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11th European Conference on Earthquake Engineering 1998 Balkema, Rotterdam, ISBN 90 5410 982 3

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Experimental dynamic tests on the first structure in the worldisolated with rubber bearings

M. GarevskiInstitute of Earthquake Engineering and Engineering Seismology, University "St. Cyril and Methodius",Skopje, Republic of Macedonia

J. M. KellyEERC, University of California at Berkeley,California, U.S.A.

M. BojadzievIZIIS, University "St. Cyril and Methodius", Skopje, Republic of Macedonia

Keywords: rubber bearings, aging effect, dynamic full scale measurements, natural frequencies,damping, 3D mathematical model

ABSTRACT: The results obtained by experimental dynamic ambent and forced vibration tests on abase-isolated structure are presented and discussed in this paper. Since the isolators of the structurewere placed 28 years ago, one of the goals of these investigations was to discover whether the iso-lators have been affected by aging, i.e., whether there has been a change of the dynamic character-istics of the structure. Rocking motion due to low vertical stiffness of the rubber isolators wasanalyzed.The influence of the reinforced concrete plates covering the seismic gap on the stifnesscharacteristics of the structure was also examined.

Based on these tests, some of the necessery characteristics of the isolators to be used for formula-tion of the 3D mathematicla model of the school were obtained.

1 INTRODUCTIONThe primary school "Pestalozzi" in Skopje built in 1969 is the first building in the world for whichnatural rubber isolators were used for its protection against strong earthquakes (Kelly, 1981, Izumi,1988). Since the isolators on which the school is placed are more than 28 years old, a project is cur-rently being carried out jointly by IZIIS (Institute of Earthquake Engineering and EngineeringSeismology, University "St. Cyril and Methodius, Skopje, Republic of Macedonia) and EERC(University of California at Berkeley) to evaluate the state of the isolators after passing of so manyyears.

The behaviour of the school building under the effect of strong earthquakes shall be analyzed byusing a 3D mathematical model that shall include all the present characteristics of the isolators. Toaccurately formulate this model, it is necessary to consider the effect of aging of the isolators, i.e.,

define their present stiffness characteristics and equivalent damping. For the purpose of definingthe mentioned characteristics, experimental laboratory tests of two isolators taken from the founda-tion of the school building "Pestalozzi" are planned to be performed in the beginning of June 1998.To define the dynamic characteristics, in-situ, ambient and forced vibration tests were performedobtaining thus the fundamental and some of the higher frequencies and the corresponding modeshapes for the three orthogonal directions of the school-building.Two torsional modes of vibrationand equivalent dampings for all the measured mode shapes were also defined.

The results from the ambient and forced vibration tests of the school building "Pestalozzi" aregiven in the subsequent text.

First of all, the school building is described and the characteristics of the bearings are given.Then, the technical data on the equipment used for measuring of the dynamic characteristics of the

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building are presented and the measuring procedures are described. At the end of the paper, someof the results from the performed measurements are presented and discussed.

2 DESCRIPTION OF THE BUILDINGThe building which is subject of the investigations, i.e., the primary school "J.H. Pestalozzi" inSkopje was donated to Skopje by the Swiss Government after the catastrophic earthquake of July26, 1963. The school building consists of several units of different number of storeys and outline.The base isolation system has been applied only for the main school building, while the other unitsare founded in the classical way.

2.1 Description of the structural system of the base isolated partThe main school building consists of a ground floor and two storeys. The proportions of the stric-ture at plan are 13.0 m/61.5 m, while its height is 10.0 m. It represents a reinforced concrete boxsystem. The bearing wall system is composed of shear walls with a thickness of 0.18 m. The floorslab is 0.20 m thick. The building has a strip foundation forming a beam grid which is sufficientlyrigid to sustain all the effects from the upper structure without heavier deformations. Along theedges of the building, there is a seismic gap with a width of 0.3 m. The gab is covered with rein-forced concrete plates.

The structure is base isolated by its placement on special rubber bearings incorporated between

Figure 1. Rubber bearings placed between the strip foundation and the building.

the foundation structure and the first floor slab (Fig.1). There is a total of 54 bearings made ofnatural rubber, with a square shape, a side of 0.7 m and a height of 0.3 m. Each bearing transfersan axial force amounting to approximately 500 kN. The rubber bearings are produced by theSwiss firm HUBER-SUHNER from Zurich. The isolators do not contain reinforcing steel plates.

They are fabricated by gluing together several rubber layers. The vertical stiffness of suchbearings is therefore not much greater than the horizontal one. In addition to the negative effect ofthe low vertical stiffness of the bearings on the possible rocking motion, there is another disadvan-tage of these bearings which is the occurrence of large lateral deformations of the rubber bearingsdue to dead load effects.

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3 DYNAMIC TESTING OF THE PRIMARY SCHOOL "J. H. PESTALOZZI"Dynamic testing of the main (isolated) school building was performed by application of two inde-pendent testing methods:

Ambient vibration method-This method was used for fast obtaining of preliminary results onthe dynamic characteristics - resonant frequencies and mode shapes of vibration of the testedstructure. The vertical vibration of the structure obtained by this method was used as a referent onesince the vibrators used for the forced vibration tests cannot induce vertical displacements.

To perform the tests for definition of the dynamic characteristics of the considered structure, anambient vibration equipment consisting basically of sensors, signal amplifier and recorder wasused.

Four Range seismometers, Model SS-1 produced by Kinemetrics from USA were used as sen-sors. The natural frequency of the seismometer is 1 Hz, while the damping is 0.7 of the critical one.

The signal adjuster model SC-1 was used for signal amplification and simultaneous control ofthe four seismometers. There was a possibility of simultaneous or independent recording at theoutputs and each channel provided a nominal increase of 100.000 times.

The records were processed in situ by a Spectrum Analyzer, Model 3582A.Forced vibration method -This method was applied to define some characteristic vibrations of

the structure in both horizontal directions as well as the torsional one. According to its function, theequipment used for these dynamic tests is divided into two parts:

Equipment for forced vibration generation, and Equipment for recording of the dynamic response of the structure.

To generate forced vibrations on the structure within a frequency range of 0.5 Hz to 9.0 Hz,two vibrators of the type of GSV-101 produced by Geotronix - USA were used. Each of the vibra-tors was provided with an electric motor with a power of 1.1 kW. The intensity of generated forceper vibration exciter was limited to 24.5 kN, depending on the mass of the baskets of the vibrationexciter and the square of the frequency by which they rotate.To induce torsional vibration of thestructure, the vibration exciters worked with a phase difference of () radians.

The dynamic response of the structure was recorded by six accelerometers type A4-0.25 pro-duced by Statham - USA. These accelerometers can record only the horizontal component of thestructural response. The measuring range of these accelerometers is 0.25 in a frequency range of0 to 16 Hz. To record the vertical component of the structural response, three accelerometers typeKistler 8710A50M1 were used in combination with a system for signal amplification type Kistler5134. The measuring range of these accelerometers is 50g in a frequency range of 1 Hz to 5000Hz. The signal recording was done by the same spectral analyzer that was used for the ambient vi-bration tests.

To define the damping coefficients, two methods were used: (1) half-power bandwidth (fre-quency-response) method and (2) logarithmic decrement (time-response) method.

4 EXPERIMENTAL RESULTSThe dynamic characteristics of the building were obtained for two characteristic states: the state ofpresence of reinforced concrete plates covering the seismic gap separating the structure from itssurrounding (state I) and the state of reinforced concrete plates being removed from the seismic gapand free motion of the isolated building in all directions (state II).

These two states were considered to determine the role of these plates in dynamic behaviour ofthe structure, i.e., whether they act as fixation of the structure.

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Fig. 2 displays the plan of the second storey of the structure with marked measuring pointsand position of vibration exciters by which it was excited during the forced vibration tests.

Figure 2. Plan of the second storey of the tested structure with presentation of measuring points and positionof the two vibration exciters.

4.1 Results from dynamic testing of the structure state IResults from the tests performed by applying the ambient vibration method:To define the resonantfrequencies of the structure in both orthogonal directions - transverse (N-S) and longitudinal (E-W), and the vertical one, gauges were placed in the central part of the structure, at the level of thesecond storey, at point "C" (Fig. 2). Gauges were placed at the ends of the structure (measuringpoints 1 and 5) and also at the level of the second storey to define the resonant frequencies of thetorsional vibration. The Fourier amplitude spectra corresponding to the torsional vibrations of thestructure were obtained by combining the signals from both gauges.

Fig. 3 shows the Fourier amplitude spectra recorded on the structure in the corresponding direc-tions of vibrations from which the resonant frequencies were defined. For such defined resonantfrequencies of the structure, the corresponding mode shapes were obtained . However, these andthe mode shapes obtained by other measurements are not presented in this paper due to lack ofspace.

Results from tests performed by using the forced vibration method: In the process of testing, theresonant frequencies of the structure were first of all defined for both orthogonal directions (N- S)and (E-W) and then for torsion. The structural response to the generated excitation, while definingthe resonant frequencies for the N-S and E-W directions, was recorded at several measuring points.

The recorded responses at the points located in the midst of the structure (point "C" or point "3")were considered referent. The referent point for definition of the torsional resonant frequencies

was point "5" on the east side of the structure.The frequency curves presented in Fig 4 were established based on the recorded amplitudes of

structural response to the generated excitations.For such defined resonant frequencies, the corresponding mode shapes of vibration were re-

corded.

. T. 1 . T.2 . T. 3 T. 4 ..T. 6

T. 7 . .T. C

T. 5 .GSV-101GSV-101

8.45 3.58 8.39 8.39 3.58 8.39 8.39 3.58 8.45

61.20

11.

42

W

S

N

E

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Figure 3. Fourier amplitude spectra recorded on the tested structure (ambient, state I).

Figure 4. Resonant curves of the structural response (force excitation, state I).

Frequency (Hz)

10.0

Am

plitude(mV)

2.0 4.0 6.0 8.0

16 0

12 0

80

40

N- Sf = 1.60 Hz

f = 3.76 Hzf = 6.08 Hz f = 6.56 Hz

10.0Frequency (Hz)

Am

plitude(mV)

2.0 4.0 6.0 8.0

40

30

20

10

E-Wf = 2.40 Hz

f = 2.72 Hzf = 4.72 Hz

f = 5.84 Hz

f = 1.60 Hz

10.0

Frequency (Hz)

Amplitude(mV)

2.0 4.0 6.0 8.0

40

30

20

10

Torsion

f = 2.48 Hz

10.0

Frequency (Hz)

Amplitude(mV)

2.0 4.0 6.0 8.0

16 0

12 0

80

40

Verticalf = 1.60 Hzf = 2.72 Hz

f = 3.76 Hz

f = 6.08 Hz

1 .00 1.50 2.00 2.50 3.00Frequency (Hz)

0.00

50.00

100 .00

150 .00

Displaceme

nt(microns)

f = 2.00 Hz

f = 2.48 Hz

= 3.8 % S4+L2

1.60 2.00 2 .40 2.80 3 .20Frequency (Hz)

80 .00

120 .00

160 .00

200 .00

240 .00

Displacement(microns)

f = 1.92 Hz

f = 2.48 Hz

= 9.7 % S4+L2

1.00 1.50 2.00 2 .50 3.00 3 .50Frequency (Hz)

0.00

50.00

100.00

150.00

200.00

250.00

300.00


f = 1.24 Hz = 6.6 %

S4+L2

N- S torsion

E- W

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4.2 Results from dynamic testing of the structure state IIIn this phase of testing of the structure, all the reinforced-concrete plates covering the seismic gapwere removed. In this way, free motion of the structure in all the directions was enabled.

The procedure, the number of tests and the number of measuring points and their locations fordetermination of the dynamic characteristics was identical to that applied in the case when theseismic gap was covered with the plates. Hence the results obtained for both cases could directly becompared.

Results from Dynamic Tests Performed by Applying the Ambient Vibration Method: Fig. 5 pre-sents the Fourier amplitude spectra recorded on the structure in the corresponding directions of vi-bration. In this case also, the mode shapes of the characteristic resonant frequencies were meas-ured.

Results from tests performed by applying the forced vibration method: Based on the recordedamplitudes of the response of the structure to the generated excitations, the corresponding resonant

Figure 5. Furier amplitude spectra recorded on the isolated part of the building (ambient, state II).

frequency curves were established. Certain tests performed for definition of the same frequency

were carried out by different excitation forces for the purpose of evaluating the relation betweenmodification of the resonant frequency and the excitation intensity. Due to the flexibility of theisolators, some of the mode shapes could quite well be excited and even visually observed. There-fore, apart from using accelerometers which is the usual practice in performing these tests, dis-placemement-meters (LVDT-s) were also used.

The frequency curves of the displacement response of the structure are presented in Fig. 6.These were used to directly define the resonant frequencies. The mode shapes for the case of anopen seismic gap were find also.

10.0Frequency (Hz)

Amplitude(mV)

2.0 4.0 6.0 8.0

80

60

40

20

N- S

f = 0.96 Hz

f = 2.24 Hzf = 3.60 Hz

f = 5.52 Hz

10.0Frequency (Hz)

Amplitude(mV)

2.0 4.0 6.0 8.0

80

60

40

20

E-Wf = 1.28 Hz

f = 2.64 Hzf = 3.60 Hz

Amplitude(mV)

2.0 4.0 6.0 8.0

80

60

40

20

Torsionf = 1.04Hz

f =3.5Hz

f =5.44Hz

10.0

Frequency (Hz)10.0

Frequency (Hz)

Amplitude(mV)

2.0 4.0 6.0 8.0

16 0

12 0

80

40

Vertical

f = 2.56 Hz

f = 3.60 Hz

f = 4.18 Hz f = 6.16 Hz

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Figure 6. Resonant curves of the structural response (force excitation, state II).

4.3 Mode shapes and dampingsBased on the data obtained from the performed testing of the structure, the following is concluded:

The values of the frequencies of the fundamental vibration modes in certain directions for state Iare: for transverse direction (N-S): f

I1f = 1.24 Hz (forced vibration tests) and f

I1a = 1.6 Hz (ambient

vibration tests); for torsion: fI2f = 1.92 Hz (forced vibration tests) and f

I2a= 2.48 Hz (ambient vibra-

tion tests); for longitudinal direction (E-W): fI3f = 2.48 Hz (forced vibration tests) and f

I3a= 2.40 Hz

(ambient vibration tests) and for vertical direction: fI4a = 2.72 Hz (ambient vibration tests).

The values of the frequencies of the fundamental vibration modes in certain directions when theplates covering the seismic gap are removed (state II) are: for transverse direction (N-S) = f

II1f =

0.76 Hz (forced vibration tests) and fII1a = 0.96 Hz (ambient vibration tests); for torsion: fII

2f = 0.84Hz (forced vibration tests) and f

II2a = 1.04 Hz (ambient vibration tests); for the longitudinal direc-

tion (E-W): fII

3f = 0.88 Hz (forced vibration tests) and fII

3a = 1.28 Hz (ambient vibration tests) andfor vertical direction: f

II5a = 2.56 Hz (ambient vibration tests).

Apart from the fundamental vibration modes, some of the higher modes were recorded for stateII. The frequency of f

II4f = 2.08 Hz represents the second natural frequency of the structure in the

(N-S) direction, while fII

6f= 3.20 represents the second natural frequency at torsional excitation.The second natural frequency of the structure in the E-W direction is very close (almost equal) tothe corresponding frequency at torsional excitation amounting to f = 3.20 Hz, i.e., f

II7f= 3.28 Hz.

0.40 0.80 1.20 1.60 2.00 2.40Frequency (Hz)

0.00

100.00

200.00

300.00

400.00


f = 0.88 Hz

= 7.3 %

S4+L4

0.50 1.00 1.50 2.00 2.50Frequency (Hz)

0.00

200.00

400.00

600.00

800.00

1000.00


f = 0.76 Hz

f = 2.08 Hz

= 3.8 % S4+L4

0.50 1.00 1.50 2.00 2.50Frequency (Hz)

0.00

400.00

800.00

1200.00

1600.00


f = 0.84 Hz

= 5.4 % S4+L4

E- W

torsionN-S

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The recorded higher frequencies of fII8f

= 3.44 Hz, f

II9f= 5.28 Hz, f

II12f= 5.92 Hz in the trans-

verse direction and fII

10f= 5.28 in the longitudinal direction, i.e., fII

11f = 5.52 Hz at torsional vibra-tions are associated with some of the higher mode shapes of vibration of the structure.

The frequencies and the corresponding damping measured by applying both methods are pre-sented in Table 1.

Table 1. Presentation of the resonant frequencies and damping of the fundamental modes of the structure .

Mode Type State I State II

No. Ambient Forced vibration Ambient Forced vibration

f f Damping (%) f f Damping (%)

(Hz) (Hz) HP* LD** (Hz) (Hz) HP* LD**

1 first (N-S) 1.6 1.24 4.0-6.6 5.9 0.96 0.76 3.8 3.52 first (E-W) 2.4 2.48 3.8 3.6 1.28 0.88 7.3 6.33 torsion 2.48 1.92 9.7 - 1.04 0.84 5.4 3.04 vertical 2.72 - - - 2.56 - - -5 second (N-S) 2.08 - 2.96 sec.torsion 3.00 2.0 2.57 second (E-W) 3.28 5.5 3.0

*Half-power bandwidth method.

**Logarithmic decrement method.

.

Analyzing the recorded mode shapes, it can be said that the translatory components dominateover the first two modes of vibration of the school building (f1f

II= 0.76 Hz, f2f

II= 0.88 Hz, State II).

Characteristic for the second vibration modes in E-W direction (f4fII

= 2.08 Hz) and N-S direc-tion (f7f

II= 3.28 Hz) is the rocking motion which is more pronounced for the mode with a frequency

of f4fII

= 2.08 Hz. Such vibration modes occur as a result of the low vertical stiffness of the rubberbearings.

5 DISCUSSIONS AND CONCLUSIONSFirst of all it may be said that the assumption about the rocking motion of the school buildingproved to be true by the recorded mode shapes. The ratio between the vertical and the horizontalfrequency indicates that the vertical stiffness of the isolators is only 3 times greater than their hori-zontal stiffness. It can therefore be said that the vertical stiffness is very low, having in mind thatnew isolators can possess more than 1000 times higher vertical stiffness than the horizontal one.

Since the fundamental modes for the two horizontal directions are not purely translatory (thereare rotation components), their frequencies are not identical. There occurs a difference of 18% inthe values of their frequencies.

The obtained results on damping presented in Table 1 point to similar damping obtained by bothmethods. The damping values (State II) lead to the conclusion that the internal damping of thebearing is small since the coefficients of viscous damping for the fundamental vibration modes are

3.5%, 3% and 6.3% of the critical one in the N-S and E-W directions as well as torsion respec-tively.

Comparing the values of resonant frequencies for the fundamental vibration modes (Table 1) re-corded in both the considered cases (the structure with and without the covering plates), it is con-cluded that the plates have a considerable effect in both orthogonal horizontal directions and rota-tion of the structure about the vertical axis. It can also be concluded that the stiffness characteristicsare considerably altered since the mass of the structure is constant. For instance, the increase instiffness due to the presence of the plates in the N-S direction is 39%. In the E-W direction, it is65%, while the torsional stiffness is increased for 56%. Only the frequencies dominating in theFourier amplitude spectra referring to the vertical direction of vibration are not deviated from theoriginal ones (from f

Ia4= 2.72 Hz to f

IIa4 = 2.56), which is a difference of about 6%. Hence, it is

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concluded that the reinforced concrete plates over the seismic gap of the structure stiffen the struc-ture in the horizontal direction only and do not have any effect in the vertical direction.

Finally it may be concluded that the results obtained from these investigations shall contribute alot to the achieving of a greater accuracy in preparing the mathematical models to be formulated foranalysis of the school building. This particularly refers to the mathematical model for State I which

cannot be formulated without such measurements. The measured dampings are also necessary forthe formulation of the models for State I and State II.

ACKNOWLEDGMENT

This publication is based on the investigations sponsored by the Macedonian US. Joint Fund incooperation with the Macedonian Ministry of Science and US - NIST under project number056/600. The authors of this paper and indebted to these two institutions for the correct evaluationof the necessity for these investigations.

REFERECES

Bridgestone corporation, 1991.Rubber technology for seismic isolation. Proc.,11th international conf. onstruc. mech. in reactor technology: 45-56, Tokyo, Japan.Garevki, M. & Kelly, M. J. 1996. Evaluation of the proper functioning of the rubber isolators of the primary

school Pestalozzi in Skopje under strong earthquake. U.S.- Macedonian science and technology program-project proposal.

Garevki, M., Kelly, M. J. & Bojadjiev, M., in prep. Evaluation of the proper functioning of the rubber isola-tors of the primary school Pestalozzi in Skopje under strong earthquake : part I- Full-scale dnamic tetingof the first rubber base-isolated building in the world. Report.

Izumi, M., 1988. Base isolation and passive seismic response control. Proc., 9th world conference onearthquake engineering, vol. VII: 386-396,Tokyo-Kyoto, Japan,.Kelly, J.M., 1981. The Development of base isolation for the seismic protection of structures. Report No.UCB/EERC-81/01, Earthquake engineering research center, University of California, Berkeley, California.

Kelly, J.M., 1988. Base isolation in Japan. Report No. UCB/EERC-88/20, Earthquake engineering researchcenter, University of California, Berkeley, California.Kelly, J.M., 1993. Implementation of base isolation in the United States. Proc., 6th symposium on seismic,

shock and vibration isolation, ASME pressure vessel and piping conference: 159-170, Denver, Colorado.Staudacher, K., 1982. The Swiss full base isolation system (3D) for extreme earthquake saifty of structures.Proc. of the 1982 convention of the structural engineerin asotiation of Californija, Sacaramento, California.

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