suspension bridge ppt

A presentation on Suspension bridge

1. SUSPENSION BRIDGES

2. INTRODUCTION The Bridge is a structure carrying a road, railway, path etc., across an obstacle (Ex:- River) There are various designs of Bridge to span different lengths. The Bridge is designed in such a way that all the forces acting on it are transferred to the ground. Bridge designers in other words are Force Balancers.

3. TYPES OF BRIDGES 1)BEAM BRIDGE 2)ARCH BRIDGE 3)TRUSS BRIDGE 4)CANTILEVER BRIDGE 5)SUSPENSION BRIDGE 6)CABLE STAY BRIDGE

4. BEAM BRIDGES Beam Bridges are simplest kind of Bridges. These Bridges consist of Horizontal with supports usually on either ends. They are constructed for short span requirements. The weight of Bridge and any traffic on it is directly supported by Piers. In Beam Bridges the top side of the deck is under compression while the bottom side of the deck is under tension . TRUSS BRIDGES Truss Bridge is a sub-type of Beam Bridge. To increase the bridges strength designers introduced truss to Bridges Beam. The Truss of a Beam is a structure of connected elements usually forming Triangular units. The Triangles help to distribute the weight of the vehicles The connected elements (typically straight) may be stressed from tension, compression , or sometimes both in response to dynamic loads

5. ARCH BRIDGES Arch bridges are arch shaped and have abutments at each end. The Arch Bridge does not need additional supports or cables. In fact it’s the shape of structure gives it the strength. The forces on Arch bridge are only compression forces. The weight on the bridge is carried to abutments at either side. Usually they are made for short span range . But often set end to end to form a large total length.

6. CANTILEVER BRIDGE A Cantilever Bridge is a Bridge built using Cantilevers, Structures that project horizontally into space, supported only on one end. For small foot bridge , the cantilever may be simple beams ; However large cantilever Bridges are designed using trusses. In the structure of Cantilever bridge at least one portion acts as an anchorage for sustaining another portion which extends beyond supporting pier. A simple Cantilever span is formed by 2 Cantilever arms extending from opposite sides of an obstacle to be crossed. The Cantilever arms do not meet in center , instead they support a central suspended span which rests on the ends of Cantilever arms. The suspended span may be built off site and lifted into place. All the weight in the middle of bridge is pushed towards piers and abutments which distribute the weight. It can span distances over 460m (1500 feet).

7. SUSPENSION BRIDGE The deck (traffic way) of Suspension Bridge is hung by Suspender cables which hang from master cables. The Suspension (main) cables are secured into solid Bed Rock (If available at sight) or into Concrete blocks called Anchorages on both ends of the bridge to stabilize structure and are key to the structure. The sequence of force transmission for a suspension bridge is 1.To the Deck 2.To the Suspender cables 3.To the main cable 4.To the Bridge tower 5.To the foundation CABLE-STAYED BRIDGE A Big drawback of Suspension bridge is that they need to be anchored to the ground on either side. A different kind (sub type) of suspension bridge , known as Cable-Stayed Bridge , does away with this by balancing two sets of suspension cables on either side of each pier, which supports load. Cable stayed bridges are stronger but significantly shorter than conventional Suspension Bridges and generally do not span distances much greater than 1 km. The sequence of force transmission for the Cable-Stayed Bridge is 1.To the Deck 2.To the Stay cable 3.To the Bridge Tower 4.To the Foundation

8. COMPONENTS OF SUSPENSION BRIDGE

9. ORIGIN Rope bridges have been used since ancient times to cross the rivers. They added planks of wood to form deck that makes bridge easy to cross. They used iron chains instead of ropes to carry heavy loads. To flatten the sagging deck they suspended Bridge from stone towers and extended the ropes downwards to level the deck. To secure the chains at each end they anchored the chains to rocky banks on either side. This is how the design of suspension bridge has originated. The first Iron chain suspension bridge was Jacobs Creek Bridge (1801) in Pennsylvania; designed by inventor James Finley. The First important modern suspension bridge is Menai Bridge (1826) designed by Thomas Telford which was the first to incorporate all of necessary components of a Modern Suspension Bridge.

10. LEAP 1:- STRONGER CHAINS In 1845, 300 people crowd are together standing on a suspension bridge in Great Yarmouth. When all crowd crossed to one side of a bridge , it overburdened the chains and they broke out and bridge collapsed into the river drowning 79 people. In 1851, when Engineers are planning 250m Niagara Bridge, they replaced Chains with cables made out of metal wires to increase its Strength.

11. LEAP 2:- BUILDING UNDER WATER For Longer spans the supporting towers must be built in water. In 1874, when Engineers are planning Brooklyn Bridge of 486m, the challenge is to anchor piers in rushing water below. The Solution is Caisson, which provides working space for men beneath the water for excavation. Technological advances now allows to dig out sea bed foundations using machinery without risking the lives of men.

12. LEAP 3:-TALLER TOWERS The next breakthrough came in Golden Gate Bridge (1280m long .) The engineers understood that to lengthen the roadway the towers height must also be raised . To raise the height of towers engineers used hollow steel shafts for construction of towers instead of stone.

13. LEAP 4:- WIND As the decks got more longer they are more likely to twist and bend. Then for construction of Verrazano Narrows Bridge of 1298m the design must resist particularly destructive force THE WIND. In 1940; Tacoma Narrows Bridge in Washington collapsed due to wind after 4 months from opening date. The solution is to introduce stream line profiles into sides of the deck. Since the Verrazano Narrows Bridge is Double deck bridge stream line profile is not a solution as it makes currents of air to collide and produce further destruction. So rather than deflecting wind they decided to resist it by stiffening the deck. This is done using Open Box Concept.

14. LEAP 5:- EARTH QUAKE To build worlds biggest suspension bridge Akashi Kaikyo Bridge of 3911m engineers have to defeat a powerful force of nature EARTHQUAKE. Japan is situated in worlds most seismically active areas. The first line of defence from Earthquakes are the Bridge towers themselves. They are built of Steel to make them flexible. The second layer of protection are 20 huge pendulums called dampers in in each towers each of weight 10 tons.

15. CONSTRUCTION SEQUENCE

16. 1. TOWER CONSTRUCTION Tower foundations are prepared by digging down to a sufficiently firm rock formation. Some Bridges are designed so that their towers are built on dry land, which makes their construction easier. If a tower will stand in water , its construction begins with lowering a Caisson ( a large watertight chamber, open at the bottom, from which the water is kept out by air pressure and in which construction work may be carried out under water ) to the ground beneath the water; Removing water from the Caissons interior allows workers to excavate a foundation without actually working in water. When excavation is complete a concrete tower foundation is formed. From the tower foundation , towers of single or multiple columns are erected using high strength reinforced concrete , stonework, or steel. Concrete is used most frequently in modern suspension bridge construction due to high cost of steel. Large devices called Saddles, which will carry the main suspension cables, are positioned at top of towers.

17. 2. CABLE CONSTRUCTION When the towers and anchorages have been completed, a pilot line must be strung along the cables eventual path, from one anchorage across the towers to other anchorage. Various methods can be used to position the pilot line: Ex:- Kites, Helicopters, Boats. When the pilot line is in place, a Catwalk is constructed for bridges entire length, about 1m below the pilot line so workers can attend cable formation. The spinning wheel mounted on the Pilot line carries the wire across the Bridges path to and fro between anchorages on either side. This process is repeated until a bundle of desired no of wire strands is formed. (It varies from about 125 strands to more than 400.) During spinning , workers standing on catwalk make sure it unwinds smoothly, freeing any kinks. When proper number have been spun, a special arrangement of rapidly positioned jacks is used to compress the bundles into compact cable, and then a steel wire is wrapped around it. Steel clamps are mounted around the cable at predetermined intervals to serve as anchoring points for vertical cables that will connect the decking to the support cable.

18. 3. ANCHORAGE CONSTRUCTION Anchorages are structures to which the ends of bridges cables are secured. They resist the Tension of cables and form as the main anchor system for entire structure. The cables are usually anchored to good quality rock. If there is no solid rock at the site to secure them into, the anchor point is built on the shore line using steel frames and concrete.

19. 4. DECK CONSTRUCTION After vertical cables are attached to main support cable, the Deck structure can be started. In one technique, a moving crane that rolls at top of suspension bridge lifts Deck sections into place , where workers attach them to previously placed sections and to the vertical cables that hung from main suspension cables. Alternatively, the crane may rest directly on the deck and move forward as each section is placed. The structure must be built in both directions from support towers at correct rate in order to keep the forces on the towers balanced at all times.

20. 5. FINISHING When the Deck Structure is complete, it is covered with a base layer (Ex:-Steel plates) and paved over. Painting the steel surfaces and installing electric lines for lighting are examples of other finishing steps. In addition, ongoing maintenance procedures begin. For example a permanent staff of 17 iron workers and 38 painters continue to work daily on Golden Gate Bridge , replacing corroding rivets and other steel components and touching up the paint that protects the Bridge.

21. CONCLUSION The design of bridge is to be selected based on span and site conditions. When spanning long distances (>1 km) Suspension Bridge is the only option for Bridge designers. The Suspension bridges had grown in span up to 4 km using power of Human Ingenuity . They must be designed very carefully considering all forces of nature . Engineers spanned bridges between cities , states and even Countries and even continents.