. The base line, which is either the centre line of the building, or a line parallel to it but clear of the building, should have been set out previously by end pegs sited well clear of the work.
It is usual to work from co-ordinates along this base line from some fixed zero point, and measuring right angle distances out from them. In this way lines of walls and column centres can be marked on the blinding concrete.
Distances may be measured by steel or fibreglass tape pulled horizontally, so it is a great convenience if the site is level. If not a plumb bob has to be used to transfer distances. Distance co-ordinates along the base line from the zero peg are set out, using the steel tape and marking a pencil line across the peg.
The theodolite is set out over the pencil line, and its position is adjusted laterally so that it transits accurately on the two outermost base line marks. The plumb bob on the theodolite gives the mark for the co-ordinate point, a round headed nail being inserted on this point.
Distances at right angles to the base line are then set out with theodolite and steel tape. The advantage of this method is that the theodolite can sight down into column bases which are usually set deeper than the general formation level. For the assistance of bricklayers and formwork carpenters, sight boards can be provided, with the cross-arm fixed at a given level above formation level and with saw cuts exactly on the lines of sight to be used.
A builder’s line can then be fixed through such saw cuts. An alternative to the foregoing is to set out two base lines at right angles to each other and use theodolite right angle settings from these to give centres for such column bases, etc.
The introduction of EDM equipment has, however, meant that accurate distance and angle measurements can now be made from a single point set up. The instruments work by measuring the time of a wave in travelling from the transmitter to a reflector and back.
Readings may be automatically repeated to improve accuracy. Built-in or add-on equipment allows for automatic data logging, reduction of distances to horizontal and vertical components and for downloading to a computer.
Accuracy over short distances is good. Over longer distances corrections may need to be made for atmospheric conditions which vary from the manufacturers’ setting. The improved accuracy available has meant that setting out on site or general survey work is often done by some form of traversing. By this method the position of two known points is extended by noting the angle to a third point and its distance from the instrument set up over one of the points. Extended traverses should be closed onto another known point to check for errors.
Even with EDM equipment, setting out of regular structures is probably best done using a marked baseline as described above. The equipment also has major advantages in ground surveying since the location and elevation of any point in the area to be surveyed can usually be determined directly from just one or two positions of the instrument.
Data from the instrument can then be downloaded into a computer and with the use of appropriate software, contoured plans of the area can be produced for design or for earthworks measurement purposes.
Acertain amount of planning is necessary to produce the best results by ensuring a regular grid of locations is used for targeting and that any individual feature, such as sharp changes in slope are picked up. As an alternative ranging poles can be used to set out a rough grid and readings at say 20 m intervals between these should give sufficient coverage for accurate plotting.
The problems between main and sub-contractors were one of the areas to benefit most from Part II of the UK Government’s Housing Grants, Construction and Regeneration Act 1996 (see Section 1.6). The introduction of adjudication under that act to deal with disputes has at least allowed sub-contractors to press their claims to an earlier conclusion, and to challenge any withholding of payment by the contractor.
The Act requires payment terms to be stated and regular payments made. It prohibits ‘pay when paid’ clauses, and requires the contractor to issue a detailed ‘withholding notice’ if he seeks to hold back payment. These measures have eased the cash flow problems of sub-contractors.
Also most standard forms of sub-contract now contain provision for payment of interest on delayed payments, but this may not be very effective because a sub-contractor may not claim interest for fear the contractor might not as a consequence give him any further work.
The Civil Engineering Contractors Association (CECA) has issued a Form of Sub-contract ‘for use in conjunction with the ICE conditions of contract.’ Contractors are, of course, not obliged to use this form and many use one of their own devising or modify the standard form.
The provisions of the CECA sub-contract illustrate the many matters which such a sub-contract has to cover and the difficulty of trying to provide rights to the sub-contractor without putting the main contractor at risk under his contract.
Provisions of the CECA sub-contract, apart from defining the work, timing and duration of the sub-contractor’s input, require the sub-contract to set out the division of risks as between contractor and sub-contractor.
It defines procedures and methods of valuing variations made by the engineer and confirmed by the contractor, or made by the contractor; and sets out procedures for notification and payment for ‘unforeseen conditions’ or other claim matters. It also stipulates requirements for insurances and so on.
Many of the provisions are similar in terms to the ICE conditions applying to the contractor, and are thus passed on to the sub-contractor in respect of his work. The subcontractor is ‘deemed to have full knowledge of the provisions of the main contract’ and the contractor must give him a copy of it (without the prices) if the sub-contractor requests it.
Of particular importance is Clause 3 of the CECA sub-contract which requires the sub-contractor to carry out his work so as to avoid causing a breach of the main contract by the contractor. He has to indemnify the contractor ‘against all claims, demands, proceedings, damages, costs and expenses made against or incurred by the contractor by reason of any breach by the subcontractor of the sub-contract.’
But a sub-contractor undertaking a small value contract may find it impossible to accept this clause. If he fails to complete his work on time and this could possibly cause a delay to the whole project, he might be liable to pay many thousands of pounds to the contractor – far in excess of the value of his sub-contract.
A further problem for the engineer is that, if a dispute arises between the contractor and his sub-contractor as to who is responsible for some defective work, the defect can remain uncorrected until the dispute is resolved. If a defect is found after the sub-contractor has left site and he is believed or known to be responsible for it, the contractor may not be able to get the sub-contractor back to site to remedy the defect, or to pay for its repair.
To guard against this, the contractor may therefore hold back full payment to the sub-contractor for many months until a certificate of completion for the whole works is issued. This will cause another dispute between contractor and sub-contractor.
The development of sub-contracting in civil engineering has therefore brought both advantages and disadvantages. However, problems rarely arise if the contractor can use sub-contractors he has worked with before whose work has proved satisfactory and he treats them fairly.
What Is Sub Contracting?
Many civil engineering contractors now use sub-contractors to do much of their work. Most conditions of contract permit a contractor to sub-let work of a specialist nature; but the ICE conditions of contract have gone further and permit the contractor to sub-contract any part of the work (but not the whole of the work), subject only to notifying the engineer of the work sub-contracted and the name of the sub-contractor appointed to undertake it.
The contractor does not have to notify any labour-only sub-contracts he uses. The engineer can object, with reasons, to the appointment of a sub-contractor, but otherwise has no rights in connection with such sub-contracts, except that he can require removal of a sub-contractor who proves incompetent or negligent, or does not conform to safety requirements.
Under FIDIC conditions for overseas work, sub-contracting requires the engineer’s prior sanction. In building work there has long been a trend to pass the majority of work to sub-contractors who specialize in various trades, and the same has now occurred in civil engineering where many operations are ‘packaged up’ and sub-let.
Thus sub-contracts may be let for excavation, formwork, reinforcement supplied and erected, and concreting. The advantage to the contractor is that this reduces the staff he needs on site and his capital outlay on plant and equipment. He can use sub-contractors with proven experience and does not have to take on a range of temporary labour whose quality may be variable.
The contractor retains responsibility for the quality and correctness of work and, of course, has to plan and co-ordinate the sub-contract inputs, and often supply any necessary materials.
But if much of the work is sub-contracted, the contractor’s or agent’s main input to a project may be that of dealing with the sub-contracts and controlling their financial outcome, so these matters may take priority over dealing with any engineering problems which arise.
The contractor may therefore tend to leave a sub-contractor to solve any problems he encounters, on the basis that these are his risks under his sub-contract and it is up to him to deal with them. But the sub-contractor may think otherwise, so a dispute arises as each considers the other responsible for any extra cost or delays caused.
Frequent disputes have also arisen in recent years when any default or presumed default by a sub contractor has resulted in the contractor withholding payment to him. Late payment by contractors to sub-contractors is another widespread source of complaint by sub-contractors, but remedies are difficult to devise.
The sub-contracts are private contracts whose terms are unknown to the engineer and the employer, so they cannot interfere in any such dispute. The engineer has only power to protect nominated sub contractors, i.e. subcontractors he directs the contractor to use.
As a building rises the vertical alignment must also be controlled. This can be done by extending building centre lines at right angles to each other out to fixed points clear of the structure.
These lines can then be projected up the building and marked, allowing accurate measurements from these marks at each floor. Alternatively an optical plumb can be used to project a fixed point up through openings in the floors of the building so as to provide a set of reference points at each level.
The standard of setting out for tunnels must be high using carefully calibrated equipment, precise application and double checking everything. An accurate tunnel baseline is first set out on the surface using the methods described above. Transference of this below ground can be done by direct sighting down a shaft if the shaft is sufficiently large to allow this without distortion of sight-lines on the theodolite.
With smaller shafts, plumbing down may be used. A frame is needed either side of the shaft to hold the top ends of the plumb-lines and to allow adjustment to bring them exactly on the baseline. The plumb-line used should be of stainless steel wire, straight and unkinked, and the bob of a special type is held in a bath of oil to damp out any motion.
By this means the tunnel line is reproduced at the bottom of the shaft and can be rechecked as the tunnel proceeds. Many tunnels are nowadays controlled by lasers, the laser gun being set up on a known line parallel to the centre line for the tunnel and aimed at a target.
Where a tunnelling machine is used, the operator can adjust the direction of movement of the machine to keep it on target so that the tunnel is driven in the right direction. For other methods of tunnelling, target marks can be set on the soffit of rings, the tunnel direction being kept on line by adjusting the excavation and packing out any tunnel rings to keep on the proper line.
Lasers are also used in many other situations, usually for controlling construction rather than for original setting out since their accuracy for this may not be good enough. The laser beam gives a straight line at whatever slope or level is required, and so can be used for aligning forms for road pavements or even laying large pipes to a given gradient. For the latter, the laser is positioned at the start of a line of pipes and focused on the required base line.
As each new pipe is fitted into the pipeline a target is placed in the invert of the open end of the pipe, using a spirit level to find the bottom point, and the pipe is adjusted in line and level until the target falls on the laser beam. Bedding and surround to the pipe are then placed to fix the pipe in position.
Rotating lasers are also widely used and once set up give a constant reference plane at a known level. Use of a staff fitted with a reflector allows spot levels to be obtained anywhere in the area covered by the laser. Earthmoving equipment fitted with appropriate sensors can also be operated to control the level of excavation or filling with minimum input other than by the machine operator.
What Are The Usual Problem In Concrete Finishing?
The skill required by carpenters to make and erect form work for concrete is seldom fully appreciated. The formwork must remain ‘true to line and level’ despite substantial loading from the wet concrete. Column and wall faces have to be strictly vertical, and beam soffits strictly level, or any departure will be easily visible by eye.
Formwork for concrete which is to remain exposed to view has to be planned and built as carefully as if it were a permanent feature of the building. Many methods have been tried to make the appearance of exposed concrete attractive: but any of them can be ruined by honeycombing, a bad construction joint, or by subsequent weathering revealing that one pour of concrete has not been identical with adjacent pours, or that the amount of vibration used in compacting one panel has been different from that used in others.
If concrete has to remain exposed to public view, then the resident engineer should endeavour to agree with the contractor what is the most suitable method for achieving the finish required if the specification or drawings do not give exact guidance on the matter. The problem is that if, through lack of detailed attention, a ‘mishap’ on the exposed surface is revealed when the formwork is struck, it is virtually impossible to rectify it.
Sometimes rendering the whole surface is the only acceptable remedy. Where concrete will not remain exposed to view, minor discrepancies can be accepted. ‘Fins’ of concrete caused by the mix leaking through butt joints in the formwork should be knocked off. Shallow honeycombing should be chiselled out, and a chase cut along any defective construction joint.
The cut-out area or chase should be washed, brushed with a thick cement grout, and then filled with a dryish mortar mix. This rectifying work should be done as soon as possible so the mortar mix has a better chance of bonding to the ‘green’ concrete.
Shrinkage cracking of concrete is a common experience. The shrinkage of concrete due to drying is of the order of 0.2–0.5mm/m for the first 28 days. Subsequently concrete may expand slightly when wet and shrink on drying.
The coefficient of temperature expansion or contraction is very much smaller, of the order of 0.007mm/m per degree centigrade of change. Rich concrete mixtures tend to shrink more than lean mixes. The use of large aggregate, such as 40 mm instead of 20 mm, helps to minimize shrinkage. To avoid cracking of concrete due to shrinkage, wall lengths of concrete should be limited to about 9 m if restrained at the base or ends.
Heavy foundations to a wall should not be allowed to stand and dry out for a long period before the wall is erected, because the wall concrete bonding to the base may be unable to shrink without cracking. Concrete is more elastic than is commonly appreciated, for example the unrestrained top of a 300 mm diameter reinforced concrete column 4m high can be made to oscillate through nearly 1 cm by push of the hand.
Labels: Concrete Design
What Are The Types Of Suspension Bridges?
Several arrangements of suspension bridges are illustrated in Fig. 1. The main cable is continuous, over saddles at the pylons, or towers, from anchorage to anchorage.
FIGURE 15.9 Suspension-bridge arrangements. (a) One suspended span, with pin-ended stiffening truss. (b) Three suspended spans, with pin-ended stiffening trusses. (c) Three suspended spans, with continuous stiffening truss. (d ) Multispan bridge, with pin-ended stiffening trusses. (e) Self-anchored suspension bridge.
When the main cable in the side spans does not support the bridge deck (side spans independently supported by piers), that portion of the cable from the saddle to the anchorage is virtually straight and is referred to as a straight backstay.
This is also true in the case shown in Fig. 1a where there are no side spans. Figure 1d represents a multispan bridge. This type is not considered efficient, because its flexibility distributes an undesirable portion of the load onto the stiffening trusses and may make horizontal ties necessary at the tops of the pylons.
Ties were used on several French multispan suspension bridges of the nineteenth century. However, it is doubtful whether tied towers would be esthetically acceptable to the general public. Another approach to multispan suspension bridges is that used for the San Francisco–Oakland Bay Bridge (Fig. 2). It is essentially composed of two three-span suspension bridges placed end to end.
This system has the disadvantage of requiring three piers in the central portion of the structure where water depths are likely to be a maximum. Suspension bridges may also be classified by type of cable anchorage, external or internal. Most suspension bridges are externally anchored (earth-anchored) to a massive external anchorage (Fig. 1a to d).
In some bridges, however, the ends of the main cables of a suspension bridge are attached to the stiffening trusses, as a result of which the structure becomes self-anchored (Fig. 1e). It does not require external anchorages.
The stiffening trusses of a self-anchored bridge must be designed to take the compression induced by the cables. The cables are attached to the stiffening trusses over a support that resists the vertical component of cable tension. The vertical upward component may relieve or even exceed the dead-load reaction at the end support. If a net uplift occurs, a pendulum link tie-down should be provided at the end support.
Self-anchored suspension bridges are suitable for short to moderate spans (400 to 1,000 ft) where foundation conditions do not permit external anchorages. Such conditions include poor foundation bearing strata and loss of weight due to anchorage submergence. Typical examples of self-anchored suspension bridges are the Paseo Bridge at Kansas City, with a main span of 616 ft, and the former Cologne-Mu¨lheim Bridge (1929) with a 1,033-ft span.
Another type of suspension bridge is referred to as a bridle-chord bridge. Called by Germans Zu¨gelgurtbru¨cke, these structures are typified by the bridge over the Rhine River at Ruhrort-Homberg (Fig. 15.11), erected in 1953, and the one at Krefeld-Urdingen, erected in 1950.
It is a special class of bridge, intermediate between the suspension and cable-stayed types and having some of the characteristics of both. The main cables are curved but not continuous between towers. Each cable extends from the tower to a span, as in a cable stayed bridge. The span, however, also is suspended from the cables at relatively short intervals over the length of the cables, as in suspension bridges.
A distinction to be made between some early suspension bridges and modern suspension bridges involves the position of the main cables in profile at midspan with respect to the stiffening trusses. In early suspension bridges, the bottom of the main cables at maximum sag penetrated the top chord of the stiffening trusses and continued down to the bottom chord.
Because of the design theory available at the time, the depth of the stiffening trusses was relatively large, as much as 1⁄40 of the span. Inasmuch as the height of the pylons is determined by the sag of the cables and clearance required under the stiffening trusses, moving the midspan location of the cables from the bottom chord to the top chord increases the pylon height by the depth of the stiffening trusses.
In modern suspension bridges, stiffening trusses are much shallower than those used in earlier bridges and the increase in pylon height due to midspan location of the cables is not substantial (as compared with the effect in the Williamsburg Bridge in New York City where the depth of the stiffening trusses is 25% of the main-cable sag).
Although most suspension bridges employ vertical suspender cables to support the stiffening trusses or the deck structural framing directly, a few suspension bridges, for example, the Severn Bridge in England and the Bosporus Bridge in Turkey, have inclined or diagonal suspenders.
In the vertical-suspender system, the main cables are incapable of resisting shears resulting from external loading. Instead, the shears are resisted by the stiffening girders or by displacement of the main cables. In bridges with inclined suspenders, however, a truss action is developed, enabling the suspenders to resist shear.
(Since the cables can support loads only in tension, design of such bridges should ensure that there always is a residual tension in the suspenders; that is, the magnitude of the compression generated by live-load shears should be less than the dead-load tension.) A further advantage of the inclined suspenders is the damping properties of the system with respect to aerodynamic oscillations.
Fabrication of steel structures usually requires cutting of components by thermal cutting processes such as oxyfuel, air carbon arc, and plasma arc. Thermal cutting processes liberate a large quantity of heat in the kerf, which heats the newly generated cut surfaces to very high temperatures.
As the cutting torch moves away, the surrounding metal cools the cut surfaces rapidly and causes the formation of a heat-affected zone analogous to that of a weld. The depth of the heat-affected zone depends on the carbon and alloy content of the steel, the thickness of the piece, the preheat temperature, the cutting speed, and the postheat treatment.
In addition to the microstructural changes that occur in the heat-affected zone, the cut surface may exhibit a slightly higher carbon content than material below the surface. The detrimental properties of the thin layer can be improved significantly by using proper preheat, or postheat, or decreasing cutting speed, or any combination thereof.
The hardness of the thermally cut surface is the most important variable influencing the quality of the surface as measured by a bend test. Plate chemistry (carbon content), Charpy V-notch toughness, cutting speed, and plate temperature are also important.
Preheating the steel prior to cutting, and decreasing the cutting speed, reduce the temperature gradients induced by the cutting operation, thereby serving to (1) decrease the migration of carbon to the cut surface, (2) decrease the hardness of the cut surface, (3) reduce distortion, (4) reduce or give more favorable distribution to the thermally induced stresses, and (5) prevent the formation of quench or cooling cracks.
The need for preheating increases with increased carbon and alloy content of the steel, with increased thickness of the steel, and for cuts having geometries that act as high stress raisers. Most recommendations for minimum preheat temperatures are similar to those for welding.
The roughness of thermally cut surfaces is governed by many factors such as (1) uniformity of the preheat, (2) uniformity of the cutting velocity (speed and direction), and (3) quality of the steel. The larger the nonuniformity of these factors, the larger is the roughness of the cut surface. The roughness of a surface is important because notches and stress raisers can lead to fracture.
The acceptable roughness for thermally cut surfaces is governed by the job requirements and by the magnitude and fluctuation of the stresses for the particular component and the geometrical detail within the component. In general, the surface roughness requirements for bridge components are more stringent than for buildings.
The desired magnitude and uniformity of surface roughness can be achieved best by using automated thermal cutting equipment where cutting speed and direction are easily controlled. Manual procedures tend to produce a greater surface roughness that may be unacceptable for primary tension components. This is attributed to the difficulty in controlling both the cutting speed and the small transverse perturbations from the cutting direction.
Failures in service rarely, if ever, occur in properly made welds of adequate design. If a fracture occurs, it is initiated at a notchlike defect. Notches occur for various reasons.
The toe of a weld may form a natural notch. The weld may contain flaws that act as notches. A welding-arc strike in the base metal may have an embrittling effect, especially if weld metal is not deposited.
A crack started at such notches will propagate along a path determined by local stresses and notch toughness of adjacent material.
Preheating before welding minimizes the risk of brittle failure. Its primary effect initially is to reduce the temperature gradient between the weld and adjoining base metal.
Thus, there is less likelihood of cracking during cooling and there is an opportunity for entrapped hydrogen, a possible source of embrittlement, to escape. A consequent effect of preheating is improved ductility and notch toughness of base and weld metals, and lower transition temperature of weld.
Rapid cooling of a weld can have an adverse effect. One reason that arc strikes that do not deposit weld metal are dangerous is that the heated metal cools very fast. This causes severe embrittlement.
Such arc strikes should be completely removed. The material should be preheated, to prevent local hardening, and weld metal should be deposited to fill the depression.
Welding processes that deposit weld metal low in hydrogen and have suitable moisture control often can eliminate the need for preheat. Such processes include use of low-hydrogen electrodes and inert-arc and submerged-arc welding.
Pronounced segregation in base metal may cause welds to crack under certain fabricating conditions. These include use of high-heat-input electrodes and deposition of large beads at slow speeds, as in automatic welding.
Cracking due to segregation, however, is rare for the degree of segregation normally occurring in hot rolled carbon-steel plates. Welds sometimes are peened to prevent cracking or distortion, although special welding sequences and procedures may be more effective.
Specifications often prohibit peening of the first and last weld passes. Peening of the first pass may crack or punch through the weld.
Peening of the last pass makes inspection for cracks difficult. Peening considerably reduces toughness and impact properties of the weld metal. The adverse effects, however, are eliminated by the covering weld layer (last pass).
(M. E. Shank, Control of Steel Construction to Avoid Brittle Failure, Welding Research Council, New York; R. D. Stout and W. D. Doty, Weldability of Steels, Welding Research Council, New York.)
The most reliable information concerning the permeability of a deposit of coarse grained material below the water table can usually be obtained by conducting pumping tests in the field.
Although such tests have their most extensive application in connection with dam foundations, they may also prove advisable on large bridge or building foundation jobs where the water table must be lowered.
The arrangement consists of a test well and a series of observation wells. The test well is sunk through the permeable stratum up to the impermeable layer.
A well sunk into a water bearing stratum, termed an aquifer, and tapping free flowing ground water having a free ground water table under atmospheric pressure, is termed a gravity or unconfined well. A well sunk into an aquifer where the ground water flow is confined between two impermeable soil layers, and is under pressure greater than atmospheric, is termed as artesian or confined well.
Observation wells are drilled at various distances from the test or pumping well along two straight lines, one oriented approximately in the direction of ground water flow and the other at right angles to it.
A minimum of two observation wells and their distances from the test well are needed. These wells are to be provided on one side of the test well in the direction of the ground water flow.
The test consists of pumping out water continuously at a uniform rate from the test well until the water levels in the test and observation wells remain stationary. When this condition is achieved the water pumped out of the well is equal to the inflow into the well from the surrounding strata.
The water levels in the observation wells and the rate of water pumped out of the well would provide the necessary additional data for the determination of k.
As the water from the test well is pumped out, a steady state will be attained when the water pumped out will be equal to the inflow into the well. At this stage the depth of water in the well will remain constant.
The draw down resulting due to pumping is called the cone of depression. The maximum draw down DQ is in the test well. It decreases with the increase in the distance from the test well.
The depression dies out gradually and forms theoretically, a circle around the test well called the circle of influence. The radius of this circle is called the radius of influence of the depression cone.
It has been discussed earlier that soil is formed by the process of physical and chemical weathering. The individual size of the constituent parts of even the weathered rock might range from the smallest state (colloidal) to the largest possible (boulders). This implies that all the weathered constituents of a parent rock cannot be termed soil.
According to their grain size, soil particles are classified as cobbles, gravel, sand, silt and clay. Grains having diameters in the range of 4.75 to 76.2 mm are called gravel. If the grains are visible to the naked eye, but are less than about 4.75 mm in size the soil is described as sand.
The lower limit of visibility of grains for the naked eyes is about 0.075 mm. Soil grains ranging from 0.075 to 0.002 mm are termed as silt and those that are finer than 0.002 mm as clay. This classification is purely based on size which does not indicate the properties of fine grained materials.
Residual and Transported Soils
On the basis of origin of their constituents, soils can be divided into two large groups:
1. Residual soils, and
2. Transported soils.
Residual soils are those that remain at the place of their formation as a result of the weathering of parent rocks. The depth of residual soils depends primarily on climatic conditions and the time of exposure. In some areas, this depth might be considerable. In temperate zones residual soils are commonly stiff and stable.
An important characteristic of residual soil is that the sizes of grains are indefinite. For example, when a residual sample is sieved, the amount passing any given sieve size depends greatly on the time and energy expended in shaking, because of the partially disintegrated condition.
Transported soils are soils that are found at locations far removed from their place of formation. The transporting agencies of such soils are glaciers, wind and water. The soils are named according to the mode of transportation. Alluvial soils are those that have been transported by running water. The soils that have been deposited in quiet lakes, are lacustrine soils.
Marine soils are those deposited in sea water. The soils transported and deposited by wind are aeolian soils. Those deposited primarily through the action of gravitational force, as in land slides, are colluvial soils.
Glacial soils are those deposited by glaciers. Many of these transported soils are loose and soft to a depth of several hundred feet. Therefore, difficulties with foundations and other types of construction are generally associated with transported soils.
Organic and Inorganic Soils
Soils in general are further classified as organic or inorganic. Soils of organic origin are chiefly formed either by growth and subsequent decay of plants such as peat, or by the accumulation of fragments of the inorganic skeletons or shells of organisms. Hence a soil of organic origin can be either organic or inorganic. The term organic soil ordinarily refers to a transported soil consisting of the products of rock weathering with a more or less conspicuous admixture of decayed vegetable matter.
Names of Some Soils that are Generally Used in Practice
Bentonite is a clay formed by the decomposition of volcanic ash with a high content of montmorillonite. It exhibits the properties of clay to an extreme degree. Varved Clays consist of thin alternating layers of silt and fat clays of glacial origin. They possess the undesirable properties of both silt and clay.
The constituents of varved clays were transported into fresh water lakes by the melted ice at the close of the ice age. Kaolin, China Clay are very pure forms of white clay used in the ceramic industry. Boulder Clay is a mixture of an unstratified sedimented deposit of glacial clay, containing unsorted rock fragments of all sizes ranging from boulders, cobbles, and gravel to finely pulverized clay material.
Calcareous Soil is a soil containing calcium carbonate. Such soil effervesces when tested with weak hydrochloric acid. Marl consists of a mixture of calcareous sands, clays, or loam. Hardpan is a relatively hard, densely cemented soil layer, like rock which does not soften when wet. Boulder clays or glacial till is also sometimes named as hardpan. Caliche is an admixture of clay, sand, and gravel cemented by calcium carbonate deposited from ground water.
Peat is a fibrous aggregate of finer fragments of decayed vegetable matter. Peat is very compressible and one should be cautious when using it for supporting foundations of structures. Loam is a mixture of sand, silt and clay.
Loess is a fine-grained, air-borne deposit characterized by a very uniform grain size, and high void ratio. The size of particles ranges between about 0.01 to 0.05 mm. The soil can stand deep vertical cuts because of slight cementation between particles. It is formed in dry continental regions and its color is yellowish light brown. Shale is a material in the state of transition from clay to slate. Shale itself is sometimes considered a rock but, when it is exposed to the air or has a chance to take in water it may rapidly decompose.
Labels: Soil Mechanics
- SURVEYING CLASSIFICATION BASIC INFORMATION AND TUT...
- SURVEYING OBJECTS AND USES BASIC INFORMATION AND T...
- CAMBER DEFINITION BASIC INFORMATION AND TUTORIALS
- ELECTROSLAG (ESW) AND ELECTROGAS (EGW) WELDING BAS...
- SHIELDED METAL ARC WELDING (SMAW) OF STEEL BASIC I...
- PLANE SURVEYING ON CIVIL ENGINEERING PROJECTS BASI...
- SUB CONTRACTING DISPUTE RESOLUTION IN CIVIL ENGINE...
- SUB CONTRACTING ON CIVIL CIVIL ENGINEERING PROJECT...
- SETTING OUT VERTICALITY, TUNNELS AND PIPELINES IN ...
- CONCRETE FINISH PROBLEMS BASIC INFORMATION
- SUSPENSION BRIDGE TYPES BASIC INFORMATION AND TUTO...
- EFFECTS OF THERMAL CUTTING ON STEELS BASIC INFORMA...
- EFFECTS OF WELDING ON STEELS BASIC INFORMATION AND...
- DIRECT DETERMINATION OF k OF SOILS IN PLACE BY PUM...
- GENERAL TYPES OF SOIL BASIC INFORMATION AND TUTORI...
- ▼ Sep (15)