quasi-static and pseudo-dynamic
TRANSCRIPT
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Quasi-static and pseudo-dynamic testing of infilled RC frames retrofitted
with CFRP material
H. Ozkaynak a, E. Yuksel a,, O. Buyukozturk b, C. Yalcin c, A.A. Dindar d
a Faculty of Civil Engineering, Istanbul Technical University, Istanbul, Turkeyb Civil and Environmental Eng., Massachusetts Institute of Technology, MA, USAc Department of Civil Engineering, Bogazici University, Istanbul, Turkeyd Department of Civil Engineering, Istanbul Kultur University, Istanbul, Turkey
a r t i c l e i n f o
Article history:
Received 21 April 2010
Received in revised form 5 July 2010
Accepted 16 November 2010
Available online 23 November 2010
Keywords:
A. Carbon fiber
B. Plastic deformation
B. Strength
B. Retrofitting
a b s t r a c t
The intact infill walls in reinforced concrete (RC) frames have beneficial effects to overall behavior in
terms of stiffness, strength and energy dissipation in the event of seismic actions. The rationale of this
paper is to increase effectiveness of the carbon fiber reinforced polymer (CFRP)-based retrofitting tech-
nique so that intact infill walls of vulnerable mid-rise RC buildings are transformed into a lateral load
resisting system. The seismic behaviors of cross-braced and cross diamond-braced retrofitting schemes
applied on infilled RC frames have been investigated experimentally. The research consisted of quasi-sta-
tic (QS) tests wheredrift-basedcyclic loading reversals were used and pseudo-dynamic (PsD) tests where
acceleration intensity-based loading was used. Twelve 1/3-scaled RC frames were built and tested as bare
and infilled control frames, and as cross-braced and cross diamond-braced retrofitted specimens. Signif-
icant findings were noted while comparing the QS and PsD tests. The maximum restoring force and drift
couples that were obtained from PsD tests showed a close behavior pattern, regardless of the level of
inertial masses, when compared with QS tests. The energy dissipation capacity of the specimens that
was obtained from PsD test resulted somewhat less than the one tested with QS for the same level of
damage. The performance of the retrofitted frames that was obtained from the experimental studywas evaluated with code-specified performance limits. Accordingly, it was concluded that the cross dia-
mond-bracing scheme is an effective retrofitting technique that brings the bare frame from collapse pre-
vention (CP) to life safety (LS) performance levels. Finally, analytical predictions as per FEMA 356
guideline were performed and good agreement was obtained with experimental results.
2010 Elsevier Ltd. All rights reserved.
1. Introduction
Past earthquakes showed that infill walls used in RC frames had
many advantages in terms of improvements in global stiffness, lat-
eral strength and energy dissipation capacities of the structures
when they are placed regularly throughout the structure and/or
they do not cause shear failures of columns, [1]. Several experi-
mental researches conducted on infilled RC frames also showed a
significant improvement in the overall behavior. Shake table tests
on infilled RC frames performed by Hashemi and Mosallam [2] re-
sulted that the infill walls increased the structural stiffness by
nearly four times, shortened natural period by nearly 50% and in-
creased the damping coefficient from 46% to 12%.
In many existing RC buildings, especially those designed and
built before the contemporary earthquake codes, there is a lack
of seismic detailing in structural load carrying system and struc-
tural members coupled with low material quality and workman-
ship [3]. Infill walls, during any credible earthquake, may
experience excessive damage and/or out of plane movements. Ret-
rofitting these walls using CFRP materials could further improve
the contribution of infills to the overall seismic behavior of the vul-
nerable RC buildings. Mosallamet al. [4] applied PsD test technique
to experimentally investigate a two-bay, two-storey gravity load-
designed steel frame infilled with unreinforced concrete block ma-
sonry walls. It was concluded that the imparted and hysteretic
energies correlated well with the observed damage state. It was
also concluded that the variation of these quantities with the in-
crease of PGA levels might be considered as a global measure to
quantify the damage state of the structure. Taghdi et al. [5] tested
four concrete block masonry and two RC walls simulating low-rise
non-ductile walls. Two masonry walls were unreinforced and two
were partially reinforced. One wall from each pair was retrofitted
using a steel strip system consisting of diagonals and vertical
strips. Stiff steel angles and anchor bolts were used to connect
the steel strips to the foundation and top of loading beam. The tests
1359-8368/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.compositesb.2010.11.008
Corresponding author. Tel./fax: +90 212 285 6761.
E-mail address: [email protected] (E. Yuksel).
Composites: Part B 42 (2011) 238263
Contents lists available at ScienceDirect
Composites: Part B
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m p o s i t e s b
http://dx.doi.org/10.1016/j.compositesb.2010.11.008mailto:[email protected]://dx.doi.org/10.1016/j.compositesb.2010.11.008http://www.sciencedirect.com/science/journal/13598368http://www.elsevier.com/locate/compositesbhttp://www.elsevier.com/locate/compositesbhttp://www.sciencedirect.com/science/journal/13598368http://dx.doi.org/10.1016/j.compositesb.2010.11.008mailto:[email protected]://dx.doi.org/10.1016/j.compositesb.2010.11.008 -
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showed that the complete steel strip system was effective in signif-
icantly increasing the in-plane strength and ductility of low-rise
unreinforced and partially reinforced masonry walls, and lightly
reinforced concrete walls. Saatcioglu and Serrato [6] carried out
an experimental investigation on gravity-load-designed RC frames,
infilled with concrete block masonry. The aim of that study was to
develop a seismic retrofit strategy involving the use CFRP sheets.
The retrofit technique consisted of CFRP sheets, surface bonded
to the masonry wall, while also anchored to the surrounding con-
crete frame by means of specially developed CFRP anchors. The re-
sults indicated that the infilled frames without a seismic retrofit
developed extensive damage in the walls and surrounding frame
elements. Furthermore, the elastic rigidity was reduced consider-
ably resulting in softer structure and failure occurred in non-duc-
tile frame elements, especially in columns. Retrofitting using
CFRP sheets controlled cracking and increased lateral bracing while
improving the elastic capacity of the overall structural system. The
retrofitted specimens exhibited approximately three times in lat-
eral force resistance than that of control specimens. Erdem et al.
[7] conducted an experimental study on 1/3-scaled, two-story,
three-bay frames to compare two types of strengthening tech-
niques. One of the frames was strengthened with RC infill while
the other one was strengthened with CFRP-strengthened hollow
clay blocks. It was observed that both strengthened frames be-
haved similarly under reversed cyclic lateral loading. The stiffness
of the strengthened frames was at least 10 times than that of the
bare frame. Although the strengths of both specimens were almost
the same, the strength degradation of the CFRP retrofitted frame
beyond the peak lateral force level was more pronounced. Almu-
sallam and Al-Salloum [8] investigated the effectiveness of glass fi-
ber-reinforced polymers (GFRP) in strengthening of unreinforced
masonry infill walls in RC frames which are subjected to in-plane
seismic loading. Test results showed great potential for externally
bonded GFRP sheets in upgrading and strengthening the infill
walls. Wei et al. [9] studied the response of different FRP orienta-
tions on the masonry wall elements. It was concluded that the
diagonally-meshed specimen had a greater ductility than others.Binici et al. [10] developed an efficient CFRP retrofitting on hollow
clay brick infill walls which could be utilized as lateral load resist-
ing elements. The practical retrofitting scheme was developed to
limit the inter-storey deformations with CFRP-strengthened infill
walls that were integrated to the boundary frame members by
means of CFRP anchors. It was observed that the CFRP retrofitting
reduced the damage-induced deficient columns by means of con-
trolling storey drifts. Yuksel et al. [11] tested infilled RC frames
with and without retrofitting. The effect of various CFRP retrofit-
ting schemes was discussed. They concluded that the cross bracing
and cross diamond-bracing type of retrofitting had more advanta-
ges compared with the others.
The rationale of this paper is to increase the efficiency of the
CFRP-based retrofitting technique in which the infill walls of vul-nerable mid-rise RC buildings could be transformed into a lateral
load resisting system. In order to achieve this goal, CFRP sheets
were used in two different schemes applied on hollow clay brick
infill walls.
The main objective of this study is to determine the seismic per-
formance of the CFRP-based retrofitted infilled RC frames. The seis-
mic performance enhancement was evaluated in terms of various
PGA levels as the input acceleration, maximum inter-storey drifts,
energy dissipation capacities, variation of strength and stiffness
and the observed damages. The performance of retrofitted RC
frames obtained through the experimental study was evaluated
with the code-specified performance levels. Also, analytical predic-
tions were made following the FEMA 356 formulations.
The scope of this study included testing of twelve 1/3-scaled in-filled RC frames. The lateral forces representing the seismic effects
were applied to the specimen in its own plane. Two testing tech-
niques, namely quasi-static (QS) and pseudo-dynamic (PsD), were
applied to the specimens. Two different inertia forces correspond-
ing to the masses exerted on higher and lover stories of a mid-rise
RC building were used in the PsD tests.
2. Description of test specimens
An experimental study was conducted on twelve identical 1/3-
scaled RC infilled frame specimens. The specimens were one-bay
and one-story type, and loaded laterally from top of column loca-
tions [12,13]. Four of these specimens were tested using QS test
method with drift-based cyclic reversals. The PsD test method
was carried out for the remaining eight specimens with low and
high inertial masses representing lower and upper storey of a
mid-rise RC building. The test program is summarized in Table 1.
2.1. Details of test specimens
The specimens were designed to reflect the old construction
practice including poor reinforcing detailing in and around the
beam-column connections. The dimensions of the test specimensand reinforcing details are given in Fig. 1. Scaled dimensions of
each test frame are 1533 1000 mm with cross-sectional dimen-
sions of 100 200 mm for columns, 100 200 mm for beams
and 300 700 mm for foundation. Typically, longitudinal rein-
forcement ratio in columns and beam was taken as 1% while trans-
verse reinforcement ratio was taken as 0.4%. No confinement
reinforcement in and around the beam-column connections were
used. Hollow brick material was used in the infill wall which had
dimensions of 88 84 57 mm, and was produced specifically
for this study in order to respect the geometric scaling of 1/3.
Specially-designed concrete mixture with small-diameter
aggregates of 10 mm and super plasticizer were used in order to
be consistent with the scaling factor and workability condition.
The specimens were casted at once in two stages; first foundationsthen followed by the frame elements.
Average compression strength of the concrete was obtained as
19 MPa from the standard cylinder tests. Yield strength of the rein-
forcing bars was obtainedas 420 MPaand 500 MPafor 8 and 6 mm
diameters, respectively.
Unidirectional carbon fiber-reinforced polymers were used in
the retrofitted specimens. As per the technical data provided by
the manufacturer, the unit weight of the CFRP is 300 g/m2, the fiber
density is 1.79 g/cm3 and the modulus of elasticity of CFRP is
230 GPa. Tensile strength and ultimate elongation capacities are
3900 MPa and 1.5%, respectively. A two-component mixture epoxy
resin was used with specified amount of 1.0 kg/m2.
Compression and shear tests were performed on 350 350
70 mm-sized bare and CFRP retrofitted wall specimens. CFRP retro-
fitting was applied in two different schemes: completely covered
and strips applied on both faces of the specimens. The tests
performed on the bare samples yielded compression strengths of
5.0 and 4.1 MPa in the two main directions and a shear strength
Table 1
Summary of test program.
Test program
Quasi-static tests Pseudo-dynamic tests
Low inertia mass M1 High inertia mass M2
Q1. Bare frame PL1. Bare frame PH1. Bare frame
Q2. Infilled frame PL2. Infilled frame PH2. Infilled frame
Q3. Cross-braced frame PL3. Cross-braced frame PH3. Cross-braced frame
Q4. Cross diamond-
braced frame
PL4. Cross diamond-
braced frame
PH4. Cross diamond-
braced frame
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100
200
400
800
200
100
700
400
1000
1400mm
b
b
aa
Section a-a
Section b-b
5 12
5 12
2 12
4 86/140 1
00
100 200 933 200 100
1533 mm
4 86/140
200
4 86/140
1400
Fig. 1. Reinforcement details of 1/3-scaled RC frame.
100 200 933 200 100
1533 mm
400
800
200
1400mm
100 200 933 2001001533 mm
400
800
200
1400mm
(b) Infilled Wall(a) Bare Frame
400
100
600
300
1400mm
300
1333
100 315 703 315 1001533 mm
465
304
304
311150
150
737 300 320 693 3201333
400
100
600
300
1400mm
100 200 120 234 224 234 120 200100
1533 mm
282
282
461
461
150
(c) Cross-Braced (d) Cross Diamond-Braced
Fig. 2. Geometry of the specimens.
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of 0.95 MPa. The full surface covered wall specimens yielded
compression strengths of 9.2 and 5.5 MPa in the two main direc-
tions with a shear strength of 2.2 MPa while the strip type CFRP
retrofitted samples yielded shear strength of 1.3 MPa.
2.2. Specimen types
Description of four different test specimens used in the experi-mental work is given in Fig. 2.
The cross diamond-bracing scheme is aimed to prevent the out
of plane movement of the infill wall. By using knee and cross brac-
ing together, additional forces due to the retrofitting are avoided to
be transferred to vulnerable RC beam-column joints. This is a
shortcoming of the cross-bracing type of retrofitting scheme.
Fig. 3 demonstrates the steps of the CFRP application. As sug-
gested by the manufacturer, after a proper surface preparation
was made, a primer coating was applied and CFRP sheets were
bonded to the surface by using a special two-component epoxy
resin. Anchorages were provided along the CFRP sheets having a
width of 150 mm, at approximately quarter distances of the
diagonal.
2.3. The test set-up and instrumentation
The lateral loading system is consisted of a servo-controlled
280 kN-capacity hydraulic ram which was positioned at the tip
of the specimen aligned with the central axis of the beam. The
actuator was fixed to the specimen tightly by using two post-ten-
sioned rods of 20 mm in diameter. Equal tightness was controlled
by strain gauges for all specimens. The footing of the specimen was
fixed to the rigid steel beamof the test frame which was connected
to the laboratorys strong floor by means of post-tensioned rods.
Possible out of plane movement of the specimens was prevented
by using special restrainers which were placed at both sides of
the test set-up. Fig. 4 illustrates the test set-up, schematically.
Load cell to measure the restoring forces was attached to the
actuator. Several strain gauges having post-yield capabilities and
displacement transducers were positioned on the specimens. Top
displacement measurements of the system (1617), end rotations
from the displacement measurements (1212) and strain mea-
surements on longitudinal reinforcements at member ends were
conducted. Global movement (15) and rocking of the foundation
(1314) and out of plane movements of the frame (1819) were
also monitored throughout the tests.
Additionally, a very high resolution optical displacement trans-
ducer which is essentially used in the PsD tests was positioned
aligning the centre of the beam. Typical instrumentation scheme
is shown in Fig. 5.
2.4. Lateral loading cycles
Two types of lateral loading were applied to the specimens. In
QS tests, various drift cycles were applied to the specimens as
shown in Fig. 6. Gradual incremental drifts were selected in order
(a) Surface preparation (b) Primer application
(c) CFRP application (d) Anchorage application
(e) Cross bracing scheme (f) Cross diamondbracing scheme
Fig. 3. The application of CFRP to the specimen.
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to be consistent with the typical loading patterns used in the liter-
ature as well as encapsulating the values specified in TEC [14] for
various performance levels.
The PsD testing method, which is utilized since 1970s, leads
towards more realistic understanding on the nonlinear behavior
of specimens, while the mass and viscous damping properties
are pre-defined, the restoring force is measured directly from
the test specimen. The equation of motion (EQM) given in Eq.
(1) is solved numerically by using the explicit method of finite
difference, [15]. The general flowchart of the test procedure isgiven in Fig. 7.
mfxg cf _xg ffg mf1gfxgg 1
in which x, m, c, f and xg are displacement, mass, viscous damping
ratio, restoring force and ground acceleration, respectively.The cal-
culated displacement xi+1 is applied to the specimen by means of
very sensitive control procedure consisting of an optical displace-
ment transducer and the control unit. The numerical integration
time intervals selected as 0.005 s which is half of the ground accel-
eration time steps.
Thepart between8 and18 s ofBol090 component ofDuzce Earth-
quake [16] was selected in this study. The part of the original recordwas modified to attain an acceleration spectrum comparable to the
one defined in TEC [14] for seismic Zone 1 and soil class Z2. The ob-
tained acceleration record is referred as design earthquake, Fig. 8.
Three earthquake scenarios are defined in TEC. These are service,
design and greatest earthquakes. The probability of exceedence in
50 years is 50%, 10% and 2% with the return periods of 72, 474
and 2475 years, respectively. The PGAs of these earthquakes are
0.2 g, 0.4 g and 0.6 g, respectively.
3. Quasi-static test results
3.1. Experimental response of bare frame
First flexural cracks were observed at 0.25% drift level. The cor-responding restoring force was 20.6 kN. At 1.5% drift, first yielding
Hydraulic Actuator
Hinge
Strong Wall
Strong Floor
Load Cell
Loading Frame
Steel Reaction Frame
Hinge
Out of Plane Restrainers
Fig. 4. The test set-up.
3 4
5
6
1 2 11 12
9 10
7
8 16
1314
15
1718 19
Fig. 5. Strain gauges (left) and displacement transducers (right) placed on various sections of the specimen to measure the deformations and displacements.
Fig. 6. The incremental drifts used in quasi-static tests.
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of the longitudinal reinforcement was observed with the maxi-
mum crack width of 1.8 mm and the corresponding restoring force
was 41.56 kN. At 3% drift, the buckling of re-bars occurred and the
restoring force was slightly reduced to 40.2 kN. Test ended at the
limits of the actuators stroke. Fig. 9 shows the forcedisplacement
relationship and the strain variation in the longitudinal rein-
forcement in bottom section of the column. Fig. 10 illustrates typ-
ical observed crack patterns and damage states on the backbonecurve.
3.2. Experimental response of infilled frame
First symmetric flexural cracks were observed at 0.15% drift le-
vel on RC members. First diagonal crack occurred on the infill wall
at 0.7% drift. The failure mode was mainly spalling of concrete at
bottom level of the column. The separation of infill wall from RC
members was observed first at 0.25% drift level. At 4% drift level
the corner crushing was highly dominated. Test ended at the limitsof the actuators stroke. Fig. 11 shows the forcedisplacement
Ground
acceleration
data
xi+1 target displacement is calculated and applied to the test specimen
Reading the restoring force from the test specimen and
Store it for the next step
Solve the EQM by numerical integration method of finite difference
Input the initial dynamic properties and ground acceleration
Data logger is triggered and the data measured form
the strain gauges and transducers on the test specimen are
stored by a software in the computer
Calculation of velocity and acceleration for this step
Fig. 7. General flowchart of pseudo-dynamic test.
Fig. 8. The modified Duzce/Bolu090 earthquake and its acceleration spectrum.
Fig. 9. Load displacement relationship and strains of column longitudinal reinforcement.
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relationship and the strain variation in the longitudinal reinforce-
ment in bottom section of the column. Fig. 12 illustrates the typical
observed crack pattern and damage states on the backbone curve.
3.3. Experimental response of cross-braced frame
First flexural crack was observed at 0.15% drift and its corre-sponding restoring force was measured as 41 kN. First separation
of infill wall from RC members was observed at 0.25% drift level.
At this drift level, CFRP partly debonded from the infill wall. The
first diagonal crack having a width of 0.2 mm was observed at
0.65% drift level. There was a substantial decrease in infill walls
damage compared with the infilled frame. After spalling of con-
crete at bottom level of columns there was a sudden decrease in
the lateral load carrying capacity of the specimen. At 2% drift, therestoring force was measured to be 107.3 kN and tearing of the
a'>3.5
b'=0.1
c'=0.2
i'3.5
a
c=0.1
d'=0.1
k=0.2
e3.5
b'>3.5
c'=0.1
d'=0.1
e'=0.1
f '=0.2
k'>3.5
g'>3.5
l'>3.5
j'>3.5
j>3.5
l>3.5
i>3.5
h'>3.5h>3.5
i'>3.5
a>3.5
e=0.2
b=1.8
f=0.4
c=0.4
d=0.2
k>3.5
g>3.5
PULLPUSH
Fig. 12. Crack formations and observed damage states.
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CFRP sheet was observed. The maximum strain measured on the
diagonal CFRP sheets was about 0.005. After tearing of the CFRP
sheets, the corner crushing mode of failure was dominant. At 3%
drift, there were shear cracks on the joints. Test ended due to
excessive decrement of lateral force level. Fig. 13 shows the
forcedisplacement relationship and the strain variation in the lon-
gitudinal reinforcement of the bottom column section. Fig. 14 illus-
trates the typical observed crack pattern and the damage stages onthe backbone curve.
3.4. Experimental response of cross diamond-braced frame
The first flexural crack and yielding of longitudinal reinforce-
ment were occurred at 0.15% and 0.8% drifts, respectively. The first
diagonal crack on infill wall was at 1.5% drift. The first separation of
knee bracing from the infill wall started when the drift ratio
reached to 0.15%. At the same drift level, the first separation of in-
fill wall from the beam was observed. At 2.5% drift, the upper kneebracing sheets torn to pieces. At 3.3% drift, the buckling of CFRP
Fig. 13. Load displacement relationship and strains of column longitudinal reinforcement.
d>3.5a3.5
i>3.5g=0.2
f=0.6
l=0.2d'>6.0
a'3.5
f '=0.4
e'>10
g'>10 h'=0.5
PULLPUSH
Fig. 14. Crack formations and observed damage states.
a'=9.0
b'=1.0
i'>3.5
d'>3.5
j'>3.5
c'>3.5
g'>3.5
m'>3.5
n'=0.3
l'>3.5
k'>3.5
e'>>3.5
f '>3.5
a=13
b=1.5
d>3.5g=16
c=24
l>3.5
k>3.5
j>3.5
m>3.5
e=9.0
f=3.0
PULLPUSH
Fig. 15. Crack formations and observed damage states.
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sheets between the anchorage points was occurred. At 4.0% drift,
shear cracking observed at bottom of the compression column.
Test ended due to excessive decrement of lateral force level.
Fig. 15 shows the forcedisplacement relationship and the strain
variation in the longitudinal reinforcement of the bottom column
section. Fig. 16 illustrates typical observed crack pattern and dam-
age states on the backbone curve.
4. Pseudo-dynamic test results
PsD tests were performed with two groups of specimens. First
group consisting of four specimens was tested with the lower iner-
tial mass of M1 = 0.0085 kNs2/mm while the second group includ-
ing four specimens was tested with the higher inertial mass of
M2 = 0.0221 kNs2/mm.
To define the initial parameters used in PsD tests i.e. lateral
stiffness and viscous damping, low intensity sine wave excitation
was introduced to the specimens. Table 2 summarizes the experi-
mental results obtained in this preliminary test. The initial stiffness
(Kin) was determined from the slope of specimens elastic loaddis-
placement response. The equivalent viscous damping (fin) was cal-
culated using energy loss per cycle.Although the experimentally obtained viscous damping ratios
were greater than 5%; damping matrix [c] in PsD algorithm is
formed by using the constant equivalent viscous damping ratio
of 5%.
4.1. Experimental response of bare frame
4.1.1. M1 mass condition
The maximum base shear and lateral top displacement were
measured as 28 kN and 7 mm, respectively, in PGA = 0.2 g case.
The flexural cracks of 0.2 mm width were observed at the column
ends. Re-bars at that location were close to yielding. In PGA = 0.4 g
loading, the maximum base shear and top displacement were mea-
sured as 35 kN and 18 mm, respectively, while the flexural crackwidths reached to 2.5 mm. The maximum base shear and top dis-
placement were measured as 36 kN and 35 mm, respectively, in
PGA = 0.6 g case.
4.1.2. M2 mass condition
The maximum base shear and lateral top displacement were
measured as 32.2 kN and 17.35 mm, respectively, in PGA = 0.2 g
case. Nearly 3.5 mm width cracks through the columns were ob-
served. For PGA = 0.4 g case, the maximum base shear was 28 kN
and the maximum top displacement was 88.0 mm. The crack
widths were about 15 mm through the columns. Due to the ob-
served severe damages the test was ended at this level.
Load vs. top displacement hysteresis for M1 and M2 cases are gi-
ven in Fig. 17. The damage patterns and strain records at the rep-
resentative column cross section are also illustrated in Figs. 18 and
19.
4.2. Experimental response of infilled frame
4.2.1. M1 mass condition
The maximum base shear and lateral top displacement were
28 kN was and 1.3 mm, respectively, in the case of PGA = 0.2 g.
There were about 0.1 mm width flexural cracks at the column
ends. For PGA = 0.4 g, the maximum base shear was 60 kN and
maximum top displacement was 2.4 mm. The crack widths were
approximately 0.15 mm through the columns at this stage. The
maximum base shear and top displacement were 90 kN and
3.6 mm, respectively. The crack width increments were
observed.
4.2.2. M2 mass condition
The maximum base shear and lateral top displacement were
measured as 92 kN and 4.42 mm, respectively. The flexural typecracks with 0.8 mm width occurred at column ends. All of the lon-
gitudinal reinforcements at the critical sections remained in the
elastic range. For PGA = 0.4 g, the maximum base shear and top
displacement were measured as 112.4 kN and 5.48 mm, respec-
tively. The flexural crack widths reached to 3.5 mm while the lon-
gitudinal re-bars of columns were yielded.
Load vs. top displacement hysteresis for M1 and M2 cases are gi-
ven in Fig. 20. The damage patterns and strain records at the rep-
resentative column cross section are also illustrated in Figs. 21
and 22.
Fig. 16. Load displacement relationship and strains of column longitudinal reinforcement.
Table 2
Initial dynamic properties of test specimens.
Mass Test specimen Kin (kN/mm) fin (%)
M1 PL1. Bare frame 9 6.0
PL2. Infilled frame 28 12.0
PL3. Cross-braced frame 60 8.0
PL4. Cross diamond-braced frame 65 12.5
M2 PH1. Bare frame 8 4.9
PH2. Infilled frame 32 10.0
PH3. Cross-Braced frame 62 9.0
PH4. Cross diamond-braced frame 70 12.0
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4.3. Experimental response of cross-braced frame
4.3.1. M1 mass condition
The maximum base shear and lateral top displacement were32 kN and 0.7 mm, respectively in PGA = 0.2 g case. The maxi-
mum crack widths observed was 0.1 mm on the columns. For
PGA = 0.4 g, the maximum base shear was 69 kN and top
displacement was 1.3mm. In the case of PGA = 0.6g, the
maximum base shear was 84 kN and top displacement was2.2 mm.
PGA M1 Mass M2 Mass
0.2g
0.4g
0.6g
Fig. 17. Load displacement relationships for bare frame.
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PGA Cumulative Damage Propagation Strain at Bottom of Column
0.2g
PULL
PUSH
a1=0.5a1'=0.5
b2=0.1
c2=0.1
d2=0.2
b2'=0.1
c2'=0.1
d3'=0.2
e3'=0.1
f3'=0.1
0.4g
a1=2.5a1'=2.5
b2=0.1
c2=2.0
d2=2.5
b2'=0.1
c2'=0.1
d3'=2.5
e3'=0.1
f3'=0.1 e4=0.1
g4=0.1
f4=0.1
h4=0.1
PULL
PUSH
0.6g
a1=0.5a1'=>3.5
b2=0.1
c2=1.2
d2=>3.5
b2'=0.1c2'=0.1
d3'=>3.5
e3'=1.0f3'=0.1
e4=1.2
g4=0.1f4=0.1
h4=>3.5
PULL
PUSH
Fig. 18. The damage patterns and strain records at the column section for M1 case.
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4.3.2. M2 mass condition
The maximum base shear and lateral top displacement were
measured as 70.8 kN and 1.2 mm in PGA = 0.2 g case. The flexural
cracks with 0.1 mm in width observed at the columns. For
PGA = 0.4 g loading, the maximum base shear and top displace-
ment were measured as 117.9 kN and 8.2 mm, respectively. InPGA = 0.6 g case, the maximum base shear and top displacement
were recorded as 121 kN and 17.1 mm, respectively. At this stage,
after debonding of CFRP, the specimen lost its lateral strength
significantly.
Load vs. top displacement hysteresis for M1 and M2 cases are
given in Fig. 23. The damage patterns and strain records at the rep-
resentative column cross section are also illustrated in Figs. 24 and
25.
4.4. Experimental response of the cross diamond-braced frame
4.4.1. M1 mass condition
The maximum base shear and lateral top displacement were
30 kN and 1.3 mm, respectively, in PGA = 0.2 g case while 62 kNand 2.1 mm, respectively, in PGA = 0.4 g case. For PGA = 0.6 g load-
ing, the maximum base shear and top displacement 90 kN and
were 3.4 mm, respectively. During the PGA = 0.4 g and 0.6 g load-
ings, some cracks observed at the end of columns as well as some
parts of the infill wall without CFRP application.
4.4.2. M2 mass conditionThe maximum base shear and lateral top displacement were
measured as 90.0 kN and 1.7 mm, respectively, during PGA = 0.2 g
loading. The observed flexural cracks width was 0.3 mmon the col-
umns. For PGA = 0.4 g case, the base shear and top displacement
were recorded as 136.9 kN and 5.6 mm, respectively. The flexural
crack width observed at the bottom sections of columns reached
to 3.0 mm. The maximum base shear and lateral top displacement
were measured as 130.0 kN and 12.0 mm, respectively, during
PGA = 0.6 g case. In this stage, the crack widths at the end of the
columns reached to 10 mm and the gaps at the wall column inter-
face was observed as 3.5 mm.
Load vs. top displacement hysteresis for M1 and M2 cases are gi-
ven in Fig. 26. The damage patterns and strain records at the rep-
resentative column cross section are also illustrated in Figs. 27 and28.
PGA Cumulative Damage Propagation Strain at Bottom of Column
0.2g
a1'=2.0
b1'=3.5
c1'=2.0
d1'=10.0
b1'>15.0
c1'=3.0
d1'=0.2
a1>3.5
b1>3.5
c1=0.7
e1'=2.0
f1'=0.2 h1'=0.2
e1=0.1
f1=0.1
d1=1.8
PULLPUSH
Fig. 19. The damage patterns and strain records at the column section for M2 case.
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5. Evaluation of the test results
The obtainedtest results were assessedin terms of loaddisplace-
ment relationship, stiffness and energy dissipation capacity.
Additionally, the test results were also evaluated in terms of perfor-
mance criteria defined in FEMA 356 [17]. In this framework; lateral
story drifts, plastic rotations measured at columnends andobserved
damages in terms of crack widths were identified in relation to the
performance levels of Immediate Occupancy (IO), Life Safety (LS)
and Collapse Prevention (CP). Table 3 summarizes the damage and
drift limits, Table 4 describes the plastic hinge rotation capacities
corresponding to the above-mentioned performance levels.
PGA M1 Mass M2 Mass
0.2g
0.4g
0.6g
Fig. 20. Restoring force vs. drift relation for infilled frame.
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5.1. Loaddisplacement relationships
For each type of specimen, envelope curve of restoring force vs.drift hysteresis in QS testing is given together in Fig. 29 with the
locus of maxima obtained from PsD tests. A significant increment
in strength is obtained in the retrofitted specimens, especially in
cross diamond-braced type which survived in PGA = 0.6 g case forM2 mass condition.
PGA Cumulative Damage Propagation Strain at Bottom of Column
0.2g
a1'=0.1
b1'=0.1
c1'=0.1
d1'=0.1
a1=0.1
b1=0.1
c1=0.1
d1=0.1
PULLPUSH
0.4g
a1'=0.15
b1'=0.15
c1'=0.15
d1'=0.5
a1=0.15
b1=0.15
c1=0.1
d1=0.5
f 3'=0.1
g3'=0.1
h3'=0.1
e3'=0.1
e3=0.1
f3=0.1
g3=0.1
i3'=0.1 h3=0.1
PULLPUSH
0.6g
a1'=0.25
b1'=0.25
c1'=0.25
d1'=0.8
a1=0.45
b1=0.45
c1=0.3f 3'=0.1
g3'=0.1
h3'=0.1
e3'=0.2
e3=0.3
f3=0.1
g3=0.1
4=0.1
d1=0.9
i3'=0.2 h3=0.2
PULL
PUSH
Fig. 21. The damage patterns and strain records at the column section for M1 case.
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Drift performance limits defined in FEMA 356 are also shown in
the plots of Fig. 29. One could observe that the scattered locus of
maxima obtained from PsD tests for bare frame is concentrated
within the ascending branch of the response curve which is limitedwith the IO region in the retrofitted specimens.
5.2. Stiffness
For each type of specimen, the stiffness envelope which is de-
fined as the slope of the line drawn from peak to peak response
coordinates, is given together with the stiffness points for various
PGA levels in PsD tests which are calculated as the ratio of maxi-
mum base shear and corresponding top displacement, Fig. 30.
The lateral stiffness calculated in QS and PsD tests are consis-
tent with each other. Also, the observed stiffness values calculated
for retrofitted specimens are well above the non-retrofitted ones.
It was observed that the stiffness values corresponding to vari-
ous PGA levels and mass conditions accumulated within the IO re-gion for the retrofitted specimens whereas these coordinates were
scattered in bare and infilled frames. This indicates the effective-
ness of the retrofitted specimens in terms of improved stiffness.
Fig. 31 shows variation of lateral stiffness of the specimens
throughout the successive PsD tests. The diagrams were normal-ized with the initial stiffness of the specimens. The rate of stiffness
degradation was higher in bare and infilled frames, and consider-
ably lower in the retrofitted frames. When the comparison was
made between the mass conditions, the one with the lower inertia
mass (M1) had low rate of stiffness degradation than that of the
higher inertia mass (M2). To highlight the lateral stiffness incre-
ments in the retrofitted specimens, the normalized initial stiffness
of infilled frame was also added as dashed straight line segments
on the same plots of Fig. 31. The stiffness degradation in the retro-
fitted specimens, relative to the infilled frame specimen resisted to
higher PGA levels. When the retrofitted specimens compared with
each other, the relative stiffness degradation in the diamond cross-
braced frame specimen was less than that in the cross-braced
frame specimen indicating an improvement within the retrofittedspecimens.
PGA Cumulative Damage Propagation Strain at Bottom of Column
0.2g
a1'=0.1
b1'=0.1
c1'=0.1
d1'=0.1a1=0.8
h1=0.1
c1=0.2
d1=0.3
b1=0.15
f1=0.3
e13.0
c1'=0.1
d1'=1.2a1>>
h1=0.8
c1=1.0
d1=0.4
b1=0.6
f1=0.8
e1>h1'=4.0
g1'=0.1
f1'=0.1
j1'=0.1
e1'=1.2 k1'3.5
k2'=1.2
i2=0.4
l2'>3.5j2>3.5
m2>>
n2=0.3
m2'=0.4
o2=0.2
n2'=0.1
PULLPUSH
Fig. 22. The damage patterns and strain records at the column section for M2 case.
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5.3. Energy dissipation capacity
The cumulative energy dissipations were calculated as the en-
closed area of restoring force vs. story drift hysteresis. The energy
dissipation capacity of the retrofitted specimens increased signifi-
cantly when compared with the non-retrofitted specimens as
shown in Fig. 32. If a comparison was made for 1% story drift, it
was obtained that the cross-braced frame dissipated 4.6 times
more energy than the bare frame. For the cross diamond-braced
frame, this ratio increased to 5.2.
As seen in Fig. 32, although the dissipated energy values ob-
tained from QS and PsD tests were close to each other and follow
a similar trend, it was observed that the PsD energy values were
slightly over the QS energy values. For the same level of drift, more
PGA M1 Mass M2 Mass
0.2 g
0.4g
0.6g
Fig. 23. Restoring force vs. drift relation for cross-braced infilled frame.
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PGA Cumulative Damage Propagation Strain at Bottom of Column
0.2 g
a1'=
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PGA Cumulative Damage Propagation Strain at Bottom of Column
0.2g
c1'=
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damage was observed in PsD tests than QS tests. This may also ex-
plain less energy dissipation in QS tests given that damage and en-
ergy dissipation is directly proportional to each other. Also, the
higher number of zero crossing in the loading pattern of PsD test
might result more accumulated energy dissipation and thus moredamage than that of QS test.
5.4. Performance evaluation
End rotations of the columns were generated from the displace-
ment measurements performed by the transducers shown in Fig. 5.
Consequently, the yield rotation hy of 0.004 radians was deter-mined. The plastic end rotations hp were calculated by subtracting
PGA M1 Mass M2 Mass
0.2g
0.4g
0.6g
Fig. 26. Restoring force vs. drift relation for cross diamond infilled frame.
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PGA Cumulative Damage Propagation Strain at Bottom of Column
0.2g
a1'=0.1 a1=0.1
PULLPUSH
0.4g
a1'=0.1 a1=2.0
b3'=0.15
e3'=2.0
d3'=3.0
c3'=0.2
b3=1.4
c3=0.2
PULLPUSH
0.6g
a1'=0.1 a1=2.0
b3'=0.3
e3'=2.0
d3'=30
c3'=0.2
b3=1.4
c3=0.2
f4'=0.5
h4'=0.1g4'=0.15
d4=0.15
PULLPUSH
Fig. 27. The damage patterns and strain records at the column section for M1 case.
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PGA Cumulative Damage Propagation Strain at Bottom of Column
0.2 g
PULLPUSH
a1'= 0.3 a13.5
b1' spalled
d2'>3.5
c1'=0.6
c2=0.1
k2'=0.1
b2=3.5
2=0.1
j2'=0.1
e2'=0.1
p2'=0.1
l2'=0.1 o2'=0.1
n2'=0.5
f2=0.6
f2'=0.2g2=0.1
e2=1.0
m2'=1.0
h2'=0.5
g2'=0.1
d2 spalled
r3'=0.2
s3'=0.1
u3'=0.7
w3'=0.1
PULLPUSH
Fig. 28. The damage patterns and strain records at the column section for M2 case.
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the maximum rotation hmax from hy. The maximum plastic rota-
tions hp and crack widths wmax obtained for various PGA levels
are compared with the criteria given in Tables 3 and 4 and the cor-
responding performance levels are presented in Table 5. Effective-
ness of the retrofitting schemes could be clearly seen in terms of
the obtained performance levels.
6. Analytical prediction of load bearing capacities
Previous studies of bare, infilled and CFRP-retrofitted infilled
frames with mostly cross-braced type of retrofitting scheme were
conducted by various researchers. Among them, Altin et al. [18]
and Kakaletsis and Karayannis [19] have conducted experimental
Table 3
Performance levels for primary elements of RC frames (according to FEMA 356).
Item Collapse prevention Life safety Immediate occupancy
CP LS IO
Damage Extensive cracking and hinge formation in ductile
elements. Limited cracking and/or splice failure in some
non-ductile columns. Severe damage in short columns.
Extensive damage to beams. Spalling of cover and shear
cracking
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studies on 1/3-scaled one bay-one story specimens that are
somewhat similar to this study.
Bare frame predictions could be derived analytically based on
strong beam-weak column assumption and occurrence of plastic
hinges at column ends. Thus, the lateral load capacity of a typicalone bay-one story frame could be predicted as 4Mp/h, where Mpis the flexural capacity of column cross section and h is the story
height. In this study, the lateral load capacity of the bare frame
was found experimentally as 40 kN. Whereas, the above formula-
tion yields 35 kN capacity.
Masonry infills have mainly three failure modes: Bed joint
crushing, diagonal crushing and corner crushing. These modes
are reliant with the aspect ratio of infill wall. The test specimens
used in this study had an aspect ratio close to 1.0 indicating
potential bed joint or diagonal crushing failure modes. According
to FEMA 356, the expected bed joint infill shear strength, Vine,could be estimated as Vine = Ani fvie,where Ani is area of net mor-
tared section across infill panel and fvie is expected shear strength
of masonry infill. Also, the diagonal crushing strength of infillwall is calculated as Vdcomp = aeff ti fci, where aeff is effective width
of the compression strut defined by FEMA 356, ti is thickness of
infill wall and fci is average compressive strength of the infill
wall. Thus, bed joint infill shear strength of the experimental
specimen was calculated as 75 kN by considering 0.95 MPa of
shear strength from material tests. On the other hand, the hori-
zontal component of the diagonal crushing resisting force was
calculated as 36 kN. Thus, the global shear resistance of the in-
filled frame is calculated as 35 kN + min (75, 36) = 71 kN. The
experimentally obtained lateral load capacity of infilled frame
was 80 kN.
The contribution of the CFRP cross bracing to the global shear
resistance could be derived as horizontal component of the tensile
tie force of the CFRP strip. Hence, the shear resistance could be cal-
culated as VCFRP= n (eCFRP ECFRP wCFRP tCFRP) cos h, where n is thenumber of CFRP strip, eCFRP is the ultimate strain of CFRP strip, ECFRPis the elastic modulus of CFRP strip, wCFRP is width of CFRP strip,
tCFRP is the theoretical thickness of CFRP strip calculated from theratio of weight per unit area to the density of carbon material,
andh is the inclination angle of CFRP strip. In this study, the follow-
ing data could be used to estimate the shear strength contribution
of CFRP retrofitting: n = 1, eCFRP= 0.006, ECFRP = 230,000 MPa, wCFRP= 150 mm, tCFRP = 0.17 mm and h = 44. The resulting contribution
is VCFRP= 3 5 k N. The global shear resistance of the retrofitted frame
is 71 + 35 = 106 kN. The corresponding experimental result of the
cross-braced retrofitted frame specimen was 120 kN. Cross dia-
mond-braced frames shear capacity was experimentally obtained
as 150 kN, which is higher than the above prediction.
7. Conclusions
Twelve 1/3-scaled RC frames were built and tested as bare andinfilled control specimens, and as cross-braced and cross diamond-
braced retrofitted specimens. Two testing techniques, namely qua-
si-static (QS) and pseudo-dynamic (PsD) tests were applied to the
specimens. Two different inertia forces corresponding to the tribu-
tary area masses obtained from higher and lower stories of a typ-
ical mid-rise building were used in the PsD tests. Based on the
results of the experimental work, the following conclusions could
be drawn:
1. A significant seismic performance enhancement in cross-braced
and cross diamond-braced frames was determined in terms of
inter-storey drift, lateral load capacity, energy dissipation
capacity, stiffness and the observed damages.
(a) Bare frame (b) Infilled frame
(c) Cross-Braced frame (d) Cross Diamond-Braced frame
Fig. 30. QS vs. PsD test results with drift-based performance limits according to FEMA 356.
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2. The locus of maxima obtained from PsD tests showed a close
behavior pattern, regardless of the level of inertial masses, com-
pared with the envelope curve of restoring force vs. drift hyster-
esis in QS tests.
3. According to the damage observations from both test tech-
niques, PsD caused more damage in primary and secondary
elements than QS tests for the same level of drifts.
4. The cumulative energy dissipation was found to be compara-
tively less in QS tests for the varying drift ratios due to the
greater number of reverse cycles used in PsD tests.
5. However in the case of infilled frame the maximum strengths
for M1 and M2 masses were observed at PGA = 0.6 g and 0.2 g,
respectively; where for cross diamond-braced frame the maxi-
mum strengths for M1 and M2 inertial masses were observed
at PGA = 0.6 g and 0.4 g, respectively.
6. According to performance evaluations, the cross diamond-brac-
ing type of retrofitting scheme proved to be an effective tech-
nique in transforming the bare frame from Collapse Prevention
to Life Safety performance level for design earthquake of
PGA = 0.4 g.
Spec. M1 Mass M2 Mass
BareFrame
InfilledFrame
Cro
ss-BracedFrame
DiamondCrossBraced
Frame
Fig. 31. Variation of the specimens lateral stiffness throughout the successive PsD tests.
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7. The analytical predictions were within the proximity of the
experimental results for bare, infilled and cross-braced retrofit-
ted frames. However, cross diamond-braced frame, has shown
comparatively higher shear resistance.
Acknowledgments
This study was conducted at the Structural and Earthquake
Engineering Laboratory of Istanbul Technical University. It was
sponsored by research Projects 106M050 of the Scientific and
Technological Research Council of Turkey (TUBITAK) and 31966
of Istanbul Technical University (ITU) Research Funds. The contri-
butions of M.Sc. E.S. Tako, I. Bastemir and Hakan Saruhan to theexperimental works are gratefully acknowledged.
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M1_0.6gM2_0.2gM2_0.4gM2_0.6g
IO LS CP
(c) Cross-Braced frame (d) Cross Diamond-Braced frame
Fig. 32. QS vs. PsD test results with drift-based performance limits according to FEMA 356.
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