5aese korea univ hslee 2 final

5th International Conference on Advances in Experimental Structural Engineering November 8-9, 2013, Taipei, Taiwan

Earthquake Simulation Tests on a 1:15 Scale 25-Story RC Flat-Plate Core-Wall Building Model

H.S. Lee1, S.H. Choi2, K.R. Hwang2, Y.H. Kim3, S.H. Lee4

1 Professor, School of Civil, Environmental, and Architectural Engineering, Korea University, Korea, E-mail: [email protected] 2 Graduate student, School of Civil, Environmental, and Architectural Engineering, Korea University, Republic of Korea 3 Engineer, Daebang construction Co. Ltd, Seoul, Republic of Korea 4 Professor, Dept. of. Architechral Engineering, Pusan National University, Busan, Republic of Korea

ABSTRACT This paper presents the results of earthquake simulation tests on a 1:15 scale 25-story RC flat-plate core-wall building model satisfying the detailing requirement of the special shear wall system. The following conclusions are drawn based on the test results: (1) The fundamental period of the test model simulates well that of the prototype obtained using the elastic mode analysis at the design phase, and the base shear coefficient of the test model shows much larger values than that estimated in accordance with KBC2009, or IBC2006. (2) In the test, the vertical distribution of acceleration under the shake table excitations reveals the effect of the higher modes with the free vibration after the termination of shake table excitations being governed by the first mode. (3) The maximum inter-story drift ratio of the test model, 0.43%, under the design earthquake in Korea was much smaller than the design limit, 1.5%. And, (4) the modes of cracks appear to be the combination of flexure and shear in the slab around the peripheral columns and in the coupling beam. KEYWORDS: reinforced concrete; flat plate; special shear wall; earthquake simulation test

1. INTRODUCTION Recently, the number of high-rise buildings (higher than 30 stories) has been increasing, for the efficient use of available housing site. For these high-rise buildings, a combined system of core shear walls: a lateral load resistance structural system, and flat-plates: a gravity load resistance structural system, has been widely used. These structural types in current seismic provisions, KBC2009 [1] and IBC2006 [2], are classified as dual frame or building frame system. For the shear walls in the building frame system, special shear walls, for which special seismic detailing requirements are imposed, or ordinary shear walls, which have a height restriction, have been generally used. However, in the case of the RC flat-plate structure, seismic detailing requirements for the connection with columns are given only as part of intermediate moment frames in ACI 318-05 [3]. Furthermore, in the dual frame or building frame systems, two vertical shear walls generally include regular openings, and are connected each other with coupling beams, which have a great effect on the lateral resistance behavior. Although a number of experimental and analytical studies [4-6] have been done on the high-rise structure, the information is still not sufficient for design. This study investigated the seismic characteristics of this type of building structure through a shaking table tests on 1:15 scale 25-story RC flat-plate core-wall building mode. 2. DESIGN OF THE MODEL AND EXPERIMENTAL SETUP Among the RC flat-plate core-wall building structures constructed in Korea, the most typical type was chosen as a prototype: This was originally a 35-story flat-plate building, where each floor has four dwelling units, and each dwelling unit has the size of 188m2, as shown in Fig. 2.1 (a). However, due to limitation in the capacity of the shake table at the earthquake simulation test center of Pusan National University (size 5m x 5m, payload 600 kN) and for the convenience of construction of the model, the number of the stories of the prototype for the shaking table test was reduced to 25 (height: 79.5m), and staircases and slabs inside the core walls were all omitted as shown in Fig. 2.1 (b). The height of the first story is 5.1m, with those of the other stories being 3.1m. In the prototype building, core walls take most of resistance to the lateral load, and peripheral frames are designed to resist only the gravity load, in accordance with the definition of the building frame system. The result of elastic analysis of the prototype building shows that the core walls resisted 87% of the total lateral load. The size of all the peripheral columns is 900900mm, and the thickness of the core walls is 600mm, with that of the slab being


300mm, all along the height of the structure. The design concrete strength (fc), 40 MPa, and the yield strength of reinforcement, 400MPa, are applied to the whole building structure. The dead load of the prototype is 12,257kN for the second floor, 10,340kN for the typical floors (third to twenty fifth floors), 10,960kN for the roof, and 261,000kN in total. The effective seismic weight is set as the dead load, and live load is not included.

Y

X

8700 9600 8700

750750

27000

8100

5400

540

081

00

750

750

3875

2450

3875

5700

2000

370

01037 1100 1038

2075 1100

2850

0

28500

Column : 900 900mmSlab thickness : 300mmWall thickness : 600mmfc = 40MPafy = 400MPa

Y

X

590 620 590

540

360

360

540

190

0

1900

76

0

265 150 265

(a) Plan of the prototype building (b) Plan of the 1:15 scale model

Figure 2.1 Plan of the prototype building and specimen The size and payload of a shaking table in the Earthquake Test Center of Pusan National University are 5m5m and 600kN, respectively, and the model was scaled down to 1/15, taking availability of model reinforcement and constructability into consideration. Corresponding to the prototype reinforcement (D29, D16, D13) of the prototype, the scale model (1:15) used steel wires, 2 and 1 for model reinforcement. (Fig. 2.2 (a)) The average compressive strength of the model concrete is 46.9MPa, which is larger than design compressive strength, 40MPa. The average split tensile strength is 4.21MPa, about 1/10 of the compressive strength. Fig. 2.2 (b) shows that the stress-strain relations of the model concrete simulate well that of the full-scale concrete.

0

0.5

1

1.5

2

0 5 10 15 20 25

For

ce(k

N)

Displacement(mm)

Fy, max = 1.48kN

Fy, min = 1.14kN

Before annealing

After annealing

0

10

20

30

40

50

60

0 0.001 0.002 0.003 0.004 0.005

Str

ess

(MP

a)

Strain (mm/mm)

Test piece 1Test piece 2Test piece 3Test piece 4Test piece 5Test piece 6Secant modulus, EcTodeschini, 1964

Full-scale

Ec = 21,800MPa1

(a) Force versus strain relation of 2 (D29) (b) Stress-strain curve of model concrete

Figure 2.2 Material test results of model reinforcement and concrete Even with a high reduction factor of 15, the required self-weight of the model is 1,160kN, which still exceeds the capacity of the shaking table, 600kN, if the true replica model were used (Table 2.1). Therefore, the models weight should be reduced further, by using a distorted model. The relationship of scale factors in physical quantities is given in Eq. 2.1. In Fig. 2.2 (b), shape and strength in the stress-strain curves in the model concrete appear to be similar to those in the full-scale concrete. Thereby, the similitude scale factor for modulus of the elasticity of concrete, SE, is assumed to be 1. [7]

1= la

E

SSS

S (2.1)

Taking into account the length similitude factor, Sl of 1/15, and the weight of available steel plates for added artificial mass, the density similitude factor, S, was chosen to be 4.18. Therefore, the acceleration similitude factor, Sa, was determined as 3.59 to satisfy Equation (1). The similitude law applied to the test model is summarized in Table 2.1. The total effective seismic weight of the prototype was 261,000kN (self-weight: 205,100kN, additional dead load: 55,900kN). According to the similitude law in Table 2.1, the total effective seismic weight of the true replica model is 1,160kN (self-weight: 60.8kN, added load: 1,099kN), and the total weight of the test model, as for a distorted model, is 323kN, (self-weight: 60.8kN, added weight: 262.2kN) 1/3.59 of the total seismic weight of the true replica model. Fig. 2.3 shows an overview of the model. Displacement transducers and accelerometers were installed at the


floors of the 6th, 10th, 14th, 18th, and 22nd stories, and at the roof, to measure the overall behavior of the model as shown in Fig. 2.5. Furthermore, displacement transducers were deployed to measure the local behaviors of the walls and foundations. Steel blocks as shown in Fig. 2.4, were attached to the model, to compensate for the difference between the weight of model itself and that required as per similitude law in Table 2.1.

Table 2.1 Similitude law Quantities Scale Factor True replica model Distorted model

Length Sl 1/15 1/15 Elastic modulus SE 1 1

Density S 15 (total weight = 1,160kN) 4.18 (total weight = 323.3kN) Acceleration Sa= SE / (S Sl) 1 1 / (4.18 1/15) = 3.59

Force SE Sl 2 1 (1/15)2 1 (1/15)2

Frequency la SS / 15 1559.3=)15/1/(59.3

Time la SS //1 15/1 1559.3/1

(a)

1197

300

A view

B view

Y

X

Displacement meter(6, 10, 14, 18, 22F)Accelerometer(6, 10, 14, 18, 22F)

(b)

shaking table

7 5

30

30

30

shaking table

A VIEW B VIEW

LVDTs

ReferenceFrame

D1,D2

D3,D4

D5,D6

D7,D8

D9,D10

D13,D14

D15,D16

D17,D18

D19,D20

D21,D22

D23,D24Load Cell

A1

D11,D12

A2

A3 A4

A5 A6

A7 A8

A9 A10

A11 A12

A13 A14

A15 A16

A17 A18

A19 A20

A21 A22

A23 A24

Accelerometer

Figure 2.3 Overview of the model

Figure 2.4 Steel block (Added mass)

Figure 2.5 Instrumentation of the 1:15 scale model (D: disp., A: accel.): (a) Plan and (b) Elevation

The program of earthquake simulation tests is summarized in Table 2.2. The target or input accelerogram of the table was based on the recorded 1952 Taft N21E (X direction) and Taft S69E (Y-direction) components, and was

formulated by compressing the time axis with the scale factor of, 59.315/1 , and by amplifying the

acceleration with the scale factor, 3.59, according to the similitude law in Table 2.1. X, Y, and XY in designation of each test mean that the excitations were implemented in the X direction only, in the Y direction only, and in the X and Y directions simultaneously, respectively.

Table 2.2 Test Program (X-dir.: Taft N21E, Y-dir.: Taft S69E)

Test Designation

Intended PGA(g) Measured PGA(g) /

3.59 Return period

in Korea

Test Designation

Intended PGA(g) Measured PGA(g) /

3.59 Return period

in Korea X-dir. Y-dir. X-dir. Y-dir. X-dir. Y-dir. X-dir. Y-dir. White Noise (0.025 X, Y) White Noise (0.025 X, Y)

0.035X 0.035 0.0243 Elastic

Behavior

0.187X 0.187 0.137 Design Earthquake

(DE) 0.035Y 0.040 0.034 0.187Y 0.216 0.167

0.035XY 0.035 0.040 0.0243 0.034 0.187XY 0.187 0.216 0.137 0.167 White Noise (0.025 X, Y) White Noise (0.025 X, Y)

0.07X 0.070 0.052 50 years

0.3X 0.300 0.226 MCE

2400 years 0.07Y 0.080 0.065 0.3Y 0.345 0.253

0.07XY 0.070 0.080 0.052 0.065 0.3XY 0.300 0.345 0.226 0.253 White Noise (0.025 X, Y) White Noise (0.025 X, Y)

0.154X 0.154 0.127 500 years

0.4X 0.400 0.300 DE in San Francisco

USA 0.154Y 0.176 0.140 0.4Y 0.460 0.354

0.154XY 0.154 0.176 0.127 0.140 0.4XY 0.400 0.460 0.300 0.354


3. TEST RESULTS AND OBSERVATIONS

3.1. Global Responses Fig. 3.1 compares the elastic design spectrum of KBC2009 and acceleration response spectra of the shaking table output, showing that the table excitation simulated well the elastic design spectrum of the design earthquake (0.187XY) and the maximum considered earthquake (0.3XY).

0.0

0.2

0.4

0.6

0.8

0 0.1 0.2 0.3 0.4 0.5

Sa

Period (sec)

Elastic, Sd (KBC2009)Inelastic, Sd (KBC2009)Output (0.187g X-dir)Output (0.187g Y-dir)Output (0.3g X-dir)Output (0.3g Y-dir)

MCE

DE(R=1.0, I=1.0)

DE(R=6.0, I=1.2)

0.035 0.07

0.1540.187

0.3

0.035

0.07

0.154

0.1870.3

0

0.05

0.1

0.15

0.15 0.25 0.35 0.45 0.55 0.65

Bas

e sh

ear

coef

fici

ent,

Cs

Period (sec)

Exp. X-dir.

Exp. Y-dir.DE(I=1.0, R=1.0)

DE(I=1.2, R=6.0)

Ty(anal.)=0.277sec

Tx(anal.)=0.357sec

Cs,(design)=0.0253

Figure 3.1 KBC 2009 design spectra and output

response spectra Figure 3.2 Relation of the natural period and base

shear coefficient (Cs) with the design spectra Mode shapes and natural periods were shown in Fig. 3.3 from the frequency response function (FRF) analysis, by using the data of accelerations obtained through white noise tests. The vibration amplitudes in the second and third vibration modes in the X- and Y-directions are considerably large, compared to those in the first vibration mode. It can be found that the vibration amplitude of the second mode was larger than that of the first mode in the X direction. Table 3.1 shows the natural periods and damping ratios obtained from the FRF analysis. The virgin first mode natural period appears to be 0.413 sec in the X direction and 0.341 sec in the Y direction. The natural periods of the prototype from the elastic mode analysis at the design phase were 2.62 sec and 2.03 sec in the X- and Y- directions, respectively. The corresponding natural periods in the 1:15 scale model according to the similitude law in Table 2.1 are 0.357 sec in the X direction and 0.277 sec in the Y direction. The virgin natural periods from the test are relatively close to these analytical results. The damping ratio in the first mode is 5.53% in the X direction, and 4.39% in the Y direction with the damping ratio in the second and third modes being approximately 1.5% in both X and Y directions. In Table 3.1, the larger the seismic intensity, the longer natural periods the specimen had, and the damping ratios were around 5% to 7% in the X direction and 4% to 7% in the Y direction.

Table 3.1 Natural periods and damping ratios obtained from the white noise test

Seismic intensity X-direction Y-direction

Period (sec) Damping ratio (%) Period (sec) Damping ratio (%) Before test 0.413 5.53 0.341 4.39

After 0.35XY 0.419 7.26 0.346 4.50 After 0.07XY 0.423 6.14 0.357 4.65

After 0.154XY 0.467 6.43 0.391 4.14 After 0.187XY 0.483 6.28 0.408 5.58

After 0.3XY 0.550 7.57 0.442 5.74 After 0.4XY 0.688 6.03 0.510 7.07

-30 -15 0 15 30

Vibration Amplitude

1st (0.413s)2nd (0.0945s)3rd (0.0486s)

X-dir.

6

10

14

18

22

Roof

Flo

or

-30 -15 0 15 30

Vibration Amplitude

1st (0.341s)2nd (0.0696s)3rd (0.0285s)

Y-dir.

6

10

14

18

22

Roof

Flo

or

(a) X-direction (b) Y-direction

Figure 3.3 The first, second, and third vibration modes before the earthquake simulation test


Fig. 3.2 shows the relation between the natural period and maximum base shear coefficient (Cs = V/W) for each level of tests with the design spectra (Fig. 3.1). In computing the value of Cs, 1,160kN of the true replica model was used as the seismic weight, W, of the test model. The prototype was designed with the response modification factor R = 6 and importance factor IE = 1.2. Fig. 3.4 shows the point given by the maximum base shear coefficient (Cs = V/W) and the corresponding maximum roof drift in each test, and comparison with the design base shear coefficient Cs,design = 0.0253. It can be noted that the base shear coefficient was 0.0361 in the X direction and 0.0518 in the Y direction in the maximum considered earthquake (0.30XY), which were 1.5 times and 2 times larger than the design values, respectively. The strength increased further under MCE (0.30XY), and exceeded the elastic design spectrum with the elongated period (approximately 1.5 times the virgin period) in Fig. 3.2.

0

0.02

0.04

0.06

0.08

0.1

0 20 40 60 80

Bas

e sh

ear

coef

fici

ent,

Cs

Roof drift (mm)

XY

XX-dir.

0.035g0.070g

0.154g

0.187g

0.30g0.40g

CS ,(design) = 0.0253

= 1.51

0

0.02

0.04

0.06

0.08

0.1

0 20 40 60 80

Ba

se s

hea

r co

effi

cien

t, C

s

Roof drift (mm)

XY

Y

CS ,(design) = 0.0253

Y-dir.

0.035g

0.070g

0.154g0.187g

0.30g0.40g

= 2.05

Figure 3.4 Correlation between maximum roof drift and base shear coefficient

Fig. 3.5 shows the time histories of the base shear and roof drift at the levels of design and maximum considered earthquakes. The time history can be divided into the duration of table excitation, and that of no table excitation. No table excitation means the duration when free vibration occurs, after the shake table excitation was terminated. It can be noted that the maximum response of the base shear and roof drift during the free vibration period reveals a level of the maximum response similar to that during table excitation. In particular, except for the Y direction of 0.3XY, all the maximum roof drifts occurred during no excitation rather than during table excitation. (Fig. 3.5(b)). This phenomenon is also observed in the vertical distribution of drift at time instant when the roof drift was maximum, as shown in Fig. 3.6. Under 0.4XY, which is assumed to represent the design earthquake in a high seismicity region, such as San Francisco, USA, the maximum drift during no excitation was about 2 and 1.5 times larger in the X- and Y-directions, than those during table excitation, respectively.

(a)

No ExcitationTable Excitation

-90-60-30

0306090

Bas

e sh

ear(

kN

)

0.187XY

Base shear in X-dir.

-41.91 -35.02

-90-60-30

0306090

7 8 9 10 11 12 13 14 15 16 17 18

Ba

se s

hea

r(k

N)

Time(sec)

0.187XY

Base shear in Y-dir.

-60.17 -46.57

No ExcitationTable Excitation

-90-60-30

0306090

Bas

e sh

ear(

kN

)

0.30XY

Base shear in X-dir.57.14

-58.76

-90-60-30

0306090

7 8 9 10 11 12 13 14 15 16 17 18

Ba

se s

hea

r(k

N)

Time (sec)

0.30XY

Base shear in Y-dir.76.19

-53.68

(b)

-20

-10

0

10

20

7 8 9 10 11 12 13 14 15 16 17 18

Dis

pla

cem

ent(

)

right middle left

Roof drift in X-dir.

0.187XY

9.20 13.24

Table Excitation No Excitation

-20

-10

0

10

20

7 8 9 10 11 12 13 14 15 16 17 18

Dis

pla

cem

ent(

)

Time (sec)

right middle left

Roof drift in Y-dir.

0.187XY

12.26

14.70

Table Excitations No Excitation

-45-30-15

0153045

7 8 9 10 11 12 13 14 15 16 17 18

Dis

pla

cem

ent(

)

Time(sec)

right middle left 0.30XY

Roof drift in Y-dir.

-18.82

24.24

-45-30-15

0153045

Dis

pla

cem

ent(

)

right middle left

0.30XY

Roof drift in X-dir.

-15.30-34.35

Figure 3.5 Time histories of the (a) base shear and (b) roof drift


1.1 0.7 0.4 0 0.4 0.7 1.1Drift/height(%)

6

10

14

18

22

Roof

Flo

or

-60 -40 -20 0 20 40 60Drift ()

0.4XY

0.3XY

0.187XY

Max. Roof drift in X-dir. Table Excitation

15.30mm, 0.29%

1.1 0.7 0.4 0 0.4 0.7 1.1Drift/height(%)

6

10

14

18

22

Roof

Flo

or

-60 -40 -20 0 20 40 60Drift (mm)

0.4XY0.3XY0.187XY

Max. Roof drift in Y-dir.

24.24mm, 0.46%

Table Excitation

1.1 0.7 0.4 0 0.4 0.7 1.1Drift/height(%)

6

10

14

18

22

Roof

Flo

or

-60 -40 -20 0 20 40 60Drift ()

0.4XY

0.3XY

0.187XY

Max. Roof drift in X-dir.

34.35mm, 0.65%

No Excitation1.1 0.7 0.4 0 0.4 0.7 1.1

Drift/height(%)

-60 -40 -20 0 20 40 60Drift (mm)

0.4XY0.3XY0.187XY

Max. Roof drift Y-dir. No Excitation

18.82mm, 0.36%

6

10

14

18

22

Roof

Flo

or

(a) Table excitation (b) No excitation

Figure 3.6 Distribution of the drift at the maximum response of roof drift Fig. 3.7 shows the vertical distribution of acceleration at instant of the maximum base shear, and roof acceleration during table excitation, and during no excitation. In this test, the weight of 1/15 scale test model was reduced to 1/3.59 of the true replica model, as mentioned before. Thus, the input earthquake table acceleration was increased by the factor of 3.59 according to the similitude law (Table 2.1). During table excitation, the acceleration distribution reveals higher modes at time instant of the maximum base shear response. In particular, the distribution at time of the maximum roof acceleration reveals clearly that the second mode governs in both of X and Y directions: the roof acceleration at the maximum considered earthquake was 0.73g in the X direction and 1.06g in the Y direction. This distribution is similar to the vibration mode obtained through the white noise test (Fig. 3.3). On the contrary, the acceleration distributions under the free vibration, during no excitation, were governed by the first mode.

6

10

14

18

22

Roof

Flo

or

-1 -0.5 0 0.5 1Acceleration (g)

0.4XY

0.3XY

0.187XY

Max. Baseshear X-dir Table Excitation

0.54g6

10

14

18

22

Roof


0.4XY

0.3XY

0.187XY

Max. Baseshear X-dir No Excitation

0.33g

6

10

14

18

22

Roof


0.4XY

0.3XY

0.187XY

Max Baseshear. Y-dir Table Excitation

0.45g

-1 -0.5 0 0.5 1

Acceleration (g)

0.4XY

0.3XY

0.187XY

Max Baseshear. Y-dir No Excitation

0.42g

(a) Maximum base shear

6

10

14

18

22

Roof

Flo

or


0.4XY

0.3XY

0.187XY

Max. Roof accel X-dir Table Excitation

0.73g

6

10

14

18

22

Roof


0.4XY

0.3XY

0.187XY

Max. Roof accel X-dir No Excitation

0.47g

Roof


0.4XY

0.3XY

0.187XY

Max. Roof accel Y-dir Table Excitation

Max: 1.06g

-1 -0.5 0 0.5 1

Acceleration (g)

0.4XY0.3XY0.187XY

Max. Roof accel Y-dir No Excitation

0.39g

(b) Maximum roof acceleration

Figure 3.7 Distribution of acceleration at instants of maximum base shear and roof acceleration

Fig. 3.8 shows the hysteretic relation between base shear and roof drift in each test, with the thick line and thin line denoting the response during table excitation, and during no excitation, respectively. The hysteretic curves during table excitation show sharp peaks and valleys, due to the higher mode effect, whereas those during no excitation reveal relatively smooth curves, having a large amount of energy dissipation through inelastic deformation. Elastic behavior was observed under test 0.070XY, with the initial stiffness in the X direction, kx = 4.71kN/mm and in the Y direction, ky = 5.26kN/mm, respectively. Under 0.187XY, design earthquake, some small amount of energy dissipation occurred, and under the maximum considered earthquake, 0.3XY, larger inelastic behaviors were observed. Under 0.4XY, the stiffness is kx = 0.97kN/mm in the X direction and ky = 1.79kN/mm in the Y direction. Whereas the stiffness was reduced significantly, and the amount of energy dissipation increased greatly, the strength degradation was not observed. The behavior in the X direction where the core wall contains openings, reveals lower values of initial stiffness and strength, and larger energy dissipation through inelastic behavior than those in the Y direction, where there is no opening in the wall.


-100

-50

0

50

100B

ase

sh

ear

(kN

)

TableExcitationNoExcitation

X-dir.0.070XY

k = 4.71kN/mm

Vmax=24kN


X-dir.0.187XY

k = 2.36kN/mm

Vmax=42kN


X-dir.0.3XY

k = 1.61kN/mm

Vmax=59kN


X-dir.0.4XY

k = 0.97kN/mm

Vmax=71kN

-100

-50

0

50

100

-50 -25 0 25 50

Bas

e sh

ear

(kN

)

Roof displacement (mm)


Y-dir.

k = 5.26kN/mm

0.070XYVmax=26kN

-50 -25 0 25 50Roof displacement (mm)


Y-dir.0.187XY

k = 3.67kN/mm

Vmax=60kN



Y-dir.0.3XY

k = 2.55kN/mm

Vmax=76kN



Y-dir.0.4XY

k = 1.79kN/mm

Vmax=83kN

Fig. 3.8 Hysteretic relation of the base shear and roof displacement under 0.070XY, 0.187XY, 0.3XY, and 0.4XY

3.2 Inter-story drift Fig. 3.9 shows the vertical distribution of inter-story drift ratios at the time instant when the roof drift reached maximum. KBC2009 specifies that the inter-story drift ratio (IDR) for buildings of importance group I shall be within 1.5% of the story height. The test result shows that IDR (0.29%) in the 10th to 13th stories in the X direction and IDR (0.43%) in the 1st to 5th stories in the Y direction, are the highest under design earthquake, 0.187XY. Under 0.4XY, IDRs are still all within 1.5%, while the maximum inter-story drifts during no excitation are larger, than those during table excitation.

-0.02 -0.01 0 0.01 0.02Interstory drift ratio (mm/mm)

0.187XY0.3XY0.4XY

X-dir.

6

10

14

18

22

Roof

Sto

ry

6

10

14

18

22

Roof

Sto

ry


0.187XY0.3XY0.4XY

Y-dir.

6

10

14

18

22

Roof

Sto

ry


0.187XY0.3XY0.4XY

X-dir.

6

10

14

18

22

Roof

Sto

ry


0.187XY0.3XY0.4XY

Y-dir.

(a) Table Excitation (b) No Excitation

Figure 3.9 Interstory drift ratio 3.3 Failure mode Fig. 3.10 shows the crack patterns in the 12th and 18th stories where major cracks were observed in the slab and coupling beams. After the DE (0.187XY), many flexural cracks were observed near the connection between column and slab, and minor cracks also occurred in the joint area between slab and core wall, as well as in coupling beams. After MCE (0.3XY), cracks were expanded, so that the column-slab connection area experienced most of intensive cracks. Flexural cracks in the slab were also observed due to lateral behavior in the Y direction, and in the core walls, cracks occurred near the opening area, with flexural and shear cracks in the coupling beams. However, the columns had no crack at all. The wall cracks were concentrated in the 7th to 16th stories intensively, and the largest wall cracks were observed in the 11th and 12th stories, where intensive slab cracks were found. In addition, no crack was found in the short wall in the first story in the X direction having special boundary elements, but a few shear cracks were observed in the long wall.

Front view at the 12th story (0.187g) Front view at the 12th story (0.30g)

The right side at the 12th story (0.187g) The right side at the 12th story (0.30g)

Figure 3.10 Crack patterns in the slab and exterior core wall


Front view at the 18th story (0.187g) Front view at the 18th story (0.30g)

The right side at the 18th story (0.187g) The right side at the 18th story (0.30g)

Figure 3.10 Crack patterns in the slab and exterior core wall (continued)

4. CONCLUSIONS This study investigated the seismic responses of a high-rise RC flat-plate core-wall building structure, namely global force-drift relations, higher-mode effect, and failure modes based on the results of the shaking table tests of a 1:15 scale 25-story RC flat-plate core-wall building model. The conclusions reached are as follows:

(1) The initial first-mode natural periods of the model obtained by using the white nose test were 0.413 sec and 0.341 sec in the X- and Y-directions, which are similar to the 0.357 sec and 0.277 sec in the X- and Y-directions, respectively, obtained through modal analysis for the design of the prototype.

(2) Under the design earthquake, 0.187XY, the base shear coefficients were 0.0361 in the X direction, and 0.0518 in the Y direction, which are 1.5 times and 2 times larger than the design base shear coefficient of 0.0253. The hysteretic curves between base shear and roof drift show elastic behavior under 0.070XY, representing the earthquake with a 50 year return period in Korea, whereas inelastic behavior increased under the design earthquake (0.187XY) and the MCE (0.3XY), with increasing energy dissipation. Under the design earthquake (0.187XY), the maximum inter-story drift ratio was 0.29% at the 10th to 13th story in the X direction, and 0.43% at the 1st to 5th story in the Y direction, all of which satisfy the allowable inter-story drift ratio (1.5%), imposed by KBC 2009 (IBC 2006).

(3) The higher modes were observed in both X and Y directions in the vertical distribution of acceleration. In particular, when the roof acceleration reached the maximum, the second and third modes effect governed, and the largest story shear was apparent in the 14th to 21st stories, instead of the first story. The middle stories experienced intensive cracks in the slabs around the columns, the coupling beams, and walls. Therefore, for the design of high-rise buildings (about above 70m), where the higher mode effect dominates, responses when the roof acceleration reaches the maximum could be more critical to the middle stories, than responses when the base shear or roof drift reaches the maximum.

(4) The model behaved in the first mode during free vibration after termination of excitation, and the maximum values of base shear and roof drift in this duration can be either similar, or sometimes larger than the values of the maximum responses during the table excitation. However, the design approach proposed in the current seismic design codes accounts for the seismic behavior in the time duration of ground excitations, and does not take into account the free-vibration behavior after excitation. A study on a design approach that takes this into consideration is required in the future.

AKCNOWLEDGEMENT The research presented herein was supported by the National Research Foundation of Korea, through the contracts No. 2009-0078771. The writers are grateful for this support.

REFERENCES

1. Architectural Institute of Korea (2009), Korean Building Code (KBC2009), Seoul, Korea. (in Korean) 2. International Code Council (2006), International Building Code, Country Club Hills, IL. 3. ACI Committee 318 (2005), Building code requirements for structural concrete and commentary (ACI

318-05), American Concrete Institute, Detroit. 4. Li, C. S., Lam, S. S., Zhang, M. Z., & Wong, Y. L. (2006). Shaking table test of a 1: 20 scale high-rise

building with a transfer plate system. Journal of structural engineering, 132(11), 1732-1744. 5. Lu, X., Zhou, Y. and Lu, W. (2007). Shaking table model test and numerical analysis of a complex high-rise

building, The Structural Design of Tall and Special Buildings, 16, 131-164. 6. Zhou, X., and Li, G. (2010). Shaking table model test of a steel-concrete composite high-rise building.

Journal of Earthquake Engineering, 14(4), 601-625. 7. Harris, H. G. (1999). Structural modeling and experimental techniques. CRC Press LLC.

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