DYNAMIC BEHAVIOR UNDER STRONG MOTIONSAND STRUCTURAL DESIGN PROCEDURES OF
SEISMICALLY ISOLATED BUILDINGS
Rocio Rivera ∗MEE12611

Supervisor:
Adviser:

Masanori IIBA∗∗
Koichi MORITA ∗∗∗ABSTRACTThis paper reviews the dynamic behavior under strong motions and structural design procedures ofseismically isolated buildings. Chile and Japan are known as countries very prone to earthquakes andhave a long history of earthquake damages and countermeasures. Seismic isolation system is a key
issue to mitigate structural disasters against strong shaking, thus it is necessary to understand thecharacteristics of ground motions and the behaviors of seismically isolated buildings to enhance theseismic safety against earthquakes.
In Chile, after last 2010 earthquake, seismically isolated buildings have begun to be accepted
into our society in order to keep structural safety and functionality, and to ensure that, base isolation
system should be examined continuously to verify the validity of structural design.
Firstly, this paper introduces the observed behavior of seismically isolated buildings during
last major earthquakes and advantages of this type of systems. Secondly, summarizes the strong
motion observation of a target building and dynamic behavior of target building based on observed
records. Finally, structural design of isolation systems is presented through a fundamental procedure.
Keywords: Seismically isolated building, Strong motion, Design procedure.

1. INTRODUCTION
Chile is a land of earthquakes, it is located in the Ring of Fire, an area with highest seismic activity.
The earthquakes occur due to release of mechanical stress of the subduction movement of Nazca Plate
below the South American Plate on the Chile-Peru Trench, which is an active area of the Ring of Fire.
Along its history large earthquakes have shaken hundreds of times the territory, including thelargest earthquake in the world in 1960 with magnitude of 9.5Mw. The last important earthquake (withmajor destructive tsunami) is also the 6th largest Earthquake in the World (USGS Web site, 2012) andit occurred off the coast of Chile’s Maule Region on February 27, 2010 (hereinafter called 2010 Chileearthquake). Its epicenter was at 35.909S, 72.733W offshore from Maule, it has a magnitude of 8.8Mw
and the rupture area (parallel to the coast) had about 550km of length, 100km of width and 50km of
depth (Pararas-Carayannis, 2010).
The development of seismic isolation in Chile was started with the first seismically isolatedbuilding as part of a project of University of Chile in 1992, where this technology was first introducedin the country. With the development of the technology in Chile, great deal of effort was put into theproject to establish Chilean seismic codes in 2003 to fulfill the minimum requirements for seismicallyisolated buildings under Chilean seismic characteristics. After the publication of the seismic isolation
code, the number of seismically isolated projects has increased. But, it was not until the occurrence of∗Santolaya Ingenieros Consultores, Chile.
BRI Research Coordinator of Building Technology, Japan.
∗∗∗BRI Chief Fellow, Japan.
∗∗1

2010 Chile earthquake when the technology acquired national connotation due to its good performancein different types of structures. After the earthquake several hospitals could not be operated due tostructural and non-structural damages. The mitigation of non-structural damages was having the same
importance to enable the building due to its repair cost. Thus, we have learnt from the last earthquake
it is important to avoid the interruption of operation in at least essential buildings.
At present, Chile has approx. 50 seismically isolated structures and the seismic isolation isused not only for private structures such as buildings, but also for bridges, harbors and gas plants, now
the technology is starting to spread to public structures such as hospitals and public housings.
This study aims to achieve two main objectives: Contribute to permanent effort of decrease
the seismic vulnerability and to protect the human life; contribute effectively on Chilean practice ofseismic isolation design by learning and comparing with actual knowledge and procedures used inJapan.
The contents of this study are summarized as: Chapter 2 presents seismic behavior ofseismically isolated buildings in Chile and Japan during last major earthquakes; Chapter 3 studies thebehavior of a target seismically isolated building based on observed records; Chapter 4 discusses
simulation analysis of target building trying to achieve same response through an analytical method;
Chapter 5 presents a fundamental procedure of seismically isolated building design; Chapter 6
corresponds to final conclusions.

2. SEISMIC BEHAVIOR OF SEISMICALLY ISOLATED BUILDINGS IN CHILE AND
JAPAN DURING LAST MAJOR EARTHQUAKES
2.1. Seismic Behavior Observation after 2010 Chile Earthquake
At the moment of the Mw 8.8 2010 Chile earthquake only one seismically isolated building wasinstrumented, which is the first isolated building in Chile built in 1992 in Santiago, called Andalucia
Community Building. This building is part of a research project developed by Sarrazin & Moroni atUniversity of Chile.
The superstructure corresponds to 4-story confined masonry building, with the first level ofreinforced concrete and equipped with 8 high damping rubber isolators. The floor area is 6m x 10m
with a total height of 9.5m.
From the structure several records have been obtained, and the most important one is the2010 Chile earthquake record and an aftershock recorded on March 11 with a magnitude 7.0Mw(317km from the epicenter). The peak accelerations are summarized in Table 1.
Table 1. Peak accelerations on Andalucia Community Building.
Direction
Horizontal
Vertical2010 Chile earthquake
Basement Acc. cm/s2
4th level Acc. cm/s2303
218
178284Aftershock on May 11
Basement Acc. cm/s2
4th level Acc. cm/s234362030

The seismically isolated building has shown an excellent seismic behavior without structuraland non-structural damages even during the 2010 Chile earthquake; on the other hand, the fixedbuilding suffered some light cracks and non-structural damages (Civil Engineering Department,
University of Chile, 2012).
2.2. Seismic Behavior Observation after 2011 Tohoku Earthquake
This subchapter is based on an investigation made in June of 2011 for a team consisted by themembers of NILIM (National Institute of Land, Infrastructure and Management) and BRI (BuildingResearch Institute) (Saito, Iiba, et. al., 2012 & Saito, Okawa, & Kashima, 2012).

2During the 2011 Tohoku earthquake, in spite of the strong shaking with JMA intensity 6upper, the seismically isolated buildings showed excellent performance without any severe structural
damage even to non-structural damages in superstructure. However, some damages to isolation
dampers and expansion joints were observed.
Some damages in dampers were observed, for example many cracks were found in leaddampers. In addition, peeling off of paint was observed in U-shape steel dampers and residual
deformation of steel was remained.
On the other hand, the one-story steel warehouse located in Miyagino-ward in Sendai City,
near to the bay, was reached by tsunami. Building and the SI floor was submerged under the watertaking 16 days to remove all water. Tsunami height was estimated in around 4m and external walls
presented damages by the collision of floating debris. There was no harmful scratch or inflation of therubber of HRB (high rubber bearing); however severe rust at the steel plates and bolts was observed.

3. BEHAVIOR OF SEISMICALLY ISOLATED BUILDING BASED ON OBSERVED
RECORDS
The target is Tsukuba City Hall Building, hereinafter called Tsukuba Building, is located in Ibaraki
Prefecture at 334km from the epicenter and 24km in focal depth in the 2011Tohoku earthquake. The
isolation system consists of 20 Natural Rubber Bearings (NRB), 45 Lead Rubber Bearings (LRB) and9 Natural Rubber Bearings with Steel Dampers.
The mainshock of 2011Tohoku earthquake was recorded in Tsukuba Building on March 11,
2011, at 14:46 (local time) with intensity 5.2 of JMA Seismic Scale. In comparison with theearthquake motion on the basement and the response on the first floor, the peak acceleration decreases
from 326.9cm/s2 to 91.7cm/s2 (in Y direction). That in Z direction shows acceleration amplification
where the peak acceleration on the sixth floor is twice that on basement (121.9cm/s2). The
superstructure suffered a small amplification from magnitude 4.6 to 4.8 in JMA scale.
This study uses records from 461 accelerograms measured effectively in Tsukuba Building,between February 05, 2011, and June 06, 2012, including the mainshock of the 2011 Tohoku
earthquake. The results will be shown below according to analysis directions.

Figure 1. Distribution of basement acceleration
ranges between 10cm/s2 and 350cm/s2

Figure 2. Basement peak acceleration less than
50cm/s2 vs. ratio 01F/B1F.

Figure 1 shows a bar graph focusing on the accelerations between 10cm/s2 and 350cm2. The
total number of events in this range is 50 and this graph is showing that the number of events isdecreasing according to increase of peak acceleration range. There are 411 events lower than 10cm/s2.

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Figure 2 presents the peak acceleration ratios (1F/B1F) with basement acceleration less than50cm/s2. When the basement accelerations are more than 10cm/s2 the ratios are less than 1.0. The
isolation effect is confirmed.
4. SIMULATION ANALYSIS OF SEISMICALLY ISOLATED BUILDING
The main objective of this chapter is to describe, with an analytical model, the behavior observed inTsukuba building during 2011 Tohoku earthquake and to compare properties considered in the design
versus properties observed during strong motions.
4.1. Fitting for characteristics of superstructure
The first step of this analysis is to make an analytical model to obtain the design period of thesuperstructure using 3% of damping factor, from this analysis the primary natural period ofsuperstructure is 0.591 second for X direction and 0.598 second for Y direction.
Then the observed frequency domain of superstructure should be established, through theFourier spectral ratio between sixth and first floors, using the average natural frequency of three smallrecords (with JMA intensity of 2.0) to obtain the elastic stiffness of the superstructure in the range ofsmall displacements. Fourier spectral ratio graphs between sixth and first floors indicating theresponse characteristics of superstructure and the peaks corresponding to the first natural frequency.
To reach the analytical case the same observed period will change the stiffness ofsuperstructure simply using the equation: T = 2π M sup k sup .
The stiffness of each story of the superstructure is modified to 2.1 times the stiffness
considered in the design process.
4.2. Fitting for first stiffness of seismically isolated system
Using the designer properties and the modified stiffness of superstructure done in the last section, iscalculated the first natural period of in both analysis directions of the seismically isolated building. For
X direction the period is 1.376 second and 1.366 second for the Y direction.
To calibrate the first stiffness of designer force-displacement total curve, it is chosen a bigrecord. It has a large JMA intensity (greater than 4.0) that causes a displacement closest to yielding
point of the system. The record is the second largest record of Tsukuba building, while the largest one
is the mainshock of 2011 Tohoku earthquake.
Therefore, the periods of entire system should be modified from 1.376 second (X direction)and 1.366 second (Y direction) to an observed period of 1.0 second according to Fourier spectral ratio
06F/B1F and phase lag. By modification of first stiffness of isolator devices with keeping the values ofQ2, δ2 and K3 of total curve, only the first stiffness of LRB and NRB with steel damper are modified.
It means that the stiffnesses K1 and K2 of total curve are changed. The results are shown below.
Table 2. Modified properties of total curve of isolators and dampers.
Characteristics
Number
NRB
20
NumberK1 (kN/mm)
LRB
45
1,218.98
NumberK1 (kN/mm)
SteelDamper
9469.24
Total Curve of Isolated Layer
NumberK1 (kN/mm)
NRB+LRB+
Steel damper
74
1,712.76

K1 (kN/mm)
24.54
Q1 (kN)K2 (kN/mm)
7,976
87.29
Q1 (kN)K2 (kN/mm)
3,070
2.30
Weight(kN) =Q1 (kN)
K2 (kN/mm)
11,207114.134Q2 (kN)
K3 (kN/mm)
9518
54.44
103,113.6
Q2 (kN)K3 (kN/mm)
13,22482.20

As the peak of Fourier spectrum (3.65 atapprox. 1Hz for entire building) is almost two times
larger than the corresponding spectral ratio, thepeaks in the modeling should be decreased applying
an additional damping factor. The additional
damping is calculated according to the maximum
relative velocity within the isolated layer and wascalculated to be approx. 0.10m/s.
Finally, Figure 3 shows a clear similarity
among waveforms of observed record measurementon the building and the response of the building
from the structural model with the record input. The
behavior of superstructure is represented by green
dashed line for observed record and blue solid line
for the modeling case, those show almost same peakvalues but have a small gap between them, which isFigure 3. Fourier spectral ratio of observed a peak of 8.62 versus 7.35 respectively. On the other
hand, black dashed line and red solid line represent
record (dashed lines) and analytical casebehavior of the entire system for the observed and(solid lines).
modeling case respectively. In this situation bothpeaks appear are in a frequency of 1Hz but they have a small difference of amplitude, at a peak of 2.38
vs. 2.04 for observed one. However, this simulation reaches remarkable approximations.

5. STRUCTURAL DESIGN OF SEISMICALLY ISOLATED BUILDING
Two design procedures are adopted in Japan according to new regulations stipulated in Notifications
1461 and 2009 in October of 2000 from the Ministry of Land, Infrastructure, Transport and Tourism.
The techniques of design are: time-history analysis method and equivalent linearization method(ELM) using a design spectrum. According to the regulation, seismically isolated building with heightequal to or less than 60m, first or second site ground classification and seismic isolation at the basedoes not need authorization by Minister (Saito, Hiramatsu, & Kani, 2011). Is this structural design thetechnique of time-history analysis is selected.
A procedure of design of seismically isolated building is briefly explained below, in which
there are some recommended values used in the practice. The adopted procedure is as follows:(1) Target building input data: preliminary sections of elements, building location and a soil type.
(2) Calculation of superstructure using simplified method by pushover analysis with base shear
coefficient between 0.15 and 0.20. It is recommended to use 0.18.
(3) Setting appropriate design of superstructure such as sections of structural elements andreinforcement.
(4) Static analysis of superstructure using long term loads such as dead (DL) and live loads (LL).
Determine building weight (W) and vertical forces to isolators (NL) due to long term loads.
(5) Preset isolators determining minimum diameters in accordance with maximum axial stress. It
is common to use minimum value of 10N/mm2.
(6) Setting target performance of seismically isolated building:a) Target period (Tf) between 2 and 5 seconds. It is recommended to the period in 4 seconds.
b) Ratio Qy/W ≤ 5.0% (Qy; yield horizontal force), whose recommended value should bebetween 1.5% and 3.0%.
c) Determine maximum target displacement. It is recommended to set 40cm.
(7) Check distance between center of gravity and center of stiffness is less than 3%.
(8) Make dynamic analysis (time-history) on entire system (superstructure and seismic isolatedlayer). Equivalent MDOF system of superstructure is obtained through Pushover analysis.

5(9) Check performance limitation of isolators using vertical forces due to long term loads andshort term loads (seismic lateral forces) and check tensile strength less than 1N/mm2.
(10) Comparison between response and performance of entire system.
(11) Check design against wind forces.

6. CONCLUSIONS
From analysis of behavior of seismically isolated building based on observed records the following
understanding are gained.
a) The input spectrum at the basement is dominant in short-periods, and predominant at the range of
0.2 to 0.3 second, while the predominant periods at the range of 3 to 4 seconds are remarkable in both
directions for the velocity response spectrum during mainshock.
b) 461 accelerograms were measured in Tsukuba building, 99% of them have maximum horizontal
acceleration less than 50cm/s2, and was observed that for accelerations larger than 10cm/s2
amplification of acceleration on first story dominates reduction.
Through simulation analysis dynamic characteristics of base isolated building werecompared designer considerations versus real parameters obtained from to observed records.
a) After setting the first stiffness of entire system according to the first natural frequencies of Fourier
spectrum analysis, it is possible to notice that the analytical stiffness of superstructure is approx. twiceof the stiffness considered in the design procedure.
b) Simulation analysis reaches remarkable approximations through changing stiffness according toobserved records.
A simple design procedure should be done to understand the seismic behavior to comprehend
dynamic characteristics and to gain better understanding and to verify the validity of structural design.
a) It is recommendable to set lead rubber bearings in external perimeter of the arrangement andnatural rubber bearing at the center to avoid torsional movement through their stiffnesses.
b) Lead rubber bearing has major influence for the calculation of the yield horizontal force andconsequently it is important to set a determined value of Qy/W.
c) To calculate the period of seismically isolated building it was used the second stiffness of the totalload-displacement curve of isolator arrangement, where natural rubber bearing has importance.

ACKNOWLEDGEMENTS
I would like to express my appreciation and gratitude to Dr. Toshihide KASHIMA for his valuableadvice to complete this study. In this study, strong motion records from seismic network of Building
Research Institute were used. I wish to express my special gratitude to Mr. Shinichi ARA, vice generalmanager of Orimoto Structural Engineers Company, for his support and giving the opportunity tolearn about the engineering practice in Japan.

REFERENCES
USGS. (2012, November 30). http://earthquake.usgs.gov/earthquakes/world/10_largest_world.php.
Pararas-Carayannis, G. (2010). International Tsunami Society, 126-156.
Civil Engineering Departament, University of Chile. (2012) (Spanish) (p. 323). Chile.
Saito, T., Iiba, M., Morita, K., Azuhata, T., & Inoue, N. (2012). 12th World Conference on Seismic
Isolation Energy Disipation and Active Vibration Control of Structures. Sochi, Russia.
Saito, T., Okawa, I., & Kashima, T. (2012). The International Symposium for CISMID 25thAniversary, (pp. No. M-3). Lima, Peru.
Saito, T., Hiramatsu, M., & Kani, N. (2011). 12th World Conference on Seismic Isolation Energy
Dissipation and Active Vibration Control of Structures. Sochi, Russia.

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