Ambient Vibration Monitoring / Edition 1 available in Hardcover
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In-operation vibration monitoring for complex mechanical structures and rotating machines is of key importance in many industrial areas such as aeronautics (wings and other structures subject to strength), automobile (gearbox mounting with a sports car body), rail transportation, power engineering (rotating machines, core and pipes of nuclear power plants), and civil engineering (large buildings subject to hurricanes or earthquakes, bridges, dams, offshore structures). Tools for the detection and the diagnosis of small changes in vibratory characteristics are particularly useful to set up a preventive maintenance policy based on the actual evolution of the state of the monitored machine or structure, as opposed to systematic a priori planning.
Ambient Vibration Monitoring is the backbone of such structural assessment monitoring and control. It provides the possibility to gain useful data under ambient conditions for the assessment of structures and components.
Written by a widely respected authority in this area, Ambient Vibration Monitoring describes the current practice of ambient vibration methodologies illustrated by a number of practical examples. Designed to aid the practical engineer with their understanding of the topic, it is the culmination of many years of practical research and includes numerous ‘real world’ examples. It also provides information on applicable solutions.
This book will enable not only practitioners (in civil, mechanical and aerospace engineering), but also researchers and students, to learn more about the theory and practical applications of this subject.
|Product dimensions:||6.24(w) x 9.51(h) x 0.88(d)|
About the Author
Dr Helmut Wenzel, Managing Director
Dieter Pichler, both of VCE Holding GmbH, Vienna, Austria
Read an Excerpt
Ambient Vibration Monitoring
By Helmut Wenzel
John Wiley & SonsCopyright © 2005 John Wiley & Sons, Ltd
All right reserved.
The increasing number of civil engineering structures in the field of traffic infrastructure entails the increasing significance of maintenance problems of such engineering structures. Bridges are monitored by periodic supervision measures with the aim of minimizing the safety risk for the user, on the one hand, and of keeping the costs for the maintenance as low as possible by the execution of rehabilitation measures at the right time, on the other hand.
Civil engineers concerned with supervision of structures for safety and maintenance reasons are aware of the limitations of their current practice of condition assessment based on visual inspections. Typical routine applications of condition assessment are carried out on structures applying rating systems. The consequence would be unbearable costs on society for replacement and retrofit tightened up by shrinking budgets. The expressed intention of the bridge owners globally is to reduce the number of bridges rated deficient by 30% within a short time through the application of sophisticated methods based on measurements.
1.1 SCOPE OF APPLICATIONS
Owing to the quickly increasing traffic density, in particular in the field of sophisticated road networks, restrictions of unhindered traffic flow because of inspection works entail high economic costs. Therefore such works must be limited to the absolutely necessary minimum. The ambient vibration method (AVM) was developed under the premise that it can be used practically without any impairment of the traffic flow. The aim is to provide a system that makes it possible to reduce the employment of inspection devices to a minimum by wellaimed specification of problem zones, therefore maintaining the traffic flow in as undisturbed a way as possible during inspection works. The procedure can be applied independently of the type and construction of the structure and materials used.
The currently used procedures for bridge monitoring are dominated by manual methods. Visual assessments have a central significance. Devices such as binoculars, magnifying glass for measuring, levelling instruments, endoscopes, etc., are used but the results are dependent on the subjective recognition of damage by the personnel carrying out the examination. It is tried to objectify the recognition by check lists, comparative patterns, etc. Additional methods such as concrete cover measurements and chemically and technical tests help to document an objective image of the maintenance condition.
The AVM has an engineering character comparable with the main manual inspection methods currently used. By measuring the vibration behaviour 'actual' values are obtained, which - with certain restrictions - are not subject to the circumstances of the personnel carrying out the test. The objective condition of the tested structure is determined by a systematic analytical evaluation.
Of course the methods used formerly do not become obsolete but rather provide an additional assessment procedure that improves the application of the method. The examples presented in this report show how AVM can be usefully applied for the support of traditional methods of bridge monitoring. Apart from the requirements for bridge and structure monitoring AVM offers an abundance of further areas of application, which owing to its flexibility are also explained by means of practical examples.
1.2 LAWS AND REGULATIONS
In Austria, according to the relevant legal regulations (federal road law, provincial road administrative laws, building regulations for Vienna), the road administration is responsible for road building and road maintenance. The competent road administration is therefore also responsible for the safety of the engineering buildings in the course of its road network. This results in the following competencies for the Austrian road network:
federal road network: the provincial governors by way of the indirect federal administration, special companies (OSAG, ASAG);
provincial road network: the provincial governments, the district administration authorities, the mayors.
With regard to road construction and road maintenance no special legal regulations exist. Roads and therefore road bridges have to be built and maintained according to the respective state of the art.
In Austria bridge monitoring in the field of the federal road network takes place on the basis of RVS 13.71, Strassenerhaltung - Uberwachung, Kontrolle und Prufung - Strassenbrucken (road maintenance - monitoring, inspection and check - road bridges) at periodic intervals . Here four types of monitoring are distinguished:
Constant monitoring includes the determination of damages that are discernible from outside and is carried out by inspection rides at least every four months. Inspections take place at least every two years with an increased examination depth for the substructure and superstructure by means of a checklist compared with constant monitoring, but usually no special devices are used for it.
The checks (also bridge checks or main checks) are, however, usually carried out every six years. For these checks bridge inspection devices are often used, which frequently require a partial road block. Special tests are carried out if a survey of the actual condition is required due to special events (e.g. after an accident). The scope of such tests corresponds to that of a regular main check; traffic restrictions therefore occur in a similar scope.
In Germany the decisive standard in this field is the DIN 1076, which is valid both for railway and for road bridges. This standard was globally introduced for road construction in all German Federal States in November 1999. For the German railways the D804 with various modules is valid for bridges and other civil engineering structures. They are, however, still being revised at the time of preparation of this publication. In particular, the D80480 is decisive for inspections and/or examinations but must be adapted to the new module system.
In France the decisive standards were published by the Division of Road Management in the Transport Ministry. They are generally known under the title Technical Instructions for the Supervision and Maintenance of Engineering Structures. What has to be pointed out is in particular fascicle 31 in part II (1990): Reinforced and Unreinforced Concrete Bridges, as well as fascicle 32 (1986): Prestressed Concrete Bridges. Fascicle 34 (1986) refers to Steel Bridges.
Furthermore, a regulation on the 'Classification of Structures' from 1996 as well as numerous publications and leaflets by LCPC (Laboratoire Central des Ponts et Chaussees), a governmental research institute dealing with building and infrastructure as well as town planning and environmental technology, are decisive. The following titles were analogously translated from French:
Handbook for the Identification and Interpretation of Reactions and Signs of Concrete Damages (Fatigue) in Engineering Structures, LCPC (1998);
Examination of Cable Forces by Using Vibration Measurements, LCPC (1993).
In Italy the decisive standards were published by the Ministry for Public Works (Ministero Lavori Pubblici) in 1971 and 1974. In particular the technical standards for the planning, execution and final approval of road bridges (Norme tecniche per la progettazione, la esecuzione e il collaudo dei ponti stradali) need to be mentioned, which were revised and complemented in 1991.
In Great Britain the decisive standards are basically divided into four groups: inspection, maintenance, repair and strengthening, and assessment of roads and bridges. In particular the codes of practice in section one (inspection), including inspection of highway structures and of post-tensioned concrete bridges, as well as inspection and records for road tunnels, and in section four (assessment), assessment of steel, concrete and composite bridges, are interesting.
In this connection the following leaflets as well as a German standard draft are interesting as important aids:
Automatisierte Daueruberwachung im Ingenieurbau, Merkblatt des DGZfp-Ausschuss fur zerstorungsfreie Prufung im Bauwesen (AB), dated 12 August 1997;
Zustandsanalyse mittels modaler Analyse (ZMA) Merkblatt des DGZfp-Ausschuss fur zerstorungsfreie Prufung im Bauwesen (AB), dated March 1998;
ISO/CD 14963, Mechanical Vibration and Shock - Guidelines for Dynamic Test and Investigation on Bridges and Viaducts, from DIN-Normen Ausschuss Bauwesen, dated 12 October 1999.
This standard is to form the regulation on which dynamic bridge monitoring will be based in the future.
Standardization is to be established at a uniform level all over Europe by means of the European network project SAMCO (structural assessment monitoring and control, project number CTG2-2000-33069 of the DG Research of the European Union). This is to be the prerequisite for a quick dissemination of dynamic methods. The progress of the project, which was started in 2001, can be followed under samco.org on the Internet.
1.3 THEORIES ON THE DEVELOPMENT OF THE AVM
The AVM is a practice-oriented procedure that closes the gap between basic research and application-oriented development by means of statistical methods and approximation methods. Therefore the following assumptions are made:
The condition of a structure represents itself in the response spectrum.
The linear static and linear dynamic approaches are not sufficient for describing correctly the dynamic characteristic mathematically.
The measuring technique has been sufficiently developed.
Data acquisition and evaluation via computers is efficiently feasible.
The existing software is appropriate for meeting the requirements.
From the preceding assessment of the status quo the following results can be derived:
The measuring technique is widely advanced and the data will be lasting.
The theoretical evaluation can still be developed further.
The aim of this work is to close this gap by an empirical solution that will last until computer mechanics is in a position to supply sufficiently reliable results. The following theory, which is represented in a simplified way here, arises from this fact:
The gap between measuring technology and calculation can be closed by simple statistic, empiric procedures based on probability.
The basic idea is to register the dynamic characteristic by means of the highly sensitive acceleration sensors and to measure absolute deformation values by means of a laser in parallel and simultaneously. The correlation of the two signals calibrates the measurement. The error arising from this can be estimated, but does not have any influence on the quality of the interpretation. This method can be replaced by more accurate interpretation procedures as soon as the required non-linear calculation methods are ready for application. The measurements are, however, so precise that they can offer reference data with a high qualitative value for every future evaluation method.
It is therefore justified to begin with periodic measurements of structures now, even if the tools for a more detailed analytical solution are not yet available. The empirical procedure already yields excellent results.
A classification of structures with a similar design is possible and can be used as the basis for a sequence of priorities. In Figure 1.1 the measuring results of 35 small pre-stressed concrete bridges, which were built between 1955 and 1965, are shown. The condition of the structures is reflected in the measuring values and therefore clearly shows the need for action.
Excerpted from Ambient Vibration Monitoring by Helmut Wenzel Copyright © 2005 by John Wiley & Sons, Ltd. Excerpted by permission.
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Table of Contents
1 INTRODUCTION 1
1.1 Scope of Applications 1
1.2 Laws and Regulations 2
1.3 Theories on the Development of the AVM 4
2 OBJECTIVES OF APPLICATIONS 7
2.1 System Identification 7
2.1.1 Eigenfrequencies and Mode Shapes 8
2.1.2 Damping 11
2.1.3 Deformations and Displacements 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 the Vibration Elements 14
2.2.3 Stress of Individual Structural Members 15
2.2.4 Determination of Forces in Tendons and Cables 15
2.3 Assessment of Stresses 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 of Load Models 23
2.4.4 Determination of Environmental Influences 24
2.4.5 Determination of Specific Measures 24
2.4.6 Check on the Success of Rehabilitation Measures 25
2.4.7 Dynamic Effects on Cables and Tendons 25
2.4.8 Parametric Excitation 27
2.5 Monitoring of the Condition of Structures 28
2.5.1 Assessment of Individual Objects 29
2.5.2 Periodic Monitoring 31
2.5.3 BRIMOS_ Recorder 31
2.5.4 Permanent Monitoring 34
2.5.5 Subsequent Measures 35
2.6 Application of Ambient Vibration Testing to Structures for Railways 35
2.6.1 Sleepers 36
2.6.2 Noise and Vibration Problems 39
2.7 Limitations 49
2.7.1 Limits of Measuring Technology 49
2.7.2 Limits of Application 51
2.7.3 Limits of Analysis 52
2.7.4 Perspectives 53
3 FEEDBACK FROM MONITORING TO BRIDGE DESIGN 55
3.1 Economic Background 55
3.2 Lessons Learned 56
3.2.1 Conservative Design 56
3.2.2 External versus Internal Pre-stressing 57
3.2.3 Influence of Temperature 57
3.2.4 Displacement 61
3.2.5 Large Bridges versus Small Bridges 64
3.2.6 Vibration Intensities 66
3.2.7 Damping Values of New Composite Bridges 68
3.2.8 Value of Patterns 68
3.2.9 Understanding of Behaviour 72
3.2.10 Dynamic Factors 72
4 PRACTICAL MEASURING METHODS 77
4.1 Execution of Measuring 78
4.1.1 Test Planning 83
4.1.2 Levelling of the Sensors 83
4.1.3 Measuring the Structure 84
4.2 Dynamic Analysis 84
4.2.1 Calculation Models 84
4.2.2 State of the Art 88
4.3 Measuring System 89
4.3.1 BRIMOS_ 89
4.3.2 Sensors 90
4.3.3 Data-Logger 91
4.3.4 Additional Measuring Devices and Methods 92
4.4 Environmental Influence 93
4.5 Calibration and Reliability 96
4.6 Remaining Operational Lifetime 97
4.6.1 Rainflow Algorithm 98
4.6.2 Calculation of Stresses by FEM 101
4.6.3 S–N Approach and Damage Accumulation 104
4.6.4 Remaining Service Lifetime by Means of Existing Traffic Data and Additional Forward and Backward Extrapolation 105
4.6.5 Conclusions and Future Work 106
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 Frequency Analysis, ANPSD (Averaged Normalized Power Spectral Density) 115
5.2.5 Mode Shapes 120
5.2.6 Damping 121
5.2.7 Deformations 123
5.2.8 Vibration Coefficients 125
5.2.9 Counting of Events 126
5.3 Stochastic Subspace Identification Method 129
5.3.1 The Stochastic Subspace Identification (SSI) Method 129
5.3.2 Application to Bridge Z24 130
5.4 Use of Modal Data in Structural Health Monitoring 134
5.4.1 Finite Element Model Updating Method 134
5.4.2 Application to Bridge Z24 141
5.4.3 Conclusions 147
5.5 External Tendons and Stay Cables 149
5.5.1 General Information 149
5.5.2 Theoretical Bases 150
5.5.3 Practical Implementation 150
5.5.4 State of the Art 151
5.5.5 Rain–Wind Induced Vibrations of Stay Cables 152
5.5.6 Assessment 152
5.6 Damage Identification 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 of SHM 159
5.7 Damage Prognosis 161
5.7.1 Sensing Developments 162
5.7.2 Data Interrogation Procedure for Damage Prognosis 162
5.7.3 Predictive Modelling of Damage Evolution 163
5.8 Animation and the Modal Assurance Criterion (MAC) 164
5.8.1 Representation of the Calculated Mode Shapes 164
5.8.2 General Requirements 164
5.8.3 Correlation of Measurement and Calculation (MAC) 164
5.8.4 Varying Number of Eigenvectors 165
5.8.5 Complex Eigenvector Measurement 165
5.8.6 Selection of Suitable Check Points using the MAC 166
5.9 Ambient Vibration Derivatives (AVD_) 168
5.9.1 Aerodynamic Derivatives 168
5.9.2 Applications of the AVM 168
5.9.3 Practical Implementation 169
6 THEORETICAL BASES 173
6.1 General Survey on the Dynamic Calculation Method 174
6.2 Short Description of Analytical Modal Analysis 176
6.3 Equation of Motion 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 for the AVM 181
6.5 Practical Evaluation of Measurements 181
6.5.1 Eigenfrequencies 181
6.5.2 Mode Shapes 183
6.5.3 Damping 185
6.6 Theory on Cable Force Determination 185
6.6.1 Frequencies of Cables as a Function of the Inherent Tensile Force 185
6.6.2 Influence of the Bending Stiffness 190
6.6.3 Influence of the Support Conditions 192
6.6.4 Comparison of the Defined Cases with Experimental Results 193
6.6.5 Measurement Data Adjustment for Exact Cable Force Determination 197
6.7 Transfer Functions Analysis 199
6.7.1 Mathematical Backgrounds 199
6.7.2 Transfer Functions in the Vibration Analysis 205
6.7.3 Applications (Examples) 214
6.8 Stochastic Subspace Identification 222
6.8.1 Stochastic State-Space Models 223
6.8.2 Stochastic System Identification 226
7 OUTLOOK 235
7.1 Decision Support Systems 236
7.2 Sensor Technology and Sensor Networks 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-tensional T-beam (1956) 245
8.2 Donaustadt Bridge, Cable-Stayed Bridge in Steel (1996) 248
8.3 F9 Viaduct Donnergraben, Continuous Box Girder (1979) 250
8.4 Europa Bridge, Continuous Steel Box Girder (1961) 252
8.5 Gasthofalm Bridge, Composite Bridge (1979) 256
8.6 Kao Ping Hsi Bridge, Cable-Stayed Bridge (2000) 258
8.7 Inn Bridge Roppen, Concrete Bridge (1936) 260
8.8 Slope Bridge Saag, Bridge Rehabilitation (1998) 263
8.9 Flyover St Marx, Permanent Monitoring 265
8.10 Mur Bridge in St Michael, Bridge Rehabilitation 270
8.11 Rosen Bridge in Tulln, Concrete Cable-Stayed Bridge (1995) 272
8.12 VOEST Bridge, Steel Cable-Stayed Bridge (1966) 275
8.13 Taichung Bridge, Cable-Stayed Bridge 279