Post on 23-Mar-2023
Hari P Pokharel, Arcadis Australia Pacific, Sydney
2nd April 2019
TRACING LOAD PATH IN A CONCRETE TRUSS BRIDGE AND
DEVELOPING DESIGN DETAILS FOR A TYPICAL NODE
REFERENCE TO THE FIU PEDESTRIAN BRIDGE THAT COLLAPSED IN MARCH 2018
Disclaimer: The views, thoughts, and opinions expressed in this paper are solely those of the author in his private capacity and are in no way
connected to, belong or represent the views, thoughts or opinions of the author’s employer, Arcadis Australia Pacific Pty Limited or any other entity
connected to the author’s employer.
© Arcadis 2018
Background and Purpose of This Presentation
1. Loss of Life, Injuries, Pain – Our Professional Obligation, Trust and Responsibility.
2. US National Transport Safety Board, NTSB, Investigating the causes of failure.
Initial investigation report, November 2018 – Confirmed strength of material as
required in design documents.
3. Selection of Bridge Type –
- Merits of concrete truss bridge
- Multi criteria analysis
4. Discussion - Design of Nodes in reinforced concrete truss bridges
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Elevation of Concrete Truss Bridge and Cross Section
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Ref-5 - Florida International University, University Prosperity Project, http://facilities.fiu.edu/projects/BT_904/MCM_FIGG_Proposal_for_FIU_Pedestrian_Bridge_9-30-2015.pdf
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Design Philosophy - Bridge Codes
Limit sate conditions
- Equilibrium and stability
- Deflection
- Crack width
- Vibration,
- etc.
- Ductility
Mode of failure to be ductile
Alternate load path, excessive deflection, warning - Robustness
- Load factors, and
- Strength reduction factors
No Collapse – Safety First
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Ductile Behavior
Ductility – Deformation before it loose load carrying capacity, CEB-FIP 43
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Axial Capacity of Concrete
Confinement reinforcement improves ductility and capacity, Mander et al
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Successful Design of Nodes in RC Structures
Connection of Cables to Concrete Deck
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Case: Axial Forces in Members at a Node
Member SW (MN) SLS LC (MN) ULS LC (MN)
T -7.5 -9.5 -12.4
B 5.2 6.4 8.3
X 4.0 5.1 6.6
Y -6.3 -7.8 -10.1
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Loads → DL – 170KN/m, LL – 45KN/m
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Design Capacity of Member T, Y
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Axial capacity of a stocky compression member, AS5100.5
N* = Σ γ F < ϕ (0.85f’c Ac + fs. As) Equation -1
Values of γ and φ are specified by bridge design codes. γ varies for various
load combinations, while φ is dependent on the mode of failure.
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Design Development for Effective Area Ac
Design is an iteration process, geometry ->loads->structural check->Ac ~ 0.4 sqm,
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Blister
Blister
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Member Capacity for T
Σ γ F < φ (0.85f’c Ac + fs. As)
SW – 7.5MN/0.4m2 = 19MPa
SLS – 9.5MN/0.4 m2 = 24MPa Ac=0.4 m2
ULS -12.4MN/0.4m2 = 31MPa
Stress Limits Based on Gross Section Area
σ = F / Ac
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Strain in Concrete at Limit Stress
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0
10
20
30
40
50
60
70
0.0E+00 5.0E-04 1.0E-03 1.5E-03 2.0E-03 2.5E-03 3.0E-03
Str
es
s (
Mp
a)
Strain
Stress_Strain Curve 0.45Fc' φ*0.85fc' Ec
Strain in Concrete at Limit Strength is within Elastic Range, Codes Set ϕ
31
24
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Stress Path at Node - 1
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Capacity of a compression strut , AS5100.5
ϕNu = Φst βs 0.9f’c Ac Equation – 2
Φst = 0.6, the strength reduction factor in compression,
βs = strut efficiency factor,
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Stress Path at Node - 2
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Capacity of a compression strut
φNu = Φst βs 0.9f’c Ac Equation – 2
Φst = 0.6, the strength reduction factor in compression,
βs = strut efficiency factor, depends on degree of confinement
For Nu=12.5MN, Ac~0.5m2, for βs=0.8
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Poisson Ratio, Splitting Stress
If σx = 31MPa, σy=σz=µσx~6MPa f,t = 0.36√f’c = 2.8MPa
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Typical Reinforcement of Node
Cross section at Node, anchor plate not shown
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Design Development – Input from Construction Operation
Strength depends on making of concrete – batching, placing curing
Often strength in structure is lower than the strength in test cylinder
Changing nature of support conditions during transportation alters
load path, affecting integrity of concrete member
Whole construction process requires – documentation, review and
input for design
Design development process – construction operation, design
verification, peer review - > minimize risk
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Summary - Conclusions
• Several factors could contribute for the collapse of bridge
• Complexities of geometry in balancing need of Architectural and
Structural requirements in Landmark bridges.
• Load path at a Node is complex
• Assessment of capacity following code requirements can create
robust design
• Concrete Truss Bridge for future bridge projects - feasible
• Replying in computer without understanding behaviour can be
design risk
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Thank You
Disclaimer: The views, thoughts, and opinions expressed in this paper are solely those of the author in his private capacity and are in no way connected to, belong or represent the views, thoughts or opinions of the author’s employer, Arcadis Australia Pacific Pty
Limited or any other entity connected to the author’s employer.
Acknowledgements – Technical reviewers for comments, ARCADIS Australia for supporting author to present this paper, organizer of this
conference, and you.
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