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 WELLAS
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

10. 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

13. 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

18. 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

19. Fig 2: Position of special formwork

20. • 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

21. • 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

22. • 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

23. 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

24. 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.

25. 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.

26. 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.

28. 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

29. 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.

30. 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

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

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

33. Results and conclusion

34. THANKYOU