Explain the Construction of Cable Stayed Bridge by Cantilever Method.
The cantilever method is normally adopted for the construction of long-span cable-stayed bridges. Here the towers are built first. Each new segment is built at site or installed with precast segment, and then supported by one new cable or a pair of new cables which balances its weight.
The stresses in the girder and the towers are related to the cable tensions. Since the geometric profile of the girder or elevation of the bridge segments is mainly controlled by the cable lengths, the cable length should be set appropriately at the erection of each segment. During construction, monitoring and adjustment of the cable tension and geometric profile require special attention.
A notable example of the construction of a major cable-stayed bridge by the cantilever method is the Yangpu bridge in Shanghai, China, built in 1994 with a main span of 602 m. The composite girders of this bridge consisted of prefabricated, wholly welded steel girders and precast reinforced concrete deck slabs.
Depending on the bridge site, cable-stayed bridges can have any one of four general layouts of spans :
(a) Cable stayed bridges with one eccentric tower, eccentric with respect to the gap to be bridged, e.g. Severin’s bridge;
(b) Symmetrical two-span cable stayed bridges, e.g. Akkar bridge :
(c) Three-span cable stayed bridges, e.g. Second Hooghly bridge, Stromsund bridge :
(d) Multi-span cable stayed bridges, e.g. Millau viaduct. Of these, the most common type is the three-span cable stayed bridge, consisting of the central main span and the two side spans. Temporary stability during construction is a major problem, particularly just prior to closure at midspan. The structure must be able to withstand the effects due to wind and accidental loads due to mishaps during erection. When intermediate piers are provided in the side spans, the stability is very much enhanced, In this case, the side spans are built first on the intermediate supports, and later the long cantilevers in the main span.
Discuss the Detail of Cable used in Cable Stayed Bridge.
The stay cables constitute critical components of a cable stayed bridge, as they carry the load of the deck and transfer it to the tower and the backstay cable anchorage. So the cables should be selected with utmost care. The main requirements of stay cables are :
(a) High load carrying capacity :
(b) High and stable Young’s modulus of elasticity :
(c) Compact cross-section ;
(d) High fatigue resistance ;
(e) Ease in corrosion protection ;
(f) Handling convenience; and
(g) Low cost. The ultimate tensile strength of wire is of the order of 1600 MPa. While locked coil stands have been used in early bridges (e.g. Stromsund bridge), the recent preference is towards the use of cables with bundles of parallel wires or parallel long lay stands. The sizes of cables are selected to facilitate a reasonable spacing at the deck
anchorages. Parallel wire cables using 7 mm wires of high tensile steel have been adopted in the Second Hooghly bridge. Corrosion protection of the cables is of paramount importance. For this purpose, the steel may be housed inside a polyethylene (PE) tube which is tightly connected to the anchorages. The cables are anchored at the deck and at the tower. The anchorage at the deck is fixed and has a provision for a neoprene pad damper to damp oscillations. The length adjustment is done at the tower end.
The cables are prestressed by introducing additional tensile force in the cables in order to improve the stress in the main girder and tower at the completion stage, to prevent the lowering of rigidity due to sagging of cable, and to optimize the cable condition for the erection. The magnitude of the prestress is determined by taking into consideration the following factors :
(i) the horizontal component there is no in-plane bending of the tower due to unbalanced horizontal fore due to dead load at the completion stage; and
(ii) the net force on the main girder member at the connection of the cable at the completion stage be zero.
Currently, the steel used for cables have ultimate tensile strength (UTS) of the order of 1600 MPa. Carbon fiber cables having UTS of about 3300 MPa are under development. The latter cables are claimed to have negligible corrosion and to possess high fatigue resistance. However, carbon fiber cables are presently very expensive.
What are Cantilever Bridges? Explain in detail using examples.
A cantilever bride with a single main span consists of an anchor arm at either end between the abutment and the pier, a cantilever arm from either pier to the end of the suspended span, and a suspended span. Such an arrangement permits a long clear span for navigation and also facilitates the erection of steelwork without the need for supporting centering from below.
Steel cantilever bridges came into general use for long-span railway bridges, because of their greater rigidity compared with suspension bridges. Three well-known examples are shown in Figure 2.6. The Firth of Forth bridge with two main spans of 521 m each became a milestone in bridge construction on its completion in 1889.
The designers, John Fowler and Benjamin Baker used tubular members of fairly large size with riveted construction for the arch ribs to withstand wind pressures of 2.68 KN/m”. Though the tubes were large in size, the weight per linear meter of the bridge was still less than that of the Quebec bridge.
The design of the Quebec bridge was first entrusted to Theodore Cooper, who was then well known for his specifications on railway bridges. The plan envisaged a main span of 549 m with anchor spans of 157 m each, making this bridge the longest span in the world. The first attempt to construct the bridge ended in the complete collapse of the south arm killing 75 men (1907). The failure was due to miscalculation of dead load and wrong design of compression members, which errors were not noticed in time. The design was revised by H.A. Voutelet and the structure was reconstructed in 1917 with the same main span.
Howrah bridge with a main span of 457 m was the third-longest span cantilever bridge in the world at the time of its construction (1943). The bridge was erected by commencing at the two anchor spans and advancing towards the center with the use of creeper cranes moving along the upper chord. The closure at the middle was obtained by means of sixteen hydraulic jacks of 800 t capacity each, The construction was successfully completed with very
Osaka Port bridge was completed in 1974 with a clear span of 510 m. The bridge is double-decked and is currently the world’s third-largest span cantilever bridge. The construction has been achieved without accidents and with great precision, testifying to the great advance in technology in bridge construction.
The weight of the structure and the labour involved in the construction of a cantilever bridge are large compared with a cable stayed bride of the same clear span. Hence the cantilever bridge is not very popular at present.
Cable stayed bridges in India
How the Cable Hayed Bridge is analysed?
The cable stayed bridge with the multi-stay configuration is a statically indeterminate structure with a high order of indeterminacy. The deck acts as a continuous beam on elastic supports of varying stiffness. Bending moments in the deck and pylons increase due to second-order effects due to deflection of the structure. The effects of creep and shrinkage during construction and service life should be considered for concrete and composite decks.
The internal force distribution in the deck and tower can be managed to be compressed with minimum bending, by adjustment of the forces in the stay cables. A rigorous analysis considering three-dimensional space action is quite complex. Approximate designs can be made using a two-dimensional approach.
Though the cable stays show a non-linear behavior due to large displacements, sag in cables, and moment-axial force interactions in stays, girders and towers, an approximate analysis assuming linear behavior leads to satisfactory girders and towers, an approximate analysis assuming linear behavior leads to satisfactory results in most cases. However, a non-linear analysis is essential for very long-span bridges.
Give Statistics of Selected Suspensions Bridges.
Some statistics of selected suspension bridges are shown in Table 2.1. It can be seen that the Span/Depth ratio has steadily increased from 94 for the Brooklyn Bridge (1863) to 168 for the Golden Gate bridge (1937). The Tacoma Narrows I bridge adopted a ratio of 350 with stiffening plate girders, and it failed due to aerodynamic instability. Conservatism dictated the use of a ratio of 85 for the replacement structure.
With gaining of confidence with the conventional stiffening truss design, the Span/Depth ratio again increased to 177 for the Verrazano Narrows bridge (1964). The Seven bridge (1966) pioneered the all-welded closed box deck with inclined suspenders, and the innovative design achieved a Span/Depth ratio of 324.
This was also followed in the Great Belt East bridge with a ratio of 406. The Akashi Kaikyo Bridge, which has the longest span of 1991 m, has adopted the conventional design for stiffening trusses and thus maintained a ratio of 142. The aerodynamic stability will have to be investigated thoroughly by detailed analysis as well as wind tunnel tests on models.
Write a short note on Deck Structure in Suspension Bridge.
While the deck is merely supported by the cables in a suspension bridge, the deck of a cable stayed bridge is an integral part of the structure resisting the axial force and bending induced by the stay cables. For bridge width greater than 15 m and spans in excess of 500 m, the need to reduce deadweight prompts the use of an all-steel orthotropic plate deck, as adopted for the Normandie bridge and the Tatara bridge.
Torsion box deck sections in prestressed concrete have been used with single-plane systems, as in Brotonne bridge and the Sunshine Skyway bridge. Composite deck sections have been employed in the Second the Sunshine Skyway bridge. Composite deck sections have been employed in the Second Hooghly bride at Kolkata, India, and the Second Severn Crossing, UK. Special attention should be devoted to the anchorage of cables to the deck. The superstructure of the main span is normally constructed using the segmental cantilever method.
The ratio of the side span (L,) to the main span (Lm) for the case of a bridge with towers on both sides of the main span usually lies between 0.3 and 0.45. The ratio LLM Can be 0.42 for concrete highway bridge decks and not more than 0.34 for railway bridges”. This ratio influences the changes in stress in the backstay cables due to variation of live load. It further influences the magnitude of vertical forces at the anchor pier, the anchor force decreasing with increasing L/Lm. The choice of depends also on the local conditions of water depth and foundation.
What is the role of Towers in Case of Cable Stayed Bridge ? Also discuss the different forms of Towers in Detail.
Towers carry the forces imposed on the bridge to the ground. They are not replaceable during the life of the bridge. Hence they should be designed to be structurally strong. constructible, durable, and economical.
The towers may take any one of the following forms:
1. Single free standing tower, as in Nordelbe bridge ;m owl es ogbind oplia
2. Pair of free-standing tower shafts, as in Dusseldorf Northbridge ;
3. Portal frame, as in Stromsund bridge and Second Hooghly bridge
4. A-frame as in Severin’s bridge or inverted Y-shape as in Yangpu bridge;
5. Diamond configuration as in Globe Island bridge, Sydney.
When the stay cables are in one place, a single free-standing tower may be adopted. In this case, the pier below the box girder should be sufficiently wide for bearings to resist the o to torsional moments of the superstructure. For bridges with cables in two planes, the towers can be a free-standing pair or a portal frame with a slender bracing.
An additional bracing append may be introduced below the deck. The A-shaped tower and the inverted Y-shaped tower banon have been favored for long bridges having shallow box girder decks in regions of strong wind forces. The land takes at the base can be reduced by adopting a diamond configuration, as used in the Tatara bridge.
Since the tower is the most conspicuous component in a cable stayed bridge, besides structural considerations, aesthetics plays a prominent part in the selection of the particular ai male shape of the tower. For example, the proximity of the Cologne cathedral influenced the adoption of the A-frame for the Severins bridge. Sometimes, an additional height is provided for the tower above the point of connection of the cable for architectural reasons, as in the Nordelbe bridge (in this case, as a tribute to the city fathers).
Anchorage of cables at the tower should follow good order. Since the cables at the deck level are anchored along a line along the edges or at the middle of the deck, it is natural that these should end along a vertical line at the tower head. In the case of an A-shaped tower, the anchorage line can be parallel to the tower leg. It is not desirable to spread the anchorages transversely in one layer at the tower.
What are the various examples of Suspension Bridges? Also, give some detail of it.
The suspension bridge is currently the only solution for spans in excess of 900 m, and is regarded as competitive for spans down to 300 m. The Wheeling suspension bridge across the Ohio River in the USA was built by Charles Ellet in 1849 with a span of 308 m and rebuilt by John Roebling in 1854 after tornado damage was the first long-span wire-cable suspension bridge in the world. The Brooklyn Bridge in New York designed by Roebling was completed in 1886 central span of 486 m. This was followed by other notable bridges such as the George Washington Bridge with a main span of 1067 m (1931), the Golden Gate bridge with a central span of 1280 m (1937), the Mackinac bridge with a span of 1158 m (1957), the Verrazano free arrows bridge of span 1298 m (1964), the Severn bridge with a span of 988 m (1966), the Humber bridge of span 1410 m (1981), and the Tsing Ma bridge in Hong Kong (1997) with a span of 1377 m. The Rodenkirschen bridge in Germany, designed and built by Fritz Leonhardt in 1941 with a modest span of 378 m, is an example of structural elegance.
The world’s longest span bridge at present is the Akashi Kaikyo bridge across Akashi Straits in Japan with the main span of 1991 m. The second-longest span bridge is the East Bridge across the Great Belt Waterway in Denmark with its main span of 1624 m. The Bosporus Bridge at Istanbul, Turkey, completed in 1973 with a central span of 1074 m,
provided the first permanent highway link between Europe and Asia.
What are the various components of Suspension Bridges? Explain with the help of Example.
The components of a suspension bridge, as shown in Fig. 2.7, are : (a) flexible main cables, (b) towers, (c) anchorages, (d) hangers, (e) deck, and (f) stiffening systems. The main cables carry the stiffening trusses by hangers and transfer the loads to the towers. The cable normally consists of parallel wires or parallel wire ropes of high tensile steel. The Akashi Kaikyo bridge has two main cables. Each cable, 1122 mm in diameter, consists of 290 parallel wire ropes, each containing 127 high strength (UTS = 1800 MPa) wires of 5.23 mm diameter. Thus each cable contains 36830 parallel wires.
The towers support the main cables and transfer the bridge loads to the foundations. Besides the primary structural function, the towers have a secondary function in giving the entire bridge a robust, graceful, and soaring visual image. While earlier bridges had steel towers, concrete towers have been used in the Humber Bridge and the Great Belt East bridge.
Anchorages are usually massive concrete structures which resist the tension of the main cables. The hangers transfer the load from the deck to the cable. They are made up of high tensile wires. The hangers are usually vertical, as also adopted in the Akashi Kaikyo bridge. Only three major suspension bridges, namely Seven, Bosporus, and Humber, have inclined hangers.
The deck is usually orthotropic with stiffened steel plate, ribs or troughs, and floor beams. The deck may be of strong steel trusses or of the streamlined steel box girder. The stiffening system, usually consisting of trusses, pinned at the towers, serves to control aerodynamic movements and to limit the local angle changes in the deck. If the stiffening system is inadequate, torsional oscillations due to the wind might result in the collapse of the structure, as illustrated in the tragic failure of the first Tacoma Narrows bridge in 1940.
What are the various conditions under which Arch Bridge is provided? Also, discuss some other advantages & disadvantages of Arch Bridge.
Arch Bridges. The arch form is best suited to deep gorges with steep rocky banks which furnish efficient natural abutment to receive the heavy trust exerted by the nibs. In the absence of these natural conditions, the arch usually suffers a disadvantage, because the construction of a suitable abutment is expensive and time-consuming.
The arch form is aesthetically the most pleasing and has been used in steel bridges in the span range of 100 to 250 m. Typical steel arch bridges are shown in Fig. 2.8. Deck-type open-spandrel arches can be particularly attractive as in the case of the Rainbow bridge across the Niagara River at Niagara Falls.
The arch profile is intended to reduce bending moments in the superstructure and will be economical in the material when compared with an equivalent straight simply supported girder or truss. The efficiency is made possible by the horizontal reactions provided by the supports and hence the site has to be suitable.
The fabrication and erection of an arch bridge would pose more difficult problems than a girder bride and should be properly taken into account by the designer. Arch ribs can be hingeless as in the case of the Rainbow bridge; or may have one, two or three hinges.
The arch rib can consist of a box section as in Rainbow bridge, of tubular section as in Askeroford bridge in Sweden, or a trussed form as in the Runcorn-Widnes bridge near Liverpool in England. The rise-span ratio of arches varies widely, but for most arches, the value lies in the range of 1:4.5 to 1:6.
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