AMBIENT VIBRATION MONITORING Ambient Vibration Monitoring . H e lmut Wenzel and Dieter Pichler Copyright  2005 John Wiley & Sons, Inc. ISBN: 0-470-02430-5 (HB) AMBIENT VIBRATION MONITORING Helmut Wenzel VCE Holding GmbH, Vienna, Austria Dieter Pichler VCE Holding GmbH, Vienna, Austria Copyright(cid:1)2005 JohnWiley&SonsLtd,TheAtrium,SouthernGate,Chichester, WestSussexPO198SQ,England Telephone (þ44)1243779777 Email(forordersandcustomerserviceenquiries):[email protected] VisitourHomePageonwww.wiley.com AllRightsReserved.Nopartofthispublicationmaybereproduced,storedinaretrievalsystem ortransmittedinanyformorbyanymeans,electronic,mechanical,photocopying,recording, scanningorotherwise,exceptunderthetermsoftheCopyright,DesignsandPatentsAct1988 orunderthetermsofalicenceissuedbytheCopyrightLicensingAgencyLtd,90Tottenham CourtRoad,LondonW1T4LP,UK,withoutthepermissioninwritingofthePublisher. 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Contents PREFACE xi ACKNOWLEDGEMENTS xiii SUMMARY xv 1 INTRODUCTION 1 1.1 Scope ofApplications 1 1.2 Laws and Regulations 2 1.3 Theories on theDevelopment ofthe AVM 4 2 OBJECTIVESOF APPLICATIONS 7 2.1 System Identification 7 2.1.1 Eigenfrequenciesand Mode Shapes 8 2.1.2 Damping 11 2.1.3 DeformationsandDisplacements 11 2.1.4 Vibration Intensity 12 2.1.5 Trend Cards 13 2.2 Stress Test 13 2.2.1 Determination of Static and Dynamic Stresses 14 2.2.2 Determination of theVibration Elements 14 2.2.3 Stress ofIndividual Structural Members 15 2.2.4 Determination of Forces inTendons andCables 15 vi Contents 2.3 Assessment ofStresses 17 2.3.1 Structural Safety 17 2.3.2 Structural Member Safety 19 2.3.3 Maintenance Requirements and Intervals 19 2.3.4 Remaining Operational Lifetime 21 2.4 Load Observation (Determination of External Influences) 21 2.4.1 Load Collective 21 2.4.2 Stress Characteristic 21 2.4.3 Verification ofLoad Models 23 2.4.4 Determination ofEnvironmentalInfluences 24 2.4.5 Determination ofSpecific Measures 24 2.4.6 Check on theSuccess of Rehabilitation Measures 25 2.4.7 DynamicEffectson Cables and Tendons 25 2.4.8 Parametric Excitation 27 2.5 Monitoring ofthe Condition of Structures 28 2.5.1 Assessmentof Individual Objects 29 2.5.2 Periodic Monitoring 31 2.5.3 BRIMOS(cid:2)Recorder 31 2.5.4 Permanent Monitoring 34 2.5.5 SubsequentMeasures 35 2.6 Application of Ambient Vibration Testing to Structuresfor Railways 35 2.6.1 Sleepers 36 2.6.2 Noise and Vibration Problems 39 2.7 Limitations 49 2.7.1 Limits ofMeasuring Technology 49 2.7.2 Limits ofApplication 51 2.7.3 Limits ofAnalysis 52 2.7.4 Perspectives 53 References 54 3 FEEDBACKFROM MONITORINGTO BRIDGE DESIGN 55 3.1 EconomicBackground 55 3.2 Lessons Learned 56 3.2.1 Conservative Design 56 3.2.2 External versus InternalPre-stressing 57 3.2.3 Influenceof Temperature 57 3.2.4 Displacement 61 3.2.5 Large Bridges versus Small Bridges 64 3.2.6 Vibration Intensities 66 Contents vii 3.2.7 DampingValuesof New Composite Bridges 68 3.2.8 Value ofPatterns 68 3.2.9 Understanding of Behaviour 72 3.2.10 Dynamic Factors 72 References 75 4 PRACTICAL MEASURING METHODS 77 4.1 Execution ofMeasuring 78 4.1.1 Test Planning 83 4.1.2 Levelling of theSensors 83 4.1.3 Measuring theStructure 84 4.2 Dynamic Analysis 84 4.2.1 CalculationModels 84 4.2.2 State of theArt 88 4.3 MeasuringSystem 89 4.3.1 BRIMOS(cid:2) 89 4.3.2 Sensors 90 4.3.3 Data-Logger 91 4.3.4 Additional Measuring Devices and Methods 92 4.4 EnvironmentalInfluence 93 4.5 Calibration and Reliability 96 4.6 Remaining Operational Lifetime 97 4.6.1 Rainflow Algorithm 98 4.6.2 Calculationof Stressesby FEM 101 4.6.3 S–NApproach and DamageAccumulation 104 4.6.4 Remaining ServiceLifetime by Meansof Existing Traffic Data and Additional Forward and Backward Extrapolation 105 4.6.5 Conclusions and FutureWork 106 References 109 5 PRACTICAL EVALUATION METHODS 111 5.1 Plausibility of Raw Data 111 5.2 AVM Analysis 112 5.2.1 Recording 112 5.2.2 Data Reduction 114 5.2.3 Data Selection 115 5.2.4 FrequencyAnalysis, ANPSD (Averaged Normalized Power Spectral Density) 115 5.2.5 Mode Shapes 120 5.2.6 Damping 121 5.2.7 Deformations 123 viii Contents 5.2.8 Vibration Coefficients 125 5.2.9 Countingof Events 126 5.3 StochasticSubspace Identification Method 129 5.3.1 The StochasticSubspace Identification (SSI) Method 129 5.3.2 Application toBridgeZ24 130 5.4 Use of ModalData in Structural HealthMonitoring 134 5.4.1 Finite Element Model Updating Method 134 5.4.2 Application toBridgeZ24 141 5.4.3 Conclusions 147 5.5 External TendonsandStay Cables 149 5.5.1 GeneralInformation 149 5.5.2 TheoreticalBases 150 5.5.3 Practical Implementation 150 5.5.4 State of theArt 151 5.5.5 Rain–WindInduced Vibrations ofStay Cables 152 5.5.6 Assessment 152 5.6 DamageIdentification and Localization 153 5.6.1 Motivation for SHM 154 5.6.2 Current Practice 155 5.6.3 Condition and Damage Indices 157 5.6.4 Basic Philosophy ofSHM 159 5.7 DamagePrognosis 161 5.7.1 Sensing Developments 162 5.7.2 Data Interrogation Procedure forDamage Prognosis 162 5.7.3 PredictiveModelling ofDamage Evolution 163 5.8 Animation and the Modal AssuranceCriterion (MAC) 164 5.8.1 Representation of theCalculated ModeShapes 164 5.8.2 GeneralRequirements 164 5.8.3 Correlationof Measurement and Calculation(MAC) 164 5.8.4 Varying Number ofEigenvectors 165 5.8.5 ComplexEigenvectorMeasurement 165 5.8.6 Selection of SuitableCheck Pointsusing the MAC 166 5.9 AmbientVibration Derivatives(AVD(cid:2)) 168 5.9.1 Aerodynamic Derivatives 168 5.9.2 Applications ofthe AVM 168 5.9.3 Practical Implementation 169 References 170 6 THEORETICAL BASES 173 6.1 GeneralSurveyon theDynamic CalculationMethod 174 6.2 Short Description ofAnalyticalModalAnalysis 176 Contents ix 6.3 Equation ofMotion of Linear Structures 178 6.3.1 SDOF System 178 6.3.2 MDOF System 179 6.3.3 Influence of Damping 181 6.4 Dynamic Calculation Method forthe AVM 181 6.5 Practical Evaluationof Measurements 181 6.5.1 Eigenfrequencies 181 6.5.2 Mode Shapes 183 6.5.3 Damping 185 6.6 Theory on Cable ForceDetermination 185 6.6.1 Frequencies ofCablesas a Function of the Inherent Tensile Force 185 6.6.2 Influence of theBending Stiffness 190 6.6.3 Influence of theSupport Conditions 192 6.6.4 Comparison ofthe DefinedCaseswith Experimental Results 193 6.6.5 MeasurementData Adjustment for Exact Cable Force Determination 197 6.7 Transfer FunctionsAnalysis 199 6.7.1 MathematicalBackgrounds 199 6.7.2 Transfer Functionsin theVibration Analysis 205 6.7.3 Applications(Examples) 214 6.8 Stochastic SubspaceIdentification 222 6.8.1 Stochastic State-Space Models 223 6.8.2 Stochastic System Identification 226 References 232 7 OUTLOOK 235 7.1 Decision SupportSystems 236 7.2 Sensor Technologyand SensorNetworks 236 7.2.1 State-of-the-Art Sensor Technology 236 7.3 Research Gaps and Opportunities 237 7.4 International Collaboration 239 7.4.1 Collaboration Framework 239 7.4.2 Activities 243 8 EXAMPLES FOR APPLICATION 245 8.1 Aitertal Bridge, Post-tensionalT-beam(1956) 245 8.2 Donaustadt Bridge, Cable-Stayed Bridge inSteel (1996) 248 8.3 F9 Viaduct Donnergraben, Continuous Box Girder (1979) 250 8.4 Europa Bridge, ContinuousSteel Box Girder(1961) 252 x Contents 8.5 Gasthofalm Bridge, CompositeBridge(1979) 256 8.6 Kao Ping Hsi Bridge, Cable-Stayed Bridge (2000) 258 8.7 Inn Bridge Roppen, ConcreteBridge (1936) 260 8.8 Slope Bridge Saag, Bridge Rehabilitation (1998) 263 8.9 Flyover StMarx, PermanentMonitoring 265 8.10 Mur Bridge inSt Michael,BridgeRehabilitation 270 8.11 Rosen Bridge inTulln, ConcreteCable-Stayed Bridge (1995) 272 8.12 VOESTBridge, Steel Cable-Stayed Bridge (1966) 275 8.13 TaichungBridge, Cable-Stayed Bridge 279 APPENDIX 283 Nomenclature 283 INDEX 289 Preface The development of methods for the monitoring and assessment of structures was driven by the demand for better as well as cheaper methods. Constraints like undisturbed traffic flow, limited access possibilities and finally limited or evenshrinkingbudgetshaveledtoadevelopmenttoanassessmentmodelwell fittedtotheconstructionsector.Ambientvibrationmeansthattheinputisnot fullyknown,leaving,asalways,amarginofuncertainty.Ifweacceptthatthese new technologies will in certain cases not provide exact or acceptable answers our expectations on the outcome will be fulfilled. It also has to be considered that this early development has explored only a minor portion of its entire technical range, leaving much still to do. It is most likely that the one or the other of the current approaches will be overruled by future research work. Nevertheless the conception proposed for data acquisition has been made in order to support any new technological development in the next 50 years. Even if the methodology changes, the old data will still be usable for the new assessment routines. These facts imply that further research work should be put into this subject. The sheer number of researchers working in this fieldmakesusoptimisticthatin10yearstimeacompleterevisionofthisbook will be due. Another aspect which has arisen during our work on this subject, which we would like to share with the reader, is that the best results have also been achieved in combination with engineering judgement. The variety of approachesandassumptionswhichcanbemadecreatethepotentialforawide range of misapplications, which should be avoided and might create disap- pointment for the users. The rules defined in this book are well suited to standard cases, but the engineer who applies them also has to be aware that there arelimitations. It is easy to be caught by the fascinating opportunities ambient vibration monitoringprovides.Atthesametimeitisveryoftenforgottenthattheeffort put in also limits the potential. The dilemma that perfect results are too