Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanThe 2010 GREAT CHILE EARTHQUAKE - CHANGESTO DESIGN CODES
Patricio BONELLI 1, José I. RESTREPO 2, Rubén BOROSCHEK 3, Jorge F. CARVALLO 41

Professor of Structural Engineering, Universidad Técnica Federico Santa María, Valparaíso, Chile.
patricio.bonelli@usm.cl
2
Professor of Structural Engineering, University of California at San Diego, United States.
jrestrepo@ucsd.edu
3
Associate Professor of Structural Engineering, University of Chile, Santiago, Chile.
rborosch@ing.uchile.cl
4
Associate Professor of Structural Engineering, Catholic University of Valparaiso, Chile.
jcarvall@uc.cl

ABSTRACT: The 27 February 2010 Mw 8.8 Maule in Chile produced a disproportionate
concentration of structural damage in newly built buildings, including the collapse of threebuildings, and widespread damage to nonstructural elements prompted the government to revise
current design practice. Results suggest that structures with post yielding positive hardening must
be used in soft deep soils since an instable response could arise. Non linear displacement spectra
must be proposed to determine the design displacement, instead of linear ones.
Key Words: 27 February 2010 Mw 8.8 Maule in Chile earthquake, long strong
shaking, several cycles of high accelerations.

INTRODUCTION
The 27 February 2010 Mw 8.8 Maule in Chile generated MM VII intensity or higher and PGA= 0.3g or higher on a 100 km wide by 600 km long corridor, as well as a tsunami. About eight
million people live in this corridor. The central government, and a large percentage of the domestic
and export industries are located within. The direct cost of damage may exceed US $25 billion, or
about 16% of GDP.
The earthquake occurred over the known Concepción-Constitución seismic gap in central Chile,where several prior studies had concluded the most likely near future occurrence of an earthquakeof the characteristics observed (Boroschek et al. 2006, Ruegg et al. 2009). The rupture models of
the main shock processed by the USGS (2010), Delouis et al. (2010) present two or three asperities
with high slips and energy release.

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Several strong motion records were obtained by the University of Chile accelerograph network
during this earthquake. Peak ground accelerations, some of them in the order of 0.9 g are presented
in Boroschek et al 2010. Figures 1 and 2 show the ground motions recorded at Viña del Mar MargaMarga and downtown Concepción stations, at epicentral distances of 340 and 100 km, respectively.
These records are long, which typifies this kind of large magnitude subduction earthquakes. For
example, the Viña del Mar Marga-Marga record present strong motion duration based on 90% of
Arias Intensity of 30 sec and an envelope with three periods of large amplitude vibrations.
For the case of downtown Concepción, Figure 2(a), the record shows a duration of 85 seconds
and include more than 10 cycles of high accelerations with amplitudes greater than 0.10 g. This
characteristic is strongly related with geotechnical conditions of the site (Class D, average Vs of
220 m/sec). Narrow band-pass filtering of this record at T = 1.9 sec shows 14 sinusoid cycles ofincreased acceleration resulting from the response of the deep alluvial soil column where the station
is located, see Fig 2(b). A more detailed description of the sites can be found in Arango et al, 2011.

Fig. 1 Corrected acceleration time histories. Station: Viña del Mar-Puente Marga Marga. Filter band
[0.05 90] Hz.

(a)

(b)

Fig. 2 Corrected acceleration time histories. Station: Concepción-Colegio Inmaculada Concepción. (a)Acceleration Signal Filter: [0.2 80] Hz. (b) Filtered Acceleration Signal - S63oW component.

DESIGN CODES AND DESIGN SPECTRAIn Chile there are several loading and design codes, differentiated by their functionality or structural
system. The loading codes are: NCh433 for residential and office buildings; NCh2369 for industrial
installations; NCh2745 for base isolated buildings. In general, the seismic demand is represented byseismic zoning, a reference design zero period acceleration (ZPA) and a design response spectrum
for elastic response. ZPA and response spectra are not strictly derived from a probabilistic or

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deterministic hazard study. The shape of the spectra has been calculated from the shape of theaverage acceleration response spectra normalized by PGA for the database of historical groundmotions of Chile. This shape has been incorporated into the NCh433 and NCh2369 loading codes.
With such spectra, tall buildings that have become common in Chile since 1985, are subjected tounrealistically small displacement demands as evidenced in the displacement response spectra
depicted in Fig. 3.
In contrast, Newmark type acceleration response spectra have been incorporated into the ETG 1015
and NCh2745 loading codes. In NCh2745 the combined effective acceleration and spectral shape isscaled, cut or amplified based on the results of analysis on several typical reference structures. Then
design spectrum in any Chilean code does not correspond to elastic seismic demands derived fromlocal recording by large factors. As can be seen in Figure 3, the criteria for the shape and scale ofeach code design spectra is quite different. The NCh433 was calibrated so it has the average shapeof the response spectra real records, but values are scale so residential buildings with period
between 0.5 and 1.5 seconds have a base shear coefficient consistent with pre 1985 design
standards. In NCh2369 the spectral shape contained in NCh433 is used as reference, but in theindustrial code the spectral values are limited by a maximum coefficient generating an extremely
low equivalent plateau. This was done so rigid structure, equipment or system, could be design withthe traditional values used prior to the development of the code.
Strong emphasis to displacement demands is given in the spectrum incorporated into NCh2745 forbuildings with periods greater than 1.5 sec. This is because it was deemed that the design of baseisolated buildings should result in large displacement demands. Moreover, the ZPA value in thiscode does reflect the results of a probabilistic study; and the plateau in the design spectra representsa typical amplification factor for the acceleration control band of the spectra using Chilean records.

(a)(b)
Fig. 3 Response spectra at 5% damping for records obtained in zone 3 and soil type II according toChilean seismic codes. Elastic demands of NCh433, NCh2369 and NCh2745 are shown. (a)Acceleration Spectra. (b) Displacement Spectra.

Lateral forces obtained from the base response spectrum in each of the loading codes are modified
by the importance of the structure, by its inherent ductility and damping. Minimum and maximum
base shear limits may control these forces. Limits are also placed to controlled inter story drifts.
All loading codes in Chile only require a linear elastic analysis. A traditional modal response
spectrum method is the preferred choice. Nonlinear pushover analyses are not mandatory but suchanalyses have been performed to support the design of some tall residential buildings as part of thepeer review verification process. Nonlinear time history analyses are rarely used, and have been
conducted to support the design of some base isolated buildings and of buildings that incorporate
supplementary damping devices.

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DESIGN LATERAL DISPLACEMENT
A difficulty exists in the calculation of the displacement demands for buildings on soft soil
conditions in Chile. The 2010 issue of the Chilean code NCh433-mod2010, calculates the lateraldisplacement at roof as:δu = 1.3 Sde (Tag)where Sde is the elastic displacement spectrum ordinate for 5% damping ratio, calculated with thefundamental natural in the analysis direction considering cracked sections.
The problem with this formulation is that, in soft soil, the amplification in the displacement demandof an inelastic building with a period shorter than the predominant period of the soil can beunderestimated. For example, the ground motion as recorded in “Concepción Centro”, on a deep
alluvial soil with a 30 m shear wave velocity of 220 m/s, induces large displacements in non-linear
structures whose period is somewhat shorter than the predominant period of the soil. When soilstructure interaction is accounted for, something that is seldom considered in design, the period of
the soil-structure moves closer to that of the soil and the structure experiences larger displacement
demands. Such demands are completely overlooked in design.
Figure 4 compares the spectral displacements obtained from the ground motion recorded in
downtown Concepcion, on Soil Type III, for elastic response and for inelastic response for aductility of 2 and 3 with those derived from the spectral ordinates of three of the Chilean codes. It
is evident that the spectral displacements calculated with NCh2369 are significantly below thosecomputed from the ground motion for elastic and inelastic response. The spectral ordinates ofNCh2745 are somewhat similar to those computed for inelastic response, but below that computedfor elastic response. The spectral displacements computed from NCh433 are greater than thosecomputed for the ground motion only for periods below 1.2 sec.

CONC S63W (Azimuth = 243°)
Displacement Response Spectra (m = 3.0, z = 2%)

CONC S63W (Azimuth = 243°)
Displacement Response Spectra (m = 2.0, z = 2%)
1.8

1.8
Elastic

1.6

Elastic

1.6

r = 2.5%

r = 2.5%
r = 0%

r = 0%

1.4

r = -5.0%
NCh433

1.2

Displacement (m)Displacement (m)1.4

NCh2369
NCh27451.0
0.8

r = -5.0%

Nch433

1.2

NCh2369
NCh2745

1.0
0.8
0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0
0

0.5

1

1.5

2

2.5

0

3

0.5

1

1.5

2

2.5

3

Period (sec)Period (sec)Figure 4. Displacement response spectra, Concepcion Centro, Maule Earthquake andChilean codes, Zone III, Soil Class II and III.

A reason for the mismatch of the code and ground motion displacement spectral demands
is the lack of recognition that in these soil conditions the motion has strong sinusoid
components. To demonstrate this, the displacement coefficients, Cμ, have been computedfor the ground motion and for a record with 14 sinusoids of increased amplitude. Cμ1781

coefficient has been defined as the ratio between the maximum displacement of a nonlinear single degree of freedom system and the maximum displacement of a linear one
with same initial stiffness (Miranda, 1999). Comparisons with those obtained from thesemi-synthetic record are illustrated in Figure 5.

Figure 5. Comparison between Cμ values for harmonic representation and the ground motion.

COUPLED WALL RESPONSE
The two reinforced concrete bearing walls coupled by the slabs shown in Figure 6, hasbeen chosen to evaluate the response to a ground motion on a soft soil such as that
registered in Concepcion. These walls are very similar to those employed in 16 story
building that collapsed during the 2010 Chile Earthquake in Concepcion Centro.
Lateral displacement at roof computed for r = 0 and r = - 0.025, are shown in Figure 6.
A semi synthetic record representing the ground motion registered in Concepcion, during
the Chile 2010 earthquake, in a soil with similar characteristics to the soil where thebuilding was founded has been considered. For r = 0, the maximum lateral roof
displacement was close to 40 centimeters, with a stable response. Analysis was done using
Ruaumoko (Carr). When a decay of resistance is considered, for r = -0.025, a non linear
resonance phenomena occurred, being the response instable. Lateral displacement
increases until collapse. Post yield hardening is represented by factor r computed from themoment curvature relationship. Slab coupling effect was taken into account through a bar
element assuming an effective web.

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Figure 6. Coupled walls

Desplazamiento [m] .

Desplazamiento de Techo Concepción NS
0.40
0.30
0.20
0.10
0.00
-0.10 0

10

20

30

40

50

60

70

80

90

-0.20
-0.30
-0.40
t [s]

Desplazamiento [m] .

deroof
Techo
Concepción
NS, r=-2.5%a) LateralDesplazamiento
displacement atcomputedfor r=0
Concepcion
NS.
0.50
0.00
0

10

20

30

40

50

60

70

80

90

-0.50
-1.00
-1.50
-2.00
t [s]

b) Lateral displacement at roof computed for r= -0.025, Concepcion NS.
Figure 7. Lateral displacement at roof computed for r = 0 and r = -0.025,
Chile 2010, Concepcion NS record, Ruaumoko.

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PROPOSED MODIFICATIONS TO ACI318-08 TO DESIGN R/C SPECIAL WALLS IN
CHILEThe overall performance of buildings in Chile to the Maule earthquake was rather
acceptable, in terms of the amount of damage observed. However, it is evident that newer
high-rise buildings suffered a disproportionate percentage of the damage observed.
Concentration of damage in high-rise buildings was observed in softer soils conditions. The
ground motion obtained from Concepción has shown sensitivity to strength degradation in
the period band corresponding to the fundamental period of damaged high-rise buildings
built in the area.
Performance based design requires a ductile behavior to permit the structures to reach adesire collapse mechanism under lateral displacements, greater than demands of the
maximum considered earthquake. If not, it is not possible to design for different damagelevels for different intensity earthquakes. Unless changes in reinforced concrete wall
design avoid the brittle nature of unexpected observed damages in the 2010 Chile
Earthquake, performance based design in this type of structures could not be applied.
Plants of damage buildings are similar to that one shown at Figure 8(a). Typical damage
in R/C walls are shown in Figures 8(b). Several R/C buildings had damage in wallboundaries as shown in Figure 8(c).

(a)

(b)

(c)

Fig. 8. (a) Typical plant of R/C wall building in Chile. (b) Typical failures in R/Cwalls, localization caused by buckling of vertical reinforcement. (c) Buckling of Bars
in Reinforced Boundary Element.
Urgent modifications were proposed to design especial R/C walls in Chile, after the
February 27th, 2010, Earthquake. A law has been promulgated on February 14th, 2011
adopting ACI 318S-08 to R/C building design, but complemented by following
requirements presented in this paper. Complementary requirements shall be applied only
to R/C special structural walls, defined in section 21.1 in ACI 318-08.
Most of observed failures occurred in thin walls, used in Chile since the eighties. Some oldbuildings that had had damages in the 1985 Chile Earthquake in Viña del Mar, had
several damages again. To avoid brittle failures under bending and axial forces, limitations
to axial force have been proposed. A conventional strain in concrete equal to 0.003 hasbeen adopted by ACI318-08 to define a nominal strength under axial load and bending.
Under large deformations, shortening strain in concrete is usually greater than thisconventional value at critical zones. Tensile strain in steel when shortening strain in
concrete is equal to 0.004 in the opposite fiber is used as an index to identify the type of

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failure. Failures controlled by compression, as defined currently in ACI-318 for members
in flexure see Fig. 9, were not permitted in walls. Sections controlled by tension arerecommended in walls, with transverse reinforcement to avoid premature buckling of
longitudinal bars in extreme fibers or in boundary elements and adequate confined begiven to concrete. Analogy to dispositions for beams in §10.3.5 in ACI318-08, tensile strain
in steel εt at nominal strength in special walls must be greater or equal to 0.004. Largelateral displacement must not be reached with large shortening strains at confinedconcrete, longitudinal steal in opposite fiber must yield before the ultimate strength
capacity is reached in concrete under compression. For effective confinement considerable
thickness are needed in walls, tests done in columns simulating boundary elements
similar to actual boundary elements in walls failed under compression during the
February 27th, 2010 Chilean Earthquake, accomplishing ACI requirements, have shownhow brittle failures can be.
Then, maximum factored axial load permitted in walls must be P4, associated to a tensile
strain in steel equal to 0.004 when shortening strain in concrete in opposite fiber is 0.003.
Then, to avoid compression failures in structural walls a special disposition was added to
ACI318-08: “The net tensile strain in the extreme tension steel, εt, must be equal or
greater than 0.004 when the concrete in compression reaches its assumed strain limit of
0.003.”

Fig. 9 Combined Flexure Axial Design Approach ACI318-08 Code.

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Figure 10 shows one of the walls of the building shown in Figure 8(b), and Figure 11 shows
the moment curvature relationships for one of the walls. It is evident that the wall isbrittle when the flange is in tension.

Fig. 10. Walls as in official drawings and state after the 2010 Chile Earthquake.

Fig. 11. Moment curvature diagrams for one of the walls
Curvature at yield can be computed according to different criteria resulting in different
results. If curvature at yield is estimated as, thenAs

then

The design lateral displacement according to the current Chilean code is 290 mms.
When the building was designed the displacement capacity did not have to be evaluated,according the codes.
A new law approved in Chile in December 13, 2011, permits to consider the elastic
deformations separately from the plastic deformation.

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But in this transverse section,

For the considered wall, strain in steel is εs=0.0017 when shortening in concrete isεc=0.003, then the wall is controlled by compression.
According to the new Chilean law, if transverse reinforcement is necessary to confine
special boundary elements, then a minimum thickness of 300 mm is required. When
thickness is increased εs=0.0038 when shortening in concrete is εc=0.003, then the wall isin the transition zone. A maximum shortening in confined concrete in boundary elements
is limited to εc=0.008, for ultimate limit state. Then, at ultimate, the wall could reach alateral displacement as large as 340 mm, 1.3% of height, with a steel strain equal toεs=0.015. Anyway, T section walls can be very brittle.

CONCLUSIONS
Results suggest that structures with post yielding positive hardening must be used in softdeep soils since an instable response could arise. Non linear displacement spectra must be
proposed to determine the design displacement, instead of linear ones.

ACKNOWLEDGMENTS
Sam Lee, graduate student at UCSD computed spectral displacement contour surfaces, figures
showing coupled wall responses were taken from Gabriel Alarcon`s thesis, student at Universidad
Santa Maria, Valparaiso. Opinion, findings, recommendations and conclusions in this papers arethose of writers, and do not necessarily represent those of any institution or others mentioned.
REFERENCES
Arango, MC, Strasser, FO, Bommer, J, Boroschek, R., Comte, D., Tavera H. (2011), “A strong
motion database from the Peru-Chile Subduction Zone”. Journal of Seismology (2011) 15: 1941.
Boroschek, R., Domb, F. (2006), “Estudio Nacional de Chile”, Project IDB-CEPAL Programa de
información para la gestión de desastres, CEPAL.
Boroschek, R., Soto, P., León, R. (2010), “Registros del Terremoto del Maule, Mw=8.8, 27 defebrero de 2010”, RENADIC 10/05, Octubre 2010. http://www.renadic.cl
https://terremotos.ing.uchile.cl
Kawashima, K., Sasaki, T., & Kajiwara, K., “Experimental Study on the Seismic Response of Bridge
Columns using E-Defense (2010)”, http://www.pwri.go.jp/eng/ujnr/tc/g/pdf/25/1-4.pdf,
Acceso: 18 de agosto 2010.
Miranda E. (1999), “Approximate lateral deformation demands in multi-story buildings subjected toearthquakes.” Journal of Structural Engineering, Vol. 125, pp. 417–425.
Panagiotou, M., Restrepo, J.I. & Conte, J.P. (2010), “Shake Table Test of a Full-Scale 7-Story
Building Slice Phase I: Rectangular Wall”, Journal of Structural Engineering, American Societyof Civil Engineers (in print).
Panagiotou, M., & Restrepo, J.I. (2010), “A Displacement-Based Method of Analysis for Regular
Reinforced Concrete Wall Buildings: Application to a Full-Scale 7-Story Building Slice Tested at
UC San Diego”, Journal of Structural Engineering, American Society of Civil Engineers, (inprint).

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