CHAPTER 1 INTRODUCTION
1.1 Introduction A bridge consists of super structure of steel or reinforced concrete member that is supported on one or more points by cables extending from one or more tower is known as cable stayed bridge. The cable-stayed bridge is one of the most modern bridges. It consists of a continuous strong beam (girder) with one or more pillars or towers (pylons) in the middle Cables stretch diagonally between these pylons and the beam, these cables forms intermediate supports for deck and the cables are anchored in the tower rather than at the end. Most of the cable stayed bridges have been build across the navigable rivers where dimensions of bridge are decided by the navigation requirements. The cable stayed bridge is ideal for spanning the natural barriers of wide rivers, deep valley and for the vehicular and pedestrian bridge crossing the wide highways because there are no pier that will form obstructions. 1.2 Basic concept The basic concept of cable stayed bridges is to provide intermediate support using inclined cables. The pre-tension force in cable is design such that the bending moment or deflection at that point is as per design requirements. Closely spaced stay cables reduces the required depth and bending stiffness of girder. This leads to simple cross section and superior dynamic behavior. The multi-cable-stayed system allows main span up to about 200m for concrete and up to 1000m for steel with considerable saving on steel over suspension bridges (Berg, 2003).
Figure 1.1: Force in cable and pylon. Courtesy: www.intel.com/.../bridge/cablestayed The tower is responsible for absorbing and dealing with compression forces Tension occurs along the cable lines, this works because a moving load is not applied evenly across the bridge, and as it moves one set or the other of the diagonals will find itself in tension (Figure 1.1). This system is advantageous because it uses single support, A Well-balanced Cables can be fabricated separately and Horizontal loads are contained within the structure. It is ideal for use when the river banks are fragile for example if the banks are alluvial mud. The cables disperse a load across more area easily. It has Greater inherent rigidity of the triangulated cable-stayed bridges. 1.3 Structural Advantage of Cable-Stayed Bridge 1) Horizontal compressive forces due the component of cable force are taken by the girder and no massive anchorage is required therefore substructure is economical.
The orthotropic deck can easily take the amount of axial force developed due to cable with almost no material.
Excellent aerodynamic stability, by the increase of damping capacity of the system due to large number of stay cables with variable lengths and different natural frequencies.
Cable-stayed bridges have a low center of gravity which makes them strong against earthquakes.
Ease of replacing the stay cables in case of deterioration, without having to interrupt the traffic on the structure. If any one cable is removed, the distribution of forces in the structure is hardly changed under reduced live load.
1.4 Comparison with Suspension Bridge A multiple-tower cable-stayed bridge may appear similar to a suspension bridge, but in fact is very different in principle and in the method of construction. In the suspension bridge, a large cable is made up by "spinning" small diameter wires between two towers, and at each end to anchorages into the ground or to a massive structure. These cables form the primary load-bearing structure for the bridge deck. Before the deck is installed, the cables are under tension from only their own weight. Smaller cables or rods are then suspended from the main cable, and used to support the load of the bridge deck, which is lifted in sections and attached to the suspender cables. As this is done the tension in the cables increases, as it does with the live load of vehicles or persons crossing the...
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