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Seismic performance of non-ductile RC frames strengthened with CFRP
Composite Structures 221 (2019) 110870
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Seismic performance of non-ductile RC frames strengthened with CFRP a
a
a
Weihong Chen , Weirong Shou , Zehui Qiao , Shuangshuang Cui a b
b,⁎
T
School of Civil Engineering, Fuzhou University, Fuzhou 350108, China School of Civil Engineering, Fujian University of Technology, Fuzhou 350118, China
A R T I C LE I N FO
A B S T R A C T
Keywords: CFRP Non-ductile RC frames Strengthened Seismic performance Lateral cyclic load
To study the seismic performance of non-ductile reinforced concrete (RC) frames strengthened with carbon fiberreinforced polymer (CFRP), two 1/2-scale, 2-bay, 2-story RC frames were prepared and tested under combined axial load and cyclic lateral load. One of the RC frames served as a control specimen (bare specimen) and the other was strengthened with CFRP sheets (strengthened specimen) at its joint regions. Based on the hysteretic curves, the envelop curves, and several important indexes (e.g., strength, stiffness, ductility, energy dissipation capacity) of the two specimens, the performance of the employed seismic strengthening technique in improving the seismic performance of RC frames was discussed in detail. The experimental results indicated that the externally bonded CFRP reinforcement had little influence on the lateral stiffness of the RC frame. While the lateral strength of the strengthened RC frame increased by about 20% compared to the control RC frame. Furthermore, it is worth noting that the energy dissipation capacity of the strengthened RC frame is as much as 3.01 times that of the control RC frame. The test results of the current study indicate that the employed strengthening method can substantially improve the seismic performance of the strengthened RC frames.
1. Introduction Numerous reinforced concrete (RC) buildings designed prior to the 1970s are still in service in seismically active regions in Central and Western China. The Chinese code for seismic design of buildings has been updated twice to address the earthquake damage to buildings since the Wenchuan earthquake in 2008. It is notable that many structures are vulnerable to earthquakes due to the obsolete design strategies, underestimation of seismic input, and lack of proper structural design details. These existing RC structures and buildings were usually designed either for gravity load alone or with little consideration of seismic forces and detailing and thus as they are insufficient in horizontal shear strength and ductility. Among such RC structures and buildings are a large number of RC frames and such RC frames are classified as non-ductile RC frames in this study. These structures have limited lateral stiffness and strength, so they tend to develop excessive sideways or soft-story mechanisms under earthquake ground motions [1]. As a result, all of these non-ductile RC frames are at a high risk of experiencing severe damage or collapse in the event of severe earthquakes. Usually, the “destroy-reconstruct” approach is taken, but it is costly. To mitigate this risk, it is proposed to retrofit the frames to be more ductile. This method is not only cost-effective, but also environmentally friendly, particularly in less developed regions.
⁎
Steel plates, bars, or cables have been typically used for strengthening, but their heavy weight and low corrosion resistance have rendered these materials as somewhat undesirable. Fiber reinforced polymer (FRP) composites have been widely used for seismic strengthening or rehabilitation of concrete structures in the past decades due to their high strength to weight ratio, excellent corrosion resistance and ease of application. The FRP retrofit of RC frame members such as beams [2,3], columns [4,5], and joints [6,7] indicates the effectiveness of the retrofitting technique. In recent decades, FRP-strengthened RC joints have been studied by many investigators. Antonopoulos et al. [8] applied simulated seismic load tests on eighteen 2/3-scale exterior RC joints strengthened with FRP; they concluded that, under simulated seismic loads, externally bonded FRP reinforcement is a viable solution to enhance the strength, energy dissipation capacity, and stiffness amplitude of RC joints with inadequate shear strengthening. Al-Salloum [9] analyzed four RC interior beam-column joints by means of pseudo-static testing. Two of the four specimens were used as baseline specimens and the other two were strengthened with CFRP sheets. The comparison revealed that CFRP sheets improve the shear resistance and ductility of the joint. Beydokhti [10] investigated the behavior of shear deficient exterior beam–column joints strengthened by CFRP. Moreover, their test revealed that FRP predominantly affects the joint behavior in high levels of damage.
Corresponding author. E-mail address: [email protected] (S. Cui).
https://doi.org/10.1016/j.compstruct.2019.04.042 Received 25 June 2017; Received in revised form 16 January 2019; Accepted 5 April 2019 Available online 13 April 2019 0263-8223/ © 2019 Elsevier Ltd. All rights reserved.
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Parghi [11] studied fragility curves of typical single circular concrete bridge piers reinforced with FRP and concluded that the amount of reinforcement, shear span-depth ratio, and the level of the axial load can significantly affect the collapse fragility curve of the strengthened bridge piers. Said [12] assessed the strength, ductility, and energy dissipation gains of substandard beam-column joints enhanced by FPR rehabilitation techniques. The results indicated that FRP joint repair schemes can generally enhance the performance of substandard joints, but they often do not meet the current standard. The experiments [13,14] revealed that for a CFRP strengthened frame node, the ductility of the joints was improved, the failure mode of the node core zone was changed, and the shear failure of the joints was delayed. Furthermore, the strength and stiffness were improved. Plenty of comprehensive researches on the FRP strengthened RC members and beam-column joints have been released, but only a limited number of studies investigated the effectiveness of the FRP on the overall seismic behavior of RC structures. Balsamo et al. [15] developed a full-scale dual system (4-story and 2-bay) subjected to pseudo-dynamic tests. The experimental tests revealed that repaired structures obtained a large displacement capacity without exhibiting any loss of strength and were able to provide energy dissipation similar to that of the original configuration. Wang et al. [16] conducted the shaking table test on two 1/2-scale 4-story and 2-bay non-ductile RC frames (one was bare; the other was FRP-strengthened) to investigate and evaluate the seismic performance of non-ductile RC frames strengthened with FRP. Based on the experimental study for seismic performance of FRP-strengthened RC beam-column joints, Peng et al. [17] carried out a static elastoplastic analysis on the joints of the FRP-strengthened concrete frames, and the result clearly indicated that effective reinforcement for the beam–column joints can improve the overall seismic performance of frame structures. Niroomandi et al. [18] carried out nonlinear pushover analysis of the full frame to evaluate its force-displacement capacity curve, and results revealed a notable improvement in the performance when the frame was strengthened at the joints by FRP. Furthermore, Cao and Ronagh [19] conducted comparative inelastic time history and damage analyses on an 8-story poorly-confined and FRP retrofit RC frame, and the results revealed that the poorly-confined frame was essentially upgraded to the level of the intermediate frame, which was designed to more restrictive requirements. Shin et al. [20] also performed a series of dynamic tests on a fullscale, non-ductile RC frame strengthened with prefabricated FRP jackets on the first-story columns, and the results demonstrated that the retrofit scheme helped develop a more uniform story drift distribution. Previous theoretical and experimental studies clearly supported the effectiveness and advantages of using FRP to prevent the failure of nonductile structural members that are seismically deficient, such as beams, columns, and joints. Furthermore, few experimental studies have been conducted on reducing the potential damage of multistory non-ductile RC frames subjected to vertical concentrated loads and horizontal cyclic loads. However, for the capacity evaluation of structural components and systems, the conventional quasi-static test continues to be performed in engineering practices [21]. Thus, it is necessary to conduct reduced-scale or full-scale tests on non-ductile RC frames strengthened with FRP, the results of which will serve as the premise for the analysis of the seismic behavior of non-ductile RC frames strengthened with FRP. The primary objective of the present study is to explore the seismic behaviors of RC frames strengthened with CFRP. Pseudo-static tests of a bare non-ductile RC frame and a CFRP strengthened non-ductile RC frame were conducted to investigate the variations of the mechanisms in failure mode, seismic behavior, and performance between bare and CFRP-strengthened frames.
Fig. 1. Outlines and reinforcement detailing of the RC frame (all dimensions are in mm).
2. Experimental program 2.1. Test specimens In this study, an existing two-bay, five-story building in Western China with non-ductile RC frames was selected as the prototype. This building was designed and detailed as per the old Chinese Standards [22] which account for only gravity loads. The width of each bay of the building is 4.0 m, and each story height is 3.0 m. In order to carry out a comparative analysis on the seismic behavior of CFRP-strengthened frame structures, two non-ductile RC frames were constructed. In view of the cost-effectiveness of the test and the limitations of the loading facilities, only the bottom two stories were physically tested using substructuring techniques. The scale ratio of the test substructure specimens to the prototype structure is 1:2. One specimen was used as the control specimen (bare specimen) and the other was strengthened with CFRP sheets (strengthened specimen) at its joint regions. The foundation beams and columns were also included in the test model to accurately simulate the constraint conditions of the structure base. Outlines and reinforcement detailing of the specimens are presented in Fig. 1. The dimensions and reinforcement detailing were determined based on the prototype frame members using similitude relationships, and are summarized in Table 1. For the strengthened specimen, the reinforced regions were polished and scrubbed with acetone. The reinforced region was then wrapped with epoxy-impregnated CFRP sheets (3 layers) via the wet lay-up method. 2.2. Test set-up As presented in Fig. 2, the test set-up includes a reaction wall, a strong floor, two vertical jacks, and two horizontal actuators. The specimens were tied to the strong floor. The two vertical jacks were used to apply constant vertical forces to the columns. The two horizontal actuators, which were fixed to the reaction wall, were used to apply horizontal forces to the two stories. All hydraulic actuators had a loading capacity of 500 kN. Furthermore, the horizontal fixed connecting pieces included steel rods, steel plates, and nuts, making the actuators work effectively. 2.3. Material properties Compressive tests of concrete were conducted after 28 days of curing time. According to the Chinese standard GB 50010-2010, six cubic samples (150 mm × 150 mm × 150 mm) and six prismatic samples (150 mm × 150 mm × 300 mm) were tested under a compression testing machine. The average compressive strength obtained from the cubic samples was 29 MPa. The average compressive strength, elasticity modulus, and Poisson’s ratio of the prismatic samples were 23 MPa, 2
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Table 1 Geometric details of various RC members of test specimens. Member
Overall size (L × B × H)
Clear cover (mm)
Longitudinal reinforcement
Transverse reinforcement (mm c/c)
Column Beam
1500 × 200 × 200 2000 × 150 × 200
15 15
10Y (4 nos.) 10Y (8 nos.)
6Y @200 6Y @200
Fig. 4. Loading procedure for low cyclic loading test.
to monitor the lateral load and displacement applied to the test specimens, respectively. Two linear variable differential transformers (LVDTs) were fixed on the middle column of the second layer to measure the lateral displacements-one was fixed at the middle of the column, and the other was fixed at the bottom of the column (see Fig. 5). An additional dial indicator was also used to measure the displacement of beam–column joints. At the section of the joint, strain gauges were attached to the longitudinal reinforcement of the beams and columns along the direction of the horizontal and vertical forces to measure the real-time longitudinal strain (see Fig. 3).
Fig. 2. Specimen with test set-up (all dimensions are in mm).
2.5. Loading history Vertical loads were applied to the top of the frame columns to simulate the gravity of the upper three stories. Accordingly, the vertical load on the middle column was 269.24 kN, and that on each of the two side columns was 191.08 kN. In the test, one of the side columns of the RC frame was connected to the two MTS actuators at the joint regions as shown in Fig. 2. The seismic forces on the structure are assumed to follow an inverted triangular pattern [23]—the proportion of the designed seismic forces from the fifth to the first story of the prototype non-ductile RC frame is 5:4:3:2:1. Since only the first and the second story were tested, the seismic forces sustained by the upper three stories (i.e., the third, fourth and fifth stories) had to be applied to the second story. Although the distribution of the designed seismic force of the second story is 2F, considering the loads transmitted form the upper stories (3–5 stories), the actual horizontal shear force in the second story is 14F. Similarly, the cyclic lateral load of F is applied to the first story, and the actual horizontal shear force of the first story is 15F. Therefore, two cyclic lateral loads with a proportion of 14:1 are applied by the two horizontal actuators at the second and first story. In the test, the lateral load was applied by the force-controlled procedure before the first yielding of the longitudinal reinforcement occurred, and a displacement-controlled procedure was adopted afterward. In addition, the actuator connected to the second story of the RC frame was regarded as the main control actuator and the loading history for the main control actuator is shown in Fig. 4. The lateral load applied on the first story is then always 1/14 of that applied on the second story. Moreover, the testing of the strengthened specimen was stopped after the residual lateral load decreased to 85% of the peak load and the
Fig. 3. The layout of the strain gauges.
33,100 N/mm2, and 0.23, respectively. In addition, tensile tests were conducted for the steel bars to determine the yield stress, ultimate strengths, and elastic modulus and the test results are listed in Table 2. Moreover, the coupon tensile tests were conducted for the employed CFRP sheets according the Chinese standard GB 50608-2010. The CFRP coupons had the dimensions of 15 mm × 230 mm and the nominal thickness of the CFRP sheet was 0.167 mm. As shown in Table 3, the elastic modulus, ultimate tensile strength, and ultimate tensile strain of the CFRP sheet were 240 GPa, 4340 MPa, and 0.017, respectively.
2.4. Instrumentation The load cell and displacement transducer in the actuator were used 3
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Table 2 Mechanical properties of steel bars. Member
Yield strength (MPa)
Ultimate strength (MPa)
Discrepancy of yield strength (%)
Discrepancy of ultimate strength (%)
Elastic modulus (GPa)
Stirrups Longitudinal bars
313 424
487 549
2.6 2.0
2.2 1.6
218 197
Table 3 Properties of CFRP sheets. Member
Grammage (g/mm2)
Thickness (mm)
Tensile strength (MPa)
Elastic modulus (GPa)
Elongation (%)
CFRP sheets
300
0.167
4340
240
1.7
Fig. 6. The second RC frame with the plastic hinge regions to be strengthened.
Fig. 5. The first RC frame (control specimen) in test.
testing of the bare specimen was stopped when the inter-story drift reached 2%. 2.6. Test procedure The test program was conducted in the National and Local United Research Center for Seismic and Disaster Informatization of Civil Engineering (NLRCSD) at Fuzhou University. The two specimens were prepared and tested in the following procedure: (1) Two RC frames were designed and built; (2) one of the RC frames was tested as control specimens as shown in Fig. 5; (3) the length of the plastic hinges on the control specimen was carefully measured after the test; (4) according to the plastic-hinge length obtained in step (3), the plastic-hinge regions of the other RC frame were prepared for strengthening (see Fig. 6) and then CFRP wraps were applied; (5) the strengthened RC frame was tested as shown in Fig. 7.
Fig. 7. The strengthened specimen in test.
force to the first concrete cracking is described as the elastic stage. In this stage, the lateral force was relatively small. Furthermore, the relationship between the lateral load and the displacement was approximately linear. Cracking stage. As the level of lateral force gradually increased, the first crack appeared in the column at joint II and the corresponding lateral load acting on the second story was −20 kN (see Fig. 8). The first crack in the beam appeared at joint IV when the second-story load approached 30 kN. When the second-story load reached −50 kN, column-penetrating cracks appeared on the column at joint II. Yield stage. Based on the available strain gauge data and visual inspection, when the second-story load reached 61 kN, the value of the steel strain gauge exceeded 1800 με, which indicated the first yield of the steel bar. The lateral displacement at the top of the specimen was measured as 12 mm (yield displacement Δ). Failure stage. After the first yield of the longitudinal steel bar, the lateral loads on the specimen became applied by displacement control.
3. Test observations As stated above, to estimate the effectiveness of the CFRP strengthening method for non-ductile frames, comprehensive comparisons of the strengthened specimen and the control specimen were made based on the test results. For ease of reference, the joints of the two specimens were named with the roman numerals I-IX as shown in Fig. 8. 3.1. Bare specimen failure modes Elastic stage. The period from the onset of the slow-cyclic lateral 4
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Cracking stage. It was difficult to observe the cracks of the strengthened specimen as the plastic hinge region was wrapped with CFRP sheets. Failure of the CFRP sheets initiated with partial debonding at the column base joint III when the lateral force reached −42.17 kN. The test was continued until the lateral force of 56 kN was reached, and the same phenomenon occurred at joint I and joint II. Yield stage. Based on the available strain gauge data and visual inspection, when the load reached 75 kN, the value of the steel strain gauge exceeded 1800 με, which means that the first yield of the steel bar occurred. Also, the lateral displacement at the top of the specimen was measured as 15.5 mm (yield displacement Δ). Failure stage. After the first yield of the longitudinal steel bar, the displacement-controlled loading stage started. During the 1Δ cyclic loading stage, additional steel bars yielded. When the displacement reached 2Δ, the column base split, leading to concrete spalling. During the 3Δ loading stage, cracks occurred in the bare region close to the reinforced area. In the 4Δ loading stage, the peeling phenomenon of CFRP sheets occurred at the joint of the strengthened frame. At the 5Δ loading stage, bulging at the base of the columns was observed due to volumetric expansion and fracturing of the concrete inside the column. The test terminated at the 6Δ cyclic loading stage with the lateral load becoming less than 85% of the peak load. During the entire experiment, no FRP rupture happened. Based on the experimental observations and experimental data, the plastic hinges in different joint formed in the following order: III → II → I → IV → V → VI → VII → IX → VIII. The failure modes are shown in Fig. 10. After the test, the FRP wraps were peeled off from the strengthened specimen to further observe its failure mode. As shown in Fig. 11, the FRP-confined concrete at the plastic hinge region of the columns was severely damaged while that at the plastic hinge regions of the beams was not or only slightly damaged. For the control specimen, however, serious crash of concrete happened at the plastic hinges of both the beams and the columns.
Fig. 8. Names of the joints in the specimen.
During the cycle of the 1Δ loading stage, a crack appeared in the column at joint I. When the lateral displacement increased from 1Δ to 3Δ, the lateral force remained nearly stable. The width of the crack in the column base of plastic hinge II reached 0.50 mm. As the load increased, spalling of the concrete cover appeared in joint II. The test was terminated when the interstory drift reached 2%. Based on the experimental observations and experimental data, plastic hinges at different joint formed in the following sequences: II → IV → VI → III → I → VII → V → IX → VIII. The failure modes are shown in Fig. 9. 3.2. Strengthened specimen failure modes Elastic stage. At the beginning of the test, the lateral force was relatively small, and the relationship between the lateral load and the displacement was approximately linear. The strengthened specimen had a longer elastic stage compared with the bare specimen.
Fig. 9. Failure modes of bare specimen: (a) elastic stage; (b) cracking stage; (c) yield stage; and (d) failure stage. 5
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Fig. 10. Failure modes of strengthened specimen: (a) elastic stage; (b) cracking stage; (c) yield stage; and (d) failure stage.
4. Test results and discussion
direction are substantially higher than that in the negative direction. At the beginning of the test, steel bars did not yield during the force control mode, but the sequence of failure of the members was asymmetrical when associated with significant lateral loading. Consequently, lateral displacements differed on the two sides of the specimens causing differences of the measured force. (3) Further details can be found in the relevant individual hysteretic loop of the strengthened specimen. The strengthened specimen reached the negative peak base shear force of −129.37 kN at the 1.53% (−3Δ) loading stage, and then reached the positive peak base shear force of 164.54 kN at the 1.8% (−3.5Δ) loading stage. At the drift ratio of 3.1% (−6Δ) (the base shear force was −108.72 kN), the shear force dropped more than 15% from its negative peak value. (4) There is a linear relationship between load and displacement in first cycles. After the yield load, the strengthened specimen revealed small residual deformation during unloading; the lateral displacement remained at a certain value, which indicates that the structure was basically in plastic working state. An increasing number of cracks appeared in the plastic hinge zones as the load increased when the column base failed. The curve veers toward the drift ratio
4.1. Hysteretic response of specimens Hysteretic curves of base shear versus controlled overall drift ratio θu and the corresponding story shear versus story drift ratio for the first and second stories of each specimen are shown in Fig. 12. The following observations can be obtained from Fig. 12: (1) The hysteretic loops of the strengthened frame are plumper than that of the bare frame, which means the strengthened frame dissipated more energy. In particular, the rheostriction in the hysteretic loop of the strengthened frame become increasingly obvious with the increase of lateral displacement. This indicates that the resistance of the strengthened frame decreased after significant damage occurred in the plastic hinge region. (2) Although the MTS electro-hydraulic servo structure tester was programed to apply symmetrical lateral load on the specimens, it can be seen in Fig. 12 that the hysteresis curves of both the bare specimen and the strengthened specimen are obviously asymmetrical: the peak load and the corresponding drift ratio in the positive
Fig. 11. Photographs of strengthened specimen after removing CFRP sheets. 6
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Fig. 13. Envelop curves of specimens: (a) first story shear versus story drift ratio; (b) second story shear versus story drift ratio; (c) base shear versus overall drift ratio.
Fig. 12. Hysteresis curves of specimens: (a) first story shear versus story drift ratio; (b) second story shear versus story drift ratio; (c) base shear versus overall drift ratio.
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Table 4 Behavior of specimens at different characteristic points. Specimen
Bare Strengthened
Cracking point
Yield point
Peak point
Ultimate point
Pcr (kN)
Δcr (mm)
θcr (%)
Py (kN)
Δy (mm)
θy (%)
Pm (kN)
Δm (mm)
θm (%)
Pu (kN)
Δu (mm)
θu (%)
NA −19.01 NA −43.38
NA −2.71 NA −5.68
NA −0.08 NA −0.19
97.53 −75.80 118.29 −90.50
21.66 −19.79 22.09 −22.71
0.72 −0.66 0.74 −0.76
134.96 −110.84 164.54 −129.37
42.31 −49.40 54.00 −46.04
1.41 −1.65 1.80 −1.53
137.32 NA NA −108.72
64.95 NA NA −77.53
2.17 NA NA −2.58
Note: Pcr, Δcr, and θcr represent the cracking load, corresponding displacement, and corresponding overall drift ratio. Py, Δy, and θy represent the yield load, corresponding displacement, and corresponding overall drift ratio. Pm, Δm, and θm represent the maximum load, corresponding displacement, and corresponding overall drift ratio. Pu, Δu, and θu represent the ultimate load, corresponding displacement, and corresponding overall drift ratio.
bonded CFRP sheets resulted in a higher lateral bearing capacity of the strengthened frames compared to the bare frame. Individually, the lateral bearing capacity of the second story of the strengthened frame was approximately 1.21 (positive) and 1.21 (negative) times of that of the corresponding bare specimen. This situation is similar to the previous overall lateral bearing capacity of the strengthened frame. In summary, the lateral bearing capacity of non-ductile RC frames can be improved by CFRP reinforcement. (2) The cracking load (Pcr) and yield load (Py) of the strengthened specimen are 2.24 times and 1.20 times than that of the bare specimen, respectively. For each load level (see Fig. 12), the peak point of the first cycle is slightly higher than the two subsequent cycles. Particularly in the 1.55% (−3Δ) loading cycles, the maximum base shear force of the first cycle is 165 kN, but the maximum base shear forces of the second and third cycles are 161 kN and 158 kN, indicating that the decrease in strength of the frame is mainly due to the deterioration of the strength of column bases (see Fig. 10b). The strength degradation indicates that the maximum load in each cycle decreases as the number of load cycles increases. The bearing capacity degradation coefficient, λ, can be calculated using Eq. (1) [25].
Fig. 14. Diagram of energy equivalent method.
axis.
λ = P3 max / P1 max
4.2. Strength and stiffness
(1)
where P1max and P3max represent the maximum load of the first cycle and third cycle during the displacement control method, respectively. The bearing capacity degradation coefficients of the strengthened specimen are presented in Fig. 15. The bearing capacity degradation coefficients of the strengthened specimen are between 0.949 and 0.991, and its average value is 0.966. This reveals that the strengthened specimen has a stable lateral bearing capacity and the lateral strength of a non-ductile RC frame strengthened with CFRP sheets causes almost no
Envelop curves were constructed from the story shear (or base shear) versus story drift ratio (or overall drift ratio) hysteresis curves by connecting end points at each loading stage, as presented in Fig. 13. The results at the key points (i.e., cracking point, yield point, peak point, and ultimate point) of the envelop curves [24] of the bare frame and the strengthened frame are listed in Table 4. The cracking point refers to the point where the first crack occured in the specimen. The structural yield point can be determined using the energy equivalent method [24], in which S (OAB) = S (BYM), as presented in Fig. 14. The peak point was related to the maximum lateral load, Pu, on the envelop curves. The ultimate point of the strengthened specimen is equal to 0.85 Pm (i.e., Pu = 0.85 Pm) at the descending branch of the envelop curves. Nevertheless, for the bare specimen, due to its poor ductility behavior, it’s backbone curve dropped sharply to the ultimate point after the peak point, as shown in Fig. 13c. The positive lateral displacement increased rapidly when the overall drift ratio reached 1.41%, but the lateral load hardly changed. Based on the test observations described above, the maximum point of the corresponding cycle can be considered as the ultimate point of the bare specimen when the overall drift ratio reached 2.17%. The behavior of the specimens at different characteristic points is summarized in Table 4. Based on Fig. 13 and Table 4, the following calculations and conclusions can be made: (1) The overall lateral bearing capacity of the strengthened frame was approximately 1.21 (positive) and 1.17 (negative) times of that of the corresponding bare specimen. To a certain extent, the externally
Fig. 15. Strength degradation with displacements. 8
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Table 5 Stiffness of specimens. Specimen
k0 (kN/mm)
kcr (kN/mm)
ky (kN/mm)
km (kN/mm)
Bare Strengthened Ratio
9.62 10.58 1.10
7.01 7.64 1.09
4.18 4.66 1.11
2.68 2.94 1.10
Note: Ratio is the ratio of stiffness of the strengthened frame to that of the bare frame.
Fig. 17. Energy dissipation with cycles.
μΔ = Δu /Δy
(2)
where Δy represents the yielding displacement, and Δu represents the ultimate displacement. The ductility coefficients of the two specimens are presented in Table 6. The displacement ductility of the strengthened specimen is 1.1 times of that of the bare specimen. This confirms that the externally bonded CFRP reinforcement effectively improves the ductility of the frame. The energy dissipation capacity of a frame is an important index to evaluate its seismic performance. In this study, energy dissipation, E, is defined as the area enclosed under the first hysteresis loop. Fig. 17 presents the energy dissipation with each cycle. The relationship between the energy dissipation and lateral displacement is shown in Fig. 18. Table 7 presents results of the key points (i.e., yield point, peak point and ultimate point) on envelop curves of the specimens. The energy dissipation, Ey, Em, and Eu, correspond to the yield point, peak point, and ultimate point mentioned above. It should be noted that Ey, Em, and Eu were indirectly obtained from Fig. 20 by the linear interpolation method. In addition, the equivalent viscous damping coefficient, he, is also an important parameter which can be used to evaluate the seismic performance of the specimens [26]. The equivalent viscous damping coefficient can be calculated using Eq. (3) [27].
Fig. 16. Curves of stiffness degeneration.
degeneration. The initial-load stiffness, k0, the cracking stiffness, kcr, the yield stiffness, ky and the peak stiffness, km, of the specimens are listed in Table 5. The stiffness degradation curves for the specimens were presented in Fig. 16. The following can be observed in Table 5 and Fig. 16: (1) During the entire test process, the stiffness of the strengthened frame is always greater than that of the bare frame. Furthermore, the stiffness degradation curves of the bare specimen and strengthened frame both show downward-sloping trends. At the early loading stage, the downward-sloping trend is clear. As the displacement increases, the trend of stiffness degradation remains relatively flat. (2) Differences of stiffness between the two frames are not easily observable, showing that the externally bonded CFRP reinforcement has limited influence on stiffness.
he =
1 SABCDEFG ∙ 2π SΔAOI + SΔDOH
where SABCDEFG represents the area enclosed by the hysteresis loop
4.3. Ductility and energy dissipation capacity When subject to seismic load, non-ductile RC frames usually show poor plastic deformation ability. Using the yield point and ultimate point listed in Table 4, the displacement ductility coefficient, μΔ , is defined as [26]: Table 6 Displacement ductility coefficients of specimens. Specimen
Δu
Δy
μΔ
Bare Strengthened Ratio
64.95 −77.53 NA
21.66 −22.71 NA
3.00 3.41 1.14
(3)
Note: Ratio is the ratio of displacement ductility coefficient of the strengthened frame to that of the bare frame.
Fig. 18. Energy dissipation with displacements. 9
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large lateral displacement. Duo to the CFRP strengthening system, the strengthened specimen has larger E and he at the same lateral displacement compared with the bare specimen when the lateral displacement becomes larger than 9 mm. (2) It is indicated that the ratio of energy dissipation varies from 2.05 to 3.01; the ratio of equivalent viscous damping coefficient of the strengthened frame to that of the bare frame varies from 1.50 to 1.92. This observation indicates that the externally bonded CFRP reinforcement results in substantial improvement in the energy dissipating capability of the RC frame. 4.4. Limitations of the current study This paper conducted a quasi-static test on a bare RC frame and a strengthened RC frame. To figure out whether this reinforcement method can effectively improve the seismic capacity of the frame structure, only two RC frame specimens without slabs were designed. The research presented in this paper has some certain limitations, which are summarized as the following three aspects:
Fig. 19. Calculation diagram of equivalent viscous damping coefficient. Table 7 Measured E and he at characteristic points. Specimen
Bare Strengthened Ratio
Yield point
Peak point
(1) The effect of infilled walls on the seismic performance of the frame structures is not considered. The presence of infilled walls may cause the uneven distribution of the vertical stiffness of the frame structures, which may result in weakness floors. Besides, the confining effect of the infilled walls may also have the influence on the seismic performance of the frame structures. (2) The influence of floor slab on the seismic performance of the frame structures is not considered. The presence of floor slabs may change the order of the appearance of the plastic hinges. Besides, the failure modes of the specimens may also change. (3) The horizontal seismic force of this experiment is determined according to the equivalent base shear method. This method ignore the influence of high-order vibration mode of the structures and thus only applicable to the relatively regular frame structures.
Ultimate point
Ey
hey
Em
hem
Eu
heu
580.41 1191.16 2.05
0.054 0.081 1.50
1945.30 5498.84 2.83
0.061 0.114 1.87
3492.35 10,500.05 3.01
0.077 0.148 1.92
Note: Ratio is the ratio of the E or he of strengthened frame to that of the bare frame.
5. Conclusions The seismic performance of CFRP-strengthened non-ductile RC frames was experimentally investigated in the current study. Critical test results such as the failure modes, failure phenomenon, hysteresis response, strength and stiffness, ductility, and energy dissipation capacity were presented and discussed. Based on the experimental results, the following conclusions are obtained: (1) No major cracks were observed in the CFRP-reinforced specimen when it was subjected to a large displacement (a total drift ratio of 3.1%). This confirms that, as a strengthening material, CFRP sheets can keep the non-ductile RC frame from serious damage in the event of strong earthquakes. (2) The effects of the externally bonded FRP reinforcement on lateral stiffness are limited, thus there is no significant influence on the original lateral stiffness of a structure. However, the lateral strength of the strengthened specimen around 20% higher than that of the bare specimen. Moreover, the bearing capacity degradation coefficients of the strengthened specimen are between 0.949 and 0.991, roughly equal to one. This indicates that the strengthened frame has a stable bearing capacity. (3) The ductility coefficient of the strengthened frame was improved by 14%. Furthermore, an remarkable improvement in energy dissipation capacity is also achieved when the frame is strengthened at joints by CFRP sheets. This demonstrates that, during an earthquake, the non-ductile RC frames strengthened with CFRP sheets can dissipate the ground motion energy and limit the damage to the structure. From the test, it can be observed that the energy dissipation of the frame strengthened with CFRP sheets is 3.01 times that of the bare non-ductile RC frame. The ratio of equivalent
Fig. 20. Curves of equivalent viscous damping coefficient.
ABCDEFG; and SΔAOI and SΔDOH represent areas of the triangle AOI and DOH, respectively, as presented in Fig. 19. Moreover, Fig. 20 illustrates the influence of displacement on the equivalent viscous damping coefficients. The equivalent viscous damping coefficient at the key points on the envelop curves of the specimens are also presented in Table 7 where hey, hem, and heu, correspond to the yield point, peak point, and ultimate point, respectively. The following observation can be obtained from Figs. 17, 18 and 20, and Table 7: (1) In the first few cycles, the energy dissipation capability of the bare specimen and that of the strengthened specimen are approximately the same. In the subsequent cycles, the strengthened specimen shows a much higher energy dissipation capability compared to the bare specimen. This is because the CFRP strengthening system is not activated until the concrete begin to expand laterally due to the 10
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viscous damping coefficient of the strengthened frame to that of the bare frame reaches 1.92, which also underscores the efficiency of the CFRP retrofitting method.
[10] Beydokhti EZ, Shariatmadar H. Strengthening and rehabilitation of exterior RC beam–column joints using carbon-FRP jacketing. Mater Struct 2016;49:5067–83. [11] Parghi A, Alam MS. Seismic collapse assessment of non-seismically designed circular RC bridge piers retrofitted with FRP composites. Compos Struct 2016;160:901–16. [12] Said AM, Nehdi ML. Use of FRP for RC frames in seismic zones: Part I. Evaluation of FRP beam-column joint rehabilitation techniques. Appl Compos Mater 2004;11(4):205–26. [13] Le-Trung K, Lee K, Lee J, Lee DH, Woo S. Experimental study of RC beam-column joints strengthened using CFRP composites. Compos B 2010;41:76–85. [14] Lee WT, Chiou YJ, Shih MH. Reinforced concrete beam-column joint strengthened with carbon fiber reinforced polymer. Compos Struct 2010;92:48–60. [15] Balsamo A, Colombo A, Manfredi G, Negro P, Prota A. Seismic behavior of a fullscale RC frame repaired using CFRP laminates. Eng Struct 2005;27:769–80. [16] Wang DY, Wang ZY, Li H, Smith ST. Shaking table test of large scale nonductile RC frames retrofitted with FRP composites 11-13 December Fourth Asia-Pacific conference on FRP in structures, Melbourne, Australia. 2013. [17] Peng Y, Ma M, Dong G. Seismic performance analysis of FRP reinforced concrete frame structure 27-29 September CICE 2010 – The 5th international conference on FRP composites in civil engineering, Beijing, China. 2010. [18] Niroomandi A, Maheri A, Maheri MR, Mahini SS. Seismic performance of ordinary RC frames retrofitted at joints by FRP sheets. Eng Struct 2010;32:2326–36. [19] Cao VV, Ronagh HR. Reducing the seismic damage of reinforced concrete frames using FRP confinement. Compos Struct 2014;118:403–15. [20] Shin J, Scott DW, Stewart LK, Yang CS, Wright TR, DesRoches R. Dynamic response of a full-scale reinforced concrete building frame retrofitted with FRP column jackets. Eng Struct 2016;125:244–53. [21] Pan P, Zhao G, Lu X, Deng K. Force–displacement mixed control for collapse tests of multistory buildings using quasi-static loading systems. Earthquake Eng Struct Dyn 2014;43:287–300. [22] TJ11-74, Code for seismic design of industrial and civil buildings, 1974. Beijing, China: Architecture& Building Press (in Chinese). [23] JGJ101-96, Specification of testing methods for earthquake resistant building, 1997. Beijing, China: China Architecture and Building Press (in Chinese). [24] Zhang J, Jia J. Experimental study on seismic behavior of composite frame consisting of SRC beams and SRUHSC columns subjected to cyclic loading. Constr Build Mater 2016;125:1055–65. [25] Xue W, Cheng B, Zheng R, Li L, Li J. Seismic Performance of nonprestressed and prestressed HPC frames under low reversed cyclic loading. J Struct Eng 2011;137:1254–62. [26] Youssf O, ElGawady MA, Mills JE. Static cyclic behaviour of FRP-confined crumb rubber concrete columns. Eng Struct 2016;113:371–87. [27] Lu Y, Hao H, Carydis PG, Mouzakis H. Seismic performance of RC frames designed for three different ductility levels. Eng Struct 2001;23:537–47.
Acknowledgments The authors gratefully acknowledge the financial support for this work from the Natural Science Foundation of China (Grant No. 51408131 and No. 51508099). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compstruct.2019.04.042. References [1] Celik OC, Ellingwood BR. Seismic risk assessment of gravity load designed reinforced concrete frames subjected to Mid-America ground motions. J Struct Eng 2009;35(4):414–24. [2] Mohamed HM, Masmoudi R. Flexural strength and behavior of steel and FRP-reinforced concrete-filled FRP tube beams. Eng Struct 2010;32:3789–800. [3] Benjeddou O, Ouezdou MB, Bedday A. Damaged RC beams repaired by bonding of CFRP laminates. Constr Build Mater 2007;21:1301–10. [4] Ozcan O, Binici B, Ozcebe G. Seismic strengthening of rectangular reinforced concrete columns using fiber reinforced polymers. Eng Struct 2010;32(4):964–73. [5] Ilki A, Demir C, Bedirhanoglu I, Kumbasar N. Seismic retrofit of brittle and low strength RC columns using fiber reinforced polymer and cementitious composites. Adv Struct Eng 2009;12(3):325–47. [6] Al-Salloum YA, Almusallam TH, Alsayed SH, Siddiqui NA. Seismic behavior of asbuilt, ACI-complying, and CFRP-repaired exterior RC beam- column joints. J Compos Constr 2011;15(4):522–34. [7] Li B, Kai Q. Seismic behavior of reinforced concrete interior beam-wide column joints repaired using FRP. J Compos Constr 2011;15(3):327–38. [8] Antonopoulos CP, Triantafillou TC. Experimental investigation of FRP-strengthened RC beam-column joints. J Compos Constr 2003;7(1):39–49. [9] Al-Salloum YA, Almusallam TH. Seismic response of interior RC beam-column joints upgraded with FRP sheets. I: Experimental study. J Compos Constr 2007;11(6):575–89.
11
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Seismic performance of non-ductile RC frames strengthened with CFRP a
a
a
Weihong Chen , Weirong Shou , Zehui Qiao , Shuangshuang Cui a b
b,⁎
T
School of Civil Engineering, Fuzhou University, Fuzhou 350108, China School of Civil Engineering, Fujian University of Technology, Fuzhou 350118, China
A R T I C LE I N FO
A B S T R A C T
Keywords: CFRP Non-ductile RC frames Strengthened Seismic performance Lateral cyclic load
To study the seismic performance of non-ductile reinforced concrete (RC) frames strengthened with carbon fiberreinforced polymer (CFRP), two 1/2-scale, 2-bay, 2-story RC frames were prepared and tested under combined axial load and cyclic lateral load. One of the RC frames served as a control specimen (bare specimen) and the other was strengthened with CFRP sheets (strengthened specimen) at its joint regions. Based on the hysteretic curves, the envelop curves, and several important indexes (e.g., strength, stiffness, ductility, energy dissipation capacity) of the two specimens, the performance of the employed seismic strengthening technique in improving the seismic performance of RC frames was discussed in detail. The experimental results indicated that the externally bonded CFRP reinforcement had little influence on the lateral stiffness of the RC frame. While the lateral strength of the strengthened RC frame increased by about 20% compared to the control RC frame. Furthermore, it is worth noting that the energy dissipation capacity of the strengthened RC frame is as much as 3.01 times that of the control RC frame. The test results of the current study indicate that the employed strengthening method can substantially improve the seismic performance of the strengthened RC frames.
1. Introduction Numerous reinforced concrete (RC) buildings designed prior to the 1970s are still in service in seismically active regions in Central and Western China. The Chinese code for seismic design of buildings has been updated twice to address the earthquake damage to buildings since the Wenchuan earthquake in 2008. It is notable that many structures are vulnerable to earthquakes due to the obsolete design strategies, underestimation of seismic input, and lack of proper structural design details. These existing RC structures and buildings were usually designed either for gravity load alone or with little consideration of seismic forces and detailing and thus as they are insufficient in horizontal shear strength and ductility. Among such RC structures and buildings are a large number of RC frames and such RC frames are classified as non-ductile RC frames in this study. These structures have limited lateral stiffness and strength, so they tend to develop excessive sideways or soft-story mechanisms under earthquake ground motions [1]. As a result, all of these non-ductile RC frames are at a high risk of experiencing severe damage or collapse in the event of severe earthquakes. Usually, the “destroy-reconstruct” approach is taken, but it is costly. To mitigate this risk, it is proposed to retrofit the frames to be more ductile. This method is not only cost-effective, but also environmentally friendly, particularly in less developed regions.
⁎
Steel plates, bars, or cables have been typically used for strengthening, but their heavy weight and low corrosion resistance have rendered these materials as somewhat undesirable. Fiber reinforced polymer (FRP) composites have been widely used for seismic strengthening or rehabilitation of concrete structures in the past decades due to their high strength to weight ratio, excellent corrosion resistance and ease of application. The FRP retrofit of RC frame members such as beams [2,3], columns [4,5], and joints [6,7] indicates the effectiveness of the retrofitting technique. In recent decades, FRP-strengthened RC joints have been studied by many investigators. Antonopoulos et al. [8] applied simulated seismic load tests on eighteen 2/3-scale exterior RC joints strengthened with FRP; they concluded that, under simulated seismic loads, externally bonded FRP reinforcement is a viable solution to enhance the strength, energy dissipation capacity, and stiffness amplitude of RC joints with inadequate shear strengthening. Al-Salloum [9] analyzed four RC interior beam-column joints by means of pseudo-static testing. Two of the four specimens were used as baseline specimens and the other two were strengthened with CFRP sheets. The comparison revealed that CFRP sheets improve the shear resistance and ductility of the joint. Beydokhti [10] investigated the behavior of shear deficient exterior beam–column joints strengthened by CFRP. Moreover, their test revealed that FRP predominantly affects the joint behavior in high levels of damage.
Corresponding author. E-mail address: [email protected] (S. Cui).
https://doi.org/10.1016/j.compstruct.2019.04.042 Received 25 June 2017; Received in revised form 16 January 2019; Accepted 5 April 2019 Available online 13 April 2019 0263-8223/ © 2019 Elsevier Ltd. All rights reserved.
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Parghi [11] studied fragility curves of typical single circular concrete bridge piers reinforced with FRP and concluded that the amount of reinforcement, shear span-depth ratio, and the level of the axial load can significantly affect the collapse fragility curve of the strengthened bridge piers. Said [12] assessed the strength, ductility, and energy dissipation gains of substandard beam-column joints enhanced by FPR rehabilitation techniques. The results indicated that FRP joint repair schemes can generally enhance the performance of substandard joints, but they often do not meet the current standard. The experiments [13,14] revealed that for a CFRP strengthened frame node, the ductility of the joints was improved, the failure mode of the node core zone was changed, and the shear failure of the joints was delayed. Furthermore, the strength and stiffness were improved. Plenty of comprehensive researches on the FRP strengthened RC members and beam-column joints have been released, but only a limited number of studies investigated the effectiveness of the FRP on the overall seismic behavior of RC structures. Balsamo et al. [15] developed a full-scale dual system (4-story and 2-bay) subjected to pseudo-dynamic tests. The experimental tests revealed that repaired structures obtained a large displacement capacity without exhibiting any loss of strength and were able to provide energy dissipation similar to that of the original configuration. Wang et al. [16] conducted the shaking table test on two 1/2-scale 4-story and 2-bay non-ductile RC frames (one was bare; the other was FRP-strengthened) to investigate and evaluate the seismic performance of non-ductile RC frames strengthened with FRP. Based on the experimental study for seismic performance of FRP-strengthened RC beam-column joints, Peng et al. [17] carried out a static elastoplastic analysis on the joints of the FRP-strengthened concrete frames, and the result clearly indicated that effective reinforcement for the beam–column joints can improve the overall seismic performance of frame structures. Niroomandi et al. [18] carried out nonlinear pushover analysis of the full frame to evaluate its force-displacement capacity curve, and results revealed a notable improvement in the performance when the frame was strengthened at the joints by FRP. Furthermore, Cao and Ronagh [19] conducted comparative inelastic time history and damage analyses on an 8-story poorly-confined and FRP retrofit RC frame, and the results revealed that the poorly-confined frame was essentially upgraded to the level of the intermediate frame, which was designed to more restrictive requirements. Shin et al. [20] also performed a series of dynamic tests on a fullscale, non-ductile RC frame strengthened with prefabricated FRP jackets on the first-story columns, and the results demonstrated that the retrofit scheme helped develop a more uniform story drift distribution. Previous theoretical and experimental studies clearly supported the effectiveness and advantages of using FRP to prevent the failure of nonductile structural members that are seismically deficient, such as beams, columns, and joints. Furthermore, few experimental studies have been conducted on reducing the potential damage of multistory non-ductile RC frames subjected to vertical concentrated loads and horizontal cyclic loads. However, for the capacity evaluation of structural components and systems, the conventional quasi-static test continues to be performed in engineering practices [21]. Thus, it is necessary to conduct reduced-scale or full-scale tests on non-ductile RC frames strengthened with FRP, the results of which will serve as the premise for the analysis of the seismic behavior of non-ductile RC frames strengthened with FRP. The primary objective of the present study is to explore the seismic behaviors of RC frames strengthened with CFRP. Pseudo-static tests of a bare non-ductile RC frame and a CFRP strengthened non-ductile RC frame were conducted to investigate the variations of the mechanisms in failure mode, seismic behavior, and performance between bare and CFRP-strengthened frames.
Fig. 1. Outlines and reinforcement detailing of the RC frame (all dimensions are in mm).
2. Experimental program 2.1. Test specimens In this study, an existing two-bay, five-story building in Western China with non-ductile RC frames was selected as the prototype. This building was designed and detailed as per the old Chinese Standards [22] which account for only gravity loads. The width of each bay of the building is 4.0 m, and each story height is 3.0 m. In order to carry out a comparative analysis on the seismic behavior of CFRP-strengthened frame structures, two non-ductile RC frames were constructed. In view of the cost-effectiveness of the test and the limitations of the loading facilities, only the bottom two stories were physically tested using substructuring techniques. The scale ratio of the test substructure specimens to the prototype structure is 1:2. One specimen was used as the control specimen (bare specimen) and the other was strengthened with CFRP sheets (strengthened specimen) at its joint regions. The foundation beams and columns were also included in the test model to accurately simulate the constraint conditions of the structure base. Outlines and reinforcement detailing of the specimens are presented in Fig. 1. The dimensions and reinforcement detailing were determined based on the prototype frame members using similitude relationships, and are summarized in Table 1. For the strengthened specimen, the reinforced regions were polished and scrubbed with acetone. The reinforced region was then wrapped with epoxy-impregnated CFRP sheets (3 layers) via the wet lay-up method. 2.2. Test set-up As presented in Fig. 2, the test set-up includes a reaction wall, a strong floor, two vertical jacks, and two horizontal actuators. The specimens were tied to the strong floor. The two vertical jacks were used to apply constant vertical forces to the columns. The two horizontal actuators, which were fixed to the reaction wall, were used to apply horizontal forces to the two stories. All hydraulic actuators had a loading capacity of 500 kN. Furthermore, the horizontal fixed connecting pieces included steel rods, steel plates, and nuts, making the actuators work effectively. 2.3. Material properties Compressive tests of concrete were conducted after 28 days of curing time. According to the Chinese standard GB 50010-2010, six cubic samples (150 mm × 150 mm × 150 mm) and six prismatic samples (150 mm × 150 mm × 300 mm) were tested under a compression testing machine. The average compressive strength obtained from the cubic samples was 29 MPa. The average compressive strength, elasticity modulus, and Poisson’s ratio of the prismatic samples were 23 MPa, 2
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Table 1 Geometric details of various RC members of test specimens. Member
Overall size (L × B × H)
Clear cover (mm)
Longitudinal reinforcement
Transverse reinforcement (mm c/c)
Column Beam
1500 × 200 × 200 2000 × 150 × 200
15 15
10Y (4 nos.) 10Y (8 nos.)
6Y @200 6Y @200
Fig. 4. Loading procedure for low cyclic loading test.
to monitor the lateral load and displacement applied to the test specimens, respectively. Two linear variable differential transformers (LVDTs) were fixed on the middle column of the second layer to measure the lateral displacements-one was fixed at the middle of the column, and the other was fixed at the bottom of the column (see Fig. 5). An additional dial indicator was also used to measure the displacement of beam–column joints. At the section of the joint, strain gauges were attached to the longitudinal reinforcement of the beams and columns along the direction of the horizontal and vertical forces to measure the real-time longitudinal strain (see Fig. 3).
Fig. 2. Specimen with test set-up (all dimensions are in mm).
2.5. Loading history Vertical loads were applied to the top of the frame columns to simulate the gravity of the upper three stories. Accordingly, the vertical load on the middle column was 269.24 kN, and that on each of the two side columns was 191.08 kN. In the test, one of the side columns of the RC frame was connected to the two MTS actuators at the joint regions as shown in Fig. 2. The seismic forces on the structure are assumed to follow an inverted triangular pattern [23]—the proportion of the designed seismic forces from the fifth to the first story of the prototype non-ductile RC frame is 5:4:3:2:1. Since only the first and the second story were tested, the seismic forces sustained by the upper three stories (i.e., the third, fourth and fifth stories) had to be applied to the second story. Although the distribution of the designed seismic force of the second story is 2F, considering the loads transmitted form the upper stories (3–5 stories), the actual horizontal shear force in the second story is 14F. Similarly, the cyclic lateral load of F is applied to the first story, and the actual horizontal shear force of the first story is 15F. Therefore, two cyclic lateral loads with a proportion of 14:1 are applied by the two horizontal actuators at the second and first story. In the test, the lateral load was applied by the force-controlled procedure before the first yielding of the longitudinal reinforcement occurred, and a displacement-controlled procedure was adopted afterward. In addition, the actuator connected to the second story of the RC frame was regarded as the main control actuator and the loading history for the main control actuator is shown in Fig. 4. The lateral load applied on the first story is then always 1/14 of that applied on the second story. Moreover, the testing of the strengthened specimen was stopped after the residual lateral load decreased to 85% of the peak load and the
Fig. 3. The layout of the strain gauges.
33,100 N/mm2, and 0.23, respectively. In addition, tensile tests were conducted for the steel bars to determine the yield stress, ultimate strengths, and elastic modulus and the test results are listed in Table 2. Moreover, the coupon tensile tests were conducted for the employed CFRP sheets according the Chinese standard GB 50608-2010. The CFRP coupons had the dimensions of 15 mm × 230 mm and the nominal thickness of the CFRP sheet was 0.167 mm. As shown in Table 3, the elastic modulus, ultimate tensile strength, and ultimate tensile strain of the CFRP sheet were 240 GPa, 4340 MPa, and 0.017, respectively.
2.4. Instrumentation The load cell and displacement transducer in the actuator were used 3
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Table 2 Mechanical properties of steel bars. Member
Yield strength (MPa)
Ultimate strength (MPa)
Discrepancy of yield strength (%)
Discrepancy of ultimate strength (%)
Elastic modulus (GPa)
Stirrups Longitudinal bars
313 424
487 549
2.6 2.0
2.2 1.6
218 197
Table 3 Properties of CFRP sheets. Member
Grammage (g/mm2)
Thickness (mm)
Tensile strength (MPa)
Elastic modulus (GPa)
Elongation (%)
CFRP sheets
300
0.167
4340
240
1.7
Fig. 6. The second RC frame with the plastic hinge regions to be strengthened.
Fig. 5. The first RC frame (control specimen) in test.
testing of the bare specimen was stopped when the inter-story drift reached 2%. 2.6. Test procedure The test program was conducted in the National and Local United Research Center for Seismic and Disaster Informatization of Civil Engineering (NLRCSD) at Fuzhou University. The two specimens were prepared and tested in the following procedure: (1) Two RC frames were designed and built; (2) one of the RC frames was tested as control specimens as shown in Fig. 5; (3) the length of the plastic hinges on the control specimen was carefully measured after the test; (4) according to the plastic-hinge length obtained in step (3), the plastic-hinge regions of the other RC frame were prepared for strengthening (see Fig. 6) and then CFRP wraps were applied; (5) the strengthened RC frame was tested as shown in Fig. 7.
Fig. 7. The strengthened specimen in test.
force to the first concrete cracking is described as the elastic stage. In this stage, the lateral force was relatively small. Furthermore, the relationship between the lateral load and the displacement was approximately linear. Cracking stage. As the level of lateral force gradually increased, the first crack appeared in the column at joint II and the corresponding lateral load acting on the second story was −20 kN (see Fig. 8). The first crack in the beam appeared at joint IV when the second-story load approached 30 kN. When the second-story load reached −50 kN, column-penetrating cracks appeared on the column at joint II. Yield stage. Based on the available strain gauge data and visual inspection, when the second-story load reached 61 kN, the value of the steel strain gauge exceeded 1800 με, which indicated the first yield of the steel bar. The lateral displacement at the top of the specimen was measured as 12 mm (yield displacement Δ). Failure stage. After the first yield of the longitudinal steel bar, the lateral loads on the specimen became applied by displacement control.
3. Test observations As stated above, to estimate the effectiveness of the CFRP strengthening method for non-ductile frames, comprehensive comparisons of the strengthened specimen and the control specimen were made based on the test results. For ease of reference, the joints of the two specimens were named with the roman numerals I-IX as shown in Fig. 8. 3.1. Bare specimen failure modes Elastic stage. The period from the onset of the slow-cyclic lateral 4
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Cracking stage. It was difficult to observe the cracks of the strengthened specimen as the plastic hinge region was wrapped with CFRP sheets. Failure of the CFRP sheets initiated with partial debonding at the column base joint III when the lateral force reached −42.17 kN. The test was continued until the lateral force of 56 kN was reached, and the same phenomenon occurred at joint I and joint II. Yield stage. Based on the available strain gauge data and visual inspection, when the load reached 75 kN, the value of the steel strain gauge exceeded 1800 με, which means that the first yield of the steel bar occurred. Also, the lateral displacement at the top of the specimen was measured as 15.5 mm (yield displacement Δ). Failure stage. After the first yield of the longitudinal steel bar, the displacement-controlled loading stage started. During the 1Δ cyclic loading stage, additional steel bars yielded. When the displacement reached 2Δ, the column base split, leading to concrete spalling. During the 3Δ loading stage, cracks occurred in the bare region close to the reinforced area. In the 4Δ loading stage, the peeling phenomenon of CFRP sheets occurred at the joint of the strengthened frame. At the 5Δ loading stage, bulging at the base of the columns was observed due to volumetric expansion and fracturing of the concrete inside the column. The test terminated at the 6Δ cyclic loading stage with the lateral load becoming less than 85% of the peak load. During the entire experiment, no FRP rupture happened. Based on the experimental observations and experimental data, the plastic hinges in different joint formed in the following order: III → II → I → IV → V → VI → VII → IX → VIII. The failure modes are shown in Fig. 10. After the test, the FRP wraps were peeled off from the strengthened specimen to further observe its failure mode. As shown in Fig. 11, the FRP-confined concrete at the plastic hinge region of the columns was severely damaged while that at the plastic hinge regions of the beams was not or only slightly damaged. For the control specimen, however, serious crash of concrete happened at the plastic hinges of both the beams and the columns.
Fig. 8. Names of the joints in the specimen.
During the cycle of the 1Δ loading stage, a crack appeared in the column at joint I. When the lateral displacement increased from 1Δ to 3Δ, the lateral force remained nearly stable. The width of the crack in the column base of plastic hinge II reached 0.50 mm. As the load increased, spalling of the concrete cover appeared in joint II. The test was terminated when the interstory drift reached 2%. Based on the experimental observations and experimental data, plastic hinges at different joint formed in the following sequences: II → IV → VI → III → I → VII → V → IX → VIII. The failure modes are shown in Fig. 9. 3.2. Strengthened specimen failure modes Elastic stage. At the beginning of the test, the lateral force was relatively small, and the relationship between the lateral load and the displacement was approximately linear. The strengthened specimen had a longer elastic stage compared with the bare specimen.
Fig. 9. Failure modes of bare specimen: (a) elastic stage; (b) cracking stage; (c) yield stage; and (d) failure stage. 5
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Fig. 10. Failure modes of strengthened specimen: (a) elastic stage; (b) cracking stage; (c) yield stage; and (d) failure stage.
4. Test results and discussion
direction are substantially higher than that in the negative direction. At the beginning of the test, steel bars did not yield during the force control mode, but the sequence of failure of the members was asymmetrical when associated with significant lateral loading. Consequently, lateral displacements differed on the two sides of the specimens causing differences of the measured force. (3) Further details can be found in the relevant individual hysteretic loop of the strengthened specimen. The strengthened specimen reached the negative peak base shear force of −129.37 kN at the 1.53% (−3Δ) loading stage, and then reached the positive peak base shear force of 164.54 kN at the 1.8% (−3.5Δ) loading stage. At the drift ratio of 3.1% (−6Δ) (the base shear force was −108.72 kN), the shear force dropped more than 15% from its negative peak value. (4) There is a linear relationship between load and displacement in first cycles. After the yield load, the strengthened specimen revealed small residual deformation during unloading; the lateral displacement remained at a certain value, which indicates that the structure was basically in plastic working state. An increasing number of cracks appeared in the plastic hinge zones as the load increased when the column base failed. The curve veers toward the drift ratio
4.1. Hysteretic response of specimens Hysteretic curves of base shear versus controlled overall drift ratio θu and the corresponding story shear versus story drift ratio for the first and second stories of each specimen are shown in Fig. 12. The following observations can be obtained from Fig. 12: (1) The hysteretic loops of the strengthened frame are plumper than that of the bare frame, which means the strengthened frame dissipated more energy. In particular, the rheostriction in the hysteretic loop of the strengthened frame become increasingly obvious with the increase of lateral displacement. This indicates that the resistance of the strengthened frame decreased after significant damage occurred in the plastic hinge region. (2) Although the MTS electro-hydraulic servo structure tester was programed to apply symmetrical lateral load on the specimens, it can be seen in Fig. 12 that the hysteresis curves of both the bare specimen and the strengthened specimen are obviously asymmetrical: the peak load and the corresponding drift ratio in the positive
Fig. 11. Photographs of strengthened specimen after removing CFRP sheets. 6
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Fig. 13. Envelop curves of specimens: (a) first story shear versus story drift ratio; (b) second story shear versus story drift ratio; (c) base shear versus overall drift ratio.
Fig. 12. Hysteresis curves of specimens: (a) first story shear versus story drift ratio; (b) second story shear versus story drift ratio; (c) base shear versus overall drift ratio.
7
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Table 4 Behavior of specimens at different characteristic points. Specimen
Bare Strengthened
Cracking point
Yield point
Peak point
Ultimate point
Pcr (kN)
Δcr (mm)
θcr (%)
Py (kN)
Δy (mm)
θy (%)
Pm (kN)
Δm (mm)
θm (%)
Pu (kN)
Δu (mm)
θu (%)
NA −19.01 NA −43.38
NA −2.71 NA −5.68
NA −0.08 NA −0.19
97.53 −75.80 118.29 −90.50
21.66 −19.79 22.09 −22.71
0.72 −0.66 0.74 −0.76
134.96 −110.84 164.54 −129.37
42.31 −49.40 54.00 −46.04
1.41 −1.65 1.80 −1.53
137.32 NA NA −108.72
64.95 NA NA −77.53
2.17 NA NA −2.58
Note: Pcr, Δcr, and θcr represent the cracking load, corresponding displacement, and corresponding overall drift ratio. Py, Δy, and θy represent the yield load, corresponding displacement, and corresponding overall drift ratio. Pm, Δm, and θm represent the maximum load, corresponding displacement, and corresponding overall drift ratio. Pu, Δu, and θu represent the ultimate load, corresponding displacement, and corresponding overall drift ratio.
bonded CFRP sheets resulted in a higher lateral bearing capacity of the strengthened frames compared to the bare frame. Individually, the lateral bearing capacity of the second story of the strengthened frame was approximately 1.21 (positive) and 1.21 (negative) times of that of the corresponding bare specimen. This situation is similar to the previous overall lateral bearing capacity of the strengthened frame. In summary, the lateral bearing capacity of non-ductile RC frames can be improved by CFRP reinforcement. (2) The cracking load (Pcr) and yield load (Py) of the strengthened specimen are 2.24 times and 1.20 times than that of the bare specimen, respectively. For each load level (see Fig. 12), the peak point of the first cycle is slightly higher than the two subsequent cycles. Particularly in the 1.55% (−3Δ) loading cycles, the maximum base shear force of the first cycle is 165 kN, but the maximum base shear forces of the second and third cycles are 161 kN and 158 kN, indicating that the decrease in strength of the frame is mainly due to the deterioration of the strength of column bases (see Fig. 10b). The strength degradation indicates that the maximum load in each cycle decreases as the number of load cycles increases. The bearing capacity degradation coefficient, λ, can be calculated using Eq. (1) [25].
Fig. 14. Diagram of energy equivalent method.
axis.
λ = P3 max / P1 max
4.2. Strength and stiffness
(1)
where P1max and P3max represent the maximum load of the first cycle and third cycle during the displacement control method, respectively. The bearing capacity degradation coefficients of the strengthened specimen are presented in Fig. 15. The bearing capacity degradation coefficients of the strengthened specimen are between 0.949 and 0.991, and its average value is 0.966. This reveals that the strengthened specimen has a stable lateral bearing capacity and the lateral strength of a non-ductile RC frame strengthened with CFRP sheets causes almost no
Envelop curves were constructed from the story shear (or base shear) versus story drift ratio (or overall drift ratio) hysteresis curves by connecting end points at each loading stage, as presented in Fig. 13. The results at the key points (i.e., cracking point, yield point, peak point, and ultimate point) of the envelop curves [24] of the bare frame and the strengthened frame are listed in Table 4. The cracking point refers to the point where the first crack occured in the specimen. The structural yield point can be determined using the energy equivalent method [24], in which S (OAB) = S (BYM), as presented in Fig. 14. The peak point was related to the maximum lateral load, Pu, on the envelop curves. The ultimate point of the strengthened specimen is equal to 0.85 Pm (i.e., Pu = 0.85 Pm) at the descending branch of the envelop curves. Nevertheless, for the bare specimen, due to its poor ductility behavior, it’s backbone curve dropped sharply to the ultimate point after the peak point, as shown in Fig. 13c. The positive lateral displacement increased rapidly when the overall drift ratio reached 1.41%, but the lateral load hardly changed. Based on the test observations described above, the maximum point of the corresponding cycle can be considered as the ultimate point of the bare specimen when the overall drift ratio reached 2.17%. The behavior of the specimens at different characteristic points is summarized in Table 4. Based on Fig. 13 and Table 4, the following calculations and conclusions can be made: (1) The overall lateral bearing capacity of the strengthened frame was approximately 1.21 (positive) and 1.17 (negative) times of that of the corresponding bare specimen. To a certain extent, the externally
Fig. 15. Strength degradation with displacements. 8
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Table 5 Stiffness of specimens. Specimen
k0 (kN/mm)
kcr (kN/mm)
ky (kN/mm)
km (kN/mm)
Bare Strengthened Ratio
9.62 10.58 1.10
7.01 7.64 1.09
4.18 4.66 1.11
2.68 2.94 1.10
Note: Ratio is the ratio of stiffness of the strengthened frame to that of the bare frame.
Fig. 17. Energy dissipation with cycles.
μΔ = Δu /Δy
(2)
where Δy represents the yielding displacement, and Δu represents the ultimate displacement. The ductility coefficients of the two specimens are presented in Table 6. The displacement ductility of the strengthened specimen is 1.1 times of that of the bare specimen. This confirms that the externally bonded CFRP reinforcement effectively improves the ductility of the frame. The energy dissipation capacity of a frame is an important index to evaluate its seismic performance. In this study, energy dissipation, E, is defined as the area enclosed under the first hysteresis loop. Fig. 17 presents the energy dissipation with each cycle. The relationship between the energy dissipation and lateral displacement is shown in Fig. 18. Table 7 presents results of the key points (i.e., yield point, peak point and ultimate point) on envelop curves of the specimens. The energy dissipation, Ey, Em, and Eu, correspond to the yield point, peak point, and ultimate point mentioned above. It should be noted that Ey, Em, and Eu were indirectly obtained from Fig. 20 by the linear interpolation method. In addition, the equivalent viscous damping coefficient, he, is also an important parameter which can be used to evaluate the seismic performance of the specimens [26]. The equivalent viscous damping coefficient can be calculated using Eq. (3) [27].
Fig. 16. Curves of stiffness degeneration.
degeneration. The initial-load stiffness, k0, the cracking stiffness, kcr, the yield stiffness, ky and the peak stiffness, km, of the specimens are listed in Table 5. The stiffness degradation curves for the specimens were presented in Fig. 16. The following can be observed in Table 5 and Fig. 16: (1) During the entire test process, the stiffness of the strengthened frame is always greater than that of the bare frame. Furthermore, the stiffness degradation curves of the bare specimen and strengthened frame both show downward-sloping trends. At the early loading stage, the downward-sloping trend is clear. As the displacement increases, the trend of stiffness degradation remains relatively flat. (2) Differences of stiffness between the two frames are not easily observable, showing that the externally bonded CFRP reinforcement has limited influence on stiffness.
he =
1 SABCDEFG ∙ 2π SΔAOI + SΔDOH
where SABCDEFG represents the area enclosed by the hysteresis loop
4.3. Ductility and energy dissipation capacity When subject to seismic load, non-ductile RC frames usually show poor plastic deformation ability. Using the yield point and ultimate point listed in Table 4, the displacement ductility coefficient, μΔ , is defined as [26]: Table 6 Displacement ductility coefficients of specimens. Specimen
Δu
Δy
μΔ
Bare Strengthened Ratio
64.95 −77.53 NA
21.66 −22.71 NA
3.00 3.41 1.14
(3)
Note: Ratio is the ratio of displacement ductility coefficient of the strengthened frame to that of the bare frame.
Fig. 18. Energy dissipation with displacements. 9
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large lateral displacement. Duo to the CFRP strengthening system, the strengthened specimen has larger E and he at the same lateral displacement compared with the bare specimen when the lateral displacement becomes larger than 9 mm. (2) It is indicated that the ratio of energy dissipation varies from 2.05 to 3.01; the ratio of equivalent viscous damping coefficient of the strengthened frame to that of the bare frame varies from 1.50 to 1.92. This observation indicates that the externally bonded CFRP reinforcement results in substantial improvement in the energy dissipating capability of the RC frame. 4.4. Limitations of the current study This paper conducted a quasi-static test on a bare RC frame and a strengthened RC frame. To figure out whether this reinforcement method can effectively improve the seismic capacity of the frame structure, only two RC frame specimens without slabs were designed. The research presented in this paper has some certain limitations, which are summarized as the following three aspects:
Fig. 19. Calculation diagram of equivalent viscous damping coefficient. Table 7 Measured E and he at characteristic points. Specimen
Bare Strengthened Ratio
Yield point
Peak point
(1) The effect of infilled walls on the seismic performance of the frame structures is not considered. The presence of infilled walls may cause the uneven distribution of the vertical stiffness of the frame structures, which may result in weakness floors. Besides, the confining effect of the infilled walls may also have the influence on the seismic performance of the frame structures. (2) The influence of floor slab on the seismic performance of the frame structures is not considered. The presence of floor slabs may change the order of the appearance of the plastic hinges. Besides, the failure modes of the specimens may also change. (3) The horizontal seismic force of this experiment is determined according to the equivalent base shear method. This method ignore the influence of high-order vibration mode of the structures and thus only applicable to the relatively regular frame structures.
Ultimate point
Ey
hey
Em
hem
Eu
heu
580.41 1191.16 2.05
0.054 0.081 1.50
1945.30 5498.84 2.83
0.061 0.114 1.87
3492.35 10,500.05 3.01
0.077 0.148 1.92
Note: Ratio is the ratio of the E or he of strengthened frame to that of the bare frame.
5. Conclusions The seismic performance of CFRP-strengthened non-ductile RC frames was experimentally investigated in the current study. Critical test results such as the failure modes, failure phenomenon, hysteresis response, strength and stiffness, ductility, and energy dissipation capacity were presented and discussed. Based on the experimental results, the following conclusions are obtained: (1) No major cracks were observed in the CFRP-reinforced specimen when it was subjected to a large displacement (a total drift ratio of 3.1%). This confirms that, as a strengthening material, CFRP sheets can keep the non-ductile RC frame from serious damage in the event of strong earthquakes. (2) The effects of the externally bonded FRP reinforcement on lateral stiffness are limited, thus there is no significant influence on the original lateral stiffness of a structure. However, the lateral strength of the strengthened specimen around 20% higher than that of the bare specimen. Moreover, the bearing capacity degradation coefficients of the strengthened specimen are between 0.949 and 0.991, roughly equal to one. This indicates that the strengthened frame has a stable bearing capacity. (3) The ductility coefficient of the strengthened frame was improved by 14%. Furthermore, an remarkable improvement in energy dissipation capacity is also achieved when the frame is strengthened at joints by CFRP sheets. This demonstrates that, during an earthquake, the non-ductile RC frames strengthened with CFRP sheets can dissipate the ground motion energy and limit the damage to the structure. From the test, it can be observed that the energy dissipation of the frame strengthened with CFRP sheets is 3.01 times that of the bare non-ductile RC frame. The ratio of equivalent
Fig. 20. Curves of equivalent viscous damping coefficient.
ABCDEFG; and SΔAOI and SΔDOH represent areas of the triangle AOI and DOH, respectively, as presented in Fig. 19. Moreover, Fig. 20 illustrates the influence of displacement on the equivalent viscous damping coefficients. The equivalent viscous damping coefficient at the key points on the envelop curves of the specimens are also presented in Table 7 where hey, hem, and heu, correspond to the yield point, peak point, and ultimate point, respectively. The following observation can be obtained from Figs. 17, 18 and 20, and Table 7: (1) In the first few cycles, the energy dissipation capability of the bare specimen and that of the strengthened specimen are approximately the same. In the subsequent cycles, the strengthened specimen shows a much higher energy dissipation capability compared to the bare specimen. This is because the CFRP strengthening system is not activated until the concrete begin to expand laterally due to the 10
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viscous damping coefficient of the strengthened frame to that of the bare frame reaches 1.92, which also underscores the efficiency of the CFRP retrofitting method.
[10] Beydokhti EZ, Shariatmadar H. Strengthening and rehabilitation of exterior RC beam–column joints using carbon-FRP jacketing. Mater Struct 2016;49:5067–83. [11] Parghi A, Alam MS. Seismic collapse assessment of non-seismically designed circular RC bridge piers retrofitted with FRP composites. Compos Struct 2016;160:901–16. [12] Said AM, Nehdi ML. Use of FRP for RC frames in seismic zones: Part I. Evaluation of FRP beam-column joint rehabilitation techniques. Appl Compos Mater 2004;11(4):205–26. [13] Le-Trung K, Lee K, Lee J, Lee DH, Woo S. Experimental study of RC beam-column joints strengthened using CFRP composites. Compos B 2010;41:76–85. [14] Lee WT, Chiou YJ, Shih MH. Reinforced concrete beam-column joint strengthened with carbon fiber reinforced polymer. Compos Struct 2010;92:48–60. [15] Balsamo A, Colombo A, Manfredi G, Negro P, Prota A. Seismic behavior of a fullscale RC frame repaired using CFRP laminates. Eng Struct 2005;27:769–80. [16] Wang DY, Wang ZY, Li H, Smith ST. Shaking table test of large scale nonductile RC frames retrofitted with FRP composites 11-13 December Fourth Asia-Pacific conference on FRP in structures, Melbourne, Australia. 2013. [17] Peng Y, Ma M, Dong G. Seismic performance analysis of FRP reinforced concrete frame structure 27-29 September CICE 2010 – The 5th international conference on FRP composites in civil engineering, Beijing, China. 2010. [18] Niroomandi A, Maheri A, Maheri MR, Mahini SS. Seismic performance of ordinary RC frames retrofitted at joints by FRP sheets. Eng Struct 2010;32:2326–36. [19] Cao VV, Ronagh HR. Reducing the seismic damage of reinforced concrete frames using FRP confinement. Compos Struct 2014;118:403–15. [20] Shin J, Scott DW, Stewart LK, Yang CS, Wright TR, DesRoches R. Dynamic response of a full-scale reinforced concrete building frame retrofitted with FRP column jackets. Eng Struct 2016;125:244–53. [21] Pan P, Zhao G, Lu X, Deng K. Force–displacement mixed control for collapse tests of multistory buildings using quasi-static loading systems. Earthquake Eng Struct Dyn 2014;43:287–300. [22] TJ11-74, Code for seismic design of industrial and civil buildings, 1974. Beijing, China: Architecture& Building Press (in Chinese). [23] JGJ101-96, Specification of testing methods for earthquake resistant building, 1997. Beijing, China: China Architecture and Building Press (in Chinese). [24] Zhang J, Jia J. Experimental study on seismic behavior of composite frame consisting of SRC beams and SRUHSC columns subjected to cyclic loading. Constr Build Mater 2016;125:1055–65. [25] Xue W, Cheng B, Zheng R, Li L, Li J. Seismic Performance of nonprestressed and prestressed HPC frames under low reversed cyclic loading. J Struct Eng 2011;137:1254–62. [26] Youssf O, ElGawady MA, Mills JE. Static cyclic behaviour of FRP-confined crumb rubber concrete columns. Eng Struct 2016;113:371–87. [27] Lu Y, Hao H, Carydis PG, Mouzakis H. Seismic performance of RC frames designed for three different ductility levels. Eng Struct 2001;23:537–47.
Acknowledgments The authors gratefully acknowledge the financial support for this work from the Natural Science Foundation of China (Grant No. 51408131 and No. 51508099). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compstruct.2019.04.042. References [1] Celik OC, Ellingwood BR. Seismic risk assessment of gravity load designed reinforced concrete frames subjected to Mid-America ground motions. J Struct Eng 2009;35(4):414–24. [2] Mohamed HM, Masmoudi R. Flexural strength and behavior of steel and FRP-reinforced concrete-filled FRP tube beams. Eng Struct 2010;32:3789–800. [3] Benjeddou O, Ouezdou MB, Bedday A. Damaged RC beams repaired by bonding of CFRP laminates. Constr Build Mater 2007;21:1301–10. [4] Ozcan O, Binici B, Ozcebe G. Seismic strengthening of rectangular reinforced concrete columns using fiber reinforced polymers. Eng Struct 2010;32(4):964–73. [5] Ilki A, Demir C, Bedirhanoglu I, Kumbasar N. Seismic retrofit of brittle and low strength RC columns using fiber reinforced polymer and cementitious composites. Adv Struct Eng 2009;12(3):325–47. [6] Al-Salloum YA, Almusallam TH, Alsayed SH, Siddiqui NA. Seismic behavior of asbuilt, ACI-complying, and CFRP-repaired exterior RC beam- column joints. J Compos Constr 2011;15(4):522–34. [7] Li B, Kai Q. Seismic behavior of reinforced concrete interior beam-wide column joints repaired using FRP. J Compos Constr 2011;15(3):327–38. [8] Antonopoulos CP, Triantafillou TC. Experimental investigation of FRP-strengthened RC beam-column joints. J Compos Constr 2003;7(1):39–49. [9] Al-Salloum YA, Almusallam TH. Seismic response of interior RC beam-column joints upgraded with FRP sheets. I: Experimental study. J Compos Constr 2007;11(6):575–89.
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