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Experimental Study on Dynamic Behavior of Multi-Story

Reinforced Concrete Frames with Non-Seismic Detailing

Authors:

Soheil Yavari, University of British Columbia, Vancouver, Canada, [email protected] J. Elwood, University of British Columbia, Vancouver, Canada

Shih-Han Lin, National Taiwan University, Taipei, TaiwanChiun-lin Wu, National Center for Research on Earthquake Engineering, Taipei, Taiwan

Shyh-Jiann Hwang, National Taiwan University, Taipei, Taiwan

Jack P. Moehle, University of California, Berkeley, USA

ABSTRACT

In order to observe the interaction of structural elements at the onset of collapse, four framespecimens were tested at the National Center for Research on Earthquake Engineering (NCREE)

in Taiwan in spring 2009. Each specimen consisted of a 1/2.25 scaled model of a two-bay and

two-story reinforced concrete frame. The specimens were tested under moderate and high gravityloads to investigate the influence of axial loads on the collapse vulnerability of the structures.

These tests will also be employed to study the interaction of the beams, columns and joints as

collapse is initiated. The current paper presents the observations during the tests and comparison

of the behavior of the frame specimens.

INTRODUCTION

Many of existing reinforced concrete buildings in the United States and worldwide do not satisfythe special seismic detailing requirements of ACI Building Codeor similar codes. When such

buildings are subjected to a strong earthquake, several inadequacies in their performance

capabilities may become apparent and in extreme cases, collapse may result. Due to therequirements from older building codes, most of these structures were designed and constructed

with strong beams and weak columns. In such buildings, columns and beam-column joints are

more vulnerable than beams to damage or failure. Such failures commonly are attributed towidely spaced and poorly anchored transverse reinforcement. Surveys of reinforced concrete

building collapses in past earthquakes and experimental research have indicated that existing

building columns with light and inadequately detailed transverse reinforcement are vulnerable toshear failure during ground shaking. Shear failure causes reduction in building lateral strength,

lower axial load carrying capacity, and potentially leading to collapse of the building.

A major engineering objective is to avoid introducing earthquake-related collapse

vulnerabilities in new constructions and to identify and retrofit those vulnerabilities in existingbuildings. To date, there have been relatively few tests on behavior of inadequate reinforced

concrete frame systems in the literature, particularly dynamic tests to collapse. As a result of the

relatively small data set, our understanding of failure and collapse mechanisms is limited.Furthermore, it is observed that in contrast with the frequent collapse predictions based on

current assessment procedures, post-earthquake reconnaissance studies show a relatively low rate

of collapse amongst older non-seismically detailed concrete structures even in major earthquakes

489

ATC & SEI 2009 Conference on Improving the Seismic

Performance of Existing Buildings and Other Structures

yright ASCE 2009Improving the Seismic Performance of Existing Buildings and Other Structures

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[Otani, 1999]. These observations suggest that current practices for assessing collapse are

conservative and need refinement in order to identify the critical buildings that are most collapse-prone.

The following describes a research project that directly addresses this issue and the

significant life-safety risk facing millions of people living and working in existing concrete

buildings worldwide. The research has been conducted in collaboration with researchers fromTaiwan, Canada and the United States leveraging knowledge gained through international

research programs. The main objective of this work is to experimentally study the seismic

collapse behavior of non-seismically detailed reinforced concrete frames. Particularly, this studytries to investigate structural framing effects on column shear and axial failures, and conversely,

the effects of column failures on frame system collapse vulnerability. Understanding these

interactions is essential in assessing the collapse vulnerability of structures. Through a betterunderstanding of mechanisms that cause collapse, improved engineering tools may be developed

for use by practicing engineers to assess the collapse vulnerability of poor detailed reinforced

concrete frame structures.

SIGNIFICANCE OF THE RESEARCH AND PROJECT DESCRIPTION

Older gravity-based design methods resulted in a system with weak columns and strong beams,and therefore, most building frames designed using such methods are expected to experience

failure of columns and/or beam-column joints. Past shaking table collapse tests have focused on

the performance of one specific component (i.e. columns) and only considered low to moderategravity loads. To fill the gaps in knowledge, this study involved dynamic testing to collapse of

four two-dimensional, two-bay, two-story, and 1/2.25 scaled reinforced concrete frames. Each

frame contained non-seismically detailed columns, commonly referred to as flexure-shear-

critical columns, whose proportions and reinforcement details allowed them to yield in flexureprior to shear strength degradation and ultimately reach axial failure. The influence of non-

confined joints on the collapse behavior of the frame was also investigated. In particular, the

tests focused on two previously unexplored issues: (1) the interaction of multiple vulnerableconcrete components (i.e. beams, columns, and joints) within a building frame as collapse is

initiated, and (2) the influence of high gravity loads on the collapse vulnerability of a structure.Figure 1 describes the types of shaking table specimens that were tested. Comparison of the

results from MCFS and HCFS reveals the influence of axial load on shear and axial behavior of

flexure-shear-critical columns, while observations from MUF and MUFS demonstrates theeffects of unconfined joints on overall behavior of the frame near the point of collapse and

sequence of failure in the elements. Details of the specimens are described in the following

section.

Specimen MCFS:

Moderate Axial Load

Confined Joints

Flexure-Shear Columns

Specimen HCFS:

High Axial Load

Confined Joints


Specimen MUFS:

Moderate Axial Load

Unconfined Joints


Specimen MUF:

Moderate Axial Load

Unconfined Joints

FlexureColumns

FIGURE 1 -DESCRIPTION OF SHAKING TABLE SPECIMENS

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SPECIMENS AND TEST SETUP

Specimen Design

Four frames with two stories and two bays were designed to be tested on the shaking table at the

National Center for Research on Earthquake Engineering (NCREE) in Taiwan. The geometries

and details (Figure 2) were selected to be representative of elements used in an existing seven-story hospital building in Taichung, Taiwan. Final dimensions and reinforcement details of the

frames were influenced by laboratory and shaking table limitations, scaling of column detailsused in the existing building, available reinforcement, and desired failure mode. The target

failure mode was intended to be damage leading to collapse that would enable examination of

gravity load redistribution during the test. The ratio of beam stiffness to column stiffness was

considered to be similar to the existing hospital building. Since the overall width of the frame,and consequently the beam length, were limited by the dimensions of the shaking table; the beam

depth was adjusted to achieve the target beam-to-column stiffness ratio. Beam transverse

reinforcement with closed stirrups and 135 hooks provided sufficient shear strength to developfull flexural strength. Beams longitudinal reinforcement was chosen to create a weak-column-

strong-beam mechanism typical of the older concrete construction. Neither beams nor columnshad lap splices to eliminate the splicing effects from the scope of this study. Slabs were cast withthe beams to include the effect of slabs on the beam stiffness and the joint demands.

Beam-column joints in non-seismically detailed concrete frames were frequently constructed

without transverse reinforcement and are vulnerable to shear and axial failure during strongground shaking. However, to separate the collapse behavior due to column failure from that

resulting due to joint failure, specimens MCFS and HCFS incorporated well-confined joints and

failure of the columns was expected to precipitate collapse of the frame. In contrast, sufficient

confinement in the columns of specimen MUF ensured a flexural column response, whileeliminating the confinement from the first-story joints was expected to lead to a collapse mode

dominated by joint failure. Constructing MUFS with no confinement in the first-floor joints and

light transverse reinforcement in the columns provides the opportunity to study the sequence offailure in a typical existing building frame with both unconfined joints and non-ductile columns.

Discontinuity of the columns above the second floor made the joints susceptible to early failure;therefore, joints at second level were confined for all specimens.

Given the complexity of desired test frame behavior and failure mechanisms, a detailed

analytical model, rather than the common design methods, was used to select the test framedetails. An extensive parametric study was performed to determine optimal test frame final

dimensions and details, considering the common full-scale columns in the existing hospital

building. Columns with 200 mm200 mm square section and eight deformed #4 bars forlongitudinal reinforcement were selected (longitudinal reinforcement ratio=2.6%). Column

transverse reinforcement was selected as 5mm hoops at 120 mm for specimens with flexure-

shear-critical columns (MCFS, HCFS, and MUFS), while the spacing was reduced to 40 mm forMUF with ductile columns. The resulting transverse reinforcement ratio for the flexure-shear-

critical and flexure-critical columns was =0.16% and 0.49%, respectively (=Ast/bs where,

Ast is the area of transverse reinforcement with spacing s, and b is the column width,perpendicular to the direction of lateral shear). Using ASCE/SEI-41 (2008), the ratio of the

plastic shear demand on the columns (Vp) to the nominal shear strength (Vn) varied between 0.9

and 1.0 for the columns in specimens MCFS, HCFS, and MUFS, which complies with theASCE/SEI-41 definition of flexural-shear-critical columns. Vp/Vn for the columns from the MUF

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the frame such that only lateral forces were transmitted to the specimens. The inertial-mass was

supported on rollers mounted on the steel supporting frames on either side of the specimen (seeFigure 3). The connection mechanism between the mass and the specimen was designed such

that the inertial force could be transferred to the specimen, while the vertical deformation of the

columns was not restrained.

The column axial loads from the upper stories were achieved by prestressing the columns tothe shaking table using pressure-controlled hydraulic jacks. A transverse steel girder, placed on a

pin at the top of each column, transferred the axial load to the columns. A clevis pin, aligned

with the intended direction of shaking, was installed on each end of the girder. A high-strengththreaded rod was used to attach the clevis pin to the hydraulic jack which was secured to the

shaking table by another clevis pin. In order to observe the effects of axial load on column and

joint behavior, the middle column of Specimens MCFS, MUFS, and MUF was subjected to amoderate axial load (0.2fcAg). The middle column of Specimen HCFS was pre-stressed to an

axial load of approximately 0.4fcAg=0.33P0=1.15Pb, where P0 is the concentric axial load

capacity and Pb is the balanced axial load. Note 0.35P0 is used by Chapter 21 of ACI-318 todistinguish gravity columns requiring seismic detailing similar to columns of the lateral force

resisting system [ACI-318, 2008]. The exterior columns of all frames were subjected to half ofthe axial loads applied to their corresponding middle columns. It should be noted that variation in

applied axial load was observed during the tests even though accumulators and fast pressurereducing and relieving valves were used to passively control the axial load at the desired level.

Steel lateral supporting frame

and Inertial-mass system

Side view of steel lateral supporting

frame and axial Pre-stressing system

FIGURE 3 PRE-STRESSING AND INERTIAL-MASS SYSTEM

The test frames were subjected to the same table motion but scaled to various peak ground

accelerations. The north-south component of the ground motion record from the 1999 Chi-Chiearthquake at station TCU047 with the Joyner/Boore distance to the rupture surface of 32.1 km

was selected for the tests (PGA=0.4g). This record was scaled to nominally the PGA of 0.3g

(Half-yield Test), 1.1g (Test1), and 1.35g (Test2). Table 1 shows the spectral acceleration for the

specimens at the natural period of 0.29 sec. Proximity of spectral accelerations allows comparingthe results from the four specimens.

Test

Description

MCFS

Sa(T1=0.29sec)

HCFS

Sa(T1=0.29sec)

MUF

Sa(T1=0.29sec)

MUFS

Sa(T1=0.29sec)

Half-Yield 0.6g 0.4g 0.6g 0.7g

Test1 1.8g 2.1g 1.8g 1.9g

Test2 2.2g 2.7g 2.2g 2.7g

TABLE 1 TEST PROTOCOL AND SPECTRAL ACCELERATIONS

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FIGURE 4 ELASTIC RESPONSE SPECTRA WITH 2% DAMPING (TEST2)

Specimen instrumentation consisted of: 1) force transducers to measure shear, axial load, andbending moments at the base of the frame footings; 2) strain gages on longitudinal and transverse

reinforcement in columns, beams, and joints; 3) accelerometers for horizontal, vertical, and

transverse accelerations of both beams; and 4) displacement transducers to measure both localcolumn and global frame deformations.

EXPERIMENTALTESTRESULTS

Results from 0.1g White-Noise tests prior to earthquake excitations were employed to attain

the natural period and damping ratio of the frame. The natural periods of the frames wereobtained in a range of 0.28 to 0.29sec, while the damping ratio was determined to be 2%. All the

frames sustained very minor flexural cracks, during the Half-Yield Test. The specimens did not

collapse during Test1, but all of them were to some extent damaged. Figure 5 demonstrates a fewsnapshots from the damaged frames after Test1. As expected, columns of frames MCFS and

HCFS experienced shear and flexural cracks, while damage was mostly concentrated at theexterior first-story joints of specimens MUF and MUFS. The specimens did not performsimilarly during Test2 and the failure mode was different for each frame. MUF did not collapse

during Test2, while the other three frames experienced complete collapse. Figure 6 summarizes

the failure modes for the four specimens. Frame MCFS collapsed due to shear and axial failure

of the first-story columns. This was not observed for the other cases, where combination ofplastic hinge development and damage to the structural elements caused the failure of the frame.

Global behavior of the frames and performance of the structural elements during the tests are

discussed in detail in the following sections.

(A) (B) (C) (D)

FIGURE 5 DAMAGED FRAMES, TEST1 (A) BASE OF COLUMN B1, SPECIMEN MCFS; (B) BASE OFCOLUMN B1, SPECIMEN HCFS; (C) JOINT A1, SPECIMEN MUF; (D) JOINT A1, SPECIMEN MUFS.

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& Axial Failure

Joint Shear Failure

Plastic Hinge

MCFS HCFS MUF MUFS

Column Shear

ABC ABC ABC ABC

Direction of PeakDisplacement

FIGURE 6 FAILURE MODE FOR EACH TEST FRAME

Frame Global Behavior

During Test1, frames experienced story drifts large enough to cause yielding and shear

cracks in specimens MCFS and HCFS, and joint shear cracks in specimens MUF and MUFS.

Table 2 compares the inter-story drifts for the four specimens during Test1 and Test2. Largestfirst-story drift (6.8%) was experienced by frame MCFS, where all of the first-floor columns

failed during Test2. It was observed for nearly all the cases that the inter-story drift for the

second floor was smaller than the first floor. Table 2 also reveals that unlike for the specimensMCFS and HCFS, the maximum drift ratio of specimens MUF and MUFS, with unconfinedjoints, remained similar during both Test1 and Test2. This can be explained by the localized

shear deformation at the joints, where large shear cracks developed in joint area decreasing the

frame stiffness and the table motion demands. Nevertheless, they did not cause the collapse ofthe specimens.

Specimen

Test1 Test2

1st

Story

Drift Ratio

(%)

2nd

Inter-story

Drift Ratio

(%)

Frame Total

Drift Ratio

(%)

1st

Story

Drift Ratio

(%)

2nd

Inter-story

Drift Ratio

(%)

Frame Total

Drift Ratio

(%)

MCFS 2.3 1.7 1.8 6.8 2.2 3.6

HCFS 2.2 1.7 1.6 3.9 2.3 2.5MUF 3.0 2.1 2.3 3.0 2.3 2.4

MUFS 3.0 2.1 2.3 3.0 3.0 2.6

TABLE 2 PEAK INTER-STORY DRIFT RATIOS OF THE FRAMES DURING TEST1 AND TEST2

Figure 7 compares the base shear hysteretic response of the four frames for Test1. It isobserved that the columns in specimens MUF and MUFS did not reach their shear capacity,

while their first-story drift ratio was larger than MCFS and HCFS. This was expected for

specimen MUF with ductile columns, however, similar performance by specimen MUFS, withflexure-shear-critical columns, suggests that the unconfined joint in these two specimens worked

as a fuse and did not allow shear to be fully transferred to the first-story columns and

accommodated much of the deformation demands. This is supported by the hysteretic responsefrom Test2 (Figure 8), where unlike MUF and MUFS columns, the columns from MCFS andHCFS failed under the lateral demand.

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-3 -2 -1 0 1 2 3-300

-200

-100

0

100

200

300

1st Story Drift Ratio (%)(A)

Bas

e

Shear(kN)

MCFSHCFS

Vp for MCFS

Vp for HCFS

-3 -2 -1 0 1 2 3-300

-200

-100

0

100

200

300

1st Story Drift Ratio (%)(B)

Base

Shear(kN)

MUF

MUFS

Vp for MUFS

FIGURE 7 COMPARISON OF BASE SHEAR HYSTERETIC RESPONSE OF THE FRAMES DURINGTEST1 (A) MCFS VS. HCFS; (B) MUF VS. MUFS.

FIGURE 8 COMPARISON OF BASE SHEAR HYSTERETIC RESPONSE OF THE FRAMES DURINGTEST2 (A) MCFS VS. HCFS; (B) MUF VS. MUFS.

-3 -2 -1 0 1 2 3 4 5 6 7 8 9-300

-200

-100

0

100

200

300

1st Story Drift Ratio (%)(A)

BaseShear(k

N)

MCFS

HCFS

-3 -2 -1 0 1 2 3-300

-200

-100

0

100

200

300

1st Story Drift Ratio (%)(B)

BaseShear(kN)

MUF

MUFS

Behavior of Structural Elements

SpecimenMCFSvs.HCFS

Due to negligible elongation of the connecting beams, columns A1, B1, and C1 underwent

almost equal story drifts. However, the initiation of shear failure was at a different time and drift

ratio for each column because of different axial loading. As discussed in previous section,columns experienced shear failure only during Test2. In specimen MCFS, shear failure of

column B1 was initiated at approximately 1.9% drift ratio and a shear of 79.5 kN. Column C1,

taking higher shear forces due to overturning compression demands, experienced shear failure at2.2% drift ratio and shear of 87.2 kN. Finally, column A1 at a drift ratio of 2.8% and shear of

65.7 kN commenced to fail. Shear failure initiation can be defined where shear strength loss

starts and large shear cracks are developed. Shear hysteretic response of first-story columns ofspecimens MCFS and HCFS are compared in Figure 9. It is observed that the onset of shearfailure happened at a larger drift ratio for specimen HCFS, where column B1 experienced shear

failure at 2.3% drift ratio and a shear of 86.8 kN and shear failure of column C1 was commenced

at 2.9% drift ratio and a shear of 81.6 kN. However, column C1 did not lose all its shear capacityin the same cycle as B1. Unlike frame MCFS, column A1 of specimen HCFS did not experience

significant shear degradation. It was noticed that after shear degradation in columns B1 and C1,

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column A1 experienced additional tension and the first-story beam between columns A1 and B1

experienced flexural yielding resulting in shear failure of column A2.

-2 0 2 4 6 8-100

-50

0

50

100

Drift Ratio (%)

ColumnA1Shea

r(kN)

MCFS

HCFS

-2 0 2 4 6 8-100

-50

0

50

100

Drift Ratio (%)

ColumnB1Shea

r(kN)

-2 0 2 4 6 8-100

-50

0

50

100

Drift Ratio (%)

ColumnC1Shea

r(kN)

FIGURE 9 COMPARISON OF SHEAR HYSTERETIC RESPONSE OF FIRST-STORY COLUMNS OFSPECIMENS MCFS AND HCFS DURING TEST2.

SpecimenMUFvs.MUFS

In contrast with specimens MCFS and HCFS, first-story beam-column joints were not

confined in specimens MUF and MUFS. Therefore, they experienced severe damage duringTest1 and Test2. Behavior of joints A1 and C1 (exterior joints) from specimens MUF and MUFS

are compared in Figure 10. Shear deformation was not recorded at joint B1. The beam-column

joints from both frames underwent large shear deformations, while the exterior joints fromspecimen MUFS showed a stiffer behavior at the beginning. Figure 10 illustrates that joint A1

from both specimens experienced shear failure during Test1, but not leading to collapse of the

frames. Joint C1 was also substantially damaged, but the damage was not too severe to causelarge permanent deformation at the beam-column joint.

As shown in Figure 11, joint A1 lost most of its shear capacity during Test1, while joint C1

was still able to carry most of the shear demand during Test2. It was observed that due to the

internal forces and moments, the direction of major shear cracks in the joints was opposite to thedirection of movement of the upper story during peak cycles of response. Furthermore, columns

of specimen MUF were designed and constructed to be ductile and consequently, none of the

columns in this frame experienced significant damage. Considering these facts, the frame MUFnever collapsed. To study the behavior of the specimen under larger lateral demands, the

damaged frame was tested with intensified table motion scaled up to Sa(0.29)=3.4g. The shear

cracks at the joints became larger and more spalling was occurred at the joint, however, thespecimen withstood the dynamic test without collapse. Due to the shaking table limitations,

increasing the intensity of the table motion was not possible. In contrast, collapse of the second

story of frame MUFS was observed during Test2. Columns of this specimen were designed and

constructed with flexure-shear-critical details similar to the two other specimens. Shear failure ofthe first-story joints released the moments at the top of the first-story columns and as a result,

these columns did not experience significant damage. The same release of moments did not

appear to occur at the base of the second-story columns possibly due to the restrain of the slab.Furthermore, the beam-column joints of the second story were well-confined and imposed fixed-

end boundary condition, so that the second-story columns experienced larger demands than first

story. This would explain the shear and axial failures which occurred at the top of columns B2and C2. Considering the fact that the peak drift ratios were similar for the two stories in Test2

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(see Table 2), the behavior of frame MUFS implies that the rotation at the ends of a nonductile

column plays a significant role in shear and axial failure of this type of columns.

-0.03 -0.02 -0.01 0 0.01 0.02 0.03-80

-60

-40

-20

0

20

40

60

8080

Joint Shear Deformation (rad)

Shear,ColumnA1Top(kN)

MUF

MUFS

-0.03 -0.02 -0.01 0 0.01 0.02 0.03-80

-60

-40

-20

0

20

40

60

8080


Shear,ColumnC1Top(kN)

FIGURE 10 COMPARISON OF SHEAR HYSTERETIC RESPONSE OF EXTERIOR JOINTS OFSPECIMENS MUF AND MUFS DURING TEST1.

-0.03 -0.02 -0.01 0 0.01 0.02 0.03-80

-60

-40

-20

0

20

40

60

8080


Shear,ColumnA1Top(kN)

MUF

MUFS

-0.03 -0.02 -0.01 0 0.01 0.02 0.03-80

-60

-40

-20

0

20

40

60

80


Shear,ColumnC1Top(kN)

FIGURE 11 COMPARISON OF SHEAR HYSTERETIC RESPONSE OF EXTERIOR JOINTS OFSPECIMENS MUF AND MUFS DURING TEST2.

SUMMARY AND CONCLUSIONS

Four frames with non-seismic details were tested in the National Center for Research onEarthquake Engineering under moderate and high axial loads. Details of the specimens and setup

of the tests were described in this paper. Observation of interaction of failure in columns and

joints during the tests suggests that frame geometry and layout of critical elements are crucial indetermining failure sequence and ultimately collapse mechanisms.

Comparison of results from testing specimens MCFS and HCFS indicates that changes in

axial loads can significantly impact the failure mode for the frame, and that larger axial load

increases the likelihood of failure in higher stories.Test results of frames MUF and MUFS suggest that unconfined beam-column joints are

playing a major role in behavior of non-ductile frames. In both cases, large deformations at thejoints enhanced the performance of the frames by reducing the rotation at the column top. Owing

to the effects of internal forces and moments at the joints, the trend of shear cracks was opposite

to the direction required for failure of the frame. This allowed the specimen to withstand the

demands without collapsing due to joint failure.

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Behavior of specimen MUFS indicates that inter-story drifts may not correlate well with

shear failure of flexure-shear-critical columns, where shear failure initiation appears to be morerelated to column end rotations than column drifts.

ACKNOWLEDGMENT

The study discussed above was part of a tri-national collaborative research effort on thecollapse of concrete frames funded in part by the National Center for Research on Earthquake

Engineering (NCREE) and the National Science Council of Taiwan, the Natural Science andEngineering Research Council of Canada, and the National Science Foundation (US). This

funding is gratefully acknowledged. All opinions expressed in this paper are solely those of the

authors and do not necessarily represent the views of the sponsors.

REFERENCES

[1] American Concrete Institute (ACI), Building Code Requirements for Structural Concrete, ACI 318-08, 2008,

Farmington Hills, USA.

[2] American Society of Civil Engineers (ASCE), Seismic Rehabilitation of Existing Buildings, ASCE/SEI-41

Supplement 1, 2008, Reston, Virginia.

[3] Otani, S., RC Building Damage Statistics and SDF Response with Design Seismic Forces, Earthquake

Spectra, 15, No 3., 1999, 485-501.

[4] Ministry of the Interior (MOI), Building Technical Regulations, Construction and Planning Agency, Ministry

of the Interior, 2009, Taipei, Taiwan, ROC, (in Chinese).

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