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,

radu@cnrrs.ro 2 National Center for Seismic Risk Reduction and University of Architecture, Bucharest, basarab@cnrrs.ro

3 Proiect Bucuresti SA, Bucharest, office@pb.ro

4 National Center for Seismic Risk Reduction and JICA, seki.matsutaro@jica.ro

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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 1

st 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/)

R. Vacareanu et al. 390

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 medium compacted state, with a 3 daN/cm

2 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.

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Figure 2. Satellite view of the building site – 90-96 Mihai

Bravu Blvd., Bucharest (from www.earth.google.com)

Figure 3. Main façade of the

building A located at 90-96 Mihai Bravu Blvd

Figure 4. Groundfloor plan view

Figure 5. Plan view of current floor

R. Vacareanu et al. 392

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 ETABS

TM

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

PGA ['g] = PGA ['g] =

Expected seismic

response 0.10 0.24 0.35 0.10 0.24 0.35

SD, cm 4.6 15.5 24.5 3.0 11.3 19.0

SA, 'g 0.22 0.24 0.24 0.26 0.33 0.33

Droof, cm 6.3 21.2 33.5 3.8 14.3 24.1

V, tf 988 1078 1078 994 1224 1224

µ 1.5 5.1 8.0 1.1 3.9 6.5

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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

0.0

0.1

0.2

0.3

0 5 10 15 20 25 30

SD, cm

SA

, g

µ=1 Capacity µ= 1.5

Performance

point

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25

SD , cm

SA

, g

µ=1 µ= 1.1 Capacity

Performance

point

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30 40 50 60

SD, cm

SA

, g

µ=1 µ= 5.1 Capacity

Performance

point

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30 40 50 60

SD, cm

SA

, g

µ=1 µ= 3.9 Capacity

Performance

point

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60 70 80

SD, cm

SA

, g

µ=1 µ= 8.0 Capacity

Performance

point

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60 70 80

SD, cm

SA

, g

µ=1 µ= 6.5 Capacity

Performance

point

Figure 7a. Expected seismic response of

existing building for PGA=0.1g (top), 0.24g (middle), 0.35g (bottom) – X direction

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.

R. Vacareanu et al. 394

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

-0.02

-0.01

0

0.01

0.02

0.03

0 2 4 6 8 10 12 14 16 18 20

Time, s

Gro

un

d F

loo

r to

p d

isp

lac

em

en

t, m

UX C=20kN*mm/s

UX no added damping

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0 2 4 6 8 10 12 14 16 18 20

Time, s

Gro

un

d F

loo

r to

p d

isp

lac

em

en

t, m

Uy C=20kN*mm/s

Uy no added damping

Figure 9. Groundfloor top displacement time-history for the building equipped with fluid viscous dampers and for the existing building

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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

PGA ['g] = PGA ['g] =

Expected seismic

response 0.10 0.24 0.35 0.10 0.24 0.35

SD, cm 1.8 6.9 11.6 1.4 4.6 7.8

SA, 'g 0.15 0.23 0.24 0.15 0.30 0.33

Droof, cm 2.5 9.4 15.9 1.7 5.9 9.9

V, tf 688 1053 1078 568 1129 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

R. Vacareanu et al. 396

0.0

0.1

0.2

0.3

0 5 10 15 20 25 30

SD, cm

SA

, g

µ=1 Capacity

Performance

point

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25

SD, cm

SA

, g

µ=1 Capacity

Performance point

0.0

0.1

0.2

0.3

0.4

0 10 20 30

SD, cm

SA

, g

µ=1 µ= 2.3 Capacity

Performance

point

0.0

0.1

0.2

0.3

0.4

0 10 20 30

SD, cm

SA

, g

µ=1 µ= 1.6 Capacity

Performance

point

0.0

0.2

0.4

0.6

0 10 20 30 40 50

SD, cm

SA

, g

µ=1 µ= 3.8 Capacity

Performance

point

0.0

0.2

0.4

0.6

0 10 20 30 40 50

SD, cm

SA

, g

µ=1 µ= 2.7 Capacity

Performance

point

Figure 11a. Expected seismic response of rehabilitated building for PGA=0.1g (top),

0.24g (middle), 0.35g (bottom) – X direction

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.

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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):

[ ]

Φ=

dsdds

SDSDds

,Sln

1P

β (1)

where: dsd,S is the median value of spectral displacement at which the building

reaches the threshold of the damage state, ds,

βds is the standard deviation of the natural logarithm of spectral displacement of damage state, ds, and

Φ 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

dsd,S , cm βds dsd,S , cm βds dsd,S , cm βds dsd,S , cm βds

5.1 0.70 8.2 0.81 20.4 0.89 51.1 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

dsd,S , cm βds dsd,S , cm βds dsd,S , cm βds dsd,S , cm βds

8.2 0.87 16.4 0.96 51.1 1.02 102.3 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.

R. Vacareanu et al. 398

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 20 40 60 80 100 120

SD, cm

P(>

DS|S

D)

Slight

Moderate

Extensive

Complete

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 20 40 60 80 100 120

SD, cm

P(>

ds|

SD

)..

Slight

Moderate

Extensive

Complete

Figure 12a. Building fragility curves for structural

damage states 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

PGA ['g] = PGA ['g] = Damage state, ds

0.10 0.24 0.35 0.10 0.24 0.35

None 5.64E-01 5.66E-02 1.28E-02 8.36E-01 1.58E-01 4.04E-02

Slight 2.00E-01 1.59E-01 7.60E-02 8.71E-02 2.29E-01 1.36E-01

Moderate 1.90E-01 4.07E-01 3.33E-01 6.71E-02 3.92E-01 3.97E-01

Extensive 3.93E-02 2.66E-01 3.53E-01 8.85E-03 1.70E-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

PGA ['g] = PGA ['g] = Damage state, ds

0.10 0.24 0.35 0.10 0.24 0.35

None 5.64E-01 5.66E-02 1.28E-02 8.36E-01 1.58E-01 4.04E-02

Slight 2.00E-01 1.59E-01 7.60E-02 8.71E-02 2.29E-01 1.36E-01

Moderate 1.90E-01 4.07E-01 3.33E-01 6.71E-02 3.92E-01 3.97E-01

Extensive 3.93E-02 2.66E-01 3.53E-01 8.85E-03 1.70E-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

PGA ['g] = PGA ['g] = Damage state, ds

0.10 0.24 0.35 0.10 0.24 0.35

None 9.34E-01 3.37E-01 1.25E-01 9.82E-01 6.08E-01 3.24E-01

Slight 3.59E-02 2.49E-01 2.15E-01 9.47E-03 1.85E-01 2.50E-01

Moderate 2.67E-02 3.04E-01 4.04E-01 7.58E-03 1.69E-01 3.11E-01

Extensive 2.72E-03 8.96E-02 1.93E-01 6.01E-04 3.27E-02 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

PGA ['g] = PGA ['g] = Damage state, ds

0.10 0.24 0.35 0.10 0.24 0.35

None 9.60E-01 5.80E-01 3.50E-01 9.87E-01 7.76E-01 5.69E-01

Slight 2.93E-02 2.37E-01 2.95E-01 9.76E-03 1.44E-01 2.42E-01

Moderate 9.89E-03 1.58E-01 2.83E-01 2.92E-03 7.19E-02 1.64E-01

Extensive 4.29E-04 1.90E-02 5.16E-02 9.49E-05 5.92E-03 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] = PGA ['g] =

0.10 0.24 0.35 0.10 0.24 0.35

Structural damage 4.95 28.84 43.64 1.40 17.99 32.34

Nonstructural damage 1.68 12.07 20.84 0.48 6.83 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

R. Vacareanu et al. 400

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)

Existing building Seismically rehabilitated

building

Reduction by seismic

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).

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REFERENCES

ATC 40 - Seismic Evaluation and Retrofit of Concrete Buildings, Report ATC-40, Applied

Technology Council, Redwood City CA., 1996 Chesca, A.B., Seki, M., Vacareanu, R., Okada, T., Georgescu, B., Kaminosono, T., Kato, H.

– Seismic Rehabilitation of an Existing Soft and Weak Groundfloor Building. Case Study - Proceedings of the International Symposium on Seismic Risk Reduction, ISSRR2007, Bucharest, 2007

Chopra A. K., Goel R. K. - Capacity-Demand-Diagram Methods For Estimating Seismic Deformation Of Inelastic Structures: SDF Systems, Report No. PEER-1999/02, Pacific Earthquake Engineering Research Center, 72 p., 1999

FEMA 356 - Prestandard and Commentary for the Seismic Rehabilitation of Buildings, Federal Emergency Management Agency, 2000

FEMA - HAZUS – Technical Manual. Earthquake Loss Estimation Methodology, 3 Vol. 1999 Hwang, J.S. – Seismic Design of Structures with Viscous Dampers, International Training

Programs for Seismic Design of Building Structures Kelly T. E. - In-Structure Damping and Energy Dissipation, Holmes Design Group,

www.holmesgroup.com, 2001 Miyamoto, H. K. & Scholl R. E. - Seismic Rehabilitation of a Historic Non-Ductile Soft Story

Concrete Structure Using Fluid Viscous Dampers, http://www.quakepro.com/new/prod_pages/dampening/rehab.htm, viewed March 14th, 2007

Ramirez, O. M. Constantinou, M. C., Gomez, J. D., Whittaker A. S., & Chrysostomou, C. Z. - Evaluation of Simplified Methods of Analysis of Yielding Structures with Damping Systems, Earthquake Spectra, Vol. 18, No. 3, pages 501–530, August 2002