Citation: Sulthan, F., M. Seki, H. Nakagawa, Y. Otsuka (2023), Seismic fragility analysis of fixed andisolated base reinforced concrete building structures in Indonesia, Synopsis of IISEE-GRIPS Master’sThesis.

SEISMIC FRAGILITY ANALYSIS OF FIXED AND
ISOLATED BASE REINFORCED CONCRETE
BUILDING STRUCTURES IN INDONESIA
Faiz SULTHAN1MEE22705

Supervisor: Matsutaro SEKI2, Hiroto NAKAGAWA2*,Yuri OTSUKA2**
ABSTRACT

Indonesia has had seismic codes for earthquake-resistant structure designs since 1970 and has been
updated five times to the latest in 2019. In updating the Indonesian seismic codes, seismic hazard maps
for design also update, and there are changes to the Peak Ground Acceleration (PGA). The Indonesian
seismic design uses the concept of building performance levels consisting of Immediate Occupancy (IO),
Life Safety (LS), and Collapse Prevention (CP). In relation to these performance levels, some cases haveshown that buildings suffer from huge damage compared to their performance targets after an earthquake.
Considering this issue, the current study aims to analyze the performance of the seismic isolation systemdesign (isolated) on existing target buildings (fixed) and analyze seismic fragility with the PGA intensity
according to Indonesia’s seismic hazard maps of 0.1–1.5g. The target building used in this study is a
prototype design eight-story medium-rise residential building that uses the reinforced concrete moment
frame structure. The analysis uses Nonlinear Time History Analysis (NLTHA) for the design andIncremental Dynamic Analysis (IDA) for seismic fragility. Both analyses use 11 selected groundmotions based on soil classification, magnitude, fault distance, and earthquake source mechanism. TheNLTHA results reveal that using a seismic isolation system can increase the target building'sperformance level from LS to IO. A comparison of the IDA results depicts a trend of significant
performance improvement. That is, with the same performance level target and risk category, the isolatedbase structure can be used at 1.46–3.20 times higher PGA than the fixed base structure. The fragility
analysis results show that the fixed base structure has a 30% safety margin and a 62.5% isolated basestructure from the PGA design. The obtained results are useful for assessing existing buildings orconsidering a new building's performance.
Keywords: Seismic isolation, fragility curve, incremental dynamic analysis, performance level.
1. INTRODUCTION
As a country prone to earthquakes, Indonesia has had seismic codes for earthquake-resistant structure
designs since 1970 and has been updated five times to the latest in 2019. In updating the Indonesian
seismic codes, seismic hazard maps also update. There are changes to the Peak Ground Acceleration(PGA) values that can decrease or increase based on the research developments related to seismic hazard
maps in Indonesia (Irsyam et al., 2017). Under these conditions, an analysis is needed to determine theeffect of changing the earthquake's intensity on the structure's behavior. The analysis that can be used is1

Implementation Unit for Building Materials and Structures, Ministry of Public Works and Housing, Indonesia.
International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan.
* Chief examiner, ** Examiner21

seismic fragility analysis. This analysis can provide an overview of the structure's behavior towards
damage limits with different earthquake intensities. It is useful for evaluating structures at various
intensity values, such as PGA (Rajkumari et al., 2022).
The Indonesian seismic design uses the concept of building performance levels comprising
Immediate Occupancy (IO), Life Safety (LS), and Collapse Prevention (CP). In relation to thisperformance, some studies found that, after an earthquake, buildings suffer from more damage than theirperformance targets. This problem often occurs because of the poor quality of the construction materials
used and the workmanship involved (Pribadi et al., 2021). A seismic isolation system is one of thetechnologies that can be used to increase the building performance level. In Indonesia, several buildings
had been built using this system, but the usage has remained relatively low (Imran et al., 2021).
Regarding seismic fragility analysis, no study in Indonesia has researched this analysis onseismic isolation systems. A study on the fragility analysis of the isolation system in Indonesia is needed
to see its behavior if the earthquake intensity increases much compared to the design earthquake
intensity. Based on the background description, the main objective of this study is to analyze the seismic
isolation system design to improve the performance level of existing target buildings and the seismic
fragility of fixed and isolated base structures with a PGA intensity according to Indonesia's seismic
hazard maps of 0.1–1.5 g.
2. DATAThe target building is a typical construction design for a high-seismicity area by the Indonesian Ministryof Public Works and Housing (Table 1 and Figure 1). The target building was chosen because it is atypical design that is widely used and has a life safety performance target that is still possible to improveand to see how significant performance increases using an isolation system.

Parameters
Occupancy
Structure system
Number of stories
Total height
Building codes
Seismic parameters design
Concrete strength (fc')
Yield strength of rebar (fy)Number of type members

Table 1. Target building information.
Information
Residential (risk category: II)
Special moment frame reinforced concrete with shear wall
8 stories
27.4 m = 3.6 m + (3.4 m x 7)
Indonesian seismic codes (SNI 1726:2019)
𝑺𝑫𝑺 = 1.00 g; 𝑺𝑫𝟏 = 0.68 g; site class D
Beam and slab = 30 MPa; column and shear wall = 35 MPa
longitudinal, shear, and cross tie = 420 MPa
Column type = 1; beam type = 5; shear wall type = 1; sslab type = 1

Figure 1. Target building typical structure plan.

23. METHODOLOGY
3.1. Seismic Isolation
Seismic isolation design was conducted on the target building without changing the structural design.
The target building structures have been designed with seismic codes so that some design parameters
can be used, such as the total building mass and base shear coefficient. In this study, three types ofisolators were used, namely natural rubber bearing, lead rubber bearing, and sliding rubber bearing. The
parameters used in seismic isolation design were base shear coefficient (𝛼1 ) of 0.1 was obtained, naturalperiods (𝑇𝑓 ), shear coefficient of the damper (𝛼𝑠 ), and design displacement of the seismic isolation
system (𝐷𝑑 ).
𝑚
𝐾𝑓

𝑇𝑓 = 2𝜋√(1)

𝐷𝑑 =𝑔𝑆𝐷1 𝑇𝑓
4𝜋2 𝐵𝐷(2)

𝛼𝑠 =

𝑄𝑦𝑚𝑔

(3)

Where 𝑚 is the total mass of the building, and 𝐾𝑓 is the stiffness of the seismic isolation
system. The total structural mass of the upper structure is 8774.14 tons. 𝑆𝐷1 is design spectral
acceleration at a period of 1, and 𝐵𝐷 is the damping coefficient related to effective damping (𝛽). The
Japan Society of Seismic Isolation (JSSI) has a range of values for seismic isolation design parameters
as shown in Table 2. In addition to the design parameter values discussed earlier, there are also force
requirements governing stability such as the wind load ( 𝐹𝑤𝑖𝑛𝑑 ) which must be lower than thecharacteristic shear strength (𝑄𝑦 ). By calculating the service load (dead load, superimposed dead load,live load, wind load, and earthquake load) following Indonesia codes (SNI 1726:2019) and design
parameters following Japanese standards (JSSI), the results of the seismic isolation design can be seenin Figure 3 and Table 2 for the design parameters.
Table 2. Design parameter result.
Design parameters
General value in Japan
0.05 – 0.20
Base shear coefficient (𝜶𝟏 )30 – 50 cm
Design displacement (𝑫𝒅 )Natural periods (𝑻𝒇)
3–5s
0.02 – 0.10
Shear coefficient of damper (𝜶𝒔 )Wind load (𝑭𝒘 ) < characteristic shear strength (𝑸𝒚 )> 1265.88 kNFigure 2. Seismic isolation design

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Design result
0.1
39 cm
3.5 s
0.02
1717.6 kN

StatusOKOKOKOKOK3.2. Concept Modeling and Analysis
The modeling and analysis of reinforced concrete building structures were performed using threedimensional modeling. The beam, column, and shear wall elements were modeled as line elements, andthe slab was modeled as a rigid diaphragm. The infill wall was modeled as a gravity load without
contributing to the lateral stiffness. Modeling of plastic hinges and acceptance criteria for each structuralelement is carried out per ASCE 41-17. The nonlinear behavior of reinforced concrete elements wasmodeled by the Takeda hysterical model. In the IDA analysis, the earthquake's intensity will be greaterthan the design earthquake, so in the isolation system, displacements will exceed the design
displacements. In the building
design plan, there is a retaining
wall around the isolation system,
the distance between the seismic
isolation system and theretaining wall is determined atshear strain (γ) 300% of theheight of the total rubber
thickness. This value is theassumption of rubber conditions
that have not yet been broken andare still performing quite well.
Thegap between the seismicFigure 3. Concept of seismic gap model for pounding effect.
isolation system and the retaining
wall is 500 mm and was modeled as a gap element. The defined stiffness only affects when thedeformation exceeds the defined seismic gap. This study used the stiffness value based on theexperimental test results of Miwada et al. (2012) as 104.6 kN/mm.
3.3. Incremental dynamic analysis (IDA)
The IDA in this research uses the Nonlinear Time History Analysis (NLTHA) principle, carried out on
several intensity measurements in the form of PGA with intervals of 0.1–1.5 g based on Indonesian
seismic hazard maps. For damage measurement in the form of inter-story drift ratio maximum (𝐼𝐷𝑅𝑚𝑎𝑥 )which is referring to performance level
IO of 1.0%, LS of 2.0%, and CP of 2.5%
(Xue et al., 2008; Ibrahim & El-Shami,2011). Based on the SNI 1726:2019 forperforming the NLTHA, 11 groundmotions are required as the minimum
number of ground motions. The groundmotions were selected referring toIndonesian deaggregation maps data
which explains the source magnitude(M) and distance (R) for each
earthquake source mechanism and soil
classification map, 11 ground motions
Figure 4. Selected ground motion scaled at PGA of 1.0 g.
were chosen for analysis, as shown in
Figure 4. Furthermore, the resulting
IDA from 11 ground motions can be statistically evaluated to be the equivalent dispersion, 𝛿𝑒𝑞 , which
can be calculated by Equation (4).

𝛿𝑒𝑞 =ln(𝐼𝐷𝑅𝑚𝑎𝑥 )84% − ln(𝐼𝐷𝑅𝑚𝑎𝑥 )16%
24(4)Using the formula from Nagae et al. (2006), the probability of the 𝐼𝐷𝑅𝑚𝑎𝑥 exceedance
against the 𝑖𝑑𝑟 limit of each performance level can be calculated by Equation (5).
ln(𝑖𝑑𝑟) − ln(𝐼𝐷𝑅𝑚𝑎𝑥 )50%
𝑃[𝐼𝐷𝑅𝑚𝑎𝑥 > 𝑖𝑑𝑟] = 1 − 𝑃[𝐼𝐷𝑅𝑚𝑎𝑥 ≤ 𝑖𝑑𝑟] = 1 − Φ ()
𝛿𝑒𝑞

(5)

The IDA is only performed in the weak direction of the building, which in this work, is theX-direction. The structure herein was modeled in three full dimensions; hence, the IDA will require
much time when using the 11 ground motions and do it on fixed and isolated bases. The weak axis
analysis is also sufficient for representing the worst possible conditions.
4. RESULTS AND DISCUSSION
The fragility curve in Figures 5 denotes the relationship of the Probabilities of Exceedance (POE) ateach performance level in the PGA range of 0.1–1.5 g. Based on ASCE 7-16, a reliability target isdetermined based on the building risk category. The target reliability is also called the target probabilityof failure or the POE. The building risk category was related to the target value because it showed alevel of building risk according to its occupancy. The risk target classification based on occupancy isdetermined by the government in the seismic codes, which, in this case, is the Indonesian government
in SNI 1726:2019.

Fixed

Isolated

Figure 5. Fragility curve result
Table 3. PGA values range of performance levels at target reliability.
Risk
category
I-II

III

IV

Occupancy(SNI 1726:2019)

Residential
OfficeFactory
PrisonMeeting hallSport Centre
Hospital
SchoolDisaster shelter

POE(ASCE 7-16)

PGA limit
Fixed base structure
Isolated base structure
IOLSCPIOLSCP

25 %≤0.23 g>0.23 g≤0.52 g>0.52 g≤0.65 g≤0.65 g>0.65 g≤0.87 g>0.87 g≤0.95 g15 %≤0.18 g>0.18 g≤0.43 g>0.43 g≤0.54 g≤0.56 g>0.56 g≤0.75 g>0.75 g≤0.81 g9%≤0.15 g>0.15 g≤0.37 g>0.37 g≤0.47 g≤0.48 g>0.48 g≤0.65 g>0.65 g≤0.70 gTable 3 explains the performance level of the target building at several PGA levels. Thetarget building was designed using DBE (2/3 MCER) PGA = 0.40 g with risk category II. Theperformance was LS for a fixed structure and IO for an isolated structure. A comparison of the PGAlimit value on each target reliability and risk category showed the significance of using an isolatedstructure, which can be employed on a PGA larger than that of fixed structures with a ratio of 1.46–3.20.

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The results also indicate the safety margin level of the design results. Fixed and isolated structures weredesigned with different performance targets. The fixed base structure was designed with LS targets,
while its isolated base was designed with IO targets. Limit values were used for each performance target.
The safety margin for the fixed base structure was 0.52 g / 0.40 g = 1.3 (30%), while that for the isolatedbase structure was 0.65 g / 0.40 g = 1.625 (62.5%).
5. CONCLUSIONSThis study analyzed the design of a seismic isolation system for a target building and determined theseismic fragility with a PGA intensity according to Indonesia’s seismic hazard maps of 0.1–1.5 g. The
following conclusions can be drawn based on the results of the performed analyses:⚫ The design results using the seismic isolation system showed increased performance in the target
building. Based on the NLTHA, the target buildings with an LS performance level can increase toIO, as evidenced by the decrease in the 𝐼𝐷𝑅𝑚𝑎𝑥 value from above 1.0% to below 0.5%. The IDA
result comparison also showed a trend of significant performance improvement. That is, with thesame performance level target and risk category, seismic isolation can be used at 1.46–3.20 times
higher PGA than that of the fixed base structure.
⚫ The fragility analysis results using the IDA were very effective in determining the fragility and thereliability of the response structure. For fragility, the fixed base structure showed a 30% safetymargin and a 62.5% isolated structure from the PGA design. The safety margin is defined as therange of structural performance targets still being achieved before the performance target decreases.
ACKNOWLEDGEMENTS
I want to express my sincere gratitude to my supervisor Dr. Matsutaro Seki, Dr. Hiroto Nakagawa, andDr. Yuri Otsuka, for supervising and advice during this research.
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