1.2 R Erasmus Cantilever
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Bridge cantilever construction with in situ concreting Civil works construction technology Civil engineering department 5th year
Sofie Johansson IST1 88853
Elin Fryklund IST1 88854
Bojan Fajdiga IST1 88915
Antonio Barhouche IST1 88870
Maxime De Almeida IST1 89099
05/12/2017
Civil Works Construction Technology
INTRODUCTION Bridge cantilever construction method can be executed both with prefabricated an in-situ concreting. The method does not require false work or centering which leaves traffic under the spans widely unobstructed during construction of the bridge. The only access from the ground is when constructing the piers and abutments and for the preparation for the start of cantilevering, that will start from these locations. The method consist in building the bridge structure by part that are called segments. When casting on the construction site the segments are concreted into a formwork that is connected to a movable structure called form traveller. Form travelers are supported by steel cantilever trusses attached to previously completed segment. When the previously cast segment has developed enough strength can it be prestressed to previous segment and to support the subsequent one. After that the form traveller can move on and start the next segment.
Figure 1: Cantilever cast-in-situ bridge construction1
1
https://theses.lib.vt.edu/theses/available/etd-120199-224950/unrestricted/11lucko_chapter4.pdf
2
Civil Works Construction Technology
TABLE OF CONTENTS Introduction............................................................................................................................................. 2 Equipment to be used ............................................................................................................................. 4 1
Segmental bridges ........................................................................................................................... 4
2
Construction of a bridge and equipment for construction ............................................................. 4 2.1
Cast in situ method and Equipment for this method .............................................................. 5
How to control movements during construction? .................................................................................. 7 1
2
Cantilever Tendons .......................................................................................................................... 7 1.1
Principle ................................................................................................................................... 7
1.2
Disposition ............................................................................................................................... 8
Continuity tendons ........................................................................................................................ 10 2.1
Principle ................................................................................................................................. 10
2.2
Disposition ............................................................................................................................. 10
3
Temporary pilings .......................................................................................................................... 11
4
Closure of the end span................................................................................................................. 11
5
Embedding on a pier ..................................................................................................................... 12
How to correct final misalignments in closing segment?...................................................................... 13 Useful references .................................................................................................................................. 15
3
Civil Works Construction Technology
Equipment to be used 1 SEGMENTAL BRIDGES A Segmental Bridge is a bridge built in short sections (called segments), that is one piece at a time. And then segments are erected to make the bridge. The bridge is either cast-in-place or precast. The first cast-in-place segmental concrete bridge, built in 1950, across the Lahn River in Germany. Segmental bridges are classified under the following categories: -
Balance Cantilever Bridges Progressive Placement Bridges Span-by-Span Constructed Bridges Incrementally Launched Bridges
Balance Cantilever Bridges Balanced cantilever construction is an economical method when access from the below is expensive or practically impossible. Segments may be cast in-situ or precast concrete normally with a box shape cross-section. Construction starts from the top of a pier, with the segment normally fixed to the pier either permanently or temporarily during the construction. For in-situ balanced cantilever construction, the formwork is suspended from the end of the last segment. The new segment is cast, and once the concrete has developed a predetermined strength, the section is post-tensioned to the rest of the bridge. The figures show the construction sequence of this type of construction. Segments are often cast and stressed in pairs, one each side of the pier.
Figure 2: Balance Cantilever Bridge
2 CONSTRUCTION OF A BRIDGE AND EQUIPMENT FOR CONSTRUCTION There are many methods for construction one bridge. This time we will say something about the insitu balanced cantilever construction. In basic terms, cast-in-situ construction describes a process whereby segments are progressively cast on site in their final positions within the structure. Construction of a bridge is a very complex process and uses special equipment. Equipment for construction of bridges is: - Form Traveler machine - Hydraulic jacks - Mobile scaffolding - Gantry crane or other crane
4
Civil Works Construction Technology
Everyone knows about crane, scaffoldings, jackets we use them in almost every construction. In the figure 2 we can see equipment. However, in next paragraph we will say something about equipment, which is specific.
Figure 3: Equipment
2.1 CAST IN SITU METHOD AND EQUIPMENT FOR THIS METHOD When we talk about this method, we can say this: -
All works are done on site To cast the segment in their final position Commonly used method for casting segments in place is with use of form traveler. Form travelers are movable forms supported by steel cantilevers trusses attached to previously completed segments. Form travelers can be used when a bridge pier can be used as a starting point For balance cantilever erection is used, we need minimum 2 form travelers.
Form Travelers On figure 4 we can see form travelers. Form Travelers are powered by hydraulic system, requiring the crews that operate them to receive specific training .The structural framework of the form traveler move forward on rails anchored to the previously constructed span segments. The individual steel structural members of each form traveler are connected with pins to facilitate mobilization/demobilizations. Form Traveler machine is easier to be use, it is easier to be modified for future usage. Form Traveler machine are with good quality modular connection, designed for future modification and repeated usage for many years.
5
Civil Works Construction Technology
Figure 4: Form travelers
This is special workshop in the top of the pillars. They are supplied with the necessary molds, substrates, screws, concreting devices. The concrete can be prepared on site or carried from the concrete factory. For the production we need to have the necessary molds, formwork, vibratory for concrete. On the pillar, we have scaffolds. If we need lifting equipment we mounted on the upper part of the pillar. On market today we can find various types of form machines. For same special bridges sometime companies design special types of equipment.
6
Civil Works Construction Technology
How to control movements during construction? During the construction of segmental concrete bridges using the balanced cantilever method, it is necessary to guarantee the stability of the cantilever arms on their pier before they are joined to the neighbouring cantilever or to the cast-on-falsework section on the end span. It is essential to prevent the cantilever from tipping on the pier crosshead. In fact, the movements have to be controlled during construction when the cantilever is unbalanced due to the weight of a segment that is built or installed before its symmetrical counterpart is in place. Five main solutions can be used to stabilize cantilevers on their piers. The choice of the method depends on the general design of the structure. The cantilever method of construction requires, as a first method, the use of two sets of prestressing tendons, the cantilever tendons and the continuity tendons. Using prestress tendons is the most widely used method for stabilising cantilevers during the construction. In principle, this is the most economical method for spans of less than 120 meters in length.
1 CANTILEVER TENDONS 1.1 PRINCIPLE2 The cantilever tendons are required primarily to resist the effects arising from the construction stages, which constitute the greater part of the hogging bending moment at the piers. They are the principal reinforcement of the structural system and are necessary to hold the segments added and to take up the negative moment.
Figure 5: Longitudinal disposition of cantilever tendons
They are located in the deck slab and are anchored at the ends of the segments. In this way, an efficient arrangement that closely balances the moment diagram can be achieved. Anchors are preferably located at the intersection of deck slab and web.
2
https://www.google.pt/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwikvqaluuHXAhVEqxoKHV8wDQQFggsMAE&url=https%3A%2F%2Feducnet.enpc.fr%2Fpluginfile.php%2F11070%2Fmod_folder%2Fcontent%2F0%2F3-2POA.pdf%3Fforcedownload%3D1&usg=AOvVaw3HO0H_T5FhdwGtX2O-srIJ
7
Civil Works Construction Technology
Figure 6: Construction load and longitudinal disposition of cantilever tendons
The cantilever tendons are within the concrete section, they are “internal tendons”. At least one pair of tendons is anchored per segment. Sometimes, the decision will be made to include the minimum number of tendons only within the concrete section, in order that the external tendons, which can usually be stressed in a more controlled manner, may provide more resistance.
Figure 7: Lenght of cantilever tendons3
When designing the bridge, it may be desirable to make provision for additional (empty) pre-stressing ducts to be installed. These would then be available to accommodate additional tendons in the event that, during construction, the stressing records indicated that insufficient pre-stress had been applied (e.g. due to exceptionally high friction in the ducts). All the ducts are filled with grout at the end of the construction, to avoid corrosion.
1.2 DISPOSITION The cantilever tendons have a straight profile and are located in the top and bottom slabs. They apply the uniform prestress, and are therefore called the launching tendons. The design of cantilever tendons is made mainly by using the construction load, the dead load of the element and the wind load. Here is the disposition of those tendons in a transversal section.
3
https://books.google.pt/books?id=4R_kt9PDcUC&pg=PA168&lpg=PA168&dq=cantilever+tendons+bridge&source=bl&ots=rvmGAw9Ch8&sig=oyAj9Ky05pt64q-SRD9quSljbI&hl=fr&sa=X&ved=0ahUKEwjTxMzVl9zXAhXGmLQKHbXRB9wQ6AEIYjAK#v=onepage&q&f=false
8
Civil Works Construction Technology
Figure 8: Disposition of cantilever tendons in a transversal section
Two or four cables can be specified per segment. If, for the aforementioned reasons, we decide not to anchor the tendons in the webs and want to avoid increasing the size of the gusset, the optimum number of tendons specified for the end of each segment is two (one tendon per web). This arrangement makes it possible to standardize the reinforcements for the segments.
Figure 9: Typical balanced cantilever segment4
4
http://www.utraconindia.com/bridges_tanks.html
9
Civil Works Construction Technology
2 CONTINUITY TENDONS 2.1 PRINCIPLE5 The continuity tendons are required to ensure adequate compression at the joint between the tips of the cantilevers, as well as to provide the necessary resistance to positive moments from superimposed dead and live loading. The continuity tendons are internal to the section of concrete.
Figure 10: Continuity tendons
2.2 DISPOSITION6 In a plan view, the external continuity cables are situated close to the webs. To take account of construction tolerances, a distance of approximately 5cm must also be allowed between the outside of the tendon sheaths and the concrete of the segment slabs and/or the anchor blocks. For curvilinear structures, it is necessary to recenter the tendons using additional deviators.
Figure 11: Disposition of continuity tendons
5
https://www.google.pt/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwikvqaluuHXAhVEqxoKHV8wDQQFggsMAE&url=https%3A%2F%2Feducnet.enpc.fr%2Fpluginfile.php%2F11070%2Fmod_folder%2Fcontent%2F0%2F3-2POA.pdf%3Fforcedownload%3D1&usg=AOvVaw3HO0H_T5FhdwGtX2O-srIJ 6 https://fr.slideshare.net/PiyushKumarSanthalia/practical-design-of-balanced-cantilever-bridges-piyush-santhalia
10
Civil Works Construction Technology
3 TEMPORARY PILINGS7 This method consists of increasing the center distance between the temporary support blocks by placing them on temporary metal or concrete pilings. This is only feasible when the bridge deck is situated at a reasonable height above the ground (less than 15 meters or so).
Figure 12: Example of a temporary piling
This method is used on terrestrial sites in the following situations: -
For total lengths in excess of 100 meters When the pier head dimensions are reduced, often for architectural reasons When the pier shafts do not have enough resistance to flexion to stabilize the cantilever on their own.
This is a viable solution for aquatic sites, provided that the pilings are supported on the foundation bulkhead for the pier. The cantilever is usually stabilized by two pilings positioned symmetrically in relation to the pier.
4 CLOSURE OF THE END SPAN If the structure has spans of very unequal lengths or very short end spans, it is possible to use the closure of the small spans to stabilize the adjacent half-cantilevers. At the start of the construction of the half- cantilever, the structure must be stabilized using traditional methods. When the cantilever reaches the middle of the shorter span, the central connection segment is assembled. The longer span is then built by over- cantilevering, using the weight of the rear span to counteract the imbalance.
7
http://www.infra-transports-materiaux.cerema.fr/IMG/pdf/US_F0308A_Prestressed_concrete_bridges.pdf
11
Civil Works Construction Technology
Figure 13: Weight of the rear span used to counteract the imbalance
5 EMBEDDING ON A PIER When the cantilever is permanently embedded on its pier, the stability calculation is given by the strength calculations for the pier shaft and foundations. For structures subjected to very strong winds, additional stays are often necessary, requiring extra verifications under dynamic wind effects. This solution is also required in the case of double pier shafts.
Figure 14: Embedding of the cantilever on the pier
Figure 15: Pier with double shafts
12
Civil Works Construction Technology
How to correct final misalignments in closing segment?8 When a bridge is nearly done and there is only the closing segment left, there is a risk that there will be misalignments between the tips of the two cantilevers. This can be handled through using hydraulic jacks or dead weights to correct the carefully smaller misalignments between the two cantilever tips. For cast-in-place construction alignment variations and corrections are more easily accommodated but more corrections will probably be necessary. When comparing cast-in-place construction and precast construction the increase in alignment corrections will be much bigger for the cast-in-place construction which relates directly to the age of the concrete when loaded. In cast-in-place construction the concrete is generally much younger when it is loaded9. The misalignment between the two cantilever tips can be corrected when molding the final closing segment where the two girders meet at midspan of the bridge. This molding can be done by three different methods. These three methods are: The first method that can be used is a hinged connection, where the structure is allowed a slight horizontal movement. Even though the system is structurally relative simple hinges are complicated details and because of the hinges, the structurally capacity of the system is reduced. Because the structure can move in the hinge it can be a slight angle between the two superstructure halfs due to deflection in the structure. This deflection is bad for the appearance of the bridge and the user’s comfort.
Figure 16: Hinged connection
The second method that can be used is by a suspended span sitting on bearings between the cantilevers. In this method the deflection angle between the shorter cantilevers and the suspended 8
https://theses.lib.vt.edu/theses/available/etd-120199-224950/unrestricted/11lucko_chapter4.pdf https://www.researchgate.net/publication/229038055_Constructability_Considerations_for_Balanced_Cantilever_Constru ction 9
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Civil Works Construction Technology span will be much smaller than with the hinge system so the differential settling of the support can be better accounted for. But still the connections in the structural system will demands special details.
Figure 17: Suspended span principle
The third usable method is by making the structure continuous at midspan. This means that the two cantilevers tips will be casted together and will be continuous at midspan. Before casting the misalignment of the two tips can be adjusted with hydraulic jacks.
Figure 18: Continuous midspan system
The use of hydraulic jacks will cause additionally imposed stresses, these stresses shoule kept small by trying to keep the correct alignment during the casting of the two cantilevers halfs. When closing the the two segments a from traveller and steel strong backs are attached to the two tips to prevent differential movements during casting. This can also help to correct both horizontal and vertical misalignments10.
10
https://books.google.pt/books?id=JpClAgAAQBAJ&pg=PA99&lpg=PA99&dq=correct+misalignment+balanced+Cantilever+ Construction&source=bl&ots=JzU42osN_L&sig=AL6ou6cWT_8K7Pn6YrHy46HOhRE&hl=sv&sa=X&ved=0ahUKEwjhiab3k9LX AhUIWBQKHfo3DekQ6AEIPzAH#v=onepage&q=correct%20misalignment%20balanced%20Cantilever%20Construction&f=fal se
14
Civil Works Construction Technology
Useful references https://www.researchgate.net/publication/299449141_Cast_inSitu_Balanced_Cantilever_for_Building_a_Bridge http://www.bbrnetwork.com/technologies/construction-methods.html https://www.youtube.com/watch?v=jdmuIjaROHA https://theses.lib.vt.edu/theses/available/etd-120199-224950/unrestricted/11lucko_chapter4.pdf http://eprints.utm.my/33326/1/MohdKhairulAzmanMFKA2012.pdf http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.502.2785&rep=rep1&type=pdf http://www.waagner-biro.com/en/divisions/bridge-construction/about
15
Sofie Johansson IST1 88853
Elin Fryklund IST1 88854
Bojan Fajdiga IST1 88915
Antonio Barhouche IST1 88870
Maxime De Almeida IST1 89099
05/12/2017
Civil Works Construction Technology
INTRODUCTION Bridge cantilever construction method can be executed both with prefabricated an in-situ concreting. The method does not require false work or centering which leaves traffic under the spans widely unobstructed during construction of the bridge. The only access from the ground is when constructing the piers and abutments and for the preparation for the start of cantilevering, that will start from these locations. The method consist in building the bridge structure by part that are called segments. When casting on the construction site the segments are concreted into a formwork that is connected to a movable structure called form traveller. Form travelers are supported by steel cantilever trusses attached to previously completed segment. When the previously cast segment has developed enough strength can it be prestressed to previous segment and to support the subsequent one. After that the form traveller can move on and start the next segment.
Figure 1: Cantilever cast-in-situ bridge construction1
1
https://theses.lib.vt.edu/theses/available/etd-120199-224950/unrestricted/11lucko_chapter4.pdf
2
Civil Works Construction Technology
TABLE OF CONTENTS Introduction............................................................................................................................................. 2 Equipment to be used ............................................................................................................................. 4 1
Segmental bridges ........................................................................................................................... 4
2
Construction of a bridge and equipment for construction ............................................................. 4 2.1
Cast in situ method and Equipment for this method .............................................................. 5
How to control movements during construction? .................................................................................. 7 1
2
Cantilever Tendons .......................................................................................................................... 7 1.1
Principle ................................................................................................................................... 7
1.2
Disposition ............................................................................................................................... 8
Continuity tendons ........................................................................................................................ 10 2.1
Principle ................................................................................................................................. 10
2.2
Disposition ............................................................................................................................. 10
3
Temporary pilings .......................................................................................................................... 11
4
Closure of the end span................................................................................................................. 11
5
Embedding on a pier ..................................................................................................................... 12
How to correct final misalignments in closing segment?...................................................................... 13 Useful references .................................................................................................................................. 15
3
Civil Works Construction Technology
Equipment to be used 1 SEGMENTAL BRIDGES A Segmental Bridge is a bridge built in short sections (called segments), that is one piece at a time. And then segments are erected to make the bridge. The bridge is either cast-in-place or precast. The first cast-in-place segmental concrete bridge, built in 1950, across the Lahn River in Germany. Segmental bridges are classified under the following categories: -
Balance Cantilever Bridges Progressive Placement Bridges Span-by-Span Constructed Bridges Incrementally Launched Bridges
Balance Cantilever Bridges Balanced cantilever construction is an economical method when access from the below is expensive or practically impossible. Segments may be cast in-situ or precast concrete normally with a box shape cross-section. Construction starts from the top of a pier, with the segment normally fixed to the pier either permanently or temporarily during the construction. For in-situ balanced cantilever construction, the formwork is suspended from the end of the last segment. The new segment is cast, and once the concrete has developed a predetermined strength, the section is post-tensioned to the rest of the bridge. The figures show the construction sequence of this type of construction. Segments are often cast and stressed in pairs, one each side of the pier.
Figure 2: Balance Cantilever Bridge
2 CONSTRUCTION OF A BRIDGE AND EQUIPMENT FOR CONSTRUCTION There are many methods for construction one bridge. This time we will say something about the insitu balanced cantilever construction. In basic terms, cast-in-situ construction describes a process whereby segments are progressively cast on site in their final positions within the structure. Construction of a bridge is a very complex process and uses special equipment. Equipment for construction of bridges is: - Form Traveler machine - Hydraulic jacks - Mobile scaffolding - Gantry crane or other crane
4
Civil Works Construction Technology
Everyone knows about crane, scaffoldings, jackets we use them in almost every construction. In the figure 2 we can see equipment. However, in next paragraph we will say something about equipment, which is specific.
Figure 3: Equipment
2.1 CAST IN SITU METHOD AND EQUIPMENT FOR THIS METHOD When we talk about this method, we can say this: -
All works are done on site To cast the segment in their final position Commonly used method for casting segments in place is with use of form traveler. Form travelers are movable forms supported by steel cantilevers trusses attached to previously completed segments. Form travelers can be used when a bridge pier can be used as a starting point For balance cantilever erection is used, we need minimum 2 form travelers.
Form Travelers On figure 4 we can see form travelers. Form Travelers are powered by hydraulic system, requiring the crews that operate them to receive specific training .The structural framework of the form traveler move forward on rails anchored to the previously constructed span segments. The individual steel structural members of each form traveler are connected with pins to facilitate mobilization/demobilizations. Form Traveler machine is easier to be use, it is easier to be modified for future usage. Form Traveler machine are with good quality modular connection, designed for future modification and repeated usage for many years.
5
Civil Works Construction Technology
Figure 4: Form travelers
This is special workshop in the top of the pillars. They are supplied with the necessary molds, substrates, screws, concreting devices. The concrete can be prepared on site or carried from the concrete factory. For the production we need to have the necessary molds, formwork, vibratory for concrete. On the pillar, we have scaffolds. If we need lifting equipment we mounted on the upper part of the pillar. On market today we can find various types of form machines. For same special bridges sometime companies design special types of equipment.
6
Civil Works Construction Technology
How to control movements during construction? During the construction of segmental concrete bridges using the balanced cantilever method, it is necessary to guarantee the stability of the cantilever arms on their pier before they are joined to the neighbouring cantilever or to the cast-on-falsework section on the end span. It is essential to prevent the cantilever from tipping on the pier crosshead. In fact, the movements have to be controlled during construction when the cantilever is unbalanced due to the weight of a segment that is built or installed before its symmetrical counterpart is in place. Five main solutions can be used to stabilize cantilevers on their piers. The choice of the method depends on the general design of the structure. The cantilever method of construction requires, as a first method, the use of two sets of prestressing tendons, the cantilever tendons and the continuity tendons. Using prestress tendons is the most widely used method for stabilising cantilevers during the construction. In principle, this is the most economical method for spans of less than 120 meters in length.
1 CANTILEVER TENDONS 1.1 PRINCIPLE2 The cantilever tendons are required primarily to resist the effects arising from the construction stages, which constitute the greater part of the hogging bending moment at the piers. They are the principal reinforcement of the structural system and are necessary to hold the segments added and to take up the negative moment.
Figure 5: Longitudinal disposition of cantilever tendons
They are located in the deck slab and are anchored at the ends of the segments. In this way, an efficient arrangement that closely balances the moment diagram can be achieved. Anchors are preferably located at the intersection of deck slab and web.
2
https://www.google.pt/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwikvqaluuHXAhVEqxoKHV8wDQQFggsMAE&url=https%3A%2F%2Feducnet.enpc.fr%2Fpluginfile.php%2F11070%2Fmod_folder%2Fcontent%2F0%2F3-2POA.pdf%3Fforcedownload%3D1&usg=AOvVaw3HO0H_T5FhdwGtX2O-srIJ
7
Civil Works Construction Technology
Figure 6: Construction load and longitudinal disposition of cantilever tendons
The cantilever tendons are within the concrete section, they are “internal tendons”. At least one pair of tendons is anchored per segment. Sometimes, the decision will be made to include the minimum number of tendons only within the concrete section, in order that the external tendons, which can usually be stressed in a more controlled manner, may provide more resistance.
Figure 7: Lenght of cantilever tendons3
When designing the bridge, it may be desirable to make provision for additional (empty) pre-stressing ducts to be installed. These would then be available to accommodate additional tendons in the event that, during construction, the stressing records indicated that insufficient pre-stress had been applied (e.g. due to exceptionally high friction in the ducts). All the ducts are filled with grout at the end of the construction, to avoid corrosion.
1.2 DISPOSITION The cantilever tendons have a straight profile and are located in the top and bottom slabs. They apply the uniform prestress, and are therefore called the launching tendons. The design of cantilever tendons is made mainly by using the construction load, the dead load of the element and the wind load. Here is the disposition of those tendons in a transversal section.
3
https://books.google.pt/books?id=4R_kt9PDcUC&pg=PA168&lpg=PA168&dq=cantilever+tendons+bridge&source=bl&ots=rvmGAw9Ch8&sig=oyAj9Ky05pt64q-SRD9quSljbI&hl=fr&sa=X&ved=0ahUKEwjTxMzVl9zXAhXGmLQKHbXRB9wQ6AEIYjAK#v=onepage&q&f=false
8
Civil Works Construction Technology
Figure 8: Disposition of cantilever tendons in a transversal section
Two or four cables can be specified per segment. If, for the aforementioned reasons, we decide not to anchor the tendons in the webs and want to avoid increasing the size of the gusset, the optimum number of tendons specified for the end of each segment is two (one tendon per web). This arrangement makes it possible to standardize the reinforcements for the segments.
Figure 9: Typical balanced cantilever segment4
4
http://www.utraconindia.com/bridges_tanks.html
9
Civil Works Construction Technology
2 CONTINUITY TENDONS 2.1 PRINCIPLE5 The continuity tendons are required to ensure adequate compression at the joint between the tips of the cantilevers, as well as to provide the necessary resistance to positive moments from superimposed dead and live loading. The continuity tendons are internal to the section of concrete.
Figure 10: Continuity tendons
2.2 DISPOSITION6 In a plan view, the external continuity cables are situated close to the webs. To take account of construction tolerances, a distance of approximately 5cm must also be allowed between the outside of the tendon sheaths and the concrete of the segment slabs and/or the anchor blocks. For curvilinear structures, it is necessary to recenter the tendons using additional deviators.
Figure 11: Disposition of continuity tendons
5
https://www.google.pt/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwikvqaluuHXAhVEqxoKHV8wDQQFggsMAE&url=https%3A%2F%2Feducnet.enpc.fr%2Fpluginfile.php%2F11070%2Fmod_folder%2Fcontent%2F0%2F3-2POA.pdf%3Fforcedownload%3D1&usg=AOvVaw3HO0H_T5FhdwGtX2O-srIJ 6 https://fr.slideshare.net/PiyushKumarSanthalia/practical-design-of-balanced-cantilever-bridges-piyush-santhalia
10
Civil Works Construction Technology
3 TEMPORARY PILINGS7 This method consists of increasing the center distance between the temporary support blocks by placing them on temporary metal or concrete pilings. This is only feasible when the bridge deck is situated at a reasonable height above the ground (less than 15 meters or so).
Figure 12: Example of a temporary piling
This method is used on terrestrial sites in the following situations: -
For total lengths in excess of 100 meters When the pier head dimensions are reduced, often for architectural reasons When the pier shafts do not have enough resistance to flexion to stabilize the cantilever on their own.
This is a viable solution for aquatic sites, provided that the pilings are supported on the foundation bulkhead for the pier. The cantilever is usually stabilized by two pilings positioned symmetrically in relation to the pier.
4 CLOSURE OF THE END SPAN If the structure has spans of very unequal lengths or very short end spans, it is possible to use the closure of the small spans to stabilize the adjacent half-cantilevers. At the start of the construction of the half- cantilever, the structure must be stabilized using traditional methods. When the cantilever reaches the middle of the shorter span, the central connection segment is assembled. The longer span is then built by over- cantilevering, using the weight of the rear span to counteract the imbalance.
7
http://www.infra-transports-materiaux.cerema.fr/IMG/pdf/US_F0308A_Prestressed_concrete_bridges.pdf
11
Civil Works Construction Technology
Figure 13: Weight of the rear span used to counteract the imbalance
5 EMBEDDING ON A PIER When the cantilever is permanently embedded on its pier, the stability calculation is given by the strength calculations for the pier shaft and foundations. For structures subjected to very strong winds, additional stays are often necessary, requiring extra verifications under dynamic wind effects. This solution is also required in the case of double pier shafts.
Figure 14: Embedding of the cantilever on the pier
Figure 15: Pier with double shafts
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Civil Works Construction Technology
How to correct final misalignments in closing segment?8 When a bridge is nearly done and there is only the closing segment left, there is a risk that there will be misalignments between the tips of the two cantilevers. This can be handled through using hydraulic jacks or dead weights to correct the carefully smaller misalignments between the two cantilever tips. For cast-in-place construction alignment variations and corrections are more easily accommodated but more corrections will probably be necessary. When comparing cast-in-place construction and precast construction the increase in alignment corrections will be much bigger for the cast-in-place construction which relates directly to the age of the concrete when loaded. In cast-in-place construction the concrete is generally much younger when it is loaded9. The misalignment between the two cantilever tips can be corrected when molding the final closing segment where the two girders meet at midspan of the bridge. This molding can be done by three different methods. These three methods are: The first method that can be used is a hinged connection, where the structure is allowed a slight horizontal movement. Even though the system is structurally relative simple hinges are complicated details and because of the hinges, the structurally capacity of the system is reduced. Because the structure can move in the hinge it can be a slight angle between the two superstructure halfs due to deflection in the structure. This deflection is bad for the appearance of the bridge and the user’s comfort.
Figure 16: Hinged connection
The second method that can be used is by a suspended span sitting on bearings between the cantilevers. In this method the deflection angle between the shorter cantilevers and the suspended 8
https://theses.lib.vt.edu/theses/available/etd-120199-224950/unrestricted/11lucko_chapter4.pdf https://www.researchgate.net/publication/229038055_Constructability_Considerations_for_Balanced_Cantilever_Constru ction 9
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Civil Works Construction Technology span will be much smaller than with the hinge system so the differential settling of the support can be better accounted for. But still the connections in the structural system will demands special details.
Figure 17: Suspended span principle
The third usable method is by making the structure continuous at midspan. This means that the two cantilevers tips will be casted together and will be continuous at midspan. Before casting the misalignment of the two tips can be adjusted with hydraulic jacks.
Figure 18: Continuous midspan system
The use of hydraulic jacks will cause additionally imposed stresses, these stresses shoule kept small by trying to keep the correct alignment during the casting of the two cantilevers halfs. When closing the the two segments a from traveller and steel strong backs are attached to the two tips to prevent differential movements during casting. This can also help to correct both horizontal and vertical misalignments10.
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https://books.google.pt/books?id=JpClAgAAQBAJ&pg=PA99&lpg=PA99&dq=correct+misalignment+balanced+Cantilever+ Construction&source=bl&ots=JzU42osN_L&sig=AL6ou6cWT_8K7Pn6YrHy46HOhRE&hl=sv&sa=X&ved=0ahUKEwjhiab3k9LX AhUIWBQKHfo3DekQ6AEIPzAH#v=onepage&q=correct%20misalignment%20balanced%20Cantilever%20Construction&f=fal se
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Civil Works Construction Technology
Useful references https://www.researchgate.net/publication/299449141_Cast_inSitu_Balanced_Cantilever_for_Building_a_Bridge http://www.bbrnetwork.com/technologies/construction-methods.html https://www.youtube.com/watch?v=jdmuIjaROHA https://theses.lib.vt.edu/theses/available/etd-120199-224950/unrestricted/11lucko_chapter4.pdf http://eprints.utm.my/33326/1/MohdKhairulAzmanMFKA2012.pdf http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.502.2785&rep=rep1&type=pdf http://www.waagner-biro.com/en/divisions/bridge-construction/about
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