SERIES SEISMIC ENGINEERING RESEARCH INFRASTRUCTURES FOR EUROPEAN SYNERGIES

Transcription

1 SEVENTH FRAMEWORK PROGRAMME Capacities Specific Programme Research Infrastructures Project No.: SERIES SEISMIC ENGINEERING RESEARCH INFRASTRUCTURES FOR EUROPEAN SYNERGIES Workpackage WP3 Deliverable 3.4 Common Protocol for the Qualification of Research Infrastructures in Earthquake Engineering and Technical Annexes Annex 2 Specific Technical Requirements for Seismic Research Tests by Shaking Table Task Leader: PeP Author: Maurizio Zola - PeP Reviewed by: Fabio Taucer JRC Authorized by: Alessandro Bonzi - PeP Revision: Final January, 2013

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3 ABSTRACT The present document is a template for the drafting of the Specific Technical Requirements for research seismic testing performed by large RTD infrastructures. The introduction gives general information about the SERIES Project whose main aim is the qualification of large seismic RTD European infrastructures. Moreover in the introduction the general criteria for the drafting of this document are given. The following clauses are identifying the specific technical requirements which should be specified with respect to the Shaking table tests. Certification The formal procedure by which an accredited or authorized person or agency assesses and verifies (and attests in writing by issuing a certificate) the attributes, characteristics, quality, qualification, or status of individuals or organizations, goods or services, procedures or processes, or events or situations, in accordance with established requirements or standards. Accreditation Accreditation is the third party attestation related to a conformity assessment body (CAB) conveying formal demonstration of its competence to carry out specific conformity assessment tasks (ISO/IEC 17000:2004). Conformity Conformity assessment shall mean the process demonstrating whether assessment specified requirements relating to a product, process, service, system, person or body have been fulfilled (REGULATION (EC) No 765/2008). Qualification The process to demonstrate the ability to fulfill specified requirements (ISO 9000). Standard A technical specification approved by a recognized standardization body for repeated or continuous application, with which compliance is not compulsory (Directive 98/34/CE). Facility/Facilities The research laboratory/laboratories dealing with seismic testing and/or monitoring of structures. Research Activity The carrying out of observations/tests on a research object, based on a research Research and Development Activity plan, and the interpretation of the results thereof. (RvA T031) The research, within the field of competence which in its design and/or execution is not of a repetitive nature, and the relevant research techniques in which the body has proven experience and the required expertise. The field of research is primarily determined by the problem with which the body has experience of and is faced with. The research field must be based on the research methods/techniques or on the inspection methods/techniques as specified in the list of already recognized activities, applied standard methods within the body. The development of new research techniques, within the research field, may form a part of the research. (RvA T031) i

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5 ACKNOWLEDGMENTS This Networking Activity leading to these results has received funding from the European Community s Seventh Framework Programme [FP7/ ] under grant agreement n iii

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7 DELIVERABLE CONTRIBUTORS EUCENTRE IZIIS LNEC NTUA PeP Simone Peloso Zoran Rakicevic, Vlatko Sesov Rogério Bairräo, Ema Coelho Ioannis N. Psycharis, Haris Mouzakis Alessandro Bonzi, Maurizio Zola v

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9 CONTENTS 1 Scope Specimen to be tested Parameters/quantities and relevant ranges general excitation control Check and reference points Transducers choice measuring points other parameters Apparatus and equipment requirements for test apparatus Characteristics Cross axis motion Rotational motion requirements for the measuring system Characteristics Data acquisition Dynamic range Reference documents test standards General definitions Mounting of the specimen...13 vii

10 5.1.3 Tests execution Excitation definition Guide to the seismic testing Definitions for data processing test equipment standards Test equipment performances technical documents Excitation equipment transfer function Environmental conditions laboratory conditions specimen conditioning Description of the test setup specimen management Identification marks Handling Transporting Storing Preparation checks on the item checks on the testing equipment safety measures Specimen protection Personnel protection Test equipment protection...17 viii

11 8 Methods of test General remarks sinusoidal excitation Sinusoidal test method Signal tolerance Sweep rate Control strategy Severities Vibration amplitude Testing Test report transient excitation Transient test method Frequency range Severities Testing Test report stationary random excitation Stationary random vibration test method Statistical accuracy Control strategy Severities Vibration amplitude Testing...24 ix

12 8.4.7 Test report shock excitation Shock test method Pulse shape tolerance Severities Testing Test report recording of observations and results Criteria for approval Criteria for specimen approval requirements for test approval Data management data to be recorded Data type Data storage formats Data organization Data documentation Data Model Method of data analysis Data processing Data analysis data presentation Presentation types Tabular form...30 x

13 Charts and graphs Multimedia files Uncertainty management uncertainty indication General remarks procedure for EVALUATING uncertainty General remarks Measuring chain Base criteria for the estimation of the measuring uncertainty Contribution to uncertainty due to the measuring chain Excitation amplitude Excitation frequency Excitation duration Uncertainty on the excitation amplitude Uncertainty on the excitation frequency Uncertainty on the excitation duration Uncertainty on the excitation amplitude in case of multi point control Uncertainty on the excitation amplitude in case of multi axes excitation Repeatability evaluation general remarks repeatability of the excitation amplitude repeatability of the excitation frequency repeatability of the excitation duration repeatability during time General remarks...40 xi

14 Repeatability during time of the measure of the excitation amplitude Repeatability during time of the measure of the excitation frequency Repeatability during time of the measure of the excitation duration...41 xii

15 The SERIES Project INTRODUCTION SERIES project aimed at bridging the two gaps of RTD in experimental earthquake engineering and structural dynamics: (a) between Europe and the US or Japan, and (b) between European countries with high seismicity but less advanced RTD infrastructures on one hand and some more technologically advanced but not so seismic Member States on the other. It did so by integrating the entire European RTD community in earthquake engineering via a lot of activities and among the others by specifying standards, protocols and criteria for qualification of RTD infrastructures in earthquake engineering. Description of WP3 NA2 WP3 NA2 was the Networking Activity of the SERIES Project whose aim was to create the conditions leading to the qualification (in the form of mutual accreditation) of Structural Testing Research Laboratories specialising in earthquake engineering and equipped for large scale testing. The conclusions of the assessment of testing and instrumentation management procedures in Tasks NA2.2 and NA2.3, respectively, led to a critical analysis of the requirements imposed by official standardization and accreditation organisations, National and European. The activity was broken down in four Tasks. Task NA2.1: Evaluation and impact of qualification of experimental facilities in Europe The task included: a critical analysis of the problems that limit the free circulation of the products of the European Industry, the solution of which will be promoted by the qualification of structural laboratories, and a study of the issues of technical, quality or commercial relevance - which in fact constitute obstacles to the mutual accreditation. Task NA2.2: Assessment of testing procedures and standards requirements The focus of this task was on Shaking Table and Reaction Wall facilities, which, owing to their special character and relatively small number, their differences in technical solutions, the wide 1

16 variety of testing procedures among their operators and the almost non-existence of knowledge of the technology itself among the Users (Industry), do not lend themselves to fully harmonised approaches. Task NA2.3: Criteria for instrumentation and equipment management This task treated the issue of the management criteria of the instrumentation (testing equipment, measuring instrumentation, acquisition systems and processing tools), including the issues related to the calibration of instruments (periodicity and technical conditions), their maintenance, the estimation of measurement uncertainty, the practice used for applying measuring instrumentation on specimens. As in Task NA2.2, a critical assessment of the presently adopted methods was made, after exchange of information among the laboratories regarding the procedures used and cross visits of the laboratory operators to the co-operating facilities on the occasion of important or benchmark tests. Task NA2.4: Development and implementation of a common protocol for qualification The starting point of this task was the Workshop Qualification of research infrastructures in the framework of the International Workshop on Role of research infrastructures in performance-based earthquake engineering, held in Ohrid (Republic of Macedonia), in conjunction with the 14th European Conference in Earthquake Engineering in Sept. of Using as baseline the outcomes of Tasks NA2.1 to NA2.3 and the conclusions of the discussion at the Workshop, the following conclusions were drawn: ISSUES Qualification of a research and technological development (RTD) infrastructure in earthquake engineering implies a process different from the qualification of a testing laboratory; International or European Standards dealing with certification or accreditation are not specifically devoted to the RTD laboratories; Lack of International or European standard or regulation requesting the qualification of RTD infrastructures; Lack of international recognition of the large capacities and associated human resources of the European RTD infrastructures; Lack of co-operation between a) more technologically advanced but less seismic countries and b) less technologically advanced but more seismic countries. OBSTACLES Lack of specific Standards for the qualification of RTD infrastructures; 2

17 Lack of specific Standards covering RTD seismic testing; Lack of specific Standards covering special seismic testing with multi-axial large shaking tables, quasi-static and pseudo-dynamic techniques or hybrid experimental & mathematical modeling techniques; Lack of a qualification-oriented mentality of the high level management of the RTD infrastructures; Underestimation by the laboratory staff of the benefits of an official qualification of the RTD infrastructures; Reduced investment capabilities of the RTD infrastructures. Then a road map towards the Common Protocol was conceived: 1. Evaluation of the suitability of the General Management Requirements of EN ISO/IEC for RTD infrastructures; 2. Evaluation of the suitability of the General Technical Requirements of EN ISO/IEC for RTD infrastructures; 3. Identification of Specific Technical Requirements (STR) for the RTD seismic testing; 4. Identification of Specific Technical Requirements relevant to documentation and data sharing to guarantee repeatability and reproducibility of test results in the framework of the activities of SERIES/NA1 aiming to the development of a Common European Database; 5. Issue of a draft Common Protocol for the qualification with respect to the General Management and Technical Requirements; 6. Drafting of RTD testing procedures; 7. Issue of a technical specification for the development of a Common European database to guarantee repeatability and reproducibility of seismic testing; 8. Implementation on a voluntary basis of the draft Common Protocol in some SERIES laboratories; 9. Development of the Final Common Protocol for the Qualification. After the completion of the first two points of the road map it was recognized that: the general management requirements of EN ISO/IEC are suitable also for the management of RTD infrastructures, and the same conclusion can be drawn for the suitability of the general technical requirements of EN ISO/IEC for the RTD infrastructures, but not for requirements of clause 5.4.2, selection of methods. Namely as it is above stated there is a lack of specific Standards covering RTD seismic testing and special seismic testing with single and multi-axial large shaking tables, quasi-static and pseudo-dynamic techniques or hybrid experimental & mathematical modeling techniques; moreover the taking in of the testing approach, as a consequence of too rigid Accreditation 3

18 requirements and procedures, is often unpractical, due to the complexity of the tests and to the hazardous character of the specimen response: in many situations, testing procedures may require real-time adjustments suggested by engineering judgments based on the acquired results of the experiments 18. To overcome this situation, in accordance with EA 2/15 17, Specific Technical Requirements for the RTD seismic testing were identified with reference to the requirements of clauses 5.4.3, and of EN ISO/IEC This choice will lead to the qualification with flexible scope of the RTD infrastructures. The adoption of technical annexes dealing with Specific Technical Requirements will account for the need to cover the special requirements for the performance of research tests. The Common Protocol will then be supplemented by the necessary technical guidance and recommendations, in the form of Technical Annexes. General criteria and references References The Specific Technical Requirements for shaking table testing are specified with reference to two lists of documents: The documents listed here below as far as the general aspects of this document, and The documents listed in clause 5 as far as the specific technical aspects. 1. COMMISSION OF THE EUROPEAN COMMUNITIES SERIES: SEISMIC ENGINEERING RESEARCH INFRASTRUCTURES FOR EUROPEAN SYNERGIES - Grant Agreement Number COMMISSION OF THE EUROPEAN COMMUNITIES SERIES: SEISMIC ENGINEERING RESEARCH INFRASTRUCTURES FOR EUROPEAN SYNERGIES - Project Number Grant Agreement Preparation Forms 3. Grant agreement No for: Integrating Activity - Combination of Collaborative Project and Coordination and Support Action SERIES - SEISMIC ENGINEERING RESEARCH INFRASTRUCTURES FOR EUROPEAN SYNERGIES Annex I: Description of Work - 29 January Grant agreement No for: Integrating Activity - Combination of Collaborative Project and Coordination and Support Action SERIES - SEISMIC ENGINEERING RESEARCH INFRASTRUCTURES FOR EUROPEAN SYNERGIES - Annex II: General Conditions - 02 October Grant agreement No for: Integrating Activity - Combination of Collaborative Project and Coordination and Support Action SERIES - SEISMIC ENGINEERING RESEARCH INFRASTRUCTURES FOR EUROPEAN SYNERGIES - Annex III: SPECIFIC PROVISIONS FOR TRANSNATIONAL ACCESS ACTIVITIES. 4

19 6. SEVENTH FRAMEWORK PROGRAMME - Capacities Specific Programme - Research Infrastructures SERIES: SEISMIC ENGINEERING RESEARCH INFRASTRUCTURES FOR EUROPEAN SYNERGIES Consortium Agreement February COMMISSION OF THE EUROPEAN COMMUNITIES - FP7 Grant Agreement N SERIES Letter RTD/B05(2008)D/ (ref.: I/24376). 8. EN ISO 9001: Quality management systems Requirements. 9. EN ISO 9000: Quality management systems - Fundamentals and vocabulary. 10. ISO/IEC 17025: General requirements for the competence of testing and calibration laboratories. 11. ISO : Statistics Vocabulary and symbols Part 1: Probability and general statistical terms. 12. ISO 2041 Vibration and Shock Vocabulary. 13. WP3NA Minutes of the meeting in Napoli (Italy). 14. WP3NA WP3NA2.1 Final Report - Evaluation and impact of qualification of experimental facilities in Europe. 15. WP3NA WP3NA2.2 Final Report - Assessment of testing procedures and standards requirements. 16. WP3NA WP3NA2.3 Final Report - Criteria for instrumentation and equipment management. 17. EA-2/15 - EA Requirements for the Accreditation of Flexible Scopes. 18. CASCADE PROJECT - Directory of European Facilities for Seismic and Dynamic Tests in Support of Industry RvA/T031 Dutch Accreditation Council (Raad voor Accreditatie) Research and Development RvA/T015 Dutch Accreditation Council (Raad voor Accreditatie) Explanation of ISO/IEC 17025: General Criteria The contents of this Specific Technical Requirements is developed in accordance with the requirements of clause of EN ISO/IEC 17025: a) Appropriate identification b) Scope c) Description of the type of item to be tested d) Parameters or quantities and ranges to be determined 5

20 e) Apparatus and equipment, including technical performance requirements f) Reference standards required g) Environmental conditions required and any stabilization period needed h) Description of the test procedure, including 1. Affixing of identification marks, handling, transporting, storing and preparation of items 2. Checks to be made before the work is started 3. Check that the equipment is working properly and, when required, calibration and adjustment of the equipment before each use 4. The method of recording the observations and results 5. Any safety measures to be observed i) Criteria and/or requirements for the approval/rejection j) Data to be recorded and method of analysis and presentation k) The uncertainty or the procedure for estimating the uncertainty. The clauses covering the above reported requirements are following. Specific criteria Reference is made to the international standards as far as possible, avoiding repetitions, and only Specific Technical Requirements are extensively reported. 6

21 1 Scope The title of the document is: Specific Technical Requirements for Seismic Research Tests by Shaking Table. The present document reports the Specific Technical Requirements (STR) to conduct shaking table tests for seismic engineering research purposes. The aim of the document is to state the STRs to allow large research testing facilities to pursue a flexible scope accreditation in accordance to EN ISO/IEC and EA 2/15. The test methods presented in this document follow International Standards. For research tests with special purposes additional STRs are stated in the following sections. Before carrying out an experimental campaign a Test Specification document should be prepared by the concerned laboratory, covering all the requirements of the present document and in particular the following list: a) Preparation of the specimen for the transportation phases b) Transportation of the specimen c) Mounting of the specimen d) Fixing points e) Choice of check points f) Choice of control points g) Control strategy h) Measuring points i) Cross-axis motion j) Rotational motion k) Vibration amplitude tolerances l) Frequency resolution m) Excitation types n) Testing axes (single axis, biaxial or tri-axial testing) o) Test severity (amplitude, frequency range, duration) p) Testing program (including tests sequence) 7

22 q) Preconditioning r) Specimen check-up s) Performance and functional check t) Acceptance or rejection criteria for the tests The requirements of the present document apply to shaking table testing of: Reduced scale models of civil structures; Full scale components of civil structures; Geotechnical models; Industrial equipment and components. Shaking table testing can be performed: for research purposes; for qualification purposes, where technical standards for qualification are not available. 2 Specimen to be tested In the test specification the specimen to be tested should be described with special references to the following points: a) Geometry and dimensions of the specimen b) Mass of the specimen c) Reproduction of restraint conditions on the shaking table (see ref. doc. iii ) d) Materials of the specimen (full characterisation) e) Description of construction and assembly of the specimen f) Lifting procedures of the specimen 8

23 3 Parameters/quantities and relevant ranges 3.1 GENERAL In the Test Specification the parameters and quantities to be determined during the test should be given with reference to their relevant ranges of measurements. Specifications should be given with reference to the following points: a) Excitation control on the shaking table b) Measuring points on the specimen c) Other parameters necessary to describe the response of the specimen 3.2 EXCITATION CONTROL Check and reference points The excitation motion will be assessed by check and reference points as defined in doc. iv. Check points should be located as stated in doc. iv Transducers choice The measuring system should be chosen taking into account the amplitude and frequency range of excitation with respect to the sensitivity, frequency response and resolution of the transducer. Generally speaking acceleration transducers will be used for the check points. For test representing seismic excitations the acceleration signal at low frequencies may reach very small amplitudes with respect to the measuring chain noise. Using appropriate accelerometers and displacement transducers may help to overcome this problem. The transducers used for the check points could be: Acceleration transducers Displacement transducers Strain gages Optical transducers In the test specification the relevant ranges should be given with special attention to: Amplitude Amplitude resolution 9

24 Frequency Sensitivity. 3.3 MEASURING POINTS Measuring points are those at which the response of the specimen or any other quantity of interest is recorded. The signal coming from these points can be used to control the excitation input, e.g. to avoid damage and degradation of the specimen. The Test Specification should specify the type of measuring transducers and the strategy to use the relevant signals. 3.4 OTHER PARAMETERS During the test it may be necessary to monitor the following parameters on the specimen: Cracking and damage progression Noise Temperature Geometric properties such as planarity, inclination, surface roughness, tapering or ovalization Chatter Backlash The Test Specification should specify the parameter to be monitored and the relevant detection device. It is noted that the experimental measuring points are limited in number and transducers give only discrete information. Nevertheless additional information may be gathered in other ways, e.g. with field vision systems or laser scanners during the tests or by integrating the measurements with a numerical model of the structure. 10

25 4 Apparatus and equipment 4.1 REQUIREMENTS FOR TEST APPARATUS Characteristics The required characteristics apply to the complete excitation apparatus, which includes the power supply system, dynamic actuators, shaking table, test fixture, specimen, control system and data acquisition system at the time of testing. For the general requirements reference is given to doc. ii, iii, iv and v. When fastening the specimen to the shaking table it is of a paramount importance that both its actual mounting or a fixture does not modify the specified excitation Cross axis motion Vertical and horizontal cross-axis motion of the shaking table should be checked both before the test by conducting a sine or random mono-axial excitation pre-test at a level prescribed by the test specification and during testing by utilising additional monitoring channels in the two perpendicular axes. There should be some caution in the testing of large size or high mass specimens or if the centre of gravity of the specimen is greatly offset from the centre of the shaking table. Such specimens may have a tendency to cause a transverse motion or rotational motion of the vibration table. In such cases it may be difficult to avoid any cross-axis motion; the test specification shall state the maximum amplitude of the cross-axis motion and the measured transverse motion shall be reported in the test report Rotational motion In the case of large size or high mass specimens, the occurrence of spurious rotational motion of the vibration table is likely to be important. A tolerable level should be prescribed in the test specification and the measured rotational motion shall be reported in the test report. 11

26 4.2 REQUIREMENTS FOR THE MEASURING SYSTEM Characteristics The characteristics of the measuring system shall be such that the true value of vibration as measured in the intended axis at the reference point is within the tolerance stated in the Test Specification. The frequency response of the overall measuring system, which includes the transducer, the signal conditioner and the data acquisition and processing device, has a significant effect on the accuracy of the measurements. The frequency range of the measuring system shall extend from at least 0,5 times the lowest frequency to 2,0 times the highest frequency of the test frequency range. The frequency response of the measuring system shall be flat within ±5 % in the test frequency range. For auxiliary quantities of interest, other than excitation and response acceleration or displacement, relaxed requirements can be applied as required in the test specification. Outside of this range any further deviation shall be stated in the test specification and the true measured deviation shall be reported in the test report. When indirect measurements are performed, e.g. velocity through integration of the signal coming from an acceleration transducer, special provisions should be taken with respect to the data processing tools and procedures; during the data acquisition process specific actions should be taken, e.g. band pass filtering Data acquisition Experimental data can be recorded by analogue or digital systems. Raw data should be kept in the data base in the time domain. For further processing analogue records should also be digitized. With the purpose of avoiding aliasing phenomena the sampling frequency should be suitable and anti-aliasing filtering should be applied on the analogue signal before A/D conversion Dynamic range The number of bits of the analogue to digital converter is directly related to the resolution of the amplitude. A preliminary analysis of the analogue signals being recorded should be carried out, with the purpose of assessing the minimum and maximum expected amplitude. It is recommended to check that the smallest value of the recorded signals can be appropriately converted into digits. Moreover phase lag due to amplifiers and multiplexing data recorders should be accounted for. 12

27 5 Reference documents 5.1 TEST STANDARDS General definitions i. ISO 2041 Vibration and Shock Vocabulary ii. ISO/IEC 17025: General requirements for the competence of testing and calibration laboratories Mounting of the specimen iii. IEC (1999) Environmental testing - Part 2: Test methods - Mounting of components, equipment and other articles for vibration, impact and similar dynamic tests Tests execution iv. EN IEC (1995) Environmental testing - Part 2-6: Tests - Test Fc: Vibration (sinusoidal) v. EN IEC (1999) Environmental testing - Part 2-57: Tests - Test Ff: Vibration - Time-history & sine-beat method vi. EN IEC (1993) Environmental testing - Part 2-64: Tests - Test Fh: Vibration, broad-band random (digital control) and guidance vii. EN IEC (2009) Environmental testing - Part 2-27: Tests - Test Ea and guidance: Shock Excitation definition viii. EN IEC (2003) Environmental testing - Part 2-81: Tests - Test Ei: Shock - Shock response spectrum synthesis ix. EN ANB: Eurocode 8: Design of structures for earthquake resistance - Part 1: General rules, seismic actions and rules for buildings Guide to the seismic testing x. EN IEC (1993) - Environmental testing - Part 3: Guidance. Seismic test methods for equipment 13

28 5.1.6 Definitions for data processing xi. ISO : Statistics Vocabulary and symbols Part 1: Probability and general statistical terms xii. ISO : Mechanical vibration and shock Signal processing Part 1: General introduction xiii. ISO Accuracy (trueness and precision) of measurement methods and results - Part 1: General principles and definitions xiv. ISO Accuracy (trueness and precision) of measurement methods and results - Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement xv. ISO Accuracy (trueness and precision) of measurement methods and results - Part 3: Intermediate measures of the precision of a standard measurement method. xvi. ISO Accuracy (trueness and precision) of measurement methods and results - Part 4: Basic methods for the determination of the trueness of a standard measurement method. xvii. ISO Accuracy (trueness and precision) of measurement methods and results - Part 5: Alternative methods for the determination of the precision of a standard measurement method. xviii. ISO Accuracy (trueness and precision) of measurement methods and results - Part 6: Use in practice of accuracy values. xix. ENV ISO Guide to the expression of uncertainty in measurement. xx. EA 4/16 - EA guidelines on the expression of uncertainty in quantitative testing. 5.2 TEST EQUIPMENT STANDARDS Test equipment performances xxi. ISO Auxiliary tables for vibration generators - Method of describing equipment characteristics xxii. ISO Servo-hydraulic test equipment for generating vibration - Method of describing characteristics xxiii. ISO 5344: Electro-dynamic vibration generating systems - Performance characteristics 5.3 TECHNICAL DOCUMENTS Excitation equipment transfer function xxiv. Allen J.Clark, Analysis of basic servo-hydraulic requirements for simulating earthquake motions, Seismovibrotechnika - 79, Erevan (Armenia),

29 6 Environmental conditions 6.1 LABORATORY CONDITIONS When critical for the specimen, environmental conditions (temperature and humidity), during day and night, should be recorded in the laboratory from the assembly of the specimen until the end of the test campaign. Environmental conditions should be specified in the test specification. 6.2 SPECIMEN CONDITIONING Test Specifications may call for the conditions necessary for preconditioning of the structure. The required period for stabilization shall be specified (e.g. concrete curing). Hygroscopic materials, as masonry, concrete and timber, absorb and lose moisture according to ambient relative humidity and temperature; then the mass of specimens can change and so their natural frequencies. Moreover, temperature variations affects the stress distribution of statically indeterminate structures, again potentially affecting their natural frequency. For repeatability evaluation care should be taken of ambient conditions. 7 Description of the test setup 7.1 SPECIMEN MANAGEMENT Identification marks All specimens should be identified by affixing of marks. 15

30 7.1.2 Handling Handling procedures should be specified in the Test Specification covering displacement, lifting, fixing Transporting Transport procedures should be specified in the Test Specification Storing Environmental conditions for storage should be specified in the Test Specification Preparation Preparation of the specimen before test should be specified in the Test Specification. 7.2 CHECKS ON THE ITEM Checks should be specified in the Test Specification before the work is started. 7.3 CHECKS ON THE TESTING EQUIPMENT The equipment shall be working properly and, when required, it shall be calibrated and adjusted before each use. 16

31 7.4 SAFETY MEASURES Specimen protection In the Test Specification it shall be stated any safety measures to be observed during the pretesting, the testing and the post-testing phases including dismounting and specimen disposal Personnel protection In the Test Specification it shall be stated any safety measures to be observed during the pretesting, the testing and the post-testing phases including dismounting and specimen disposal Test equipment protection In the Test Specification it shall be stated any safety measures to be observed during the pretesting, the testing and the post-testing phases including dismounting and specimen disposal. 8 Methods of test 8.1 GENERAL REMARKS In the following four different types of vibration excitation are considered: Sinusoidal vibration Transient vibration Stationary random vibration Shock These different types of vibration can be used mainly for two different purposes: Vibration Response Investigation (VRI) and 17

32 Seismic Excitation A Vibration Response Investigation is defined as a vibration test where the response of the specimen is investigated in order to study the behavior of the specimen under a given frequency range. The most suitable type of vibration for reproducing earthquake excitation is transient excitation. In the following clauses the requirements for the different types of excitation are stated in terms of a transient excitation, both for seismic and other types of VRI excitations. Special care should also be taken when performing tests for special applications as in the case of sub-structuring where a high quality of the reproduction of the time domain excitation is needed. 8.2 SINUSOIDAL EXCITATION Sinusoidal test method A sinusoidal test consists in subjecting the specimen to a vibration of harmonic pattern specified by a sinusoidal vibration (acceleration, velocity, displacement or force) over a given frequency range or at discrete frequencies. Tests with sinusoidal excitation will be conducted with reference to doc. iv. Care should be taken when using the sinusoidal excitation for the earthquake reproduction because the stationary response of a specimen in resonance reaches higher values of amplification than in the case of a transient multi-frequency excitation (see docs. v and x ). In the following clauses some additional requirements or comments are reported with special reference to research purposes Signal tolerance Signal tolerance as defined in reference doc. ii gives a measure of the ability of representing a rigorous mono-frequency sinusoidal excitation. This information is very important in order to correlate the dynamic behavior of the specimen with the excitation. The presence of spurious frequencies in the excitation can excite modes of vibration at frequencies different from the fundamental ones. With special reference to specimens with non-linear behavior this may result in a response much different from that corresponding to the fundamental frequency of the excitation. The frequency response function is measured by rejecting all the frequency components having frequencies different from the excitation frequency. Moreover, in cases where the signal tolerance is high, the measuring system will indicate a vibration level that may be incorrect, since it will contain, apart from the required frequency, many unwanted frequencies. This will result in lower acceleration amplitudes at the required frequency. 18

33 Therefore, in order to overcome the above related problems, a limit in the signal tolerance should be specified in the Test Specification and the signal tolerance should always be measured at the check points. In general, and depending on the test being performed, where the signal tolerance is less than 5% there is no practical difference between the actual force and specified force. If the signal tolerance is in excess of 5%, the reproducibility and repeatability of the test may be significantly affected Sweep rate If a vibration response investigation (VRI) with sweeping frequency is carried out with a sinusoidal excitation, the sweep rate should be chosen such that transient vibrations of the response due to the changing frequency damp out: specimens with low damping ratio should be excited at low sweep rates. Nevertheless, particular attention should be paid when getting close to the structural resonant frequencies since important dynamic amplification of the motion could take place, particularly with low damping ratio Control strategy In the following the requirements are stated with reference to the check points on the shaking table or on the fixture in order to control the motion of excitation. Anyway some measuring points on the specimen can be used in order to stop the control system in order to guarantee the safety of the specimen and of the equipment. If it is not possible to achieve the basic motion by a single point control, then multipoint control shall be used by controlling the average or extreme value of the signals at the check points. In either of these cases of multipoint control, the reference point is a fictitious reference point. Use of multipoint control does not assure that the tolerances of each checkpoint are met. In the averaging strategy, the control amplitude is computed from the signal from each check point. A composite control amplitude is formed by a linear combination of the signal amplitudes from the check points. In the extremal strategy, a composite control amplitude is computed from the maximum (MAX) or the minimum (MIN) extreme amplitudes of the signal amplitude measured at each check point. This strategy will produce a control amplitude that represents the envelope of the signal amplitudes from each check point (MAX) or a lower limit of the signal amplitudes from each check point (MIN). In both cases the computed control amplitude is then compared with the specified excitation amplitude. 19

34 It should be kept in mind that the multipoint control strategy does not guarantee a uniform excitation motion of the shaking table and this could result in a low reproducibility and repeatability of the test Severities A vibration severity is defined by the combination of the following three parameters: frequency range vibration amplitude and duration of the vibration. Generally speaking, for seismic purposes the frequency range may extend from 0.5 to 30 Hz, nevertheless the frequency range should be defined on the basis of the actual characteristics of the specimen Vibration amplitude The amplitude of the applied displacement, velocity or acceleration or combinations of those shall be stated in the Test Specification. Below a certain frequency known as the cross-over frequency, all amplitudes are specified as constant displacement, whilst above this frequency, amplitudes are given as constant velocity or constant acceleration. Each value of displacement amplitude is associated with a corresponding value of acceleration amplitude so that the amplitude of vibration is the same at the cross-over frequency. On the other hand sometimes, it could be useful to conduct the test by adopting a force control strategy, in this case the amplitude shall be specified in term of maximum positive and negative force. When adopting the sinusoidal excitation for the earthquake reproduction reference should be done to doc. x to state an equivalence between the seismic response spectrum and the sinusoidal excitation Testing The structural modal parameters, i.e. natural frequencies, damping and modal shapes can be extracted from the Frequency Response Function (FRF). When digital control and data acquisition system is used, care shall be taken when determining the critical frequencies as a function of the number of data points per sweep (frequency resolution). 20

35 When performing a manual sweeping over a natural frequency care should be taken so that the frequency resolution is sufficiently high so as to determine the resonant peaks. Besides the modal quantities, structural cyclic behaviour can be determined too: dynamic cyclic tests are particularly useful to explicitly account for rate dependent effects Test report A test log should be written for the testing campaign, including a chronological list of meaningful test runs, test parameters, observations during testing, actions taken and data sheets on measurements made. The test log should be attached to the test report. 8.3 TRANSIENT EXCITATION Transient test method The transient test consists in subjecting the structure to a vibration specified by a time history simulating the effects of the dynamic forces for which the structure is being studied. A special case of transient excitation is represented by a sine-beat. The test with transient excitation shall be conducted with reference to docs. v and viii. In the following clauses some additional requirements or comments are reported with special reference to research purposes Frequency range The signal from the reference point shall not contain any frequency higher than the frequency range of the vibrations imposed during the test, except those induced by the test equipment and the specimen. The maximum value of the signal falling outside the test frequency range induced by the test equipment shall not exceed 20% of the maximum value of the signal specified at the reference point. Frequencies outside the test frequency range shall not be taken into account when evaluating the response spectrum of the test Severities The test severity for transient excitation is determined by a combination of the following parameters: frequency range; 21

36 required response spectrum; number and duration of the time-histories; number of high stress response cycles (if applicable). Generally speaking, for seismic purposes the frequency range may extend from 0.5 to 30 Hz, nevertheless the frequency range should be defined on the basis of the actual characteristics of the specimen Testing The test specification shall also state whether single axis, biaxial or triaxial testing is required. During a transient test the specimen should be visually monitored. The dynamic behaviour is determined by examining the vibration response data. When digital control and data acquisition system is used, care shall be taken when choosing the sampling frequency in order: to determine high frequency response resulting from shock phenomena; in general 1,000 Hz is a suitable value for the sampling frequency when shocks inside the structure are expected; to avoid computation errors in the calculation of the test response spectrum; a sampling frequency equal to five times the maximum analyzed frequency should be chosen. Moreover for the calculation of the excitation test response spectrum as a function of the reference point, attention should be paid to distinguish between the frequency components of the signal due to the effective controlled excitation and the frequency components due to shocks inside the specimen which could be transmitted to the acceleration transducer Test report See STATIONARY RANDOM EXCITATION Stationary random vibration test method A stationary random vibration test consists in subjecting the structure to a broadband random vibration specified by a acceleration Power Spectral Density (PSD) of a given duration. The test with stationary random excitation shall be conducted with reference to doc. vi. 22

37 The stationary random vibration should be a long lasting vibration, which is not suitable for the reproduction of a transient excitation such as that of an earthquake. Nevertheless the stationary random excitation of suitable duration can be adopted successfully for the VRI of specimen with non linear behaviour. In the following clauses some additional requirements or comments are reported with special reference to research purposes Statistical accuracy To guarantee the reproducibility of a stationary random excitation test is often not an easy task. Because of the statistical nature of the random signal, the complex response of the specimen and the errors arising from the analyzing process, it is not possible to predict with certainty whether the true acceleration spectral density of the random input at the structure will match the indicated acceleration spectral density at the specimen within a predefined set of tolerances. A complex, time-consuming analysis after the test is required, as an on line estimation is not possible. Nevertheless, with a careful selection of parameters of the vibration control equipment, a preliminary calculation can be made to estimate the statistical accuracy associated with the difference between the measured and the true acceleration spectral density. The statistical accuracy is determined by the statistical degrees of freedom (DOF) of the data analysis for a specified confidence level. These parameters, which are dependent on each other, may be chosen to achieve an optimum similarity between the two acceleration spectral densities. For a specified confidence level, the higher the number of DOFs, the higher the statistical accuracy. Considering that for linear averaging the DOFs equal twice the number of samples and the frequency resolution is the inverse of the duration of each sample, long duration tests are often needed. A narrow resolution bandwidth will yield a higher control accuracy, however, this will result in a longer test duration. In order to minimize the deviation between the measured and the true acceleration spectral density at the specimen, optimization of the statistical degrees of freedom are required. A minimum limit of the statistical accuracy and of the confidence level should then be specified in the Test Specification in order to guarantee the reproducibility and repeatability of the test Control strategy If it is not possible to achieve the basic motion by a single point control, then multipoint control shall be used by controlling the average or extreme value of the signals at the check points. In either of these cases of multipoint control, the reference point is a fictitious reference point. Use of multipoint control does not assure that the tolerances of each checkpoint are met. In the averaging strategy, the control acceleration spectral density (ASD) is computed from the signal from each check point. A composite control ASD is formed by a linear combination of the signal ASDs from the check points. 23

38 In the extremal strategy, a composite control ASD is computed from the maximum (MAX) or the minimum (MIN) extreme amplitudes of the signal ASD measured at each check point. This strategy will produce a control ASD that represents the envelope of the signal ASDs from each check point (MAX) or a lower limit of the signal ASDs from each check point (MIN). In both cases the computed control ASD is then compared with the specified excitation ASD. It should be kept in mind that the multipoint control strategy does not guarantee a uniform excitation motion of the shaking table and this could result in a low reproducibility and repeatability of the test Severities A vibration severity is defined by the combination of the following four parameters: frequency range root mean square value of acceleration shape of acceleration spectral density and duration of the vibration. For seismic purposes the frequency range should extend from 0,5 to 30 Hz Vibration amplitude The amplitude of displacement, velocity or acceleration or combinations of those, shall be stated in the Test Specification. Below a certain frequency known as the cross-over frequency, all amplitudes are specified as constant displacement, whilst above this frequency, amplitudes are given as constant velocity or constant acceleration. Each value of displacement amplitude is associated with a corresponding value of acceleration amplitude so that the amplitude of vibration is the same at the cross-over frequency Testing See When digital control and a data acquisition system are used, care shall be taken when determining the critical frequencies since the accuracy of the determination is dependent upon the duration of each sample (frequency resolution). 24