BALANCED CANTILEVER BRIDGE

BALANCED CANTILEVER BRIDGE

1. BALANCED CANTILEVER BRIDGE SRINIVAS REDDY.S

2. What is balanced Cantilever method of Bridge Construction : The balanced cantilever method of bridge construction used for bridges with few spans ranging from 50 to 250m. The bridge can be either cast-in-place or precast. Moreover, the basic concept of balanced cantilever construction method is to attach the segments in an alternate manner at opposite ends of cantilevers supported by piers . Furthermore, this method is easily adaptable to irregular and long span lengths, congested project sites, rough and water terrain, rail crossings, and environmentally sensitive areas. Additionally, it is highly suitable for building cable-stayed bridges. This is because once segments are placed, they will be supported by new cable-stays in each erection stage. Therefore, no auxiliary supports are required, and hence it is both economical and practical method for long cable-stayed bridges.

3. ADVANTAGES OF SIMPLY SUPPORTED AS WELL AS CONTINUOUS STRUCTURES The structures are statically determinate and the moments, shears etc., may be found out by the basic rules of statics and The possibility of cracks due to unequal settlement of the foundations is eliminated. This type of structure is also comparable to some extent with continuous structures since the free positive moment at mid-span is partly balanced by the negative moment caused by the cantilever and thereby leads to economy in materials. Balanced cantilever bridges also require one line of bearings over the piers similar to continuous bridges. For bridging smaller channels, usually one central longer span with two shorter end spans of the types as shown in Fig 1 a and 1 b are adopted but where the bridge length is more, repetition of the type of span illustrated in Fig 2 is resorted to.

4. FIG 1 : BALANCED CANTILEVER BRIDGES FIG 2 : MULTISPAN BALANCED CANTILEVER BRIDGES

5. PROPORTIONING OF MEMBERS To get the most economical design, the proportioning of the members should be such that the sections at mid-span and at support satisfy both the structural and architectural requirements and at the same time require minimum quantity of materials. To achieve this, the cantilever lengths are usually made from 0.20 to 0.30 of the main span. This ratio depends on the length of the main span and the type of suspended span the cantilever has to support as well as the number of cantilevers (single or double) available for balancing the mid-span positive moment etc.

6. Types of Superstructure: The superstructures may be of solid slab, T-beam and slab, hollow box girder etc. Photograph 3 shows one hollow-box balanced cantilever bridge.

7. CANTILEVER CONSTRUCTION METHOD Cantilever construction method Very ancient technique Structure is built component by component above ground level Most recent : construction of cable stayed bridges, extra-dosed bridges etc Prestressed concrete bridges: Cast in-situ segments or pre-cast segments Integral with pier or on bearing

8. 3.Construction sequences : A. Pier head: on ground supported staging B. Most of segments: Erect/cast using segment lifter/from traveller Cantilevered out from preceding segment Prestressing tendons running one of the cantilever to the other are stressed Symmetrical construction to minimize unbalanced moment on substructure and foundation : balanced cantilever Cast portion beyond 0.5 x L of both ends spans ground supported staging Cast stitch segments: Stitch in the end span Stitch in the mid span Levels of the cantilever arms being stitched should be matched C. Segmentation : 2.5m to 4m or even 5m Construction cycle Capacity of form traveller/segment lifter

9. 5. Support conditions Box girder- on simple bearing - stability check during construction -Minimal secondary effect of creep, shrinkage and prestressing Box girder integral with intermediate piers -Check pier for unbalanced moment during construction -Pronounced secondary effect

10. Procedure for balanced cantilever method of cast-in-situ bridge Construction After the construction of lower infrastructure of the bridge is completed, fig.1 Bridge construction begins at each pier. Special formwork is positioned and cast-in-situ pier segment is begun, fig.2. The complete pier segment is then used as an erection platform to support a form traveler for cast-in-place segments. Fig 1: Construction of lower infrastructure of bridge

11. Fig 2: Position of special formwork

12. Thereafter, soffit shuttering, shuttering for web & deck shuttering is fixed on both sides of pier as shown in fig.3 and fig.4. Fig.3: Soffit, web, and Deck shuttering Fig.4: shuttering soffit, web, and decks

13. Then concreting is done on both sides of the pier as shown in fig.5 and fig.6. The segment production rate for form travelers is usually one segment every 5 days per traveler. Fig.5: concrete placement Fig.6: Concrete placement

14. Cast-in-situ segments range between 3m to 5m in length with formwork moving in tandem with each segment. Segment construction is continued until a joining midpoint is reached where a balanced pair is closed as demonstrated in fig.7. The construction of closer section of a bridge is shown in fig.8. Fig.7: bridge construction progression Fig.8: Construction of closer section of the bridge

15. Casting of precast segments There are two methods for precast segment casting which include: Short line method: In this rate of segment production is slow. Three or four segments cast at a time. Long line method: In this rate of segment production is fast. Segments equal to one span cast at a time. Fig.14: Short line segment casting Fig.15: Long line segment casting

16. Cast-in-Place Segments Vs precast segments Cast-in-place construction proves to be very beneficial when large, considerably heavy segments are required to be constructed. So, instead of handling the segments, only materials have to be transported thus influencing the type and size of required equipment. Alignment variations and corrections are more easily accommodated in cast-in-place construction; but more corrections will probably be necessary. The increase in alignment corrections for cast-in-place construction compared to precast construction relates directly to the age of the concrete when loaded. By and large, the concrete is much younger when loaded in cast-in-place construction.

17. STUDY AND ANALYSIS OF BALANCED CANTILEVER BRIDGE AT KOCHI METRO the type of bridge in this study is balanced cantilever bridge (Railway bridge), a part of Kochi Metro Project of Maharajas- Petta stretch at Ernakulum. A cantilever bridge is a bridge built using cantilevers, structures that project horizontally into space, supported on only one end. It is the fifth balanced cantilever bridge and first curved balanced cantilever bridge of India. The 90metre span will be balanced on either end by 65-metre-long concrete spans, taking the total length of the structure to 220 metres and a curve of 152 m long radius. The reason for adopting a cantilever bridge instead of ordinary bridges are the presence of overhead electric lines of the nearby railway station, underground pipes causing difficulties for piling, non- availability of vacant space between the tracks for erecting pillar, and to avoid interruptions of the underneath railway transportation. Due to long span the bridge is constructed as segments. Segmental bridge construction is one of its specialty. it is even more an engineering challenge since the span is located at a curve having a 152-metre-long radius. Such spans are generally made for over bridges carrying vehicles.

18. methodology The details required for analysis are collected from the site and certain parameters are assumed as per the standard specifications. The study is focused on the deflections of cantilever span under different loading conditions such as dead load and live loads. The analysis of balanced cantilever bridge is done using STAAD Pro software and behavior of cantilever bridge is studied by considering construction methodology, load-deflection characteristics. Since beams curved in plan cannot be analyzed using STAAD Pro software, the bridge is analyzed as straight.

19. CAPACITY DMC : 191 passengers (Sitting-35, Crush Standing-156) TC: 218 passengers (Sitting-44, Crush Standing-174) 3 Car Train: 600 Passengers (Sitting-114, Crush Standing-486) LOAD CALCULATIONS Average weight of passenger = 65kg Coach and bogie is designed for 13 T Weight of one passenger = 65 x 10 = 650 N Weight of 600 passengers = 650 x 600 = 390 kN Weight of coach and bogie = 130 kN Total weight = 390 + 130= 520 kN

20. Construction stage analysis Since during the construction stage, only self-weight will be acting, therefore self-weight with factor -1 was assigned to the structure. At this stage bridge will act as a cantilever bridge, so at the free end moments were released. Using this first load case, bridge was analysed and deflection and bending moment diagrams were obtained.

21. Working stage analysis During working stage, after prestressing , the bridge will act as a continuous bridge. So in this stage moment releases were removed. Here live load of metro rolling stock is considered. From the Kochi Metro DPR it was obtained that train consisting of three-car was 60m in length with concentrated load of 520kN. Thus a UDL of 8.67kN/m is considered as live load. Two cases were studied to get maximum deflection. Case:1- Live load given only on one track

22. Case:2- Live load given on supports on both tracks Case 3: Load given at mid span on single track

23. Case 4: Load given at mid span on both the tracks

24. Results and conclusion

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