TRACING LOAD PATH IN A CONCRETE TRUSS BRIDGE ...

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

FIU Pedestrian Bridge, Collapsed – 15th March 2018

2 April, 2019 2Photo - SMH

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Florida International University, FIU, Pedestrian Bridge, March 2018

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© Arcadis 2018

Failure of Node, Video - internet

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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

5

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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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End Support Arrangement

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Section A-A

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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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Concentrated Load

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Stress Concentration at Node

Load Path in Roof Canopy at Node

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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

19

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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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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

© Arcadis 2018

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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