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Experimental study of earthquake-resilient prefabricated beam-column steel joint with L-shaped plate
Journal of Constructional Steel Research 166 (2020) 105928
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Journal of Constructional Steel Research
Experimental study of earthquake-resilient prefabricated beam-column steel joint with L-shaped plate Zi-Qin Jiang a,b, Chao Dou c,⁎, Hang Zhang a, Qi Wang a, Yi-huan Yang a a b c
Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing University of Technology, Beijing 100124, China Beijing Advanced Innovation Center for Future Urban Design, Beijing 100044, China School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China
a r t i c l e
i n f o
Article history: Received 17 August 2019 Received in revised form 31 December 2019 Accepted 3 January 2020 Available online xxxx Keywords: Earthquake-resilient Prefabricated beam-column steel joint Damage control Seismic performance Flange cover plate L-shaped plate
a b s t r a c t The research of prefabricated steel structure have become a hot topic in international structural engineering, which accelerates its promotion and application. Based on the idea of damage control, an earthquake-resilient prefabricated beam-column steel joint (PBCSJ) has been proposed, which can make the plastic damage occur at the flange cover plate (FCP) by reasonable design of the FCP thickness, middle bolts interval and other parameters. It is ensured that no plastic damage occurs at the main components such as beams and columns. Thus, joint can be repaired by replacing the connection devices to restore its use function. In this article, four specimens are designed, and the quasi-static loading tests and the numerical simulations are conducted to the four specimens and two repaired specimens. The impact on the seismic performance caused by the FCP thickness, FCP connecting bolts number, beam gap and other parameters, as well as the post-earthquake resilience are emphatically investigated. The failure mode, hysteretic curves, skeleton curves and other main performance indexes are obtained. The study shows that: PBCSJ can dissipate energy through the FPC plastic deformation; the thickness of FCP has a relatively big influence on the joint bearing capacity and energy dissipating performance; the decreasing number of FCP connecting bolts would cause the FCP sliding; the insufficient gap of beams would cause the local plastic damage due to the extrusion of the beams. In addition, the joint before and after the repairing would have excellent bearing capacity and energy dissipating performance. The results of the refined finite element (FE) analysis basically match the test results, and can be used as an important tool for the performance study of such type of joints. © 2020 Elsevier Ltd. All rights reserved.
1. Introduction Steel structures are widely used in high-rise building structures due to their good seismic performance [1,2]. However, in the recent strong earthquakes, the traditional welded joints of steel structural beams and columns had a lot of brittle failure, and the structure and components were greatly deformed [3,4]. Even after long time development, current design standards in the special moment steel frame connections are highly ductile, but it was difficult to repair after the earthquake and cause huge loss of property, which had aroused widespread concern among scholars of steel structure research around the world. In order to avoid the overall damage in the existing bolt-welded joints, the design of reasonable prefabricated beam-column steel joints and the earthquake-resilient function of the joints have become a hot spot in the field. The earthquake-resilient prefabricated joint proposed in this paper belongs to the beam-column joint with cantilever beam, that is, the ⁎ Corresponding author. E-mail address: [email protected] (C. Dou).
https://doi.org/10.1016/j.jcsr.2020.105928 0143-974X/© 2020 Elsevier Ltd. All rights reserved.
beam was disconnected at a certain position away from the end of the column, and one part of the beam was directly connected with the column, called cantilever beam, the other part is called common beam. For the joint with cantilever beam, many scholars carried out many researches. Astaneh-Asl [5] proposed a semi-rigid joint design method by theoretical calculation of joint with cantilever beam, which dissipates energy through the friction of bolts and squeezing the bolts against the holes. Oh et al. [6,7] improved the rotation of the joint by weakening the RBS dog bone of the cantilever beam. Popov et al. [8] compared the two types of joints where the upper and lower flanges of the beam were weakened and only the lower flange was weakened. Zhang and Jiang et al. [1,9–12] proposed a series of beam-column joints with cantilever beams, and carried out theoretical derivation, experiment and finite element (FE) simulation. Tests showed that the main components can be protected from damage by adjusting the parameters of flange cover plate (FCP) and bolt. Liu et al. [2,13] proposed two types of bolt-welded and full-bolt beam-column joints, and carried out quasistatic test and FE simulation to obtain various indicators. The results show that these joints have good seismic performance. Ma et al. [14] based on the idea of bolt slip energy dissipation to change the bolt
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Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928 Table 1 Main parameters of specimens.
Flange cover plates
L-shaped plates Common beam
Specimen FCP thickness tcov,f (mm)
Gap between two beams g (mm)
Number of bolt rows at one side nbolt
Loading law
SJ1 SJ1R SJ1RR SJ2 SJ3 SJ4
20 20 20 20 20 10
5 5 5 3 5 5
S1 S1 S2 S1 S1 S1
16 16 16 16 12 16
Table 2 Material properties of the plate.
High-strength bolts Circle tubular steel column with cantilever beam
Fig. 1. Schematic diagram of earthquake-resilient prefabricated beam-column joint. (a) Front view of the specimen. (b) Top view of the specimen.
holes in the joints. The research shows that the longitudinal slotted holes will improve the ductility and deformation ability of the joints. Mou and Li et al. [15,16] performed an experimental investigation to research the elasto-plastic behavior of the steel beam-column connections with unequal depth of outer annular stiffener. The earthquake-resilient structure is a new way of development for seismic design of structures [17]. Lu et al. [18] set up replaceable components in the beam. Through reasonable design, it will yield under the large earthquake before the other parts, and the structure can be quickly repaired by replacing the components. Farsi [19]. investigated a new kind of beam that can be repaired after earthquake and two full-scale
Plate position
Thickness (mm)
Yield strength fy (MPa)
Tensile strength fu (MPa)
fu/fy
Flange cover plate L-shaped plate H-shaped beam
12 16 8 6 12 20
256 270 365 359 368 372
398 412 518 504 526 546
1.55 1.53 1.42 1.40 1.43 1.54
tests were carried out to test the seismic performance. Calado et al. [20,21] proposed a combined beam-column joint and carried out a low cycle reciprocating load test. It is known from the experiment that such combined joint can dissipate seismic energy through plastic deformation of the plate and bolt slippage, and avoid plastic deformation of the main body member. Oh et al. [22] installed a damper on the traditional T-connection joint to concentrate the plastic deformation on the damper while the steel beam body remained elastic. Chen et al. [23] conducted a test on new type of cast steel connectors. It is a replaceable energy dissipation element for beam-column joint and it has realized rapid repair of the joints by simple replacement of these connectors after earthquakes. Fang et al. [24,25] introduced the shape memory alloy (SMA) into the seismic design of beam-column joints, and the
Fig. 2. Geometric dimensions of the basic specimen.
Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928
3
Hydraulic jack Force sensor Beam Column
Fig. 3. Loading device of the test.
test results showed that the beam-column joints with SMA had good self-resetting performance and ductility. He et al. [26] proposed a kind of moment-resisting joint with replaceable angle, and the repairable joint after the earthquake has been achieved. On the basis of the plastic damage theory, Zhang and Jiang [27] proposed a type of earthquake-resilient prefabricated beamcolumn steel joint (PBCSJ), which consists of a column with cantilever beam, common beam, flange cover plates (FCPs), L-shaped plates and high-strength bolts (Fig. 1). The joint is give full play to the characteristics of the prefabricated steel structure that can be replaced and assembled rapidly, and transfers the plastic hinge to the FCPs for energy dissipation by increasing the flange thickness of cantilever beams, weakening the FCP, setting the certain distance between cantilever beam and common beam, which was called the gap between beams, to control the position of plastic damage occurs, and to ensure that the plastic failure does not occur in the main components such as beam and column. Meanwhile, the out-of-plane deformation of the L-shaped plate can be utilized to increase the rotation capacity of the joint. After an earthquake, the post-earthquake restoration can be realized by replacing connection devices. In this article, to study the influence of the key design parameters such as the FCP thickness, FCP connecting bolts number, gaps between two beams, as well as the post-earthquake restoration on the seismic
Table 3 Loading law S1. Load step
Joint rotation (rad)
Number of cycles (cycle)
Displacement amplitude (mm)
1 2 3 4 5 6 7 8 9
0.00375 0.005 0.0075 0.01 0.015 0.02 0.03 0.04 0.05
6 6 6 4 2 2 2 2 2
7.76 10.35 15.53 20.70 31.05 41.40 62.10 82.80 103.50
performance of joints, four basic specimens and two repaired specimens were designed for the quasi-static loading tests and the numerical simulations. It provides experimental and numerical basis for the subsequent establishment of such type joint design theory.
2. Test program 2.1. Dimension characters and material properties Four PBCSJ specimens were designed, and the impact on seismic performance caused by the FCP thickness, FCP connecting bolts number, gap between two beams and other parameters, as well as the postearthquake resilience, were emphatically investigated. The length of columns in all specimens was 3000 mm, and that of cantilever beam and common beam were 650 mm (the length from the cantilever beam end to the column center) and 1500 mm, respectively (Fig. 2). The upper and lower sections of the circular tube column were 299 × 14 mm, and the column in the joint panel zone was 299 × 16 mm. H-shaped steel of 300 × 200 × 6 × 12 mm was adopted as the section of common beam. To ensure that the cantilever beam does not undergo plastic damage during the earthquake, the cantilever beam was strengthened to 300 × 200 × 12 × 20 mm. Q235B [28] steel was adopted for the weakened FCP, and Q345B steel was adopted for other plates of the specimen, the sand blasting was conducted to the contact surface between each plates. Grade 10.9 frictional and diameter 22 mm highstrength bolts were adopted, and the pretension force of 190kN were applied for high strength bolts. In Table 1, the parameters investigated in this paper have been shown, where tcov,f indicates FCP thickness, g indicates the gap between beams, and nbolt indicates the number of FCP Table 4 Loading law S2. Load step
Joint rotation (rad)
Number of cycles (cycle)
Displacement amplitude (mm)
1
0.03
30
62.10
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Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928 D1
D3
D5 D4 D2
D7
D6
(a) Deformation of the entire specimen Fig. 4. Arrangements of displacement meters.
connecting bolt rows at one side of cantilever beam. Additionally, to study the restoring performance of PBCSJ, the repaired specimen SJ1R was formed by replacing the FCPs, L-shaped plates and high-strength bolts of tested specimen SJ1. When the repaired specimen SJ1R were completely loaded, the specimen SJ1R was repaired again according to the same operation steps to form SJ1RR. The basic parameters and material properties of the repaired specimens SJ1R and SJ1RR were the same as those of specimen SJ1. Refer to GB/T228.1–2010 [29], the tensile tests of steel were conducted on various thickness plates of all the specimens. The test results for material properties of all plates are as shown in Table 2. 2.2. Scheme of loading and measuring
Fig. 6. Failure mode of the specimen SJ1. (a) Deformation of the entire specimen. (b) Rotation as L-shaped plate.
common beam end, and the lateral supporting devices were set on the common beam to avoid the out-of-plane instability of the common beam during the loading procedure. The loading law was based on ANSI/AISC 341–10 [30], with displacement-control loading adopted for the whole procedure. The loading law S1 shown in Table 3 was adopted for the rest specimens, except for the second repaired specimen SJ1RR. The low cycle fatigue performance of the second repaired specimen SJ1RR was mainly investigated. Therefore, the corresponding loading law was the cyclic loading of 30 cycles and the joint rotation was 0.03 rad. The loading law S2 was as shown in Table 4. Several displacement meters were set at each specimen to control the loading (Fig. 4). The displacement meters for measuring the displacement at the end of the common beam and cantilever beam were numbered D1 and D2. That for measuring the slip of the left FCP relative to the common beam was numbered D3. Those for measuring the displacement between the common beams at both sides relative to the cantilever beam were numbered D4 and D5. Those for measuring the
40
In this test, the column horizontal loading method was adopted, and the left end of column was the universal hinge, while the right end was connected to the reaction frame with the hinged ring flange, as shown in Fig. 3. The axial pressure applied by the hydraulic jack at the left column end was 0.3 [10] axial compression ratio, which was kept constant. The quasi-static loading was conducted with the hydraulic jack at the
(b) Rotation as L-shaped plate
L1 L2L3
X1X2X3
X5 Z1 Z2 Z3
(a) Back view Fig. 5. Arrangement plan of strain gauges. (a) Back view. (b) Left view.
(b) Left view
Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928
(a) Deformation of the entire specimen
(b) Rotation as L-shaped plate
Fig. 7. Failure mode of the specimen SJ1R. (a) Deformation of the entire specimen. (b) Rotation as L-shaped plate.
(a) Deformation of the entire specimen
5
(b) Slipping of the bolts
Fig. 9. Failure mode of the specimen SJ2. (a) Deformation of the entire specimen. (b) Slipping of the bolts.
slip of FCPs at both sides relative to the cantilever beam were numbered D6 and D7. In order to measure the strain change, strain gauges were set on each specimen, and the three-direction 45°strain rosette was set on the core region of the joint. The layout of the strain gauges is shown in Fig. 5. 3. Test results and analyses 3.1. Test phenomena Each specimen presented a good ductility in the test. The elasticity of the main components was maintained during the whole loading procedure. The obvious plastic deformation occurred at the FCPs of all specimens, with the final failure mode as shown in Figs. 6–11. The elasticity of specimen SJ1 was maintained at the initial stage of loading, and the load-displacement curve linearly changed. When the joint rotation was 0.015 rad, the curve reached the peak, and at the intermediate zone of the FCPs, slight plastic deformation could be observed. With the load increasing in the test, the deformation of FCPs was more obvious, and the deformed zone extended. When the joint rotation was 0.05 rad, the deformation of
(a) Deformation of the entire specimen
(b) Fracture of the FCP
Fig. 8. Failure mode of the specimen SJ1RR. (a) Deformation of the entire specimen. (b) Fracture of the flange cover plate.
(a) Deformation of the entire specimen
(b) No slipping of the bolts
Fig. 10. Failure mode of the specimen SJ3. (a) Deformation of the entire specimen. (b) No slipping of the bolts.
(a) Deformation of the entire specimen
(b) Extrusion contact of the flange
Fig. 11. Failure mode of the specimen SJ4. (a) Deformation of the entire specimen. (b) Extrusion contact of the flange.
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FCPs was obvious, and the L-shaped plate rotated. However, no plastic deformation appears at any other main components in addition to this. When the loading of specimen SJ1 was completed, replace the FCPs, L-shaped plate and high-strength bolts to form the first repaired
specimen SJ1R. The test of repaired specimen SJ1R was carried out according to the loading law of specimen SJ1, and the test phenomenon was basically the same as that of the specimen SJ1. When the loading of the repaired specimen SJ1R was completed, the second repaired specimen SJ1RR was formed by replacing the deformed connection devices
150
150
100
100
Load (kN)
200
Load (kN)
200
50 0
50 0
-50
-50
-100
-100
-150 -200
Experiment FEA Model -5
-4
-3
-2
-1
0
1
2
3
4
5
-150 -200
Experiment FEA Model -5
-4
-3
-2
(a) Specimen SJ1
150
100
100
Load (kN)
150
Load (kN)
200
50 0
-100
-100
-150
Experiment FEA Model -3
-2
-1
0
1
2
3
4
5
-150 -200
-5
-4
-3
-2
100
100
Load (kN)
150
Load (kN)
200
50 0
-100
-100
-150
Experiment FEA Model 0
1
1
2
3
4
5
2
3
4
5
2
3
4
5
Experiment (g=14) FEA Model (g=14) FEA Model (g=10)
0 -50
-1
0
50
-50
-2
-1
(d) Specimen SJ2
150
-3
5
Rotation (% rad)
200
-4
4
Experiment FEA Model
(c) Specimen SJ1RR
-5
3
Platform segment
Rotation (% rad)
-200
2
0
-50
-4
1
50
-50
-5
0
(b) Specimen SJ1R
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-200
-1
Rotation (% rad)
Rotation (% rad)
-150 -200
-5
-4
-3
-2
-1
0
1
Rotation (% rad)
Rotation (% rad)
(e) Specimen SJ3
(f) Specimen SJ4
Fig. 12. Hysteretic curve of test specimens. (a) Specimen SJ1. (b) Specimen SJ1R. (c) Specimen SJ1RR. (d) Specimen SJ2. (e) Specimen SJ3. (f) Specimen SJ4.
Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928
200
100
Load (kN)
positive loading procedure, and the ultimate bearing capacity happened later. For the specimen SJ3, the FCP was relatively thin, the bearing capacity and energy dissipating performance were significantly lower than that of other specimens, and the entire hysteretic curve was similar to a scaled-down version of that of the specimen SJ1. The design value of beam gap of the specimen SJ4 was relatively small as 10 mm, in the later stage, the extrusion contact of both beams would occur. Therefore, the bearing capacity of SJ4 was anticipated to rapidly rise and the hysteretic curve would show an upward sharp corner. However, the sharp corner of hysteretic curve obtained was not obvious since the actually measured value of beam gap of the specimen SJ4 was 14 mm due to the error of specimen manufacturing and on-site installation in the test. Thus, the hysteretic curves of the specimens SJ4 and SJ1 were similar.
SJ1 SJ1R SJ2 SJ3 SJ4
150
50 0
-50 -100 -150 -200
-5
-4
-3
7
-2
-1
0
1
2
3
4
5
Rotation (% rad)
3.3. Skeleton curves
again with new ones, and carried out the second repair test. Being different from the loading law of the previous two tests, the cyclic loading of 30 cycles was carried out to the second repaired specimen SJ1RR, and the rotation of the joint was 0.03 rad. The obvious buckling deformation occurred at the FCP of the SJ1RR in the first cycle of cyclic loading procedure. With the loading continued, the cyclic tensile deformation and compressive buckling deformation were discovered at the FCP. At the 29th cycle of the loading, the test stopped when the FCP showed cracks and then fractured. The failure modes of specimens were as shown in Figs. 6–8. The loading law of specimen SJ2-SJ4 was the same as that of specimen SJ1, and the difference of test phenomenon was that as the specimen SJ2 has less bolts, the FCP slipped during the loading procedure with continuous noise. The specimen SJ3 had a thinner FCP, which had yielded when the joint rotation was 0.05 rad, and in the later stage, the buckling deformation of FCP was obvious. Because of the relatively small beam gap of specimen SJ4, at the later stage of test, the extrusion contact would occur. The failure mode of the specimens of SJ2-SJ4 are as shown in Figs. 9–11.
The skeleton curves of all specimens are as shown in Fig. 13. According to the figure, the skeleton curves of all specimens are straight and basically coincides in the initial stage, which indicates the basically same of the specimens' initial stiffness. When the basic specimen SJ1 was loaded to 135 kN, the buckling occurred at the FCP, and the bearing capacity started to decrease gradually. The trends of skeleton curves of the repaired specimen SJ1R and the basic specimen SJ1 are basically the same. Considering the impact of initial defect and installation error of the plate, the curves of both specimens do not exactly coincide. For SJ2, since there were three rows of bolts at one side of the flange, the slipping of FCP would occur in case of relatively small joint rotation, and make the yield load decrease in comparison with the former two specimens. Meanwhile, the ultimate bearing capacity would appear later, when the joint rotation was larger. Since the FCP of the specimen SJ3 was 12 mm thick, which was less than that of other specimens, the curve was far lower than other ones, and the yield load was 100 kN. Because of the relatively small beam gap, the small middle bolt interval and the increased stiffness of FCP, the bearing capacity of skeleton curve of SJ4 was slightly higher. However, due to the manufacturing and assembling error, the actually measured gap of specimen was wider than the design value, therefore the extrusion contact did not completely occur between two beams, and the curve shows no obvious rising segment.
3.2. Hysteretic curves
3.4. Strain curves
Fig. 12 is the hysteretic curve for the common beam end load and the joint rotation of each specimen. Take the specimen SJ1 as the example, at the initial stage, a slight rotation only occurred at the beam of the specimen, and the hysteretic curve basically presents linearly. With the increase of joint rotation, the bearing capacity of FCPs reached maximum value with it gradually entered the strengthening stage. Then the bearing capacity of specimen gradually decreased because of the obvious buckling deformation of FCPs. When the loading of the specimen SJ1 was completed, no obvious plastic deformation was found in the main structures such as beams and columns. The repaired specimen SJ1R had basically the same hysteretic curve as the specimen SJ1, which indicates that the specimens before and after repaired have basically the same bearing capacity and energy dissipation performance, and the post-earthquake restoring function of the joint can be realized. The load-rotation curve of the second repaired specimen SJ1RR after continuous 30 cycles loading at the rotation of 0.03 rad is relatively full, which indicates the specimen has a stable energy dissipating performance and can assume the role of displacement-related damper. The specimen SJ2 had less bolts and had a similar elastic stage to the basic specimen SJ1. In later loading, the obvious slipping occurred at the FCP, and the yield load (the beam end load value when the load-rotation curve enters the yield stage) decreased in comparison with the specimen SJ1. The specimen SJ2 had a longer platform segment in the
Fig. 14 indicates the strain developing at the beams, and at the key positions of column while loading. It lists the strain of key position at each joint rotation of each specimen. The main components in the test were Q345B steel. According to the test results of material properties, the minimum yield strain was approximately 1740 με. As shown in the figure, each position of joint domain of all specimens is in elastic state. It indicated that the main components of each specimen remained elastic during the whole loading procedure. Therefore, the plastic hinge appeared on the replaceable FCP by the strengthening of cantilever beam and the weakening of FCP to ensure that the beams and column were always in the elastic stage, in favor of the post-earthquake functional repair of joint.
Fig. 13. Skeleton curves.
3.5. Main indicators of joint performance The yield load P y of specimen, the yield displacementΔ y , the ultimate load P max , corresponding displacement Δ 0.8P max when the beam end load reduces to 0.8Pmax, and the ductility coefficient μ = Δ0.8P max/Δy were obtained by processing the test data of each specimen. These main indicators of each specimen are as shown in Table 5. In Table 5, PLD represent the positive loading, and NLD represent the negative loading. According to the table, the ductility coefficient of each specimen are all larger than 3.0,
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Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928
Fig. 14. Strain distribution curves at each joint rotation of each specimen. (a) Specimen SJ1. (b) Specimen SJ1R. (c) Specimen SJ2. (d) Specimen SJ3. (e) Specimen SJ4.
which indicates that such type of joints has good ductility and seismic performance [31]. The yield load of specimen SJ1, repaired specimen SJ1R and specimen SJ4 are basically around 135 kN; the yield load of specimen SJ3 is slightly lower than other specimens' yield load because of the thinner FCP, and the peak load of each specimen is slightly higher than the yield load.
The equivalent viscous damping coefficient he [32,33] of each specimen is as shown in Fig. 15. According to the figure, when the joint rotation was 0.03 rad, the equivalent viscous damping coefficient is basically larger than 0.3 for all specimens, which indicates all specimens have good energy dissipating performance. The cumulative energy dissipation curves of all specimens are as shown in Fig. 16. It can be seen
Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928
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Table 5 Main indicators of specimen performance. Specimen
Δy(mm)
Py(kN)
SJ1 SJ1R SJ2 SJ3 SJ4
μ
NLD
PLD
NLD
PLD
NLD
PLD
NLD
PLD
NLD
131.89 136.71 124.09 89.94 134.75
−124.47 −122.85 −107.65 −100.58 −131.85
21.55 22.97 22.85 16.45 22.99
−19.82 −18.92 −16.92 −17.54 −20.63
135.12 142.84 140.21 96.07 144.42
−135.44 −135.10 −126.02 −101.23 −137.98
80.95 85.15 N101.0 N101.0 100.35
−84.57 −60.6 N101.0 N101.0 82.03
3.76 3.70 N4.42 N6.14 4.36
4.26 3.20 N5.76 N5.76 3.98
from that, the energy dissipation indicators of the basic specimen SJ1 and the repaired specimen SJ1R are basically the same, and the basically coinciding energy dissipation curves indicate that the joint can still remain the energy dissipating performance of the original structure after being repaired. The bearing capacity of SJ2 decreased slightly because of using less bolts. However, since the bolt slipping dissipated some energy, the energy dissipation capability of the specimen SJ2 still has little difference from the former two specimens. Because the specimen SJ3 had a thinner FCP than other specimens, the plastic deformation in the initial stage was obvious, and the energy dissipation was relatively large at the initial stage. The bearing capacity of SJ3 was lower in the later stage and thus when the joint was
0.4
loaded to 0.05 rad, the equivalent viscous damping coefficient was small, and the energy dissipation was obviously lower than other specimens. In comparison with the specimen SJ1, the specimen SJ4 had a smaller gap between beams and the basically same energy dissipation at the initial stage. At the final stage, the bearing capacity of SJ4 increased because of the extrusion contact between two beams, and the energy dissipating performance of SJ4 was better than that of the specimen SJ1. As shown in Fig. 17, carry out the summation of hysteretic curve area at every five cycles to obtain the energy dissipation curve of repaired specimen SJ1RR. According to the figure, with the progress of loading, the energy dissipation of specimen gradually decreased and tended to be stable, and the total energy dissipation in final five cycles was the smallest due to the final fracture of FCP. The total energy dissipation of repaired specimen SJ1RR during the whole loading procedure reached 320.9 kJ.
75
Energy dissipation (kJ)
0.3
he
Δ0.8P max(mm)
Pmax(kN)
PLD
0.2
SJ1 SJ1R SJ2 SJ3 SJ4
0.1
0.0
1
2
3
4
60
45
30
15
5
Rotation (% rad) 0 1~5
Fig. 15. Energy dissipation indicators of the specimens.
6~10
11~15
16~20
21~25
Cumulative energy dissipation (kJ)
Cycles
200
Fig. 17. Energy dissipation curve of specimen SJ1RR
SJ1 SJ1R SJ2 SJ3 SJ4
180 160 140 120 100 80 60 40 20 0 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Rotation (% rad) Fig. 16. Cumulative energy dissipation curves of each specimen.
Fig. 18. Finite element model.
26~30
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(a) Test and FE results of the specimen SJ1
(b) Test and FE results of the specimen SJ1R
(c) Test and FE results of the specimen SJ1RR
(d) Test and FE results of the specimen SJ2
(e) Test and FE results of the specimen SJ3
(f) Test and FE results of the specimen SJ4
Fig. 19. Failure mode comparison of test and FE analysis. (a) Test and FE results of the specimen SJ1. (b) Test and FE results of the specimen SJ1R. (c) Test and FE results of the specimen SJ1RR. (d) Test and FE results of the specimen SJ2. (e) Test and FE results of the specimen SJ3. (f) Test and FE results of the specimen SJ4.
Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928
4. FE analyses 4.1. FE models The general FE analysis software ABAQUS [34] has been adopted to carry out the numerical simulation of above specimens. The element partitioning was carried out to the beams, columns and bolts with C3D8R element [34], and the contact relationship was established between each plate in the model. When editing tangential behavior and normal behavior in contact property, the “Normal Behavior” of all contact surface was “Hard” contact and the “Tangential Behavior” of all contact surface was “Penalty” and the friction coefficient of each contact surface of 0.45 [35]. The pretension force of 190 kN [35] were applied at the high-strength bolts. The yield strengthens and the tensile strengthen of the steel plates in the numerical analysis are adopted from the materials results in Table 2. Meanwhile, the elastic modulus of each specimen is 206 GPa, the tangent modulus of the plastic stage is 2% elastic modulus [9]. The refined FE model is shown in Fig. 18. The hinged boundary conditions were set at the column ends, and displacement load was applied at the loading point. In addition, the out-of-plane instability of the common beam can be avoided by setting the lateral restraints.
11
that such new-type PBCSJ can be used as a type of displacementrelated damper. (3) The decrease of FCP thickness will make the bearing capacity and energy dissipation performance decrease. The decreased number of FCP connecting bolts will cause the slipping of the FCP at the earlier stage, but the entire energy dissipation of the specimen will be enhanced. The decrease of beam gap can enhance the bearing capacity at the later stage, the extrusion contact between beams will cause the local plastic damage on beams, and there is no guarantee of joint performance after repair. (4) The results of numerical verification and the test results properly match. The hysteretic curves and failure mode of specimen obtained by above methods are basically the same. The FE analysis can be used as an essential tool for researching the joints performance.
Declaration of Competing Interest Zi-Qin Jiang, Chao Dou, Hang Zhang, Qi Wang and Yi-huan Yang declare that they have no conflict of interest.
4.2. Numerical verification
Acknowledgements
Fig. 12 shows the load-displacement curves comparison between the tests and numerical results. According to the figure, the two hysteretic curves of each specimen properly match, basically with the same change trend. However, the bearing capacity would be slightly higher than the test result since the FE result is ideal. Fig. 19 shows the comparison between the test and numerical failure mode at the later stage of loading. The numerical simulations deformation and the deformation in test of each specimen can basically match. According to the stress nephogram of numerical simulations, the columns and beams of each specimen were elastic, only the FCP and L-shaped plate part entered the plastic stage. To investigate the impact on the seismic performance of specimen caused by the gap between beams again, two numerical models with the beam gaps as 10 mm and 14 mm were established respectively. The comparison between the numerical results and the test results indicated that (Fig. 12), with the decrease of beam gap, the extrusion contact of both beams would occur at the later stage of loading, and the bearing capacity of joint would be enhanced while the loaddisplacement curve of joint would appear an obvious sharp corner. Then the flange root zone of cantilever beam and the extrusion zone of both beams would enter the plastic stage, and the specimen could not realize the earthquake-resilient. It indicated that the reasonable gap between beams was one of the important factors to ensure the earthquake-resilient of the specimen.
The authors appreciated the funding supported by National Natural Science Foundation of China (Grant No. 51608014 and 51808032), and China Postdoctoral Science Foundation (Grant No. 2017 T100020).
5. Conclusion In this article, the quasi-static loading test and FE analyses have been carried out on a type of earthquake-resilient PBCSJ. The following conclusions are obtained by analyzing the hysteretic performance and failure mode, etc. of all specimens: (1) The PBCSJ has good bearing capacity and energy dissipation capability. The plastic damage of PBCSJ is concentrated on the replaceable FCP, and the normal function can be restored through replacing the relevant plates and bolts. (2) The specimen after repaired has outstanding bearing capacity and energy dissipating performance, and can complete the lowcycle constant amplitude test loading of 29 cycles with the 0.03 rad joint rotation. During the whole loading procedure, the stable energy dissipating performance of the specimen indicates
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Contents lists available at ScienceDirect
Journal of Constructional Steel Research
Experimental study of earthquake-resilient prefabricated beam-column steel joint with L-shaped plate Zi-Qin Jiang a,b, Chao Dou c,⁎, Hang Zhang a, Qi Wang a, Yi-huan Yang a a b c
Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing University of Technology, Beijing 100124, China Beijing Advanced Innovation Center for Future Urban Design, Beijing 100044, China School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China
a r t i c l e
i n f o
Article history: Received 17 August 2019 Received in revised form 31 December 2019 Accepted 3 January 2020 Available online xxxx Keywords: Earthquake-resilient Prefabricated beam-column steel joint Damage control Seismic performance Flange cover plate L-shaped plate
a b s t r a c t The research of prefabricated steel structure have become a hot topic in international structural engineering, which accelerates its promotion and application. Based on the idea of damage control, an earthquake-resilient prefabricated beam-column steel joint (PBCSJ) has been proposed, which can make the plastic damage occur at the flange cover plate (FCP) by reasonable design of the FCP thickness, middle bolts interval and other parameters. It is ensured that no plastic damage occurs at the main components such as beams and columns. Thus, joint can be repaired by replacing the connection devices to restore its use function. In this article, four specimens are designed, and the quasi-static loading tests and the numerical simulations are conducted to the four specimens and two repaired specimens. The impact on the seismic performance caused by the FCP thickness, FCP connecting bolts number, beam gap and other parameters, as well as the post-earthquake resilience are emphatically investigated. The failure mode, hysteretic curves, skeleton curves and other main performance indexes are obtained. The study shows that: PBCSJ can dissipate energy through the FPC plastic deformation; the thickness of FCP has a relatively big influence on the joint bearing capacity and energy dissipating performance; the decreasing number of FCP connecting bolts would cause the FCP sliding; the insufficient gap of beams would cause the local plastic damage due to the extrusion of the beams. In addition, the joint before and after the repairing would have excellent bearing capacity and energy dissipating performance. The results of the refined finite element (FE) analysis basically match the test results, and can be used as an important tool for the performance study of such type of joints. © 2020 Elsevier Ltd. All rights reserved.
1. Introduction Steel structures are widely used in high-rise building structures due to their good seismic performance [1,2]. However, in the recent strong earthquakes, the traditional welded joints of steel structural beams and columns had a lot of brittle failure, and the structure and components were greatly deformed [3,4]. Even after long time development, current design standards in the special moment steel frame connections are highly ductile, but it was difficult to repair after the earthquake and cause huge loss of property, which had aroused widespread concern among scholars of steel structure research around the world. In order to avoid the overall damage in the existing bolt-welded joints, the design of reasonable prefabricated beam-column steel joints and the earthquake-resilient function of the joints have become a hot spot in the field. The earthquake-resilient prefabricated joint proposed in this paper belongs to the beam-column joint with cantilever beam, that is, the ⁎ Corresponding author. E-mail address: [email protected] (C. Dou).
https://doi.org/10.1016/j.jcsr.2020.105928 0143-974X/© 2020 Elsevier Ltd. All rights reserved.
beam was disconnected at a certain position away from the end of the column, and one part of the beam was directly connected with the column, called cantilever beam, the other part is called common beam. For the joint with cantilever beam, many scholars carried out many researches. Astaneh-Asl [5] proposed a semi-rigid joint design method by theoretical calculation of joint with cantilever beam, which dissipates energy through the friction of bolts and squeezing the bolts against the holes. Oh et al. [6,7] improved the rotation of the joint by weakening the RBS dog bone of the cantilever beam. Popov et al. [8] compared the two types of joints where the upper and lower flanges of the beam were weakened and only the lower flange was weakened. Zhang and Jiang et al. [1,9–12] proposed a series of beam-column joints with cantilever beams, and carried out theoretical derivation, experiment and finite element (FE) simulation. Tests showed that the main components can be protected from damage by adjusting the parameters of flange cover plate (FCP) and bolt. Liu et al. [2,13] proposed two types of bolt-welded and full-bolt beam-column joints, and carried out quasistatic test and FE simulation to obtain various indicators. The results show that these joints have good seismic performance. Ma et al. [14] based on the idea of bolt slip energy dissipation to change the bolt
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Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928 Table 1 Main parameters of specimens.
Flange cover plates
L-shaped plates Common beam
Specimen FCP thickness tcov,f (mm)
Gap between two beams g (mm)
Number of bolt rows at one side nbolt
Loading law
SJ1 SJ1R SJ1RR SJ2 SJ3 SJ4
20 20 20 20 20 10
5 5 5 3 5 5
S1 S1 S2 S1 S1 S1
16 16 16 16 12 16
Table 2 Material properties of the plate.
High-strength bolts Circle tubular steel column with cantilever beam
Fig. 1. Schematic diagram of earthquake-resilient prefabricated beam-column joint. (a) Front view of the specimen. (b) Top view of the specimen.
holes in the joints. The research shows that the longitudinal slotted holes will improve the ductility and deformation ability of the joints. Mou and Li et al. [15,16] performed an experimental investigation to research the elasto-plastic behavior of the steel beam-column connections with unequal depth of outer annular stiffener. The earthquake-resilient structure is a new way of development for seismic design of structures [17]. Lu et al. [18] set up replaceable components in the beam. Through reasonable design, it will yield under the large earthquake before the other parts, and the structure can be quickly repaired by replacing the components. Farsi [19]. investigated a new kind of beam that can be repaired after earthquake and two full-scale
Plate position
Thickness (mm)
Yield strength fy (MPa)
Tensile strength fu (MPa)
fu/fy
Flange cover plate L-shaped plate H-shaped beam
12 16 8 6 12 20
256 270 365 359 368 372
398 412 518 504 526 546
1.55 1.53 1.42 1.40 1.43 1.54
tests were carried out to test the seismic performance. Calado et al. [20,21] proposed a combined beam-column joint and carried out a low cycle reciprocating load test. It is known from the experiment that such combined joint can dissipate seismic energy through plastic deformation of the plate and bolt slippage, and avoid plastic deformation of the main body member. Oh et al. [22] installed a damper on the traditional T-connection joint to concentrate the plastic deformation on the damper while the steel beam body remained elastic. Chen et al. [23] conducted a test on new type of cast steel connectors. It is a replaceable energy dissipation element for beam-column joint and it has realized rapid repair of the joints by simple replacement of these connectors after earthquakes. Fang et al. [24,25] introduced the shape memory alloy (SMA) into the seismic design of beam-column joints, and the
Fig. 2. Geometric dimensions of the basic specimen.
Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928
3
Hydraulic jack Force sensor Beam Column
Fig. 3. Loading device of the test.
test results showed that the beam-column joints with SMA had good self-resetting performance and ductility. He et al. [26] proposed a kind of moment-resisting joint with replaceable angle, and the repairable joint after the earthquake has been achieved. On the basis of the plastic damage theory, Zhang and Jiang [27] proposed a type of earthquake-resilient prefabricated beamcolumn steel joint (PBCSJ), which consists of a column with cantilever beam, common beam, flange cover plates (FCPs), L-shaped plates and high-strength bolts (Fig. 1). The joint is give full play to the characteristics of the prefabricated steel structure that can be replaced and assembled rapidly, and transfers the plastic hinge to the FCPs for energy dissipation by increasing the flange thickness of cantilever beams, weakening the FCP, setting the certain distance between cantilever beam and common beam, which was called the gap between beams, to control the position of plastic damage occurs, and to ensure that the plastic failure does not occur in the main components such as beam and column. Meanwhile, the out-of-plane deformation of the L-shaped plate can be utilized to increase the rotation capacity of the joint. After an earthquake, the post-earthquake restoration can be realized by replacing connection devices. In this article, to study the influence of the key design parameters such as the FCP thickness, FCP connecting bolts number, gaps between two beams, as well as the post-earthquake restoration on the seismic
Table 3 Loading law S1. Load step
Joint rotation (rad)
Number of cycles (cycle)
Displacement amplitude (mm)
1 2 3 4 5 6 7 8 9
0.00375 0.005 0.0075 0.01 0.015 0.02 0.03 0.04 0.05
6 6 6 4 2 2 2 2 2
7.76 10.35 15.53 20.70 31.05 41.40 62.10 82.80 103.50
performance of joints, four basic specimens and two repaired specimens were designed for the quasi-static loading tests and the numerical simulations. It provides experimental and numerical basis for the subsequent establishment of such type joint design theory.
2. Test program 2.1. Dimension characters and material properties Four PBCSJ specimens were designed, and the impact on seismic performance caused by the FCP thickness, FCP connecting bolts number, gap between two beams and other parameters, as well as the postearthquake resilience, were emphatically investigated. The length of columns in all specimens was 3000 mm, and that of cantilever beam and common beam were 650 mm (the length from the cantilever beam end to the column center) and 1500 mm, respectively (Fig. 2). The upper and lower sections of the circular tube column were 299 × 14 mm, and the column in the joint panel zone was 299 × 16 mm. H-shaped steel of 300 × 200 × 6 × 12 mm was adopted as the section of common beam. To ensure that the cantilever beam does not undergo plastic damage during the earthquake, the cantilever beam was strengthened to 300 × 200 × 12 × 20 mm. Q235B [28] steel was adopted for the weakened FCP, and Q345B steel was adopted for other plates of the specimen, the sand blasting was conducted to the contact surface between each plates. Grade 10.9 frictional and diameter 22 mm highstrength bolts were adopted, and the pretension force of 190kN were applied for high strength bolts. In Table 1, the parameters investigated in this paper have been shown, where tcov,f indicates FCP thickness, g indicates the gap between beams, and nbolt indicates the number of FCP Table 4 Loading law S2. Load step
Joint rotation (rad)
Number of cycles (cycle)
Displacement amplitude (mm)
1
0.03
30
62.10
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Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928 D1
D3
D5 D4 D2
D7
D6
(a) Deformation of the entire specimen Fig. 4. Arrangements of displacement meters.
connecting bolt rows at one side of cantilever beam. Additionally, to study the restoring performance of PBCSJ, the repaired specimen SJ1R was formed by replacing the FCPs, L-shaped plates and high-strength bolts of tested specimen SJ1. When the repaired specimen SJ1R were completely loaded, the specimen SJ1R was repaired again according to the same operation steps to form SJ1RR. The basic parameters and material properties of the repaired specimens SJ1R and SJ1RR were the same as those of specimen SJ1. Refer to GB/T228.1–2010 [29], the tensile tests of steel were conducted on various thickness plates of all the specimens. The test results for material properties of all plates are as shown in Table 2. 2.2. Scheme of loading and measuring
Fig. 6. Failure mode of the specimen SJ1. (a) Deformation of the entire specimen. (b) Rotation as L-shaped plate.
common beam end, and the lateral supporting devices were set on the common beam to avoid the out-of-plane instability of the common beam during the loading procedure. The loading law was based on ANSI/AISC 341–10 [30], with displacement-control loading adopted for the whole procedure. The loading law S1 shown in Table 3 was adopted for the rest specimens, except for the second repaired specimen SJ1RR. The low cycle fatigue performance of the second repaired specimen SJ1RR was mainly investigated. Therefore, the corresponding loading law was the cyclic loading of 30 cycles and the joint rotation was 0.03 rad. The loading law S2 was as shown in Table 4. Several displacement meters were set at each specimen to control the loading (Fig. 4). The displacement meters for measuring the displacement at the end of the common beam and cantilever beam were numbered D1 and D2. That for measuring the slip of the left FCP relative to the common beam was numbered D3. Those for measuring the displacement between the common beams at both sides relative to the cantilever beam were numbered D4 and D5. Those for measuring the
40
In this test, the column horizontal loading method was adopted, and the left end of column was the universal hinge, while the right end was connected to the reaction frame with the hinged ring flange, as shown in Fig. 3. The axial pressure applied by the hydraulic jack at the left column end was 0.3 [10] axial compression ratio, which was kept constant. The quasi-static loading was conducted with the hydraulic jack at the
(b) Rotation as L-shaped plate
L1 L2L3
X1X2X3
X5 Z1 Z2 Z3
(a) Back view Fig. 5. Arrangement plan of strain gauges. (a) Back view. (b) Left view.
(b) Left view
Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928
(a) Deformation of the entire specimen
(b) Rotation as L-shaped plate
Fig. 7. Failure mode of the specimen SJ1R. (a) Deformation of the entire specimen. (b) Rotation as L-shaped plate.
(a) Deformation of the entire specimen
5
(b) Slipping of the bolts
Fig. 9. Failure mode of the specimen SJ2. (a) Deformation of the entire specimen. (b) Slipping of the bolts.
slip of FCPs at both sides relative to the cantilever beam were numbered D6 and D7. In order to measure the strain change, strain gauges were set on each specimen, and the three-direction 45°strain rosette was set on the core region of the joint. The layout of the strain gauges is shown in Fig. 5. 3. Test results and analyses 3.1. Test phenomena Each specimen presented a good ductility in the test. The elasticity of the main components was maintained during the whole loading procedure. The obvious plastic deformation occurred at the FCPs of all specimens, with the final failure mode as shown in Figs. 6–11. The elasticity of specimen SJ1 was maintained at the initial stage of loading, and the load-displacement curve linearly changed. When the joint rotation was 0.015 rad, the curve reached the peak, and at the intermediate zone of the FCPs, slight plastic deformation could be observed. With the load increasing in the test, the deformation of FCPs was more obvious, and the deformed zone extended. When the joint rotation was 0.05 rad, the deformation of
(a) Deformation of the entire specimen
(b) Fracture of the FCP
Fig. 8. Failure mode of the specimen SJ1RR. (a) Deformation of the entire specimen. (b) Fracture of the flange cover plate.
(a) Deformation of the entire specimen
(b) No slipping of the bolts
Fig. 10. Failure mode of the specimen SJ3. (a) Deformation of the entire specimen. (b) No slipping of the bolts.
(a) Deformation of the entire specimen
(b) Extrusion contact of the flange
Fig. 11. Failure mode of the specimen SJ4. (a) Deformation of the entire specimen. (b) Extrusion contact of the flange.
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FCPs was obvious, and the L-shaped plate rotated. However, no plastic deformation appears at any other main components in addition to this. When the loading of specimen SJ1 was completed, replace the FCPs, L-shaped plate and high-strength bolts to form the first repaired
specimen SJ1R. The test of repaired specimen SJ1R was carried out according to the loading law of specimen SJ1, and the test phenomenon was basically the same as that of the specimen SJ1. When the loading of the repaired specimen SJ1R was completed, the second repaired specimen SJ1RR was formed by replacing the deformed connection devices
150
150
100
100
Load (kN)
200
Load (kN)
200
50 0
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(f) Specimen SJ4
Fig. 12. Hysteretic curve of test specimens. (a) Specimen SJ1. (b) Specimen SJ1R. (c) Specimen SJ1RR. (d) Specimen SJ2. (e) Specimen SJ3. (f) Specimen SJ4.
Z.-Q. Jiang et al. / Journal of Constructional Steel Research 166 (2020) 105928
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positive loading procedure, and the ultimate bearing capacity happened later. For the specimen SJ3, the FCP was relatively thin, the bearing capacity and energy dissipating performance were significantly lower than that of other specimens, and the entire hysteretic curve was similar to a scaled-down version of that of the specimen SJ1. The design value of beam gap of the specimen SJ4 was relatively small as 10 mm, in the later stage, the extrusion contact of both beams would occur. Therefore, the bearing capacity of SJ4 was anticipated to rapidly rise and the hysteretic curve would show an upward sharp corner. However, the sharp corner of hysteretic curve obtained was not obvious since the actually measured value of beam gap of the specimen SJ4 was 14 mm due to the error of specimen manufacturing and on-site installation in the test. Thus, the hysteretic curves of the specimens SJ4 and SJ1 were similar.
SJ1 SJ1R SJ2 SJ3 SJ4
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0
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3.3. Skeleton curves
again with new ones, and carried out the second repair test. Being different from the loading law of the previous two tests, the cyclic loading of 30 cycles was carried out to the second repaired specimen SJ1RR, and the rotation of the joint was 0.03 rad. The obvious buckling deformation occurred at the FCP of the SJ1RR in the first cycle of cyclic loading procedure. With the loading continued, the cyclic tensile deformation and compressive buckling deformation were discovered at the FCP. At the 29th cycle of the loading, the test stopped when the FCP showed cracks and then fractured. The failure modes of specimens were as shown in Figs. 6–8. The loading law of specimen SJ2-SJ4 was the same as that of specimen SJ1, and the difference of test phenomenon was that as the specimen SJ2 has less bolts, the FCP slipped during the loading procedure with continuous noise. The specimen SJ3 had a thinner FCP, which had yielded when the joint rotation was 0.05 rad, and in the later stage, the buckling deformation of FCP was obvious. Because of the relatively small beam gap of specimen SJ4, at the later stage of test, the extrusion contact would occur. The failure mode of the specimens of SJ2-SJ4 are as shown in Figs. 9–11.
The skeleton curves of all specimens are as shown in Fig. 13. According to the figure, the skeleton curves of all specimens are straight and basically coincides in the initial stage, which indicates the basically same of the specimens' initial stiffness. When the basic specimen SJ1 was loaded to 135 kN, the buckling occurred at the FCP, and the bearing capacity started to decrease gradually. The trends of skeleton curves of the repaired specimen SJ1R and the basic specimen SJ1 are basically the same. Considering the impact of initial defect and installation error of the plate, the curves of both specimens do not exactly coincide. For SJ2, since there were three rows of bolts at one side of the flange, the slipping of FCP would occur in case of relatively small joint rotation, and make the yield load decrease in comparison with the former two specimens. Meanwhile, the ultimate bearing capacity would appear later, when the joint rotation was larger. Since the FCP of the specimen SJ3 was 12 mm thick, which was less than that of other specimens, the curve was far lower than other ones, and the yield load was 100 kN. Because of the relatively small beam gap, the small middle bolt interval and the increased stiffness of FCP, the bearing capacity of skeleton curve of SJ4 was slightly higher. However, due to the manufacturing and assembling error, the actually measured gap of specimen was wider than the design value, therefore the extrusion contact did not completely occur between two beams, and the curve shows no obvious rising segment.
3.2. Hysteretic curves
3.4. Strain curves
Fig. 12 is the hysteretic curve for the common beam end load and the joint rotation of each specimen. Take the specimen SJ1 as the example, at the initial stage, a slight rotation only occurred at the beam of the specimen, and the hysteretic curve basically presents linearly. With the increase of joint rotation, the bearing capacity of FCPs reached maximum value with it gradually entered the strengthening stage. Then the bearing capacity of specimen gradually decreased because of the obvious buckling deformation of FCPs. When the loading of the specimen SJ1 was completed, no obvious plastic deformation was found in the main structures such as beams and columns. The repaired specimen SJ1R had basically the same hysteretic curve as the specimen SJ1, which indicates that the specimens before and after repaired have basically the same bearing capacity and energy dissipation performance, and the post-earthquake restoring function of the joint can be realized. The load-rotation curve of the second repaired specimen SJ1RR after continuous 30 cycles loading at the rotation of 0.03 rad is relatively full, which indicates the specimen has a stable energy dissipating performance and can assume the role of displacement-related damper. The specimen SJ2 had less bolts and had a similar elastic stage to the basic specimen SJ1. In later loading, the obvious slipping occurred at the FCP, and the yield load (the beam end load value when the load-rotation curve enters the yield stage) decreased in comparison with the specimen SJ1. The specimen SJ2 had a longer platform segment in the
Fig. 14 indicates the strain developing at the beams, and at the key positions of column while loading. It lists the strain of key position at each joint rotation of each specimen. The main components in the test were Q345B steel. According to the test results of material properties, the minimum yield strain was approximately 1740 με. As shown in the figure, each position of joint domain of all specimens is in elastic state. It indicated that the main components of each specimen remained elastic during the whole loading procedure. Therefore, the plastic hinge appeared on the replaceable FCP by the strengthening of cantilever beam and the weakening of FCP to ensure that the beams and column were always in the elastic stage, in favor of the post-earthquake functional repair of joint.
Fig. 13. Skeleton curves.
3.5. Main indicators of joint performance The yield load P y of specimen, the yield displacementΔ y , the ultimate load P max , corresponding displacement Δ 0.8P max when the beam end load reduces to 0.8Pmax, and the ductility coefficient μ = Δ0.8P max/Δy were obtained by processing the test data of each specimen. These main indicators of each specimen are as shown in Table 5. In Table 5, PLD represent the positive loading, and NLD represent the negative loading. According to the table, the ductility coefficient of each specimen are all larger than 3.0,
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Fig. 14. Strain distribution curves at each joint rotation of each specimen. (a) Specimen SJ1. (b) Specimen SJ1R. (c) Specimen SJ2. (d) Specimen SJ3. (e) Specimen SJ4.
which indicates that such type of joints has good ductility and seismic performance [31]. The yield load of specimen SJ1, repaired specimen SJ1R and specimen SJ4 are basically around 135 kN; the yield load of specimen SJ3 is slightly lower than other specimens' yield load because of the thinner FCP, and the peak load of each specimen is slightly higher than the yield load.
The equivalent viscous damping coefficient he [32,33] of each specimen is as shown in Fig. 15. According to the figure, when the joint rotation was 0.03 rad, the equivalent viscous damping coefficient is basically larger than 0.3 for all specimens, which indicates all specimens have good energy dissipating performance. The cumulative energy dissipation curves of all specimens are as shown in Fig. 16. It can be seen
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Table 5 Main indicators of specimen performance. Specimen
Δy(mm)
Py(kN)
SJ1 SJ1R SJ2 SJ3 SJ4
μ
NLD
PLD
NLD
PLD
NLD
PLD
NLD
PLD
NLD
131.89 136.71 124.09 89.94 134.75
−124.47 −122.85 −107.65 −100.58 −131.85
21.55 22.97 22.85 16.45 22.99
−19.82 −18.92 −16.92 −17.54 −20.63
135.12 142.84 140.21 96.07 144.42
−135.44 −135.10 −126.02 −101.23 −137.98
80.95 85.15 N101.0 N101.0 100.35
−84.57 −60.6 N101.0 N101.0 82.03
3.76 3.70 N4.42 N6.14 4.36
4.26 3.20 N5.76 N5.76 3.98
from that, the energy dissipation indicators of the basic specimen SJ1 and the repaired specimen SJ1R are basically the same, and the basically coinciding energy dissipation curves indicate that the joint can still remain the energy dissipating performance of the original structure after being repaired. The bearing capacity of SJ2 decreased slightly because of using less bolts. However, since the bolt slipping dissipated some energy, the energy dissipation capability of the specimen SJ2 still has little difference from the former two specimens. Because the specimen SJ3 had a thinner FCP than other specimens, the plastic deformation in the initial stage was obvious, and the energy dissipation was relatively large at the initial stage. The bearing capacity of SJ3 was lower in the later stage and thus when the joint was
0.4
loaded to 0.05 rad, the equivalent viscous damping coefficient was small, and the energy dissipation was obviously lower than other specimens. In comparison with the specimen SJ1, the specimen SJ4 had a smaller gap between beams and the basically same energy dissipation at the initial stage. At the final stage, the bearing capacity of SJ4 increased because of the extrusion contact between two beams, and the energy dissipating performance of SJ4 was better than that of the specimen SJ1. As shown in Fig. 17, carry out the summation of hysteretic curve area at every five cycles to obtain the energy dissipation curve of repaired specimen SJ1RR. According to the figure, with the progress of loading, the energy dissipation of specimen gradually decreased and tended to be stable, and the total energy dissipation in final five cycles was the smallest due to the final fracture of FCP. The total energy dissipation of repaired specimen SJ1RR during the whole loading procedure reached 320.9 kJ.
75
Energy dissipation (kJ)
0.3
he
Δ0.8P max(mm)
Pmax(kN)
PLD
0.2
SJ1 SJ1R SJ2 SJ3 SJ4
0.1
0.0
1
2
3
4
60
45
30
15
5
Rotation (% rad) 0 1~5
Fig. 15. Energy dissipation indicators of the specimens.
6~10
11~15
16~20
21~25
Cumulative energy dissipation (kJ)
Cycles
200
Fig. 17. Energy dissipation curve of specimen SJ1RR
SJ1 SJ1R SJ2 SJ3 SJ4
180 160 140 120 100 80 60 40 20 0 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Rotation (% rad) Fig. 16. Cumulative energy dissipation curves of each specimen.
Fig. 18. Finite element model.
26~30
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(a) Test and FE results of the specimen SJ1
(b) Test and FE results of the specimen SJ1R
(c) Test and FE results of the specimen SJ1RR
(d) Test and FE results of the specimen SJ2
(e) Test and FE results of the specimen SJ3
(f) Test and FE results of the specimen SJ4
Fig. 19. Failure mode comparison of test and FE analysis. (a) Test and FE results of the specimen SJ1. (b) Test and FE results of the specimen SJ1R. (c) Test and FE results of the specimen SJ1RR. (d) Test and FE results of the specimen SJ2. (e) Test and FE results of the specimen SJ3. (f) Test and FE results of the specimen SJ4.
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4. FE analyses 4.1. FE models The general FE analysis software ABAQUS [34] has been adopted to carry out the numerical simulation of above specimens. The element partitioning was carried out to the beams, columns and bolts with C3D8R element [34], and the contact relationship was established between each plate in the model. When editing tangential behavior and normal behavior in contact property, the “Normal Behavior” of all contact surface was “Hard” contact and the “Tangential Behavior” of all contact surface was “Penalty” and the friction coefficient of each contact surface of 0.45 [35]. The pretension force of 190 kN [35] were applied at the high-strength bolts. The yield strengthens and the tensile strengthen of the steel plates in the numerical analysis are adopted from the materials results in Table 2. Meanwhile, the elastic modulus of each specimen is 206 GPa, the tangent modulus of the plastic stage is 2% elastic modulus [9]. The refined FE model is shown in Fig. 18. The hinged boundary conditions were set at the column ends, and displacement load was applied at the loading point. In addition, the out-of-plane instability of the common beam can be avoided by setting the lateral restraints.
11
that such new-type PBCSJ can be used as a type of displacementrelated damper. (3) The decrease of FCP thickness will make the bearing capacity and energy dissipation performance decrease. The decreased number of FCP connecting bolts will cause the slipping of the FCP at the earlier stage, but the entire energy dissipation of the specimen will be enhanced. The decrease of beam gap can enhance the bearing capacity at the later stage, the extrusion contact between beams will cause the local plastic damage on beams, and there is no guarantee of joint performance after repair. (4) The results of numerical verification and the test results properly match. The hysteretic curves and failure mode of specimen obtained by above methods are basically the same. The FE analysis can be used as an essential tool for researching the joints performance.
Declaration of Competing Interest Zi-Qin Jiang, Chao Dou, Hang Zhang, Qi Wang and Yi-huan Yang declare that they have no conflict of interest.
4.2. Numerical verification
Acknowledgements
Fig. 12 shows the load-displacement curves comparison between the tests and numerical results. According to the figure, the two hysteretic curves of each specimen properly match, basically with the same change trend. However, the bearing capacity would be slightly higher than the test result since the FE result is ideal. Fig. 19 shows the comparison between the test and numerical failure mode at the later stage of loading. The numerical simulations deformation and the deformation in test of each specimen can basically match. According to the stress nephogram of numerical simulations, the columns and beams of each specimen were elastic, only the FCP and L-shaped plate part entered the plastic stage. To investigate the impact on the seismic performance of specimen caused by the gap between beams again, two numerical models with the beam gaps as 10 mm and 14 mm were established respectively. The comparison between the numerical results and the test results indicated that (Fig. 12), with the decrease of beam gap, the extrusion contact of both beams would occur at the later stage of loading, and the bearing capacity of joint would be enhanced while the loaddisplacement curve of joint would appear an obvious sharp corner. Then the flange root zone of cantilever beam and the extrusion zone of both beams would enter the plastic stage, and the specimen could not realize the earthquake-resilient. It indicated that the reasonable gap between beams was one of the important factors to ensure the earthquake-resilient of the specimen.
The authors appreciated the funding supported by National Natural Science Foundation of China (Grant No. 51608014 and 51808032), and China Postdoctoral Science Foundation (Grant No. 2017 T100020).
5. Conclusion In this article, the quasi-static loading test and FE analyses have been carried out on a type of earthquake-resilient PBCSJ. The following conclusions are obtained by analyzing the hysteretic performance and failure mode, etc. of all specimens: (1) The PBCSJ has good bearing capacity and energy dissipation capability. The plastic damage of PBCSJ is concentrated on the replaceable FCP, and the normal function can be restored through replacing the relevant plates and bolts. (2) The specimen after repaired has outstanding bearing capacity and energy dissipating performance, and can complete the lowcycle constant amplitude test loading of 29 cycles with the 0.03 rad joint rotation. During the whole loading procedure, the stable energy dissipating performance of the specimen indicates
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