CABLE AND SUSPENSION BRIDGES
CABLE AND SUSPENSION BRIDGES —— CABLE :The cables are flexible structures which carry loads in tension only. The cables carry vertical loads and are suspended between the supports. Cable stayed bridges utilize tensioned cables to transmit major loads of the structure. The tension in the cables enables the structure to be stable Cable structures allow displacement of loads from one part of the structure to the another cable stayed bridges are supported by cables at one or more points from one or more towers
Characteristics of A Cable
Cables are widely used to support building roofs, suspension bridges, trolley wheels, electrical transmission lines, derricks and various other engineering structures. The purpose of providing a cable is to support and to transmit loads from one member to another member. Cables in the form of fixed ropes can be permanently provided for transmission towers and chimneys and temporarily during erection.
The cables are preferred for structures (especially for trusses), whose spans are greater than 50 m. The cables with stiffening girders are suitable for road ways and are not recommended for railways. The cable tension due to given loading is reduced if the provided dip of the cable is greater.
Due to its flexible nature, the shape of the cable varies according to the nature and position of the loading and offering resistance to shear bending which is due to flexibility. The cable under the given loading takes the shape of a funicular polygon.
The tension in the cable is resisted by concrete mass at culated supports. The traffic load acting over the deck of the bridge is carried by vertical suspenders (one set at each side) which in turn transfer the loads to the suspension cables. These are the main loading members and they do not take up any bending moment. By providing or stiffening girder (three hinged or a two hinged girder), the initial shape of the cable can be retained. The cable used in suspension bridges are available in the form of wire ropes or parallel wires which are grouped together with dips.
Assumptions in Analysis of Cable
(i) Cable is stable and flexible.
(ii) When external loads act on the cable, self weight of cable is not considered.
(iii) The length of cable is always constant and therefore it is assumed as a rigid body.
(iv) The force in the cable is tangential to the cable profile as it carries only axial tensile forces.
V. The load acting on the cable is assumed to be uniformly distributed even if it is moving load.
Suspension Bridge
Suspension bridge has got two cables which are stretched over the span. Each cable run over two towers and is anchored by anchor to have a firm foundation. Cable is flexible throughout the span and its bending moment at every point is taken as zero. The load transferred by hangers or suspenders are assumed to be UDL. When the span is more than 200 mts for a road way and 300 mts for light way traffic suspension bridge is preferred.
Suspension bridge consists of the length of the bridge. The suspended cable is supported by two ore more towers In suspension bridges, the load bearing portion is hung below the suspension cable on vertical suspenders. The main forces in suspension bridges are tension in the cables and compression in pillars. Suspension bridg
Components and function of suspension bridge
Suspension Cable :The suspension cables are flexible members and are considered as main load bearing structural component of a suspension 1 1 bridge. The central dip of the suspension cable is about 10 to 15 of the horizontal span. The suspension cables take only direct tension and cannot take any bending moment.
Suspenders :The suspenders are provided to transfer the traffic load on the deck slab to the suspension cable as a UDL. These suspenders are closely spaced
Supporting Towers :The supporting towers are basically designed for strength, stability and for architectural value of the structure. It provides foundation and glory to the bridge. The suspension cable is supported on the towers on its either sides and the height of the tower is about 20 to 200 m. For passing the suspension cable on the other side of the tower a saddle placed on rollers or a guide pulley in provided over the towers.
Anchor Cable :The cable tensions in suspension cable are of the order of several hundered tonnes and due to this reason, the anchoring of the suspension cable becomes a perplex task. The suspension cables are to be anchored to the bed rock, after they have been passed over the tall pylons.
Stiffening girder :
Stiffening girders are the major load bearing members in suspension bridges. As they are flexible, they change their shape with the nature and position of the moving live load on deck slab. Stiffening girders may be classified into two types,
1.Three hinged stiffening girders
2. Two hinged stiffening girders.
1.Three hinged stiffening girders : If the bridge is stiffened with three hinged stiffening girder it maintains its parabolic shape during the movement of loads over the bridge.If moving loads are involved, then the cables are assumed to carry uniform load and hence the stiffening girder will be subjected to bending moment and shear force.
2. Two hinged stiffening girders: 1. These are used to decrease the sag under the rolling load. 2.Suspension cable bridges are stiffened with two hinged stiffening girders to make them stiff. 3.These structures are statically indeterminate and by using energy methods, the forces in the cable may be obtained. 4. When the girder is assumed to be rigid, the load at any position is transferred in the form of udl (to the cable)
Functions of Stiffening Girders
1. Stiffening girders allow the suspension bridge deck to remain in its actual position after the application of load also.
2. The live load acting on the deck slab, (point load or udl) is converted into equivalent udl and is transferred to the deck slab. It enables the cable to maintain its parabolic shape while the loads pass through the deck slab.
3. The cables take directly the dead load of the girder.
4. The dead load of the girder does not cause any shear force or bending moment in the girder.
5. The stiffening girders are subjected to shear force and bending moment due to live loads and they should resist them safely.
Suspension bridge with a stiffening girder
