Download Case study on the expected seismic losses of soft and weak

International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
Oct. 4-6, 2007, Bucharest, Romania
CASE STUDY ON THE EXPECTED SEISMIC LOSSES
OF SOFT AND WEAK GROUNDFLOOR BUILDINGS
R. Vacareanu1, A.B. Chesca2, B. Georgescu3, M. Seki4
ABSTRACT
Bucharest, one of the earthquake prone largest cities in Europe, has a large building stock
with soft and weak groundfloor buildings erected before the March 4, 1977 Vrancea
subcrustal earthquake. This paper presents a case study on the expected seismic losses of
a soft and weak groundfloor building. The case study is conducted for both existing situation
of the building as well as for the seismically rehabilitated building. The seismic rehabilitation
solution is prepared within JICA Technical Cooperation Project on Reduction of Seismic Risk
for Buildings and Structures in Romania by a team from JICA/NCSRR in partnership with
Proiect Bucharest design company. The seismic rehabilitation solution consists of
introducing fluid viscous dampers in the groundfloor and steel jacketing of the groundfloor
columns and of the upper stories structural walls. The seismic evaluation of the building is
performed using capacity spectrum method [ATC, 1996] and the evaluation of the expected
seismic losses is conducted according to [HAZUS, 1999] methodology. The important
reduction of the expected seismic losses for seismically rehabilitated building is highlighted.
Only economic losses are considered; the paper does not discuss the life losses.
INTRODUCTION
Romania is one of the earthquake prone countries in Europe. During the March 4, 1977
Vrancea subcrustal earthquake 31 buildings collapsed in Bucharest. Out of these 31
buildings, 28 were built before 1940-1945 and 3 buildings were built in 1960-1970’s. The
most affected buildings were the reinforced concrete high-rise buildings built until 1940-1945
with no seismic design. The structural system of the buildings consists of RC columns and
RC beams not acting as spatial RC frames due to the weak joints. Within this typology 23
residential buildings collapsed in Bucharest on March 4 1977. Out of the 3 newer buildings
collapsed on March 4 1977, one belongs to the typology of soft and weak groundfloor
buildings. This building typology consists of a dual structural system in the vertical direction,
i.e. RC columns in the ground floor (commercial area) and RC shear walls in the upper
floors (residential area). The major vulnerabilities of this building typology come from the
concentration of most of seismic lateral displacement and seismic dissipated energy in the
groundfloor, insufficient ductility of RC columns in the groundfloor and insufficient shear
capacity of the RC upper shear walls.
The collapsed soft and weak groundfloor building was located in Bucharest at the corner of
Stefan cel Mare Boulevard with Lizeanu Street. The 10-stories building was designed and
erected in 1961-1962. During March 4, 1977 earthquake, the western part of the body “A” of
the building had been displaced over a length of 10 m and needed to be demolished
afterward, Figure 1. The collapse of the building consisted in the failure of the first two floors
along a portion of part “A” on the entire height of the building.
1
National Center for Seismic Risk Reduction and Technical University of Civil Engineering, Bucharest,
[email protected]
2
National Center for Seismic Risk Reduction and University of Architecture, Bucharest, [email protected]
3
Proiect Bucuresti SA, Bucharest, [email protected]
4
National Center for Seismic Risk Reduction and JICA, [email protected]
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
389
As a result of the failure the damaged body become totally independent from the rest of the
building, having the upper structure virtually undeformed. The ground floor and the 1st floor
of this independent part had sunk in the basement. This dislocation and collapse has been
generated by the failure of the groundfloor columns, that generated the sunk of the above
slab and together with it the failure and collapse of the rest of the above slabs and the
breaking of the slabs of the remaining part of the building. Likewise, during the collapse, the
building had been displaced 50 cm from the ground floor level to the northeast direction and
stopped in an equilibrium position inclined with 10º from the vertical axes. Some of the
ground floor columns were still standing, after the failure of the superior support, punching
the above slabs that sunk vertically. The failure of the ground floor columns generated the
failure of other columns that in response determined the dislocation and the severe
inclination of the damaged part to the south. All these conclusions reported by Eng. Emilian
Titaru were made following the investigations made on site and in the office after the
earthquake and from the photos that were taken. It is most likely that the columns failure
was caused by the most disadvantageous stress combination, bending with shear force,
which resulted due to the low ductility of the section (caused by the under-reinforcement of
the column), including the insufficient resistance to shear force (caused by the transversal
under-reinforcement of the column as well because of the cross size dimensions). The flaws
in the earthquake resistant design of the building were due the limitations of the regulations
at that time. Following the earthquake, the part of the building that was mostly affected was
completely demolished because of the severe damage.
Figure 1. Stefan cel Mare no. 33 building 30 “A” collapsed in March 4 1977 Vrancea earthquake
(from: http://nisee.berkeley.edu/)
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For the rest of the building, that did not collapsed during the earthquake, there has been
severe damage of the ground floor columns, consisting either in crushing and spalling of
concrete together with the buckling of the reinforcements, either in cracks and fissures at
65º from the vertical axis that were located at the base of the column or at the superior end
of the column under the slab above the ground floor.
Given the lessons learnt in March 4 1977 earthquake on the very poor seismic response of
the soft and weak groundfloor buildings and the large existing building stock in this typology
in Bucharest, it is a stringent need for searching a seismic rehabilitation strategy. Within the
JICA Technical Cooperation Project in Romania it was decided to take actions towards the
aim of seismic rehabilitation of soft and weak groundfloor buildings. In order to proceed with
the seismic rehabilitation project it was selected a representative building with soft and weak
groundfloor built in 1960’s in Bucharest and located at 90-96 Mihai Bravu Boulevard in
Bucharest. For the preparation of the feasibility study, technical project and detailed design
of the seismic rehabilitation project, National Centre for Seismic Risk Reduction, NCSRR
and Japan International Cooperation Agency, JICA made a partnership with Proiect
Bucuresti SA design institute. In this paper it is presented the seismic rehabilitation strategy
and the solution for the soft and weak groundfloor selected building based on modern
techniques. The seismic rehabilitation solution consists in introducing fluid viscous dampers
in the groundfloor and in steel jacketing of the columns in the groundfloor and of the
structural walls in the upper stories.
SEISMIC EVALUATION AND SEISMIC REHABILITATION OF THE BUILDING
The building to be seismically rehabilitated is located on Mihai Bravu Boulevard, at number
90-96, in the city of Bucharest, Figure 2. The building was erected in 1960’s, it has 11
storeys (B+GF+10S) and its main destination is residential in the upper floors and
commercial in the groundfloor, Figure 3. The structural system consists of reinforced
concrete frames in the groundfloor, Figure 4 and RC structural walls in the upper floors,
Figure 5. The groundfloor is a soft and weak story, with no structural walls. The building
consists of 3 parts (A, B and C). In what concerns the seismic evaluation and retrofitting part
A is under discussion hereinafter. The A building has a rectangular base of 11.42x32.85 m.
A staircase connects the buildings “A” and “B”. Amongst all 3 buildings there are seismic
joints of 3 cm.
The ground floor height is 4.80m while the height of the rest of the floors is 2.73m.
The soil underneath the building is made of layers of sand with gravel fractions in the
2
medium compacted state, with a 3 daN/cm conventional pressure. The building’s footprint
area is 380.07 m2.
The concrete used is B250 (equivalent to C16/20) and the reinforcing steel used is OB37
(equivalent to S235).The RC beams sections vary from 15x55 cm up to 37,5x60 cm. The
RC columns sections vary from 40x55 cm to 80x50 cm; there are also two elongated
columns of 170x50cm transversally placed in the axes 9 and 10. The structural wall
thickness varies from 15 to 20 cm. The slabs are made of reinforced concrete of 8 to 11 cm
in thickness. The infrastructure system consists of a rigid box in the basement with
continuous foundations under all the structural elements.
The total weight of the building is 5528 t. The building first eigenvalue is 0.68s, the second
eigenvalue is 0.56s and the third eigenvalue is 0.45s. The first eigenvector is a translation in
the longitudinal direction; the second eigenvector is a translation in transversal direction
together with a rigid body rotation around the basement while the third is a rotation around
the vertical axis, Figure 6.
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
Figure 2. Satellite view of the building site – 90-96 Mihai
Bravu Blvd., Bucharest (from www.earth.google.com)
Figure 4. Groundfloor plan view
Figure 5. Plan view of current floor
391
Figure 3. Main façade of the
building A located at 90-96 Mihai
Bravu Blvd
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R. Vacareanu et al.
T1=0.68s
T2=0.56s
T3=0.45s
Figure 6. Modal shapes and eigenvalues of the building
Details on the seismic evaluation and seismic rehabilitation can be found elsewhere
(Chesca et.al., 2007). Only information relevant for the present paper are presented
hereinafter.
The seismic evaluation of the existing building and as well as of the seismically rehabilitated
building was performed using capacity spectrum method, CSM [ATC40, 1996] with the
alternative approach using strength reduction factors proposed by (Chopra&Goel, 1999).
Pushover analyses were performed for each direction of the building using ETABSTM
computer software. The application of the capacity spectrum method for the existing building
is presented hereinafter. The seismic action considered is according to the Romanian Code
for Earthquake Resistant Design of Buildings, P100-1/2006, considering two levels of
seismic hazard with 80% exceedance probability in 50 years and 40% exceedance
probability in 50 years. Nevertheless, for the completeness of the analysis, a third level of
seismic hazard with 10% exceedance probability in 50 years was considered. The design
peak ground accelerations in Bucharest for the previously mentioned levels of seismic
hazard are 0.1g, 0.24g and 0.35g. The corresponding building performance levels are:
damage limitation, life safety and collapse prevention, respectively. The results obtained
from capacity spectrum method are presented in Table 1 and Figure 7.
Table 1. Expected seismic response of existing building
X direction
Y direction
Expected
seismic
PGA ['g] =
PGA ['g] =
response 0.10
0.24
0.35
0.10
0.24
0.35
SD, cm
SA, 'g
Droof, cm
V, tf
4.6
0.22
6.3
988
15.5
0.24
21.2
1078
24.5
0.24
33.5
1078
3.0
0.26
3.8
994
11.3
0.33
14.3
1224
19.0
0.33
24.1
1224
µ
1.5
5.1
8.0
1.1
3.9
6.5
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
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Note:
X Direction – Longitudinal; Y Direction - Transversal
SD - spectral displacement; SA - spectral acceleration
V - base shear force; W - weight of the building
Droof - lateral displacement at top of building; PGA - peak ground acceleration
µ - displacement ductility
0.3
0.4
0.3
SA, g
SA, g
0.2
0.2
Performance
point
0.1
Performance
point
0.1
0.0
0
5
10
15
20
25
0.0
30
0
5
10
SD, cm
µ= 1.5
Capacity
µ=1
0.7
0.7
0.6
0.6
0.5
0.5
0.4
SA, g
SA, g
µ=1
Performance
point
0.3
15
20
25
SD , cm
µ= 1.1
Capacity
Performance
point
0.4
0.3
0.2
0.2
0.1
0.1
0.0
0.0
0
10
20
30
40
50
0
60
10
20
30
µ=1
40
50
60
SD, cm
SD, cm
µ= 5.1
µ=1
Capacity
1.0
1.0
0.8
0.8
µ= 3.9
Capacity
Performance
point
0.6
SA, g
SA, g
0.6
Performance
point
0.4
0.4
0.2
0.2
0.0
0.0
0
10
20
30
40
50
60
70
80
0
10
µ=1
µ= 8.0
20
30
40
50
60
70
80
SD, cm
SD, cm
Capacity
Figure 7a. Expected seismic response of
existing building for PGA=0.1g (top), 0.24g
(middle), 0.35g (bottom) – X direction
µ=1
µ= 6.5
Capacity
Figure 7b. Expected seismic response of
existing building for PGA=0.1g (top), 0.24g
(middle), 0.35g (bottom) – Y direction
The seismic rehabilitation/retrofitting objective is to improve the seismic performance of the
building without much disturbance for the residents of the building. In this respect it was
considered that any kind of intervention shall be made in the basement and ground floor
while limiting the amount of works in the upper stories.
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The structural walls in the upper floors lack shear capacity while the columns in the ground
floor mainly lack ductility.
The seismic rehabilitation solution adopted for Mihai Bravu 90-96 building “A” consists of
introducing fluid viscous dampers in the groundfloor, steel jacketing of the columns in the
groundfloor and steel jacketing of the structural walls in the upper stories. The fluid viscous
dampers layout was chosen taking into account the position of the upper stories structural
walls. One or two dampers are placed under each structural wall. The reason for this layout
is the proper transfer of the shear force between the dampers and the structural walls atop
them. The dampers are placed in the openings beneath the upper structural walls. In order
to avoid any supplemental forces added to the groundfloor columns, the final dampers
configuration was a chevron one, with dampers placed in horizontal position, at the upper
part of the groundfloor. In Figure 8 it is presented the layout of the dampers in the ground
floor. The damping constant was chosen to be C=20 kN*s/mm producing an overall
damping ratio of 30% for the rehabilitated building.
Figure 8. Damper layout in the groundfloor (solid thick lines)
Figure 9. represents the time-history of the groundfloor top displacement for the building
equipped with linear fluid viscous dampers and for the existing building. One may notice a
50% reduction in the maximum displacement demand as well as the reduction of the
number of cycles at high amplitudes.
0.03
0.03
Uy C=20kN*mm/s
Uy no added damping
UX C=20kN*mm/s
UX no added damping
0.02
0.01
0
0
2
4
6
8
10
-0.01
-0.02
12
14
16
18
20
G round Floor top displacem ent, m
G round Floor top displacem ent, m
0.02
0.01
0
0
2
4
6
8
10
12
14
16
18
20
-0.01
-0.02
-0.03
-0.03
Time, s
Time, s
Figure 9. Groundfloor top displacement time-history for the building equipped with fluid viscous
dampers and for the existing building
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
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Figure 10. presents the dampers connection details beneath a structural wall. Several
horizontal damper connections in the same frame were analysed. The optimal solution was
to connect the dampers to the middle RC column in order to avoid the connection of the
dampers to tensioned shear weakened corner columns.
The other retrofitting measures consisted of steel jacketing of the ground floor RC columns
and steel jacketing of the first 3 stories RC structural walls.
Figure 10. Damper connection detail
The results obtained with capacity spectrum method for the seismically rehabilitated building
are presented in Table 2 and Figure 11. The code elastic response spectrum for Bucharest
was reduced according to the provisions of Annex A of P100-1/2006 Code to take into
account the damping ratio of 30% for the seismically rehabilitated building.
Table 2. Expected seismic response of seismically rehabilitated building
X direction
Y direction
Expected
seismic
PGA ['g] =
PGA ['g] =
response 0.10
0.24
0.35
0.10
0.24
0.35
SD, cm
SA, 'g
Droof, cm
V, tf
1.8
0.15
2.5
688
6.9
0.23
9.4
1053
11.6
0.24
15.9
1078
1.4
0.15
1.7
568
4.6
0.30
5.9
1129
7.8
0.33
9.9
1234
µ
1.0
2.3
3.8
1.0
1.6
2.7
Note:
X Direction – Longitudinal; Y Direction - Transversal
SD - spectral displacement; SA - spectral acceleration
V - base shear force; W - weight of the building
Droof - lateral displacement at top of building; PGA - peak ground acceleration
µ - displacement ductility
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R. Vacareanu et al.
0.4
0.3
0.3
0.2
SA, g
SA, g
Performance
point
Performance point
0.2
0.1
0.1
0.0
0.0
0
5
10
15
20
25
0
30
5
10
µ=1
Capacity
20
µ=1
0.4
25
Capacity
0.4
Performance
point
0.3
SA, g
0.3
SA, g
15
SD, cm
SD, cm
0.2
0.1
0.2
Performance
point
0.1
0.0
0.0
0
10
20
30
0
10
20
SD, cm
µ=1
30
SD, cm
µ= 2.3
µ=1
Capacity
0.6
µ= 1.6
Capacity
0.6
0.4
0.4
SA, g
SA, g
Performance
point
0.2
0.2
Performance
point
0.0
0.0
0
10
20
30
40
50
0
10
SD, cm
µ=1
µ= 3.8
20
30
40
50
SD, cm
Capacity
Figure 11a. Expected seismic response of
rehabilitated building for PGA=0.1g (top),
0.24g (middle), 0.35g (bottom) – X direction
µ=1
µ= 2.7
Capacity
Figure 11b. Expected seismic response of
rehabilitated building for PGA=0.1g (top),
0.24g (middle), 0.35g (bottom) – Y direction
One can notice the important reduction of the ductility demands for the rehabilitated building
(more than two times reduction), thus alleviating one of the major deficiencies of the
structural system.
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
397
EVALUATION OF THE EXPECTED SEISMIC DAMAGE AND LOSSES
In order to investigate the expected seismic damage and losses for the existing and for the
rehabilitated building, the methodology presented in [HAZUS, 1999] is applied. The building
typology was assigned to C1H (high-rise reinforced concrete frames) designed according to
a low code (P13-63 Romanian Earthquake Resistant Design Code), even the structural
system is dual on the height of the building. Nevertheless, given the lack of the soft and
weak groundfloor typology in HAZUS and the overall behavior of the building given by the
RC frames in the groundfloor, the above-mentioned decision was made.
The building fragility curves are computed for Slight, Moderate, Extensive and Complete
structural and nonstructural damage states. Each fragility curve is characterized by median
and lognormal standard deviation (β ) values. Median values of spectral displacement define
the thresholds of Slight, Moderate, Extensive and Complete damage states. The probability
of being in or exceeding a given damage state is modeled as a cumulative lognormal
distribution. For structural and nonstructural damage, given the spectral displacement, SD,
the probability of being in or exceeding a damage state, ds, is modeled as, (HAZUS, 1999):
 1  SD
P ds SD = Φ 
ln
β
 ds  S d ,ds
[
]
where: Sd,ds
β ds
Φ



(1)
is the median value of spectral displacement at which the building
reaches the threshold of the damage state, ds,
is the standard deviation of the natural logarithm of spectral
displacement of damage state, ds, and
is the standard normal cumulative distribution function.
The parameters of the building fragility curves for structural damage states for C1H building
typology designed according to a low code are [HAZUS, 1999], Table 3:
Table 3. Values of building fragility curve parameters for structural damage states
Slight
Moderate
Extensive
Complete
Sd,ds , cm
5.1
β ds
0.70
Sd,ds , cm
8.2
β ds
0.81
Sd,ds , cm
20.4
β ds
0.89
Sd,ds , cm
51.1
β ds
0.98
The parameters of the building fragility curves for nonstructural damage states for C1H
building typology designed according to a low code are [HAZUS, 1999], Table 4:
Table 4. Values of building fragility curve parameters for nonstructural damage states
Slight
Moderate
Extensive
Complete
Sd,ds , cm
8.2
β ds
0.87
Sd,ds , cm
16.4
β ds
0.96
Sd,ds , cm
51.1
β ds
1.02
Sd,ds , cm
102.3
β ds
1.06
The building fragility curves for both structural and nonstructural damage states are
presented in Figure 12.
Given the expected spectral displacements presented in Table 1 and Table 2 and the
building fragility curves presented in Figure 12, the probabilities of being in a given
structural/nonstructural damage state are reported in Tables 5 and 6 for the existing building
and in Tables 7 and 8 for the seismically rehabilitated building.
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1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.6
P(>ds|SD)..
P(>DS|SD)
0.7
Slight
Moderate
Extensive
Complete
0.5
0.4
0.6
0.5
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0.0
Slight
Moderate
Extensive
Complete
0.0
0
20
40
60
SD, cm
80
100
120
Figure 12a. Building fragility curves for structural
damage states
0
20
40
60
SD, cm
80
100
Figure 12b. Building fragility curves for
nonstructural damage states
Table 5. Probabilities of being in a given structural damage state – existing building
X direction
Y direction
Damage
PGA ['g] =
PGA ['g] =
state, ds
0.10
0.24
0.35
0.10
0.24
0.35
None
Slight
Moderate
Extensive
5.64E-01
2.00E-01
1.90E-01
3.93E-02
5.66E-02
1.59E-01
4.07E-01
2.66E-01
1.28E-02
7.60E-02
3.33E-01
3.53E-01
8.36E-01
8.71E-02
6.71E-02
8.85E-03
1.58E-01
2.29E-01
3.92E-01
1.70E-01
4.04E-02
1.36E-01
3.97E-01
2.92E-01
Complete
6.87E-03
1.12E-01
2.25E-01
1.15E-03
5.12E-02
1.35E-01
Table 6. Probabilities of being in a given nonstructural damage state – existing building
X direction
Y direction
Damage
PGA ['g] =
PGA ['g] =
state, ds
0.10
0.24
0.35
0.10
0.24
0.35
None
Slight
Moderate
Extensive
5.64E-01
2.00E-01
1.90E-01
3.93E-02
5.66E-02
1.59E-01
4.07E-01
2.66E-01
1.28E-02
7.60E-02
3.33E-01
3.53E-01
8.36E-01
8.71E-02
6.71E-02
8.85E-03
1.58E-01
2.29E-01
3.92E-01
1.70E-01
4.04E-02
1.36E-01
3.97E-01
2.92E-01
Complete
6.87E-03
1.12E-01
2.25E-01
1.15E-03
5.12E-02
1.35E-01
Table 7. Probabilities of being in a given structural damage state – seismically rehabilitated
building
X direction
Y direction
Damage
PGA ['g] =
PGA ['g] =
state, ds
0.10
0.24
0.35
0.10
0.24
0.35
None
Slight
Moderate
Extensive
9.34E-01
3.59E-02
2.67E-02
2.72E-03
3.37E-01
2.49E-01
3.04E-01
8.96E-02
1.25E-01
2.15E-01
4.04E-01
1.93E-01
9.82E-01
9.47E-03
7.58E-03
6.01E-04
6.08E-01
1.85E-01
1.69E-01
3.27E-02
3.24E-01
2.50E-01
3.11E-01
9.38E-02
Complete
3.05E-04
2.02E-02
6.32E-02
5.86E-05
5.47E-03
2.15E-02
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Table 8. Probabilities of being in a given nonstructural damage state – seismically
rehabilitated building
X direction
Y direction
Damage
PGA ['g] =
PGA ['g] =
state, ds
0.10
0.24
0.35
0.10
0.24
0.35
None
Slight
Moderate
Extensive
9.60E-01
2.93E-02
9.89E-03
4.29E-04
5.80E-01
2.37E-01
1.58E-01
1.90E-02
3.50E-01
2.95E-01
2.83E-01
5.16E-02
9.87E-01
9.76E-03
2.92E-03
9.49E-05
7.76E-01
1.44E-01
7.19E-02
5.92E-03
5.69E-01
2.42E-01
1.64E-01
2.01E-02
Complete
6.60E-05
5.40E-03
1.94E-02
1.25E-05
1.32E-03
5.79E-03
The cost of damage is expressed as a percentage of the complete damage state. The
assumed relationship between damage states and repair/replacement costs, for both
structural and non-structural components, is as follows [HAZUS, 1999]:
Slight damage:
2% of complete
Moderate damage:
10% of complete
Extensive damage:
50% of complete
These values are consistent with and in the range of the damage definitions and
corresponding damage ratios presented in ATC-13 Earthquake Damage Evaluation Data for
California.
Given the repair/replacement costs previously mentioned and the distribution of probabilities
in Tables 5-8, the expected cost of damage given the incidence of an earthquake can be
obtained. The expected costs of damage presented in the following are for structural and
nonstructural elements and are expressed as percentage of the replacement cost obtained
as weighted averages. The expected costs of damage for the existing building are reported
in Table 9 and the expected costs of damage for the seismically rehabilitated building are
presented in Table 10.
Table 9. Expected cost of damage for the existing building (% of replacement cost)
X direction
Y direction
PGA ['g] =
Structural damage
Nonstructural damage
PGA ['g] =
0.10
0.24
0.35
0.10
0.24
0.35
4.95
1.68
28.84
12.07
43.64
20.84
1.40
0.48
17.99
6.83
32.34
13.96
Table 10. Expected cost of damage for the seismically rehabilitated building (% of
replacement cost)
X direction
Y direction
PGA ['g] =
PGA ['g] =
0.10
0.24
0.35
0.10
0.24
0.35
Structural damage
0.51
10.03
20.45
0.13
4.24
10.45
Nonstructural damage
0.19
3.55
7.94
0.05
1.44
3.71
400
R. Vacareanu et al.
The design earthquake with 40% exceedance probability in 50 years (PGA=0.24g) is
considered in the following for comparison purposes. Considering the occurrence of the
seismic action in X and Y direction equally probable events forming a complete set, then the
expected cost of damage for the building (structural and nonstructural) is simply obtained as
the arithmetic mean of the expected costs in X and Y direction, Table 11. The reduction of
the expected cost of damage by the seismic rehabilitation is also reported in Table 11.
Table 11. Expected cost of damage for PGA = 0.24g (% of replacement cost)
Seismically
Reduction by
Existing building
rehabilitated
seismic
building
rehabilitation
Structural damage
23.42
7.14
16.28
Nonstructural damage
9.45
2.49
6.96
One can notice from Table 11 the important decrease of the expected losses (2.5 to 3
times) due to the seismic rehabilitation of the building. If one considers the replacement cost
of the structural components at 400 Euro/sq.m. and the replacement cost of the
nonstructural components at 500 Euro/sq.m, the expected reduction of the cost of damage
in the case of incidence of the design earthquake might amount 100 Euro/sq.m
(16.28%x400+6.96%x500). Moreover, if the building content is evaluated as the sum of the
replacement costs of structural and nonstructural components [HAZUS, 1999], the expected
reduction of the losses likely amount 200 Euro/sq.m.
CONCLUSIONS
The seismic rehabilitation solution presented in this paper provides good results for an
existing soft and weak groundfloor building in Bucharest. The employment of the fluid
viscous dampers in the groundfloor of the building provides supplemental energy
dissipation. The damper configuration is chosen in such a way as to use as much as
possible the interstory velocity in the groundfloor. The upper structural walls are retrofitted
with steel plates in order to increase the capacity in shear. The assessment of the seismic
risk for both existing building and seismically rehabilitated building revealed the important
reduction of the expected seismic losses (2.5…3 times) for the latter case and the net
economic benefit in the case of incidence of the design earthquake. Only economic losses
are assessed in this case study. The life losses are far beyond any economic losses and the
life safety of the residents is the strongest incentive for seismic rehabilitation of existing
vulnerable buildings. Nevertheless, the economic issue is an important one for the residents
for making the decision of seismic rehabilitation of the building. Moreover, the soft and weak
groundfloor buildings, along with pre 1940 RC high rise buildings, are now included in the
Romanian legal framework for urgent retrofitting.
ACKNOWLEDGEMENTS
NCSRR deeply acknowledge the generous, continuous and long-lasting financial support of
Japan International Cooperation Agency, JICA during the implementation of the Technical
Cooperation Project for Reduction of Seismic Risk for Buildings and Structures in Romania.
The authors also acknowledge the strong technical support provided by engineer Tateyoshi
Okada (Affect Engineering, Japan – short term JICA expert at NCSRR) in performing
nonlinear analyses for the building and the valuable technical advices given by engineer
Takashi Kaminosono (Center for Better Living, Japan – former JICA expert at NCSRR) and
engineer Hiroto Kato (Building Research Institute, Tsukuba - JICA expert at NCSRR).
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
401
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