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Structural Health Monitoring of Steel Bridges

Structural Health Monitoring of Steel Bridges

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Structural Health Monitoring of Steel Bridges

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

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

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

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

  5. Example Old Railway Bridge

  6. Material Properties UTS (MPa) 413 YS (MPa) 235 Elongation (%) 29 Poisson`s Ratio 0.28 E (MPa) 2.09x105

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

  8. Instrumentation Scheme L'7 L'6 L'5 L'4 L'3 L'2 L'1 Gauges on Stringers L7 L6 L5 L4 L3 L2 L1 Gages on L7U7 L'7U'7 Gauges on Cross Girders U7 U1 U7 U6 U5 U4 U3 U2 U1 L1 L1 L7 L6 L5 L4 L3 L2 L1 L7 Fixed Gages on L6L5 Free Gages on L6U7 Electronic Tilt Sensors Gauges on both bearings Gauges on L7L8 L'7L'8 Vibrating Wire Strain Gauge (Location to be determined after site visit

  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) Raw Data File Signal Conditioning Data Processing Data Acquisition Data Packaging Raw Data File Data Analysis DATA ACQUISITION AT SITE DATA ANALYSIS OFFSITE Data Acquisition & Analysis

  11. Centre Span Deflection *Difference due to problems of site measurement and inability to numerically simulate actual joint conditions. Pinned connections – 19.8 mm

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

  17. Vibration Measurement • Instrumentation • Six-channel Pulse System, B & K, Netherlands • Six DeltaTron Accelerometers, B & K make • Miniature cables, dot connectors etc. • Location of Accelerometers • 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) • 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) • 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. 1st mode (plan) lateral vibration 2nd mode (plan) lateral vibration 3rd mode (elevation) vertical vibration 4th mode (plan) torsional vibration

  20. 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). Natural Vibration Frequencies

  21. Fatigue Tests 10 samples at 3 stress levels (R = 0) Stress 100 MPa 200 MPa 300 MPa Min. >10 million 3.5 million 1.8 million Avg. >10 million 4.2 million 2.1 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 rainfall counting procedures to estimate stress histograms • Maximum dynamic stress (incl. DL) Chords 150 MPa (5 million cycles) Bracings 80 MPa (below endurance limit) • Estimate of traffic over last 95 years = 900,000 • Remaining life at current traffic - 45 years

  23. Conclusions • Objective of SHM has to be clear • Comprehensive procedure for condition and remaining life assessment is illustrated • 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