Seismic flexural behavior of concrete connections reinforced with CFRP bars and grids

Composite Structures 93 (2011) 2439–2449

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Review

Seismic flexural behavior of concrete connections reinforced with CFRP bars and grids M. Kazem Sharbatdar a,⇑, M. Saatcioglu b, B. Benmokrane c a

Faculty of Civil Engineering, Semnan University, Semnan 3513119111, Iran Dept. of Civil Engineering, University of Ottawa, Ottawa, Canada c Dept. of Civil Engineering, University of Sherbrooke, Canada b

a r t i c l e

i n f o

Article history: Available online 15 April 2011 Keywords: Seismic loading CFRP bars FRP RC joints Hysteretic behavior

a b s t r a c t The corrosion of steel reinforcement in concrete and the resulting deterioration of structures prompted research on fiber reinforced polymers (FRP) as potential reinforcement for concrete members, for use in new construction. FRPs have more favorable advantages for new concrete buildings subjected to seismic loads particularly in corrosive environment. A comprehensive experimental research program was conducted at Ottawa University in Canada to investigate the behavior of FRP reinforced concrete joints to develop design and detailing requirements for FRP reinforced concrete joints under seismic loading. Three large-scale FRP reinforced concrete structural joints were designed, constructed, and tested under cyclic loading. The specimens were T-shape joints consisting of two columns and one beam representing half portion of the first and the second floor of one-bay reinforced concrete frame, or exterior joint of frames with more than one bay. The columns were subjected to the constant axial load and the beams were under reversed cyclic loading. The reinforcement cage was consisted of CFRP bars as longitudinal reinforcement and CFRP grids as transverse reinforcement. The paper presents the details and results of the experimental programs. The results indicate that FRP reinforcement can be used effectively in new concrete buildings. Photographs taken at the selected stages of loading illustrated the performance of each joint. The hysteretic behavior was presented in terms of force–displacement and moment–drift relationships and other hysteretic relationships. Spacing of CFRP grids and arrangement of longitudinal CFRP bars were the main test parameters. Ó 2011 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Research significance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Properties of test specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Material properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Specimens design principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Preparation, test set-up and instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Observed behavior and test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +98 231 3335405; fax: +98 231 3335404. E-mail address: [email protected] (M.K. Sharbatdar). 0263-8223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2011.04.009

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1. Introduction Fiber reinforced polymer (FRP) reinforcement, in the form of longitudinal and transverse reinforcement, are currently being developed for use in new buildings and bridges [1–5], and also several design codes were presented to suggest new expressions for this purpose [6–9]. The major driving force behind this development is the superior performance of FRPs in corrosive environments [6,9]. FRP reinforcement has high strength-to-weight ratio, favorable fatigue strength, electro-magnetic transparency and low relaxation characteristics when compared with steel reinforcement, offering a structurally sound alternative in most applications. However, FRP reinforcement shows linear stress–strain characteristics up to failure, without any ductility. Barris et al. have presented the results of an experimental program concerning on the concrete beams reinforced with glass-FRP (GFRP) bars with a relatively high modulus of elasticity in order to evaluate the short-term flexural behavior by considering the different the reinforcement ratio and the effective depth-to-height ratio. They concluded that the current provisions predict reasonably well the behavior up to service load [10]. Ferreira et al. have modeled the concrete beams reinforced with FRP re-bars and proposed the first-order shear – deformation theory for these beams and then verified that theory with some experimental research [11]. More analytical and experimental research were observed related to RC beams reinforced with FRP re-bars, indicating that general behavior of FRP and steel reinforced beams were almost same with only difference at their final brittle or flexible failure modes [12,13]. Beside of application of FRP bars at new concrete elements, few experimental and numerical research were conducted to present models for RC columns retrofitted with advanced composite jacketing (sheets) with the purpose of decreasing the seismic vulnerability of this kind of structures [14,15]. A simplified damage model based on the continuum damage mechanics has been developed for seismic assessment and retrofit design of columns under

flexural-axial combined loads and then validated through the experimental tests performed on bridge columns [15]. Limited experimental and analytical works were done on RC columns reinforced with FRP bars, indicating the current ACI code was not adequate for this kind of RC columns [2,16]. Several researches were conducted on steel RC connections at concrete frames particularly under seismic loadings [17–19]. Different design expressions were developed and suggested and used during past two decade [20–22]. Some of them were concentrated on the confined concrete at structural elements such as columns that is one of the important elements at RC connections [23,24]. Even though several researches were conducted on the separate beams or columns reinforced with FRP re-bars, but the real behavior of these elements should be investigated at RC frames. So far there is limited research on the structural behavior of concrete joints (connected beams and columns together) reinforced with composite FRP re-bars, therefore a comprehensive experimental research program was conducted at Ottawa University in Canada to investigate the behavior of FRP reinforced concrete joints to develop design and detailing requirements for FRP reinforced concrete joints under seismic loading. The details of the specimens, loading set-up, the results and the discussions are presented in this paper as following. 2. Research significance Experimental research is needed to verify the applicability of those individual members for concrete frame reinforced with FRP bars and stirrups under different stress conditions particularly subjected to seismic reversed cyclic loading, and indeed the joints are essential parts of frames particularly to maintain theses frames integrity. The new concrete frames reinforced with FRP are needed for some industrial and parking buildings to reduce the building deterioration due to steel corrosion. This poses serious concerns about their applicability to earthquake resistant structures, where

1680

Column Column

1680 mm

450

Beam

483 Fig. 1. Geometry of beam–column joint specimen.

450

2100 mm

2441

483 mm

M.K. Sharbatdar et al. / Composite Structures 93 (2011) 2439–2449

450 mm

(a) Columns

350 mm 8-Bar Arrangement

350 mm

12-Bar Arrangement

10-Bar Arrangement

(b) Beams Fig. 2. Column and beam cross-section.

seismic energy is expected to be dissipated by inelasticity in members. Experimental research has been conducted to investigate the behavior of FRP reinforced concrete joints to develop design and detailing requirements for these joints under seismic loading. Three large-scale FRP reinforced concrete structural joints were designed, constructed, and tested under cyclic loading. The results of the selected tests are summarized in the following sections, with the assessment of their significance from seismic performance perspective. 3. Experimental program 3.1. Properties of test specimens The specimens were T-shape joints consisting of two columns and one beam representing half portion of first and second floor of one-bay reinforced concrete frame, or exterior joint of frames with more than one bay. The specimens were reinforced with carbon FRP bars and carbon FRP grids as longitudinal and transverse reinforcement, respectively. General geometry of the beam– column joint specimen is shown in Fig. 1. The length of the beam, subjected to lateral cyclic loading, was 2100 mm. Two columns with 1680 mm lengths and 350 mm by 483 mm cross-section were above and bottom of the joint and reinforced with either 8 or 12 longitudinal 12.7 mm CFRP bars. Fig. 2 illustrates the crosssections of the columns and beams. Since the lateral loads on the

Table 1 Properties of test specimens. Specimens

Column bars (12.7 mm)

Beam bars (12.7 mm)

Grid spacing at joint (mm)

JFRP 1 JFRP 2 JFRP 3

Total 8 Total 12 Total 12

5 top and 5 bottom 5 top and 7 bottom 5 top and 7 bottom

100 100 200

elements were applied at the specified distance from the tip of each element, resulted shear span of the beam and columns were respectively about 1900 mm and 1500 mm. The beam crosssection was 350 mm by 450 mm, reinforced with 5–12.7 mm longitudinal bars at the bottom as positive reinforcement and 5 or 7 same size bars 12.7 mm were put at the top as negative reinforcement, respectively at the three different beams. Three-cell grids and 12-cell grids were respectively used for the beams and columns. The test parameters included the arrangement of longitudinal reinforcement and the grid spacing at the beams and columns. Constant axial load was applied on the columns of all specimens. The specimen characteristics are given at the Table 1. 3.2. Material properties Early strength Portland cement concrete was ordered from a local ready-mix supplier to cast the specimens. Specified 3-days

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(a) Big Grid

(b) Column Grid

(c) Carbon Beam Grid

(d) Glass Beam Grid

Fig. 3. Grids used for columns and beams.

Table 2 Anticipated beams maximum moment resistances. Beam

MF (kN m)

c (mm)

efrpu

Mn (kN m)

c (mm)

efrpu

A (5-bar)-JFRP1 B, C(5-bar)-JFRP2 and JFRP3 B, C(7-bar)-JFRP2 and JFRP3

215 245 245

90 110 110

0.0096 0.0083 0.0083

300 340 340

80 100 100

0.0111 0.0094 0.0094

Table 3 Specimens characteristics. Specimens Beams G1 = 12  80 mm (s = 200 mm) G2 = 6  8 mm (s = 100 mm) Columns G = 6  8 mm s = d/4 = 100 mm at the distance of 600 mm from supports, s = d/2 at the middle portion

FRPJ1

FRPJ2

FRPJ3

Bar arrang. M pb ¼ M nb (kN m) T max (kN) Bar arrang.

10-bar 300, G1, G2 808 8-bar

12-bar 340, G2 957 12-bar

12-bar 340, G1, G2 957 12-bar

M nc (kN m)

235 (s = d/2), (s = d/4) 150

253 (s = d/2), (s = d/4) 172

253 (s = d/2), (s = d/4) 172

100

114.7

114.7

100 470 Ok 708 Ok Beam failure

100 506 Ok 842.3 Not ok Joint failure

200 506 Ok 842.3 Not ok Joint failure

M fc ¼ M2nb (kN m) V nc ¼

M fc 1:5

(kN)

Grid spacing at the joint (mm) P M (kN m) P nc P M nc P Mpb V fj ¼ T  V nc (kN) V frj ¼ 795 P V fj (kN) Expected failure

strength (equal to 28-days normal Portland cement concrete) of concrete, fc0 , was 36 MPa. Maximum aggregate size used in all batches was 10 mm and the slump of concrete was 100 mm. Pultrall CFRP bars with the sand-coated surface and a nominal diameter of 12.7 mm were used as longitudinal reinforcement. The bars were made from high strength carbon fibers and durable vinyl ester resin. The bond performance between the FRP bars and concrete was very important, so the bars used in these specimens were coated completely with the special sand and then their development length was tested and measured to ensure preventing any probable slippage. Some concrete columns and beams reinforced with these sand-coated FRP bars and tested under seismic loading

by this paper writers, so no slippage was observed at the previous works conducted by these paper writers particularly under seismic performance [2]. The maximum tensile strength and modulus elasticity of these bars were 1450 MPa and 115 GPa, respectively. The rupture strain was 0.0126. CFRP transverse reinforcement used in this investigation consisted of NEFMAC grids with a specific gravity of 1.4 and a modulus of elasticity of 100 GPa, reported by the manufacturer. The previous research tests have showed that the maximum tensile strength and the modulus elasticity of the single cross element were almost 1240 MPa and 80 GPa. Those consist of cross CFRP reinforcement elements with either 6  8 mm rectangular or 12  8 mm

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130 mm

130 mm

110

160 mm

Beam 45 o

160 mm

45 o

30 o

240 mm

Column

(a) Schematic view

nominal moment resistance. S806-02, 8.2.1 recommends that the failure of the section is initiated by crushing of concrete in the compression zone, thus the maximum compressive strain of concrete at the both resistances is limited to 0.0035. And also compression bars are conservatively ignored in the calculation of the flexural resistances (S806-02, 8.4.1.8). The capacity reduction factor for concrete, /c = 0.6 and the capacity reduction factor for FRP reinforcement /F = 0.75 shall be used whenever the factored moment resistance is calculated. Whenever the section is overreinforced in only one layer, the crushing of concrete occurs prior to rupture of the FRP bars. In this case, the stress in the FRP bars at the failure, ffrp, which is smaller than the tensile strength can be calculated from the following expression:

2

ffrp

4a1 b1 fc0 ¼ 0:5Efrp ecu 4 1 þ qfrp Efrp ecu

!1=2

3  15

ð1Þ

A

(b) FRP Cages inside Formwork Fig. 4. Strain gauge locations on FRP bars and grids.

Fig. 5. Positions of actuators and other parts of test set-up.

cross-section. The distance of center to center of the cross element was 100 mm. The grids used for the columns had a rectangular 300  400 mm (out-to-out dimension) configuration. The same size and configuration was used for the beams, but some the cross ties were cut to simulate with the assumed configuration for beams. Fig. 3 shows the different transverse reinforcement. 3.3. Specimens design principles The beams should be designed and calculated for two different resistances, MF, the factored moment resistance and Mn, the

frp in which qfrp ¼ bd (reinforcement ratio), Efrp = tensile modulus elasticity of FRP reinforcement, ffrpu = ultimate tensile strength of FRP (MPa), ecu = ultimate compression strain of concrete, 0.0035, efrpu = ultimate tension strain of FRP in the extreme layer. The depth of neutral axis, c, is calculated from equilibrium and then the moment resistance of the section can be calculated. Tensile failure is caused by the rupture of FRP bars, so the FRP bars f strain is efrpu ¼ Efrpu and the corresponding strain, ec, at the extreme frp compressive fiber will be less than 0.0035. Therefore minimum five bars were required for the beams of tests joints to provide the code conditions for preventing FRP bar rupturing prior to the concrete crushing; therefore FRP tensile strain of five-bar section was 0.0093 for the factored moment resistance that this strain is close to 0.0094, which is 0.75efrpu. The anticipated factored and the nominal moments of the beams at each joint specimen are given at Table 2. The strong column-weak beam design approach should be considered to have resistance joint during severe seismic loadings. The energy dissipation necessary for an R/C multi-story frame to survive a severe earthquake should be occurring by formation of ductile plastic hinges in R/C beams. Canadian Design Code CSA A23.3-04 indicates that the flexural resistance of the columns P P and beams at the joint shall satisfy the equation M nc P M pb in order to consider the contribution of the members to lateral P resistance of the lateral force resisting system [22]. Mnc is the sum nominal resistance moments of the columns, at the center of the joint, and RMpb is the sum of probable resistance moments of the beams, at the center of the joint. This moment shall be calculated based on /c = 1 and /s = 1 and 1.25fy. Since there is no yielding for FRP bars, it is assumed that the moment capacity of beam is limited to nominal moment, Mnb, with ffrpu (ultimate tensile strength of FRP). Induced tensile force, T, at the tensile bars was equal to real tensile strain of bars time tensile modulus elasticity of FRP bars. Amounts T shown in the Table 3 were calculated based on the bar strains for the nominal moment resistance from Table 2. This T would be transferred to the joints and applied as factored shear. One more factored shear was transferred to the joints from the columns, Vnc, that was equal to the nominal moment at the column, Mnc, divide to the shear span of the column (almost 1.5 m), because it was assumed that one end of the column was hinge support. The nominal moment of columns, Mnc was half of the nominal moment resistance of the beam, Mnb. The factored shear in the joints, Vfj was equal to the following expression:

V fj ¼ T  V nc

ð2Þ

According to CSA A23.3-94, 21.6.4 the factored shear resistance of the unconfined exterior joint shall not be assumed to exceed the following amount:

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M.K. Sharbatdar et al. / Composite Structures 93 (2011) 2439–2449

(a) LVDT’s Position

(b) Position of Temposonic

Fig. 6. Positions of LVDT’s and load control and data acquisition systems.

8

Moment, M (kN-m)

280

Lateral Drift (%)

6

210

4

140

2

70

0 -2

-5

-4

-4

-3

-2

-1

0

1

2

3

4

0

3

6

9

12

15

18

-210

21

-280

Loading Cycles

(a) JFRP1

Fig. 7. The deformation history for cyclic lateral loading.

V fj 6 V frj ¼ 1:5/c

qffiffiffiffi fc0 Aj

350

qffiffiffiffi fc0 Aj

210 140 70 -5

-4

-3

-2

3.4. Preparation, test set-up and instrumentation After initial preparation, the tied FRP cages, with electric resistance strain gauges mounted on some FRP bars and grids, were

-1

0 -70

0

1

2

3

4

5

Drift (%)

-140

ð4Þ

Even though some previous research indicated that the shear resistance of joint was conservatively depended on concrete strength, some other experimental showed that reducing the hoop spacing affected the shear resistance of joints. So, it was expected that Vfrj would be reduced to less than 830 (or even 719) kN if the grid spacing was doubled to almost 180 mm. Shear failure was expected at the joint prior to the beam failure due to the crushing of concrete even if the applied factored shear in the joint with wide grid spacing, Vfj, was less than 830 (or even 719) kN. The characteristics of the three specimens based on the design code are respectively shown at Table 3. It was expected to have earlier failure at the first specimen, FRPJ1, because it was assumed that the shear resistance of joint was higher than the applied shear. Failure at the joints of two other specimens, FRPJ2 and FRPJ3, would be happened earlier because the shear capacities of joints were less than the maximum transferred shears from the members.

Moment, M (kN-m)

280

ð3Þ

where Aj is minimum cross sectional area within a joint in a plane parallel to the axis of the reinforcement generating the shear in the joint, this was equal to the column area in this specimen (350 mm  483 mm). But this equation was changed at the new design code at CSA A23.3-04, 21.5.4.1 as follows:

V frj ¼ 1:3k/c

5

Drift (%)

-140

-6 -8

0 -70

-210 -280 -350

(b) - JFRP2 350

Moment (kN-m)

280 210 140 70 -5

-4

-3

-2

-1

0 -70

Drift (%)

0

1

2

3

4

-140 -210 -280 -350

(c) - JFRP3 Fig. 8. hysteretic moment–drift relationship of joints.

5

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Before Loading

(a) - JFRP1

Before Loading (b) - JFRP2

Before Loading

2445

At 4% Drift

At 4% Drift

At 4% Drift-Failure (c) - JFRP3

Fig. 9. Observed photos of joints at the different stages.

putted at the formworks. Each corner bar of the beam had three strain gauges, one strain gauge was 160 mm above of the interface, one at the interface, and third one was 160 mm below the beam– column interface to measure the variation of strains along the length of the bar within the critical region for flexure. The first two grids in each beam and column, immediately above the joint, were also instrumented with strain gauges. Fig. 4 illustrates the locations of strain gauges. The test set-up is shown in Figs. 5. Two 1000 kN capacity MTS actuators were used to apply the loads. One of the actuators was positioned horizontally at the column level to apply the constant axial compression to the columns during testing. The columns were horizontally connected to the hinge supports. The second 1000 kN capacity MTS actuator was used to apply the lateral load directly on the beams without any axial load. The columns tested in these tests were subjected to a constant axial load, which was established as a percentage of column concentric compressive capacity, Po.

Pro ¼ 0:85/c fc0 ðAg  A0FRP Þ þ 0:002/F Efc AFRP

ð5Þ

The axial compressive load was applied first on the column and was maintained at a constant level of 530 kN (20% of its concentric capacity) through the test. The horizontal load was applied on the beam in the deformation reversals control mode and starting with three elastic cycles at 0.5% lateral drift, which approximately corresponded to the displacement at the first flexural cracking, followed by three cycles at 1% drift. The subsequent stages of the loading included three cycles at each of the incrementally increasing drift level. Each specimen was instrumented with strain gauges, LVDTs, and Temposonics to measure strains and displacements. LVDTs were placed in pairs vertically or horizontally near the critical section of the column or the beam to measure the total rotations of hinging region. The difference between the readings of the two LVDTs on either side divided by their horizontal distance would give the total rotation of hinging region. A Temposonic LVDT was placed horizontally at the point of application of top horizontal load to measure the beam tip displacement. All instrumentations were connected to a data acquisition systems and MTS controller for data collection. Fig. 6 shows the positions of LVDT’s and the Load Control and Data Acquisition

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M.K. Sharbatdar et al. / Composite Structures 93 (2011) 2439–2449

210

210

140

140

70 -0.3

-0.2

-0.1

280 Moment, M (kN-m)

Moment, M (kN-m)

280

70

Strain (%)

0 0.0 -70

0.1

0.2

-0.03

0.3

-0.02

-0.01

-140

-140

-210

-210

-280

-280

350

280

210

210

70 -0.2

0.1

0.2

Beam Total Rotation (rad)

0 0.00 -70

0.3

-140

-140

-210

-210

-280

-280

-350

-350

0.01

350

Moment (kN-m)

280

210

210

140

140

70 -0.4

-0.2

-70

Strain (%)

0.0

0.2

0.4

0.02

0.03

(b) - Beam2

280

0

Beam Total Rotation (rad)

70

Strain (%)

(b) JFRP2 350

0.03

140

140

-0.3

0.02

350 Moment, M (kN-m)

Moment, M (kN-m)

280

0 -0.1 0.0 -70

0.01

(a) - Beam1

(a) JFRP1

-0.4

0 0.00 -70

0.6

0.8

1.0

1.2

70 0 0.00 -70

-140

-140

-210

-210

-280

-280

-350

(c) JFRP3 Fig. 10. Moment-strain hysteretic relationship of diagonal LVDT at joints.

System. The beams were tested following the loading history shown in Fig. 7. 3.5. Observed behavior and test results Observed behavior of the columns and the beams and the general behavior of T-shape joint tests is presented and discussed in this section. Photographs taken at the selected stages of loading illustrate the performance of each joint. The hysteretic behaviors are presented both in terms of the force–displacement and the moment–drift relationships. The moments plotted in these relationships were computed from the recorded test data as the net lateral force times the shear span. The hysteretic moment–drift relationship of the joint specimens shown in Fig. 8 indicated that the joints showed stable hysteresis loops up to almost 3% drift ratio. Fig. 8-a indicates that JFRP1 had significant and sudden strength decay after 3% drift ratio due to bar slippage and following the crushing of concrete. The hysteretic relationships recorded during the test, shown in Fig. 8-b, indicates

Moment, M (kN-m)

Beam Total Rotation (rad)

0.01

0.02

0.03

-350

(c) - Beam3 Fig. 11. Moment–rotation hysteretic relationships of beams at joints.

that the joint JFRP2 after stable hysteresis loops up to almost 3% drift ratio, showed a gradual degradation up to 4% at the strong side and almost significant and sudden strength decay at the weak side of the beam during 4% drift ration due to some bars rupturing and following the crushing of concrete. The specimen JFRP3 was companion to the specimen JFRP2 except with grid spacing inside the joint. The spacing of grids at the beam and the column was 100 mm, which was approximately equal to maximum spacing required by CSA S806-02 for earthquake resistant frames. The joint grid spacing was 200 mm that was almost twice of the grid spacing of the column. After stable hysteresis loops up to almost 3% drift ratio, JFRP3 joint showed stable hysteresis loops up to almost 4% drift ratio at the strong side of the beam and up to 3% drift at the weak side of beam in Fig. 8c, followed by sudden decay during 5% and 4% drift ratio, respectively. Observations during testing shown in Fig. 9 indicated that initial flexural and shear cracking at all joints started at 0.5% drift cycles at the beam above the joint, No significant diagonal crack was observed up to 1% drift ratio while the cracks of beam was

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280

350

Moment, M (kN-m)

210

210

140

140

70 -0.01

Moment, M (kN-m)

280

70

0 0.00 -70

0 0.00 -70

-0.01

0.01

-140

-140

-210

-210

0.02

Column Total Rotation (rad)

-280

Column Total Rotation (rad)

-280

0.01

-350

(a) - Column-1

(b) - Column2 350

Moment, M (kN-m)

280 210 140 70 0 0.00 -70

-0.01

0.01

-140

0.02

Column Total Rotation (rad)

-210 -280 -350

(b) - Column3 Fig. 12. Moment–rotation hysteretic relationships of columns at joints.

Table 4 Comparison of anticipated and recorded moment resistances at beams. Beam

A (5-bar)-JFRP1 B, C(5-bar)-JFRP2 and JFRP3 B, C(7-bar)-JFRP2 and JFRP3

MF (kN m)

Mn (kN m)

MRecorded (kN m)

Weak side

Strong side

Weak side

Strong side

Weak side

Strong side

215 215 215

215 245 245

300 300 300

300 340 340

253 281 278

253 296 306

widened. Significant diagonal cracks of the joint JFRP1 with almost 45° that passed all the joint grids and also some horizontal cracks of the beam were observed during 2% drift ratio as Fig. 9a. The crack parallel with longitudinal bar indicated possible bar slippage at one side of beam. Complete bar slippage was happened at the one side of beam bars at the first cycle of 3% drift ratio in the joint JFRP1. More diagonal cracks consisting of one big and more small parallel diagonal cracks at the end of 2% drift ratio stage was observed at the specimen JFRP2. The complete diagonal cracks that passed all grids and the crack at the interface happened at 3% drift ratio stage. Gradual strength degradation was observed at the strong side of beam up to 4% drift, but sudden drop happened at the weak side during 3.5% drift with significant crack at this side at the beam–column interface that shown in Fig. 9b. The appearance of bar failure was more like a bar ruptured subjected high tension. At the end of 2% drift ratio stage of the specimen JFRP3, the main diagonal crack with almost 45° that passed three joint grids, one crack parallel the beam longitudinal inside the joint, more diagonal cracks consisting of one big and more small parallel diagonal cracks, were observed as Fig. 9c. Cracks parallel with the beam longitudinal bars at other side were observed at this stage, but no significant bar slippage and no strength degradation or new cracks at the interface were happened. The complete diagonal cracks that passed all grids happened at 3% drift ratio stage. Joint diagonal

crack width was increased and possible slippage of bars was observed from the cracks around the longitudinal bars. The crack at the interface started during this stage and sudden strength degradation was happened at the weak side of beam due to bar rupturing or possible bar slippage. And also sudden strength degradation was observed at the strong side of the beam during 5% drift, followed the joint failure due to the bar rupturing. Two diagonal LVDTs were instrumented at the joints to measure diagonal strains of concrete particularly at the crushing phase. These strains in Joint1, Joint2, and Joint3 are shown at Fig. 10 were respectively with a maximum 0.25%, 0.35%, and 1.1% compressive strains at the end of testing when drift ratio reached 3%, 4%, and 4%. Maximum tensile strains of theses joints were 0.1%, 0.3% and 0.20% respectively. Moment-total rotation hysteretic relationships of the beams and the columns, recorded during the test, are shown respectively in Figs. 11 and 12. The past research results have indicated that total hinging region rotation could be attributed to anchorage slip and flexure. 4. Results discussion Unsymmetrical bar arrangements were considered to simulate real behavior of the beams at the top and bottom side due to

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unequal negative and positive moments during seismic loads on RC frames. Anticipated factored moment resistance and nominal moment resistance of three different beams were calculated based on the design code and given at the Table 2. As shown in the Table 4, maximum moment recorded during testing JFRP1 joint was 253 kN m. These moments at the JFRP2 and JFRP3 joints were 281 and 278 kN m at the weak sides and also 296 and 306 kN m at strong sides of the beams. It was obvious that the real moments should be bigger than the resistance moment with considering the reduction resistance factors and also less than the nominal moments. Test results showed that the maximum resistant moments were between 10% and 15% less than the expected nominal moments because the FRP re-bars did not reach to their maximum tensile strains, so a reduction factor approximately 0.85 was observed here for the joints subjected to cyclic loadings. The bar arrangement had more important role in increasing of the moment capacity compare to the grid spacing at the joints. No significant difference was observed between the companion specimens JFRP2 and JFRP3 with two different grid spacing at the joints, therefore one grid with the spacing of one-half of the column effective depth is enough. Hysteretic moment–drift of the concrete frame joints reinforced with FRP bars and grids shown and compared at Fig. 8 had generally same behavior. All joints behaved stable up to almost 4% drift ratio that was within the accepted ratio for seismic design codes. Comparison of hysteretic relationship of the cantilever beam connected to a very rigid steel reinforced concrete connection [4] and the joints tested in this research showed that both had almost same behavior, even thought if the joint would be less rigid so it’s behavior should be different. National Building Code of Canada (1995, 2005) indicates that lateral drift ratio limits specified for earthquake resistant columns is 2% and 2.5%, some previous investigations showed that the FRP reinforced concrete columns were able to develop drift capacities higher than those specified at NBCC code [2,3]. Even thought the NBCC code concentrate on column behavior, it was shown in this research that the FRP reinforced concrete beams were able to have sustainable drift ratios up to 4% subjected to the cyclic loading. The hysteretic relationships indicated progressive stiffness degradation due to concrete cracking, and inelasticity due to gradual concrete crushing. However, the shape of hysteresis loops was distinctly different than those of steel reinforced beams. The unloading branch of hysteresis loops for FRP reinforced concrete members aim towards the origin, due to the elastic behavior of FRP bars in tension. Therefore, earthquake resistant design of FRP reinforced concrete structures and particularly joints must be based on elastic member behavior while taking advantage of relative flexibility of FRP reinforced concrete and associated elongation in the fundamental period of structures, attracting lower spectral accelerations. The bar arrangements of columns and indeed inside of joint was effective at increasing the number and shape of stable hysteretic relationships. The specimen JFRP1 with eight-bar column was less confined than the specimen JFRP2 with 12-bar column, so the latter joint showed the stable behavior but sudden failure was observed at the specimen JFRP1 with less ductility due to less confined joint even with close grids spacing. 5. Conclusion The following conclusions can be drawn from the experimental research and result discussion reported in this paper: 1. Initial idea about the new industrial buildings and parking buildings concrete frame reinforced with FRP was reducing

2.

3.

4.

5.

6.

7.

8.

the building deterioration due to steel corrosion, but many of these buildings should be tolerate earthquake effects and dissipate the seismic energy by inelasticity behavior. FRP reinforced joints can be designed to satisfy strength and deformability requirements of earthquake resistant structures. Tests performed under the reversed cyclic loadings indicated that the joint drift capacities can be in excess of 3%. FRP reinforced concrete joints can be confined to develop inelastic deformations. The CSA S806-02 requirements for confinement showed good correlations with the test data. FRP rebars were capable of resisting the significant compression and tension–compression cycles without any distress. The strength and elastic modulus of FRP bars in compression were approximately equal to 20% of the values in tension. The failures in tension were observed to occur at about 1.0% strain. Seismic design strategies for FRP reinforced concrete elements may be to design them remain elastic, with sufficient lateral deformability. The design approach may be improved by providing sufficient confinement for compression members and joints by means of closely spaced transverse FRP reinforcement. Hysteretic behavior of FRP reinforced concrete elements and joints can be substantially different than that for steel reinforced concrete members. Inelastic response of FRP reinforced concrete joints can only occur in well confined and over-reinforced elements. Maximum moment capacities of FRP RC joints were 15% less than maximum nominal moment without reduction material factors, so moment resistant can be calculated by applying coefficient 0.85 to nominal moment. Generally in order to prevent reduce the disadvantage of this kind of joint, probable brittle failure, lower coefficient is proposed. The more bar arrangements of columns continued inside of the joints JFRP2 and JFRP3 could provide more confinement for joint concrete compare to joint JFRP1 and therefore effective at having the stable and ductile hysteretic relationships without the sudden failure what observed at the specimen JFRP1. The specimens JFRP2 and JFRP3 with more longitudinal bars behaved better compared to the specimen JFRP1 due to better confined concrete inside of the joint. No significant difference was observed between these two specimens, therefore very closed spacing grid inside of the joints was not required.

Acknowledgements This research was financially supported by the National Science and Engineering Research Council (NSERC) of Canada. The authors would like to express their gratitude to Pultrall Inc., and technical staff of Faculty of Engineering in University of Ottawa particularly to Mr. Muslim Majeed. References [1] Fukuyama H, Masuda Y. Structural performances of concrete frame reinforced with FRP reinforcement. In: Taerwe E, Spon FN, editors. Non-metallic (FRP) reinforcement for concrete structures, London; 1995. p. 275–86. [2] Sharbatdar MK. Concrete columns and beams reinforced with FRP bars and grids under monotonic and reversed cyclic loading. Ph.D. thesis, University of Ottawa, Ottawa, Canada; 2003. [3] Sharbatdar MK, Saatcioglu M. Behavior of FRP reinforced concrete under simulated seismic loading. In: 13th World conference on earthquake engineering, Vancouver, BC, Canada; 2004. [4] Sharbatdar MK, Saatcioglu M. Flexural behavior of FRP reinforced concrete beams under reversed cyclic loading. In: Fourth international conference on advanced composite materials in bridges and structures, ACMBS-IV, Calgary, Canada; 2004. [5] Sharbatdar MK, Saatcioglu M. Design of FRP reinforced concrete structures for seismic effects. In: 1st Canadian conference on effective design of structures, Hamilton, Ontario, Canada; 2005.

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