Ambient Vibration Instrumental Investigations on Full Scale Structures

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ABSTRACT: The ambient vibration tests describe the linear behavior of structures, since the amplitudes of vibration are small.They can be used also to describe the linear behavior of damaged structures and can help to perform more accurate structuralmodels of analysis in the design process of strengthening. Therefore, the development of instrumental methods for in-situmeasurement of full-scale partially damaged structures is of considerable interest. The ambient vibration investigations areperformed for use in health monitoring and in structural control studies within the technical assessment of different kinds ofstructures. During the past ten years, the Romanian National Center for Earthquake Engineering and Vibration (RNCEEV) has

performed many ambient vibration tests on new and old buildings, chimney stacks and dams. An ambient vibration test duringthe demolition of two wings of an old building is also presented on short in the paper. It is intended to present the results ofsome of these investigations and to show how these can be used in the technical assessments of existing structures. Finally, asDr. Gary C. Hart pointed out, perhaps the most important reason for measuring full-scale structure response is that it recordsthe real motion of the building, and therefore, documents what really happened and not just what the computer structural modelof analysis says happened.

KEY WORDS: Ambient vibration; Seismic noise; Damage detection; Eigencharacteristics of vibration; Full-scale experiments.

1 INTRODUCTION

The main concern of the present paper is the dynamic testingof full-scale structures from the point of view of knowledge

needed for earthquake-resistant design. The structural modelsof analysis used in the design process of buildings andengineering structures are idealizations conceived torepresent the response of real structures to loads generated bystrong earthquakes. The most sophisticated and brilliantstructural analysis methods are easily defeated by poor,

inaccurate, or inappropriate data. Professor Mete A. Sozen, ina summary of a talk about the importance of the structuralanalysis entitled A Way of Thinking, has stated: Today,

ready access to versatile and powerful software enables the

structural engineer to do more and think less.In order to develop a better understanding of the earthquake

motions of buildings and engineering structures it is desirable

to have experimental measurements of the actual motions andof stresses and strains which occur during strong earthquakes.

Information of this kind is difficult to obtain, as in mostlocations earthquakes are infrequent and strong motions occurat large time intervals.

System identification using ambient vibration measurementpresents a challenge requiring the use of special identificationtechniques, which can deal with very small magnitudes of

ambient vibration contaminated by noise without theknowledge of input forces.

The structural models of analysis of buildings andengineering structures can be verified by conducting full scaleambient and forced vibration experiments. Both of these can

be used to identify the dynamic characteristics of a structuralsystem, i.e. eigenfrequencies of vibration, damping ratio andmode shapes.

The beginning of the ambient and forced vibration tests ofstructures dates 1936 and is due to the U.S. Coast andGeodetic Survey for determining the fundamental periods ofvibration for some high-rise buildings, and 1937 for

determining the fundamental periods of vibration of somebridges [1].

A procedure for obtaining information on the physicalproperties of the buildings and engineering structures was toperform dynamic measurements while the structure wasexcited into motion by a shaking machine installed in thestructure and which exerted dynamic forces upon it. The

forced vibration testsrequired large forces to produce useful(larger) response amplitudes of full-scale structures. The

vibration exciter (the shaker) was usually located on the top ofthe building. This led to more prominent excitation of themodes of vibration that had large amplitudes at the higherlevels of the structures. The paths of waves propagating

through the structure are different from those in case ofearthquake ground shaking, ambient noise, or wind excitation,

and cautious interpretation of the results is required to takesuch differences into account [2], [3].

The ambient vibration testsdescribe the linearbehavior ofbuildings and engineering structures, since the amplitudes ofvibration are small. When a structure is behaving linearly, themaximum response will depend on the fundamentaleigenperiod of vibration and on the magnitude of the actualdamping. An advantage of the ambient vibration over theforced vibration instrumental investigations is that usually

only light equipment and smaller number of operators arerequired. An excellent literature review on the subject of

ambient vibration testing which illustrates the state-of-the-artin the application of the ambient vibration method was written

Ambient vibration instrumental investigations on full scale structures

Ion Vlad1, Mihnea Vlad11Romanian National Center for Earthquake Engineering and Vibrations, TUCEB, 124 Lacul Tei, sector 2, Bucharest, Romania

email: [email protected], [email protected]

Proceedings of the 8th International Conference on Structural Dynamics, EURODYN 2011

Leuven, Belgium, 4-6 July 2011

G. De Roeck, G. Degrande, G. Lombaert, G. Muller (eds.)

ISBN 978-90-760-1931-4

2424


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4 AMBIENT VIBRATION APPLICATIONS

4.1 Structural monitoring of a new building

The instrumental investigations of the dynamic characteristicsof a new office building, during different stages of itsconstruction, are presented in this paragraph.

The architectural project established that this new buildingshould have, in the horizontal plane, the shape of an

equilateral triangle, having the sides of 50 m. In each of thethree peaks of the triangle structural cores (A1, A2 and A3)were placed, each of them being realized with structuralreinforced concrete walls.

The office building has a dual type structural system,consisting of a reinforced concrete subsystem and a steelsubsystem.

The superstructure of the building, of composite type, hastwo different parts:

a reinforced concrete structural subsystem formed by thethree structural cores placed in the peaks of the equilateral

triangle, their main role being that of assuring the lateralstrength of the building to the seismic loadings;

a steel structural subsystem, having a structural rolemainly towards the gravity loadings, and assuring thetransmission of the horizontal seismic loadings to thethree resistant and rigid vertical reinforced concrete cores.

The substructure of the buildingand its foundation structurehas a unitary concept able to ensure the base fix jointing of thevertical structural elements (the columns and the structuralcores).

The main stages of the ambient vibration measurementswere carried out after the construction of the reinforced

concrete structural cores (the first three tests), after theconstruction of the steel structural subsystem (the fourth test)

and after the complete finishing of the office building. Theprincipal goalsof these investigations were that of verifyingthe accuracy of the structural models of analysis of the threereinforced concrete cores after their individual construction bysteel sliding formwork, after the achievement of the steelstructural subsystem and after the complete erection of the

office building [6]. A complete modern Kinemetrics dataacquisition system was used and alternative settings of sensors

(SS-1 Ranger seismometers) were performed during all abovementioned stages.

The structural analysis of this office building was performed

using the ETABS and ANELISE 2D software.In what concerns the structural models of analysis with

finite elements, by using ETABS, a natural period for thefundamental mode of vibration equal to 0.4024 s (Figure 2)was obtained, and by using ANELISE 2D a value equal to

0.4270 s was obtained.The examination of the natural period for the fundamental

mode of vibration for the structural core (A1), based onrecording of its vibrations (T1 = 0.43 s), shows that the structuralmodels of analysis conceived for each software were correctlycalibrated. In Figure 3 it is shown how the 3 SS-1 Rangerseismometers recording velocities were positioned at the top

level of each core, in separate configurations corresponding to

the first three stages, and Figure 4 presents samples of thetime domain and the corresponding amplitude Fourier spectra.

Figure 2. First eigenmode of vibration.

Figure 3. The first three stages of ambient vibrations tests.

Figure 4. First stage (core A1). Ambient vibration testing.Time domain and amplitude Fourier spectra representations.

Proceedings of the 8th International Conference on Structural Dynamics, EURODYN 2011 2426


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Similar results for the other two structural cores (A2) and (A3)were instrumentally obtained. In the fourth stage of theinstrumental investigations (after the construction of the steelstructural subsystem), a shortening of the fundamental eigenperiodof vibration was observed (0.41 s) and, finally, after all floor

structures (steel deck type) and curtain walls were mounted,

the overall fundamental eigenperiod of vibration of the officebuilding was essentially in the same range [7]. In Figure 5location of sensors during the final stage are presented.

Figure 5. Location of sensors during the final stage.

Samples of the time domain and amplitude Fourier spectrarepresentations are presented in Figure 6.

Figure 6. Final stage. Ambient vibration testing; velocities.Time domain and amplitude Fourier spectra representations.

4.2 Ambient vibration testing in case of existing buildings

The building under discussion is one of the eight bodies

pertaining to the Emergency Hospital Bucharest designed in1967, known as body C2, having nine levels (basement,

ground floor and seven floors) and an irregular L-shaped planconfiguration, as seen in Figure 7 [8].

Figure 7. View of the North-East wing of body C2.

The structural system of the building consists of cast-in-place reinforced concrete moment resisting frames, with in-filled walls of unreinforced masonry. The floors are typically

reinforced concrete two-ways slabs which extent to concreteframe beams. The beam spans are different in the two wingsof the L-shaped building, with reasonable dimensions able tosupport the gravity loading (depths on the order of 40 to 60cm). The cross-section of the columns, adapted for theparticular in-plan shape of this building, varies. Theunreinforced masonry is used for constructing external

structural walls and internal partition walls.The foundation system of the building consists of two

subsystems, each located under the two individual wings:

a system of individual footings made of plain concreteand reinforced concrete blocks interconnected by

foundation beams for one wing;

foundations type colonnade filled with concrete alsointerconnected by foundation beams, for the other wing.

A technical assessment of the building was necessary to be

carried out as the hospital has been severely damaged duringthe March 4

th, 1977 Vrancea earthquake. The technical

assessment showed high values for the fundamentaleigenperiods of vibration on both directions, being known thatin the design process, the computed periods are likely to belonger than those of the actual structure.

The main objectives of the instrumental investigationscarried out before retrofitting the C2 building, were related

to the following aspects:

establishing of modal dynamic characteristics fromambient vibration tests (eigenperiods, damping);

identification of possible elastic and/or inelasticdiscontinuities induced by cumulative damage;

pointing out the vulnerable potential zones to future

seismic actions;

dynamic characterization of the structural properties ofthe whole building, with the intent to diagnose its own

dynamic identity.



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The acquisition of the experimental data was also achievedwith six SS-1 Ranger seismometers, widely recognized asexcellent short-period field instruments and a VSS-3000, afully portable acquisition system designed for ambient andforced vibration field measurements (Kinemetrics). The

location and orientation of the sensors, installed at the last

floor, is presented in Figure 8.Typical time domain velocities and corresponding

amplitude Fourier spectra are shown in Figure 9.

Figure 8. Body C2 (upper floor). Location of sensors before

retrofitting the hospital wing.

Figure 9. Body C2. Ambient vibration testing; velocities.

Time domain and corresponding amplitude Fourier spectrarepresentations, before retrofitting the hospital wing.

After performing the entire program of instrumental

investigations the following results were obtained:

the fundamental eigenperiod on the longitudinal directionof measurement was T1,L = 0.53 s, while the fundamental

eigenperiod on the transversal direction of measurementwas T1,T = 0.49 s;

the fundamental eigenshape (vibration deformationpattern, as shown in Figure 10);

the values of the fraction of critical damping obtained byspecific processing pertained to the interval 35%;

the asymmetrical shape in plan of the building led tosignificant rotational motions and modal coupling(T1,TORSION = 0.36 s);

the reinforced concrete floors of the building resulted tobe stiff and strong enough to distribute lateral loads in the

floor to the lateral load resisting elements;

the building presented a high degree of flexibility on bothdirections.

Figure 10. Body C2. Fundamental shape of vibration, before

retrofitting the hospital wing (transversal direction).

Thus, the analysis of the recorded data emphasized a widefrequency content characterizing the recorded signals, a clearindication that the existing building did not have a steadydynamic identity.

The strengthening solution concept consisted of the

shortening of the computed fundamental eigenperiods ofvibrations, together with the increasing of the strength

capacity of the building. An adequate strengthening on bothdirections of the existing structural system, doubled by a newadditional structural system, appeared to be the only rationalsolution to be adopted.

Considering all technical aspects that were emphasizedduring the technical assessment, together with the owner-

imposed restriction for the undisturbed continuity of the hospitalactivity, a set of technical solutions were adopted as follows:

the introduction of reinforced concrete structural walls

with coupling beams, disposed along the perimeter of theexisting building, as well as the introduction of one

structural wall at the interior, on the transversal direction;these walls are connected together with the existing

structural system by installing epoxy resins chemicalconnectors at the floor beam levels, and sometimes on thecolumns;



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the addition of an extension on the N-E corner of the

building (a new wing), connected with the existing

structural system of the building; two more reinforcedconcrete structural walls were placed in this part of thebuilding with the intent to improve the general capacityand structural stiffness of the new building, on both its

directions.

After the strengthening of body C2 the objectives of theinstrumental evaluation of the building were similar to the

above mentioned ones, though additional tasks wereconsidered:

the verification of the accuracy of the results obtained inthe numerical investigation of the structural model of

analysis used in the design;

the identification of dynamic structural properties of theentire building;

the verification of the joint work (the connection)between the existing and the new structural systems;

the elimination the initial deficiencies of the building(shape and structural configuration) generated by the

initial layout.

The locations and the orientations of the sensors were similarwith those employed in the investigation of the old bodybuilding. For the new extension, new locations wereestablished for the seismic sensors, one of the layouts beingshown in Figure 11. The new fundamental eigenperiods, on

the longitudinal and transversal directions of measurement,are T1,L = 0.39 sand T1,T = 0.34 s, respectively. Some samplesof the outcome obtained during the numerical processing ofthe records are given in Figure 12 (with red line old

retrofitted building; with blue line the new extension).

Figure 11. Body C2 (upper floor). Location of sensors afterperforming the strengthening by extension.

As it can be observed, before strengthening the records put toevidence a large frequency band, after the strengtheningprocess the records showed a narrowband of frequencies, so

one can speak of an elastic and homogeneous behavior onboth directions, in ambient vibration conditions. The obtained

frequencies and mode shapes were determined for smallamplitude vibrations and, therefore, indicate the structuralbehavior in the range of linear response.

In Table 1 the evolution of the fundamental eigenperiods/eigenfrequencies of vibration after performing the complexinstrumental program, before and after the retrofitting of thebuilding, is synthetically presented.

Figure 12. Body C2 (retrofitted building). Samples ofvelocity records and corresponding Fourier amplitude spectra.

Table 1. Body C2. Fundamental eigenvalues, before andafter retrofitting, from ambient vibration testing.

Fundamental eigenvalues

DirectionEigenfrequency

(Hz)

Eigenperiod

(s)

L 1.90 0.53

T 2.05 0.49Old

buildingTorsion 2,80 0.36

L 2.62 0.38

T 3.07 0.33

Newretrofitted

building Torsion

As a conclusion, the instrumental investigations confirmed thevalidity of the structural model of analysis, and quantified the

efficiency of the design process.

4.3

Ambient vibration testing in case of engineering

structures

This paragraph focuses on the instrumental investigations

carried out in view of identifying the eigencharacteristics of a

250 m high reinforced concrete chimney stack, erected byusing sliding forms [4].



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The number of measuring points was established at theintermediate bridges along the height of the chimney stack, ona horizontal radial direction, as shown in Figure 13. Thevibration sources considered were: microseisms combinedwith traffic and the in-plant operation of the equipment in the

vicinity of the chimney stack.

The time domain representations (velocities and displacements)were performed in view of getting an overall image of thespatial motion of the ensemble chimney stack foundation.Typical time domain representations and the correspondingamplitude Fourier spectra are shown in Figures 14 and Figure15. Long time intervals of time were recorded, thuscontributing to a higher resolution of the results. Afterprocessing and interpreting the data obtained by instrumental

investigations, the values of the eigenfrequencies/eigenperiodsof vibration corresponding to the first five eigenmodes ofvibration were obtained. These measured values aresummarized in Table 2.

Figure 13. General view of a 250 m chimney stack and thelocation of the SS1 Ranger seismometers.

Figure 14. Running Fourier spectra representation (+246 m).

Figure 15. Ambient vibration testing; displacements.Time domain and amplitude Fourier spectra representations.

Table 2. Chimney stack (250 m).

Eigenmodes of vibration

Mode of vibration no. 1 2 3 4 5

Eigenfrequency (Hz) 0.24 1.10 2.44 3.03 4.88Eigenperiod (s) 4.14 0.91 0.41 0.33 0.20

Considering the spectral composition of the Vrancea

earthquakes (characterized by intermediate focal depths) towhich the amplification of the dominant components

correspond to periods in the range 11.6 s, the chimney stack

in discussion presents a relatively reduced degree ofvulnerability, taking into account the value of the fundamental

eigenperiod of vibration (T1 = 4.16 s).To this tall chimney stack, the ratios between the first three

eigenperiods of vibration, instrumentally obtained, correspondto the theoretical established values in the technical literature

(T2 0.25 T1; T3 0.10 T1). Considering the eigenvalues andthe eigenshapes of vibration one can state that, in the actualtechnical state, the structural system of the chimney doesntshow inertial and elastic discontinuities. Figure 16 illustrates

the first three eigenmodes of vibration of the high reinforcedconcrete chimney stack, identified by means of instrumental data.



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Figure 16. Chimney stack. Eigenmodes of vibration.

The instrumental investigations allowed us to assign to the

chimney stack a well-defined dynamic identity, as thedynamic eigencharacteristics corresponding to its eigenmodesof vibration pertain to an expected range of results.

4.4 Ambient vibration tests and controlled explosions

RNCEEV has monitored the partial demolition by controlledexplosions of a commercial complex, which consisted in threemain units, separated one from another by aseismic joints (15

cm). RNCEEV has accomplished the dynamic monitoring ofthe structural system of the central part, named Body B

(reinforced concrete moment resisting frame type with aspecial steel structure dome), during the Body A and BodyC (reinforced concrete moment resisting frame types)

demolition by small controlled explosions (Figure 17).

Figure 17. Photo during the demolition of Body C.

The records have been carried out in the central unit withdome (that was kept by the owner), taking into account thefollowing vibration sources: ambient vibrations and vibrationsinduced by the explosions during the demolition of the bodiesA and C (Figure 18).

The small controlled explosions had two simultaneous

effects: the destruction of the gravity loadbering of thestructural systems of the lateral bodies, by the collapse of the

columns at the basement level, and the generation of a soilvibration, similar to an earthquake motion. After each

explosion a gravitational collapse mechanism was initiated, whichconstituted a second shock applied to the soil, having a muchsmaller intensity and being non-simultaneous with the explosions.

Figure 18. Dome level: maximum recorded values during thedemolition of Body C.

5 CONCLUSIONS

The use of the ambient vibration measurements has beenproved as a quick, efficient and economic method for thedetermination of periods and associated damping. This papershows the potentialities of the experimental techniques based

on in situ records, with highly sensitive dynamic sensorslocated at well established levels and positions in an existingstructure, which can capture with great accuracy the maintrends of the way the structure is vibrating. These show greatadvantages in terms of effectiveness, accuracy and cost.Besides informing on the eigencharacteristics of a givenbuilding, or an engineering structure, and from this point

allowing some indications on the possible resonant effectswith soil, the knowledge of eigenperiods is of great

importance to calibrate structural models of analysis.

REFERENCES

[1] D.S. Carder, Observed vibrations of buildings,BSSA, Vol.26, No.3, 1936.[2] S.S. Ivanovi S.S., M.D. Trifunac and M.I. Todorovska, Ambient

vibration tests of structuresA review, ISET Journal of Earthquake

Technology, Paper no. 407, Vol. 37, No. 4, pp. 165-197, 2000.[3] J.E. Luco, M.D. Trifunac and H.L. Wong, On apparent change in

dynamic behaviour of a nine-story reinforced concrete building, BSSA,

Vol. 77, No. 6, pp.1961-1983, 1987.[4] I. Vlad, Ambient vibration measurements. Current stage and

perspectives,Structural Monitoring and Status-Dependent Maintenance

and Repair of Constructed Facilities, Proceedings of Workshop

MEMSCON Bucharest, APC Publishing House, 2011.[5] M.D. Trifunac, S.S. Ivanoviand M.I. Todorovska,Apparent periods of

a building, I: Fourier analysis, Journal of Structural Engineering, Vol.

127, No.5, 2001.[6] I. Vlad and M. Vlad, Determination of modal parameters of structures

by ambient vibration measurements, 14ECEE, Ohrid, Macedonia, 2010.

[7] I. Vlad, M.N. Vlad and H. Sandi, Instrumental data and analytical

considerations on the performance of a peculiar structure. A case study. ,

1stEC on Earthquake Engineering and Seismology, Geneva, 2006.

[8] I. Vlad and M. Vlad, The Influence of the 1906 San Francisco

earthquake on seismology and earthquake engineering in Romania, 8th

US National Conf. on Earthquake Engineering, San Francisco, 2006.

[9] I. Vlad and M.N. Vlad, Health monitoring of a building during thedemolition of its lateral wings by controlled explosions, 3rdEuropean

Conference on Structural Control, 3ECSC, Vienna, Austria, 2004.


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Ambient Vibration Instrumental Investigations on Full Scale Structures