TYPE OF BRIDGE
ABDUL MALIK KHADAFI
CIVIL AND ENGINERING
2010
Abstract
Abstract
The main types of bridges are arches, beam bridges, cable-stayed bridges, cantilever bridges and suspension bridges. You will have noticed that this list does not include truss bridges. These are usually arches, beams or girders, or cantilevers, or they may be parts of bridges, for example the suspended span of a cantilever bridge, or the deck of a cable-stayed bridge or a suspension bridge. The phrase "truss bridge", however, is sometimes reserved for those which act primarily as beams, while the others are discussed under the heading of the bridges of which they form a part. You could say that a truss, like a box-girder or a pre-stressed span, is more a type of construction than a type of structure.
At various places in this Paper there are sections which explain that the boundaries between the various types of bridges are not completely impervious, and that in principle at least, bridges can be built that are not obviously in a simple category. The reason that the types of most bridges are obvious is that these types have become popular because they are successful, and success is greatest in the broad central regions of the available variable-space. For example, if you make an extremely flat suspension bridge, you could put the wires in a concrete matrix, and you would have a pre-stressed beam requiring no anchorages. The same is true of arches - an extremely flat arch would generate enormous thrust, and a beam would be a better solution.
The same difficulty applies to many other other human activities, and indeed of many natural groups of species: although there are many genera and species which tax the powers of biologists to classify them, the vast majority fall more easily into groups. On the other hand, where there are very many closely related species, there may be sporadic disputes between "lumpers" and "splitters"
1. Intoduction
Bridge is one of the important things in life. in general, the bridge used to connect a separate path. usually the road is cut by rivers. but now the bridge is not only used to connect a road that was blocked by the river, however there is also a bridge connecting one island to another island.
there are several types of bridges including: Beam bridges, cable stayed bridges, truss bridges and suspension bridges. actually there are many types of bridges, but in this paper will only discuss the four types of bridges only. because it is basically another type bridge is a development of this bridge fourth.
course of each bridge has its pros and cons of each. and several bridges have also been built at a cost of more expensive than the other bridges.
there are several types of bridges including: Beam bridges, cable stayed bridges, truss bridges and suspension bridges. actually there are many types of bridges, but in this paper will only discuss the four types of bridges only. because it is basically another type bridge is a development of this bridge fourth.
course of each bridge has its pros and cons of each. and several bridges have also been built at a cost of more expensive than the other bridges.
Bridges that have been established at this time is built using advanced technology, so the bridge is built is also more advanced course. bridge was originally built with a very simple example is only built of wood and tied together only by rattan. but the current bridge was built using concrete and steel and also has the appearance of a very beautiful.
2. Purpose
Purpose of writing this paper is to:
2.3 To inform the people about the kinds of bridges
2.4 Explain the benefits and advantages of a bridge
2.5 To explain where the best bridge
3. KEY WORD: cable stayed , cantilever , beam, arch,
4. Type of Bridge
The essence of an arch is that ideally there should be no tendency for it to bend, except under live loads. It should be purely in compression, and for that reason it can be made of materials such as, masonry, cast iron and concrete, that perform poorly in tension. Of course, in a trussed arch there will be some tension members, but the main ones are always in compression. These main members are always much thicker than than the cross-members.
On the other hand, in a deck-stiffened arch, the deck is much thicker than the arch, because the deck is resisting any tendency to bend or buckle, leaving the arch chord to resist pure compression. In such a bridge, the deck can be very much thinner than a simple beam across the gap, because its weight is supported by the arch, and the arch can be very much thinner than a simple arch, because it is stiffened by the beam.
These two types of arch are shown below.
Picture: www.design-technology.orgIn any structure, except a simple pier or column, it is impossible to have compression without tension. In the case of an arch, the tension is in the ground, which is therefore a member that costs nothing. If we take this argument further, it can prove that arch spans can be made longer than beam spans. Although the ground under an arch is in tension, the ground just outside the abutments is compressed by the thrust of the arch. Between the regions of tension and compression, the ground is subject to complicated mixtures of tension, compression and shear stresses.
Although an arch is generally not under stress to make it bend, it has curvature designed in, because it is in a gravitational field. The amount of curvature at any point is designed so that the whole thing is perfectly balanced, neither tending to increase the curvature or to decrease it. The ideal shape is called the funicular, the exact shape of which depends on the weight distribution, so the funicular is not necessarily a simple mathematical curve such as a circle or a parabola. The arch and the suspension bridge are generally closer to the funicular, or natural curve, than any other type. In this they imitate the path of projectiles, which also follow curved natural paths, and even light, which curves in a gravitational field. Nevertheless, although the cause, gravity, is the same for both arches and projectiles, the detailed reasons for the curvature are different. We must always beware of making false analogies, though similarities have on occasion been valuable in science and mathematics in finding solutions to problems.
Why must an arch be curved? If we consider any section of an arch, the forces comprise two distinct kinds - those pulling down (the weight of the section pulling down, and the load, if any) - and the forces from the sections on either side. In order to balance the downward forces, the forces from the side must not be exactly in line: the angle between them, repeated throughout the arch, is the reason for the curvature. A beam, because it is straight, cannot work like this - it has to balance the downward forces by means of shear stress.
In one sense, the arch is one of the simplest of all bridges, because if you build it against hard rock, you only need the arch, and no other parts. The rock acts as abutments, provided that you cut the rocks to the right shape so that they are at right angles to the arch. In practice, abutments would be used to spread the load so that the stresses in the ground would be safely small.
4.1.1 Advantages of arches
The entire arch is in compression. The compression is transferred into the abutments, and ultimately resisted by tension in the ground under the arch. The absence of tension in the arch means that it can sustain much greater spans than beams can achieve, and it can use materials that are not strong in tension, such as masonry and cast iron. In older times, before the advent of wrought iron, many cast iron arches were built, some of which are still in use. Relatively few arches are now built in masonry, and none are built in cast iron. Masonry is labour intensive, and for the shorter spans, over roads and railways, it is often cheaper and simpler to lift whole beams and cantilevers into position.
4.1.2 Disadvantages of arches
An arch cannot stand until it is complete. Therefore it must either rest on falsework (centring) until it is complete, or the two halves must be cantilevered from the springing, using cables. The cantilever method cannot be used for masonry arches or concrete arches. Clickt o see photographs of centring for the Nicholson bridge.
The thrust of a big arch has a horizontal component, which the abutments must withstand without significant movement. The pictures below show the results of movement.
The cable-stayed bridge is related to the cantilever bridge. The cables are in tension, and the deck is in compression. The spans can be constructed as cantilevers until they are joined at the centre. A big difference between cantilever bridges and cable-stayed bridges is that the former usually have a suspended span, and the latter do not.
A cable stayed-bridge lacks the great rigidity of a trussed cantilever, and the continuous beam compensates for this to some extent. Indeed, while a long cable-stayed span is under construction, there may be great concern about possible oscillations, until the cantilevers are joined. For the Pont de Normandie, there was even thought of using active correctors if things threatened to get out of hand. In fact, the construction went smoothly.
The cables are of high tensile steel. In a few examples these are encased in concrete. Towers are often made in concrete, though steel is also used.
Picture: www.design-technology.org
4.2.1 Advantages of cable-stayed bridges
The two halves may be cantilevered out from each side. There is no need for anchorages to sustain strong horizontal forces, because the spans are self-anchoring. They can be cheaper than suspension bridges for a given span. Many asymmetrical designs are possible.
4.2.2Disadvantages of cable-stayed bridges
In the longer sizes, the cantilevered halves are very susceptible to wind induced oscillation during construction. The cables require careful treatment to protect them from corrosion.
In the arch and beam we saw that the bridges were supported at two places - the ends. In fact, if you want to hold any object in position in two dimensions, you always need two points of attachment. In three dimensions you need three points. Two-legged animals need feet in order to achieve stability in the three dimensions. This statement is not strictly true, because animals can balance using their muscles to change their position in response to perceived movement. Nevertheless, feet make the process easier.
A cantilever differs from the arch and the beam in that the attachment points are not necessarily at opposite ends. The cantilever is rather like a bracket, projecting out into space. The two forces almost always act in opposite directions. In the lower half of the first photograph, the oscillation in the wind is revealed by the longer exposure. Whenever there is both mass and elasticity, there are natural resonant frequencies. The second photograph shows a vertical cantilever deflecting in a wind, with oscillation in the right hand half of the picture.
The diagrams below show two basic types of cantilever, though in fact the second includes the first - above the support pier, there are forces like those in the first example, holding the two arms together, as we see in the third diagram. The connection to the pier may be a hinge or a rigid support. The central cantilever pair of the Forth bridge has to be supported rigidly, because both cantilever ends are free. Almost all other cantilever bridges have only two pairs, each of which has a fixed, end, and therefore hinged supports are sufficient, except during construction.
As with the beam, the bending stresses and shear stresses vary throughout the structure.
Most cantilever bridges have two cantilevers, with a beam suspended between their free ends, like the example shown below, typical of many motorway bridges, usually built from reinforced concrete or pre-stressed concrete. The largest cantilever bridges are made of steel, though medium sized ones are sometimes in pre-stressed concrete. Ancient ones in Asia were made of wood.
The cantilevers can be maintained in position in two different ways. Firstly, they can be supported by pivots or hinges at the balance point, with the fixed end held in place at the abutment; secondly they can be supported at the balance point by a tower with a base so wide that no practical load can tip the structure. The central part of the Forth railway bridge is of the second type, which is why it has a wider tower than the outer parts. Most cantilevers are of the first type. In the first type there are two ways of holding the structure in position. One is to make the anchored span so heavy that no practical load at the free end can tip the structure. The other is to fix the anchored end to the ground. The outer ends of the Forth railway bridge are high up on masonry piers, which cannot withstand tension. The steel structures therefore have heavy weights attached, which hang down inside the piers. These weights are so heavy that the spans cannot be tipped by any likely load.
The longest cantilever span is the centre span of the Quebec bridge, in Canada, part of which is shown here.
Picture: www.brantacan.co.uk
4.3.1 Advantages of cantilevers
Building out from each end enables construction to be done with little disruption to navigation below. The span can be greater than that of a simple beam, because a beam can be added to the cantilever arms. Cantilever bridges are very common over roads. Because the beam is resting simply on the arms, thermal expansion and ground movement are fairly simple to sustain. The supports can be simple piers, because there is no horizontal reaction. Cantilever arms are very rigid, because of their depth.
4.3.2 Disadvantages of cantilevers
Like beams, they maintain their shape by the opposition of large tensile and compressive forces, as well as shear, and are therefore relatively massive. Truss construction is used in the larger examples to reduce the weight.
The cable of a suspension bridge is in tension, enabling it to be much narrower and cheaper than an arch of the same span.
With the deck high above the floppy cables, this looks unstable, and it is. This construction can be used only for spans that are short enough for a stiff deck to transmit lateral forces to the anchorages. Some early iron trusses were made like this, one advantage being the that the top chord of a truss is in compression, and has to be thick, and so it may as well carry the deck, especially for railways. The Traversina footbridge in Switzerland was built like this, using wood among various other materials.
As we all know, the standard suspension bridge looks more like the picture below (showing one half only) with the attendant cost of the towers and anchorages.
Picture: www.design-technology.org
The functions of the various parts are easy to understand.
The towers hold the cable up.
The anchorages pull the cable outwards and downwards.
The hangers connect the deck to the main cable.
And the deck is there to carry the traffic.
The diagram below shows an anchorage that relies on its weight to hold the cable. The moment of the weight about the toe must be greater than the moment of the pull of the cable about that same point. The diagram includes the inequality. The strands of the cable splay out from the saddle to a very large number of attachments into the concrete. This type of anchorage may be used when the ground is not good enough for a buried anchorage. In good rock, the saddle and the attachments are in a cavity in the ground. The anchorages of the Clifton bridge are in tapering tunnels that are filled with masonry which acts as wedges.
Picture: www.design-technology.org
The deck has also to possess enough local rigidity in bending and torsion to prevent undue flexure as vehicles pass. Locally, around each vehicle, it acts as a beam with rather diffuse supports, namely the hangers for some distance in each direction. This same rigidity must be sufficient to help in the task of preventing undesirable amplitudes of oscillation. Some hangers are provided with small devices that help to damp oscillations in them.
Before the invention of steel, many suspension bridges were based on wrought iron eye-bars, resting on masonry towers. The invention of the steel wire cable, spun in place, changed everything. Though the great Brooklyn bridge had masonry towers, steel became the normal material for a long period, until concrete became a popular alternative. The Humber bridge, one of the longest, has towers of concrete. Decks are almost always made of steel, usually in the form of a steel truss or an aerodynamic box girder, which can be much lighter than a truss.
4.4.1 Advantages of suspension bridges
The main sustaining members, the cables or chains, are purely in tension, and are not required to be rigid, so they can be only as thick as needed to resist the tension. The towers are almost purely in compression, so their design is relatively simple.
4.4.2 Disadvantages of suspension bridges
They are only as rigid as the deck structure, which in older structures was usually of truss construction. This makes them generally unsuitable for railway traffic. Great attention is required in the design stage to deal with aerodynamic loads and, in the smaller sizes, periodic loads applied by pedestrians. During construction, the cables and towers may be susceptible to wind induced oscillations. The anchorages must sustain very strong horizontal forces as well as vertical ones. Constructing the cables or chains across the gap can be a lengthy process.
4.4.3 Functions of the Parts
The towers hold up the cables. They have to be rigid enough to act as struts between the downward forces from the cables and the upward forces from the foundations. But modern cables are fixed to the towers at the saddles: there is no sliding. So the towers have to be flexible enough to allow for changes in length due to live loads and temperature.
The anchorages have to hold the ends of the cables against the enormous tension, either by sheer weight, or by taking the tension into the ground. At the time of construction, they have to include means of adjusting each strand to the correct tension.
The cables hold up the deck and the traffic, via the hangers or suspenders. They have to be strong enough to do this without undue stretching. They have to withstand vibration and be resistant to corrosion and wind-borne dust.
The deck has to be as light as possible, but rigid enough to prevent a dip as each vehicle passes. It is very difficult to make a suspension deck stiff enough so that a railway locomotive doesn't spend the whole crossing in trying to climb out of a valley. The deck must also be stable in winds of any possible direction and magnitude, whether steady or gusting.
4.5 Truss bridge
A truss bridge is a bridge composed of connected elements (typically straight) which may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges. The basic types of truss bridges shown in this article have simple designs which could be easily analyzed by nineteenth and early twentieth century engineers. A truss bridge is economical to construct owing to its efficient use of materials.
Design
Picture: www.wikipedia.org
4.5.1 The integral members of a truss bridge
The nature of a truss allows for the analysis of the structure using a few assumptions and the application of Newton's laws of motion according to branch of physics known as statics. For purposes of analysis, trusses are assumed to be pin jointed where the straight components meet. This assumption means that members of the truss (chords, verticals and diagonals) will act only in tension or compression. A more complex analysis is required where rigid joints impose significant bending loads upon the elements, as in a Vierendeel truss.
In the bridge illustrated in the infobox at the top, vertical members are in tension, lower horizontal members in tension, shear, and bending, outer diagonal and top members are in compression, while the inner diagonals are in tension. The central vertical member stabilizes the upper compression member, preventing it from buckling. If the top member is sufficiently stiff then this vertical element may be eliminated. If the lower chord (a horizontal member of a truss) is sufficiently resistant to bending and shear, the outer vertical elements may be eliminated, but with additional strength added to other members in compensation. The ability to distribute the forces in various ways has led to a large variety of truss bridge types. Some types may be more advantageous when wood is employed for compression elements while other types may be easier to erect in particular site conditions, or when the balance between labor, machinery and material costs have certain
| 4.6 Beam bridges A beam or "girder" bridge is the simplest kind of bridge. In the past they may have taken the form of a log across a stream but today they are more familiar to us large box steel girder bridges. There are lots of different types of beam bridges. A beam bridge needs to be stiff. It needs to resist twisting and bending under load. In its most basic form, a beam bridge consists of a horizontal beam that is supported at each end by piers. The weight of the beam pushes straight down on the piers. Under load, the beam's top surface is pushed down or compressed while the bottom edge is stretched or placed under tension. If we imagine that there is an imaginary line running down the centre of the beam this line remains at its original length while the material above is compressed and the material below is stretched. The farther apart its supports, the weaker a beam bridge gets. As a result, beam bridges rarely span more than 250 feet. This doesn't mean beam bridges aren't used to cross great distances it only means that there may be a series of beam bridges joined together, creating what's known as a "continuous span." | ||||
| 4.6.1Types of Beam Bridges
Picture: www.wikipedia.org 4.6.2Beams Beams used in buildings may vary in cross sectional shape. Some may be solid or hollow. Below are three different shaped beams. The first beam is a box section, the second an I section beam and the third an L section beam. Solid beams are heavier than hollow beams. Beams like the one's below are given a special cross section for strength and rigidity. They may be as strong as the solid beams but are a lot lighter. |
The inclusion of the elements shown is largely an engineering decision based upon economics, being a balance between the costs of raw materials, off-site fabrication, component transportation, on-site erection, the availability of machinery and the cost of labor. In other cases the appearance of the structure may take on greater importance and so influence the design decisions beyond mere matters of economics. Modern materials such as prestressed concrete and fabrication methods, such as automated welding, and the changing price of steel relative to that of labor have significant.
5. Conclusion
Bridge at the time was built with more modern technology. bridge which was built not only has strong endurance, and equipment also has a beautiful architectural value. Each type of bridge has its own advantages and disadvantages. Because each bridge is built with materials and different costs.
The main types of bridges are arches, beam bridges, cable-stayed bridges, cantilever bridges and suspension bridges. You will have noticed that this list does not include truss bridges. These are usually arches, beams or girders, or cantilevers, or they may be parts of bridges, for example the suspended span of a cantilever bridge, or the deck of a cable-stayed bridge or a suspension bridge. The phrase "truss bridge", however, is sometimes reserved for those which act primarily as beams, while the others are discussed under the heading of the bridges of which they form a part. You could say that a truss, like a box-girder or a pre-stressed span, is more a type of construction than a type of structure.
Bibliography
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