This testing has featured particular implementations of ... · Experimental Modal Analysis testing - a powerful tool for identifying the in-service condition of bridge structures

Experimental Modal Analysis testing -

a powerful tool for identifying the in-service

condition of bridge structures

N. Haritos

Department of Civil & Environmental Engineering, The University

of Melbourne, Grattan St, Parkville, Victoria 3052, Australia

Email: n.haritos @ engineering, unimelb. edu. au

Abstract

The Experimental Modal Analysis (EMA) testing technique provides a great deal moreinformation about the in-service condition of a bridge under test than is possible from themore traditional forms of static testing through identification of the modal properties (naturalfrequencies, mode shapes and damping levels) of the bridge concerned. This information isof immense value to the "tuning" of Finite Element Method (FEM) based models which canthen be used with confidence for assessing bridge load carrying capacity and response toseismic and other dynamic load inputs. This paper outlines some of the key features of thetechnique highlighting results obtained from its application to the field testing of severalbridges by the University of Melbourne structural engineering research team.

1 Introduction

A common method of predicting the behaviour of real systems in many areasof science and engineering is to use relevant mathematical models. One of thefundamental problems faced by scientists and engineers is the verification ofsuch models using data from experimental measurements. In particular,specific parameters of the mathematical model describing the system have tobe found that provide the best possible match between the predicted behaviourfrom the model and actual experimental observations.

A team of researchers at the University of Melbourne has beencollaboratively involved over the past few years with the Principal BridgeEngineer's department of VicRoads, the State of Victoria, Australia, road andbridge authority, in the field testing of a number of ageing ReinforcedConcrete (RC) bridge superstructures as part of the VicRoads corporatesponsored R&D project: "Load Capacity of In-service Bridges",' ^^.

Transactions on Modelling and Simulation vol 16, © 1997 WIT Press, www.witpress.com, ISSN 1743-355X

214 Computer Methods and Experimental Measurements

This testing has featured particular implementations of the ExperimentalModal Analysis (EMA) dynamic testing technique* in which the bridgesuperstructures concerned were dynamically excited using forcing from aLinear Hydraulic Shaker (LHS). Basic parameters (such as the value ofYoung's modulus of aged RC; the flexibility of pier supports; supportconditions at the abutments, etc), in numerical models of these structures basedupon the Finite Element Method (FEM) modelling technique were able to be"tuned" so as to obtain as good a fit as conveniently possible between thepredicted modal properties from these models and those obtained from theEMA testing. This paper highlights some of the experience gained and thebenefits realised from application of the EMA testing technique to a number ofbridge structures in the State of Victoria, Australia, by our research team.

2 Description of the EMA testing technique

2.1 Principles of EMA testing

EMA is essentially the process by which the natural modes (mode shapes,frequencies and damping) of a structure under test are identified fromperformance of a Modal Experiment. Time domain traces of responsemeasurements over a grid of points captured simultaneously with excitationforce at a single point, "k", are transformed in the frequency domain to obtainestimates of the Frequency Response Functions (FRFs), A#(co) , via:

£M (l)

where X. fcoj and F*(co) are the Fourier Transforms of the displacementresponse at point "j", given by Xj(t), and this excitation force, respectively,whereas the theoretical form of the FRF, hjk(co), is given by:

(2)

in which %„ and 9 represent the j* and k* elements of the complexeigenvector for the n* mode of vibration, and X% is the complex eigenvalue forthis mode, with the "*" representing the process of conjugation.

The complex eigenvalue X% in eqn. (2) is related to %„ and coj, thedamped and undamped circular frequencies for the n* mode respectively, and£n, the ratio to critical damping for that mode, via:

Aw = -GnWfz + fadn (3)

Through "ensemble averaging" of several realisations of fr/*(a>) obtainedfrom repeat test records, the "quality" of the FRF can be improved forsubsequent processing by suitable EMA algorithms that attempt to fit modalparameters (%, In and hence co n and £„)» to the theoretical model of eqn. (2).The DSMA algorithm , developed for performing EMA by the University of


Computer Methods and Experimental Measurements 215

Melbourne team, achieves "best" estimation of the complex eigenvectors %(mode shapes) and eigenvalues Xn (natural frequencies and damping ratios) byminimising on the square of the difference between all of the "ensemble-averaged" forms of FRFs and their corresponding theoretical counterparts viaa non-linear least squares fitting procedure.

In practice, only a small number of transducers (say 10 to 15 accelero-meters) would be available for measuring the response, so these would need bere-located a sufficient number of times to "cover" the required measurementgrid. The excitation would then be repeated as necessary to produce the"ensemble averaged" realisations of A#(oo) for all points "j" on the grid.

2.2 The University of Melbourne EMA testing package

The University of Melbourne system for performing EMA testing comprisesthe following major components:(i) A Linear Hydraulic Shaker(LHS)/actuator rated at lOOkN for a 20

MPa line operating in the frequency range: 0 - 50 Hz.(ii) A 120 litre/min hydraulic power supply to the actuator.(iii) A three phase electrical power unit normally hired for field use.(iv) A remote console for controlling the actuator in displacement/force

mode of controlled excitation via "TSPECTRA"* software.(v) "TSPECTRA" - Real time data acquisition/experimental control

software/hardware system that captures up to 16 simultaneouschannels with programmable anti-aliasing filters, gain and offset.

(vi) "DSMA"* - a fully non-linear least squares package for EMA.(vii) A set of vibration measurement transducers - choice of 4 servo-

based Sundstrand accelerometers (sensitivities of ~5V/g), 6 PCBaccelerometers (sensitivities of ~10mV/g) and, more recently, 15Dytran piezo-electric accelerometers (sensitivities of ~0.5V/g).

2.3 Practical implementation of EMA testing to bridge superstructures

Three major stages necessary to the design and successful performance ofEMA testing in field applications have been identified, viz:(i) Selection of the actual grid to be used for response measurement.(ii) Selection of the location(s) for positioning the exciter and(iii) details of the performance of the dynamic testing itself:

characteristics of the excitation; time length and frequency of datasampling; number of repeat tests for ensemble averaging, etc.

An "initial" FEM model of the structure to be tested in order to gain somesome preliminary understanding of its dynamic characteristics is necessary toalgorithms that attempt to optimally perform the first two of these tasks/°.(This model is termed "initial" in the sense that it contains parameters whichwe initially would be specifying using design estimates but which we wouldlater be revising based upon actual field results obtained from EMA testing).

A record length of 2048 data points per data channel at a simultaneouschannel sampling rate of 128 Hz corresponding to a time length of record of16 seconds has been chosen as the basic data capture condition forimplementing the EMA testing technique on short span bridges using ourEMA testing system. This choice permits identification of the most important



dynamic modes of bridge structures in the ~0 to 50 Hz frequency band. Up to16 repeat test records of Swept Sine Wave (SSW) forcing (frequency linearly"swept" from 0 to 50 Hz in the 16 second recording period) from the LHS aretaken for the purpose of ensemble averaging of the FRFs using "TSPECTRA"which are suitably identified and stored at the end of data collection.

A typical experiment, involving the fixing of the exciter, the setting up ofsteel plates over the measurement grid on which to locate the accelerometers(say a 7 x 7 grid), and the performance of the testing itself assuming that onehalf of the bridge will be open to clear traffic at periodic intervals betweenrecord capture, can all be performed on the one day.

3 EMA testing: application to several bridge superstructures

3.1 Objectives of EMA testing

The principal objective of the earlier EMA testing by the University ofMelbourne was to determine the viability of the technique as a method forestablishing analytical models of bridge superstructures that could then bereliably used to perform assessments of the Load-carrying Capacity of thesestructures. The relative performance of this testing technique compared to themore traditional static testing methods for establishing such analytical models,was ascertained on a number of these occasions as the bridge superstructuresconcerned were often statically tested by VicRoads immediately or soon aftercompletion of EMA testing depending upon static test vehicle availability.

A second major objective was to gauge whether the EMA testing methodexhibited potential as a "condition monitoring" technique by gaining a "feel"for the level of accuracy to which descriptive parameters of the structuralfeatures of the bridge superstructure could be established using such testing.

The descriptive parameters for distinctive structural features of aparticular bridge superstructure that naturally emerge when using EMA testingare the natural frequencies, mode shapes and associated damping levels thatare determined from the DSMA algorithm. The complex mode shapes(amplitudes and respective phases) determined at the discrete positions chosenfor the measurement grid can be animated via an option in the DSMA softwarepackage to better be able to visualise their properties. Simple inspection ofthese mode shapes during animation permits the appreciation of some basicfeatures of the bridge superstructure to be realised such as: the condition ofrestraint at the abutments and over supports (degree of compliancy) and any"anomalous" features that may be present (such as the effects of localdegradation, deck delamination, or non-linear structural characteristics).

3.2 "Tuning" FEM model parameters

The "initial" FEM model of the bridge superstructure is normally based on afiner mesh of grid points than can be used for the EMA testing (usually anintegral factor of two or three times the density in mutually orthogonaldirections for a regular bridge superstructure), and assumed values forboundary conditions at the supports and for the effective Young's modulus E^for an "uncracked" concrete deck cross-section, amongst others, which havebeen chosen using design assumptions. This FEM model can also be used to



predict the modal properties ("real" mode shapes and associated frequencies)that are dependent upon the structural parameters used in the FEM description.

"Tuning" of structural parameters of the FEM model so as to produce asgood a fit as conveniently possible to the observed (determined from EMAtesting) modal properties of the bridge superstructure concerned leads to amuch more reliable model than the "initial" form as it would better reflect thein-service behaviour and characteristics of the bridge superstructure tested.

The so-called "Modal Index" (MI) value, is an indication of the "goodnessof fit" between any two mode shape vectors and and is given by:

(4)

When and <^ represent the mode shape prediction from the FEM modeland from the EMA testing respectively, ideally one would attempt to "tune"structural parameters in the FEM model to achieve a value of MI^ as close tounity as possible for all modes in the frequency range of interest whilstmatching the modal frequency corresponding to each. (It should be noted thatfor "real" structures, the orthogonality principle suggests that for ( and (representing any two distinct modes, MI = 0).

Experience has shown that a simple "trial and error" sensitivity approachof the predicted modal properties to the structural parameters adopted in theFEM model is normally adequate for performing this "tuning" procedure,especially for short span bridges with simple support conditions,

4 Results of EMA testing of some selected bridges

4.1 First EMA test - adjacent test spans of the La Trobe River Bridge

Figure 1 depicts a sample set of traces for two of the ten accelerometers andthe corresponding excitation force from the LHS (3 tonne mass suspendedfrom beneath the bridge deck at the excitation point) for the case of span#l ofthe La Trobe River bridge, the first bridge tested using our EMA testingsystem. Figure 2 depicts the form of the FRF (amplitude and phase) taken forthe excitation point as an example of such a plot. (The "dots" in Fig. 2represent the experimental values of the "ensemble averaged" FRF and the"solid" line, the variation corresponding to eqn. (2) optimally fitted to theseobservations by program "DSMA").

In the case of the La Trobe River Bridge, comparisons of results for datasets obtained from: "shaker" (actuator vibrates a 3 tonne mass "seismically")and "force" (actuator reacts directly against the ground) modes of vibration onone simply supported span of the bridge (15.2m long x 7.7m wide) and fromforce mode of vibration of an adjacent nominally identical span were possible.

Figure 3 compares results for the "tuned" STRAND6.1" FEM modelmode shapes to those obtained experimentally from the three separaterealisations on the adjacent test spans for the first two vibration modes: asimple flexural mode and a torsional mode respectively, by way of example.Only 28 points were used for the measurement grid so that missing points onthe outside edges of the span are depicted in "dotted" form for the EMA


Computer Methods and Experimental Measurements

results in this figure. (Grid points were principally positioned along thecentrelines of each of the three longitudinal stiffening beams and midwayalong the slab in between these beams at l/6th span increments).

The influence of the shaker mass of 3 tonne in these tests was observed toconsistently produce slightly lower natural frequencies than for force mode ofoperation in all modes with only a "mild" influence on the mode shape, (seeexamples in Fig. 3). Ml values between modes were found to be better than90% for most cases whether performed between separate corresponding EMAestimates or between corresponding EMA and FEM mode shape predictions.

The "tuned" FEM model obtained from the EMA testing was used topredict deflections at measurement points for a number of static load casesperformed independently by VicRoads. The agreement was found to beexcellent.

15.00

Time (s)

Figure 1 Sample traces for accelerometers SSI and SS2 and SSW excitation

180.090.000.0000-90.00-180.0

10.00 20.00 30.00 40.00

10.00 20.00 30.00Frequency (Hz)

40.00

Figure 2 Frequency Response Function for location of SSI in Fig. 1



fo = 8.03 Hz

FEM Mode #1

fo = 10.5 Hz

FEM Mode#2

EMA Mode # 1 (First Span tested Shaker and Force Modes; Second Span Force Mode)

EMA Mode # 2 (First Span tested Shaker and Force Modes; Second Span Force Mode)

Figure 3 Sample modal results for La Trobe River Bridge test spans

This "first off experience in EMA testing using the University ofMelbourne EMA testing system in early 1993, was encouraging and promptedthe continued support of the collaborative EMA test program between theUniversity of Melbourne and VicRoads on an annual basis.

4.2 Results for other bridges with separate simply supported spans

Several other short span (< 20m) simply supported bridges were tested usingthe University of Melbourne EMA testing system. These included:(i) The Yarriambiack Creek Bridge - a bridge with a 32" skew and RC

deck cast integrally with 5 longitudinal stiffening beams for whichindependent data sets were obtained from two separate locations ofthe LHS on the test span*.

(ii) The Campaspe River Bridge - of composite RC beam on steelbeam construction (5 beams) where a single span over thefloodplains was tested

(iii) McCoy's Bridge over the Goulburn River - also of composite RCbeam on steel beam construction where two spans were tested: one15.2m long over the floodplains (3 steel beams) and another -17mlong over the river itself (4 steel beams)

In the case of the Yarriambiack Creek Bridge, all response measurementswere performed using accelerometers mounted on the underside of the bridgedeck with the LHS operating from the top of the bridge. In all of theremaining bridges tested, all of the response measurements as well as theexcitation from the LHS were performed on the top of the bridge deck.

Figures 4 and 5 presents the first mode match from the "tuned" FEMmodel prediction and the EMA test results for the two separate data sets forYarriambiack Creek Bridge (LHS located at position "A" then "B") and forthe remaining simply supported bridge spans listed above, respectively. In allcases MI values very close to unity were able to be achieved for the first modeafter "tuning" parameters in the FEM model whilst exactly "matching" theobserved first mode natural frequency.



EMA (LHS at A)fo= 13.37 Hz£ = 5.37%

EMA (LHS at Brfo= 13.45 Hz

Figure 4 First mode results for Yarriambiack Creek Bridge (separate data sets)

Campaspe River Bridge McCoy's Bridge - 3 beam McCoy's Bridge - 4 beam

Figure 5 First mode results for Campaspe River and McCoy's Bridge spans

The level of agreement achieved with higher modes (mode shape andnatural frequency value), was in most cases very good to excellent (MI betterthan -0.9 and -0.95 and frequencies within -20% and -10%, respectively).Figure 6 presents a comparison of results for an additional three modes of theCampaspe River Bridge by way of example.

4.3 Results for continuous multi-span bridges

The EMA testing of Fuge's and Concongella Creek Bridge, provided anopportunity to apply the EMA testing system to continuous multi-span (3-span) bridge configurations. In the case of Fuge's Bridge an 80 point grid wasused whereas for the Concongella Creek Bridge, a total of 118 points,inclusive of the 8 horizontal acceleration measurement points at mid-height on

Figure 6 Additional modal results for the Campaspe River Bridge



the two pairs of 4 column supporting piers, was adopted for the EMA testing.Figures 7 and 8 provide sample plots of the "tuned" FEM model and

EMA mode shapes obtained for these two bridges. Simple inspection of theEMA mode shapes for both bridges suggests the support conditions at theabutments to be much closer to "pinned" than "en-castre" as assumed in theoriginal design. In addition, local "perturbations" in the animated mode shapesat points "A" and "B" in Fig. 8 were able to be attributed to a discontinuity inthe crash-barrier of the Concongella Bridge and to delamination of itsbituminous overlay, respectively.

That such "anomalous" conditions can be simply evidenced from theDSMA fitted mode shapes is another powerful attribute of the EMA testingtechnique demonstrating that the method has the potential to act as a"condition monitoring" tool.

Figure 7 Sample modal results for Fuge's Bridge

FEM6= 12.6Hz

LHS

Figure 8 Sample modal results for Concongella Creek Bridge

5 Concluding Remarks

The experience gained from the testing of a number of bridge superstructuresin the State of Victoria, Australia, using the University of Melbourne EMAtesting system has demonstrated that the technique is capable of:• producing "tuned" FEM models of the bridges that accurately

describe their in-service performance/behaviour• identifying major (and even minor) structural features such as the

conditions of support at the abutments; delamination of thebituminous overlay from the deck; local discontinuity in the crashbarrier at the edge of the bridge, etc

The value of the testing system to bridge engineers concerned withmaintaining an ageing bridge stock has been obviated.



Acknowledgment

The author would like to acknowledge the active participation of a number ofmembers of the research team in the work reported in this paper, that include:Mr. Hussein Khalaf, Dr Tom Chalko, Dr Vladimir Gershkovich, Mr MarcusAberle, Mr Emad Gad. Financial support and assistance in kind received fromVicRoads Principal Bridge Engineer's Department and the regional offices ofVicRoads relevant to the bridge testing is also gratefully acknowledged.

References

1. Haritos, N., Khalaf, H. & Chalko, T. Modal testing of bridge structuresusing a linear hydraulic shaker, Proc. 13th Australasian Conf. on theMechs. ofStructs. and Mats., Wollongong, July 1993, pp 349-356.

2. Khalaf, H. & Haritos, N. Dynamic testing of the Latrobe River Bridges,Proc. AUSTROADS'94 Conf., Melbourne, Feb., 1994, pp 17.1-17.10.

3. Haritos, N., Khalaf H. & Chalko, T. Modal testing of a skew reinforcedconcrete bridge, Proc. International Modal Analysis Conference IMAC-XIII, Nashville, Feb, 1995, pp 703-709.

4. Haritos, N. & Khalaf, H. Dynamic testing of the Rutherglen Bridges,UNIMELB Report, The University of Melbourne, June, 1995.

5. Haritos, N. & Chalko, T. Dynamic testing of three bridges, UNIMELBReport, The University of Melbourne, May, 1996.

6. Ewins, D. J. Modal Testing: Theory and Practice, John Wiley, NewYork, 1985.

7. Chalko, T., Gershkovich, V. & Haritos, N. Direct Simultaneous ModalApproximation Method, Proc International Modal Analysis ConferenceIMAC-XIV, Dearbourn, February, 1996, pp 1130-1136.

8. TSPECTRA - 16 Channel Spectrum Analyser, User Manual, ScientificEngineering Research P/L, Melbourne, 1992.

9. DSMA - Modal Analysis Program, User Manual, Scientific EngineeringResearch P/L, Melbourne, 1996.

10. Chalko, T., Gershkovich, V. & Haritos, N. Optimal design of modalexperiments for bridge structures, Proc. IXIHth International ModalAnalysis Conference, Nashville, February, 1995 , pp 571-577.

11. Haritos, N., Giufre, A. & Wells, J. Static and dynamic testing of two RCbridges, Proc. Roads'96 Conf, Christchurch, NZ, Sept. 1996, Vol. 3, pp259-274.

12. STRAND 6.1 - Finite Element Analysis Package. Ref. Guide, G+DComputing P/L., Ultimo NSW, Australia, 1992.


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This testing has featured particular implementations of ... · Experimental Modal Analysis testing - a powerful tool for identifying the in-service condition of bridge structures