Bridge+notes+II

Bridge Notes II:



__Beam bridge: __

Beam bridges are the most simple of structural forms being supported by an abutment at each end of the deck. No moments are transferred through the support hence their structural type is known as simply supported. The simplest beam bridge could be a slab of stone, or a plank of wood laid across a stream. Bridges designed for modern infrastructure will usually be constructed of steel or reinforced concrete, or a combination of both. The concrete used can either be reinforced, prestressed or post-tensioned. Beam bridges operate with two types of beams, some in the direction of the slab ranging from column to column and others that are perpendicular which rely on the first beams. 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.



__Cable- stayed bridge: __

A cable-stayed bridge is a bridge that consists of one or more columns (normally referred to as towers or pylons), with cables supporting the bridge deck. There are two major classes of cable-stayed bridges: In a harp design, the cables are made nearly parallel by attaching them to various points on the towers so that the height of attachment of each cable on the tower is similar to the distance from the tower along the roadway to its lower attachment. In a fan design, the cables all connect to or pass over the top of the towers. Compared to other bridge types, the cable-stayed is optimal for spans longer than typically seen in cantilever bridges and shorter than those typically requiring a suspension bridge. In a cable-stayed 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. Bridges is that the former usually have a suspended span, and the latter do not. 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. Some of the advantages of the cable-stayed bridges are that there is no need for anchorages to sustain strong horizontal forces, because the spans are self-anchoring. They can be cheaper than suspension bridges. Many asymmetrical designs are possible. Some of the disadvantages of this type of bridge are that in the longer sizes, the cantilevered halves are very susceptible to wind induced oscillation. The cables require careful treatment to protect them from corrosion.

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__<span style="font-family: Arial,Helvetica,sans-serif; font-size: 150%;">Cantilever bridge: __

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">A cantilever bridge is a bridge built using cantilevers, structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from <span class="wiki_link_ext">structural st e<span class="wiki_link_ext">el, or <span class="wiki_link_ext">box girders built from <span class="wiki_link_ext">prestressed concrete. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">A simple cantilever span is formed by two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the center. In a common variant, the suspended span, the cantilever arms do not meet in the center; instead, they support a central <span class="wiki_link_ext">truss bridge which rests on the ends of the cantilever arms. The suspended span may be built off-site and lifted into place, or constructed in place using special traveling supports. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">Thus, in a bridge built on two foundation piers, there are four cantilever arms: two which span the obstacle, and two anchor arms which extend away from the obstacle. Because of the need for more strength at the balanced cantilever's supports, the bridge superstructure often takes the form of towers above the foundation piers. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">Commonly, the structure distributes the tension via the anchor arms to the outermost supports, while the compression is carried to the foundations beneath the central towers. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">Usually this kind of bridge is 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. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">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. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">Some of the advantaged of cantilevers bridges are that Building out from each end enables construction to be done with little disruption to navigation below. 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. But this kind of bridge also have some disadvantaged such as 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.

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__<span style="font-family: Arial,Helvetica,sans-serif; font-size: 150%; line-height: 115%;">Suspension bridge: __

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">A suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">This type of bridge has cables suspended between towers, plus vertical suspender cables that carry the weight of the deck below, upon which traffic crosses. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">The suspension cables must be anchored at each end of the bridge, since any load applied to the bridge is transformed into a tension in these main cables. The main cables continue beyond the pillars to deck-level supports, and further continue to connections with anchors in the ground. The roadway is supported by vertical suspender cables or rods, called hangers. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">In this kind of bridge is very important that deck has also to possess enough local rigidity in bending and torsion to prevent undue flexure as vehicles pass. This same rigidity must be sufficient to help in the task of preventing undesirable amplitudes of oscillation. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">Since almost all the force on the pillars is vertically downwards and they are also stabilized by the main cables, the pillars can be made quite slender. Among of the main important advantages of suspension bridge are the longer main spans are achievable than with any other type of bridge <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">Less material may be required than other bridge types, even at spans they can achieve, leading to a reduced construction cost <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">Except for installation of the initial temporary cables, little or no access from below is required during construction, for example allowing a waterway to remain open while the bridge is built above <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">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. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">Some of the disadvantages are that they need a considerable stiffness or aerodynamic profiling may be required to prevent the bridge deck vibrating under high winds <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;"> They are only as rigid as the deck structure. This makes them generally unsuitable for railway traffic. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 120%;">During construction, the cables and towers may be susceptible to wind induced oscillations. Constructing the cables or chains across the gap can be a lengthy process.