1. Ying Tung, PhD Candidate
Investigation of Distortion-Induced Fatigue Cracking in a Seismically-Retrofitted Bridge
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Riyadh Hindi, PhD, PEng
Will Lindquist, PhD, PE
Ahmed Ibrahim, PhD, PE
Ying Tung, PhD Candidate

2. Presentation Outline Brief Introduction and Background
Linear Elastic Finite Element Analysis
Moving Live Loads
Stationary Live Loads
Temperature Loads
Preliminary Fatigue Calculations
Potential Repair Options
Field Monitoring
Summary and Conclusions
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3. Section 3: Project Location Section 1 Section 2 Section 3 Section 4
AADT approximately 94k, 14k trucks
Composite reinforced CIP deck
Cracks found on interior girders in the web-
gap region near new diaphragms
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Project Location

4. Retrofit Plan & Cross-Section Views
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Retrofit Plan & Cross-Section Views

5. Retrofit Plan & Cross-Section Views
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Retrofit Plan & Cross-Section Views

6. Retrofit Plan & Cross-Section Views
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Retrofit Plan & Cross-Section Views

7. *Cracks are located on interior girders
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Crack Locations

8. Finite Element Analysis
Loads / Load Application
Dead Load
Live Load
Temperature Load
Finite Element Models (FEMs) evaluated to date
SAP2000
Abaqus
Future Work
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9. Longitudinal Steel Girders
Concrete Deck
Longitudinal Steel Girders
Rigid Links
Shared nodes in Stiffener-Web intersection
Web-gap region
Top flange
Connection plate (stiffener)
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10. Load Applied Loading Dead Load: Total weight of the structure
Temperature Load :
± 80˚F applied to the entire model
Moving Load / Point Truck Load :
Exterior girder
Interior girder
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Loading

11. Bent 64 16k 16k 4k 16k 16k 320 lb/ft 4k on both
* Wheels are 6 ft. apart
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Point Loads / Moving Loads

12. FEMs Developed with SAP2000
Loads
Temperature Loads
Moving Loads + Lane Load
Point Load + Lane Load
Models
Original
Retrofit
Retrofit Options G5 G4 G3 G2 G1
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Point Loads / Moving Loads

13. SAP2000 Model List Dead load Moving Load (G1) Moving Load (G2)
Original Model
Retrofit
(Fixed braces)
(Braces free to rotate)
(Weld web gap at crack location)
(Remove retrofit links at B64)
Dead load
Moving Load (G1)
Moving Load (G2)
Moving Load (G4)
Moving Load (G5)
Temperature Load (±80˚F)
Point Truck + Lane Load (G1)
Point Truck + Lane Load (G2)
Point Truck + Lane Load (G4)
Point Truck + Lane Load (G5)
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SAP2000 Model List

14. Retrofit Cross-Bracing left of Bent 64
Crack Locations near B64 have
slightly higher stresses
Fine mesh and shell elements for
the bracing at B64.
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Retrofit Cross-Bracing left of Bent 64
SAP2000 Model

15. Temperature Load = 80° F | B64
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Average Principal Tensile Stress

16. Truck Point Load + Lane Load
Near B63
Near B64
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Average Principal Tensile Stress

17. Truck Point Load vs. Moving Load | B64
Moving load on G2
Truck + Lane load on G2
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Average Principal Tensile Stress

18. Interior vs. Exterior Girders | B64
Truck
Truck
Truck + Lane load on G1
Truck + Lane load on G2
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Average Principal Tensile Stress

19. Girder Deflections Girder Deflections Differential Girder Deflections
Out-of-Plane Distortion
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Vertical Deflection | Differential Deflection

20. Vertical Deflection | Truck Load
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Vertical Deflection | Truck Load

21. Differential Girder Deformation
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Vertical Differential Deflection

22. Out-of-Plane Displacement
“X-Bracing”
“K-Bracing”
“X-Bracing”
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Y-axis
Out-of-Plane Displacement

23. Preliminary Fatigue Analysis
Assumptions
ADTT = 14,000; ADTTSL = 14,000×0.85=11,900
AASHTO HS-15 Fatigue Truck (54 kips)
Category C to C’ detail; CAFT = 10 to 12 ksi
2 lanes available to trucks
Stress Range = 14.2 ksi
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24. 14.2
1,537,000
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AASHTO S-N Curve

25. Potential Repair Options
Continue Monitoring / Drill Crack Arrestor Holes
Stiffen the Stiffener Plates
Remove retrofit links / diaphragms
Loosen diaphragm bolts / Install under-sized bolts
Soften the Stiffener Plates
Field Monitoring
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26. Crack Arrestor Holes Δσ = 5.7 ksi Δσ = 13.0 ksi Δσ = 25.2 ksi
AASHTO Fatigue Truck
Δσ = 5.7 ksi
Δσ = 13.0 ksi
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Δσ = 25.2 ksi
Crack Arrestor Holes

27. Positive Connection / Remove Links
AASHTO Fatigue Truck
Δσavg = 14.2 ksi
Δσavg = 0.7 ksi
Δσavg = 13.8 ksi
Positive Connection / Remove Links

28. Modify Connections / Loosen Bolts
AASHTO Fatigue Truck
Δσavg = 6.0 ksi
Δσavg = 14.2 ksi
Δσavg = 10.8 ksi
Modify Connections / Loosen Bolts

29. Repair Summary Average Stress (ksi) Average Stress Reduction (ksi)
Crack Initiation (cycles)
Repair 1: Stiffen Plate
(positive connection)
0.7
95%
<CAFT
Repair 2: Remove Seismic Retrofit Links
13.8 3%
1.7×106
Repair 3: Loosen Bolts
10.8
24%
3.5×106
No Repair (Retrofit)
14.2 --
1.5×106
Original Bridge
6.0

30. Proposed Field Monitoring
Validate Finite Element Analysis
Directly measure the stress-range cycles produced by random variable live-load spectrum
Directly measure the number of cycles applied per interval of time
Wireless monitoring of strain for approx. 1 month
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31. Executive Summary
Seismic retrofit resulted in unintended out-of-plane distortion
Cracks likely initiated within 1 year after the retrofit was completed
At a minimum, continue drilling crack arrestor holes and monitor for continued crack growth
Field monitoring (approx. 1 month) recommended
The key with any retrofit is to balance long-term durability with seismic safety
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32. Repair Methodology / Category
Repair Option
Brief Description
Repair Methodology / Category 1
Remove the seismic retrofit links at bent 64 in an effort to increase flexibility.
Increase Flexibility 2
Loosen the bolts at the cross-frame to stiffener plate connection to allow rotation. 3
Provide a positive connection between the stiffener plate and the top flange.
Increase Stiffness 4
Reduce the stiffness of the “×” cross frames by 50% by removing one of the angles from the double-angle section. 5
Continue to drill 2 ⅞-in. crack arrestor holes. 6
Remove original “K” cross frames located at the center of the spans between bents 62 and 63 and bents 63 and 64. 7
Replace the “×” braces added as part of the seismic retrofit with the original “K” braces. 8
Increase the web-gap length by removing a portion of the connection plate.
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