A very informative guide related to construction of cantilever Bridges.

1. 2016 MIDAS Technical Chat
Practical Design of
Balanced Cantilever
Bridges
midas
Civil
Piyush Santhalia
ProjectEngineer, AECOM

2. 1. Introduction
2. Longitudinal Span Configuration
3. Construction Sequence
4. Cross Section
5. Support Conditions
6. Sub-Structure and Foundation
7. Prestressing Details
8. Design Check
9. Pre-Camber
10. Modelling & Other Suggestions
midas
Civil

3. Technical
Lecture
by AECOM
Introduction
• Cantilever construction method
1) Very ancient technique
2) Structure is built component by component above
ground level.
3) More recently: Construction of Cable Stayed Bridge
s, Extra-dosed Bridges etc.
4) Prestressed Concrete Bridges
• Cast in situ Segments or Pre-cast segments
• Integral with Pier or On Bearings
• 60m – 300m span

4. Technical
Lecture
by AECOM
Balanced Cantilever Bridge
- Delhi Metro Phase III
- Cast-in-Situ Segments
- Integral with Pier
Introduction

5. Technical
Lecture
by AECOM
• Typical 3 Span system
– Mid Span: L
– End Spans: 0.6L to 0.7L (to control uplift in bearing)
L
0.6 L to 0.7L
0.6 L to 0.7L
• Typical 4 or more Span (varying) system
0.6 L1 to 0.7L1 L1 (L1 + L2)/2 0.6 L2 to 0.7L2
Longitudinal Span Configuration

6. Technical
Lecture
by AECOM
• No such luxury in today’s congested urban area
I. 34 + 60 + 34 m
II. 60 + 60 m
III. 37 + 70 + 67 + 55 + 34m
Longitudinal Span Configuration

7. Technical
Lecture
by AECOM
A. Pier head : On ground supported staging
B. Most of Segments:
 Erect/cast using Segment Lifter/Form Traveller
 Cantilevered out from preceding segment.
 Prestressing tendons running one of the cantilever to the other are str
essed.
 Symmetrical construction to minimize unbalanced moment on sub-str
ucture and foundation: Balanced Cantilever
 Cast portion (beyond 0.5 x L) of both End-spans Ground Supported st
aging.
 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
Construction Sequence

8. Technical
Lecture
by AECOM
Balanced Cantilever Bridge :
Delhi Metro Phase III, 60 + 60m span
Construction Sequence

9. Technical
Lecture
by AECOM
Casting of Stitch at Mid-
span using suspender.
Balanced Cantilever Bridge :
Delhi Metro Phase III
(34 + 60 + 34m)
Construction Sequence

10. Technical
Lecture
by AECOM
L
H1 H2
• Highway Bridges
– Depth at Face of Pier, H1 : L/15 – L/18 (roughly)
– Depth at Mid-Span, H2 : L/30 – L/35 (roughly)
• Highway vs Railway Bridge
– 34+60+34m span CLC Load Metro Highway
DL 463 463
SIDL 240 85
LL 262 134
Shear Force at Pier Face (ton)
Cross Section

11. Technical
Lecture
by AECOM
• Depth may vary
– Parabolic
– Cubically: need to check for insufficient depth around L/4
– Linearly varying depth
• Local thickening of soffit is required.
Cross Section

12. Technical
Lecture
by AECOM
• 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 un-balanced moment during construction.
– Pronounced secondary effect.
Support Conditions

13. Technical
Lecture
by AECOM
• Flexibility
– High time period (lesser seismic force)
– Lower force due to secondary effects of creep, shrinkage and Pr
estressingTendons
– Twin Piers
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3𝐸𝐼
𝐿
δ
Sub-Structure and Foundation

14. Technical
Lecture
by AECOM
• Cantilever Tendons
– For holding the segments added during cantilever construction
– To take up the negative moment due to SW of Segments, SIDL and/or
Live Load
– At least 1 pair of tendon is anchored per segment.
• Continuity Tendons:
– To take up the force due to effects after the cantilever have been stitched.
Prestressing Details

15. Technical
Lecture
by AECOM
• Top Tendons
– Try to keep the web clear of the
Tendons
• Bottom Tendons
– Keep the webs clear of the tendons
as much as possible
– Keep tendons nearer to the webs as
much as possible
– Enough prestressingfor sections at
mid-span to hog.
– Blister Blocks for anchoring of tendons
Prestressing Details

16. Technical
Lecture
by AECOM
• Why Construction Stage Analysis
Bending Moment Diagram due to SW: Simultaneous Analysis
Bending Moment Diagram due to SW: Sequential Analysis
Analysis

17. Technical
Lecture
by AECOM
– Time Dependent Effects of Creep and Shrinkage
• No secondary effect of Creep and Shrinkage before stitching
Structure before casting of stitch segment
Deformation due
to Shrinkage.
Residual Shrinkage Strain:
i) After 3 days – 4.3 x 10-4
ii) After 14 days – 2.5 x 10-4
• Why Construction Stage Analysis
Analysis

18. Technical
Lecture
by AECOM
– Time Dependent Effects of Creep and Shrinkage
• Different age of concrete at different loading
• Modulus of Elasticity increases with time
• Why Construction Stage Analysis
Analysis

19. Technical
Lecture
by AECOM
• Sub-structure & Foundation
– Regular Checks for Foundation & Piers
– Secondary effects of CR, SH & PS should be considered
– Check during construction (stability or adequacy)
i) Imbalance of 1 segment
ii) Accidental Fall of Empty Form Traveller
Imbalance of 1 Segment
Fall of empty FT
Design Check

20. Technical
Lecture
by AECOM
• Super Structure
– Check during construction (ULS & SLS)
• Maximum Compression at each stage
– Maximum compression: 0.48fck (IRC 112-2011)
• Maximum Tension at each stage
– Minimum compression of 0.2fck - Precast segments (temporary Prestressing)
– Maximum tension of 1 MPa – Cast in situ segments.
• Loads
– SW of Segments
– Form Traveller (usually half the weight of heaviest segment) + Shutter
– Weight of Green Concrete
– Construction Live Load
– Wind / EQ (cantilever)
Design Check

21. Technical
Lecture
by AECOM
Stress check during Construction:
37 + 70 + 67 + 55 + 34m
Stresses at Top Fibre due to DL + PS + CR + SH (N/mm2)
Design Check

22. Technical
Lecture
by AECOM
Stresses at Top Fibre due to DL + PS + CR + SH (N/mm2)
Stress check during Construction:
37 + 70 + 67 + 55 + 34m
Design Check

23. Technical
Lecture
by AECOM
Stresses at Top Fibre due to DL + PS + CR + SH (N/mm2)
Stresses at Bottom Fibre due to DL + PS + CR + SH (N/mm2)
Stress check during Construction:
37 + 70 + 67 + 55 + 34m
Design Check

24. Technical
Lecture
by AECOM
Stresses at Top Fibre due to DL + PS + CR + SH (N/mm2)
Stresses at Bottom Fibre due to DL + PS + CR + SH (N/mm2)
Stress check during Construction:
37 + 70 + 67 + 55 + 34m
Design Check

25. Technical
Lecture
by AECOM
Stresses at Top Fibre due to DL + PS + CR + SH (N/mm2)
Stresses at Bottom Fibre due to DL + PS + CR + SH (N/mm2)
Stress check during Construction:
37 + 70 + 67 + 55 + 34m
Design Check

26. Technical
Lecture
by AECOM
Stresses at Top Fibre due to DL + PS + CR + SH (N/mm2)
Stresses at Bottom Fibre due to DL + PS + CR + SH (N/mm2)
Stress check during Construction:
37 + 70 + 67 + 55 + 34m
Design Check

27. Technical
Lecture
by AECOM
• Super Structure
– Check during Service (0 - Design life)
• Loads
– Regular Loads (SW, SIDL, LL)
– Prestressing
» Losses up to design life should be considered
» Secondary effects are usually significant
– CR & SH: Secondary effects are significant.
– Temperature Variation
• SLS Checks
– Maximum Compression
» Maximum compression: 0.48fck (IRC 112-2011)
– Maximum Tension
Design Check

28. Technical
Lecture
by AECOM
• Super Structure
– Check during Service (0 - Design life)
• ULS Checks
– Moment at the intermediate support
» Hogging for Normal Case
» Reversible in Seismic case
– Shear Check
» Varying depth: Should be checked at regular interval
• Critical at locations with kink
Design Check

29. Technical
Lecture
by AECOM
• Super Structure
– Check during Service (0 - Design life)
• ULS Checks
– Shear Check
» Vertical component of Prestressing: reduces shear
» Resal Effect: Part of Shear is balanced by the component of Nor
mal force in the soffit slab.
Design Check

30. Technical
Lecture
by AECOM
Pre-Camber

31. Technical
Lecture
by AECOM
• Why pre-camber
– Under permanent loads the deck should have achiev
ed the desired level.
• Desired Level at what time
– Concrete continues to sag/hog because of creep
– Achieving desired level at the end of design life: not
logical
Pre-Camber

32. Technical
Lecture
by AECOM
• For very wide or very deep section
– Line Beam modelling: up to 20% error
• Shear Lag effect
• Difference in rates of shrinkage and drying creep because of different thicknesses
of slabs.
• 3D model always yields larger deflections and larger Prestress losses
Ref: Excessive Long-Time Deflections of Prestressed Box Girders. I: Record-Span Br
idge in Palau and Other Paradigms - Zdeněk P. Bažant, Qiang Yu and Guang-Hua Li
• Modelling of Piles
• Give concrete more time to gain strength before prestressing
Modelling &other Suggestions

33. 2016 MIDAS Technical Chat
Practical Design of
Balanced Cantilever Bridges
midas
Civil
Piyush Santhalia
Project Engineer, AECOM
THANK YOU
Happy Designing