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

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Presentation on theme: "Investigation of Distortion-Induced Fatigue Cracking in a Seismically-Retrofitted Bridge Riyadh Hindi, PhD, PEng Will Lindquist, PhD, PE Ahmed Ibrahim,"— Presentation transcript:

1 Investigation of Distortion-Induced Fatigue Cracking in a Seismically-Retrofitted Bridge 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

3 Project Location Section 1 Section 2 Section 3 Section 4 Section 3: AADT approximately 94k, 14k trucks Composite reinforced CIP deck Cracks found on interior girders in the web- gap region near new diaphragms

4 Retrofit Plan & Cross-Section Views

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7 Crack Locations *Cracks are located on interior girders

8 Finite Element Analysis Loads / Load Application Dead Load Live Load Temperature Load Finite Element Models (FEMs) evaluated to date SAP2000 Abaqus Future Work

9 Concrete Deck Longitudinal Steel Girders Rigid Links Shared nodes in Stiffener-Web intersection Web-gap region Top flange Connection plate (stiffener)

10 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 Load Applied

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

12 Point Loads / Moving Loads 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

13 SAP2000 Model List Original Model Retrofit (Fixed braces) Retrofit (Braces free to rotate) Retrofit (Weld web gap at crack location) Retrofit (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)

14 SAP2000 Model Bent 61 Bent 62 Bent 63 Bent 64 Retrofit Cross-Bracing left of Bent 64 Bent 64 - Crack Locations near B64 have slightly higher stresses Fine mesh and shell elements for the bracing at B64.

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

16 Truck Near B63Near B64 Truck Point Load + Lane Load Truck

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

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

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

20 Vertical Deflection | Truck Load

21 Vertical Differential Deflection Differential Girder Deformation

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

23 Preliminary Fatigue Analysis Assumptions ADTT = 14,000; ADTT SL = 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

24 AASHTO S-N Curve ,537,000

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

26 Crack Arrestor Holes AASHTO Fatigue Truck Δσ = 5.7 ksi Δσ = 13.0 ksi Δσ = 25.2 ksi

27 AASHTO Fatigue Truck 27 Positive Connection / Remove Links Δσ avg = 14.2 ksi Δσ avg = 0.7 ksi Δσ avg = 13.8 ksi

28 AASHTO Fatigue Truck 28 Modify Connections / Loosen Bolts Δσ avg = 6.0 ksi Δσ avg = 14.2 ksi Δσ avg = 10.8 ksi

29 29 Average Stress (ksi) Average Stress Reduction (ksi) Crack Initiation (cycles) Repair 1: Stiffen Plate (positive connection) 0.795%

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

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

32 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. Increase Flexibility 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. Increase Flexibility 5 Continue to drill 2 ⅞-in. crack arrestor holes. Increase Flexibility 6 Remove original “K” cross frames located at the center of the spans between bents 62 and 63 and bents 63 and 64. Increase Flexibility 7 Replace the “×” braces added as part of the seismic retrofit with the original “K” braces. Increase Flexibility 8 Increase the web-gap length by removing a portion of the connection plate. Increase Flexibility

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