Download SEISMIC STRUCTURAL PERFORMANCE INDEX EVALUATION OF

SEISMIC STRUCTURAL PERFORMANCE INDEX EVALUATION
OF EXISTING REINFORCED CONCRETE BUILDINGS
by
Andres W.C. Oreta, Benito M. Pacheco and William T. Tanzo*
ABSTRACT: A method for evaluating the basic structural performance index, Io, of existing
reinforced concrete (RC) buildings is presented. The index, which is evaluated using a Level-1
screening procedure by the Architectural Institute of Japan (AIJ), with some modification, may be
applied to medium and low-rise buildings up to six stories. The index of the building may be used
as a measure of the expected seismic performance of a building.
KEYWORDS: Seismic Capacity, RC Building, Earthquake, Metro Manila
1. INTRODUCTION
The problem of possible hazards brought about by earthquakes such as building collapse and
loss of lives, especially in major urban metropolis like Metro Manila, must be addressed by city
planners, building officials and structural engineers. Although our present structural design codes
may have incorporated special provisions for the earthquake resistant design of new buildings, there
is still a danger for possible collapse or life-threatening damage in existing buildings, especially the
old and deteriorated ones. Buildings with large occupancy such as schools and buildings which are
essential after an occurrence of an earthquake such as hospitals and other emergency service
facilities must be checked urgently on their expected performance against major earthquakes. Civil
and structural engineers must address the need of assessing the seismic safety of buildings, possibly
for retrofitting or strengthening.
Given the large number of existing buildings in the metropolis, it is clear that seismic
upgrading cannot be advanced simultaneously. It is thus important that a brief method of seismic
screening of buildings be done to determine which buildings are at greatest risk, and hence must be
given priority in detailed evaluation and possible retrofitting. This paper presents one method of
evaluating the seismic performance of reinforced concrete (RC) buildings by ocular inspection and
simple computations.
2. PROCEDURE FOR BRIEF SEISMIC DIAGNOSIS
The Japanese standard for evaluating the seismic performance of existing RC buildings,
especially low and medium-rise buildings up to six stories, uses a seismic index, Is, for judging
whether a building has sufficient capacity or not. This method has been used by the Architectural
Institute of Japan (AIJ) in the inspection of the safety of existing buildings in past earthquakes such
as the 1980 Miyagi-Ken-Oki Earthquake, 1990 Philippine Earthquake and the 1995 Kobe
Earthquake. The most general form of the equation for computing Is was given by Umemura (1980)
as
(1)
I s = Io ⋅ SD ⋅T ⋅G
*
Presented at the 10th ASEP International Convention, May 23-24, 2003, Manila Pavilion, pp. 1-11
1
where Io = basic structural performance index calculated by the ultimate strength, ductility, number
of story and story level considered,
SD = structural design index to modify Io due to degree of irregularity of the building shape
and the distribution of stiffness,
T = time index to modify Io due to the degree of deterioration of strength and ductility,
G = local geology index to modify Io.
The Io-index is the dominant value in the computation of Is. The indices SD, T and G, are
reduction factors less than or equal to 1.0. In the latest 1995 publication of the International Institute
of Seismology and Earthquake Engineering (IISEE), Hirosawa et al (1995) gives guidelines on
appropriate values for SD and T, while the G-index is not included in the seismic index equation.
The Is-index is calculated at each floor in each direction of the building. This value
corresponds to the maximum elastic response shear coefficient that the floor can resist. A larger
value of Is indicates a higher seismic performance of the building. The seismic safety of the
buildings is determined by comparing the computed Is with a critical value. Nakano (1986) stated
that an index larger than about 0.6 (using Level-2 screening) should be required in order to survive
a severe earthquake, the intensity of which is nearly equal to Tokachi-Oki or Miyagi-Ken-Oki
earthquakes. The critical value of 0.6 is thus recommended as determined from correlation studies
from damaged buildings of past severe earthquakes in Japan. Buildings designed by the current
Japanese design codes have Is more than 0.6. The Japanese standard has three levels of screening
procedures - the simple Level-1 screening based on the horizontal strength of the building to a
more detailed Level-3 screening based on the failure mechanism of building frames. The higher
level screening procedure is expected to give a more reliable estimate of the seismic performance of
buildings.
The present study adopts the AIJ Level-1 screening procedure with some modification.
Since the structural performance Io-index is the most important value in the seismic index
computation, this study focuses on the computation of Io-index. Moreover, although the Io-index
can be computed for each story level, the Io-index at the ground floor level, being the most critical
level, will be used.
2.1 Classification of vertical elements
The structural performance Io-index for Level 1 screening is calculated from the horizontal
strength of the vertical elements of the building, based on the sum of the horizontal cross-sectional
areas of columns and shear walls and on their respective average unit strengths. Columns are first
classified based on the ratio of the clear height to dimension of column (h/d) as long columns, short
columns or extremely short columns. Similarly, shear walls are also grouped as Wall-Type 1, 2 or 3.
Wall-Type 1 has one boundary column or no peripheral column. Wall-Type 2 has a peripheral
column, while Wall-Type 3 has two boundary columns. Refer to Table 1. The unit shear stress
given in Table 1 corresponds to minimum values obtained in experimental data of RC elements in
Japan. A compressive strength of 200 kg/cm2 (2,800 psi or about 20 MPa) is used in determining
the unit shear stress.
2
Table 1. Type of Elements and Shear Stresses
Element
Section Property
Shear Stress
(kg/cm2)
kPa
Long Column
h/d > 6
7
687
Short Column
2 <= h/d <= 6
10
981
h/d < 2
15
1472
Wall - Type 1
10
981
Wall - Type 2
20
1962
Wall - Type 3
30
2943
Extremely Short Column
2.2 Strength Index
The C–index or strength index of each group is calculated by Eqns. 2-a, b and c.
C c = (∑ τ c Ac + ∑ τ s As ) / W
(2-a)
C e = ∑ τ e Ae / W
(2-b)
C w = (∑ τ 1 A1 + ∑ τ 2 A2 + ∑ τ 3 A3 ) / W
(2-c)
where Cc = strength index of long and short columns
Ce = strength index of extremely short columns
Cw = strength index of walls
τc = shear stress of long columns in kPa
τs = shear stress of short columns in kPa
τe = shear stress of extremely short columns in kPa
τ1, τ2, τ3 = shear stress of walls of type 1, 2 and 3, respectively
A = cross-sectional area of corresponding columns and walls in m2
W = building weight above the story under consideration in kN
2.3 Story Index
The story index (β) indicates the ratio of the response base shear coefficient and the i-th
story response shear coefficient and is given as β = (n + 1)/(n + i). Here, n is the total number of
stories and i is the story level under consideration. At the ground floor level (i=1), the value of
β = 1.
3
2.4 Structural Performance Index
The structural performance index considers the contribution of the vertical elements in
resisting lateral forces through the strength index. Incorporated in the computation of this index are
the ductility index and strength reduction factors of the vertical elements.
For a building without extremely short columns (Ce = 0), the index is computed as
I1 = β (0.7C c + C w ) ⋅1.0
or
I1 = β (C c ) ⋅1.0
(3)
Τhe second equation in Eq. 3 is used only if there are also no walls (Cw = 0).
For a building with extremely short columns (Ce ≠ 0), either Eq. 3 or Eq. 4 below will be
used depending on a specified condition.
I 2 = β (0.5C c + 0.7C w + C e ) ⋅ 0.8
(4)
Umemura (1980) stated the following condition. If the failure of the extremely short
columns results to the fatal damage of the building, then use Eq. 4, otherwise use Eq. 3. This
condition is subject to various interpretations depending on the judgement of the engineer.
However for simplicity, it is assumed here that a value of Ce ≥ 0.5 will result to fatal damage of the
building. Based on some numerical simulations, when Ce = 0.5, about 30% of the columns are
extremely short, and failure of 30% of the columns may lead to possible collapse. Hence when Ce <
0.5, the index is computed using Eq. 3. Notice that Eq. 3 does not include Ce even if it is not equal
to zero. If the number of extremely short columns is minimal, Eq. 3 assumes that these extremely
short columns fail first and only the long and short columns and the walls will be left to resist the
shear forces.
The values 0.7 and 0.5 in Eq.3 and Eq. 4 are strength reduction factors due to displacement
compatibility. The values 1.0 and 0.8 are ductility indices. Notice that the first equation of Eq. 3
considers only 70% contribution of the long and short columns compared to 100% contribution of
the walls. However, when there are no walls, then we let the columns carry 100% of shear forces as
shown in the second equation. Eq. 4, on the other hand reflects the contribution of the various
elements with the extremely short columns resisting 100%, followed by walls, 70% and the long
and short columns, 50%. Long and short columns and walls are considered more ductile than
extremely short columns as reflected by the factors 1.0 in Eq. 3 and 0.8 in Eq. 4.
2.5 Concrete Hollow Blocks (CHB) Infill Walls
The present study recognizes the effect of CHB or masonry infill on the seismic behavior of
frames. The true influence of infill on frames will be to stiffen the infilled frames relative to the
other frames. The proportion of the total seismic shear transmitted by the infilled frames will
increase because of the increased stiffnesses of these frames relative to the other frames. The
stiffening due to infill results to a reduction of the effective clear height of the columns - at the top
and bottom ends of this height, the plastic hinges are developed. The higher shear force will be
developed, however, at the expense of ductility (Paulay and Priestley 1992). This increase in shear
force may be considered in the present study by simply reducing the effective clear height of the
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columns, which may result to the column being classified to a shorter column with a larger shear
stress as shown in Table 1. In the present method, a long column with more than 30% infill will be
re-classified as a short column. A short column with more than 70% infill will be re-classified as an
extremely short column.
2.6 Procedure of Level 1 Screening
The general procedure for Level 1 Screening of RC buildings is summarized below:
1) Ocular inspection of the building is first done to gather information about the building such as
the floor area, number of stories, type of framing and flooring system, location and dimensions
of columns and shear walls, infill CHB walls and the clear height of floor to ceiling.
2) A simple diagrammatic plan, similar to Figure 1, is drawn for the ground floor showing the
main lateral force resisting system which consists of columns, shear walls and infill walls. The
lateral force resisting frames are defined for both x and z axes.
A
B
C
D
E
F
G
H
I
J
K
L
5.8 m
5
2.9 m
2.8 m
4
4.7 m
3
2
1
5.57 m (typical)
[email protected] = 73.2 m
4 Story Bldg.
Floor Area = 1010 sq m
All columns = 0.5 m x 0.6 m
Figure 1. Ground Floor Plan of an Inspected Building
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3) Using the spreadsheet program, data are inputted in Tables 2, 3 and 4. Inputted in Table 2 are
the dimensions of the column cross-sections and the number of columns of the same crosssection for each frame. The average percentage of CHB infill per frame is also inputted. In
Table 3, the required data are the shear wall dimensions (length and thickness) and the type of
shear wall (Type 1, 2 or 3). The data for Table 4 are used for the computation of the seismic
dead load or weight of the building. The data needed are the floor area, estimated dead load per
unit area that includes the weight of floor slab, floor finish, ceiling, partitions and other
permanent loads. The clear height and the average column dimension are also necessary input
data.
Table 2. Input for Columns
Frames along X-direction
Column Section A
Column Section B
Label b (m)
d (m)
No. b (m)
d (m)
No.
1
0.5
0.6 10
2
0.5
0.6
4
3
0.5
0.6 10
4
0.5
0.6 13
5
0.5
0.6 13
Frames along Z-direction
Column Section A
Column Section B
Label b (m)
d (m)
No. b (m)
d (m)
No.
A
0.5
0.6
3
B,C
0.5
0.6
6
D
0.5
0.6
5
E,F
0.5
0.6
8
H
0.5
0.6
4
J
0.5
0.6
4
M
0.5
0.6
2
G,I
0.5
0.6
8
K,L
0.5
0.6
8
Column Section C
b (m)
d (m)
No.
% CHB
Infill
20
90
10
10
70
Column Section C
b (m)
d (m)
No.
% CHB
Infill
100
0
50
0
40
0
100
80
40
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Table 3. Input for Shear Walls
Frames along X-direction
Label
Wall Section A
w (m)
t (m)
No. Type w (m)
5
5.57
0.3
2
1
1
5.57
0.3
1
1
4'
5.57
0.3
1
1
Frames along Z-direction
Label
Wall Section A
w (m)
t (m)
No. Type w (m)
D'
4.7
0.3
1
1
G'
5.8
0.3
1
1
L'
5.8
0.3
1
1
F
2.9
0.3
1
1
G
2.9
0.3
1
1
Wall Section B
t (m)
No.
Type
w (m)
Wall Section C
t (m)
No.
Type
Wall Section B
t (m)
No.
Type
w (m)
Wall Section C
t (m)
No.
Type
2.54
0.5
h/d
5.1
Table 4. Input for Weight Computation
WEIGHT COMPUTATION
Level
6
5
4
3
2
1
Add'l Loads
Load 1
Load 2
Load 3
Load 4
Area (m2)
height of 1st level , h (m) =
average col. dimension (m) =
DL (kPa)
w (kN)
1011
1011
1011
1011
Description
Storage/warehouse 25% LL
Partitions
Equipment
Others
Total Weight at Ground Floor
4
5
5
5
0.00
0.00
4042.88
5053.60
5053.60
5053.60
6
5
4
3
2
1
0
h
0.00
19203.68
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4) Based on the input data, the columns and shear walls are grouped automatically. Columns are
grouped as long columns, short columns or extremely short columns based on the h/d ratio and
% CHB infill. Shear walls, on the other hand, are grouped as Wall Type 1, 2 or 3. The sums of
the cross-sectional areas of the columns and walls per group are then computed for two
directions (x and z) as shown in Table 5.
Table 5. Grouping of Columns and Walls
SUMMARY OF SHEAR AREAS (m2) OF COLUMNS AND WALLS
Frames along X -direction
Label
COLUMNS
WALLS
Long
Short
Extreme
Type 1
Type 2
1
0.00
3.00
0.00
3.34
0.00
2
0.00
0.00
1.20
1.67
0.00
3
0.00
3.00
0.00
1.67
0.00
4
0.00
3.90
0.00
0.00
0.00
5
0.00
0.00
3.90
0.00
0.00
6
0.00
0.00
0.00
0.00
0.00
7
0.00
0.00
0.00
0.00
0.00
8
0.00
0.00
0.00
0.00
0.00
9
0.00
0.00
0.00
0.00
0.00
10
0.00
0.00
0.00
0.00
0.00
Total
0.00
9.90
5.10
6.68
0.00
1
2
3
4
5
6
7
8
9
10
Frames along Z -direction
Label
COLUMNS
Long
Short
0.00
0.00
0.00
1.80
0.00
1.50
0.00
2.40
0.00
1.20
0.00
1.20
0.00
0.00
0.00
0.00
0.00
2.40
0.00
0.00
Total
0.00
10.50
Extreme
0.90
0.00
0.00
0.00
0.00
0.00
0.60
2.40
0.00
0.00
3.90
Type 1
1.41
1.74
1.74
0.87
0.87
0.00
0.00
0.00
0.00
0.00
6.63
WALLS
Type 2
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Type 3
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Type 3
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
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5) Table 6 shows the results of the computation of the strength indices, Cc, Ce and Cw based on
the total area and the average strength per group and the total weight of the building at ground
level. The structural performance indices, Io, for both directions are then computed using the
applicable formula. The smaller value of Io is then used as the structural performance index of
the building.
Table 6. Strength Index and Structural Performance Index Computation
INDEX CALCULATED FOR GROUND FLOOR
Element Type
19204
w(kN) =
COL
COL
COL
WALL
WALL
WALL
COL
COL
COL
WALL
WALL
WALL
LONG
SHRT
E.SHRT
TYP1
TYP2
TYP3
LONG
SHRT
E.SHRT
TYP1
TYP2
TYP3
X
X
X
X
X
X
Z
Z
Z
Z
Z
Z
0.0
Direction of EQ
Area (m2)
S. Stress (kN/m2)
S. Force (kN)
Strength Index: C
0.0
9.9
5.1
6.7
0.0
0.0
687
981
1472
981
1962
2943
0
9712
7505
6557
0
687
0
0
10.5
3.9
6.6
0.0
0.0
981
1472
981
1962
2943
10301
5739
6504
0
Cc
Ce
Cw
Cc
Ce
Cw
0.51
0.39
0.34
0.54
0.30
0.34
I1
I2
Index wrt X & Z
Index
0.70
N/A
0.70
0
0.71
N/A
0.71
0.7
3. NUMERICAL APPLICATION
To understand the factors which affect the value of the Io –index, the numerical procedure is
applied to the building with the ground floor plan shown in Figure 2. Assuming a dead load of 5
kPa and %CHB infill less than 30%, the Io –index is computed for the building using column
dimensions of 0.3 m and 0.4 m for different number of stories.
A
B
5.0 m
D
C
4.0 m
5.0 m
3
5.0 m
Clear height = 2.5
Square columns
2
Z
2.0 m
1
X
Figure 2. Ground Floor Plan of a Building
9
( D L = 5 kN , 0% CHB infill)
No. of Stories
6
d = 0.30 m
d = 0.40 m
5
4
3
2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Seism ic Structural Perform ance Index
Figure 3. Index vs. No. of Stories for the Building in Figure 2.
Figure 3 shows the Io –index for the building in Figure 2 for different number of stories and
for column dimensions of 0.3 m and 0.4 m. Observe that the index decreases with an increase of the
number of stories because of the increase in the weight of the building being supported by the
columns. The building with a 0.4 m x 0.4 m columns has a significantly larger index than the
building with 0.3 m x 0.3 m columns. For the building with 0.3 m x 0.3 m columns, the index at two
stories is 0.8 and was reduced to 0.25 at six stories, while for the building with 0.4 m x 0.4 m
columns, the index is reduced from 1.35 at two stories to 0.40 to six stories.
Based on the example given, it is important that the dimensions of the columns and walls
must be accurately measured in the site and compared with the plans (if available). Moreover, the
engineer must reasonably estimate the building weight based on the inspection of the materials used
in the floors, partitions and roof of the building.
The present Level-1 AIJ screening procedure is presently being applied to existing RC
buildings in Metro Manila. Results of the brief diagnosis of existing RC buildings using this
method will be presented in a future report. Further studies in the future (including post-earthquake
damage inspections) should also aim to check the limitations of the method.
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REFERENCES
Nakano, Y. (1986). “Seismic Capacity of Existing Reinforced Concrete Buildings,” Proc. 7th
Japanese Earthquake Engineering Symposium, pp. 2041-2046.
Hirosawa, M., Sugano, S. and Kaminosono, T. (1995). Essentials of Current Evaluation and
Retrofitting for Existing and Damaged Buildings in Japan, Japan International Cooperation Agency
(JICA), Japan
Paulay, T. and Priestley, M. (1992). Seismic Design of Reinforced Concrete and Masonry Buildings.
Sect. 7.4, pp. 584-594, Wiley and Sons, Canada.
Umemura, H. (1980). “A Guideline to Evaluate Seismic Performance of Existing Medium- and
Low-Rise Reinforced Concrete Buildings and Its Application,” Proc. 7WCEE, pp. 505-512.
ABOUT THE AUTHORS
Andres W.C. Oreta, D. Eng., M. ASEP, M.PICE
Associate Professor in Civil Engineering
De La Salle University, 2401 Taft Ave., Manila
Tel/Fax: (632) 5240563, Tel. (632) 5244611 loc. 226, Email: [email protected] or
[email protected]
Benito M. Pacheco, PhD, PE, F.ASEP, M.PICE, M.ASCE
President and Co-founder of Vibrametrics, Inc. Technology Resource Group
Professorial Lecturer, Dept. of Civil Engineering, UP-Diliman, QC
Special Lecturer in Earthquake Engineering, PUP, Manila
Tel/Fax: (632) 4260044, Email: [email protected]
William T. Tanzo, D.Eng.
Vice-President and Co-founder of Vibrametrics, Inc. Technology Resource Group
Associate Professor, Dept. of Civil Engineering, UP Los Banos, College, Laguna
Professorial Lecturer, Dept. of Civil Engineering, DLSU-Manila
Tel/Fax: (632) 4260044 or (6349) 5365614 Email: [email protected]
ACKNOWLEDGEMENT
The support of the engineers and staff of Vibrametrics, Inc. in the preparation of the paper and slide
presentation is deeply appreciated.
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