Ambient vibration testing of a 3-storey substandard RC building at

different levels of structural seismic damage

P. Inci, C. Goksu, U. Demir, A. Ilki PhD Candidate, Civil Engineering Faculty, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey

Postdoctoral Researcher, Civil Engineering Faculty, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey

PhD Candidate, Civil Engineering Faculty, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey

Prof. Dr., Civil Engineering Faculty, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey

e-mail address: pinarinci@itu.edu.tr (Pinar Inci – Corresponding author)

ABSTRACT:

In this paper, the effects of structural damage on the modal characteristics of a substandard full-scale reinforced

concrete (RC) building were investigated. The RC building was a representative of large number of existing

substandard RC buildings. The building was subjected to different levels of structural damage through quasi-static

reversed cyclic lateral loading. Ambient vibration tests were carried out not only before and after quasi-static

lateral loading cycles, but also at a certain damage level. The vibration test survey showed that modal frequencies

decreased while damping ratios increased with the increasing levels of damage. More importantly, since the

structural damage pattern due to quasi-static cyclic lateral loading was similar to ones observed in existing RC

structures after earthquakes, determining the changes in dynamic characteristics at different levels of structural

damage can be useful for estimating the residual performance of the structure after an earthquake.

Keywords: Ambient vibration test, Damage, Damping, Frequency, RC

1. INTRODUCTION

Structural health monitoring (SHM) has been employed as a routine application in some industries such

as aerospace, automotive, manufacturing, etc. for several decades. It has also been considered for civil

infrastructure systems, such as dams, bridges, tall buildings in recent years. Eventually, since the

existing building stock, which includes huge number of aged RC residential and public structures,

represent vulnerability against earthquakes, a great knowledge for SHM has being used extensively for

obtaining dynamic characteristics of these civil structures. Various experimental and analytical vibration

based techniques have been considered to identify the dynamic characteristics of structures for model

updating issues, which has a key role for seismic performance assessment of concerned structures

(Brownjohn 2000, Foti et al. 2012, Sanayei et al. 2015). Similarly, well-known SHM applications were

adopted and developed to identify the change in dynamic characteristics of structures, which have been

subjected to seismic actions (Ivanovic et al. 2000; Kusunoki et al. 2012; Vidal et al. 2014). In this study,

similar to these condition assessment studies, it was aimed to present the change in dynamic

characteristics of a substandard RC building due to damage on structural members. An important point

of this study, was i to quantify the influence of different types and extents of structural damages on the

dynamic characteristics of building. The test building was a single bay, 3-storey RC frame building

(Figure 1). The building was designed and constructed to represent the common characteristics of

existing seismically vulnerable building stock in Turkey (Goksu et al. 2015, Comert et al. 2016). In the

study of Goksu et al. 2015, the change of the modal frequencies and the modal damping ratios of the

building with increasing structural damage was identified through forced vibration tests. Different than

Goksu et al. 2015, in this study, acceleration response of the building was measured under ambient

vibrations. Additionally, the mode shapes of the building were introduced.

A set of quasi-static cyclic lateral loading was applied in x direction of the building in terms of

incrementally increasing reversed displacement cycles. Figure 2 displays the x and y directions of the

building in plan view. The building experienced structural damages similar to ones commonly observed

in existing RC structures after earthquakes. Ambient vibration tests were carried out before and after

quasi-static lateral loading cycles as well as at certain damage levels. During these dynamic tests, an

output only system identification algorithm was followed to obtain a relationship between the modal

parameters and the level of structural damage based on the response of the structure to ambient

vibrations. The system identification process of the building showed that the modal frequencies

decreased while damping ratios for different modes increased with the increasing levels of damage.

Since the test building is representative of large number of substandard RC buildings, the test results

would be useful for the seismic performance assessment of existing RC buildings.

Figure 1. Test building

2. DESCRIPTION AND INSTRUMENTATION OF THE TEST BUILDING

The test building was a full-scale, 3-storey RC moment resisting bare frame building with a single bay

in x and y directions. It was designed to be representative of a large number of substandard RC buildings.

It has insufficient construction characteristics, such as low compressive strength of concrete, inadequate

thickness of concrete cover, plain reinforcing bars and poor reinforcement detailing. Moreover, the

columns were weaker than beams. The height of each story was 3 m. The concrete compressive strength

(fc') was 10 MPa and the yield stress of the plain longitudinal and transverse reinforcing bars was 350

MPa. The typical plan view of the building is presented in Figure 2. More detailed information about

the test building can be found in Goksu et. al (2015).

Ambient vibration measurements were performed through 6 uniaxial piezoelectric voltage

accelerometers and a 24-bit delta-sigma dual core data acquisition system with anti-aliasing filter. Each

accelerometer was mounted at the center of each story and mat foundation (A2, A4, A5 and A6) while,

2 additional accelerometers were mounted on 2 opposite corners of the third story (A1 and A3) (Figure

3). The configuration given in Figure 3 illustrates the arrangement of accelerometers for capturing the

vibration response of the building in x direction. For capturing the response of the building in y direction,

the accelerometers at the centers of the slabs (i.e. A2, A4, A5 and A6) were rotated 90°.

Figure 2. Typical plan view of the test building (Goksu et al. 2015)

Figure 3. Instrumentation of the buildings.

3. AMBIENT VIBRATION TEST SURVEY

3.1 Ambient vibration tests

Ambient vibration tests were performed before and after quasi-static reversed cyclic loading.

Additionally, for observing the rate of changes of the dynamic characteristics with gradually increasing

damage, it was also carried out at a certain damage level. The quasi-static reversed cyclic loading was

applied to the building only in the x direction using one hydraulic actuator at the lower first and two

other hydraulic actuators at the second story slab levels of the building (Figure 3). Actuators enforced

the loading in terms of incrementally increasing reversed lateral displacement cycles. Hence, targeted

damage levels were achieved by pushing and pulling the building to specific ground story drift ratios

(d.r.), which were calculated as the ratios of the lateral displacement of the top floor to the height of the

building. It should be noted that the building was unloaded and the hydraulic actuators were detached

from the buildings before dynamic measurements. The dynamic tests were carried out for x and

y

x

A1 A2 A3

A4

A5

A6

Hydraulic

actuators

subsequently y direction before and after quasi-static reversed cyclic loading (undamaged state and 3%

d.r., respectively) while for only x direction at the damage level corresponding to 0.5% d.r..

3.2 Damage pattern of the building at the time of ambient vibration tests

Damage pattern of the one of the columns of the building at the time of dynamic measurements is given

in Figure 4. The damage started with bending cracks at upper and lower ends of the columns at around

0.5 % d.r.. Although, the crack widths increased at around 3% d.r., they did not cause crushing of

concrete and spalling of concrete cover. However, the base shear-drift ratio relationship showed a major

decrease in lateral load bearing capacity. (at around 20% at 3% d.r.) (Figure 5).

Figure 4. Damage propagation of one of the columns the building (S11 column)

Figure 5. Base shear-drift ratio relationship of the building and the vibration testing stages

3.3 Modal identification

System identification of the building for the undamaged and damaged states was carried out using the

enhanced frequency domain decomposition (EFDD) method introduced by Brincker et al. (2001a,b).

The EFDD follows an output-only modal identification algorithm, which gives natural frequencies,

modal damping ratios and mode shapes. The EFDD derives power spectral density (PSD) functions of

the many single degree of freedom (SDOF) systems whose modal properties are equivalent to the

inspected original system. This algorithm basically computes singular value decomposition (SVD) of

the PSD function matrix of output channels, which are the vibration responses measured by the

accelerometers from A1 to A6 in this study. The PSD function is taken back to the time domain using

inverse fast fourier transform (IFFT) in order to obtain logarithmic free-decay function of the SDOF so

that the modal frequency and the corresponding damping ratio can be determined.

In this study, the sampling frequency of the data was selected to be 100 Hz. For obtaining the PSD

relationship, hanning window was utilized to minimize the leakage effects and the number of the fast

fourier transform points was taken as 1024. Figures 6 and 7 display the singular values of the PSD

Undamaged

After 0.5% d.r.

After 3% d.r.

matrix of the response and the mode shapes in x and y directions for the undamaged and damaged states

of the building, respectively.

a)

b)

c)

Figure 6. Modal frequencies and mode shapes of building in x direction, a) undamaged state, b) after %0.5 d.r.

and c) after 3% d.r.

1st mode

2.26 Hz 3rd mode

9.47 Hz

2nd mode

3rd mode

8.11 Hz

2nd mode

3rd mode

6.44 Hz

2nd mode

Frequencies corresponding to the modal frequencies of the building are shown on these figures.

However, the magnitudes of the peaks of the singular values of the PSD matrices for the damage states

are significantly weak (Figures 6b-c and 7b). This is probably caused by the increasing damping forces

of the building due to the increasing damage, for instance, increase in friction between the crack

interfaces (Figure 4). Therefore, this complication in the peak picking process was discarded by

inspecting the coherence function of the responses (Figure 8). As seen in this figure, the frequency peaks

can be captured with ease. Additionally, they are well-matched with the ones determined through EFDD

algorithm. It should be noted that the first frequency peak for the damaged state of the building could

not be captured due to technical limitations.

a)

b)

Figure 7. Modal frequencies and mode shapes of building in y direction, a) undamaged state, b) after 3% d.r.

The shifting of the peaks toward lower frequencies with the increasing damage can be clearly seen in

Figures 6, 7 and 8. This shifting of the peaks was expected since the building was enforced to exhibit

lateral deformations beyond its elastic limit. As aforementioned, the building was subjected to quasi-

static cyclic loading only in x direction. Thus, as expected, the reduction of vibration frequencies due to

structural damage observed in y direction was smaller than those observed in x direction (Table 1). For

example, the third frequency peak as 9.47 Hz for x direction at undamaged state (Figure 6a) shifted to

the frequency peak as 6.44 Hz after the building was subjected to displacement cycles at 3 % d.r. at the

ground story (Figure 6c). However, for the y direction, the third frequency peak as 9.47 Hz at undamaged

state (Figure 7a) shifted to 7.04 Hz at the damaged state after the building was subjected to displacement

cycles of 3 % d.r. at the ground story (Figure 7b). As also seen in Table 1, the modal damping ratios

increased with increasing damage. Furthermore, the change in modal damping ratio as a function of

achieved maximum lateral drift (damage state) is given in Figure 9. As seen in this figure, in contrary

1st mode

2.27 Hz

3rd mode

9.47 Hz

2nd mode

1st mode

1.56 Hz

3rd mode

7.04 Hz

2nd mode

to the modal damping ratios for y direction, the ones in x direction yielded significant jump with

increasing in damage (approximately 78% and 42% increase in damping ration for the second and third

mode, respectively). This can be explained with the application of quasi-static loading only in x direction

of the building causing more significant damage in this direction. It should be noted that the finite

element model (FEM) of the building was established to perform a modal analysis with the purpose of

obtaining modal frequencies of the building by Goksu et al. (2015). The modal frequencies predicted

through FEM and identified from the EFDD method are in good agreement for the first two modes in x

and y directions of the undamaged state of the building.

It is worth to note that, the lateral load bearing capacity of the building was degraded by approximately

20% as it was subjected to the target ground story drift level of 3% (Figure 5). However, the visual

inspection made after the building was unloaded from the 3% d.r. did not show any indication of severe

structural damage, which could have caused such a major decrease in lateral load bearing capacity (at

around 20%). This observation indicates that the vibration based inspection may be able to provide a

more realistic condition assessment for the damaged structures.

a) b) c)

Figure 8. Coherence functions in x direction, a) undamaged state, b) after %0.5 d.r. and c) after 3% d.r.

Table 1. Identified modal frequencies and corresponding modal damping ratios

Damage

states

Modal frequencies (Hz) Modal damping ratios

x direction y direction x direction y direction

1st

mode

2nd

mode

3rd

mode

1st

mode

2nd

mode

3rd

mode

1st

mode

2nd

mode

3rd

mode

1st

mode

2nd

mode

3rd

mode

Undamaged 2.26 6.07 9.47 2.27 6.06 9.47 0.018 0.028 0.033 0.048 0.047 0.045

0.5% d.r. - 5.36 8.11 No test - 0.048 0.039 No test

3% d.r. - 3.81 6.44 1.56 4.73 7.04 - 0.050 0.047 0.049 0.048 0.046

1st mode

2.05 Hz

3rd mode

8.10 Hz

2nd mode 1st mode

2.25 Hz

3rd mode

9.47 Hz

2nd mode

2nd mode

3.81 Hz

3rd mode

Figure 9. Change in modal damping ratio as a function of achieved maximum drift ratio (damage state)

4. CONCLUSION

The dynamic characteristics of a full-scale substandard RC building were determined through ambient

vibration tests before and after subjecting the building to quasi-static reversed cyclic loading. The modal

characteristics of the building was obtained using the EFDD method. The results of the study are listed

below;

- The modal frequencies decreased, while the modal damping ratios increased with increasing structural

damage.

- Increase in the modal damping ratios of the building in x direction was more significant compared to

the ones in y direction. This is due to the lateral loading, which was applied only in x direction and

caused more significant damage this direction.

- Ambient vibration tests clearly showed that the building had significant structural damage while very

limited information could be gathered by visual inspection. This results emphasize that the vibration

based inspection may be able to give a more reliable condition assessment for the damaged structures

comparing to the visual damage inspection. It should be noted that these results are limited by the

specimen considered in this study.

ACKNOWLEDGEMENTS

The authors are thankful to financial supports of Istanbul Development Agency (Project No: TR10/12/AFT/0050)

and ITU Scientific Research Fund Department. The contributions of Prof. Dr. Z. Celep, Prof. Dr. Z. Polat, Prof.

Dr. T. Kabeyasawa, Assoc. Prof. Dr. K. Kusunoki, Assoc. Prof. Dr. K. Orakcal, Assoc. Prof. Dr. E. Yuksel, Assist.

Prof. Dr. U. Yazgan, Dr. C. Demir, AN. Sanver, AO. Ates, M. Comert, Dr. C. Yenidogan, O. Ozeren, E. Tore, S.

Khoshkholghi, A. Moshfeghi, S. Hajihosseinlou, HF. Ghatte, I. Sarıbas, Tech. A. Sahin and 2014 summer trainees

are greatly appreciated. The authors are also thankful to supports of Akcansa Co., Art-Yol Co., Boler Celik Co.,

Hilti-Turkey Co., Tasyapi Co., Urtim Co., and the staff of Kadikoy Municipality.

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