Paper No. M-2

SEISMIC RESPONSE OF ISOLATED STRUCTURES SUBJECTED TO
Mw 8.8 CHILE EARTHQUAKE OF FEBRUARY 27, 2010
Ruben BOROSCHEK1, Rodrigo RETAMALES2 and Antonio AGUILAR3

SUMMARY
The growing interest of investors and stakeholders in protecting building contents and operation beyond
the goals of current Chilean seismic design codes is generating an increased demand for the use ofnonconventional seismic protection systems such as base isolation and energy dissipation devices. This
paper focuses on the seismic responses recorded in an instrumented isolated building during the Mw 8.8
earthquake that struck the central south region of Chile on February 27, 2010. Furthermore, the effects ofusing seismic isolation devices in a 20-story residential building and in a high tech facility are presented.
The main observations, conclusions and implications for design derived from the recorded data are
discussed. The critical issues related to seismic isolation design and implementation, along with its effects
on the building response and economical costs are described.

INTRODUCTION
As part of the research projects developed at the University of Chile by Sarrazin & Moroni, a low costisolated system was implemented in 1992 as an attempt to introduce this technology in developing
countries. The Andalucia Condominium project was a pilot project conformed by one isolated building
and a couple of similar but conventional (fixed base) buildings, as described by Moroni [1]. The baseisolated and one of the fixed base buildings were instrumented. This paper presents an analysis of theseismic response of the isolated building recorded during the Mw 8.8 Chile earthquake of February 27,
2010. In addition, this paper describes the basic design requirements of the Chilean Base Isolated
Building Design Code and some recent progress on seismic isolation applications.
CURRENT TRENDS ON SEISMIC BASE ISOLATION OF STRUCTURESThe Maule earthquake of February 27, 2010 caused economic losses exceeding US$30 billion. A highpercentage of these losses resulted from damage to nonstructural systems and components, such asarchitectural elements, building contents, and electric and mechanical equipment, detailed by Miranda [2].
These losses, in addition to the public’s perception regarding the earthquake’s impact on its properties,
has encouraged investors and real estate developers to incorporate seismic protection systems into
buildings in order to achieve a performance target that exceeds the standards of current national codes.
Seismic isolation is among those seismic protection systems that have experienced the most growth indemand. This type of system has been used extensively, both nationally and internationally, to protect
critical and strategic infrastructure, structures of high heritage value, and structures with high-value
1

University of Chile, Department of Civil Engineering, Chair Structural, Construction and Geotechnical Areas.
Civil Engineer University of Chile, Leader Seismic Isolation and Energy Dissipation Ruben Boroschek & Associates.
3
Civil Engineer University of Chile, Leader Structural Health Monitoring Ruben Boroschek & Associates.
2

content, among which include hospitals, datacenters, museums, government buildings and emergency
response structures, among others. However, most existing seismic base isolation systems in Chile arefound in buildings of relatively low height. Notable Chilean projects include Andalucia Building,developed by Moroni and Sarrazin in 1992; the Marga-Marga Bridge developed by G+V Engineers in
1998; and the Military Hospital developed by Hoehmann Stagno & Associates in 2001. This paper
describes the effects of implementing seismic isolation technology in two new special buildings.
National Design Code Characteristics
The Chilean standard for the seismic design isolated buildings, NCh2745.Of2003 (INN 2003) [3], ispartially based on the ASCE/SEI 7-98 provisions, adapted to the local seismicity conditions and to more
stringent analysis and testing requirements. Currently, the standardization committee’s work is aiming atease and promoting the use of this type of seismic protection technologies. The future version of the code
will allow for treating reinforced concrete moment frames as intermediate instead of special moment
frames (based on the intrinsic ductility capacity of intermediate moment frames and the low inelastic
demands expected on isolated structures), and using a response modification factor of 1.5 for the designof the substructure (based on local design practices and the overstrength resulting from using current
design methods), among others.
Torre del Sol Condominium
The first building, shown in Figure 1, corresponds to the Torre del Sol building, a residential building
currently under construction in Copiapo. This project was developed by Inmobiliaria Santo Domingo
Ltda. and built by Constructora ALCORP S.A. The structural design of the building was developed byPatricio Bonelli & Asociados Ltda. The design of the seismic isolation system, conducted in accordancewith the provisions of NCh2745.Of2003, was developed by Ruben Boroschek & Asociados Ltda.
(www.rbasoc.cl). The isolation system was manufactured and tested by Dynamic Isolation Systems Inc.
(www.dis-inc.com), the world leader in seismic isolation systems that has isolated nearly 400 structures
worldwide, including a dozen high-rises up to 33 stories tall. The excellent seismic performance ofbuildings with seismic isolation technology has been demonstrated during earthquakes in Northridge(EEUU, 1994), Kobe (Japan, 1995), Maule (Chile, 2010), and recently in Tuhoku (Japan, 2011). A
detailed description of the performance of seismically isolated buildings affected by the earthquake in
Tuhoku, Japan can be found in Saito [4].
The Torre del Sol building, which will span approximately 19,000 m2 and 19 stories (1 level for theisolation system, 2 underground parking levels, 14 residential floors, 1 level for equipment and 1 for theengine room), will be the tallest residential building with seismic isolation in Latin America. A typicalfloor covers 53x12.9 m. The total height of the building, including the undergrounds, is 49.6 m. The
seismic weight of the structure supported by the isolation system is 15,500 Tonf. According to thegeotechnical report, the foundation soil corresponds to a stiff soil with average shear wave velocity in thefirst 30 m beneath the surface of 598 m/s.
For this project, lead rubber bearings were chosen for a number of reasons, among which are: thesesystems have been extensively tested in the laboratory and their behavior has been thoroughly
demonstrated in real seismic events; the ease in predicting and modeling their behavior for the purpose ofanalysis; the stability of the devices against environmental agents; and the independence of device’s
performance from the loading rate. The number of isolators required was determined largely by thegeometry of the building. In order to provide stability to the structure and to counteract the tension of theisolators, 60x197 cm beams needed to be installed underneath the parking area on the second
underground floor. While tensile forces up to 2G (where G is the shear module for a shear strain of 100%)are permitted by the design codes, the test for tensile loading can be conducted in a limited number oflaboratories worldwide. Seismic isolators were arranged at the ends of beams and, generally, at the endsof all bearing walls. As an alternative to using large size beams under the parking area, the use of Cross
Linear Bearings (CLB) was also evaluated, but was discarded due to cost. In other cases, CLB’s may be
the only alternative for implementing seismic base isolation systems in high-rise buildings. Anotheralternative was tested to withstand incidental tension; tensile cables were used within the isolators by
Kasalanati [5] in order to exert pre-compression and to prevent tension. However, this alternative wasalso discarded due to the length of the cables required to prevent yield during the movement of theisolation interface. In order to reduce costs, the 45 seismic isolators used in the structure are all identical.
The maximum compressive force on the isolators (considered for design and for testing) is 820 Tonf,
while the tensile force (or minimum compression load) is 0 Tonf.
The seismic isolator prototypes and all production isolators were subjected to an exhaustive test series
combining compression and shear in order to validate the properties assumed for the design and to verify
the stability of those isolators against extreme seismic loads. The tests were performed at Dynamic
Isolation Systems’ laboratories in McCarran, Nevada. In the design of the isolation system, a series ofnonlinear dynamic analyses were carried out taking into consideration 7 seismic records compatible withthe demands associated with the maximum considered earthquake according to the Chilean national
standard for the seismic zone and ground conditions of the structure’s site location. A wide variety of seedrecords were selected in a manner that allows analysis of the different seismic events to which thestructure could be exposed during its lifetime. The analysis showed that the use of isolation allows forachieving a reduction of 88% in the average base shear of the structure. The design of the superstructure
was made taking into consideration the maximum shear that could be transmitted through the isolation
system. In this case, the critical area of the structure, where the demand of seismic deformations is mostconcentrated, corresponds to the isolation interface; therefore, it was not necessary to include theconfinement according to ACI318. The demand of displacement is accommodated by the deformation of
the isolators and not by the rotation of sections of concrete walls. Figure 2 shows comparisons of the
distribution of floor displacement, interstory drifts, and absolute floor accelerations of the base-isolated
and fixed-base structures. The values shown represent the mean values obtained for the 7 records
considered.

Figure 1. 3-D View analysis
model

Figure 2. Comparison seismic response Torre del Sol building with andwithout base isolation

A critical component in the implementation of seismic isolation systems is their initial cost. While theirsuperior structural response during severe seismic events is evident, the decision to invest in protection isgenerally difficult. As reference, the cost of each seismic isolator for a project of this size is around
US$8000-US$9000. Additional costs include those of the V60x197 cm beams required for thestabilization of the structure and the costs of excavations required to accommodate the height of theisolators and the aforementioned beams. Moreover, other considerations include the costs the flexible
elements required for basic installations and other distributed systems that cross the isolation joint.
However, the initial investment costs are offset undeniably by the reduced likelihood of damage orcollapse of the structure. Moreover, the implementation of isolation systems may allow a reduction in theamount of structural walls in a building, given that, as described in previous sections, the elastic interstory
drifts in the isolated structure does not exceed the 0.002 of the story height. Similarly, the use of isolation
could allow for eliminating boundary elements due to the low demand of relative deformation between
the roof and the isolation level. Additionally, this last measure would allow a reduction in the thickness of
the walls. These measures, coupled with the minimum-shear design of a structure, could offset the costs
of investment in seismic protection.
Claro Chile Data Center
The second interesting and recent application of seismic isolation is the Claro Data Center shown in
Figure 4. This building will host one of the largest high-tech facilities in Chile. The total area of thebuilding is approximately 35,000 m2, distributed in 2 levels and a mechanical floor. The architectural,
structural and isolation system designs, carried out by +Arquitectos and Gatica Jimenez Engineers and
Ruben Boroschek and Associates, respectively, considered the stringent requirements of ANSI/TIA-942[6] for Tier level 4. According to TIA’s requirements, the infrastructure shall be fault-tolerant, withelectrical power storage and distribution facilities with an expected availability beyond 99.995%. In orderto protect the operation of the building following extreme seismic events, 340 lead rubber isolators, also
manufactured and tested by Dynamic Isolation Systems, are used under the structure. The seismic
responses estimated using the signals recorded during the Mw 8.8 2010 Chile earthquake is shown in
Figure 5.

Figure 4. 3-D View Analysis Model
Claro Data Center

Figure 5. Comparison seismic response Claro Data
Center with and without base isolationRESPONSE OF AN INSTRUMENTED BUILDING DURING THE FEBRUARY 27, 2010 CHILE
EARTHQUAKEThe instrumented building, shown in Figure 1, is a 4-story confined masonry structure mounted on top of
8 high damping rubber isolators. The first level considers reinforced concrete shear walls. The building
has a typical floor plan of 10x6 m. The weight of the structure is 1630 kN. The typical slab thickness is 10

cm. The structure’s roof is a wooden truss. The isolators are 31.5 cm in diameter and 32 cm in height. The
thickness of the rubber layers is 6.7 mm. Four of these isolators are located in the corners of the building
and two at each long side of the plan. The isolators have a vertical load capacity of 350 kN at 20 cm
lateral displacement (100% strain). An extensive test series was carried out at the University ofCalifornia, Berkeley [1]. The isolated building’s design period is 2 seconds.
Instrumentation
The instrumentation consists of an array of triaxial accelerometers with common triggering. Given thateach sensor has a 12 bit analog to digital converter, the recording system is limited to earthquake events
only. Figure 6 shows the position of accelerometers C, L and F. Signals registered from C and F sensors
are presented in this paper. Sampling frequency is 200 samples per second. Measures directions ofaccelerometer channels are shown in Table 1.

Table 1. Sensors direction
SensorChannelDirection
Level
1
EW
F
Ground
2
NS
3
UD
4
EW
th
C
4
5
NS
6
UD

Figure 6. Andalucia Community Building. Global
dimensions and isolators layout.

Method of Analysis
To identify the dynamic properties of the building an equivalent parametric viscoelastic model is used andadjusted using the MOESP (Multivariable Output-Error State Space) system identification method,
devolped by Yoshimoto [7]. In this method the state space equilibrium and observation equations are
given by:ሼ‫ݖ‬௞ାଵ ሽ = ሾ‫ܣ‬ሿሼ‫ݖ‬௞ ሽ + ሾ‫ܤ‬ሿሼ‫ݑ‬௞ ሽሼ‫ݕ‬௞ ሽ = ሾ‫ܥ‬ሿሼ‫ݖ‬௞ ሽ + ሾ‫ܦ‬ሿሼ‫ݑ‬௞ ሽ

(1)
(2)

ሼ‫ݕ‬௞ ሽሼ‫ݑ‬௞ ሽሼ‫ݕ‬௞ାଵ ሽሼ‫ݑ‬௞ାଵ ሽ൞ൢ = ሾࣩ௦ ሿሼ‫ݖ‬௞ ሽ + ሾ࣮௦ ሿ ൞ൢ⋮⋮ሼ‫ݕ‬௞ା௦ିଵ ሽሼ‫ݑ‬௞ା௦ିଵ ሽ(3)

Considering that every step {zk} can be expressed as a function of previous values, the relation with {yk}can be written as:where ሾࣩ௦ ሿ௟௦ ௫ ௡ is the extended observability matrix with order “s” and ሾ࣮௦ ሿ௟௦ ௫ ௠௦ is a Toeplitz matrix.
Given that [Os] depends on [A] it is possible to obtain from this matrix the natural properties of thestructure.
Seismic Response of the Structure and Identification Process
Several records have been obtained in this building. The largest seismic event recorded is the February
27th, 2010 Chile earthquake. Despite the Mw 8.8 magnitude, the earthquake did not cause any damage tothe isolated structure or its contents. A strong aftershock was registered in March of the same year. Bothcases are studied in following subsections.
The Mw 8.8 February 27th, 2010 Chilean Earthquake
Figures 7 and 8 show the recorded data. The strong motion phase lasts about 60 seconds. The peakaccelerations at ground level were 0.30g and 0.18g in the horizontal and vertical directions, respectively.
The maximum horizontal acceleration recorded within the isolated building was 0.22g. In the verticaldirection amplification was observed since the maximum acceleration recorded at the top building level
was 0.29g. The high frequency content of the horizontal signals is considerably reduced when theacceleration reaches its peak value. This does not happen for the vertical signal.

Figure 7. Base acceleration records Andalucia
buildingth

Figure 8. 4 level acceleration records
Andalucia building

The identification process was performed considering the time frame from 30 to 118 sec. Three time
windows for the analysis were selected, each one 8 sec long. This time intervals are from 30 to 38 sec, 70
to 78 sec, and 110 to 118 sec, respectively. This procedure allows the identification of variations ofstructure’s modal properties between time windows selected, considering different accelerations
amplitudes situations.
Table 2 shows the variations of the modal frequencies and damping ratios identified, for each time
window of analysis. In Table 2 it can be observed that the first modal frequencies strongly depend on theamplitude of vibration. For example, the natural period at the beginning of the strong motion phase is0.54 sec and gets longer during the second time window, between 70 sec and 78 sec, achieving a value of
1.61 sec. It is important to note that after the earthquake the structure recovered the dynamic properties it
had before the earthquake. Modal damping ratios show that, at the same time that the natural frequencieschanges in order to increase the structure fundamental period, damping ratios change too. In the case of
the isolated structure, the high damping ratio is associated with the first modes.

Table 2. Identified Modal Properties: February 27th, 2010 Chile Earthquake Record in Andalucia
Building
Modal Frequencies (Hz)Modal Damping Ratios (%)
30 sec – 38
70 sec – 78
110 sec – 118
30 sec – 38
70 sec – 78
110 sec – 118
1.86
0.62
1.89
2.7
23.7
22.3
2.23
0.78
2.00
12.7
41.8
--2.99
2.34
3.06
17.9
50.5
19.9
14.11
16.42
18.63
3.9
3.0
2.3

March 11th, Aftershock 2010 Chile Earthquake
The March 11th was one of the strongest aftershock after main earthquake. It generated peak groundacceleration in horizontal plane of 0.03g for both directions. In vertical direction, maximum groundacceleration registered was of 0.02g. The total duration of the earthquake was around 60 seconds. The
identification process was performed considering the time frame from 20 to 70 sec. Three time windows
were considered beginning from 20 to 30 sec, continuing from 40 to 50 sec, and finishing from 60 to 70
sec. The strong motion phase extends over 20 seconds from 30 to 50 sec. The aftershock records areshown in Figures 9 and 10.

Figure 9. Base acceleration records Andalucia
buildingth

Figure 10. 4 level acceleration records
Andalucia building

From the analysis and system identification process, it can be observed that the first two frequencies are
related to the isolation system, as is presented in Table 3. This is reflected in the decrease of modal
frequencies below 2 Hz in the second time window in comparison to the corresponding two frequenciesof the first time window. The highest modal damping ratios are observed in the beginning and ending of
the record, the first and third time windows, achieving over 20% of damping. In relation with the mainearthquake analysis, the high nonlinearity imposed by the high seismic demand causes that first two mode
to vary as a function of amplitude.
It is important to note that the decrease of the modal damping ratio during the strong motion and anincrement of the natural periods. This shows that the isolation system effectively acts over the structural
system increasing the flexibility while demand is the highest. In this case the linear equivalent model is
not able to separate the two first modes. So values present the contribution of the first two translationalmodes. An appropriate nonlineal elastic model is now been work as part of the identification algorithm tobe able to capture the high nonlinearity observed in isolated systems.

thTable 3. Identified Modal Properties: March 11 , 2010 Chile Earthquake Aftershock Record in
Andalucia Building
Modal Frequencies (Hz)Modal Damping Ratios (%)
20 sec – 30
40 sec – 50
60 sec – 70
20 sec – 30
40 sec – 50
60 sec – 70
2.34*
1.51
2.07
21.6
16.6
22.6
2.62
1.65
2.12
24.1
14.3
30.9
18.26
14.05
10.54
4.1
17.0
2.1

CONCLUSIONSThis practical application has demonstrated the feasibility of using seismic isolation systems in a 19-floor
building, contrary to the belief of professionals and of investors in the impossibility of incorporating
seismic isolation in mid-rise buildings. Moreover, the assessment allows us to conclude that incorporating
seismic protection systems in real estate can increase structural safety. The isolation system reduces
maximum accelerations and deformations of the structure to 75 and 85%, respectively, with a consequent
reduction in the probability of damage and economic losses during exceptionally severe earthquakes. The
costs of investing in protection may be partly offset by the potential cost reductions in the structure, byreducing design requirements for minimum and maximum shear and by eliminating the containment
requirements for wall edges.
ACKNOWLEDGMENTS
The authors wish to express their gratitude to engineer Jose Luis Larenas and to the civil constructor
Nicolas Larenas from Constructora ALCORP S.A.; to the architects Carlos Belmar, Jorge Game, andMacarena Garcia from BGL Arquitectos; to the engineers Luis Pinto and Cesar Abuauad from Claro
Chile; to the architect Maite Bartolome from +Arquitectos; to the engineers Rafael Gatica and Carlos
Dupont from Gatica-Jimenez Engineers; to the engineers Amarnath Kasalanati, Konrad Eriksen, andTung Ng from Dynamic Isolation Systems Inc.; to the engineer Pedro Soto from the Department of Civil
Engineering at the University of Chile; and to the University of Chile.
REFERENCES
1. Moroni, M., Sarrazin, M., and R. Boroschek. Experiments on a base-isolated building in Santiago,
Chile, Engineering Structures, 20(8), 1998, p720-725.
2. Miranda, E., Mosqueda, G., Retamales, R. and G. Peckan. Performance of Nonstructural Components
during the February 27, 2010 Chile Earthquake, in Earthquake Spectra. (accepted, in press), 2012.
3. INN (2003). NCh2745.Of2003: Análisis y Diseño de Edificios con Aislación Sísmica, Instituto
Nacional de Normalización, 2003.
4. Saito, T., Iiba, M., Morita, K., Azuhata, T. and N. Inoue. Seismic Performance of Seismically Isolated
Buildings at the Great East Japan Earthquake, in proceedings of the 12th World Conference on Seismic
Isolation, Energy Dissipation and Active Vibration Control of Structures, September 20-23, 2011, Sochi,
Russia, 2012.
5. Kasalanati, A., and M. Constantinou. Testing and Modeling of Prestressed Isolators, in Journal of
Structural Engineering, Vol. 131, N°6, 2005, pp857-866.
6. TIA (2005). ANSI/TIA-942-2005: Telecommunications Infrastructure Standard for Data Centers,
Telecommunications Industry Association, 2005.
7. Yoshimoto, R., Mita, A., Keiichi Okada,K.,Damage detection of base-isolated buildings using multiinput multi-output subspace identification. Earthquake Engng Struct. Dyn. 2005; 34:307–324