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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

H. Ozkaynak et al. / Composites: Part B 42 (2011) 238263 239


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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.

240 H. Ozkaynak et al. / Composites: Part B 42 (2011) 238263


4/26

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.



5/26

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.



6/26

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.



7/26

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.



8/26

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


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


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




9/26

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.



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



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.



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.



resentative column cross section are also illustrated in Figs. 21

and 22.


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



10/26

4.3. Experimental response of cross-braced frame


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.



11/26

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.



12/26



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



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.



resentative column cross section are also illustrated in Figs. 27 and28.


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




13/26

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.



14/26

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.


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




15/26

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.


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




16/26

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.



17/26


0.2 g

a1'=


18/26


0.2g

c1'=


19/26

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.



20/26


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




21/26


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




22/26

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


23/26

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.



24/26

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.



25/26

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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Mass Specimen PGA = 0.2 g PGA = 0.4 g PGA = 0.6 g PGA = 0.2 g PGA = 0.4 g PGA = 0.6 g

hpmax wmax hpmax wmax hpmax wmax Performance levels

Radian mm Radian mm Radian mm

M1 Bare frame 0.000 0.2 0.008 2.5 0.012 >5.0 IO LS CP

Infilled frame 0.000 0.1 0.000 0.2 0.001 2.0 IO IO LS

Cross-braced frame 0.000 0.1 0.000 0.8 0.001 1.0 IO IO IO

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0

5000

10000

15000

20000

0 1 2 3 4 5

CumulativeDissipatedEnergy(kN.mm)

Story Drift (%)

QS

M1_0.2gM1_0.4g

M1_0.6g

M2_0.2g

M2_0.4g0

5000

10000

15000

20000

0 1 2 3 4 5

CumulativeDissipatedEnergy(kN.mm)

Story Drift (%)

QS

M1_0.2 g

M1_0.4g

M1_0.6g

M2_0.2g

M2_0.4g

IO LS CP

(b) Infilled frame(a) Bare frame

0

5000

10000

15000

20000

0 1 2 3 4 5CumulativeDissipatedEnergy

(kN.mm)

Story Drift (%)

QS

M1_0.2 g

M1_0.4g

M1_0.6g

M2_0.2g

M2_0.4g

IO LS CP

0

5000

10000

15000

20000

0 1 2 3 4 5

CumulativeDissipatedEnerg

y(kN.mm)

Story Drift (%)

QSM1_0.2 gM1_0.4g

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.



26/26

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