Proceedings of the 9th International Conference on Structural Dynamics, EURODYN... Porto, Portugal, 30 June - 2 July 2014

Proceedings of the 9th International Conference on Structural Dynamics, EURODYN... Porto, Portugal, 30 June - 2 July 2014

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
Response assessment of an existing building designed for earthquake loading
Araliya Mosleh1, Humberto Varum1, Hugo Rodrigues1, Aníbal Costa 1
Department of Civil Eng., Faculty of Engineering, University of Aveiro, 3810-193 Aveiro, Portugal
email: [email protected], [email protected], [email protected], [email protected]
1
ABSTRACT: Recent earthquakes confirm the significant seismic vulnerability of existing building specially those designed
according to the older codes. This paper investigates the seismic performance of the 6-story building, representative of common
new reinforce concrete building in Portugal. Twelve different artificially generated time history records are used with increasing
peak ground acceleration (PGA) values. The frame structure is evaluated by using both a nonlinear static (push-over) and time
history analysis with 3-D models in longitudinal and transversal directions. The assessment of seismic performance is based on
both global and member level criteria. For global response: maximum inter-story drift, displacement and rotation are considered,
however for member level an earthquake with 2000-year return period are selected and the biaxial demand of four columns
namely :corner, center, facadein X direction and facade in Y direction is studied.
KEY WORDS: RC buildings; Seismic vulnerability; Non linear analyses; Variation of axial load
1
INTRODUCTION
In recent years, the widespread damage to older buildings in
different earthquake (e.g. Northridge-1994 in California, and
Kobe-1995 in Japan, Laquila-2009, Emilia Romagna-2012 in
Italy, Lorca-2011 in Spain) [1-5], revealed the importance of
taking action to prevent damage to existing structures in future
earthquakes. The trend of seismic performance of reinforce
concrete (RC) have illustrated by previous researchers.
Rodrigues et al. proposed an experimental and numerical
simulation to represent the non-linear response of reinforced
concrete members due to biaxial bending combined with a
constant axial load [6-8]. Panagiotakos and Fardis are
proposed two methods for yielding rotation and quantifying
ultimate. The first one considers the plastic hinges, however
empirical in character, based on the multiple regression of a
database composed by about 1300 tests results on beamcolumn sections is considered in the second method [9]. Ciro
Faella et al. evaluated displacement based procedures for
assessing seismic behavior of structures according to both
EuroCode 8 (EuroCode 8, 2003) and the recent Italian
Seismic Provisions (New Italian Seismic Code, 2003) [10].
The seismic demand of reinforce concrete special momentresisting frame according to IBC 2003 proposed by Kim and
Kim [11]. Chaulagain et al. conducted a numerical
investigation on the seismic performance of four- story RC
building [12]. Varum et al. evaluated numerical tools for the
assessment and redesign of concrete buildings capable of
estimating the optimum distribution of strengthening needs for
a specific performance objective [13].
In this research an existing concrete building as a
representative of common RC in Portugal which is designed
with existent codes is chosen and proposed for nonlinear
analyses in longitudinal and transversal direction. The
building responses are analyzed in terms of max displacement,
inter-story-drift, and floor rotation for each story for global
response. For member level, an earthquake with 2000 (yrp) is
selected and the response of the columns are studied.
2
2.1
CASE STUDY BUILDING
Description of the 6-storey RC building
The 6-storey RC building is considered as a represent a
typical residential RC building in urban area in the center of
Portugal. The mentioned building has three bays in the
transversal (Y) direction and six bays in the longitudinal (X)
direction. The global dimension in X direction is 32.5 (m),
and in Y direction is 17.6 (m). The total structure height is
20.8 m, with a first story height of 5.4 m and 3.06 m story
heights for the middle stories and 3.16m for the upper one.
The strong axis of the rectangular columns is in longitudinal
direction. The cross section of largest column which is located
in the first story is (0.8*0.4) m2 with 18Φ25. The smallest
column is located in the 6th story with the damnation
(0.5*0.25) m2 with 12 Φ16.
2.2
Material properties
The mentioned building is designed by National codes and
other applicable technical documents, including: Safety
Regulations and Actions for Buildings and Structures Bridges,
Regulation of Structures Concrete and Prestressed, Regulation
of Steel Structures for Buildings, Eurocode 2, Reinforced
Concrete - Efforts normal and bending (REBAP-83) - LNEC,
CEB-FIP Model Code 1990. The materials properties are
assumed for the construction is the following: concrete
compressive strength, f′c=30 MPa; reinforcing steel yield
strength, fey = 400 MPa; for the first story: live load=4kN/m2
(30% for earthquake), dead load=1.5 kN/m2; for 1-5 stories:
live load=2 kN/m2(30% for earthquake), dead load=2.5
kN/m2; roof live load =0.7 kN/m2 (Nil for earthquake); roof
dead load = 1.5kN/m2. The total dead load for beams, columns
and floors are calculated by the software and the total weight
of stories due to difference dead and live loads in each floor is
between (410-470) Ton. The 3-D view of the six-story
building is shown in Figure 1.
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Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
3.2
Static pushover analysis
The push over analysis was considered in order to assess the
seismic capacity of the structure in longitudinal and
transversal directions. It is a series of incremental nonlinear
static analyses carried out to find the damage pattern and
lateral deformation during inelastic range of behavior. It could
be performed as either displacement control or force
controlled. The First approach is used when the building
expected to lose its strength, or when specified drifts are
examined where the amount of the applied load is not known
[20]. However, the second one is used when the load is known
and the structure expected to support the load [24]. In this
paper the first approach is used and the validity of pushover
method is verified based on the result of time history analysis.
Capacity curves for the building in the longitudinal and
transversal directions are presented in Figure 2.
11000
Figure 1. The 3-D view of the six-story building (m).
10000
2000-yrp
9000
3.1
ANALYTICAL MODEL
Modeling accepts and calibration
The finite element (FE) structural analysis program SAP2000
[14] was used to perform the push-over and time history
analyses with different earthquake records, in two directions.
In the following section the assumption considered for the
analysis are briefly described. Time period corresponding to
modal analysis is presented in Table 1.
7000
Time (s)
1.68
1.2
1.02
4000
X EQ-Y
975-yrp
2000-yrp
3000
475-yrp
2000
73-yrp
1000
0
0
3.3
(1)
(2)
In Esqs. (1) and (2), Lp: plastic hinge length, H: section
depth, L: critical distance from the critical section of the
plastic hinge to the point of contra flexure, fye: expected yield
strength and dbl: diameter of longitudinal reinforcement.
538
73-yrp
Y
5
10
15
20
25
30
35
40
45
Figure 2. Capacity curves in longitudinal (X) and transversal
(Y) direction.
For modeling shear wall three different methods were
proposed by previous researchers, namely: a) Equivalent
Frame Model [15-17], b) Braced Frame Analogy [18] and c)
Two – Column Analogy [19], in this paper first method is
used. Every shear wall is modeled as an idealized frame
structure with rigid beams at the floor levels. The PMM
plastic hinges with axial force-moment interaction were
assigned at the wall ends and shear hinges were assigned at
the mid-height level of walls. Since the structure is modeled
with the loads, section properties and steel content, therefore
default hinges are assigned to the columns as PMM, and to the
beams as M3 in order to FEMA-356 code [20].
Plastic hinge length is used to obtain ultimate rotation
values from the ultimate curvatures. Several plastic hinge
lengths have been proposed with previous researchers, [2123]. In this research two equations as bellow are considered,
which is recommended by (i.e. ATC-32 [20])
Lp  0.08L  0.022 f ye dbl  0.044 f ye dbl
EQ-X
475-yrp
5000
Roof displacement (cm)
Frequency (HZ)
0.59
0.83
0.98
Lp  0.5H
975-yrp
6000
Table 1. Time period and frequency of the building
Mode
1st mode(X direction)
2nd mode (rotation)
3rd mode (Y direction)
Transversal direction (Y)
Longitudinal direction (X)
Yield point
8000
Base shear (kN)
3
Dynamic time history analysis
The twelve of ground motion data were carried out in this
study by considering the similarity of the soil type for the
selected ground motion and building site. The selected ground
motion records were scaled to different maximum PGA levels
(0.09g to 0.44g). According to the FEMA code, a PGA of
0.09g corresponds to earthquakes having probability of
exceedance of about 50% in 50 years, and a PGA of 0.41g
corresponds to a probability of exceedance of about 2% in 50
years. Artificially generated PGA for various return periods
and PGA value are presented in Table 2 [25].
Table 2. Hazard curves for European scenario [25].
PGA (g)
0.09g
0.11g
0.14g
0.18g
0.22g
0.26g
0.29g
0.33g
0.38g
0.44g
PGA (m/s2)
0.889
1.06
1.402
1.796
2.18
2.543
2.884
3.265
3.728
4.273
Return period (years)
73
100
170
300
475
700
975
1370
2000
3000
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
4
RESULTS ANALYSES
Building global response
6
73-yrp
100-yrp
170-yrp
300-yrp
475-yrp
700-yrp
975-yrp
1370-yrp
2000-yrp
Storey
4
o
Y
2
X EQ-Y
0
0
5
10
15
20
(b)
Figure 3. Max. displacement profile for earthquakes in: (a)
longitudinal direction, (b) transversal direction.
7
6
5
73-yrp
100-yrp
170-yrp
300-yrp
475-yrp
700-yrp
975-yrp
1370-yrp
2000-yrp
3000-yrp
4
3
EQ-X
2
o
Y
1
X
0
0
1
2
3
4
IS-drift (%) longitudinal direction
(a)
7
6
5
73-yrp
100-yrp
170-yrp
300-yrp
475-yrp
700-yrp
975-yrp
1370-yrp
2000-yrp
4
3
o
2
Y
1
X EQ-Y
0
6
25
Max. displacement - transversal direction (cm)
Storey
The results of the time history analyses and push over
analyses were analyzed in terms of: max displacement, interstory drift and inter-story rotation for global response. For
member-level the biaxial demand in four columns namely:
center, corner, facadeX, facadeY for PGA=0.38g in
longitudinal direction is considered.
Figure 2 presents the results of the pushover analysis for
both directions: longitudinal and transversal. By applying the
triangular load pattern, the first exceedance of yield
displacement occurred at base shear 2208 kN and 4996 kN for
longitudinal and transversal direction respectively. The
corresponding PGA is between 0.11g-0.14g for both
directions. Before 0.11g the structure remain in linear region,
but after 0.14g the building follows a nonlinear behavior in
both directions. The push over curve and Figure 3 show that
the maximum displacement in longitudinal direction is about
2 times more than the transversal direction. It is shows that the
building has a higher initial stiffness and strength in
transversal direction.
To evaluate seismic performance inter-story drift is
important factor, because it is directly related to level of
structural damage. Inter-story drift IDR is computed as the
difference in lateral displacement IDR= (δi _ δi_1)/hi between
two adjacent floor levels, divided to story height. Figure 4
shows inter story drift for mentioned building in two
directions. The magnitude and distribution of inter-story drift
in longitudinal and transversal direction shows the stiffness
and strength in the transversal direction and longitudinal
direction is more flexible. The maximum inter-story drift is
1.57% and 3% for transversal and longitudinal direction
respectively as expected. Figure 5 shows the comparison of
IDR in both directions.
Figure 6 illustrates the maximum inter-story acceleration in
terms of maximum acceleration. It can be seen that for
longitudinal direction the building follows a linear behavior.
However for transversal direction, up to 975-year return
period structure shows the same pattern. From 975 to 1370
years return period the graph represents a jump from 0.0058 to
0.0181 (degree/m) which follows by a slight reduction to
0.0157 (degree/m) on 2000-year return period. Based on the
above discussion, the maximum rotation happens in 1370-year
return period on the 6th floor.
Storey
4.1
0.0
0.5
1.0
1.5
2.0
IS-drift (%) transversal direction
73-yrp
100-yrp
170-yrp
300-yrp
475-yrp
700-yrp
975-yrp
1370-yrp
2000-yrp
3000-yrp
Storey
4
EQ-X
o
Y
2
X
0
0
5
10
15
20
25
30
35
(b)
Figure 4. Max. IS-drift in: (a) longitudinal direction, (b)
transversal direction.
40
Max. displacement - longitudinal direction (cm)
(a)
539
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
4.5
3.5
o
EQ-X
Y
3.0
My
400
EQ-X
Intraction Surface
300
o
Y
X EQ-Y
Mx
X
200
100
2.5
M (kN.m)
y
Max. IS-drift (%)
500
Transversal direction (Y)
Longitudinal direction (X)
4.0
2.0
0
-100
1.5
-200
1.0
-300
0.5
-400
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
-500
-1000
-800
-600
-400
2
Max. acceleration (m/s )
-200
0
200
400
600
800
1000
Mx (kN.m)
(a)
500
Figure 5. Max. IS-drift in different earthquakes.
My
400
Intraction Surface
EQ-X
300
0.018
Transversal direction (Y)
Longitudinal direction (X)
EQ-X
0.014
Y
0.012
X
100
M (kN.m)
y
Max. IS-rotation (degree/m)
0.016
X EQ-Y
0
-100
0.010
-200
0.008
-300
0.006
-400
0.004
-500
-1000
-800
-600
-400
0.002
-200
200
400
600
800
1000
(b)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
2
500
Max. acceleration (m/s )
My
400
Figure 6. Max. IS-rotation in different earthquakes.
EQ-X
Intraction Surface
Y
300
Mx
X
200
First story column response
100
M (kN.m)
y
For member level, an earthquake with 2000-year return period
and with PGA=0.38g in longitudinal direction is selected.
Figure 7 shows the biaxial demand of four columns which are
located in 1st storey namely: center, corner, façade (x) and (y)
with corresponded interaction surpasses. The most important
biaxial demand happens in the corner column while for the
center column the lower demand is represented. In façade
columns, only uniaxial behavior was stated. However, it is
assumed that the earthquake force applied in the longitudinal
direction, so it is expected that Mx should be greater than My,
but for interior column in X direction the analysis results it is
reverse. Hence, it could be concluded that torsion happened in
the building. Since the response of the columns is stayed in
the middle of interaction surface, the columns are in elastic
area during this earthquake.
540
0
Mx (kN.m)
0.000
4.2
Mx
Y
200
0
-100
-200
-300
-400
-500
-1000
-800
-600
-400
-200
0
200
Mx (kN.m)
(c)
400
600
800
1000
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014
500
[4]
My
400
Intraction Surface
EQ-X
Y
300
[5]
Mx
X
200
[6]
M (kN.m)
y
100
0
-100
[7]
-200
-300
-400
-500
-1000
[8]
-800
-600
-400
-200
0
200
400
600
800
1000
Mx (kN.m)
(d)
[9]
Figure 7. The interaction Mx-My in: (a) center, (b) corner, (c)
facadeX, (d) facadeY columns (PGA=0.38g).
[10]
5
[11]
SUMMARY AND CONCLUSIONS
The seismic performance of the 6-story building,
representative of common reinforces concrete building in
Portugal was studied. The frame structure is evaluated by
using both a nonlinear static (push-over) and time history
analysis with 3-D models in longitudinal and transversal
directions. Based on the results of non-linear analyses the
following conclusions can be drawn:
 For global response push-over and time history
analesys show that the building is more flexible in
the longitudinal direction, while it is more sttiff in
the transversal direction.
 Rotation in longitudinal direction is followed by
linear pattern. The maximum rotation is occered in
transversal direction conreresponded to the 6th story
in 1370-year returen period.
 By considering the local response for four chosen
colums, corner and facade columns show higher
demand, but the center column represents the lower
demand. Since the initional axial load in corner
column in less than the other, so as it is expect the
most important biaxial demand happens in corner
column.The intraction surfaces show all the columns
staied in elastic area.
ACKNOWLEDGMENTS
This paper reports research developed under financial
support provided by “FCT - Fundação para a Ciência e
Tecnologia,” Portugal, of the first author through the
research project PTDC/ECM/102221/2008.
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