Experimental Testing of Buildings in Skopje After the Catastrophic Earthquake IZIIS Experience

L. Krstevska & LJ. Tashkov Institute of Earthquake Engineering and Engineering Seismology, University "Ss Cyril and Methodius", Skopje, Republic of Macedonia

ABSTRACT: Presented in the paper is the activity of the Institute of Earthquake Engineering and En-gineering Seismology (IZIIS) in experimental testing of high-rise buildings in Skopje that have been built after the catastrophic earthquake. The main objective of the testing was definition of the dynamic behaviour of the structures depending on the applied structural system and their height. Forty build-ings have been tested applying forced vibrations or ambient vibration methodology for full-scale in-situ testing of structures. In this way, important data have been obtained for definition of their seismic stability and earthquake resistance. Some characteristic results obtained during testing of representa-tive structural systems are also presented in this paper.

KEYWORDS: Experimental testing, natural frequencies, mode shapes, damping, forced vibrations, ambient vibrations

1 INTRODUCTION

Immediately after the earthquake an intensive construction of individual GF and GF+1 buildings has started in Skopje. Most of these buildings were constructed of light-weight precast panels for fast and easy construction. Thus, within one year more than 14.000 individual GF houses were constructed in 12 satellite settlements in the close vicinity of Skopje, capable to accommodate about 100.000 people.

According to the Master Plan, the construction of the "City Wall" started in the very center of the city in 1969, consisting of multistorey residential buildings, constructed as two types: residential blocks of GF+M+6 stories and residential towers of GF+M+12 stories. Construction of another large housing complex for accommodation of 80 000 inhabitants ("Jane Sandanski" complex), started in 1975. In general, there were two types of structures: residential blocks and residential tower buildings. In any place of the new settlements there were other types of structures. A large part of structures be-longed to the type with reinforced concrete walls, while the other large part of structures belonged to the combined type consisting of reinforced concrete walls and reinforced concrete frames. In these set-tlements there were also prefabricated buildings of precast panels constructed in the "Karpos" factory in Skopje.

According to the international aseismic design practice, the resonant frequencies, mode shapes and damping coefficients of the structures have been considered as the most important parameters in aseismic design. Based on that fact, an experimental procedure for definition of dynamic properties of the structures has been introduced in Yugoslav design practice since the year of 1970. This procedure has been very popular in USA and Japan in that time. IZIIS provided two shakers from USA and started with testing of large series of structures (buildings, dams, bridges, schools, etc.). Since the year 1970 till now, more than 150 structure have been tested by this method.

In order to define the dynamic behavior of the buildings with different structural systems, number of storeys and used material, many experimental tests have been performed in Skopje since 1970. More than 40 buildings in different settlements of Skopje have been tested (Ostrovo, Novo Lisice, Aerodrom, Zelezara, Hipodrom etc., Table 1). Besides forced and ambient vibration testing, additional

investigations have been undertaken such as: definition of the site parameters (local soil conditions), instrumentation of structures by strong motion accelerographs etc. These data (besides the data ob-tained by dynamic analysis performed in design stage, geomechanical testing and quality control of construction material) enable more realistic definition of seismic stability and resistance of structures.

Table 1. Different structural systems in Skopje tested experimentally by full scale testing methods forced and ambient vibration

Structural system No. of tested buildings

RC Frames 17

RC Frames with central core 4

RC Frames and RC Diaphragms 8

RC Diaphragms 9

RC Prefabricated Large-panel system 2

Steel Frame Structure 1

Some of the results obtained for characteristic buildings tested experimentally by use of the full scale method for definition of dynamic properties of particular structure (resonant frequencies, mode shapes, damping coefficients and soil-structure interaction effects) are presented below.

2 IN SITU FULL SCALE TESTING OF TYPICAL STRUCTURAL SYSTEMS

2.1 In situ testing by ambient and forced vibration testing methods

For determination of the actual state of the buildings, i. e. definition of their dynamic characteristics: natural frequencies, mode shapes of vibration, damping coefficients and soil-structure interaction, two different experimental testing methods are applied: ambient vibration testing method and forced vibra-tion testing method.

The ambient vibration testing procedure is a testing method based on recording and processing of the response of structures to wind and other ambient excitations. The advantages of this method are the light equipment easy for transportation and management, Fig.1 and the fact that it doesn't affect normal functioning of the structure and can be applied on structures in use. The experimental and theoretical procedure is based on the assumption that the exciting force is a stationary stochastic proc-ess with relatively flat amplitude spectrum. The testing consists of real time recording of vibrations and processing of records by means of the Fast Fourier Transform i. e. obtaining of Fourier Amplitude Spectra. The peaks on the spectra define the natural frequencies of the structure, and the mode shapes are obtained by normalization of each recorded spectrum amplitude for different levels, in respect to the amplitude of the reference point at the corresponding frequency. The damping ratio for each mode shape can be obtained directly from the amplitude spectrum applying the half power method which is based on the difference between the response magnitude at resonance and at particular frequencies near the resonant frequency.

During dynamic testing by forced vibrations, harmonic sinusoidal force is generated on the struc-ture by specially constructed vibrators and/or shakers, Fig.1. By gradually increasing of excitation fre-quency and measuring the amplitudes of the response of the structure in selected points, frequency re-sponse curves can be constructed and resonant frequencies for the structures defined. Mode shapes of vibration are obtained measuring the response amplitudes at resonance state in the selected points along the height of the building and normalizing them to the amplitude of the reference level. The damping ratio can be obtained by two methods: the half power method (mentioned above) and the logarithmic decrement method. The second method is based on recording of free damped vibration in which the structure enters after the steady state vibration in resonance, by sudden interruption of the applied force.

one of the GSV-101 shakers

equipment for ambient vibration measurements

Figure 1. Equipment for full scale testing of structures

2.2 Characteristic examples for tested buildings in Skopje

2.2.1 Example 1. Five storey large pannel building

The building is located in Karpos 4 settlement. It is a prefabricated large panel system having 12 transversal and 3 longitudinal diaphragms in plan (Fig. 2). It was tested in 1971 by use of the forced vibration testing method. The obtained mode shapes for the two orthogonal directions are presented in Fig. 2 also. Some of the conclusions obtained from the results show that soil conditions have a domi-nant influence on the dynamic characteristics of the structure, especially the rotation of the foundation. The analysis showed unfavorable influence of local type of earthquakes.

Figure 2. Example 1 - Five storey large pannel building in Karpos 4 Settlement

2.2.2 Example 2." ZOIL Makedonija" Building

Dynamic testing of the structure, which is located in the central part of Skopje was performed in 1979 applying the forced vibration testing method. The aim of the testing, besides definition of the dynamic characteristics, was to estimate the validity of the adopted analytical model in the project and define the most appropriate locations for placing the SMA 1 strong motion instruments. The building is a RC frame structure. Its plan and cross section are shown in Fig. 3. In the frequency range of up to 9 Hz ob-tained were several resonant frequencies and mode shapes in both orthogonal directions and for tor-sion, Fig. 4. The experimental results showed that the adopted analytical model for definition of the

seismic forces during the design was correct, i. e. the structure vibrated with shear type of deformation and the soil-structure effect was negligible.

Figure 3. Example 2 "ZOIL Makedonija" Building plan and cross section

Figure 4. Example 2 "ZOIL Makedonija" Building mode shapes of vibration

2.2.3 Example 3. Residential Building K-1, "Aerodrom" Settlement

This building has seven floors and presents a system of diaphragms, i.e. RC vertical walls and slabs, Fig. 5. In 1981 the structure was tested applying the ambient vibration testing method whereat pro-nounced resonant frequencies and mode shapes were obtained. The mode shapes proved the statement that it is a clear structural system vibrating with bending type of deformation and that the structure will behave as dynamically stable in earthquake conditions.

Figure 5. Example 3 - Residential Building K-1, Aerodrom Settlement

2.2.4 Example 4. The Building of RT2 (Radio and Television)

The structure has G+23 levels and it is located in the central part of the city. It is a RC frame structure with multiple change in proportions at plan and along the height, Fig. 6. The dynamic characteristics were obtained applying both full scale testing methodologies. Several natural frequencies and mode shapes were defined for orthogonal directions and for torsion. Figure 7 presents the mode shapes ob-tained in E-W direction. A distinguished bending type of deformation is evident in the upper part of the building and the reason for this is the change in stiffness and the greater flexibility of this struc-tural part. According to the obtained results, the structure has good dynamic characteristics and the soil-structure interaction is negligible. The comparison of the results obtained by dynamic analysis and the results obtained by experimental measurements pointed to very good correlation with minor differ-ences in the calculated seismic forces, which is on the side of safety.

Figure 6. Example 4 - The Building of RT2

Figure 7. Example 4 - The Building of RT2 mode shapes in E-W Direction

2.2.5 Example 5. The Business Center in Skopje

The Business Center building is located in the city center and it represents a RC frame structure inte-grated in the complex structural unit of the Trade Center, Fig. 8. In 1997 the structure has been tested applying forced and ambient vibration testing methods for definition of its dynamic characteristics, as well as definition of the vibration transmission effects on the Trade center through the foundations. Resonant frequencies and mode shapes of vibration have been obtained for both translational direc-tions and for torsion. The mode shapes are of shear type, characteristic for frame structures, Fig.8. The main conclusion from the obtained results is that the values of the resonant frequencies for the first two modes for the three directions are very close, which means that the structure will exert complex dynamic behaviour during earthquakes. Significant transmition of vibrations of the Business Center to the Trade Center has been noticed, especially during torsional vibrations.

Figure 8. Example 5 - The Business Center in Skopje plan, vertical cross section and comparative presentation

of mode shapes obtained by forced amd ambient vibrations

3 CONCLUSIONS

After the catastrophic earthquake in Skopje, 1963, experimental in-situ full scale testing of build-ings has been introduced by IZIIS for the next 40 years. It was the first applied methodology in Europe in that time. It was a very fruitful period of intensive research in the field of earthquake en-gineering in Macedonia, Former Yugoslavia and Europe with IZIIS having a leading position. This testing method is still applicable in the world, and IZIIS is still using it.

The selected and tested buildings by forced or ambient vibration method presented in this paper representing different structural systems with different number of storeys, enable the obtaining of a better insight into the influence of the above mentioned parameters on the dynamic behavior of the structures.

Structures with symmetrical distribution of structural elements in plan, possess clearly expressed dynamic characteristics offering simple and realistic analytical definition of their resistance as well as stability under seismic action.

The number of storeys has a significant influence on their dynamic properties. By increasing the number of storeys, the structure becomes more flexible i.e. the resonant frequencies of the struc-tures become lower.

The type of the structural system has also a remarkable influence on the dynamic properties of the structures. The most flexible system is the frame structure, then combined frame-diaphragm sys-tem, while the monolithic RC walls and floor slab system is the most rigid one.

The structures with a rectangular base have a higher ratio between longer/ shorter side, and a more complicated dynamic behavior than structures with square shape at base.

The foundation conditions also have an influence on the dynamic characteristics of the structures. If the structure is founded on soft soil, the stability of the structure will be considerably reduced and flexibility increased, because of bad fixation conditions at the base.

The soil-structure interaction effect can considerably change the dynamic properties of the structure and can produce more complicated dynamic behavior in earthquake conditions.

The building construction in Skopje is still very intensive. Most of the new buildings are with smaller number of stories, and some of them are being only reconstructed. The greatest portion of construction works is performed by private companies, but some of them are not specialized enough in aseismic design and construction of buildings. The investors are mostly private compa-nies and they don't pay enough attention to earthquake aspects. Dynamic testing of the structures is a rare practice in Skopje, which is a big mistake, because the calculations and the design analyses are not experimentally verified. Consequently, all the errors in design and construction will be dis-covered too late - during a possible strong earthquake.

REFERENCES

Jurukovski, D., Taskov, Lj., 1976. "Dynamic Testing of ZO 'Madedonija' Building by Forced Vibra-tions", IZIIS Report No. 76-3, Skopje, Republic of Macedonia

Jurukovski, D., Taskov, Lj., Krstevska, L., Bojadziev, M., 1981. "Experimental Investigations for Definition of Seismic Stability of Structures in Skopje", IZIIS Report No. 81-129, Skopje, Repub-lic of Macedonia

Jurukovski, D., Taskov, Lj., Krstevska, L., Bojadziev, M., 1983. "Definition of the Dynamic Charac-teristics of Radio-Television Building RT2 Applying Forced and Ambient Vibration Testing Methods", IZIIS Report No. 83-90, Skopje, Republic of Macedonia

Petrovski, J., Kapsarov, R., Jurukovski, D., 1973. "Experimental and Analytical Investigations of Dy-namic Response of Five Storey Large-Pannel Building", IZIIS Publication No. 36, Skopje, Re-public of Macedonia

Taskov, Lj., Krstevska, L., Rakicevic, Z., Jurukovski, D., 1997. "Dynamic Testing of Business Center in Skopje Applying Forced and Ambient Vibration Testing Methods", IZIIS Report No. 97-51, Skopje, Republic of Macedonia

Talaganov, K., Taskov, Lj., Jordanovski, K., 2000. "Skopje Earthquake, July 26, 1963", International Seminar in Agadir, Morocco

INTRODUCTIONIN SITU FULL SCALE TESTING OF TYPICAL STRUCTURAL SYSTEMSIn situ testing by ambient and forced vibration testing methodsCharacteristic examples for tested buildings in SkopjeExample 1. Five storey large pannel buildingExample 2." ZOIL Makedonija" BuildingExample 3. Residential Building K-1, "Aerodrom" SettlementExample 4. The Building of RT2 (Radio and Television)Example 5. The Business Center in Skopje

CONCLUSIONS