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Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
Porto, Portugal, 30 June - 2 July 2014
A. Cunha, E. Caetano, P. Ribeiro, G. Müller (eds.)
ISSN: 2311-9020; ISBN: 978-972-752-165-4
Fundamental period and earthquake damage in RC buildings of Viña del Mar (Chile)
C. Aranda1, F. Vidal1, G. Alguacil1, M. Navarro1,2, & I. Valverde-Palacios3
Instituto Andaluz de Geofísica. University of Granada. C/ Profesor Clavera Nº12, C.P. 18071 Granada, España
2
Dep. of Applied Physics. University of Almería. C. Sacramento s/n, C.P. 04120, La Cañada, Almería, España
3
Dep.of Architectural Buildings, University of Granada. Campus Fuentenueva, C.P. 18071 Granada, España
email: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]
1
ABSTRACT: The Great 2010 Chile Earthquake severely shook the city of Viña del Mar. The surface soil characteristics and
local strong motion records were analyzed in order to find out the influence of local conditions on the level and spatial
distribution of damage. 2054 building were inspected and structurally classified to determine their vulnerability and damage
grade using criteria based in the EMS and Risk-UE methodologies. The fundamental period of buildings (T) was empirically
obtained from their storey number. Building damage was analyzed as a function of the materials, the height H, the building age,
the rigidity (H/T), the density of walls. The local site effects were determined, as well as the predominant time periods of the
seismic shaking of the sites, computing the 1-D transfer functions of the shallow structure. The response spectra of the ground
show the highest energy in the range of 0.4 and 1.1 s. The majority of the buildings had a good response; only 252 buildings
(12.3%) of the 2054 inspected buildings had visible damage, and only 1.6% suffered grade 3 or 4 (EMS scale). Most of the
seriously damaged buildings (grade ≥ 3) were RC structures built before 1985 earthquake, with a height between 9 and 24
storeys and fundamental period near predominant period of the soil. The worst building damage was observed in zones of soft
soils (fluvial and marine deposits, with a shallow phreatic level) near the coast and the river Marga Marga, being the spatial
distribution of damage similar to that of the 1985 earthquake. The high values of density of walls (0.015 to 0.035) and building
rigidity H/T (between 40 and 70) explain the absence of collapses and the limited structural damage.
KEY WORDS: Maule 2010 earthquake, building typologies and periods, vulnerability factors, earthquake damage.
1
INTRODUCTION
A gigantic earthquake with mw 8.8 occurred in central Chile
on February 27, 2010. Central and south regions of Chile have
a long history of large (Mw ≥ 8.0) and destructive earthquakes
(1570, 1575, 1647, 1657, 1730, 1751, 1822, 1835, 1837, 1914,
1906, 1928, 1939, 1943, 1960 and 1985). The 1960 Valdivia
earthquake (located south of 2010 event) was the largest one
(Mw 9.5) with ground effects and damage much greater.
The 2010 event severely shook a wide area of high seismic
hazard level [1] and tested with high intensities an immense
building stock designed following Chilean building code
provisions [2]. About 370,000 houses were damaged and 2
millions of people were affected.
The peak ground acceleration (PGA) of the main shock
recorded at Cauquenes city, the nearest station to the
epicentre, exceeded 1 g, and reached 0.65g at Concepcion
city, where extensive damages were observed. The quake
reached a PGA of 0.35g and 0.43 g in two strong motion
stations at Viña del Mar city.
Viña del Mar is a city located on central Chile's Pacific
coast, on the northern edge of the rupture zone of the 2010
earthquake, and mostly founded on marine and alluvial
deposits. The intensity degree in Viña del Mar (Figure 1)
varies from VI to VII-VIII (EMS) [3], being VII-VIII in a
reduced zone of the plain (of thick sedimentary deposits) and
lower than VII in surrounding zones and hills (where the
bedrock is near the ground surface). The acceleration was
higher than 0.15 g for more than 20 s there and serious
damage on several buildings were observed, especially tall
reinforced concrete (RC) buildings, mainly those sited along
the coast and river Marga Marga shores, already an indicator
of the influence of site conditions and possible resonance
effects. Viña del Mar was repeatedly damaged by large
historical earthquakes and also by recent ones, most notably
those of the 1960 and 1985. This city is placed at one of the
most dangerous seismic zones in Chile, being expected there a
PGA > 0.7 g in 475 years [1].
Given the importance of the amplification and resonance
phenomena in this city [4], this paper analyses: 1) Geological
and geotechnical data from soil mechanics and ground
predominant periods in the city. 2) The strong motion records
to estimate a set of key earthquake engineering parameters in
order to see the earthquake ground motion and its influence on
building damage, 3) The characteristics of buildings, and key
structural parameters (natural period T, stiffness (h/T), wall
density, etc.) being influence on their dynamic behaviour. 4)
Damage related to ground motion and building characteristics.
Figure 1. Isoseismal Map (MSK scale) of Viña del Mar (after [3]).
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Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
2
GEOLOGICAL AND GEOTECHNICAL DATA.
Viña del Mar is a coastal city located at the mouth of the river
Marga Marga, in the region of Valparaíso. Most of the city is
built on a plain (named Plan of Viña) formed by deposits of
marine and alluvial sedimentary materials consisting mainly
of sand and gravel, sand mixed with silt (in some places), and
anthropic filling. These sedimentary deposits reach up to 100
m thick. The water table is very shallow, and it is generally
located about less than 6 m deep. The other districts of the city
are placed on surrounding hills (which mainly are rock or hard
soil out-crops) (Figure 2).
The soil of the plain of Viña del Mar consists of
Quaternary deposits of different composition and location,
which will cause a different earthquake ground response
according to differences in their subsurface structure. This
inhomogeneous ground motion were reported in large historic
earthquakes [5 and 6] and have also been detected from
damage distribution and extent of recent earthquakes (1985
and 2010) in Viña del Mar [4],
The analysis of 22 geotechnical reports (provided by the
Municipality of Viña del Mar), carried out to study soil
conditions to build in the flat area of the city, has allowed us
to obtain the surface ground structure to a depth 20-30 m.
One of the most important geomorphological features of
Viña del Mar is the presence of marine abrasion and
sedimentation terraces. The relief of the coastal massif is
partly interrupted by sandy beaches and deep ravines cutting
the terraces. On both sides of the estuary terraces are placed at
different heights. NE of the Marga Marga river there is a large
terrace, product of marine abrasion of gneissic rocks, which
are located between 200 and 240 m above sea level. This
terrace broadly consists of heterogeneous estuary deposits
formed by conglomerate and clays. The terrace sited south of
the estuary is shorter and is above 250 m altitude [7].
The geological map of the Valparaiso area [8] shows the
main geological units of Viña del Mar and the faults in the
environment of the city (Figure 2). The Marga Marga fault is
aligned with the river (NW-SE direction) and establishes
significant differences between the areas to the S and N of the
river. A simple soil classification focused on seismic response
establishes three types: intrusive rocks, consolidated and
unconsolidated sediments [3] (Figure 3).
Figure 2. Geological map of Viña del Mar (after [6]).
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Figure 3. Soil classification map of Viña del Mar (after [3]).
A set of relevant geotechnical and geophysical parameters
have been estimated by the analysis of geological sections,
borehole and geotechnical data from the geotechnical reports
mentioned before. Firstly, we have detected: 1) the presence
of anthropogenic fills of variable thickness (between 0.5 and
4.5 m); 2) the existence of discontinuous of muddy clay layers
with thicknesses between 1 and 3 m, and very low bearing
capacity; 3) in some places a vegetable soil and/or edaphic
variable thickness is recognized between 0.5 and 1 m, located
in most cases above the infill layer, but sometimes under it. 4)
The water table depth generally varies between 2.7 and 6 m
(Figure 4), being the most representative depth about 4 m.
This very low depth is a major risk factor in case of
earthquake.
Figure 4. Map of water table depth estimated from geotechnical
surveys. Black dots indicate the location of survey sites.
Mechanical and physical properties of unconsolidated
materials and hard-rocks are valuable data for a preliminary
dynamic seismic response of geological formations. Using the
experimental relationship (1) of Hasancebi Ulusay [9]
between mechanical resistance – deduced from the N-value
from Standard Penetration Test (N SPT) and S-wave velocity
(VS) for sandy soils:
Vs = 90.82 * NSPT 0.319
(1)
a Vs of the upper 10 m (VS10) map have been obtained (Figure
5). The VS- NSPT relationship was tested with the velocities
measured in situ in site number 9 formed by these type of
soils. The estimated VS10 values are low, between 210 and 280
m/s.
S wave velocities in the upper 30 meters (V S30) have been
estimated in 3 borehole sites (Figure 6). The results are: Site
6, VS30 = 331.9 m/s, site 10, VS30 = 312.3 m/s and site 11,
VS30 = 265.1 m/s. The lowest VS30 value was obtained in the
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
site 11 located near the river Marga Marga, in the SW part of
the city. According to Eurocode EC-8 [10], these analysed
sites can be classified as stiff soils (class C).
Aranda et al [13] also show shake intensity at MMVM was
higher than at CEVM.
The acceleration and velocity response spectra (SA, SV)
of the mainshock (Figure 9) show peak amplitude at different
periods at these stations, pointing to different site effect.
Similarly, the elastic input energy spectra (IES) show even
more clearly this different soil behaviour (Figure 10).
Figure 5. Vs velocities of the upper 10 m (VS10) from NSPT data.
Figure 7. Ground predominant period of Viña del Mar, after [3].
Figura 6. Vs Structure in 3 sites of Viña del Mar
The site predominant frequency is a key factor in the
prediction of earthquake damage especially when it is close to
the fundamental frequency of buildings founded on it, since
seismic motion shall produce a resonance with building that
can greatly increase the stresses in the structure [11].
One of the most used technique to easily find the ground
predominant period (Tp) is to estimate the spectral ratio
between the horizontal and vertical components of ground
noise (also called HVSR technique) [12].
Carrasco and Nuñez [3] applied this technique to perform
a seismic microzoning of Viña del Mar area (Figure 7). The
predominant periods of the plain city area are between 0.4 and
1.1 s, the highest values on zones near the river and coast
shores. The surrounding zones present Tp lower than 0.4 s.
3
CHARACTERISTICS OF
MOTION IN VIÑA DEL MAR
STRONG
GROUND
We analysed the characteristics of the 2010 mainshock ground
motion recorded at two stations in Viña del Mar (Viña Centro
(CEVM) and Viña El Salto (MMVM) (figure 8) and other two
nearby the city: El Almendral y Valparaiso UFSM). the
recorded PGA was 0.33 and 0.43 g at CEVM and MMVM,
respectively.
A set of energy related engineering parameters (Arias
intensity, CAV, Spectral intensity, Arms) calculated by
Figure 8. 2010 mainshock three component records at Viña Centro
(top) and Viña El Salto (bottom) stations.
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Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
was CEVM station) (Figure 13). They found also significant
amplification of ground motion in sites of the plain in similar
period range we obtained. The differences among sites found
by these authors in period ranges where there was maximum
amplification show dependence on site conditions (Figure 13).
HORIZONTAL VELOCITY RESPONSE SPECTRA
250
RELATIVE ACCELERATION RESPONSE SPECTRA
1800
03:34 CEVM
03:34 CEVM
03:34 MMVM
03:34 MMVM
1600
200
VELOCITY cm/s
ACELERATION cm/s
2
1400
1200
1000
150
100
800
600
50
400
200
0
0.5
1
1.5
PERIOD s
2
2.5
3
0.5
1
1.5
2
2.5
3
PERIOD s
Figure 9. Acceleration (left) and velocity (right) response spectra of
the ground motion in the stations of Viña Centro (blue) and Viña El
Salto (green) for mainshock and aftershocks.
IES gives good information about earthquake energy
transferable to the structures and is a way of characterizing the
ground motion focused on building damage potential [14].
The calculated input energy spectra show different level of
energy with peak for different periods in CEVM and MMVM
stations (0.4-1.1 s and 0.4-1.5 s, respectively). Both IES could
be representative of sites of the plain city areas with
sedimentary deposits of different thickness. IES of two
stations of Valparaiso, neighbouring to Viña del Mar, are
plotted in Figure 10. The Universidad Tecnica Federico Santa
Maria station (UTFSM) is on bedrock and El Almendral is on
sandy soil.
Figure 11. H / V Spectral ratios obtained for CEVM (left) and
MMVM (right) stations.
Figure 12. Spectral ratio CEVM/ UTFSM (left) and
MMVM/UTFSM (right) stations.
Figure 10. Input energy spectra of the mainshock in Viña del Mar
stations (CEVM (blue) and MMVM (green)) and in Valparaiso
stations (UTFSM (black) and El Almendral (magenta)).
An empirical method to obtain the characteristics of the
transfer function at each site is to calculate the spectral ratio
between the Horizontal (H) and the vertical (V) components
of earthquake ground motion (HV spectral ratio technique HVSR). The spectral ratios for the main shock were obtained
in CEVM and MMVM stations (Figure 11). Both sites present
site effects with spectral amplification above a factor of 4 for
periods of 0.4-1.2 s (CEVM) and 0.35-1.6 s (MMVM).
Another method to estimate empirically the characteristics
of the local ground response is to calculate the spectral ratio
between the Horizontal components of site and a reference
rock station (standard spectral ratio method –SSR). UTFSM
was the only station without site effect and was use as
reference station. The SSR for CEVM and MMVM stations
are plotted in Figure 12. Both methods show ground
amplification for similar period bands, even though only one
event was used in the analyses.
Midorikawa and Miura [16] applied the HVSR method to
several aftershocks recorded at 4 sites of the city (one of them
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Figure 13. Changes in amplification period bands obtained in four
sites of the Viña del Mar downtown (left) applying HVSR method to
several aftershocks (righ) [16]. S corresponds to CEVM station.
4 KEY FEATURES AND PERIOD OF BUILDINGS
To analyse the earthquake damage in Viña del Mar by the
2010 earthquake it is essential to consider the buildings
structural characteristics mainly those affecting their seismic
response. After the earthquake, 2054 buildings on the plain
area of the city were inspected. 252 of them were damaged,
and required a more detailed analysis [4]. To assess
vulnerability of buildings we collected information about
materials and structural system, typology, building data (to
take into account code provisions applied), vulnerability index
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
Iv and behaviour modifiers (according to Risk-UE) and then
vulnerability class EMS-98 was assigned (Table 1).
Several factors conditioning the buildings response were
also considered. The height H (or number of storeys N) was
one of them, making a general classification within low (N ≤
3), medium (N between 4 and 9) and high (N from 10 to 24).
The structure building materials have been another of the
factors (wood, W; masonry, M; and reinforced concrete, RC).
The structures were classified in four historical periods
corresponding to changes in the applied Chilean earthquake
resistant regulation version (NCh433).
Building fundamental period T, height/building period
ratio (H/T) and wall density (ratio of wall area in one
direction/floor area) (dn) were also considered.
The H/T parameter is usually used to estimate the
translational stiffness of a reinforced concrete (RC) building.
This relationship has been used to analyse the behaviour of
Chilean buildings with less than 40 storeys [15]. The RC
buildings can be classified with the H/T parameter value
according to [17] as: too flexible H/T< 20 [m/s], flexible
20<H/T<40 [m/s], normal 40<H/T<70 [m/s], stiff
70<H/T<150 [m/s], and too stiff H/T>150 [m/s]. Most of
Viña del Mar RC buildings have a normal stiffness with H/T
values between 40 and 70 [m/s].
The H/T relationship for the city of Viña del Mar was
estimated by Guendelman et al. [17], and the T/N by
Midorikawa [18], (Figure 13). The average ratio between T
and N is approximately the same for both authors:
T = 0.045 N
(2)
The average periods of the inspected buildings of Viña del
Mar have been estimated with this formula. These natural
periods joint to ground predominant periods, amplification
period range and earthquake ground motion frequency content
have been analysed to explain the prevalence of serious
damage to buildings within a range of storey number.
Figure 13. Relationship between building height H and period T of
RC buildings after [17] (left) and between T and number of storeys
N proposed by Midorikawa [18] (right).
Most of Viña del Mar structures belong to the ECh 3 and 4
typological types (~75 %) and the remainder correspond to the
ECh 1 and 2 types. ECh 5 structures barely exist.
RC Chilean buildings have high values of density of walls,
dn, generally ranging from 0.015 to 0.035 [19]. These values
are much higher than those of other countries (e.g. United
States dn < 0.005 or Japan), because most of tall Chilean
buildings have a housing use, and the General Ordinance of
Urbanism and Construction demanded the laying of partition
walls between the dwellings [20]. This way, reinforced
concrete buildings, both those with structural walls and those
with frames with shear walls, are more stiff, thus reducing
total displacement and relative displacement between storeys,
and reducing the seismic damage level.
The predominant dn values estimates for different periods
of construction are: prior to 1960 earthquake dn < 0.040, for
1960-1985 period dn ~ 0.043, and after 1985 earthquake
values ranging from 0.043 to 0.060. The damage assumed by
Moroni and Astroza [21] according to the wall density (dn) for
an intensity of VIII (MM) is shown in Table 2.
5
ANALYSIS OF DAMAGE
The methodologies of post-seismic damage assessment have
been generally focused on quantifying the level of damage
and the classification of the habitability of buildings affected
(e.g. ATC20 [22], FEMA [23], HAZUS99 [24]) or the recently
proposed [e.g. 25 and 26]. These methods were taken into
account in Viña del Mar damage assessment.
Although the shaking had amax ≥ 0.35 g and it exceeded 0.1g
for more than 25 s, reaching an intensity of grade I = VII-VIII
(EMS) in sedimentary soils, the performance of buildings in
Viña del Mar was generally quite satisfactory in this
earthquake. It’s worth noting that only 252 (12.3 %) of the
2054 analysed buildings suffered visible damage, most of
them (el 86.9 %) had only damage of grade 1 or 2 and none
collapsed (Table 3). 8 % suffered very light damage (grade 1,
EMS), 2.6 % suffered light damage (grade 2), 1.1 % suffered
moderate damage (grade 3), and 0.5 % suffered severe
damage (grade 4). Most of damaged buildings (86.9%) had
only damage of grade 1 or 2 (EMS-98 scale). The low
percentage of damage to RC buildings and the absence of
collapses (grade 5, EMS) is justified by the high values for
parameter wall density ranging from 0.015 to 0.035 [4].
Grade 3 buildings were 8.7 % of the damaged buildings,
most of them Reinforced Concrete (RC) structures (63.6 %),
with N ≥ 10 storeys and constructed before 1985. Grade 4
buildings were 4.3 % of the damaged buildings, and most of
them (81.8 %) were also found in RC buildings with more
than 10 storeys (Table 3). None of the buildings constructed
between 1985 and 1993 suffered severe structural damages
(grade≥ 4).
The existence of fluvial and marine deposits and a shallow
phreatic level influenced the seismic intensity distribution,
consistent with previous seismic microzonation of the city
carried out by Moehle et al [10]. The more damaged buildings
(grade ≥ 3) were located mainly in the city areas with the
softest soils along the river Marga Marga and close to the
coast (Figure 14), formed by alluvial deposits belonging to the
delta of river and to mixed deposits, respectively, showing the
soil influence. This damage distribution was also observed by
the EERI work teams [27] who visited the damaged areas.
This spatial distribution of building damage was similar to the
one observed in the earthquake of 1985 [26].
The dependence of damage level with the building date of
construction, height, materiality and stiffness parameter h/T
has also been analysed. The time of building allow us to
estimate the seismic standards applied in its construction. The
results indicate that 52.7 % of all of the damaged buildings
were built before the earthquake of 1985, 15.5 % date from
1985-1993 (when significant changes were introduced in the
Chilean Standard Seismic after the earthquake 1985), 22.6 %
date from 1994-2003 and finally 9.1 % date from 2004-2010.
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Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
Table 1. Comparison of the analysed Chilean typologies found in Viña del Mar to the typologies of HAZUS, EMS and Risk-UE
and vulnerability class according to the EMS scale and vulnerability index Iv. (italics mean type that do not exist in the city).
Chilean Structures
ECh- 1. Concrete
frame and shear
wall building
ECh- 2. Concrete
shear wall
buildings
ECh- 3. Buildings
with hybrid
masonry walls
ECh- 4.
Reinforced
brick/concrete
block masonry
building.
(reinforced
masonry or
confined masonry)
ECh- 5. Steel
frame with shear
walls
*Probable value
HAZUS
typology
C1H. Concrete
moment frame
C2H Concrete
shear walls
W1. Wood,
light frame
URM L
Unreinforced
masonry
bearing walls
RM2L.
Reinforced
masonry
bearing walls
RM1M.
Reinforce
masonry with
wood or metal
deck
diaphragms
RM2L.
Reinforced
masonry
bearing walls
S2H. Steel
braced frame
EMS-98
typology
RC frame with
RC4. RC dual systems,
high level of
RC frame and walls
ERD
RC frame with
high level of
RC2. RC shear walls
ERD
Timber
W. Wooden structures
structures
Iv* index
of RiskUE
EMS-98
Vulnerability
class
0.386
E*, F+,D-
0.386
E*, F+,D-
0.447
D*, C+, E+, B-
Bearing walls
masonry with
RC floors
M3.4. URM bearing
walls with reinforced
concrete slabs
0.616
C*, B-, D+
Reinforced
masonry or
confined
M5. Overall
strengthened masonry
0.694
D*, E+, C-
Reinforced or
confined
masonry
M3.1. Wooden slabs
URM
0.74
B*, A+, C-
Reinforced or
confined
masonry
M4. Reinforced or
confined masonry
0.451
D*, C+, E+, B-
Steel structures
S4. Steel frame and
cast in place shear
walls
0.224
E*, D+, F-, C-
+ Upper limit
- Lower limit
ERD: Earthquake resistant design
Table 2. Relationship between damage level of and
wall density (dn) for an intensity of VIII (MM).
dn (%)
dn ≥ 1.15
1.15 > dn ≥ 0.85
0.85 > dn ≥ 0.50
dn < 0.50
Risk-UE typology
Grade of damage
Light (0 and 1)
Moderate (2)
Severe (3)
Heavy (4 and 5)
URM: Unreinforced masonry
RC: Reinforced concrete
A striking result is the fact that the buildings with less than
4 storeys (of different typologies, ECh types 3 and 4, which
were 74.25% of the total) presented a lower percentage of
damaged buildings (0.9 %) than the buildings with N > 3
storeys (mostly RC structures, ECh 1 and 2 types). There was
also a lower percentage of damage of grade 3 and 4 in low
storeys (0.4 %) than in high storeys (Table 3).
Table 3. Damaged buildings classified by their storey number and damage grade.
All
Damaged Undama
VL
L
M
S
buildings buildings ged (G0) (G1)
(G2)
(G3)
(G4)
Number of buildings
2054
252
1802
165
54
22
11
% of total
100%
12.3%
87.7%
8.0%
2.6%
1.1%
0.5%
Buildings N ≥ 4 storeys
529
233
296
164
44
16
9
% of total
25.7%
11.3%
14.4%
8.0%
2.1%
0.8%
0.4%
Buildings N< 4 storeys
1525
19
1506
1
10
6
2
% of total
74.3%
0.9%
73.3%
0.1%
0.5%
0.3%
0.1%
VL: Very Light
L: Light
M: Moderate
S: Severe
C: Collapse
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C
(G5)
0
0.0%
0
0.0%
0
0.0%
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
The number of storeys N (related to the fundamental
period of buildings) and materiality (structural type) were
other parameters analysed in relation to the damage. Buildings
with grade 3 of damage were 8.7% of damaged ones (1% of
total of structures), most 63.6 % were RC structures built
before 1985 and N ≥ 10 storeys. Buildings with grade 4 were
4.3 % of damaged ones (0.5 % of the total), most of them
were also RC structures (81.8 %) and N ≥ 10 storeys. Any
building built between 1985 and 1993 suffered serious
structural damage (grade ≥ 4).
Figure 14. 3D view (from the SW) of the plain area of Viña del Mar
city. Buildings with their grade of damage (EMS) and height are
shown. Damage dependence with the buildings height and location
on the softest soils is observed.
If we take records of two CEVM and MMVM acceleration
stations as representative of earthquake ground motion in the
plain area of the city, the input energy spectra indicate us the
great transfer of energy from the ground to the structures is for
periods in the range of 0.4-1.5 s. This means that those
buildings with fundamental period in that range, which
corresponds to structures between 9 and 33 storeys could have
suffered relevant damage. Response spectra and the transfer
functions obtained with HVSR and SSR methods the
predominant soil period as well as the predominant periods of
soils tell us about the period range were amplification of
ground motion should occur and resonant effect may appear.
6
CONCLUSIONS
The analysis of geological and geotechnical data show that
the plain area of the city is formed by alluvial deposits
consisting of sands and silty sands deposits reaching up to 100
m thick. The VS10 calculated are less than 280 m/s and VS30 are
greater than 300 m / s, except in the vicinity of the river where
they are lower. The hill zones consist of rocks of different
ages with high Vs greater than those of the plain. This
geological differences joint to water table depth (generally
between 2.7 and 6-7 m) had a clear influence in the spatial
macroseismic intensity distribution in the city in the 2010
earthquake and were also observed in previous ones. The
observed intensities were almost one degree lower at hills than
in the plain area of the city.
The predominant ground periods, Tp, obtained with
ambient noise and HVSR technique show the above
mentioned heterogeneous response of the ground. The Tp of
the plain city area (on Quaternary sediments) are between 0.4
and 1.1 s, the highest values (Tp > 0.8 s) on zones near the
river and coast shores. The surrounding zones present Tp
lower than 0.4 s. These results are relevant information to
analyze the resonance.
The H/V spectral ratios of the mainshock and some
aftershocks also indicate a different response of the two sites
of permanent seismic station. In both, spectral amplification
above a factor of 4 for periods of 0.4-1.2 s (CEVM) and 0.351.6 s (MMVM) was found. Response spectra verify this
different ground response. Results obtained by [16] applying
HVSR method to aftershocks recorded at 4 sites of the city
also found significant amplification among sites of the plain in
similar period range here obtained.
The results obtained with the SSR method, using
Technical University Federico Santa María (UTFSM) as
reference station, indicate again a similar period range of
significant amplifications in Viña del Mar.
The input energy spectra show greater motion energy
transferable to the structures in periods between 0.4 and 1.1 s
at CEVM station and from 0.4 to 1.5 s at MMVM station. If
the relationship building period-number of storeys (T/N)
estimated by Midorikawa [18] for RC structures of the city,
the buildings of 9 or more storeys could suffer more damage.
This is contrasted with the analysis of damage, showing that
buildings with major damage (grades 3 and 4) are reinforced
concrete with 9 or more storeys and the buildings of less than
4 storeys (the most abundant) had a lower percentage damage.
The most noteworthy aspect is the fact that, in spite of
clearly reaching degree of VII-VIII (MM) in the plain area of
the city, most Viña del Mar buildings (87.7%) did not suffered
substantial damage and only 252 (out of the 2054 revised
constructions) suffered visible enough damages to be
assessed. Moreover, very light and light damages (grades 1
and 2 EMS scale) affected 10.6 % of the total percentage, but
were 86.9 % of the damaged buildings. None of the buildings,
either old or new, collapsed.
A certain dependence on the level of damage to the time of
construction (here characterized as a function of applied
seismic regulations) was appreciated. Most of the damaged
buildings (52.7 %) were built before the 1985 earthquake,
only 15.5% for the period 1985-1993 (where significant
changes were made to the earthquake resistant regulation
NCh433 after the 1985 earthquake), in spite of being only
6.5% of the total of 2054 buildings. The damaged buildings
percentage decreases in the later periods, being the damaged
ones by 15.5 % for the 1985-1993 period, by 22.6 % for the
1994-2003 period (where there were improvements in the
NCh433 and structural control was incorporated) and finally
by 9.1 % for the 2004-2010 period.
A dependence of damage to the fundamental period of the
building and the structural type were observed, by 0.9% of the
low buildings of less than 4 storeys and typology types ECh 3
and ECh 4 (74.2 % of all inspected buildings) had damage and
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Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
only 0.4 % damage of grade 3 or 4. However, buildings
N> 3 storeys, mainly RC structures, ECh types 1 and 2 and
less abundant (25.75 % of total), by 11.3 % were damaged
and 1.2 % with severe damage (grade 3 or 4 EMS).
Damaged buildings of grade 4 were 0.5 % of the total and
4.3 % of the damaged ones.
These severe damages concentrated in RC structures
having more than de 10 storeys (81.8 % of this grade). In
many of these tall buildings, the ground floor has a greater
height than the higher storeys, thus suffering “soft storey”
failures.
The damage low percentage to RC constructions and the
absence of RC collapses are justified by the wall density
high values, ranging from 0.015 to 0.035, and by the H/T
parameter ranging from 40 to 70 of the immense majority
of the Viña del Mar buildings.
ACKNOWLEDGMENTS
This research was carried out within the framework of
research coordinated project CGL2011-30187-C02-01-02
funded by the Spanish Ministry of Science and Innovation.
The authors wish to express their sincere gratitude to the
professors Carlos Aguirre (Technical University Federico
Santa Maria) and Jorge F. Carvallo (Catholic University of
Valparaíso) who provided us soil and building data. Also
thank the Municipality of Viña del Mar, especially to
architect Pablo Rodriguez-Diaz and geographer Rodrigo
Flores, for the soil mechanics reports and collaboration
offered during microtremor measurements in soils and in
buildings. Finally, thanks to all those who helped during
the building damage assessment and microtremor surveys.
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