1. Structural Health Monitoring of Steel Bridges Pradipta Banerji Professor of Civil Engineering, IIT Bombay CE 152 LECTURE

2. Overview Why Structural Health Monitoring? Why Structural Health Monitoring? How Structural Health Monitoring? How Structural Health Monitoring? Investigation for an Example Steel Bridge Investigation for an Example Steel Bridge Outcomes from the Investigation Outcomes from the Investigation

3. Why SHM? Health Assessment for Increased Service Loads Health Assessment for Increased Service Loads Condition Assessment for Aged Structures Condition Assessment for Aged Structures Life Extension Beyond Design Life Life Extension Beyond Design Life Experimental Verification of Design Procedure Experimental Verification of Design Procedure

4. How SHM? Measure Sensitive Structural Responses to Loads Measure Sensitive Structural Responses to Loads Use Mathematical Model of Structure Optimize Information and Sensor Requirements Optimize Information and Sensor Requirements Determine Critical Sensor Locations Determine Critical Sensor Locations Determine Sensor and DAQ Requirements Determine Sensor and DAQ Requirements

5. Example Old Railway Bridge

6. Material Properties UTS (MPa)413 YS (MPa)235 Elongation (%)29 Poisson`s Ratio0.28 E (MPa)2.09x10 5 Element Content (Wt. %) Carbon0.160 Sulphur0.020 Phosphorous0.050 Silicon0.086 Manganese0.620 Chromium0.042 Nickel0.052 Titanium0.020 AluminiumTraces Copper0.092 Iron (Remainder) 98.858

7. Numerical Modelling Fig: 3-D Model of Bridge Span

8. Instrumentation Scheme L' 1 L' 7 L' 2 L' 6 L' 3 L' 5 L' 4 L1L1 U7U7 U6U6 U5U5 U4U4 U3U3 U2U2 U1U1 U7U7 U1U1 L1L1 L7L7 L2L2 L6L6 L3L3 L5L5 L4L4 L7L7 L1L1 Electronic Tilt Sensors Gauges on both bearings Vibrating Wire Strain Gauge (Location to be determined after site visit Gages on L 6 U 7 Gages on L 6 L 5 L1L1 L7L7 L2L2 L6L6 L3L3 L5L5 L4L4 Fixed Gauges on L 7 L 8 L' 7 L' 8 Gages on L 7 U 7 L' 7 U' 7 Free Gauges on Stringers Gauges on Cross Girders

9. Instrumentation Instrumentation mainly includes equipments and accessories for 20-channel strain measurement; 8-channel vibration measurement and; 8-channel LVDT display for deflection measurement 2-channel thermocouple

10. Sensors (Strain Gages, Accelerometers, Thermocouples) Signal Conditioning Data Acquisition Raw Data File Data Processing Data Packaging Data Analysis Raw Data File DATA ACQUISITION AT SITE DATA ANALYSIS OFFSITE Data Acquisition & Analysis

11. Centre Span Deflection Average Max. Deflection in mm at the center of the spans under controlled static loading Outer Girder (Up line) Central Girder Outer Girder (Dn line) Experimental Values 17.219.216.8 Numerical Values 17.6* 17.6* 17.6* * Difference due to problems of site measurement and inability to numerically simulate actual joint conditions. Pinned connections – 19.8 mm

12. Strain Measurement Instrumentation Instrumentation 20-channel System 6000, Vishay, USA 20-channel System 6000, Vishay, USA Uniaxial strain gages, Korean make Uniaxial strain gages, Korean make Triple coated strain gage wires etc. Triple coated strain gage wires etc. Location of Strain Gages..? Location of Strain Gages..? To measure axial strains in critical members To measure axial strains in critical members To measure presence of bending strains To measure presence of bending strains

13. Fig : Goods train (uniform strain)

14. Fig : Passenger up train (higher strain level while engine on span)

15. Fig: Data Processing and Analysis

16. Axial Strains in Critical Members Numerical Values Experimental Values %age difference Location of strain gage -136-138-1.5 Post-support (OG) -136-137-0.7 Post-support (CG) 169180-6.1 Diagonal-support (CG) 170175-2.9 BC-support (OG) 170179-5.0 BC-support( CG) 180188-4.2 BC-center (OG) 180184-2.1 BC-center (CG) 182179+1.6 BC-center (OG) -131-105+25.0 TC-center (OG)

17. Vibration Measurement Instrumentation Instrumentation Six-channel Pulse System, B & K, Netherlands Six-channel Pulse System, B & K, Netherlands Six DeltaTron Accelerometers, B & K make Six DeltaTron Accelerometers, B & K make Miniature cables, dot connectors etc. Miniature cables, dot connectors etc. Location of Accelerometers Location of Accelerometers A1V-At the center of outer girder (Dn line) on bottom chord (Dir- Vertical) A1V-At the center of outer girder (Dn line) on bottom chord (Dir- Vertical) A2H- At the center of outer girder (Dn line) on bottom chord (Dir- Horizontal) A2H- At the center of outer girder (Dn line) on bottom chord (Dir- Horizontal) A3V- At the center of central girder on bottom chord (Dir-Vertical) A3V- At the center of central girder on bottom chord (Dir-Vertical) A4H-At the center of outer girder (Dn line) on top chord (Dir- Horizontal) A4H-At the center of outer girder (Dn line) on top chord (Dir- Horizontal) A5H-Near support of outer girder (Dn line) on bottom chord (Dir- Horizontal) A5H-Near support of outer girder (Dn line) on bottom chord (Dir- Horizontal)

18. Fig: FFT of a typical time history recorded by vertical accelerometer at the center of the span (A1V, A3V) Fig: FFT of a typical time history recorded by horizontal accelerometer near the support of the span (A5H)

19. 1 st mode (plan) lateral vibration 2 nd mode (plan) lateral vibration 3 rd mode (elevation) vertical vibration 4 th mode (plan) torsional vibration

20. Natural Vibration Frequencies Observations: *Structure is weak in lateral direction (as first two mode shapes are in lateral direction) More accelerometers required for mode shape comparison Movement in lateral direction is predominant when train passes over the bridge with a speed of 10-20 kmph (resonance). Experimental Values Numerical Values *2.7*5.86.77.4*2.5*6.06.26.7

21. Fatigue Tests 10 samples at 3 stress levels (R = 0) Stress100 MPa200 MPa300 MPa Min.>10 million3.5 million1.8 million Avg.>10 million4.2 million2.1 million  In log stress terms, very little variation from average values  100 MPa below the endurance limit for steel  Ductile crack propagation

22. Remaining Life Assessment Use Miner’s Rule for estimating remaining life Use Miner’s Rule for estimating remaining life Use rainfall counting procedures to estimate stress histograms Use rainfall counting procedures to estimate stress histograms Maximum dynamic stress (incl. DL) Maximum dynamic stress (incl. DL) Chords150 MPa(5 million cycles) Bracings80 MPa (below endurance limit) Estimate of traffic over last 95 years = 900,000 Estimate of traffic over last 95 years = 900,000 Remaining life at current traffic - 45 years Remaining life at current traffic - 45 years

23. Conclusions Objective of SHM has to be clear Objective of SHM has to be clear Comprehensive procedure for condition and remaining life assessment is illustrated Comprehensive procedure for condition and remaining life assessment is illustrated Metallurgy, physical and fatigue test show the ductile crack propagation phenomenon Metallurgy, physical and fatigue test show the ductile crack propagation phenomenon Experimentally validated numerical model used to determine current condition and estimate remaining life based on current traffic conditions Experimentally validated numerical model used to determine current condition and estimate remaining life based on current traffic conditions