METHODS OF CONSTRUCTION OF BRIDGES. different types application importance and various case studies

Method of construction of bridges

1. METHODS OF CONSTRUCTION OF BRIDGES 1

2. CONSTRUCTION OF SHORT SPAN BRIDGES For bridges spans up to about 40m, the super structure is built on staging supported on ground. For alternative spans, the girders may be precast for the full span length and erected using launching girders or cranes. Precast concrete bridge construction techniques are used for bridges of small and medium spans speedy erection elimination of obstructive formworks design standardization achievement of high quality. Example: Sone Bridge at Dehri comprising 93 spans of 32.9m each. 2/23

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4. CONSTRUCTION OF CONTINOUS CONCRETE BRIDGES 4/23

5. INCREMENTAL PUSH LAUNCHING METHOD Highly mechanized erection method. Construction of false work is not feasible- crossing over busy traffic corridors, or spanning deep gorges or where the pier heights are large. The total construction deck is subdivided into segments of 10 to 30m length depending on the length of spans and the time available for construction. The forms are so designed as to be capable of being moved transversely or rotated on hinges to facilitate easy stripping after sufficient hardening of concrete. 5 /23

6. INCREMENTAL PUSH LAUNCHING METHOD At the head of the first segment, a steel nose consisting of a light truss or stiffened plate girder assembly is attached. To facilitate reaching of the first and subsequent piers without inducing a very large cantilever moment during construction. The second and the following segments are concreted directly on the face of the hardened portion and the longitudinal reinforcement can continue across the construction joint. The pushing is achieved by hydraulic jacks which act against the abutment. 6 /23

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8. INCREMENTAL PUSH LAUNCHING METHOD Temporary sliding bearings are used on each pier and in the casting area to launch the deck. When the spans are more than 15 times the girder depth, auxiliary piers are employed- to reduce the bending moments in the superstructure during launching. The web should be thickened at the junction with the top slab to accommodate tendon couplers At the junction with the bottom slab to carry safely the reactions of the launching bearings. Longitudinal sloping graders can be accommodated, the launching being in the downward direction. 8 /23

9. SEGMENTAL CANTILEVER CONSTRUCTION Suited for the construction of prestressed concrete superstructure of long span bridges without the use of staging from below. The segments may be either cast-in-place or precast. Cast in place method uses cantilever form traveller (CFT) When the precast segments are used they are erected with the use of launching trusses and cranes. An innovative concept of economic construction- is to split the deck to be constructed as a spine of box section and two cantilevers. The spine is prestressed to take longitudinal moments and transverse action is catered predominantly by membrane action. 9 /23

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11. SEGMENTAL CANTILEVER CONSTRUCTION ADVANTAGES Only relatively little formwork is required due to the segmental nature of the superstructure. Use local labour and materials Give aesthetical view or good quality DISADVANTAGES Complete calculations are required for the construction stage, The influence of stepwise construction needs to be considered 10/23

12. PRECAST GIRDERS MADE CONTINOUSLY BY CAST IN PLACE SLAB AND DIAPHRAGM Used in urban flyovers where bridges built simply supported precast prestressed girders are made continuous through the cast in situ deck slab. Two generic types of connections are: The deck slab is continued monolithically over the piers, without continuing the girders. The deck slab is separated from the girders for a length of about 1m by introducing neoprene pads of about 12mm thickness. This provides flexibility to accommodate the rotation of the girders. 12/23

13. PRECAST GIRDERS MADE CONTINOUSLY BY CAST IN PLACE SLAB AND DIAPHRAGM 13/23

14. PRECAST GIRDERS MADE CONTINOUSLY BY CAST IN PLACE SLAB AND DIAPHRAGM The deck slab is hinged over the pier using partly debonded dowelling. The purpose of introducing continuity in such situations is to reduce the number of expansion joints and to enhance riding quality. This arrangement permits rotation but does not provide moment continuity. 14/23

15. PRECAST GIRDERS MADE CONTINOUSLY BY CAST IN PLACE SLAB AND DIAPHRAGM Require little to no falsework - Accelerated Bridge Construction. PC girders employ high performance concrete for strength, durability, and tend to be more economical and competitive when significant repeatability exists on a job 15/23

16. SPAN-BY-SPAN METHOD The most economic and rapid method of construction available for long bridges and with individual spans up to 60m. Decks are begun at one abutment and constructed continuously by placing segments to the other end of the bridge. Segments can be positioned by either a temporary staying mast system through more commonly using an assembly truss. Before segments are placed the truss with sliding pads is braced over two piers. Each segment is then placed on the sliding pads and slide into its position. Once all segments are in position the pier segment is then placed. 16/23

17. SPAN-BY-SPAN METHOD The final stage is then begun by running longitudinal prestressing tendons through segments ducts and prestressing entire span. Deck joints are then cast and closed and ducts grouted. When the span is complete the assembly truss is lowered and moved to the next span where construction cycle begins until the bridge is complete. 17/23

18. SPAN-BY-SPAN METHOD 18/23

19. SPAN-BY-SPAN METHOD ADVANTAGES Quick, Simple Erection (2 - 3 Spans / Week) Easy Geometry Control Minimum User Delays Cost Effective Simple Design Durable Structures DISADVANTAGE Special gantries are required 19/23

20. CAST IN PLACE CONSTRUCTION ON STAGING If the clearance between the ground and the bottom of deck is small and the soil is firm. The superstructure can built on staging. Currently free cantilevers and movable scaffold systems are increasingly used to save time and to improve safety. This movable scaffold system employs movable forms stiffened by steel frames . These forms extend one span length and are supported by steel girders which rests on pier on one end. can be moved from span to span on a second set of auxiliary steel girders. 20/23

21. CAST IN PLACE CONSTRUCTION ON STAGING The construction joint is placed at the point of inflection for dead load moment which is about 0.2L from pier. After concreting, curing and stressing the form for deck slab is first lowered. The inner web form is next loosened and rotates to free it from the deck. Lastly the outer web form and the form truss are lowered and shifted outward. The form truss is then moved lengthwise to the next stage. 21/23

22. CAST IN PLACE CONSTRUCTION ON STAGING ADVANTAGES Alignment variations and corrections are easily accommodated. Reducing direct cost Reducing the required material Reducing labour cost, rebar, and transportation costs Increasing the speed of formwork.   DISADVANTAGE Time consuming. 22/23

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