Structural Health Structural Health Monitoring ofMonitoring of Steel Steel

BridgesBridgesPradipta BanerjiPradipta BanerjiProfessor of Civil Professor of Civil

Engineering, IIT BombayEngineering, IIT Bombay

CE 152 LECTURECE 152 LECTURE

OverviewOverview

Why Structural Health Monitoring?Why Structural Health Monitoring?

How Structural Health Monitoring?How Structural Health Monitoring?

Investigation for an Example Steel Investigation for an Example Steel

BridgeBridge

Outcomes from the InvestigationOutcomes from the Investigation

Why SHM?Why SHM?

Health Assessment for Increased Service Health Assessment for Increased Service

LoadsLoads

Condition Assessment for Aged StructuresCondition Assessment for Aged Structures

Life Extension Beyond Design LifeLife Extension Beyond Design Life

Experimental Verification of Design Experimental Verification of Design

ProcedureProcedure

How SHM?How SHM?

Measure Sensitive Structural Responses to Measure Sensitive Structural Responses to

LoadsLoads

Use Mathematical Model of StructureUse Mathematical Model of Structure

Optimize Information and Sensor Optimize Information and Sensor

RequirementsRequirements

Determine Critical Sensor LocationsDetermine Critical Sensor Locations

Determine Sensor and DAQ RequirementsDetermine Sensor and DAQ Requirements

Example Old Railway Bridge

Material PropertiesMaterial Properties

UTS (MPa)UTS (MPa) 413413

YS (MPa)YS (MPa) 235235

Elongation (%)Elongation (%) 2929

Poisson`s RatioPoisson`s Ratio 0.280.28

E (MPa)E (MPa)2.09x102.09x1055

ElementElement Content (Wt. Content (Wt. %)%)

CarbonCarbon 0.1600.160

SulphurSulphur 0.0200.020

PhosphorousPhosphorous 0.0500.050

SiliconSilicon 0.0860.086

ManganeseManganese 0.6200.620

ChromiumChromium 0.0420.042

NickelNickel 0.0520.052

TitaniumTitanium 0.0200.020

AluminiumAluminium TracesTraces

CopperCopper 0.0920.092

Iron Iron (Remainder)(Remainder)

98.85898.858

Numerical ModellingNumerical Modelling

Fig: 3-D Model of Bridge Span

Instrumentation SchemeInstrumentation SchemeL'1L'7 L'2L'6 L'3L'5 L'4

L1

U7 U6 U5 U4 U3 U2 U1

U7U1

L1L7 L2L6 L3L5 L4 L7L1

Electronic Tilt Sensors Gauges on

both bearings

Vibrating Wire Strain Gauge(Location to be determined after site visit

Gages on L6U7

Gages on L6L5

L1L7L2L6 L3L5 L4

Fixed

Gauges on L7L8

L'7L'8

Gages on L7U7

L'7U'7

Free

Gauges on Stringers

Gauges on Cross Girders

InstrumentationInstrumentation

Instrumentation mainly Instrumentation mainly includes equipments includes equipments and accessories for and accessories for 20-channel strain 20-channel strain

measurement;measurement; 8-channel vibration 8-channel vibration

measurement and; measurement and; 8-channel LVDT display 8-channel LVDT display

for deflection for deflection measurementmeasurement

2-channel thermocouple2-channel thermocouple

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 & Data Acquisition & AnalysisAnalysis

Centre Span DeflectionCentre Span Deflection

Average Max. Deflection in mm at the Average Max. Deflection in mm at the center of the spans under controlled static center of the spans under controlled static

loadingloadingOuter Outer Girder (Up Girder (Up line)line)

Central Central GirderGirder

Outer Outer Girder (Dn Girder (Dn line)line)

ExperimentExperimental Valuesal Values

17.217.2 19.219.2 16.816.8

Numerical Numerical ValuesValues

17.6*17.6* 17.617.6** 17.6*17.6*

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

Strain MeasurementStrain Measurement InstrumentationInstrumentation

20-channel System 6000, Vishay, USA20-channel System 6000, Vishay, USA Uniaxial strain gages, Korean makeUniaxial 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 membersTo measure axial strains in critical members To measure presence of bending strainsTo measure presence of bending strains

Fig : Goods train (uniform strain)

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

Fig: Data Processing and Analysis

Axial Strains in Critical Axial Strains in Critical MembersMembers

NumericNumerical Valuesal Values

ExperimenExperimental Valuestal Values

%age %age differendifferen

cece

Location of strain Location of strain gagegage

-136-136 -138-138 -1.5-1.5 Post-support (OG)Post-support (OG)

-136-136 -137-137 -0.7-0.7 Post-support (CG)Post-support (CG)

169169 180180 -6.1-6.1 Diagonal-support Diagonal-support (CG)(CG)

170170 175175 -2.9-2.9 BC-support (OG)BC-support (OG)

170170 179179 -5.0-5.0 BC-support( CG)BC-support( CG)

180180 188188 -4.2-4.2 BC-center (OG)BC-center (OG)

180180 184184 -2.1-2.1 BC-center (CG)BC-center (CG)

182182 179179 +1.6+1.6 BC-center (OG)BC-center (OG)

-131-131 -105-105 +25.0+25.0 TC-center (OG)TC-center (OG)

Vibration MeasurementVibration Measurement InstrumentationInstrumentation

Six-channel Pulse System, B & K, NetherlandsSix-channel Pulse System, B & K, Netherlands Six DeltaTron Accelerometers, B & K makeSix DeltaTron Accelerometers, B & K make Miniature cables, dot connectors etc.Miniature cables, dot connectors etc.

Location of AccelerometersLocation of Accelerometers A1V-At the center of outer girder (Dn line) on bottom chord A1V-At the center of outer girder (Dn line) on bottom chord

(Dir-Vertical)(Dir-Vertical) A2H- At the center of outer girder (Dn line) on bottom chord A2H- At the center of outer girder (Dn line) on bottom chord

(Dir-Horizontal)(Dir-Horizontal) A3V- At the center of central girder on bottom chord (Dir-A3V- At the center of central girder on bottom chord (Dir-

Vertical)Vertical) A4H-At the center of outer girder (Dn line) on top chord (Dir-A4H-At the center of outer girder (Dn line) on top chord (Dir-

Horizontal)Horizontal) A5H-Near support of outer girder (Dn line) on bottom chord A5H-Near support of outer girder (Dn line) on bottom chord

(Dir-Horizontal)(Dir-Horizontal)

Autospectrum(Signal 1) - InputWorking : Input : Input : FFT Analyzer

0 4 8 12 16 20 24 28 32

0

0.2

0.4

0.6

[Hz]

[m/s²]OVERLOAD

Autospectrum(Signal 1) - InputWorking : Input : Input : FFT Analyzer

0 4 8 12 16 20 24 28 32

0

0.2

0.4

0.6

[Hz]

[m/s²]Fig: FFT of a typical time history recorded by vertical accelerometer at the center of the span (A1V, A3V)

Autospectrum(Signal 5) - InputWorking : Input : Input : FFT Analyzer

4 8 12 16 20 24 28 32

0

0.2

0.4

0.6

0.8

[Hz]

[m/s²]OVERLOAD

Autospectrum(Signal 5) - InputWorking : Input : Input : FFT Analyzer

4 8 12 16 20 24 28 32

0

0.2

0.4

0.6

0.8

[Hz]

[m/s²]Fig: FFT of a typical time history recorded by horizontal accelerometer near the support of the span (A5H)

1st mode (plan)

lateral vibration2nd mode (plan)lateral vibration

3rd mode (elevation)vertical vibration

4th mode (plan)torsional vibration

Natural Vibration FrequenciesNatural Vibration Frequencies

Observations:Observations: *Structure is weak in lateral *Structure is weak in lateral

direction (as first two mode direction (as first two mode shapes are in lateral direction)shapes are in lateral direction)

More accelerometers required More accelerometers required for mode shape comparisonfor mode shape comparison

Movement in lateral direction Movement in lateral direction is predominant when train is predominant when train passes over the bridge with a passes over the bridge with a speed of 10-20 kmph speed of 10-20 kmph (resonance). (resonance).

ExperimenExperimental Valuestal Values

Numerical Numerical Values Values

*2.7*2.7

*5.8*5.8

6.76.7

7.47.4

*2.5*2.5

*6.0*6.0

6.26.2

6.76.7

Fatigue TestsFatigue Tests

10 samples at 3 stress levels (R = 0)10 samples at 3 stress levels (R = 0)

StressStress 100 MPa100 MPa 200 MPa200 MPa 300 300

MPaMPa

Min.Min. >10 million>10 million3.5 million3.5 million 1.8 million1.8 million

Avg.Avg. >10 million>10 million4.2 million4.2 million 2.1 million2.1 million

In log stress terms, very little variation In log stress terms, very little variation

from average valuesfrom average values

100 MPa below the endurance limit for 100 MPa below the endurance limit for

steelsteel

Ductile crack propagationDuctile crack propagation

Remaining Life Remaining Life AssessmentAssessment

Use Miner’s Rule for estimating remaining lifeUse Miner’s Rule for estimating remaining life

Use rainfall counting procedures to estimate Use rainfall counting procedures to estimate

stress histogramsstress histograms

Maximum dynamic stress (incl. DL)Maximum dynamic stress (incl. DL)

ChordsChords 150 MPa150 MPa (5 million cycles)(5 million cycles)

BracingsBracings 80 MPa (below endurance limit)80 MPa (below endurance limit)

Estimate of traffic over last 95 years = 900,000Estimate of traffic over last 95 years = 900,000

Remaining life at current traffic - 45 yearsRemaining life at current traffic - 45 years

ConclusionsConclusions Objective of SHM has to be clearObjective of SHM has to be clear Comprehensive procedure for condition Comprehensive procedure for condition

and remaining life assessment is and remaining life assessment is illustratedillustrated

Metallurgy, physical and fatigue test Metallurgy, physical and fatigue test show the ductile crack propagation show the ductile crack propagation phenomenonphenomenon

Experimentally validated numerical Experimentally validated numerical model used to determine current model used to determine current condition and estimate remaining life condition and estimate remaining life based on current traffic conditionsbased on current traffic conditions