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Experimental study on three-dimensional reinforced concrete frames subjected to dynamic loading
Structures 24 (2020) 835–850
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Experimental study on three-dimensional reinforced concrete frames subjected to dynamic loading
T
Fatih Bahadir School of Higher Education Construction Technology Program, Necmettin Erbakan University, Konya, Turkey
ARTICLE INFO
ABSTRACT
Keywords: Shaking table Failure mode Short-column Soft-storey Response spectrum Elastic seismic load Energy flux
During an earthquake, soft-storey and short-column behaviours, which usually occur around window openings, can cause significant damage to structures, or even collapse, if the measures specified in regulations are not applied. This study focuses on band-type window spaces that can cause short-column and soft-storey behaviours. Reinforced concrete frames were produced on a 1/6 geometric scale, and two-storey, single-span frames were tested on a shaking table. The second storeys of all specimens had infill walls and the same window openings. The first storeys of the specimens were produced with different band-type window heights. The specimens were produced with band-type window space ratios at the infill brick wall of the first storey of 0%, 25%, 50%, 75%, and 100%, respectively. These specimens were tested on a shaking table under sinusoidal dynamics. The behaviour of the specimens, failure mechanisms, response spectra, elastic seismic loads, and energy flux values were compared by subjecting the specimens to the same ground motion. The experimental results demonstrated that the short-column and the soft-storey behaviours are effective when the band-type window space ratio in the infill brick walls decreases.
1. Introduction It is known that most structures built in seismic regions in Turkey are defective in terms of design and construction. More than 30,000 buildings were damaged because of these defects during the Van–Erciş earthquake. Many of the reinforced concrete (RC) buildings that collapsed in the region were observed to have construction defects or were irregular buildings [1]. Studies indicated that soft-storey irregularities were present in 61% of the collapsed structures and short-column behaviour was exhibited in 6% of the collapsed structures. There are shops and parking lots on the ground floor of many buildings in Turkey. The other floors of these buildings are built with filling walls. Infill walls of the floor are much more rigid than walls or windows with floors. These are attributes not only of new constructions but also of converted old houses. Such weak floors are called “soft storeys.” If the height of one floor is higher than that of the other floors, a soft-storey irregularity is present. Furthermore, as a result of the outof-plane overturning of the walls on the floor, soft-storey behaviour is observed. The soft-storey mechanism steps are given in Fig. 1.
Short columns may develop owing to structural arrangements or openings provided in infill walls between columns [2]. Such spaces are often seen in the case of band-type windows at the basement floor level and mezzanine floor at the ground floor level. The short-column behaviour, which is usually occurring around the window opening, can cause significant damage to the structure during an earthquake, or even collapse, if the measures specified in regulations are not applied. Because the short column is more rigid than the other columns in the structure, it has a very high shear strength value. Because of shear failure, the column may lose its load-carrying capacity and the structure may collapse. Damage resulting from the short column of Specimen 2 is given in Fig. 2. Since 1960, shaking tables have been used for scientific research on earthquakes and for structural engineering studies [3–17]. Generally, experiments on short-column and soft-storey behaviours are rarely conducted in three dimensions and on shake tables [18]. However, twodimensional experimental studies on soft storeys [19–24] and short columns [25–31] have been conducted many times. In this study, twostorey, single-span RC frames produced on a 1/6 geometric scale were
E-mail address: [email protected]. https://doi.org/10.1016/j.istruc.2020.01.045 Received 18 October 2019; Received in revised form 5 December 2019; Accepted 28 January 2020 2352-0124/ © 2020 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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Fig. 1. The soft story failure mechanism.
tested on a shaking table. The first storey of the specimens was produced with different band-type windows heights. These specimens were produced with band-type window space ratios at the infill brick wall of the first storey of 0%, 25%, 50%, 75%, and 100%, respectively. Soft-storey and short-column behaviours, which usually occur around the window opening, can cause significant damage to a structure, or even collapse, during an earthquake, if the measures specified in the regulations are not applied. The aim of this study is to investigate the effect of the size of the band windows that cause short-column and soft-storey behaviours in the seismic response of the structures. In this study, the failure mechanisms, acceleration values, and energy flux values of the specimens were compared by applying the same ground motion to the specimens.
2. Materials and method 2.1. Description of test specimens In this experimental study, five three-dimensional (3D) RC frames with two stories produced on a 1/6 geometric scale, which contained deficiencies commonly observed in residential buildings in Turkey, were tested on a shaking table. The second storey of all specimens contained brick walls and the same window openings, while the first storey of all specimens was produced with different band-type window spaces of brick walls (except Specimen 1 and Specimen 6). Specimen 1 was also produced as reference specimen RS1, which featured a full
Fig. 2. The short column behaviour.
Fig. 3. General photo of the test set-up for the experimental study.
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brick wall on the first storey. Specimen 6 was produced as reference specimen RS2, and had no brick wall on the first storey. The specimens were manufactured and tested at the Structural Testing Laboratory at the Necmettin Erbakan University–Konya, Turkey (Fig. 3).
In the five test specimens, the width and height of the structural elements, concrete qualities, and reinforcement forms of the frames were the same. Test frames were detailed and constructed deliberately with some deficiencies such as low strength concrete usage, strong
Fig. 4. Dimensional and reinforcements details of the general specimen (dimension: mm).
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Table 1 Parameters of test specimens. Test Specimen
Space Ratio of 1st storey The band windows dimensions of 1st storey The brick wall dimensions of 1st storey
Specimen 1
Specimen 2
Specimen 3
Specimen 4
Specimen 5
0% – 400 × 600 × 30 mm
25% 100 × 600 mm 300 × 600 × 30 mm
50% 200 × 600 mm 200 × 600 × 30 mm
75% 300 × 600 mm 100 × 600 × 30 mm
100% – –
beam–weak column formation, wide spacing of beam and column stirrups, no column stirrups at the beam–column joints, and no confinement zones at the end of the columns and beams. In addition to these deficiencies, the stirrups were prepared with 90° hooks and
placed at free ends of the columns and beams of the test specimens [32,33,34]. The height of one storey was 500 mm (3000 mm in 1:1 scale real dimensions). The length of the frame was 700 mm from one column to
Fig. 5. Production stages of specimens.
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Table 3 Equation Factors of Ground Motion. Ground Motion
Table 2 Scale factors for experimental study at shaking tables. Dimension
Scale Factors
Gravitational acceleration, g Velocity, v Time, t Linear Dimension, l Displacement, δ Frequency, ω Area, A Volume, V Modulus, E Energy, EN Mass density, ρ
LT−2 LT−1 T L L T−1 L2 L3 FL−2 FL FL−4T2
1 Sl1/2 Sl1/2 Sl Sl Sl−1/2 Sl2 Sl3 1 Sl3 1/Sl
Amplitude (mm)
Period P (sec)
PhaseShift PS (rad)
Frequency f
0–2.82 2.82–5.13 5.13–6.39 6.39–7.54 7.54–8.99 8.99–9.61 9.61–11.27 11.27–12.35 12.35–13.86 13.86–15.61 15.61–19.82
16.52 16.45 18.77 16.34 19.30 18.90 19.09 17.72 17.16 18.56 18.19
1.421 0.938 0.420 0.562 0.299 0.304 0.318 0.667 0.753 0.870 1.054
12.77 6.540 33.03 6.874 9.044 −0.629 −10.27 37.36 25.57 22.10 73.65
0.704 1.067 2.379 1.780 3.341 3.290 3.143 1.500 1.328 1.150 0.949
specimens was produced with different band-type window heights. These specimens were produced with band-type window ratio spaces of 0%, 25%, 50%, 75%, and 100%, respectively. The parameters of the test specimens are given in Table 1. The production stages of specimens are shown in Fig. 5. The test specimens were cast of micro-concrete and steel wire [36]. The average yield strengths of ϕ2 and ϕ3 steel wires were ~580 MPa and that of ϕ5 steel wire was ~590 MPa. The average cylinder compressive strength of the micro-concrete used was 6 MPa. The maximum aggregate size of micro-concrete was chosen as 3 mm. Experiments used to determine micro-concrete properties were conducted under the TS EN-12390-3 standard [37]. Because of the nonhomogeneous structures of the brick walls, it is very difficult to obtain the modulus of elasticity and Poisson ratio using the test data for brick walls. Infill brick walls of specimens were tested under diagonal compression. The average diagonal compressive strength of the infill wall was ~0.5 MPa. The test setup for the brick wall is presented in Fig. 6. The scale factors used for measurements and the dimensions of specimens on the shake table are given in Table 2. The same ground motion was applied to specimens over short time periods at high frequencies [5].
Fig. 6. Test setup of the brick wall.
Parameter
Time (sec)
2.2. Test setup another column. Plain bars were used for longitudinal reinforcement and stirrups. The column dimensions were 50 × 80 mm, and four 3mm-diameter bars were used as longitudinal reinforcement. The beam dimensions were 50 × 90 mm, and six 3-mm-diameter plain bars were used. Reinforcements with a diameter of 2 mm with 50-mm spacing were used as stirrups at columns and beams of specimens. Dimensional and reinforcement details of the specimens are shown in Fig. 4. The second storey of all specimens contained 200 × 300 mm dimension window openings in the mid-span of the two long faces and 200 × 200 mm dimension window openings in the mid-span of the two short faces. The middle axis of the brick walls and frame did not coincide; rather, the external surface of the brick wall shared the same axis as the external surface of the beams. The thickness of the wall was 30 mm while the depth of the columns was 50 mm or 80 mm. The bricks used in the infill walls were produced with plaster. The brick dimensions were 30 × 50 × 25 mm [35]. The first storey of the
The shaking table used in experiments operates under the working principle of converting rotary motion to linear motion. The specimens were tested under constant axial load and sinusoidal cyclic motion was imposed to simulate seismic action. The sinusoidal cyclic motion was processed through a data acquisition card [38]. The specimens were tested with a ground motion on the shaking table until failure. The general sinusoidal function applied was
A (t ) = A sin[(2 t /
L)
+ P ],
(1)
where A is the amplitude (the height of the top of the waves), t is time (of ground motion), A(t) is the displacement value of the shaking table, ωL is the angular velocity of the motor disc, is the wavelength (the time required for a complete cycle), and P is the phase shift (for which a value of 0 rad sets A(t) equal to 0 at t = 0 and a value of π rad sets A(t) equal to its maximum when t = 0). These equation factors are given Table 3. The graph of this function is shown in Fig. 7.
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Fig. 7. General sinusoidal function and graph.
The main parameters of the shaking table are given in Table 4. The actual block diagram of the experiments is shown in Fig. 8. The views used to describe the damage and cracks to the specimens are shown in Fig. 9. Acceleration data for the test frames were measured with ADXL345 accelerometers in the experiments (Fig. 10). These accelerometers were situated on each floor at two locations. These were used to determine the acceleration of each storey level. The ADXL345 [39] is a complete threeaxis acceleration measurement system with a selectable measurement range of ± 2, ± 4, ± 8, or ± 16 g. It measures both dynamic accelerations resulting from motion or shock and static acceleration. The shake table accelerations were measured in both X and Y directions.
Table 4 Main parameters of shaking table. Parameters
Values Units
Size of the platform Maximum mass of load Maximum displacement of the platform Maximum acceleration of the platform Frequency Maximum power of motor Maximum output torque of gear speed reducer Maximum input rotational speed of gear speed reducer Maximum output rotational speed of gear speed reducer
80 mm × 1200 mm 1500 kg ± 20 mm ±4 g 0–50 Hz 4 kW 100518 mNm 3000 rpm 380 rpm
Fig. 8. Actual block diagram of control system.
Fig. 9. 3D and plan view of Specimen 3. 840
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3. Experimental study Acceleration data for the specimens on the first storey and second storey levels in the X direction (the direction of ground movement) were measured. The measured acceleration data for Specimens 1–5 are shown in Figs. 11–15, respectively. The maximum positive and negative accelerations, duration, and failure mode of the specimens are given in Table 5. The first specimen was the reference frame (Specimen 1). The bandtype windows were not placed at the first storey of Specimen 1. The ratio of the band-type window height to the brick wall height is 0%. Specimen 1 was tested to observe the reference behaviour for comparison purposes. According to acceleration data of Specimen 1, this experimental study lasted 19.52 s. No damage occurred on the first storey of this specimen, whereas the whole damage occurred at the second storey. As the brick walls at the window level of the second storey were destroyed, short-column behaviour occurred in the direction of ground movement. Specimen 1 did not completely collapse and only the second storey collapsed at the end of the test. Because there are
Fig. 10. Location points of accelerometers.
Fig. 11. Acceleration data of Specimen 1 (RS1-%0 space).
Fig. 12. Acceleration data of Specimen 2 (%25 space).
Fig. 13. Acceleration data of Specimen 3 (%50 space).
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Fig. 14. Acceleration data of Specimen 4 (%75 space).
Fig. 15. Acceleration data of Specimen 5 (%100 space). Table 5 The failure mechanisms of specimens. Specimen
Failure Type
Duration (sec)
Story
Max Positive Acceleration
Max Negative Acceleration
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp. (mm)
1
0%
No Damage Plastic Hinge Formation
19.52
1 2
7.2 6.45
2.21 4.44
7.5 8.75
14.32 6.18
−2.12 −3.29
10 15
2
25%
Short Column No Damage
5.01
1 2
3.72 3.66
1.99 0.79
201 116
4.25 4.4
−1.78 −1.39
157 234
3
50%
Short Column
9.2
1 2
5.01 5.07
2.49 2.33
40 −25
4.86 4.61
−2.27 −1.99
−15 25
4
%75
Short Column No Damage
4.43
1 2
3.83 3.25
2.00 2.54
97.5 15
4.3 3.2
−1.99 −2.0
217.5 62.5
Failure Mode
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Table 5 (continued) Specimen
5
%100
Failure Type
Plastic Hinge Formation No Damage
Duration (sec)
13.27
Story
1 2
Max Positive Acceleration
Max Negative Acceleration
Failure Mode
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp. (mm)
4.19 4.26
1.59 1.42
17.5 20
3.41 3.69
−1.3 −1.27
10 12.5
Table 6 First Cracks of Specimen 1. Specimen No
Story
First Crack of Frame
First Crack of Walls
Forward
1
0%
Backward
Forward
Backward
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
1 2 Fig.
6.68
1.75 3.17
5 14
7.06
−1.51 −2.67
7.5 8.75
6.66
1.63 3.65
10 11.25
7.06
−1.51 −2.67
7.5 8.75
View
a
c
c
Fig. 16. Failure mode of Specimen 1 (0% space).
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Table 7 First Cracks of Specimen 2. Specimen No
Story
First Crack of Frame
First Crack of Walls
Forward
2
25%
Backward
Forward
Backward
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
1 2 Fig.
1.62
0.7 0.23
10.83 15
1.83
−0.99 −0.02
8.33 11.67
1.68
0.23 0.72
7.5 6.664
1.83
−0.99 −0.02
8.33 11.67
View
a
b-c
b
b–c
Fig. 17. Failure mode of Specimen 2 (25% space).
Table 8 First Cracks of Specimen 3. Specimen No
Story
First Crack of Frame
First Crack of Walls
Forward
3
50%
Backward
Forward
Backward
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
1 2 Fig.
4.88
0.48 2.49
20 25
4.72
−0.57 −2.27
15 10
4.88
0.48 2.49
20 25
4.72
−0.57 −2.27
15 10
View
a
a-b
b
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Fig. 18. Failure mode of Specimen 3 (50% space).
Fig. 19. Failure mode of Specimen 4 (75% space).
no first storey windows, only the second storey experienced damage and collapse. The measured acceleration values at which the first cracks appeared and damage occurred are given in Table 6. Cracks and damages to Specimen 1 are shown in Fig. 16 at the end of the test. The ratio of the band-type window height to the brick wall height is
25% for Specimen 2. The 25% band-type window space on the first storey caused a great shear force as a result of the ground movement, thus causing short-column behaviour in the column in this section. In this test, short-column behaviour was clearly observed. Specimen 2 completely collapsed in a short time. According to acceleration data of
Table 9 First Cracks of Specimen 4. Specimen No
Story
First Crack of Frame
First Crack of Walls
Forward
4
75%
Backward
Forward
Backward
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
1 2 Fig.
1.33
1.3 −0.01
12.5 12.5
1.5
−0.95 0.03
47.5 45
1.33
1.3 −0.01
12.5 12.5
1.5
−0.95 0.03
47.5 45
View
a
b
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Fig. 20. Failure mode of Specimen 5 (100% space). Table 10 First Cracks of Specimen 5. Specimen No
Story
First Crack of Frame
First Crack of Walls
Forward
5
100%
Backward
Forward
Backward
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
1 2 Fig.
4.15
1.38 0.13
10 10
4.67
−0.2 −0.41
60 62.5
–
– –
– –
–
– –
– –
View
c
Specimen 2, this test lasted 5.01 s. Accelerations and displacements of the first cracks in the specimen are given in Table 7. Cracks and damages to Specimen 2 at the end of the test are shown in Fig. 17. The ratio of the band-type window height to the brick wall height is 50% for Specimen 3. Although there were cracks on both stories of Specimen 3, more cracks formed on the second storey. Although there were cracks in the frame at the level of the band-type window of the first storey, the frame of the second storey was previously damaged and the second storey collapsed. The second storey of Specimen 3 completely collapsed at the end of the test. According to acceleration data of Specimen 3, this test lasted 9.2 s. First storey accelerometers recorded no measurements after 7.43 s. The measured acceleration values at which the first cracks appeared and damage occurred are given in Table 8. Cracks and damages to Specimen 3 are shown in Fig. 18 at the end of the test. The ratio of the band-type window height to the brick wall height is 75% for Specimen 4. Both short-column and soft-storey behaviours were observed in the first storey. The first storey of Specimen 4
–
–
completely collapsed at the end of the test. Cracks and damages in Specimen 4 are shown in Fig. 19 at the end of the test. According to acceleration data of Specimen 4, this test lasted 4.43 s. The measured acceleration values at which the first cracks appeared and damage occurred are given in Table 9. The fifth specimen was the other reference specimen. The first storey of this specimen did not contain any brick walls and was tested to record the reference behaviour for comparison purposes. According to acceleration data of Specimen 5, this test finished at 13.27 s. Specimen 5 did not collapse at the end of the test. However, this specimen will collapse under small acceleration because the column–beam joints are hinged. Soft-storey behaviour was observed in the first storey. Cracks and damages to Specimen 5 are shown in Fig. 20 at the end of the test. The measured acceleration values at which the first cracks appeared and damage occurred are given in Table 10. After reinforcement of the right column of the “a” view of the first storey were broken at 4.97 s, Specimen 5 was observed to undergo torsion and column–beam joints of the first storey underwent plastic hinge behaviour.
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4. Comparison of experimental results
4.2. Elastic seismic load
The purpose of the measurement of seismic motion is to measure acceleration as a period or frequency and to observe the behaviours of structures during an earthquake. For this reason, acceleration records constitute an important database for engineering applications and scientific studies.
The acceleration response spectrum is the envelope curve plotted from the peaks of maximum acceleration with the corresponding vibration time (T) of each single-degree-of-freedom system. In the spectrum to be used in the calculation of the inertial force of the internal forces formed in the structure, the acceleration value to be used is not the total acceleration value (u¨ (t ) + u¨g (t ) , where u(t) is the displacement u at time t and ug(t) is the relative displacement ug at time t) but the acceleration value (ω2SD) at maximum displacement. This acceleration is called the pseudo acceleration and its spectrum is called the pseudo spectral acceleration. The relationship between the pseudo spectral acceleration PSA and spectral displacement SD is
4.1. Response spectrum A building exposed to an earthquake load first vibrates during the period that it is subjected to the earthquake and then vibrates at its own period. If the members of the building do not have damping properties, resonance occurs in the building and causes the building to collapse. The basic items that reduce the period of the building are the weight of the building and irregularities in the building. The exposure of a building to earthquakes at different times can cause a building’s period to be greater during the next same magnitude earthquake. The vibrations in different periods during the earthquake can be described by a response spectrum graph [2], which shows the characteristics of the behaviour in terms of maximum displacement, acceleration, and velocity. These graphs of the test specimens are given in Fig. 21. In the analysis of the dynamic behaviour of buildings, it must be considered that each earthquake has its own unique acceleration spectrum. According to the natural period and damping rate of the building, the maximum response value obtained from the acceleration response spectrum is the absolute acceleration value that influences the building. The mean period (Tm) of the ground motion has been regarded as the preferred frequency content parameter. According to this parameter, the second storey of Specimen 4 had the longest mean period compared to that of the other specimens. In addition, the second storey of Specimen 2 was measured to have the longest mean period compared to that of the other specimens, except for Specimen 4. The main periods for the second storey of the specimens were calculated as 0.301 s for Specimen 1, 0.370 s for Specimen 2, 0.287 s for Specimen 3, 0.559 s for Specimen 4, and 0.308 s for Specimen 5. The main periods for the first storey of the specimens were calculated as 0.303 s for Specimen 1, 0.280 s for Specimen 2, 0.233 s for Specimen 3, 0.248 s for Specimen 4, and 0.351 s for Specimen 5. During these periods, significant damage occurred to Specimens 2 and 3.
PSA =
2SD,
(2)
where ω is the angular frequency (frequency of vibration). Under the influence of seismic activity, the building carrier system must be able to withstand at least the following elastic seismic load Fel:
Fel = m PSA,
(3)
where m is the mass. If the masses of the specimens are approximately the same, these values can be used as a comparison. Accordingly, the maximum elastic seismic load was calculated for the first and second storeys of Specimen 1 (0% space), given that the specimens were given the same ground motion and that the mass of the specimens was approximately the same. The minimum elastic seismic load was also calculated for the second storey of Specimen 2 (25% space). Because the first storey columns of Specimen 5 underwent plastic hinge behaviour in the early period, the shear loads of Specimen 5 occurred at quite a low level. According to the PSA data obtained from the experiments (Fig. 22), Specimen 1 was subjected to the highest elastic seismic load compared to that of the other specimens. Because the seismic load acting on this specimen lasted ~ 18.50 s, the highest elastic seismic load occurred on both the first and second stories. Compared to other specimens (except Specimen 1), Specimen 2 did not provide the expected seismic performance in terms of both seismic effect time and elastic load.
Fig. 21. The response spectrum graphs of the test specimens at the 5% damping.
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Fig. 22. The response Pseudo Spectral Acceleration graphs of test specimens by 5% damping.
4.3. Energy flux
Table 11 Summary of test results as energy flux.
Energy flux values can provide important numerical data to investigate the effect of seismic energy on the ground and the response of buildings. Energy flux is correlated with the damage caused by the earthquake. The cumulative energy flux Esum (in joules) can be defined as the amount of seismic energy passing through a unit cross-sectional area per unit time [40]:
Esum
1 = AV 2
b a
v 2 (t ) dt
Test Specimens
(4)
where ρ is the mass density of the specimen (in kg f /m3 ) (taken as 1 unit mass density in energy flux calculations), A is the cross-sectional area of the soil layer (in m2) (taken as 1 unit area in energy flux calculations), V is the velocity of seismic waves (in m/s), v (t ) is the velocity response of the specimen (in m/s), anda b is the duration of the earthquake releasing 5% and 95% of the total energy (in seconds). To use the energy flux value of a seismic wave, it is necessary to know the magnitude and distance of this wave. Because each sample was tested under the same conditions on the shaking table, the propagation velocity of seismic waves is the same. The energy flux values of test specimens are given in Table 11. The maximum energy flux value occurred for Specimen 1 and the minimum energy flux value occurred for Specimen 2. Because the seismic load acting on Specimen 1 lasted ~18.50 s, the highest energy flux occurred on the second storey. Because of the high elastic seismic load and the short failure duration, the energy absorption of Specimen 2 was not sufficient to prevent damage.
Name
Space Ratio
Specimen 1
%0
Specimen 2
%25
Specimen 3
%50
Specimen 4
%75
Specimen 5
%100
Story
Time (sec)
Energy Flux (J/cm3)*
1 2 1 2 1 2 1 2 1 2
2.47–17.82 2.43–18.50 0.98–4.72 0.70–4.52 1.92–6.32 1.56–8.71 0.41–4.52 2.46–4.84 1.20–10.30 1.04–10.23
402295 103149824 114931 26174 67242 1359396 45981 618195 1179970 244503
* These values were calculated at significant duration.
5. Summary and conclusion In this study, 3D RC frames were tested on a shaking table to determine their dynamic behaviour. Two-storey, single-span frame specimens were constructed at 1/6 geometric scale with different bandtype window space ratios. The specimens were tested under the same ground motion until failure occurred. According to the test results, the failure mechanisms, first cracks, and acceleration values of all specimens can be summarised as follows:
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• When the location and type of the first cracks of the test specimens • • • •
• •
[6] Sullivan TJ, Pinho R, Pavese A. An introduction to structural testing techniques. Earthquake Engineering Research Report Rose School; 2004. [7] Turer A, Korkmaz SZ, Korkmaz HH. Performance improvement studies of masonry houses using elastic post-tensioning straps. Earthquake Eng Struct Dyn 2007;36(5):683–705. https://doi.org/10.1002/eqe.649. [8] Hanazato T, Minowa C, Narafu T, Imai H, Ali Q, Kobayashi K, Nakagawa T. Shaking Table Test of Model House of Brick Masonry for Seismic Construction. In: Proceedings of 14th World Conference of Earthquake Engineering (14WCEE), 2008 October 12-17. Beijing, China; 2008. p. 1–8. [9] Ersubasi F, Korkmaz HH. Shaking table tests on strengthening of masonry structures against earthquake hazard. Nat Hazards Earth Syst Sci 2010;10(6):1209. [10] Kamanli M, Balik FS. The behaviour of roof gable walls under the effect of earthquake load. Nat Hazard Earth Syst Sci 2010;10(2):251–63. https://doi.org/10. 5194/nhess-10-251-2010, 2010. [11] Türker HT, Mertayak C, Kocaman S. Application of Digital Image Processing Technique on the Study of Dynamic Response of Structural Models on an Educational Shake Table, 9th International Congress on Advances in Civil Engineering, Karadeniz Technical University; 27–30 September 2010, Trabzon, Turkey. [12] Tanrikulu AK, Baran T, Dundar C, Tanrikulu AH. Construction and performance test of a low-cost shake table. Exp Tech 2011;35:8–16. https://doi.org/10.1111/j.17471567.2010.00631.x. [13] Torun A, Çunkaş M. Implementation and design of a shaking table oscillating in two-axis. Afyon Kocatepe Univ J Sci 2011;9(2):85–96. [14] Leite JC, Lourenco PB. Solutions for infilled masonry buildings: shaking table tests. 15th International Brick and Block Masonry Conference. Universidade Federal de Santa Catarina (UFSC); 2012. p. 1–10. [15] Rao KN, Ramesh Babu R. Assessment of Seismic Performance of Reinforced SMB Masonry Building Models through Shock Table and Shaking Table Tests, CiSTUP Indian Institute of Science, Bangalore; 2012. [16] Sharma A, Reddy GR, Vaze KK. Shake table tests on a non-seismically detailed RC frame structure. Struct Eng Mech 2012;41(1):1–24. [17] Saito T, Moya L, Fajardo C, Morita K. Experimental study on dynamic behavior of unreinforced masonry walls. J Dis Res 2013;8(2). [18] Kabir MZ, Shadan P. Seismic performance of 3D-panel wall on Piloti RC frame using shaking table equipment. Proceedings of the 8th International Conference on Structural Dynamics. EURODYN; 2011. [19] Dogan M, Kirac N. Soft Story Behaviour in Earthquake and Samples of Izmit – Duzce. ECAS 2002 Uluslarararası Yapı ve Deprem Mühendisliği Sempozyumu. Ankara, Turkey: Middle East Technical University; 2002. [20] Setia S, Sharma V. Seismic response of R.C.C building with soft story. Int J Appl Eng Res 2012;7(11). ISSN 0973-4562. [21] Dolšek M, Fajfar P. Soft story effects in uniformly infilled reinforced concrete frames. J Earthquake Eng 2001;5(1):1–12. [22] Wibowo A, Wilson JL, Gad EF, Lam NTK, Collier P. Drift capacity of a precast softstorey building in Melbourne. Aust J Struct Eng 2010;11(3):177–93. [23] Miss Desai Pallavi T. Seismic performance of soft story composite column. Int J Sci Eng Res 2013;4(1). ISSN 2229-5518. [24] Li B, Wang Z, Mosalam KM, Xie H. Wenchuan earthquake field reconnaissance on reinforced concrete framed buildings with and without masonry infill walls. The 14th World Conference on Earthquake Engineering. Beijing, China. 2008. [25] Alqatamin A, Talpoşi A. The action of short columns at reinforced concrete building constructions. Bull Transilvania Univ of Braşov 2009;2:51. [26] Chiou YJ, Tzeng JC, Liou YW. Experimental and analytical study of masonry infilled frames. J Struct Eng 1999;125(10):1109–17. https://doi.org/10.1061/(ASCE)07339445(1999) 125:10(1109). [27] Kheyroddin A. Seismic behavior of short columns in RC structures on slope surface. J Model Eng 2009;4(18):59–65. [28] Vahidi EK, Malekabadi MM. Conceptual investigation of short-columns and masonary infill frames effect in the earthquakes. World Acad Sci Eng Technol 2009;59:119–24. [29] Suresh Babu R, Venkatsubramani R, Venkatasubramani GS. Seismic strengthening of partially infill RC buildings using brick inserts–experimental investigation on 3D model structur. J Eng Appl Sci 2011;6(12):123–30. [30] Ramin K, Mehrabpour F. Study of short column behavior originated from the level difference on sloping lots during earthquake (Special Case: Reinforced Concrete Buildings). Open J Civ Eng 2014;4(01):23. https://doi.org/10.4236/ojce.2014. 41003. [31] Surendran S, Kaushik HB. Masonry infill RC frames with openings: review of inplane lateral load behaviour and modeling approaches. Open Constr Build Technol J 2012;6(Suppl. 1–M9):126–54. [32] Bahadir F, Kamanli M, Korkmaz HH, Balik FS, Unal A, Korkmaz SZ. Strengthening of gravity load designed reinforced concrete frames with the external RC shear walls. Adv Mater Res 2013;747:265–8. https://doi.org/10.4028/www.scientific. net/AMR.747.265. [33] Balik FS, Korkmaz HH, Kamanli M, Bahadir F, Korkmaz SZ, Unal A. An experimental study on reinforced concrete infilled frames with openings. Adv Mater Res 2013;747:429–32. https://doi.org/10.4028/www.scientific.net/AMR.747.429. [34] Unal A, Kaltakcı MY, Balik FS, Korkmaz HH, Bahadir F, Korkmaz SZ, et al. Strengthening of reinforced concrete frames not designed according to TDY2007 with external shear walls. Adv Mater Res 2013;747:433–6. https://doi.org/10.
are examined, it provides information about the failure mechanisms of the specimens. It is not the maximum acceleration that occurs in the building but the acceleration value at the first crack that determines the characteristics and location of the failure mode. Specimen 1 had the highest peak ground acceleration among all specimens. Because plastic hinge behaviour occurred at the columns, the lowest acceleration was recorded for Specimen 5. Because the first storey of Specimen 1 (space ratio of 0%) had no band-type windows, its stiffness was lower than that of the second storey. Therefore, it is the second storey of Specimen 1 that collapsed. Short-column and soft-storey behaviours were not observed. For Specimen 2 (band-type window space ratio of 25%), shortcolumn behaviour was clearly observed. For Specimen 3 (band-type window space ratio of 50%), shortcolumn behaviour began to appear in the direction of the band-type windows of the first storey. At the same time, the column–beam joints of the second storey also underwent plastic hinge behaviour. Because of this plastic hinging, the second storey of Specimen 3 collapsed. Specimen 4 (band-type window space ratio of 75%) collapsed because of short-column and soft-storey behaviours. For Specimen 5 (space ratio of 100%), soft-storey behaviour was clearly observed.
In conclusion, short-column behaviour in the structure became more prevalent as the band-type window space ratio in the infill wall decreased. In addition, as the ratio of the band-type window space in the infill wall increased, soft-storey behaviour also occurred. The bandtype window space ratio in the infill walls of the ground floor significantly affected the seismic performance of the structures. For this reason, band-type window applications should definitely be avoided to improve building safety. However, if a band-type window is an architectural feature, transversal reinforcement must be extended along the full storey length of the columns, thereby preventing short-column behaviour [2]. In existing structures with band-type windows, a strengthening application should be implemented to reduce the negative effects of band-type windows. 6. Data availability statement All raw data generated or used during the study are available in the DesignSafe-CI repository online in accordance with funder data retention policies. Bahadir, F., (2019-07-07) “PRJ-2442|SHORT COLUMN AND SOFT STORY BEHAVIOR AT 3D RC FRAMES ON THE SHAKING TABLE.”. DesignSafe-CI. doi: https://doi.org/10.17603/ds2-z4hp-nv28. References [1] Yakut A, Binici B, Canbay E, Erberik A, Sarıtaş A, Aldemir A, Demirel İO, Erdil B, Ay BÖ, Özçelik R. Chapter 3 – B site observation of seismic and structural damage on Van earthquake of Mw = 7.2, October 23, Report No: METU/EERC 2011-04. Ankara, Turkey: EERC, Middle East Technical University; 2011. [2] TEC-2007. Specification for Structures to be Built in Disaster Areas PART III EARTHQUAKE DISASTER PREVENTION TEC-2007, Ministry of Public Works and Settlement Government of Republic of Turkey, Ankara, Turkey. [3] Penzien J, Bouwkamp JG, Clough RM, Dixon R. Feasibility Study Large-Scale Earthquake Simulator Facility, EERC Report No: 67/01, Earthquake Engineering Research Center, University of California, Berkeley; 1967.p. 3–17. [4] Stephen RM, Bouwkamp JG, Clough RW, Penzien J. Structural dynamic testing facilities at the university of California. Berkeley: Earthquake Engineering Research Center, College of Engineering, University of California; 1969. [5] Başaran H, Demir A, Bağcı M, Ercan E. Shaking table study of masonry buildings with reinforced plaster. Građevinar 2014;66(07.):625–33. https://doi.org/10. 14256/JCE.1036.2014.
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F. Bahadir 4028/www.scientific.net/AMR.747.433. [35] Bahadir F, Balik FS. Seismic performance improvement of 3D reinforced concrete frames with different strengthening applications, applied mechanics and materials manufacturing science and technology VI chapter: 8. Civ Eng 2015;789–790:1140–4. [36] Zhang Hao, et al. Experimental and numerical investigations on seismic responses of reinforced concrete structures considering strain rate effect. Constr Build Mater 2018;173:672–86. https://doi.org/10.1016/j.conbuildmat.2018.04.085.
[37] Testing hardened concrete-Part 3: Compressive strength of test specimens, TS EN 12390-3, Turkish Standardization Institute, Ankara, Turkey; 2010. [38] Labjack U3-HV, https://labjack.com/support/datasheets/u3; 2015. [39] ADXL345 Evaluation Board, 2009. https://www.sparkfun.com/datasheets/ Sensors/Accelerometer/ADXL345.pdf. [40] Safak E, Harmsen S. Seismic hazard and design by using energy flux. Proc Twelfth World Conf Earthquake Eng 2000;658:1–8.
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Experimental study on three-dimensional reinforced concrete frames subjected to dynamic loading
T
Fatih Bahadir School of Higher Education Construction Technology Program, Necmettin Erbakan University, Konya, Turkey
ARTICLE INFO
ABSTRACT
Keywords: Shaking table Failure mode Short-column Soft-storey Response spectrum Elastic seismic load Energy flux
During an earthquake, soft-storey and short-column behaviours, which usually occur around window openings, can cause significant damage to structures, or even collapse, if the measures specified in regulations are not applied. This study focuses on band-type window spaces that can cause short-column and soft-storey behaviours. Reinforced concrete frames were produced on a 1/6 geometric scale, and two-storey, single-span frames were tested on a shaking table. The second storeys of all specimens had infill walls and the same window openings. The first storeys of the specimens were produced with different band-type window heights. The specimens were produced with band-type window space ratios at the infill brick wall of the first storey of 0%, 25%, 50%, 75%, and 100%, respectively. These specimens were tested on a shaking table under sinusoidal dynamics. The behaviour of the specimens, failure mechanisms, response spectra, elastic seismic loads, and energy flux values were compared by subjecting the specimens to the same ground motion. The experimental results demonstrated that the short-column and the soft-storey behaviours are effective when the band-type window space ratio in the infill brick walls decreases.
1. Introduction It is known that most structures built in seismic regions in Turkey are defective in terms of design and construction. More than 30,000 buildings were damaged because of these defects during the Van–Erciş earthquake. Many of the reinforced concrete (RC) buildings that collapsed in the region were observed to have construction defects or were irregular buildings [1]. Studies indicated that soft-storey irregularities were present in 61% of the collapsed structures and short-column behaviour was exhibited in 6% of the collapsed structures. There are shops and parking lots on the ground floor of many buildings in Turkey. The other floors of these buildings are built with filling walls. Infill walls of the floor are much more rigid than walls or windows with floors. These are attributes not only of new constructions but also of converted old houses. Such weak floors are called “soft storeys.” If the height of one floor is higher than that of the other floors, a soft-storey irregularity is present. Furthermore, as a result of the outof-plane overturning of the walls on the floor, soft-storey behaviour is observed. The soft-storey mechanism steps are given in Fig. 1.
Short columns may develop owing to structural arrangements or openings provided in infill walls between columns [2]. Such spaces are often seen in the case of band-type windows at the basement floor level and mezzanine floor at the ground floor level. The short-column behaviour, which is usually occurring around the window opening, can cause significant damage to the structure during an earthquake, or even collapse, if the measures specified in regulations are not applied. Because the short column is more rigid than the other columns in the structure, it has a very high shear strength value. Because of shear failure, the column may lose its load-carrying capacity and the structure may collapse. Damage resulting from the short column of Specimen 2 is given in Fig. 2. Since 1960, shaking tables have been used for scientific research on earthquakes and for structural engineering studies [3–17]. Generally, experiments on short-column and soft-storey behaviours are rarely conducted in three dimensions and on shake tables [18]. However, twodimensional experimental studies on soft storeys [19–24] and short columns [25–31] have been conducted many times. In this study, twostorey, single-span RC frames produced on a 1/6 geometric scale were
E-mail address: [email protected]. https://doi.org/10.1016/j.istruc.2020.01.045 Received 18 October 2019; Received in revised form 5 December 2019; Accepted 28 January 2020 2352-0124/ © 2020 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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Fig. 1. The soft story failure mechanism.
tested on a shaking table. The first storey of the specimens was produced with different band-type windows heights. These specimens were produced with band-type window space ratios at the infill brick wall of the first storey of 0%, 25%, 50%, 75%, and 100%, respectively. Soft-storey and short-column behaviours, which usually occur around the window opening, can cause significant damage to a structure, or even collapse, during an earthquake, if the measures specified in the regulations are not applied. The aim of this study is to investigate the effect of the size of the band windows that cause short-column and soft-storey behaviours in the seismic response of the structures. In this study, the failure mechanisms, acceleration values, and energy flux values of the specimens were compared by applying the same ground motion to the specimens.
2. Materials and method 2.1. Description of test specimens In this experimental study, five three-dimensional (3D) RC frames with two stories produced on a 1/6 geometric scale, which contained deficiencies commonly observed in residential buildings in Turkey, were tested on a shaking table. The second storey of all specimens contained brick walls and the same window openings, while the first storey of all specimens was produced with different band-type window spaces of brick walls (except Specimen 1 and Specimen 6). Specimen 1 was also produced as reference specimen RS1, which featured a full
Fig. 2. The short column behaviour.
Fig. 3. General photo of the test set-up for the experimental study.
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brick wall on the first storey. Specimen 6 was produced as reference specimen RS2, and had no brick wall on the first storey. The specimens were manufactured and tested at the Structural Testing Laboratory at the Necmettin Erbakan University–Konya, Turkey (Fig. 3).
In the five test specimens, the width and height of the structural elements, concrete qualities, and reinforcement forms of the frames were the same. Test frames were detailed and constructed deliberately with some deficiencies such as low strength concrete usage, strong
Fig. 4. Dimensional and reinforcements details of the general specimen (dimension: mm).
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Table 1 Parameters of test specimens. Test Specimen
Space Ratio of 1st storey The band windows dimensions of 1st storey The brick wall dimensions of 1st storey
Specimen 1
Specimen 2
Specimen 3
Specimen 4
Specimen 5
0% – 400 × 600 × 30 mm
25% 100 × 600 mm 300 × 600 × 30 mm
50% 200 × 600 mm 200 × 600 × 30 mm
75% 300 × 600 mm 100 × 600 × 30 mm
100% – –
beam–weak column formation, wide spacing of beam and column stirrups, no column stirrups at the beam–column joints, and no confinement zones at the end of the columns and beams. In addition to these deficiencies, the stirrups were prepared with 90° hooks and
placed at free ends of the columns and beams of the test specimens [32,33,34]. The height of one storey was 500 mm (3000 mm in 1:1 scale real dimensions). The length of the frame was 700 mm from one column to
Fig. 5. Production stages of specimens.
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Table 3 Equation Factors of Ground Motion. Ground Motion
Table 2 Scale factors for experimental study at shaking tables. Dimension
Scale Factors
Gravitational acceleration, g Velocity, v Time, t Linear Dimension, l Displacement, δ Frequency, ω Area, A Volume, V Modulus, E Energy, EN Mass density, ρ
LT−2 LT−1 T L L T−1 L2 L3 FL−2 FL FL−4T2
1 Sl1/2 Sl1/2 Sl Sl Sl−1/2 Sl2 Sl3 1 Sl3 1/Sl
Amplitude (mm)
Period P (sec)
PhaseShift PS (rad)
Frequency f
0–2.82 2.82–5.13 5.13–6.39 6.39–7.54 7.54–8.99 8.99–9.61 9.61–11.27 11.27–12.35 12.35–13.86 13.86–15.61 15.61–19.82
16.52 16.45 18.77 16.34 19.30 18.90 19.09 17.72 17.16 18.56 18.19
1.421 0.938 0.420 0.562 0.299 0.304 0.318 0.667 0.753 0.870 1.054
12.77 6.540 33.03 6.874 9.044 −0.629 −10.27 37.36 25.57 22.10 73.65
0.704 1.067 2.379 1.780 3.341 3.290 3.143 1.500 1.328 1.150 0.949
specimens was produced with different band-type window heights. These specimens were produced with band-type window ratio spaces of 0%, 25%, 50%, 75%, and 100%, respectively. The parameters of the test specimens are given in Table 1. The production stages of specimens are shown in Fig. 5. The test specimens were cast of micro-concrete and steel wire [36]. The average yield strengths of ϕ2 and ϕ3 steel wires were ~580 MPa and that of ϕ5 steel wire was ~590 MPa. The average cylinder compressive strength of the micro-concrete used was 6 MPa. The maximum aggregate size of micro-concrete was chosen as 3 mm. Experiments used to determine micro-concrete properties were conducted under the TS EN-12390-3 standard [37]. Because of the nonhomogeneous structures of the brick walls, it is very difficult to obtain the modulus of elasticity and Poisson ratio using the test data for brick walls. Infill brick walls of specimens were tested under diagonal compression. The average diagonal compressive strength of the infill wall was ~0.5 MPa. The test setup for the brick wall is presented in Fig. 6. The scale factors used for measurements and the dimensions of specimens on the shake table are given in Table 2. The same ground motion was applied to specimens over short time periods at high frequencies [5].
Fig. 6. Test setup of the brick wall.
Parameter
Time (sec)
2.2. Test setup another column. Plain bars were used for longitudinal reinforcement and stirrups. The column dimensions were 50 × 80 mm, and four 3mm-diameter bars were used as longitudinal reinforcement. The beam dimensions were 50 × 90 mm, and six 3-mm-diameter plain bars were used. Reinforcements with a diameter of 2 mm with 50-mm spacing were used as stirrups at columns and beams of specimens. Dimensional and reinforcement details of the specimens are shown in Fig. 4. The second storey of all specimens contained 200 × 300 mm dimension window openings in the mid-span of the two long faces and 200 × 200 mm dimension window openings in the mid-span of the two short faces. The middle axis of the brick walls and frame did not coincide; rather, the external surface of the brick wall shared the same axis as the external surface of the beams. The thickness of the wall was 30 mm while the depth of the columns was 50 mm or 80 mm. The bricks used in the infill walls were produced with plaster. The brick dimensions were 30 × 50 × 25 mm [35]. The first storey of the
The shaking table used in experiments operates under the working principle of converting rotary motion to linear motion. The specimens were tested under constant axial load and sinusoidal cyclic motion was imposed to simulate seismic action. The sinusoidal cyclic motion was processed through a data acquisition card [38]. The specimens were tested with a ground motion on the shaking table until failure. The general sinusoidal function applied was
A (t ) = A sin[(2 t /
L)
+ P ],
(1)
where A is the amplitude (the height of the top of the waves), t is time (of ground motion), A(t) is the displacement value of the shaking table, ωL is the angular velocity of the motor disc, is the wavelength (the time required for a complete cycle), and P is the phase shift (for which a value of 0 rad sets A(t) equal to 0 at t = 0 and a value of π rad sets A(t) equal to its maximum when t = 0). These equation factors are given Table 3. The graph of this function is shown in Fig. 7.
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Fig. 7. General sinusoidal function and graph.
The main parameters of the shaking table are given in Table 4. The actual block diagram of the experiments is shown in Fig. 8. The views used to describe the damage and cracks to the specimens are shown in Fig. 9. Acceleration data for the test frames were measured with ADXL345 accelerometers in the experiments (Fig. 10). These accelerometers were situated on each floor at two locations. These were used to determine the acceleration of each storey level. The ADXL345 [39] is a complete threeaxis acceleration measurement system with a selectable measurement range of ± 2, ± 4, ± 8, or ± 16 g. It measures both dynamic accelerations resulting from motion or shock and static acceleration. The shake table accelerations were measured in both X and Y directions.
Table 4 Main parameters of shaking table. Parameters
Values Units
Size of the platform Maximum mass of load Maximum displacement of the platform Maximum acceleration of the platform Frequency Maximum power of motor Maximum output torque of gear speed reducer Maximum input rotational speed of gear speed reducer Maximum output rotational speed of gear speed reducer
80 mm × 1200 mm 1500 kg ± 20 mm ±4 g 0–50 Hz 4 kW 100518 mNm 3000 rpm 380 rpm
Fig. 8. Actual block diagram of control system.
Fig. 9. 3D and plan view of Specimen 3. 840
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3. Experimental study Acceleration data for the specimens on the first storey and second storey levels in the X direction (the direction of ground movement) were measured. The measured acceleration data for Specimens 1–5 are shown in Figs. 11–15, respectively. The maximum positive and negative accelerations, duration, and failure mode of the specimens are given in Table 5. The first specimen was the reference frame (Specimen 1). The bandtype windows were not placed at the first storey of Specimen 1. The ratio of the band-type window height to the brick wall height is 0%. Specimen 1 was tested to observe the reference behaviour for comparison purposes. According to acceleration data of Specimen 1, this experimental study lasted 19.52 s. No damage occurred on the first storey of this specimen, whereas the whole damage occurred at the second storey. As the brick walls at the window level of the second storey were destroyed, short-column behaviour occurred in the direction of ground movement. Specimen 1 did not completely collapse and only the second storey collapsed at the end of the test. Because there are
Fig. 10. Location points of accelerometers.
Fig. 11. Acceleration data of Specimen 1 (RS1-%0 space).
Fig. 12. Acceleration data of Specimen 2 (%25 space).
Fig. 13. Acceleration data of Specimen 3 (%50 space).
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Fig. 14. Acceleration data of Specimen 4 (%75 space).
Fig. 15. Acceleration data of Specimen 5 (%100 space). Table 5 The failure mechanisms of specimens. Specimen
Failure Type
Duration (sec)
Story
Max Positive Acceleration
Max Negative Acceleration
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp. (mm)
1
0%
No Damage Plastic Hinge Formation
19.52
1 2
7.2 6.45
2.21 4.44
7.5 8.75
14.32 6.18
−2.12 −3.29
10 15
2
25%
Short Column No Damage
5.01
1 2
3.72 3.66
1.99 0.79
201 116
4.25 4.4
−1.78 −1.39
157 234
3
50%
Short Column
9.2
1 2
5.01 5.07
2.49 2.33
40 −25
4.86 4.61
−2.27 −1.99
−15 25
4
%75
Short Column No Damage
4.43
1 2
3.83 3.25
2.00 2.54
97.5 15
4.3 3.2
−1.99 −2.0
217.5 62.5
Failure Mode
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Table 5 (continued) Specimen
5
%100
Failure Type
Plastic Hinge Formation No Damage
Duration (sec)
13.27
Story
1 2
Max Positive Acceleration
Max Negative Acceleration
Failure Mode
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp. (mm)
4.19 4.26
1.59 1.42
17.5 20
3.41 3.69
−1.3 −1.27
10 12.5
Table 6 First Cracks of Specimen 1. Specimen No
Story
First Crack of Frame
First Crack of Walls
Forward
1
0%
Backward
Forward
Backward
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
1 2 Fig.
6.68
1.75 3.17
5 14
7.06
−1.51 −2.67
7.5 8.75
6.66
1.63 3.65
10 11.25
7.06
−1.51 −2.67
7.5 8.75
View
a
c
c
Fig. 16. Failure mode of Specimen 1 (0% space).
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Table 7 First Cracks of Specimen 2. Specimen No
Story
First Crack of Frame
First Crack of Walls
Forward
2
25%
Backward
Forward
Backward
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
1 2 Fig.
1.62
0.7 0.23
10.83 15
1.83
−0.99 −0.02
8.33 11.67
1.68
0.23 0.72
7.5 6.664
1.83
−0.99 −0.02
8.33 11.67
View
a
b-c
b
b–c
Fig. 17. Failure mode of Specimen 2 (25% space).
Table 8 First Cracks of Specimen 3. Specimen No
Story
First Crack of Frame
First Crack of Walls
Forward
3
50%
Backward
Forward
Backward
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
1 2 Fig.
4.88
0.48 2.49
20 25
4.72
−0.57 −2.27
15 10
4.88
0.48 2.49
20 25
4.72
−0.57 −2.27
15 10
View
a
a-b
b
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Fig. 18. Failure mode of Specimen 3 (50% space).
Fig. 19. Failure mode of Specimen 4 (75% space).
no first storey windows, only the second storey experienced damage and collapse. The measured acceleration values at which the first cracks appeared and damage occurred are given in Table 6. Cracks and damages to Specimen 1 are shown in Fig. 16 at the end of the test. The ratio of the band-type window height to the brick wall height is
25% for Specimen 2. The 25% band-type window space on the first storey caused a great shear force as a result of the ground movement, thus causing short-column behaviour in the column in this section. In this test, short-column behaviour was clearly observed. Specimen 2 completely collapsed in a short time. According to acceleration data of
Table 9 First Cracks of Specimen 4. Specimen No
Story
First Crack of Frame
First Crack of Walls
Forward
4
75%
Backward
Forward
Backward
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
1 2 Fig.
1.33
1.3 −0.01
12.5 12.5
1.5
−0.95 0.03
47.5 45
1.33
1.3 −0.01
12.5 12.5
1.5
−0.95 0.03
47.5 45
View
a
b
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Fig. 20. Failure mode of Specimen 5 (100% space). Table 10 First Cracks of Specimen 5. Specimen No
Story
First Crack of Frame
First Crack of Walls
Forward
5
100%
Backward
Forward
Backward
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
Time (sec)
Acce (g)
Disp (mm)
1 2 Fig.
4.15
1.38 0.13
10 10
4.67
−0.2 −0.41
60 62.5
–
– –
– –
–
– –
– –
View
c
Specimen 2, this test lasted 5.01 s. Accelerations and displacements of the first cracks in the specimen are given in Table 7. Cracks and damages to Specimen 2 at the end of the test are shown in Fig. 17. The ratio of the band-type window height to the brick wall height is 50% for Specimen 3. Although there were cracks on both stories of Specimen 3, more cracks formed on the second storey. Although there were cracks in the frame at the level of the band-type window of the first storey, the frame of the second storey was previously damaged and the second storey collapsed. The second storey of Specimen 3 completely collapsed at the end of the test. According to acceleration data of Specimen 3, this test lasted 9.2 s. First storey accelerometers recorded no measurements after 7.43 s. The measured acceleration values at which the first cracks appeared and damage occurred are given in Table 8. Cracks and damages to Specimen 3 are shown in Fig. 18 at the end of the test. The ratio of the band-type window height to the brick wall height is 75% for Specimen 4. Both short-column and soft-storey behaviours were observed in the first storey. The first storey of Specimen 4
–
–
completely collapsed at the end of the test. Cracks and damages in Specimen 4 are shown in Fig. 19 at the end of the test. According to acceleration data of Specimen 4, this test lasted 4.43 s. The measured acceleration values at which the first cracks appeared and damage occurred are given in Table 9. The fifth specimen was the other reference specimen. The first storey of this specimen did not contain any brick walls and was tested to record the reference behaviour for comparison purposes. According to acceleration data of Specimen 5, this test finished at 13.27 s. Specimen 5 did not collapse at the end of the test. However, this specimen will collapse under small acceleration because the column–beam joints are hinged. Soft-storey behaviour was observed in the first storey. Cracks and damages to Specimen 5 are shown in Fig. 20 at the end of the test. The measured acceleration values at which the first cracks appeared and damage occurred are given in Table 10. After reinforcement of the right column of the “a” view of the first storey were broken at 4.97 s, Specimen 5 was observed to undergo torsion and column–beam joints of the first storey underwent plastic hinge behaviour.
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4. Comparison of experimental results
4.2. Elastic seismic load
The purpose of the measurement of seismic motion is to measure acceleration as a period or frequency and to observe the behaviours of structures during an earthquake. For this reason, acceleration records constitute an important database for engineering applications and scientific studies.
The acceleration response spectrum is the envelope curve plotted from the peaks of maximum acceleration with the corresponding vibration time (T) of each single-degree-of-freedom system. In the spectrum to be used in the calculation of the inertial force of the internal forces formed in the structure, the acceleration value to be used is not the total acceleration value (u¨ (t ) + u¨g (t ) , where u(t) is the displacement u at time t and ug(t) is the relative displacement ug at time t) but the acceleration value (ω2SD) at maximum displacement. This acceleration is called the pseudo acceleration and its spectrum is called the pseudo spectral acceleration. The relationship between the pseudo spectral acceleration PSA and spectral displacement SD is
4.1. Response spectrum A building exposed to an earthquake load first vibrates during the period that it is subjected to the earthquake and then vibrates at its own period. If the members of the building do not have damping properties, resonance occurs in the building and causes the building to collapse. The basic items that reduce the period of the building are the weight of the building and irregularities in the building. The exposure of a building to earthquakes at different times can cause a building’s period to be greater during the next same magnitude earthquake. The vibrations in different periods during the earthquake can be described by a response spectrum graph [2], which shows the characteristics of the behaviour in terms of maximum displacement, acceleration, and velocity. These graphs of the test specimens are given in Fig. 21. In the analysis of the dynamic behaviour of buildings, it must be considered that each earthquake has its own unique acceleration spectrum. According to the natural period and damping rate of the building, the maximum response value obtained from the acceleration response spectrum is the absolute acceleration value that influences the building. The mean period (Tm) of the ground motion has been regarded as the preferred frequency content parameter. According to this parameter, the second storey of Specimen 4 had the longest mean period compared to that of the other specimens. In addition, the second storey of Specimen 2 was measured to have the longest mean period compared to that of the other specimens, except for Specimen 4. The main periods for the second storey of the specimens were calculated as 0.301 s for Specimen 1, 0.370 s for Specimen 2, 0.287 s for Specimen 3, 0.559 s for Specimen 4, and 0.308 s for Specimen 5. The main periods for the first storey of the specimens were calculated as 0.303 s for Specimen 1, 0.280 s for Specimen 2, 0.233 s for Specimen 3, 0.248 s for Specimen 4, and 0.351 s for Specimen 5. During these periods, significant damage occurred to Specimens 2 and 3.
PSA =
2SD,
(2)
where ω is the angular frequency (frequency of vibration). Under the influence of seismic activity, the building carrier system must be able to withstand at least the following elastic seismic load Fel:
Fel = m PSA,
(3)
where m is the mass. If the masses of the specimens are approximately the same, these values can be used as a comparison. Accordingly, the maximum elastic seismic load was calculated for the first and second storeys of Specimen 1 (0% space), given that the specimens were given the same ground motion and that the mass of the specimens was approximately the same. The minimum elastic seismic load was also calculated for the second storey of Specimen 2 (25% space). Because the first storey columns of Specimen 5 underwent plastic hinge behaviour in the early period, the shear loads of Specimen 5 occurred at quite a low level. According to the PSA data obtained from the experiments (Fig. 22), Specimen 1 was subjected to the highest elastic seismic load compared to that of the other specimens. Because the seismic load acting on this specimen lasted ~ 18.50 s, the highest elastic seismic load occurred on both the first and second stories. Compared to other specimens (except Specimen 1), Specimen 2 did not provide the expected seismic performance in terms of both seismic effect time and elastic load.
Fig. 21. The response spectrum graphs of the test specimens at the 5% damping.
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Fig. 22. The response Pseudo Spectral Acceleration graphs of test specimens by 5% damping.
4.3. Energy flux
Table 11 Summary of test results as energy flux.
Energy flux values can provide important numerical data to investigate the effect of seismic energy on the ground and the response of buildings. Energy flux is correlated with the damage caused by the earthquake. The cumulative energy flux Esum (in joules) can be defined as the amount of seismic energy passing through a unit cross-sectional area per unit time [40]:
Esum
1 = AV 2
b a
v 2 (t ) dt
Test Specimens
(4)
where ρ is the mass density of the specimen (in kg f /m3 ) (taken as 1 unit mass density in energy flux calculations), A is the cross-sectional area of the soil layer (in m2) (taken as 1 unit area in energy flux calculations), V is the velocity of seismic waves (in m/s), v (t ) is the velocity response of the specimen (in m/s), anda b is the duration of the earthquake releasing 5% and 95% of the total energy (in seconds). To use the energy flux value of a seismic wave, it is necessary to know the magnitude and distance of this wave. Because each sample was tested under the same conditions on the shaking table, the propagation velocity of seismic waves is the same. The energy flux values of test specimens are given in Table 11. The maximum energy flux value occurred for Specimen 1 and the minimum energy flux value occurred for Specimen 2. Because the seismic load acting on Specimen 1 lasted ~18.50 s, the highest energy flux occurred on the second storey. Because of the high elastic seismic load and the short failure duration, the energy absorption of Specimen 2 was not sufficient to prevent damage.
Name
Space Ratio
Specimen 1
%0
Specimen 2
%25
Specimen 3
%50
Specimen 4
%75
Specimen 5
%100
Story
Time (sec)
Energy Flux (J/cm3)*
1 2 1 2 1 2 1 2 1 2
2.47–17.82 2.43–18.50 0.98–4.72 0.70–4.52 1.92–6.32 1.56–8.71 0.41–4.52 2.46–4.84 1.20–10.30 1.04–10.23
402295 103149824 114931 26174 67242 1359396 45981 618195 1179970 244503
* These values were calculated at significant duration.
5. Summary and conclusion In this study, 3D RC frames were tested on a shaking table to determine their dynamic behaviour. Two-storey, single-span frame specimens were constructed at 1/6 geometric scale with different bandtype window space ratios. The specimens were tested under the same ground motion until failure occurred. According to the test results, the failure mechanisms, first cracks, and acceleration values of all specimens can be summarised as follows:
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• When the location and type of the first cracks of the test specimens • • • •
• •
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Assessment of Seismic Performance of Reinforced SMB Masonry Building Models through Shock Table and Shaking Table Tests, CiSTUP Indian Institute of Science, Bangalore; 2012. [16] Sharma A, Reddy GR, Vaze KK. Shake table tests on a non-seismically detailed RC frame structure. Struct Eng Mech 2012;41(1):1–24. [17] Saito T, Moya L, Fajardo C, Morita K. Experimental study on dynamic behavior of unreinforced masonry walls. J Dis Res 2013;8(2). [18] Kabir MZ, Shadan P. Seismic performance of 3D-panel wall on Piloti RC frame using shaking table equipment. Proceedings of the 8th International Conference on Structural Dynamics. EURODYN; 2011. [19] Dogan M, Kirac N. Soft Story Behaviour in Earthquake and Samples of Izmit – Duzce. ECAS 2002 Uluslarararası Yapı ve Deprem Mühendisliği Sempozyumu. Ankara, Turkey: Middle East Technical University; 2002. [20] Setia S, Sharma V. Seismic response of R.C.C building with soft story. Int J Appl Eng Res 2012;7(11). ISSN 0973-4562. [21] Dolšek M, Fajfar P. Soft story effects in uniformly infilled reinforced concrete frames. J Earthquake Eng 2001;5(1):1–12. [22] Wibowo A, Wilson JL, Gad EF, Lam NTK, Collier P. Drift capacity of a precast softstorey building in Melbourne. Aust J Struct Eng 2010;11(3):177–93. [23] Miss Desai Pallavi T. Seismic performance of soft story composite column. Int J Sci Eng Res 2013;4(1). ISSN 2229-5518. [24] Li B, Wang Z, Mosalam KM, Xie H. Wenchuan earthquake field reconnaissance on reinforced concrete framed buildings with and without masonry infill walls. The 14th World Conference on Earthquake Engineering. Beijing, China. 2008. [25] Alqatamin A, Talpoşi A. The action of short columns at reinforced concrete building constructions. Bull Transilvania Univ of Braşov 2009;2:51. [26] Chiou YJ, Tzeng JC, Liou YW. Experimental and analytical study of masonry infilled frames. J Struct Eng 1999;125(10):1109–17. https://doi.org/10.1061/(ASCE)07339445(1999) 125:10(1109). [27] Kheyroddin A. 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are examined, it provides information about the failure mechanisms of the specimens. It is not the maximum acceleration that occurs in the building but the acceleration value at the first crack that determines the characteristics and location of the failure mode. Specimen 1 had the highest peak ground acceleration among all specimens. Because plastic hinge behaviour occurred at the columns, the lowest acceleration was recorded for Specimen 5. Because the first storey of Specimen 1 (space ratio of 0%) had no band-type windows, its stiffness was lower than that of the second storey. Therefore, it is the second storey of Specimen 1 that collapsed. Short-column and soft-storey behaviours were not observed. For Specimen 2 (band-type window space ratio of 25%), shortcolumn behaviour was clearly observed. For Specimen 3 (band-type window space ratio of 50%), shortcolumn behaviour began to appear in the direction of the band-type windows of the first storey. At the same time, the column–beam joints of the second storey also underwent plastic hinge behaviour. Because of this plastic hinging, the second storey of Specimen 3 collapsed. Specimen 4 (band-type window space ratio of 75%) collapsed because of short-column and soft-storey behaviours. For Specimen 5 (space ratio of 100%), soft-storey behaviour was clearly observed.
In conclusion, short-column behaviour in the structure became more prevalent as the band-type window space ratio in the infill wall decreased. In addition, as the ratio of the band-type window space in the infill wall increased, soft-storey behaviour also occurred. The bandtype window space ratio in the infill walls of the ground floor significantly affected the seismic performance of the structures. For this reason, band-type window applications should definitely be avoided to improve building safety. However, if a band-type window is an architectural feature, transversal reinforcement must be extended along the full storey length of the columns, thereby preventing short-column behaviour [2]. In existing structures with band-type windows, a strengthening application should be implemented to reduce the negative effects of band-type windows. 6. Data availability statement All raw data generated or used during the study are available in the DesignSafe-CI repository online in accordance with funder data retention policies. Bahadir, F., (2019-07-07) “PRJ-2442|SHORT COLUMN AND SOFT STORY BEHAVIOR AT 3D RC FRAMES ON THE SHAKING TABLE.”. DesignSafe-CI. doi: https://doi.org/10.17603/ds2-z4hp-nv28. References [1] Yakut A, Binici B, Canbay E, Erberik A, Sarıtaş A, Aldemir A, Demirel İO, Erdil B, Ay BÖ, Özçelik R. Chapter 3 – B site observation of seismic and structural damage on Van earthquake of Mw = 7.2, October 23, Report No: METU/EERC 2011-04. Ankara, Turkey: EERC, Middle East Technical University; 2011. [2] TEC-2007. Specification for Structures to be Built in Disaster Areas PART III EARTHQUAKE DISASTER PREVENTION TEC-2007, Ministry of Public Works and Settlement Government of Republic of Turkey, Ankara, Turkey. [3] Penzien J, Bouwkamp JG, Clough RM, Dixon R. Feasibility Study Large-Scale Earthquake Simulator Facility, EERC Report No: 67/01, Earthquake Engineering Research Center, University of California, Berkeley; 1967.p. 3–17. [4] Stephen RM, Bouwkamp JG, Clough RW, Penzien J. Structural dynamic testing facilities at the university of California. Berkeley: Earthquake Engineering Research Center, College of Engineering, University of California; 1969. [5] Başaran H, Demir A, Bağcı M, Ercan E. Shaking table study of masonry buildings with reinforced plaster. Građevinar 2014;66(07.):625–33. https://doi.org/10. 14256/JCE.1036.2014.
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