Structural Health Monitoring of Steel Bridges

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Structural Health Monitoring of Steel Bridges

• Pradipta BanerjiProfessor of Civil Engineering,
  IIT Bombay

CE 152 LECTURE
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Overview

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

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Why SHM?

• Health Assessment for Increased Service Loads
• Condition Assessment for Aged Structures
• Life Extension Beyond Design Life
• Experimental Verification of Design Procedure

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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

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Example Old Railway Bridge
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Material Properties
Element Content (Wt. )
Carbon 0.160
Sulphur 0.020
Phosphorous 0.050
Silicon 0.086
Manganese 0.620
Chromium 0.042
Nickel 0.052
Titanium 0.020
Aluminium Traces
Copper 0.092
Iron (Remainder) 98.858

• UTS (MPa) 413
• YS (MPa) 235
• Elongation () 29
• Poissons Ratio 0.28
• E (MPa) 2.09x105

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Numerical Modelling
Fig 3-D Model of Bridge Span
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Instrumentation Scheme
L'1
L'7
L'2
L'6
L'3
L'5
L'4
Gauges on Stringers
L1
L7
L2
L6
L3
L5
L4
Gages on L7U7 L'7U'7
Gauges on Cross Girders
U7
U7
U6
U5
U4
U3
U2
U1
U1
L1
L1
L7
L2
L6
L3
L5
L4
L7
L1
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
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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

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Data Acquisition Analysis
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Centre Span Deflection
Average Max. Deflection in mm at the center of the spans under controlled static loading Average Max. Deflection in mm at the center of the spans under controlled static loading Average Max. Deflection in mm at the center of the spans under controlled static loading 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 17.2 19.2 16.8
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
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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

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Fig Goods train (uniform strain)
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Fig Passenger up train (higher strain level
while engine on span)
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Fig Data Processing and Analysis
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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)
169 180 -6.1 Diagonal-support (CG)
170 175 -2.9 BC-support (OG)
170 179 -5.0 BC-support( CG)
180 188 -4.2 BC-center (OG)
180 184 -2.1 BC-center (CG)
182 179 1.6 BC-center (OG)
-131 -105 25.0 TC-center (OG)
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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)

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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)
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1st mode (plan) lateral vibration
2nd mode (plan) lateral vibration
3rd mode (elevation) vertical vibration
4th mode (plan) torsional vibration
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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 5.8 6.7 7.4 2.5 6.0 6.2 6.7
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Fatigue Tests

• 10 samples at 3 stress levels (R 0)
• Stress 100 MPa 200 MPa 300 MPa
• Min. gt10 million 3.5 million 1.8 million
• Avg. gt10 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

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Remaining Life Assessment

• Use Miners 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

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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