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Cable Stayed Bridges Acknowledgement I sincerely express my gratitude to Mr. Santosh Rai, planning manager, Bandra Worli Sea Link for his guidance and help in writing this report.
Introduction Cable-stayed bridges are constructed along a structural system that comprises an orthotropic deck and continuous girders, which are supported by stays, that is, inclined cables passing over or attached to towers or pylons located at the main piers. Modern cable-stayed bridges present a three-dimensional system consisting of stiffening girders, transverse and longitudinal bracings, orthotropic type deck and supporting parts such as towers (or pylons) in compression and inclined cables in tension. Therefore, we can say for the vast majority of all cable-stayed bridges the structural system can be divided in to main components as follows: 1) 2) 3) 4)
The stiffening girder (or truss) with the bridge deck, The cable system supporting the stiffening girder, The towers (or pylons) supporting the cable system, The anchor blocks (or the anchor piers) supporting the cable system vertically or horizontally.
The cables are connected directly to the deck and induce significant axial forces into the deck. The structure is consequently self-anchoring and depends less on the foundation conditions than the suspension bridge. The cables and the deck are erected at the same time, which speeds up the construction time and reduces the amount of temporary works required.
Arrangement of the stay cables According to the various longitudinal cable arrangements, cable-stayed bridges could be divided into the following four systems: a) Radial or Converging System In this type of system all cables are connected to the top of the tower. Structurally, this system can be said as the best among all, as by taking all the cables to the tower top the maximum inclination to the horizontal is achieved and consequently amount of steel for the cables is minimized. However, at the top the tower the cable supports or saddles within the tower may become very congested and also a considerable amount of vertical force is to be transferred. Thus the detailing of the structure becomes quite complex.
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b) Harp or Parallel System In this type of the system the cables are connected to the tower at different heights, and also placed parallel to each other. This type of system cause bending moments in the structure also it is necessary to study whether the support of the lower cables can be fixed at the tower legs or must be made in a horizontal direction. The harp-shaped give an excellent strength for the main span, if each cable is anchored to a pier on the riverbanks for example in the Knie Bridge at Dusseldorf, Germany. The quantity of the steel required for a harp-shaped cable arrangement is slightly higher than for a fan-shaped arrangement, thus if we choose a higher tower (or pylon) it will increase the stiffness of the cable system against deflections. c) Fan or Intermediate System This system represents a combination of the radiating and harp system. The cables emanate from the top of the tower with equal spacings and connect with equal spacings along the superstructure. As the spaces are small near the top of the tower the cables are not parallel and the forces remain small so that single ropes can be used and all the ropes have fixed connections in the tower. The Nord Bridge, Bonn, Germany is a typical example of this kind of arrangement. d) Star System The star system is an aesthetically attractive cable arrangement. However, it contradicts the principle that the points of attachments of the cables should be distributed as much as possible along the girder.
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Selection of Cable Configuration The selection of cable configuration and number of cables is dependent mainly on length of the span, type of loadings, number of roadway lanes, height of towers, and the designer’s individual sense of proportion and aesthetics. Cost factors also have a great influence on the selection of the cable arrangements, as using less number of cables results large cable forces, which requires massive and complicated anchorage systems connecting to the tower and superstructure. These connections become sources of heavy concentrated loads requiring additional reinforcement of webs, flanges, and stiffeners to transfer the loads to the bridge girders and distribute them uniformly throughout the structural system. A large number of cables distribute the forces more uniformly throughout the deck structure without major reinforcement, which provides continuous support thus, permitting the use of shallow depth girder that also tends to increase the stability of the bridge against dynamic wind forces. In case of radiating cable arrangement as the cable stays are at the maximum angle of inclination to the bridge girders, the cables are in an optimum position to support the gravity dead and live loads and simultaneously produce minimum axial component acting on the girder system. On the other hand as this system have all the cables at the top, thus concentrating the entire load on the tower which produces large shear and moments in the tower and also presenting difficulties in anchoring all the cables at the top or over the saddle, thus complicating the transfer of the vertical force.
Positions of the cables in space There are two basic arrangements out of the many planes in which the cable stays are disposed which are: two-plane and single-plane systems. Among which the two-plane systems can be further divided in two types as follows: The Two Planes System a) Two Vertical Planes System In this type of system there are two parallel sets of cables and the tower on the either sides of the bridge, which lie in the same vertical plane. Here two alternative layouts may be adopted when using this system:
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i)
ii)
The cable anchorages may be situated outside the deck structure, which is better than the other in terms of space as no deck area of the deck surface is obstructed by the presence of the cables and the towers. There is however, a disadvantage in that the transverse distance of the cable anchorage points from the webs of the main girders requires substantial cantilevers to be constructed in order to transfer the shear and the bending moment into the deck structure. Also the substructure, especially the piers for the towers have to be longer, because in this case the towers stand apart and outside the cross-section of the bridge. When the cables and tower lie within the cross-section of the bridge, the area taken up cannot be utilized as a part of the roadway and may be only partly used for the sidewalk. Thus as area of the deck surface is made non-effective and has to be compensated for by increasing overall width of the deck.
b) Two Inclined Planes System In this system the cables run from the edges of the bridge deck to a point above the centerline of the bridge on an A-shaped tower or λ-shaped or diamond shaped pylon. This arrangement can be recommended for very long spans where the tower has to be very high and needs the lateral stiffness given by the triangle and the frame junction. Joining all cables on the top of tower has a favorable effect regarding wind oscillations, as it helps to prevent the dangerous tensional movement of the deck. The Single Plane System This type of system consists of bridges with only one vertical plane of stay cables along the middle longitudinal axis of the superstructure and as the cables are located in a single centre vertical strip thus all the space is utilized by the traffic. This arrangement requires a hollow box main girder with considerable tensional rigidity in order to keep the change of cross-section deformation due to eccentric live load within allowable limits. This system also creates a lane separation as a natural continuation of the highway approaches to the bridge. It should be noticed that all the possible variations regarding the longitudinal arrangements of the cables used with two planes bridges are also applied to single center girder bridges. This is an economical and aesthetically acceptable solution, providing an unobstructed view from the bridge; it also offers advantage of relatively small piers as their size is determined by the width of main girder. A possible disadvantage of this system is the fact that maximum cable load is transferred to the main superstructure girder, thereby requiring additional reinforcement and stiffening of the deck. Web plates and bottom flange would be also required in order to distribute the load uniformly throughout the cross-section of the superstructure members.
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Tower (or Pylon) Types:
The various basic tower shapes are illustrated in above figures major shapes includes, Aframe, Single, H-frame, Double towers. There are many modifications to these shapes, each with its own advantages, disadvantages and aesthetic appeal. Towers can be of steel, concrete or a composite. In the composite type, the inner shell can be of steel with an outer shell of concrete for aesthetic purposes or a steel cable anchoring system composite with the concrete shell. Most towers have hollow columns that accommodate ladders, hoists and power feeds.
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Investigations of cable-stayed bridges indicated that the horizontal forces of the cables were relatively small, so that freely standing tower legs could be used without disadvantage. The inclined cables even give a stabilizing restraint force when the top of the tower is moved transversely. With single tower or twin towers with no cross-member, the tower is stable in lateral direction as long as the level of the cable anchorages is situated above the level of the base of the tower. In case of displacement of the top of the tower due to wind forces, the length of the cable increases resulting increase in tension, which provides a restoring force. Longitudinal movement of the tower is restricted by the restraining effect of the cables fixed at the saddles or tower anchorages. The towers can have three different kinds of supports as follows: a) Towers Fixed at foundation Towers with fixed legs are relatively flexible, and loading and temperature do not cause significant stress in the structure. However, large bending moments are produced in tower in this case. The main girders pass between the frame legs and are supported on the transverse beam. b) Towers Fixed at Superstructure In case of the single-box main –bridge system, the towers are generally fixed to the box. With arrangement it is necessary not only to reinforce the box but also to provide strong bearings. The supports also may resist the additional horizontal forces caused by the increased friction forces in the bearings. c) Hinged Towers For structural reasons, the towers may be hinged at the base in the longitudinal direction of the bridge. This arrangement reduces the bending moments in the towers and the number of redundants, which simplifies analysis of the overall structure. Also, in case with bad soil conditions, linear hinges at the tower supports are provided, allowing longitudinal rotation, so that bending moments are not carried by the foundation.
Deck Types Most cable-stayed bridges have orthotropic decks that differ from another only as far as the cross-sections of the longitudinal ribs and spacing of the cross-girders is concerned. The orthotropic deck performs as the top chord of the main girders or trusses. It may be considered as one of the main structural elements that lead to the successful development of the modern cable-stayed bridges.
Cable Types A cable may be composed of one or more structural ropes, structural strands, locked coil strands or parallel wire strands. A strand is an assembly of wires formed helically around centre wire in one or more symmetrical layers. A strand can be used either as an individual load-carrying member, where radius or curvature is not a major requirement, or as a component in the manufacture of the structural rope.
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A rope is composed of a plurality of strands helically laid around a core. In contrast to the strand, a rope provides increased curvature capability and is used where curvature of the cable becomes an important consideration. There are three types of strand configuration: 1) Helically-wound strand 2) Parallel wire strand 3) Locked Coil strand
Structural Advantages Introduction of cable-stayed bridges has resulted introduction of a totally new direction to the bridge engineering possessing outstanding structural characteristics, efficiency and wide range of application. The basic characteristics of a cable-stayed bridge which steps it above all other includes: a) Structurally a cable-stayed bridge consists of stiffening girders, steel or concrete deck and supporting parts as towers acting in compression and inclined cables in tension, which puts them between girder type and suspension type bridges.
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b) The main structural characteristic of this system is the integral action of stiffening girders and Prestressed or post-tensioned cables, which from the tower tops down to the anchor points at stiffening girders. Horizontal compressive forces due to the cable action are taken y the girders and no massive anchorages are required. c) Introduction of the orthographic system has resulted in the creation of new types of superstructure, which can easily carry horizontal thrust of cables with almost no additional material, even for very long spans. d) Another structural characteristic of this system is that it is geometrically unchangeable under any load position on the bridge, and all the cables are always in a state of tension. This characteristic of the cable-stayed systems permits them to be built from relatively light flexible elements-cables. e) In this kind of bridge there is full participation of the transverse structural parts in the work of the main structure in the longitudinal direction. This considerably increases the moment of inertia of the bridge thus permitting a reduction of the depth of the girders and a consequent saving in steel.
Economics There is no simple formula for deciding under what conditions the cable-stayed bridge should be adopted as an economical solution. However, a survey on steel requirements of typical cable-stayed bridges shows the upper limits of the steel weights per unit area are greater for structures having portal type towers. The variation of the weight is influenced by many factors, such as the quality of steel, loading requirements, structural systems, the width of the bridge, etc. In observations made by Thul H. in which he compared the length of center spans to the total length of three-span continuous girder, cable-stayed and suspension bridges, found the limits of economical application as follows: a) 700 ft for the center span of a three-span continuous girder, the ratio of center span to total length is 30%-50% b) The suspension bridge has an economical center span of 1000 ft, which is between 60% and 70% of the total length. c) The cable-stayed bridges are between continuous girders and suspension bridges with a center span ranging from 700 ft and 50 to 60% of the total length. Thus, Thul’s investigation indicates that the cable-stayed bridges may be economical for bridges with intermediate spans. But in practice, some cable-stayed bridges have been built with longer spans. According to Homberg H., and provided that multiple stay systems are used, cable-stayed bridges are economically more advantageous than suspension bridges for spans of as much as 1600-2600 ft (490-800 m).
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Case Study: Bandra Worli Sea Link Introduction Bandra Worli Sea Link Project has been one of the most highly recommended projects of all the transport studies done for the metropolitan region during the last forty years. The sea link consists of approach viaducts and two aesthetically pleasing cable-stayed bridges. The link, which is 5.86 km long, 8-lane freeway connecting Worli and Bandra, will serve the purpose of providing an additional fast moving outlet from the South Mumbai to the Western Suburbs and thereby providing much needed relief to the congested Mahim Causeway.
Major Components of the Sea-Link 1) 2) 3) 4) 5) 6) 7) 8) 9)
449m long embankment with 20m wide promenades for 16-lane toll plaza. 800m long Precast Segmental Approach Bridge on Bandra Side. 600m long Cable Stayed Bridge 125m high Towers including transition spans. 200m long Precast Segmental Approach Bridge between Bandra Cable Stayed Bridge and Worli Cable Stayed Bridge. 350m long Cable Stayed Bridge on the Worli with Pylons including transition spans 1400m Precast Segmental Approach Bridge on Worli Side. 811m long link to Khan Abdul Ghaffar Khan Road comprising 510m Precast Segmental Bridge and 310m Cast-in-Situ Bridge. Setting up Traffic Monitoring, Surveillance, Information and Control Systems. Drainage, Street Lighting, Signage Making, Landscaping and Arboriculture.
Major Steps Involved in Bridge Erection 1) 2) 3) 4) 5)
Soil Investigation and Load Test on Piles Foundation Construction Sub Structure Construction Super Structure Construction Cable Stayed Bridges
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SOIL INVESTIGATION AND LOAD TEST ON PILES The presence of the type of soil strata under the seabed was determined by a series of geotechnical investigation, as it was necessary to establish the type of method, equipment to be finally deployed for the construction of pile in the sea. The methods, equipments, procedures sufficiency and the shaft capacities were determined by using test shafts. The test shafts were loaded to their maximum capacity and Osterberg system of load test was used to determine the shaft capacities. The platforms were constructed at four different places and the casing for the test shafts was driven into the seabed. The shaft was drilled and the reinforcement cage for the test pile was fitted with Linear Wire Displacement Transducers and Vibrating Wire Strain Gauges for measuring and analyzing the performance of the test shaft. Dr. Jorj O. Osterberg, Professor Emeritus of Civil Engineering at Northwestern University, invented and developed a deep foundation load testing device called as the Osterberg Cell, commonly called the "O-cell", to meet the construction industry's need for an innovative effective method for testing high capacity drilled shafts and piles. The O-cell is a hydraulically driven, high capacity, sacrificial jacking device installed within the foundation unit. Working in two directions, upward against side-shear and downward against end bearing, the O-cell automatically separates the resistance data. By virtue of its installation within the foundation member, the Osterberg Cell load test is not restricted by the limits of overhead structural beams and tie-down piles. Instead, the O-cell derives all reaction from the soil and/or rock system. End bearing provides reaction for the side shear portion of the Ocell load test, and side shear provides reaction for the end12
bearing portion of the test. Load testing with the O-cell continues until one of three things occurs: ultimate side shear capacity is reached, ultimate end bearing capacity is reached, or the maximum O-cell capacity is reached. Successful load testing using the O-cell requires precision monitoring of numerous test parameters. These include concrete strain using strain gauges, compression using telltale rods, shaft movement using displacement transducers and load, by way of hydraulic pump pressure monitoring using pressure transducers. The output of these tests can be easily get on an excel sheet and thus a tabulated result can be used to extrapolate the response of the pile to various conditions prevailing at the site. The O-cell can be used in production and non-production foundations. Multiple O-cells can be used and placed on the same plane to increase the available test capacity to virtually any load. Specially constructed O-cells can be attached to driven piles or cast within concrete precast piles. Bi-directional load testing (O-test) can be performed on: a) Pre-stressed concrete piles b) Steel pipe piles c) Concrete shell piles
Advantages Economy: The Ocell is usually less expensive to perform than a conventional static test despite sacrificing the Ocell. Savings are realised through reduced construction time and capital outlay for a test, no topof-pile reaction equipment requirements and less test design effort.
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High Load Capacity: The O-cell tests can have a capacity of loading up to 160 MN and even more load capacity is also possible if required. The capacity of O-cell varies from 5 to 22,000 ton. Rock Sockets: Conventional load tests often have difficulty adequately testing rock sockets because of limited reaction capacity and load shedding in the soils above the socket. Instrumentation interpretation problems often preclude any accurate separation of socket shear and bearing. The O-cell places its large test load capability directly at the bottom of the socket, and also gives an automatic separation of components. Reduced Work Area: The work area required to perform an O-cell test, both overhead and laterally, is much smaller than the area required by a conventional load system. For example, a 56 MN O-cell test, conducted in a 3-meter wide median strip of a busy Interstate Highway, would have been impossible with any other method. Over-water and Battered Shafts/Piles: Although often impractical to test conventionally, testing over water or on a better poses no special problems for O-cell testing. Static Creep and Setup (Aging) Effects: As the O-cell testing is static, the test can be held for any desired length of time and also separate data about the creep behavior of the side shear and end bearings components is obtained. This implies that one can get any desired value at any point of time after installation.
FOUNDATION CONSTRUCTION The foundations being the most important elements of the bridge, it’s also one of the most challenging activities at the project due to geological conditions. All the Piles in the Project are vertical and cast-in-situ permanent steel liners and are friction and end-bearing type of PIles. The piles are driven with RCD Rigs mounted on the Jackup Platforms. Construction of cofferdam followed by the placement of tremie seal after the dewatering is required for the erection of a pile. Concrete produced at the Batching Plant under controlled conditions is transported by agitator drums on concrete barges and is placed at the required location using Concrete Pumps.
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The number of M60 piles and its depth along with its diameter in a foundation varies from Pier to Pier, and depends mainly on the load it takes from the Sub-structure. Each Pier is given an id as which represents its position from the Bandra end of the Bridge, the ith Pier is given id as Pi for example the first Pier from the Bandra side is given an id as P1. So, number of Piles from P1 to P18 and P20 to P60 are 4 and each are of diameter 1500mm except for P17, P18, P20 and P21 which has Piles of diameter 2000mm. The number of Piles for P19 is 52 including both north and south carriageways, and the diameter for each Pile is 2000mm. Lastly, P27 and P30 have 6 Piles each of diameter 2000mm which includes both carriageways. The depth of each Pile depends on the depth of sea at the particular place, which in general varies from 5.15m to 663.4m at P19.
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RCD Drill Bit used in RCD Rigs for foundation construction.
Pile Cap Reinforcement at fabrication yard.
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Pile Cap Reinforcement
SUB STRUCTURE CONSTRUCTION The Piers for the bridge are hollow but the Piercaps are solid mass of concrete. Prefabricated reinforcement cages are brought at site for the construction of the piers and sacrificial concrete lines are installed with a top cover so as to create the hollow part inside them. Once inner liners are installed the cage is aligned in the position and placed as requisite and concreting is done after installing the outer form. The recess for bearing installation is cast with Piercap.
Picture showing a concrete pump pumping concrete in Pier-Cap rebar 17
Picture showing the formwork for the Sub structure construction
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Picture showing the bearing between a Pier and a Pier
SUPER STRUCTURE CONSTRUCTION The segments are cast at a centralized pre-casting yard using short line method of casting, which means once a segment is casted its conjugate segment is casted right after it so as the two of them matches. Sophisticated software is used to arrive at the correct casting curves for the segments. The concrete for the segments is supplied by the Batching Plant and is pumped in each module after which the segments are transported to the construction site. The Erection Gantry, a picture of which is illustrated, does the erection of span. A typical 50m span comprises of 15 numbers of precast segments, a Pier segment and 200mm (nominal) in-situ wet joints. For the construction all the spans, all the segments are suspended from the Gantry, glued and temporarily stressed together, on completion of which span alignment to the piers is followed. After the alignment, wet joints are casted including grouting of bearings top plinth. Once the wet joints achieve the required strength, stressing of longitudinal PT is commenced followed by the load transfer of span to Piers. A complete construction of a segment takes about a month, which includes its curing as well as it is being casted by the short line method of casting. A cell is the place where a segment is casted, before casting a segment a survey team decides its position in cell. Once the survey team sets the bulkhead, the rebar setup takes place which takes different amount of time depending upon the type of segment and could be as much as 20 to 30 days. After rebar setup is done, the concrete is poured in the formwork and is allowed to setout, after which again a survey team confirms its successful casting. Then the segment is displaced from its position and it position is taken by another formwork and rebar setup which is of its conjugate segment. The two segments are being linked through a male-female joint, in the meanwhile the casted segment is allowed to gain strength and also its curing is done side by side. Once the casted
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has gained enough strength its initial tensioning is done with force about 30% of its maximum capacity. Then we further wait for it to gain strength of about 40Mpa or more and final testing with rest of the 70% load is done which marks the completion of the procedure. Once a segment is completed it’s transferred to the stock where it is given a particular and it waits for its turn to come. As for the conjugate segment, as its concreting is completed it takes place of the completed segment and another matching segment takes its place and so the process goes on. Here at Bandra Worli Sea Link we have 8 cells to makes all the different types of segments. There are a number of equipments which deployed in the pre-casting yard which mainly includes the following: 1) 2) 3) 4) 5) 6)
Hydraulic Jacks of different capacities Turn Buckles Gantry, for lifting segments Tower Crain Concrete Pumps Cutting and bending machines for rebar of segments.
The tensioning test that is being done here at casting yard plays a very significant role for a segment to be perfect, as it decides the that segment thus casted can be lifted at all or not. Once a segment passes this test, this means that it can be easily lifted by any gantry or through any other process during the construction of the bridge. There are many other advantages of using the process of short line method which can be tabulated as follows: 1) As already stated, it prepares a segment for lifting 2) It also gives enough time for the curing of a segment 3) It also helps to start the other segment as soon as one is completed So, it basically less time consuming and efficient process for the building of segments. The pictures of various important components of pre-casting yard and other important processes are given below:
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Casting Yard Stock, the lowermost and the one above it are conjugate segments.
Picture showing PT cables used after the Post Tensioning of the segments 21
Picture showing a segment on barge being taken to Derrick
Picture showing the Batching
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Picture showing a segment being cast and its conjugate.
Picture showing a segment at the Erection Gantry
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Picture showing a segment being taken form the casting yard to jetty
Picture showing the Top View of the Casting Yard
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Picture showing Erection gantry
Picture showing the details of the Erection Gantry 25
Picture showing The Asian Hercules and Erection Gantry, the Asian Hercules was hired to displace the Erection Gantry.
Picture showing a cable-stayed segment with blister
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CABLE STAYED BRIDGES There are two Cable Stayed Bridges in the Sea Link Project with a complex geometry as one pair of the towers is inclined in two planes and the entire cross section reduces continuously over the entire height of the Towers. The other concern is the Casting and Geometry of the Bridge, as the bride has to attain reference geometry after 2000 days in the service stage which requires a detailed analysis of the time dependent factors such as creep, shrinkage and temperature. The design of the Stay Cables, anchorages and stressing forces are all governed by this parameter. The Stay Cables for the bridge were subjected to a series of quality and engineering tests to meet special requirements including Fatigue Test. The concrete for the Bridge is M60 grade and Microsilica is added to improve the quality of the concrete. The construction of a cable-stayed bridge requires high engineering skills and it’s the most challenging part of the whole Bridge. Its construction can be divided among following components:
Picture showing complete cable stayed bridge at Bandra-side of the Sea Link
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a) Construction of foundation The P19 Pylon of Bandra Worli Sea Link stands on a foundation comprising of 52 nos. M50 Piles, each of 2m diameters arranged in definite framework as illustrated in the following picture,
b) Construction of Tower or Pylon below deck The construction of Pylon below includes 6 lifts of M60 grade concrete each of 3.0m and 1 lift of 3.260m. The rebar layout for every leg is predesigned and the steel bars are being cut and bent as per the requirements of the prefabricated reinforcement cages at the rebar fabrication yard. These bars are then taken to the Pilecap P19 through the means of Barge where they will be fixed in the requisite fashion. Reference checks shall be made with respect to the centre line of the carriageway and control points of tower to align the cage of bars to its desired position. Then the arrangements for concreting are checked so that no interruption occurs during concreting. The concreting will be done in layers through a concrete pump placed on a temporary platform followed by vibrating with needles of appropriate diameters. The pouring concrete is done in a peripheral manner in which it’s done from the centre towards the wall. Hesian cloth rolls immersed in water are used for the curing of concrete once it’s initially set, for greater heights, a wet skirt of suitable length can kept in contact with the concrete. Dripping of water due to excess application should be avoided and a higher relative humidity will be maintained. The final surface will be given finishing touch by masons. The geometry of the
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tower is controlled by an integrated set of procedures undertaken during the construction planning and construction stages so as to ensure that any environmental or any other actions does not effects the target geometry that is to be attained by the Pylon. A stage-by-stage construction analysis of all the different types of forces and other actions is already carried out during the planning so to establish the optimum construction sequence and the requirements for temporary supports, restraints, and jacking forces. This will be helpful during the construction of the tower as it will define the target incremental and total geometry profiles at each construction stage, and estimated jacking displacements for pylons and alignment of components, together with associated jacking forces. The formwork for every lift is already preplanned and designed; also the sequence of the erection of the various lifts is predefined. A trial mock up fitting shall be conducted to check the dimensional fitting of the formwork system before installing the same at location.
Stage I
Stage II
Stage III
(First Lift from Pile Cap)
(Second Lift from Pile Cap including First Lift)
(Third Lift from Pile Cap including First and Second Lift)
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c) Construction of Diaphragm The formwork of diaphragm and adjacent segments will be supported on the staging truss of pier table beneath with the help of a system of supporting brackets spanning between the tower legs. The reinforcement cage for the diaphragm will be cut and bent as per the preset design at the rebar fabrication yard from there to the desired place and tied. The cage is then aligned to its required position with respect to various survey points and all the requisite inserts shall be fixed in position and required checks shall be made. The formwork is designed in such a manner that the 8th Lift of the tower, adjacent segments and the diaphragm will be cast a single unit, and also it shall be custom made to suit the requirements of geometry, inserts and Post tensioning. Survey Checks will be made at appropriate stages to confirm the exact position of form before casting of the section. The concreting will be done with M60 Grade Concrete and will be carried out in layers. The placed concrete is vibrated using needle vibrators of appropriate diameters. The final top surface of tower leg is given a texture finish using brooms. As early as possible afters initial setting, hessian cloth will be spread and sprinkled with water. Once the casting is done pre stressing will be commenced as soon as the adjacent segments are being casted.
d)
Construction of Pier Table
The Pire Table constitutes the cast-in-situ diaphragms, adjacent segments and segments between and outside Tower Legs.
DIAPHRAGM & ADJACENT SEEGMENTS & TOWER LIFT
PRECAST SEGMENTS OF PIER TABLE LOWER TOWER LEGS
PIER TABLE AT TOWER P19
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The construction of the Pier Table begins with the temporary arrangements for the Erection of Pier Table Segments and the Construction of Diaphragm explained followed by method for construction of Permanent Structures. The temporary arrangement includes five columns four of which are on the Pilecap and fifth lies in the sea. The pedestals for each column is of size 600mm X 600mm are casted at the pre marked required positions with the anchor bars positioned in them. The top levels of each pedestal are then matched before fixing of the top plates. The construction of a column starts with proper positioning and marking and followed by the lower modules. Once lower modules are completed, upper modules are erected and the top levels of all the columns are checked. Following this will be the erection of the truss modules from one end to the other. The modules of the truss resting directly over the columns will be erected using the crawler crane as weight of the module exceeds the capacity of the tower crane at desired working radius. The intermediate modules would be erected using the tower crane. The truss is then leveled and aligned for installation of the lifting device of segments. The leveling and alignment of the segments will be done by screw jacks mounted on the sliding shoe. This marks the completion of the temporary part of the Pier Table the permanent structure includes pre casted segments at the casting yard. The segments are delivered to the Pylon P19 through barge from where it’s lifted on the top of the truss with Strand Lifting Unit (SLU) mounted on the lifting arrangement. The segments are then slided to their positions where the longitudinal and transverse slopes would be adjusted by the combination of screw jacks and shims mounted on the sliding chair. The middle 7 segments inside the tower legs are then stressed and glued to their final position, immediately after that segments on both sides of tower legs will be placed at 6.0m distances on both sides from the end segments inside the tower legs and restrained against any movement. The procedure for the further construction of the PierTable is already being discussed in the previous section “construction of diaphragm” besides that the only part left is Post Tensioning which is done once the diaphragms are being casted together with the adjacent segments. Post tensioning includes stressing the segments and transferring the load from the truss to the pylon legs and at the time of pre stressing the segments in the longitudinal direction it would have already be seen that the horizontal props erected between the tower legs are removed. The main truss is then dismantled and modules and PierTable for the other carriageway are then to be erected with the same procedure.
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e) Construction of Tower or Pylon above deck The construction of the Pylon above the deck includes steps starting from the making of kickers to the top of the Pylon. The Kicker for the Bandra Tower Legs will be cast using custom made formwork to suit for kickers of each leg of the Pylon. These forms shall be lined with those, which would be there already after the casting of the diaphragms and 8th lift of the Tower. Once the kickers are set the DOKA formwork is positioned and aligned according to the pre calculated theoretical values. Followed by which concreting is done and the data for the position is again taken and matched with the theoretical values, in case of any discrepancy the corrections are made to the following lift. This cycle is continued till the 24 Lifts of the Tower are complete. Meanwhile Compression struts and cross bracings are provided at the required and pre calculated positions if the Pylon P19.
CABLE ANCHOR ZONE 10 LIFTS
UPPER TOWER LEGS 24 LIFTS PIER TABLE
LOWER TOWER LEGS 8 LIFTS
SIDE VIEW OF TOWER P19 FOR BANDRA CABLE STAYED
The operations of DOKA Formwork are subject to the conditions mentioned below. In no case, these operating conditions will be overruled.
The Following wind speeds are binding according to the static calculations: During Working and climbing process: Max Wind Speed = 70kmph Wind speed-exceeding 70kmph: ALL WORKING AND CLIMBING PROCESS TO BE STOPPED Wind speed-exceeding 100kmph: CLOSE FORMWORK Wind speed-exceeding 164kmph: FORM SYSTEM MUST BE TIED TO REINFORCEMENT
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The Compression struts will be erected according to the following stages; these stages are being calculated during the planning with great accuracy of forces and precision.
SNo
Position of To Be Installed PJack at Compression Strut (kN) Compression Strut after completion of Longitudinal Transverse
1.
Lift No. 11
Lift No. 15
967
563
2.
Lift No. 15
Lift No. 19
1149
687
3.
Lift No. 19
Lift No. 23
1300
782
4.
Cross Bracing between Lift No. 15 & Lift No. 19
Lift No. 23
5.
Lift No. 21
27AD and 25BC
1134
747
6.
Lift No. 23
29AD and 27BC
1203
801
Picture showing positions of various Compression Struts and Cross Bracings at P19 34
The struts are being checked for alignments, match fitting of components and other requirements at the fabrication yard and are erected at the location in preassembled conditions by the tower cranes. Once a strut is erected it is not supposed to make any kind of movement neither the forces are supposed to be altered. The walkway mounted on the compression struts will be used for accessing the legs of tower at same level and DOKA stair towers would be mounted at each leg for accessing the legs at various levels in between compression struts. For accessing the compression strut at various levels platforms would be provided, which could be accessed with the elevator. It is important here to note that the deformations shall be governing, not the PJack, In case of conflict between the PJack and deformations. Now for a particular cycle applied for each lift there are certain steps to be followed which includes, a) Fixing and alignment of the reinforcement cage b) Deshuttering, modifications, climbing and fixing of DOKA Formwork c) Concreting of the Tower Legs The geometry of the Pylon Legs would be also considered step by step as discussed earlier which will be achieved by an integrated set of procedures undertaken during the construction planning and construction stages. This is to ensure that any environmental or any other action does not affect the target geometry, which is to be attained by the Pylon. The bars for the reinforcement cage would be cut and bend at the rebar fabrication yard from where they will be take to Pylon P19 and would be fixed sequentially for each lift. The vertical bars shall be connected to the bars of the previous lifts using couplers whereas links shall be tied manually at job site. Due to the inclination of the tower legs towards the centre of the tower, the reinforcement cage will tend to incline because of its selfweight. In order to control the alignment of the rebar cage, it is
Picture showing the Diamond shaped Pylon P19 35
proposed that 4 Nos. 100mm NB pipes would be embedded into the tower lifts. These pipes shall run for total length of tower legs. On a typical lift, few of the vertical bars would be fixed on the couplers and then an adjustable template shall be fixed on the top of the pipes so as to suit for each lift configuration. This template on the top of the pipes will be surveyed and adjusted to correct position and the vertical bars shall be aligned so. After setting the template in correct position, all the remaining vertical bars would be screwed on the couplers at their correct position. The inserts required for DOKA formwork could also be fixed using the reference plates attached with the template. After ascertaining the correct position of the vertical bars link, set tying would be commenced.
Once the reinforcement is set, concreting of M60 Grade would be done in layers and will be done in a single pours. For this purpose concrete pipeline will be connected to the placer boom (42m radius of operation and 60m high). This placer boom would be stationed on south carriageway pier table towards Bandra approach and will be capable of pouring concrete up to lift no. 25 and can cater to all 4 legs of south carriageway tower and 2 legs (on Bandra side) of north carriageway tower. For onwards lift on south and north carriageway tower, concreting will be done using tower crane. The placement of the concrete would be through one concrete pump placed on temporary platform on the side of pile cap P19, as and when required. The placed concrete will be vibrated using needle vibrators of appropriate diameters. The final top surface would be given a texture finish using brooms. Pouring of concrete shall commence from the centre of the section towards walls in a peripheral manner. Hesian cloth rolls immersed in water are used for the curing of concrete once it’s initially set, for greater heights, a wet skirt of suitable length can kept in contact with the concrete. Dripping of water due to excess application should be avoided and a higher relative humidity will be maintained. The final surface will be given finishing touch by masons.
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Regarding the Geometric control of the Pylon, reference points are being marked at each stage of construction taken from the root or anchor points marked at the jetty banks or on complete structures. The survey measurements taken on the corners of the Doka shutters will be used to set the shutter at the correct location during as-set surveys, and will identify movements or construction errors in as-built surveys. The measurements are always taken with respect to the P19 Pylon datum including the 35mm of over height allowance. Minimum of 4 corners would be sufficient to define the perimeter shape of the shutter, which would be taken with high precision, although the accuracy may decrease with increase in height. The top of the tower is made up of M60 concrete and a steel anchorage box, which is hollow from inside and is made of 12mm steel plates with stiffeners, is provided as a medium to transfer load to concrete. This box is further divided into 9 elements of about 3.1m in length and the concrete outside in 10 lifts of 3m each, so this makes the total height of the tower as 30m. Each element of the anchorage box is first fabricated at fabrication yard which consists of steel plates with pipes, for the cables, pipes are at particular angle defined as sag angles. The sag angle is basically the angle, which a tangent to the curve makes with the horizontal at the anchorage point and it varies with the distance of the anchorage point from the tower. The sag is a function of self-weight of the cable and the tension it carries and therefore defines the position and alignment of the pipes with the anchorage box. The pipes and steel plates are placed at definite pre-calculated position in the anchorages box so that cables are directly brought in through the pipe and anchored with the steel plate through bearing plates and lock nuts and welding is done. The other end of the cables goes into the blister a segment at the deck, detailing of cable erection is explained in the next section. Once the element of the anchorage box is brought into position by matching each detail from the pre-evaluated values, concreting is done in same fashion as that for other lifts that is using Doka shutters. 37
f) Erection of Deck and Stay Cables The Bandra Cable Stayed Bridge spans for 600m comprising of the main bridge with a 500m Cable stayed span and 50m transition spans on either side. The cables used in the construction of the Bridge are multi-strand parallel wire cables, the number of wires in each cable varies with the amount of tension it caries. These cables are pre-fabricated which also includes the anchorages provided on the both sides and these are being imported from China. These cables are imported in exactly pre-calculated length and nos. of wire containing; the length also includes that of anchorages at the two ends of the cable. A detail drawing of cross-section of a cable used in the bridges is given below:
The transition spans together with the cantilever segments will be erected using launching girder, whereas the segments between the pier table and the closure pour shall be erected using balance cantilever construction method. The construction of the deck is carried out with the Derrick as shown in the picture, the segments are carried from the casting yard beneath derrick from where it picks it up and places it. The detail working of this is shown on the next page.
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STAGE: 1 Open the Support Bracket and Lift the Segment. Close the Support Bracket, Slide in the Trolley and Lower the Segment on Sliding Trolley.
STAGE: 2 Open the Lifting Boom & Strut, Slide out the Segment and Close the Lifting Boom and & Strut.
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3-D VIEW OF LIFTING FRAME
Note: The above Derrick is used for the Pier Table Erection.
OPERATION OF Derrick
Picture showing a Derrick used for the Deck Erection of Cable-Stayed Deck.
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As illustrated above the derrick places a segment in two stages, but before placing the segment it’s being dry matched with the previous segment and then epoxy glue is applied, followed by which it’s stressed. Once the epoxy is dried Post Tensioning (PT) cables with very high Ultimate Tensile Strength (UTS) are applied at the joint. The segment is then released from the derrick so that it moves forward to take the next one. This process can be explained in detail as follows: Let us consider that we have to erect Nth segment on the Deck which is a segment without blister that is, which is not a cable-stayed segment. So, we go through following procedure: 1) Advance the segment erection derrick from Segment N-3 to Segment N-1 (cable stayed segment). 2) Load the Segment N on the Segment Carrying Barge (Segment without stay pipe) and Haul and park the barge below the Derrick lifting points, from where the segment is lifted. 3) Dry match segment N with Segment N-1, apply epoxy, stress for gluing and wait for epoxy to dry. 4) Apply Temporary PT at Joint between Segment N and N-1 for cantilever construction and release the segment from the derrick. 5) Stress the cable anchored in Segment N-1 to second stage. 6) Forward the Segment Lifting Beam on the derrick and position for lifting of Segment N+1, after which Load the Segment N+1 on the Segment Carrying Barge (Segment with stay pipe). 7) Haul and Park the barge below the Lifting Point, from where it is lifted and dry match with Segment N+1is done, apply epoxy, stress for gluing and wait for epoxy to dry. 8) Apply Temporary PT at Joint between Segment N and N+1 for cantilever construction and release the segment from the derrick. 9) Stress the stay cable anchored in segment N+1 to first stage installation force. 15. Survey the predetermined survey points on the deck at dawn. 10) Repeat the above steps for further segments The segments used here are pre-casted segments, which are already fabricated and are kept at the Casting yard; once a segment is casted it is given an id number. When its turn comes the is loaded on the trailer only after all the accessories such as connection beams, steel shoes have been fitted and post concreting inspection have cleared the same. The segment is then loaded on a barge at jetty and is hauled to the point where the Derrick will lift it. Before the Nth segment is glued, a dry matching of the segment is done. After dry matching the segment is taken back by 400mm, and then epoxy is applied on the face of the same. Once the epoxy is applied the segment is glued with the N-1 segment and then the segments are stressed with the PT bars. Before Lifting the Segment N + 1, the stay cable anchored in the Segment N-1 shall be stressed to the provisional tension force as per the values based on the required deformation to be achieved by the segment N-1, as per the data given in Construction
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Manual. Therefore, in order not to bring the survey in the critical path of the operations (as stressing could occur of any time of day and survey has to be necessarily carried out in the dawn), the cable shall be stressed according to the expected deformation of deck. The corresponding force (ΔF) shall be computed and applied to the cable in the steps. Now, process of erection of the N+1 segment is similar to that of N but with a difference that once the tensioning of the segment is done, the erection of a stay cable comes into picture. Therefore the process is kept repeating in a Balanced Cantilever fashion and every time when a segment is brought under the derrick and as it’s lifted up from the barge it also has to avoid any possible collision with the barge. To avoid any such collisions small wooden planks of pre calculated thickness, on the basis of swell in sea, would be kept under the segment as soon as it’s lifted up from the barge. The process cable erection like derrick also follows a particular methodology, which is as follows: a) Uncoil the cable on the bridge deck using an Uncoiler and Winch b) Fix the upper (fixed) anchorage into the tower top using a deviator device c) Pull the lower end of the cable near anchorage using an winch and then guide into anchorage d) Stress the cable from the deck and adjust the force with hydraulic jacks As already stated once the cable is erected it will be stressed to the installation force only, a detail illustrative procedure is given as follows:
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g) Wet Joint Construction The main span of the Cable-Stayed bridges would be consisted of proposed 50mm joints at specific predefined locations. The segments at these locations are first slided by the derrick and a gap of 50mm is made which is surveyed for final adjustments. On completing the final adjustments temporary PT bars are placed in the gap so as to keep the segment in place, after which the steel form is placed at specified position and M60 Grade concrete is poured into it. When the concrete gains strength temporary PT bars are removed and the required PT bars are installed to get the required fore for cantilever construction. After carrying the Prestressing of temporary PT Bars to the required force release the segment from the derrick and continue further with main span erection as per normal procedure. The main purposes of these wet joints are as follows: 1) The deck units between the wet joints can be cast in different cells independent of other units. Thus, a number of such units can be cast simultaneously in different casting cells. This will reduce the time required to complete casting of segments and allows flexibility while casting segments of Bandra and Worli Cable Stay bridges in same set of casting cells. 2) Due to the long length of the span deck erection, need some finer corrections to maintain the desired geometry. Wet joints at predetermined location will allow this flexibility to apply corrections, if any.
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h) Continuity PT and Grouting Once the Deck is complete Post Tensioning of all the segments is done so as to bring them to a specific predetermined geometry. This geometry was calculated during the planning of the bridge and was found with great care and precision, as it will determine the final shape and geometry of the whole bridge. The grouting of the bridge includes a major task of fill up the space left in the holes for the PT cables. It is a mixture of various kinds of available admixtures and water as a result they form slurry which is then filled to complete the holes in the segments. This is a very important process as grouting secures that rusting of PT bars does not take place at all. Therefore one can say that the life of bridge depends on the material used for grouting.
i) Cable force adjustment and Fine Tuning This is an iterative process, which is to be done as the last stage once the bridge is complete. It includes rechecking of tension forces in each cable so as to confirm that it equals the forces, which were determined theoretically at the time of planning with a least 1 to 2 percent of variation. Sometimes it may get tedious to check force in each cable again as again as a small change in one cable will bring changes to other cables as well.
PROJECT BENEFITS a) Savings in vehicle operating cost to the tune of Rs.100 crores per annum due to reduction in congestion in the existing roads and lower vehicle operating cost on the bridge. b) Considerable savings in travel time due to reduced accidents, increased speed and reduced delays at intersections at existing roads. c) Improvement in environment especially in terms of reduction in carbon monoxide, oxides of nitrogen and reduction in noise pollution in areas of Mahim, Dadar, Prabhadevi and Worli. d) Project to have no adverse effect on fisheries, marine life and livelihood of fisherman. e) Proper landscaping measures along the approaches and promenade along waterfront to enhance environment of the area. f) Ease in driving with reduced mental tension and overall improvement in the quality of life.
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SALIENT FEATURES a) An 8-lane bridge with 2 lanes dedicated for buses. b) Unique bridge design for the Link Bridge to emerge as a landmark structure in the city. c) Single tower supported 500 meters long Cable Stayed Bridge at Bandra Channel and Twin tower supported 350m Cable Stayed Bridge at Worli Channel for each carriageway. d) Modern toll plaza of 16 lanes with automated toll collection system. e) An intelligent bridge with state-of-art systems for traffic monitoring, surveillance, information and guidance, instrumentation, emergency support etc.
References: 1) Cable-Stayed Bridges – Theory and Design by M.S. Troitsky. 2) Construction and Design of Cable-Stayed Bridges by Walter Podolny and John B. Scalzi. 3) Cable Supported Bridges – Concept and Design by Niels J. Gimsing. 4) Documents on the Project provided by Mr. Santosh Rai, planning manager, Bandra Worli Sea Link. Made By: Ravi Haldania, Dept of Civil Engineering, IIT-Powai.
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