Suspension bridge

A suspension bridge is a type of bridge in which the deck is hung below suspension cables on vertical suspenders. The basic structural components of a suspension bridge system include stiffening girders/trusses, the main suspension cables, main towers, and the anchorages for the cables at each end of the bridge.

1. Suspension Bridge By- Mohit Kumar 2019CET2065 Course: CVL776

2. Contents: Introduction Background Types of Bridges Types of Suspension bridges Components of Suspension Bridge Structural Analysis How do suspension bridges work? Raw Materials Construction of suspension bridge Construction steps of Cable Stayed Bridge Pros and cons of Suspension bridges Case Study Conclusion References

3. INTRODUCTION A suspension bridge is a type of bridge in which the deck is hung below suspension cables on vertical suspenders. The basic structural components of a suspension bridge system include stiffening girders/trusses, the main suspension cables, main towers, and the anchorages for the cables at each end of the bridge . 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. This arrangement allows the deck to be level or to arc upward for additional clearance. Like other suspension bridge types, this type often is constructed without falsework .

4. 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. In some circumstances, the towers may sit on a bluff or canyon edge where the road may proceed directly to the main span, otherwise the bridge will usually have two smaller spans, running between either pair of pillars and the highway, which may be supported by suspender cables or may use a truss bridge to make this connection. In the latter case there will be very little arc in the outboard main cables. The Brooklyn Bridge in New York City and the Golden Gate Bridge in San Francisco are two of the most famous suspension bridges. The Akashi Kaikyo Bridge in Japan, which was completed in 1998, contains the world's longest suspension span (distance between support towers)—6,529 ft (1,991 m); the entire bridge, including the portions between the towers and the shores, totals nearly 2.5 mi (4 km). Construction of the Akashi Kaikyo Bridge took ten years, cost $3.6 billion . The Brooklyn Bridge in New York City

5. BACKGROUND By the eighth century, Chinese bridge builders were constructing suspension bridges by laying planks between pairs of iron chains, essentially providing a flexible deck resting on cables. Similar bridges were built in various parts of the world during subsequent centuries. But the modern era of suspension bridges did not begin until 1808 when an American named James Finley patented a system for suspending a rigid deck from a bridge's cables. Although Finley built more than a dozen small bridges, the first major bridge that incorporated his technique was built by Thomas Telford over the Menai Straits in England. Completed in 1825, it had stone towers 153 ft (47 m) tall, was 1,710 ft (521 m) long, and boasted a span of 580 ft (177 m ). Another American, John Roebling, developed two major improvements to suspension bridge design during the mid-1800s. One was to stiffen the rigid deck platform with trusses (arrays of horizontal and vertical girders that are cross-braced with diagonal beams). Roebling's other important innovation involved construction of the bridge's supporting cables. Around 1830, French engineers had shown that cables consisting of many strands of wire worked better than chains to suspend bridges. Roebling developed a method for "spinning," or constructing, the cables in place on the bridge rather than transporting ungainly prefabricated cables and laboring them into position. His method is still commonly (though not exclusively) used on new bridges .

6. The history of suspension bridges is liberally sprinkled with examples of successful bridges that were widely believed to be impossible when proposed by a visionary engineer. One example was a railway bridge Roebling constructed between 1851-1855 across the Niagra River gorge. The first truss-stiffened suspension bridge, it was supported by four 10 in (250 cm) diameter cables strung between stone towers. Forty years after completion, the bridge was successfully carrying traffic 2.5 times as heavy as it was designed for; at that point it was retired and dismantled. Another landmark suspension bridge was built across the Golden Gate—the mouth of San Francisco Bay—from 1933-1937 by Joseph Strauss. The Golden Gate Bridge is 6,450 ft (1,966 m) long, with a main span of 4,200 ft (1,280 m). Its two towers are 746 ft (227 m) tall; they support two 7,125-ton (6.5 million kg) cables that contain a total of 80,000 mi (129,000 km) of steel wire. Despite rigorous safety precautions, 11 workers died; 19 were saved by a safety net hanging below the deck during construction—an innovation that became standard on later bridge projects. The diagram of the chain bridge over the Menai constructed near Bangor, Wales, in 1820

7. TYPES OF BRIDGES Arch bridges – These bridges uses arch as a main structural component (arch is always located below the bridge, never above it). They are made with one or more hinges, depending of what kind of load and stress forces they must endure. Examples of arch bridge are “Old Bridge” in Mostar, Bosnia and Herzegovina and The Hell Gate Bridge in New York. Beam bridges – Very basic type of bridges that are supported by several beams of various shapes and sizes. They can be inclined or V shaped. Example of beam bridge is Lake Pontchartrain Causeway in southern Louisiana. The Hell Gate Bridge in New York Lake Pontchartrain Causeway in southern Louisiana

8. Truss bridges – Very popular bridge designs that uses diagonal mesh of posts above the bridge. The two most common designs are the king posts (two diagonal posts supported by single vertical post in the center) and queen posts (two diagonal posts, two vertical pots and horizontal post that connect two vertical posts at the top). Cantilever bridges – Similar in appearance to arch bridges, but they support their load not trough vertical bracing but trough diagonal bracing. They often use truss formation both below and above the bridge. Example of cantilever bridge is Queensboro Bridge in New York City. Tied arch bridges – Similar to arch bridges, but they transfer weight of the bridge and traffic load to the top chord that is connected to the bottom cords in bridge foundation . The arch has a shape of its bending moment diagram. They are often called bowstring arches or bowstring bridges. Tied arch bridges Queensboro Bridge in New York City Warren Truss Bridge

9. Suspension bridges – Bridges that use ropes or cables from the vertical suspender to hold the weight of bridge deck and traffic. Example of suspension bridge is Golden Gate Bridge in San Francisco . Cable-stayed bridges – Bridge that uses deck cables that are directly connected to one or more vertical columns. Cables are usually connected to columns in two ways – harp design (each cable is attached to the different point of the column, creating harp like design of “strings” and fan design (all cables connect to one point at the top of the column). Golden Gate Bridge in San Francisco Cable-stayed bridge

10. Types of Suspension bridges Suspension bridges are bridges whose deck is held in place by suspender cable which hang vertically from suspension cables. But they are not all the same. They use different techniques and materials to achieve the same thing – span distances that could not be crossed differently . 1. Simple suspension bridge is a bridge that has no towers nor piers and is suspended on the cables that are anchored at their ends and nothing else. It is also known as a rope bridge, swing bridge, suspended bridge, hanging bridge and catenary bridge and is the oldest variant of the suspended bridge. The deck of this bridge follows is arched downwards and upwards and has additional ropes at a higher level which form the handrail. It is a pedestrian bridge and cannot carry modern roads and railroads. " Simple suspension bridge" can also be name for a suspended deck bridge that has a deck which is not stiffened, hence - "simple". Simple suspension bridge

11. 2. Suspension bridge is a name for a modernly designed suspension bridge – a suspended-deck suspension bridge. It has towers and, from them, cables that hold up the road deck. These cables transfer the weight of the deck, by tension, to the towers and then to the ground by cables whose ends are anchored. This type can carry heavy vehicles and light rail. The first designs of this type of bridge appeared in 16th century but they were not built until 18th century when more materials appeared which allowed for this type of bridge to be made. Longest suspension bridges of today are of this design . Suspension Bridge

12. 3. Underspanned suspension bridge is a type of bridge that was developed in the early 19th century and which has deck that is raised on posts above the main cables which are, at their ends, anchored. It is a very rare design in practice because its deck is not too stable. Some of the bridges built like this are Pont des Bergues (built in 1834), are James Smith’s Micklewood Bridge at Doune in Scotland (it was probably the first one built like this and had chains instead of cables which also makes it chain bridge). Hammersmith Bridge has parts of the roadway built in this manner. Underspanned suspension bridge

13. 4. Stressed ribbon bridge is a modern, improved variant of a classical simple suspension bridge. It has a rigid deck which lays on suspension cables which are in turn embedded in the deck. Deck follows a catenary arc between supports and is stressed in traction, which adds to its stiffness and prevents swaying and bouncing like at simple suspended bridge. This bridge is usually made of concrete reinforced by steel tensioned cables and can carry vehicle traffic. Concrete plates are premade and placed to form the initial structure. Sandbags are place upon the tiles to prestress cables that hold the tiles and gaps between the tiles are filled with concrete. When the concrete dries, sandbags are removed and cables compress, stiffening the bridge and making it more durable . Stressed ribbon bridge

14. 5. Self-anchored suspension bridge has its main cables attached to the ends of the deck rather than ground like standard suspended bridge which allows for construction on elevated piers, or in areas of unstable soils where anchors would be loosen over time. This method of building appeared in mi-19th century and was designed by Austrian engineer Josef Langer in 1859. American engineer Charles Bender patented this method in United States in 1867. Earliest bridges built with this method in United States were Three Sisters Bridges of Pittsburgh, built between 1924 and 1928. Suspension cables cannot be anchored until the deck is finished with this design so a false-work is used to hold them. Three Sisters Bridges of Pittsburgh

15. Differences Between Cable Stayed Bridges and Suspension Bridges The main difference between cable stayed bridges and suspension bridges is in the way that they transfer loads from deck to pylon. As depicted in Figure 1, in cable stayed bridges straight cables transfer deck loads directly to the pylon. But as shown in Figure 2, in suspension bridges, there are main cables (suspension cables) that carry vertical cables. These vertical cables behave as restraints for the deck and transfer deck loads to the main cables.

16. Usually main spans of suspension bridges are longer than cable stay bridges; therefore, decks of the suspension bridges have less stiffness in comparison with cable stay bridges. As a result, suspension bridges have more vibration concerns. In addition, design and construction of suspension bridges are more complicated rather than cable stay bridges; and that's the reason why most of the failures of the cable bridges happened in suspension bridges.

17. Components of Suspension Bridge A suspension bridge should consist of the components shown on the diagram, other elements are added for aesthetic purposes and design. Deck: The deck on a suspension bridge is also referred to as a roadway, where vehicles are allowed to pass to and from points A and B. They can carry motorists, pedestrians, rail traffic etc. They are made out of steel reinforced concrete and each deck is of a large span. Steel Cables: The decking or the roadway is suspended by steel cables. They can be as thick as a tall male human, and are made up of many smaller steel cables; steel is used instead of iron because it is an alloy, which makes it superior in tension and compression and it is stronger. The smaller cables are fastened to one another forming one huge cable enough to hold up to 150,000 tonnes .

18. Suspenders: The suspenders connect the decking to the steel cables and help shape the bridge. Without the suspenders, the roadway would sway out of control; they help reinforce the decking even more as well as having steel cables. Towers : The heavy weight of the steel cables are transferred onto the towers that help the bridge stay standing; the weight that is now supported by the towers is focused onto the ground, reinforcing the tower feet into the ground and keeping the bridge upright . Anchorage Block: These weigh more than the amount of cables that is holding up the deck; this is because it has to withstand a huge proportion of the roadway. Not only this, but it must be strong enough to endure the amount of road traffic and vehicles crossing the bridge at any time. They are often made out of concrete as it is extremely heavy and strong. They appear at both ends of the bridge and preserve the tension from the steel cables.

19. Foundation of Tower: because of the weight pushing down on the towers, they must have a secure foundation. The foundations are pushed far below the soil to keep the towers from tilting and to make sure that they are vertical and strong enough to withstand the weight from the cables. Depending on the softness of the soil, depends on how far down the foundations go; if the soil is soft, then the foundation would be pushed further down. Truss : The truss if found to be underneath the roadway/ decking to support it. Not only this, but it helps stiffen the decking which reduces the probability of it swaying vertically just like it did in the Tacoma Narrows Bridge example

20. Structural Analysis Loads: Three kinds of forces operate on any bridge: The dead load, Live load and dynamic load. Dead load: Dead load refers to the self weight of the bridge itself. Like any other structure, a bridge has tendency to collapse simply because of the gravitational forces acting on the materials of which the bridge is made. Live load: live load refers to the traffic that moves across the bridge as well as normal environmental factors such as changes in temperature (Thermal expansion or contraction). Dynamic load: Dynamic load refers to environmental factors that go beyond normal weather conditions, factors such as sudden gusts of wind and earthquakes.

21. Behavior of loads: Compression: The force of compression pushes down on the suspension bridge's deck, but because it is a suspended roadway, the cables transfer the compression to the towers, which dissipate the compression directly into the earth where they are firmly entrenched. Tension: The supporting cables, running between the two anchorages, are the lucky recipients of the tension forces. The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage to anchorage. The anchorages are also under tension, but since they, like the towers, are held firmly to the earth, the tension they experience is dissipated. The main forces in a suspension bridge of any type are tension in the cables and compression in the pillars.

22. Almost all suspension bridges have, in addition to the cables, a supporting truss system beneath the bridge deck (a deck truss). This helps to stiffen the deck and reduce the tendency of the roadway to sway and ripple. Suspension bridge come in two different designs: the suspension bridge, recognized by the elongated 'M' shape, and the less-common cable-stayed design, which has more of an 'A' shape. The cable-stayed bridge does not require two towers and four anchorages as does the suspension bridge. Instead, the cables are run from the roadway up to a single tower where they are secured. Load Transfer in suspension bridge

23. HOW DO SUSPENSION BRIDGES WORK? On the Severn Bridge, the two main cables act a bit like a washing line. The tension in a washing line supports the weight of the clothes that are pegged to it. In the same way, the tension in the main cables supports the weight of the deck and traffic. The bridge deck is hung from the main cables using wire hangers (rather than clothes pegs). And because the main cables are held up by the towers, the weight of the whole bridge is carried down through the towers, on to the underlying foundations . If you put something heavy on a washing line, it will sag at that point. With a suspension bridge, the road is supported by a stiffening girder, which spreads out the weight of the traffic, so avoiding excessive sag under an exceptional load. If you hang something on a washing line away from the centre , the point will not only sag but it will also move towards the nearest end. Similarly, as a heavy load travels over a suspension bridge, it will not only dip downwards at the point of the load, it will also move longitudinally towards the nearest tower.

24. If you stand on the walkway of the Severn Bridge, you can feel it moving as the traffic travels over it. If you stand by one of the towers and watch the expansion joint, you can sometimes see the whole bridge moving as the weight of the traffic travels across. We should not worry that the bridge moves. It is meant to do this. This is how it absorbs the weight of the traffic and transfers it into the main cables. The tension in the main cables carries the whole weight of the bridge deck and the traffic. This tension is resisted by the anchorages at each end, just as the tension in a washing line is resisted by whatever it is tied to at each end. And because the main cables are held up by the towers, the weight of the whole bridge is transferred through the towers to the ground. Diagram showing the main loads in a suspension bridge

25. Raw Materials Many of the components of a suspension bridge are made of steel. The girders used to make the deck rigid are one example. Steel is also used for the saddles, or open channels, on which the cables rest atop a suspension bridge's towers. When steel is drawn (stretched) into wires, its strength increases; consequently, a relatively flexible bundle of steel wires is stronger than a solid steel bar of the same diameter. This is the reason steel cable is used to support suspension bridges. For the Akashi Kaikyo Bridge, a new low-alloy steel strengthened with silicon was developed; its tensile strength (resistance against pulling forces) is 12% greater than any previous steel wire formulation. On some suspension bridges, the steel wires forming the cables have been galvanized (coated with zinc). The towers of most suspension bridges are made of steel, although a few have been built of steel reinforced concrete. Cross-section of a suspension bridge showing material used

26. CONSTRUCTION OF SUSPENSION BRIDGE 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 construction easier. If a tower will stand in water, its construction begins with lowering a caisson (a steel and concrete cylinder that acts as a circular damn) to the ground beneath the water; removing the water from the caisson's interior allows workers to excavate a foundation without actually working in water. If the bedrock is too deep to be exposed by excavation or the sinking of a caisson, pilings are driven to the bedrock or into overlying hard soil, or a large concrete pad to distribute the weight over less resistant soil may be constructed, first preparing the surface with a bed of compacted gravel. Caisson Foundation over loose soil

27. The piers are then extended above water level, where they are capped with pedestal bases for the towers of single or multiple columns are erected using high-strength reinforced concrete, stonework, or the steel. Concrete is used most frequent in modern suspension bridge construction due to the high cost of steel. Construction details vary with each unique bridge. As an example, consider the Akashi Kaikyo Bridge. Each of its two steel towers consists of two columns. Each column is composed of 30 vertical blocks (or layers), each of which is 33 ft (10 m)

28. Equipment used in construction of towers: Tower Cranes : Each caisson has a tower crane built with it to transport concrete in the caisson columns from ships Ships: Caissons are build at the shores of waterbody and being transported to location of towers with the help of ships . Excavators: are used to excavate soil underneath water upto rock.

29. Pumping of concrete : concrete is transported to tower location in ships and pumped in caissons upto water level Precast columns sections then brought on tower and erected with help of cranes.

30. 2. Saddles: Large devices called saddles , which will carry the main suspension cables, are positioned atop the towers. Typically of cast steel, they can also be manufactured using riveted forms, and are equipped with rollers to allow the main cables to shift under construction and normal loads

31. 3. Anchorage: Anchorages—structures that support the bridge's cables—are massive concrete blocks securely attached to strong rock formations. When the towers and anchorages have been completed, a pilot line must be strung along the cable's eventual path, from one anchorage across the towers to the other anchorage . Anchorages are the structures to which the ends of the bridge's cables are secured. They are massive concrete blocks securely attached to strong rock formations. During construction of the anchorages, strong eyebars (steel bars with a circular hole at one end) are embedded in the concrete. Mounted in front of the anchorage is a spray saddle, which will support the cable at the point where its individual wire bundles fan out—each wire bundle will be secured to one of the anchorage's eyebars . Anchorage of Akashi Kaikyo bridge

32. 4. Catwalks: Temporary suspended walkways, called catwalks , are then erected using a set of guide wires hoisted into place via winches positioned atop the towers. These catwalks follow the curve set by bridge designers for the main cables. Typical catwalks are usually between eight and ten feet wide, and are constructed using wire grate and wood slats. Gantries are placed upon the catwalks, which will support the main cable spinning reels.

33. Equipment used anchorage and catwalks Lift is used to lift up suspended walkways and guide cable atop of towers. Then main cable is laid on catwalks and joint to anchor and tower.

34. 5. Cable construction: To begin spinning the cable, a large spool of wire is positioned at the anchorage. The free end of the wire is looped around a strand shoe (a steel channel anchored to an eyebar ). Between the spool and the strand shoe, the wire is looped around a spinning wheel that is mounted on the pilot line. This wheel carries the wire across the bridge's path, and the wire is looped around a strand shoe at the other anchorage; the wheel then returns to the first anchorage, laying another strand in place. The process is repeated until a bundle of the desired number of wire strands is formed (this varies from about 125 strands to more than 400). During the spinning, workers standing on the catwalk make sure the wire unwinds smoothly, freeing any kinks. As spools are exhausted, the end of the wire is spliced to the wire from a new spool, forming a continuous strand. When the bundle is thick enough, tape or wire straps are applied at intervals to keep the wires together. The wire coming off the spool is cut and secured to the anchorage. Then the process begins again for the next bundle. Spinning of cable

35. The number of bundles needed for a complete cable varies; on the Golden Gate Bridge it is 61, and on the Akashi Kaikyo Bridge it is 290. When the proper number have been spun, a special arrangement of radially positioned jacks is used to compress the bundles into a compact cable, and steel wire is wrapped around it. Steel clamps are mounted around the cable at predetermined intervals to serve as anchoring points for the vertical cables that will connect the decking to the support cable.

36. 6. Hangers/ Vertical Cables: At specific points along the main cable devices called “cable bands” are installed to carry steel wire ropes called Suspender cables. Each suspender cable is engineered and cut to precise length, and are looped over the cable bands. In some bridges, where the towers are close to or on the shore, the suspenders cables may be applied only to the central span.

37. Equipment used in hangers: Suspenders are installed at specific location on main cable by rollers attached on main cable. Cable bands device is used to hang suspended cables.

38. 7. Deck Construction: After vertical cables are attached to the main support cable, the deck structure can be started. The structure must be built in both directions from the support towers at the correct rate in order to keep the forces on the towers balanced at all times. In one technique, a moving crane that rolls atop the main suspension cable lifts deck sections into place, where workers attach them to previously placed sections and to the vertical cables that hang from the main suspension cables, extending the completed length. Alternatively, the crane may rest directly on the deck and move forward as each section is placed. Upon completion of the deck the added load will pull the main cables into an arc mathematically described as a parabola, while the arc of the deck will be as the designer intended.

39. Equipment used in deck construction: Segments if deck are pre-casted on ground Then transported to location using ships and gantry crane on bridge. A crane is moved on main cable which carry deck from ship and lift up to the location where it attached to suspended cables.

40. Design of Road deck The road deck of a suspension bridge is very important. Most deck designs are made from open trusses that allow wind to pass through. It is important to build the deck aerodynamically or else it will twist and could snap . one of the failure example is the Tacoma Narrows Bridge. The truss work of the deck was too flexible and it snapped in strong winds.

41. 8. Finishing: When the deck structure is complete, it is covered with a base layer (e.g., 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 ironworkers and 38 painters continue to work daily on the Golden Gate Bridge, replacing corroding rivets and other steel components and touching up the paint that protects the bridge.

42. Summary of building steps: 1.First huge concrete caissons are sunk into the bedrock to provide a solid base for the towers. 2.Next the towers are constructed on top of the caissons. 3.Giant anchor points are created on both ends of the bridge to keep tension in the cables. 4.Then the main cables are strung across the span of the bridge . 5.A temporary walkway is constructed beneath the main cables so that construction can begin on the road deck. 6.Suspender cables are put into place as the road deck is built to provide strength. 7.When the road deck is finished, a layer of concrete is poured over the steel, followed by a layer of asphalt.

43. Construction steps of Cable Stayed Bridge Stage 1: The pylon above the main piers are erected. Stage 2: A balanced free cantilever is initiated by using derrick cranes which operate on the deck too lift up the girder segments. These are transported to the site on barges. Stage 3: As the cantilevers grow, the stay cables are installed and tensioned to their initial forces to carry the weight of the newly erected segment. Stage 4: The bridge is closed at mid span and the additional loading is applied.

44. Pros of Suspension Bridges 1 . Low Construction Costs: What makes suspension bridges practical is the inexpensiveness of these bridges due to required materials needed for construction. With three basic necessities such as cables, anchorages and roadways, suspension bridges are possible to construct. Having said, this, suspension bridges are great solutions to provide communities with functioning and useful bridges without much need for funding. These are beneficial in areas that lack infrastructure funds. And in the case of allotting budget for projects, the inexpensive costs in building these types of bridges can allow for other projects to be financed. 2. Long Span Another advantage of suspension bridges is the possibility to construct them at different lengths, from 2,000 to 13,000 feet and is lengthier than other types of bridges. This makes it possible to build suspension bridges to connect very long distance locations. Depending on the demand and possibility given, these bridges can be underspanned like the Pont des Bergues and the Micklewood Bridge. On the other hand, three long suspension bridges are in Denmark, Japan and China.

45. 3. Ease of Maintenance Apart from inexpensive construction costs, suspension bridges are known for their minimal maintenance requirements. Once construction is completed, there are no immediate needs for additional materials like cables. What is called for is simply regular maintenance. Moreover, it is known for durability and longevity, making major repairs not needed as often. Consequently, maintenance costs are also not that high . 4. Versatility Suspension bridges do not only cost less to build, they can also be built practically anywhere so long as there are places for building support towers and anchorages. This is also because of the design which is suspended in the air, no inflow restrictors are needed to be placed underneath. They can also bear the beatings of earthquakes. 5 . Attractive Tourists , local and foreign in America love to cross the Brooklyn Bridge and visit the Golden Gate Bridge in San Francisco. Compared to truss and beam bridges, suspension bridges are more aesthetically pleasing because of the different shapes of these bridges.

46. 4. Has Flexibility One common reason that the choice to build a suspension bridge is reached is if it is being built in a high earthquake zone, like California. This is because suspension bridges are flexible due to the cable system they are held up by. The bridge can “move” with the wind and during natural disasters such as an earthquake . 5. Simple Construction No access is needed from below the bridge while it is being constructed, making it a great choice for areas that ships and waterways need to stay clear. 6. Can Be Built High Up Suspension bridges can be built very high up over waterways. This is essential for any area that needs to be able to allow passing ships to come through.

47. Cons of Suspension Bridges 1. Loss of Income Despite the low costs of constructing suspension bridges and the job opportunities they offer, the length of time needed to finish building these bridges are long. What happens is that the businesses that are within the vicinity will be affected since business operations will be hampered. Consequently, there will be loss of sales and profit. This can have a negative impact on the economy of the city or town. Also, bridges built to connect locations between bodies of water can affect the course of ships carrying supplies since they need to divert their routes. This can also result to loss of money since deliveries of goods can take longer. 2. Weak in Winds Despite flexibility and strength to withstand earthquakes, these bridges are not too strong when it comes to powerful winds caused by hurricanes. Too much strong winds can result to damages to suspension bridges. A classic example is the Tacoma Narrows Bridge which collapsed on November 7, 1940 in winds of at only 40 miles per hour. Although the disaster was blamed on design and construction, what happened that time presented risks associated with suspension bridges.

48. 3. Load Limitations Another disadvantage of suspension bridges is the material used which are the cables. These cables have limitations when it comes to bearing the weight of loads. Although it can allow a minimal weight with regard to vehicles passing through, too much weight can lead to the breaking of cables 4. Limited Applications Suspension bridges, despite their cost-effectiveness in construction and flexibility when it comes to site location, have limitations when it comes to its use. This is because they can be destroyed by strong winds and not durable enough to hold limitless weight, careful consideration should be taken before construction. That said, they can only be used by general traffic . 5. Soft Ground Issues If the suspension bridge needs to be built in an area that has soft ground, like over water, very extensive foundation work in order to make it safe for heavy loads. 6 . Too Flexible Flexibility of the suspension bridge design is a major advantage, until conditions become severe. Underneath extreme winds or very heavy the load the bridge can move so much that the bridge would need to be closed.

49. CASE STUDY: Vidyasagar Setu Bridge Kolkata Introduction: Vidyasagar Setu , also known as the second Hoogly Bridge, is a bridge over the Hoogly River in West Bengal, India. It links the city of Kolkata to Howrah. It was the second bridge to be built across the H oogly river, after the H owrah bridge. Design: Suspension Bridge Total length: 822.96 meters (2700 ft ) Width: 35 meters (115 ft ) Longest span: 457.2 meters (1,500 ft ) Clearance below: 26 meters (85 ft ) Designed by Schlaich Bergermann & Partner Constructed by Gammon India Ltd

50. Features: The bridge is a cable-stayed bridge (121 cables) with a fan arrangement built on steel pylons 127.62 meters in height. It has composite steel reinforced concrete deck having two carriage ways with a total width of 35 meters, with 3 lanes each way and with a foot path of 1.2 meters on ether side. The deck is over the main span of 457.20 meters (1500 ft ) length and two side spans of 182.88 meters (600 ft ) each, supported by wire cables.

51. Construction: Towers: made of 4 x 4 m steel boxes of riveted construction were raised on the two side spans of the bridge, one set on the Calcutta and the other on the Howrah side pylons. Deck & Cable : Construction of the main span of the bridge was to erect it from both ends, as cantilevers. A deck crane was used for this erection. Cables were erected from the four pylon heads with the help of hoist frames which were mounted on top of each pylon.

52. Conclusions Longer main spans are achievable than with any other type of bridge Less material may be required than other bridge types, even at spans they can achieve, leading to a reduced construction cost. 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. May be better to withstand earthquake movements than heavier and more rigid bridges. Bridge decks can have deck sections replaced in order to widen traffic lanes for larger vehicles or add additions width for separated cycling/pedestrian paths. Considerable stiffness or aerodynamic profiling may be required to prevent the bridge deck vibrating under high winds. The relatively low deck stiffness compared to other (non-suspension) types of bridges makes it more difficult to carry heavy rail traffic where high concentrated live loads occur. Some access below may be required during construction, to lift the initial cables or to lift deck units. This access can often be avoided in cable-stayed bridge construction.

53. References: "Bluff Dale Suspension Bridge". Historic American Engineering Record. Library of Congress . "Barton Creek Bridge". Historic American Engineering Record. Library of Congress. Troyano , Leonardo (2003). Bridge Engineering: A Global Perspective. Thomas Telford. pp. 650–652. ISBN 0- 7277-3215-3 . "Cable Stayed Bridge". Middle East Economic Engineering Forum "Bridging To The Future Of Engineering" (Press release). American Society of Civil Engineers. 12 March 2007. Retrieved 8 March 2008. "Outstanding Civil Engineering Achievement Awards". Texas Section-American Society of Civil Engineers. Archived from the original on February 18, 2016. Retrieved January 5, 2017 . Kingston, Jeremy.  How Bridges are Made.  New York: Facts on File, 1985 Wikipedia Madehow.com For video: https://www.youtube.com/watch?v=0gtUkIlm5Jk