Development and experimental verification of damage controllable energy dissipation beam to column connection

Engineering Structures 199 (2019) 109660

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Development and experimental verification of damage controllable energy dissipation beam to column connection

T

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Long-He Xua, , Ge Zhanga, Shui-Jing Xiaoa, Zhong-Xian Lib a b

School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China Key Laboratory of Coast Civil Structure Safety of China, Ministry of Education, Tianjin University, Tianjin 300072, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Beam to column connection Precast concrete Replaceable steel connector Damage controllable Energy dissipation Seismic performance

The present study develops and experimentally verifies a new type of precast concrete beam to column connection. The proposed damage controllable energy dissipation (DCED) beam to column connection places the replaceable steel connectors with low yield point in predetermined areas of beams to achieve the aim of controllable plastic development, energy dissipation and replaceability after the earthquake. Two 1/2 scale interior beam-column connections were designed, fabricated and tested under cyclic reversed loadings, including one monolithic specimen and one precast specimen. The hysteresis curves were recorded during the test, and the seismic indicators, such as strength, stiffness and energy dissipation capacity, were determined and analyzed. Test results demonstrate that the precast DCED connection has similar strength to the monolithic specimen, exhibits stable and plump hysteretic responses, and fails in a typical flexural mode, which is consistent with the criteria of strong column and weak beam. The plastic hinges in the steel connectors greatly reduces the damage in the DCED connection core and mitigates the deterioration in global stiffness of connection, meeting the requirements of damage controllable. Additionally, the DCED connection exhibits satisfactory energy dissipation capacity and has better seismic performance. It is a viable alternative to the seismic beam to column connections of precast concrete buildings in medium and high seismic intensity regions.

1. Introduction In recent years, precast concrete structures are widely used due to various advantages, including rapid construction progress, high production efficiency, better allowance for quality control, and ready-made good quality aggregates supply, which reflects the concept of environmentally friendly building development. Beam to column connections significantly affect the constructability, stability, strength and flexibility of structure. Furthermore, connections also play an important role in energy dissipation and loads redistribution as the structure is loaded. However, the connections are considered as the weakest and the most critical members of a precast concrete structure, it is difficult to meet the force requirements under earthquakes, so the application of the precast frame structures in high seismic intensity regions is limited [1–5]. The relevant experimental researches and earthquake damage investigations indicate that the precast concrete frame structures have good seismic performance, as long as the precast concrete connections are reliable and stable. Therefore, the challenge in precast frame structures is to find the reasonable and practical methods to connect the precast concrete members together to ensure adequate stiffness,

⁎

strength, stability and so on. At present, a large number of experimental research programs on assembly connections have been carried out, which greatly promotes the development of precast structures. Generally, the commonly used types of concrete beam to column connection are monolithic and dry pinned. The monolithic connections are achieved by connecting or anchoring reinforcement bars and section steels at the joints of precast beams and columns, while casting concrete in place at the joints. Im et al. [6] conducted an experimental study on a precast concrete connection with U-shaped beam shells, which had the similar performance to the traditional cast-in-place connections. Parastesh et al. [7] proposed a new ductile moment-resisting connection for precast concrete frames in seismic regions and conducted an experimental study and theoretical analysis, this connection used the diagonal reinforcement bars to improve the strength of the connection core to satisfy the seismic principle of strong column and weak beam. Some researches [8–11] presented the suggestions for connecting and anchoring measures of reinforcement bars (steel strands, etc.) between precast members, and proved the reliability and effectiveness of the proposed connection measures by experimental studies. Vasconez et al. [12] and

Corresponding author. E-mail address: [email protected] (L.-H. Xu).

https://doi.org/10.1016/j.engstruct.2019.109660 Received 4 June 2019; Received in revised form 26 July 2019; Accepted 8 September 2019 0141-0296/ © 2019 Elsevier Ltd. All rights reserved.

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Maya et al. [13,14] used the high performance fiber reinforced materials instead of the ordinary concrete in the core area and studied the effect of the materials on splice length of reinforcement bars and performance of the connection. Yang et al. [15,16] described a simple ductile connection system with a hybrid H-steel precast concrete beam and conducted the experimental study and analyzed its flexural capacity and ductility. The above researches indicate that reasonable design methods can achieve great mechanical and deformation performance of the whole connection. Generally, the dry connections are achieved by using welding, bolting and other methods to connect precast members. Ersoy et al. [17] conducted the cyclic reversed loading test and studied the seismic performance of precast concrete members with welded plate connections. Kim et al. [18] used steel bands to connect the centrifugal hollow precast concrete columns to steel beams, and studied the connecting methods of steel bands and the columns. Li et al. [19] proposed a new precast beam to column connection with end-plates and investigated its seismic performance, also analyzed the flexural capacity of unbonded prestressed confined concrete beam and presented the formula of flexural bearing capacity based on the theory of concentrated plastic zone. However, welding connection has a complicated welding process, and it is difficult to guarantee the welding quality, the brittle failure of the connection is easy to occur under repeated loading. By comparison, the bolt connections have fast installation process, and stable deformation capacity and energy dissipation capability, avoiding the brittle failure. Referring to the connection mode of steel structure, Vidjeapriya et al. [20–22] proposed a precast concrete connection with bracket and stiffened angle steel and performed the horizontal cyclic loading test on it, in which the angle steel was connected to precast concrete beam and column by bolts. Ghayeb et al. [23] proposed a novel hybrid connection with the demount ability for precast concrete frames, and studied the hysteretic behavior and energy dissipation capacity, the deflection, plastic hinges, crack pattern and shear deformation are improved in the tested precast connection. Bahrami et al. [24] performed finite element analysis on two new moment resisting precast concrete connections, in which precast beams were connected to continuous precast column with corbels using inverted E (bolted connection) and box section (welded connection), and studied the connection responses associated with the lateral resistance, lateral stiffness, ductility, and energy dissipation. This paper developed a new type of dry connection, damage controllable energy dissipation (DCED) beam to column connection, in which steel connectors with low yield point are placed in predetermined areas of beams and bolted to the embedded steel components in both sides. The steel connectors enter plastic state earlier and dissipate the seismic energy, achieving the aim of controllable plastic development and replaceability after earthquake. The cyclic revered loading tests were conducted on 1/2 scale interior specimens to investigate the seismic performance of the DCED connection.

as fasteners with high strength stiffness are welded to the pre-embedded steel to increase friction and connecting strength between the steel and the concrete. For the purpose of controlling plastic damage, the steel connectors with low yield point are placed in predetermined areas of the beams close to the connection core, and connected to the pre-embedded steel components on both sides of the beam by high-strength bolts and end plates, avoiding the problems of fatigue damage and weld quality caused by on-site welding. During the earthquake, the steel connector area enters the plastic stage earlier and dissipates energy. Most of the damage is concentrated on this area and which can be replaced after an earthquake. In a properly designed subassemblage, the desired behavior is yielding of the steel connector and little damage in the connection core and the column. After the precast members are installed, the floor slabs, precast beam to column connections are integrated by secondary concreting. Fig. 2 illustrates the details of installation of DCED connection system. Besides, it is necessary to guarantee sufficient strength and stiffness of the high strength bolts before the connection fails completely, such a steel connector could resist predicted load and its performance can be fully utilized. 3. Test program 3.1. Details of the test specimens and material properties Two types of beam to column connections were tested to compare the behavior of cast-in-place connection with precast DCED connection. The connections were classified into a monolithic reference connection (MRC) specimen and a DCED connection (DCEDC) specimen as shown in Figs. 3 and 4. Both the specimens were from the standard story of a reinforced concrete frame designed to meet the Chinese specification GB 50010-2010. The story height and clear span between column and column were designed to be 3000 mm and 4800 mm, respectively, and all the columns had dimensions of 500 × 500 mm and beams had dimensions of 300 × 550 mm. The dimensions of the samples were scaled to 1/2 to fit them to the test setup. The reinforcement and dimensional details of the two specimens are shown in Figs. 5 and 6. Note that, closed stirrups and decreased spacing provided an extra confinement to concrete, and the stirrups spacing at the critical column region for the two specimens was decreased from 150 mm to 100 mm. The stirrups of the beam and column of the MRC specimen were arranged continuously on the connection core to ensure sufficient shear capacity. The stirrups in the connection core of DCEDC specimen were welded to the steel components to maintain force continuity. In addition, in order to get closer to the actual situation, the influence of the floor slab on the connection was considered, and the thickness of slab was 70 mm. The longitudinal reinforcements of the columns and the beams of the MRC specimen were continuous without splicing, using steel bars with diameters of 14 mm, 12 mm and 10 mm. In the DCEDC specimen, a cross-shaped steel composed of two I 130 × 38 × 8 × 8 steels was inserted into the precast column, and a H 140 × 80 × 8 × 8 steel was partially inserted into the precast beams, the thickness of the connecting end plate was 10 mm. Q235B steel was used for the steel connectors and Q345B steel was used for all other steel components. The beams were installed in the horizontal direction and fixed by twenty-four high strength bolts with 12 mm in diameter and the fastening strength of bolts improved the stiffness of the connection. To determine the actual mechanical properties of the material used in this test, material properties tests of concrete and steel were conducted and the results are listed in Tables 1 and 2, and the material strength of steel is the mean value of three samples of each group of test.

2. DCED connection system The configuration of the DCED connection is shown in Fig. 1. The connection details have the advantages of simple installation and excellent constructability. The cross-shaped steel and H-shaped steel are respectively pre-embedded in the precast columns and beams. The reinforcements are tied to make the basic steel skeleton, and then concrete is poured to produce the precast members, in which the steel flanges and longitudinal reinforcements of the beams are both welded to the steel flanges of the columns to provide adequate shear strength and stability and transform the effective beam bending moments. Additionally, the cross-shaped steel and longitudinal reinforcements in the column ensure the continuity and reliability of the transmission of vertical force. In order to prevent the large relative slip between the embedded steel and the concrete under external force and maintain the stability of the whole mechanical performance of the system, the studs

3.2. Connection design As part of a structure, a beam to column connection is substantially 2

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Longitudinal reinforcements Reinforcements skeleton

Concrete

Slab Stirrups

Steel connector Studs

End plate Steel components

H-shaped steel in beam

High-strength bolt

Precast beam Precast column

cross-shaped steel in column Fig. 1. Configuration of DCED connection.

Stirrups

Precast beam

Precast column

Steel connector

High strength bolts (b)

(a)

Fig. 2. The DCED connection system (a) before and (b) after installation.

beam for plastic hinges formed in the beam. The design values of combined bending moments and shear force of column ends of the connection are respectively calculated as follows:

crucial since the connection failure can cause quite severe damages to a structure. Therefore, the design objectives the DCED connection is to achieve plasticity controllable and an expected yielding order of the components to protect the beam to column connection core with little damage. To satisfy the seismic design principle of “strong column and weak beam”, the capacity of a connection core should be larger than that of a

∑ Mc = ηc ∑ Mb

(1)

V = ηvc (Mct + Mcb)/ Hn

(2)

Fig. 3. The view of the MRC specimen for: (a) test setup and (b) casted MRC specimen. 3

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Fig. 4. The view of the DCEDC specimen for: (a) test setup, (b) the steel pre-embedded in the DCEDC column, (c) the steel pre-embedded in the DCEDC beam and (d) the steel connector.

cast-in-place connection, Bn. Accurate design of plastic hinges can ensure the expected yielding order of components and realize plasticity controllable. Specific design principles are expressed as follows:

where Mc and Mb are the design values of combined bending moments at the top or bottom column ends and the left or right beam ends, respectively. Mct and Mcb are the design values of bending moments at the top and bottom column end, respectively. ηc is the amplification factor of bending moment at column end, V is the design value of shear force at column end, ηvc is the amplification factor of the shear force at column end, Hn is the clear height of the column. In order to ensure that the frame structure meets the design ductility criterion during strong earthquakes and has good deformability and energy dissipation capacity, so that the connection does not undergo shear failure before bending failure, and the principle of “strong shear and weak bending” must be met. Therefore, the shear strength of the connection core, Vp, should be larger than that of the beam end, Vb. Besides, the DCED connection should have similar dynamic characteristics to those of the cast-in-place connection under frequent earthquakes and meet the stiffness equivalent principle, that is the initial bending stiffness, Bm, of the DCED connection is equal to that of the

Mp < Ms

(3)

∑ Ma

(4)

∑ Mb = mΣyi2 Nt

(5)

Ms <

2

2

⎛⎜ Nv ⎞⎟ + ⎜⎛ Nt ⎟⎞ ⩽ 1 b b ⎝ Nv ⎠ ⎝ Nt ⎠

(6)

where Mp is the plastic bending moment at the artificial plastic hinges which also are the steel connectors of the DCED connection. Ms is the bending moment at the non-plastic hinge region. ∑Ma is the combined

Fig. 5. Details of MRC specimen (mm). 4

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Fig. 6. Details of DCEDC specimen (mm). Table 1 Material properties of concrete.

Table 2 Material properties of steel.

Sample number

1

2

3

4

5

6

Compression strength (cube)/MPa Average value/MPa

47.9 46.1

45.6

44.8

48.2

42.5

47.6

Material property

Yield strength/ MPa Ultimate strength/ MPa

bending moments of the bolts at the steel connector. m is the number of columns of the bolts, yi is the distance from the ith row of bolts to the neutral axis. Nv and Nt are the design values of shear force and tensile force of each bolt, respectively. Nvb and Ntb are the design value of shear strength and tensile strength of each bolt, respectively. The number and size of the bolts were designed according to Chinese specification GB50017-2017.

Diameter (mm)

Type (thickness:8mm)

6

8

10

12

14

Q235B

Q345B

372.2

414.4

427.2

434.8

442.3

282.5

412.1

502.2

573.9

585.6

608.5

618.9

316.8

553.4

3.3. Loading setup and scheme The cyclic revered loading tests were conducted on the two 5

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Reaction frame

Lateral brace

Connecting rod

Hydraulic Jack

Negative direction

Actuator1

column. Each specimen was first loaded axially to the level of 0.2fc0Ag. Based on the result from a simple elastic analysis reflecting the dead and live loads of the prototype structure, the axial force applied to the specimen was calculated to be approximately 20% of the product of compressive strength and cross-sectional area of the column. Two 500 kN hydraulic actuators were connected to the beam ends and applied the cyclic loads. The loading program used during the tests was shown in Fig. 8, loading was carried out at a multiple of 5 mm yield displacement, and the loading cycle was repeated twice per stage. When the applied axial pressure cannot maintain the initial value due to the damage of the member or the bearing capacity of the specimen falls below 85% of its peak load, the test will be stopped. In addition, the displacement transducers W1 and W2 were respectively placed at the loading positions of the left and right beam ends, which were used to measure the displacement of the beam ends accurately.

Connecting rod

Actuator2

(-) (+)

Reaction column

Positive direction

Specimen

Spherical hinge connection

4. Test result and analysis

Loading displacement/mm

Fig. 7. Test setup.

35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35

0

1

3

5

7

Cycle number

4.1. Failure mode

9

11

The failure mode and the crack development patterns of each specimen are presented in Figs. 9 and 10. Typical shear failure occurred in the monolithic MRC specimen due to the most unfavorable failure situation considered during its design, lots of cracks were concentrated at the connection core and surrounding beams. For the DCEDC specimen, the failure mode was that concrete peeled at the loading end of the beam, the cracks in the connection core and surrounding beams were significantly reduced and evenly distributed. For MRC specimen, the initial cracks appeared subsequently in the beams near the column, slab-to-column and slab-to-beam interface when a loading displacement reached 5 mm. Most of the cracks in the beams were skewed with the axis of the members and propagated to the right and left ends at similar intervals. With the increase of the loading displacement, the cracks in beams extended upward and downward, horizontal cracks were observed in the column, some diagonal cracks appeared in the connection core at an angle of about 30–45 degrees and the width of the cracks was getting larger. When the loading displacement increased to 30 mm, the cracks extended through the whole section of the beam, the number and width of diagonal shear cracks in the connection core were increasing and rapidly developing, concrete began to peel off and the specimen eventually failed, and the loading stopped. For DCEDC specimen, the initial cracks appeared in the slab and were perpendicular to the axis of the members when a loading displacement reached 3.5 mm. As the loading displacement increased, some fine oblique cracks were found at the loading end of the beams and beam-column interface and gradually widened and extended to the bottom of the beam. The cracks in the slab increased and extended to the beam. When the loading displacement increased to 25 mm, the fine

13

Fig. 8. Loading scheme.

specimens to evaluate their seismic performance. Fig. 7 shows the test setup and this type of loading represents the actual case of a connection subassemblage in a moment frame. The spherical hinge support was pinned at the bottom of the column and constraints were imposed in the three directions of x, y and z. The column end was clamped by the lateral brace, which was fixed to the big reaction column and prevented out-of-plane movement of the specimen during the loading process. The hydraulic jack was used to exert the axial pressure to the top of the

Fig. 9. (a) Failure mode and (b) crack development patterns of MRC specimen. 6

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Failure area Yielding of steel connect or

(a)

(b) Fig. 10. (a) Failure mode and (b) crack development patterns of DCEDC specimen.

displacement of 24.5 mm, and then bearing force drops rapidly. Due to the large diameter of the high-strength bolt holes caused by the construction, the steel connectors and the embedded steel in the beam were slightly displaced during loading. However, this influence of the inevitable assembly tolerances can be minimized by sandblasting on the contact surface of the bolts and end plates. Large force on both sides of the beam caused the embedded steel and the concrete to slip, the concrete at the edge of the steel was cracked, and the cracks extended to the slab, resulting in the stability of the hysteretic performance of the DCEDC specimen declined slightly. In addition, when the MRC specimen and DCEDC specimen reach similar peak strength, DCEDC specimen has a larger corresponding displacement due to the yielding of steel connectors. However, the DCEDC specimen has residual deformations after unloading, which may affect the replacement of damaged steel connectors. In further studies, shape memory alloys [25,26] can be employed to reduce the residual deformations and improve the replaceability of DCED connections. The skeleton curves of the MRC specimen and the DCEDC specimen are compared and shown in Fig. 12. It can be seen that the two specimens exhibit the oblique S-shaped skeleton curves, showing that the connection specimens experience three stages of elasticity, plasticity and failure during the cyclic reversed loadings. The initial skeleton curves of the two specimens are basically identical, and the load increases linearly with displacement before cracking, after specimens enter the plastic stage, the skeleton curve of the MRC specimen experiences a long horizontal segment, and that of the DCEDC specimen experiences a continuously increasing segment until the peak load is reached. The two specimens have similar yield displacement and maximum displacement, indicating their ductility is similar. Due to the existence of the slab, there are asymmetric phenomenon in the skeleton

oblique cracks appeared in the connection core, and some vertical cracks was observed in the junctions of the steel embedded in the beam and the concrete, these cracks were widening and a little concrete began to peeled off. Finally, when a displacement increased to 30 mm, the concrete at the loading end of the beam was crushed locally and began to peel off, the steel plates of the steel connectors staggered, the upper end of the left actuator shifted and twisted. Meanwhile, the bearing capacity of DCEDC specimen reduced from a maximum of 134.03 kN to 105.09 kN, which fell below 85% of its maximum value, the loading stopped. The steel components improved the performance of the connection core, this was reflected by the significant reduction in the number of cracks. The steel connectors enhanced the local strength and stiffness of the beam, also could control the position of the plastic hinges to keep them away from the connection core and ensured its safety. 4.2. Hysteretic response and skeleton curves Fig. 11 shows the hysteretic curves of the beam end load and displacement relationship to illustrate the cyclic behavior of specimens. As shown in Fig. 11(a), the MRC specimen exhibits reverse S-shaped hysteretic curve, it has a significant pinch phenomenon after the cyclic reversed loadings and the degree of pinching increases with the increase of displacement. The peak strength of 143.53 kN is observed at a loading displacement of 19.9 mm and hysteretic behavior is stable until a loading displacement of 30 mm without displaying a rapid deterioration in bearing capacity. As shown in Fig. 11(b), the hysteretic curve of DCEDC specimen has no significant pinch phenomenon and presents plump shapes. The ideal hysteretic behavior is displayed until reaching the peak strength of 134.03 kN measured at a loading 150

Strength2.5%

Pmax=143.52kN P=139.91kN

150 100

Load at beam end/kN

Load at beam end/kN

100 50 -25.0mm

0

19.9mm

-50

-100

P=-101.57kN Pmax=-103.26kN

Strength1.6%

-40

50 -24.1mm

0

24.5mm

-50

-100

P=-83.05kN Pmax=-96.26kN

Strength 13.7%

-150

-150

(a)

Strength 21.6%

Pmax=134.03kN P=105.09kN

-30

-20

-10

0

10

20

30

40

(b)

Displacement at beam end/mm

-40

-30

-20

-10

Fig. 11. Hysteretic curves of (a) MRC specimen and (b) DCEDC specimen. 7

0

10

20

Displacement at beam end/mm

30

40

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Load at beam end/kN

150

was effectively transferred to the column by the steel connector like a cast-in-place continuous beam. Fig. 16 shows the strain gauge readings in the junctions of steel connector and beam of DCEDC specimen at a loading displacement of 20 mm. The D10 stands for longitudinal bars with diameters of 10 mm, the yield strain of the Q235B steel, Q345B steel and D10 are 1.413 × 10−3, 2.061 × 10−3 and 2.135 × 10−3, respectively. It was observed that the strain of S20, S21 and S23 were all far larger than the yield strain of Q235B steel, and the strain at other positions were within the elastic range, indicating that the plastic hinges have been formed in the steel connectors. The stress of steel component outside the beam increased due to the sudden change of beam section and the discontinuity of longitudinal reinforcements at the bottom of beam, resulting in a large strain of S12. The distance from the loading end also affected the stress distribution. The farther away from the loading end, the larger the stress was, and it explained the phenomenon that the strain of S15 was smaller than that of S12. Additionally, the strain gauge readings of S23, R17 and R5 at each loading displacement are presented in Fig. 17 to analyze the yielding order of DCEDC specimen. It was observed that the strain of S23 and R17 reached the yield strain in sequence at a displacement of 10 mm and 25 mm respectively, and the strain of R4 maintained small values during the whole loading process. It was concluded that the steel connector first entered the yielding state and had great deformability, avoiding the early yielding of the beam and column, and meeting the expected design goals and damage controllable requirements. Furthermore, the strains of S23 and R17 had similar rising trend during the loading process, it also proved that the load was effectively transferred to the beam from the steel connectors by the bolts and end plates.

DCEDC MRC

100 50 0 -50

-100 -150 -30

-20

-10

0

10

20

Displacement at beam end/mm

30

Fig. 12. Skeleton curves of two specimens.

curves of the both specimens and the positive bearing capacity is obviously higher than the negative bearing capacity at the later stage of loading process. 4.3. Strain analysis To achieve good seismic performance, plastic hinges should not be transferred to a connection core but to the inside of beams. The strain distribution of partial reinforcements and steel components is analyzed and the position of corresponding strain gauges is shown in Fig. 13. The strain variations of the strain gauges R20, R21 and R3 in the connection core of the two specimens are shown in Fig. 14. The D6 and D14 stand for stirrups with diameters of 6 mm and longitudinal bars with diameters of 14 mm, respectively. The yield strain of D6 and D14 are 1.864 × 10−3 and 2.21 × 10−3, respectively, and the strain at the monitoring points remained elastic before yielding. For MRC specimen, the strain of R21 reached 1.870 × 10−3 at a loading displacement of 8.5 mm. As the loading displacement increased, the strain of R20 reached yield strain at a displacement of 13.5 mm. At the end of loading process, the three strain gauges suddenly increased to infinity, indicating that the reinforcements were all pulled off and the connection core of MRC specimen was severely damaged. For DCEDC specimen, the strains of R20 and R3 were both less than the yield strain, the maximum strain of R21 was 1.97 × 10−3, which was only more than 5.4% of the yield strain, indicating that the connection core was almost in the elastic state during the whole loading process and had a good performance. Fig. 15 shows the strain variations of the strain gauge R4 in the column of two specimens, the shapes of the curves were similar to the hysteretic curves of specimens, indicating that the load at the beam end

4.4. Stiffness and strength evaluation The stiffness evaluation can reflect the cumulative damage of the specimens during the loading process. The connection stiffness Kj at each level of loading displacement is evaluated as: n

∑ F ij Kj =

i=1 n

∑ Dij

(8)

i=1

F ij

Dij

where and are the peak load and corresponding displacement of the ith cycle at the jth level of loading displacement, respectively. Fig. 18 shows the change in stiffness in the specimens. The DCEDC specimen and MRC specimen had the relatively similar initial stiffness of 11.86 kN/mm and 12.30 kN/mm, respectively. The stiffness of both specimens deteriorated as inelastic hysteretic cycles increased. The large amount of accumulated damage in the connection core seriously affected the integral stiffness of MRC specimen, and the deterioration

The beam end

(a)

(b)

Fig. 13. Position of strain gauges in the (a) column and connection core of two specimens and (b) junctions of steel connector and beam of DCEDC specimen. 8

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Fig. 14. Strain variations of reinforcements in the connection core of (a) MRC specimen and (b) DCEDC specimen.

4 3 DCEDC MRC

Strain/×10-3

2 1 0 -1 -2 -40

-30

-20

-10

0

10

Displacement/mm

20

30

40

2.2

The yield strain of D10

2.0

The yield strain of Q345B

1.8

The yield strain of D6

Fig. 17. Strain gauge readings of S23, R17 and R5 at each loading displacement.

14 12 10

Stiffness Kj/(kN/mm)

Strain/×10-3

Fig. 15. Strain variations of R5 of two specimens.

1.6 1.4

The yield strain of Q235B

1.2 1.0

8 6 4 DCEDC MRC

2 R4 S11 R17 R11 S12 S20 S23 S21 S15 S16 R12

Strain gauge

0

Fig. 16. Strain gauge readings in the junctions of steel connector and beam of DCEDC specimen at a loading displacement of 20 mm.

-30

-20

-10

0

10

Displacement/mm

20

30

Fig. 18. Stiffness deterioration of two specimens.

rate was always maintained at a high level. For the DCEDC specimen, the deterioration in stiffness was mitigated after a loading displacement of 10 mm, mainly due to the strain hardening of the concrete surrounding the steel components embedded in the beam and column. This

effect was caused by the cohesion between the concrete and the embedded steel components, which reduced the deformation of the steel components, delayed the cracking of the concrete, improved the tensile 9

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1.2

performance of the connections under the cyclic loadings. It is defined as the ratio of strength at the second cycle to the strength of the first cycle at each level of the loading displacement. Fig. 19 shows the strength ratio at each loading displacement to describe the strength deterioration of specimens. Based on the test results, the strength ratio recorded in two specimens was between 0.81 and 1.00. For the MRC specimen, the strength deterioration values were 0.91 and 0.86 under the positive and negative loadings, and the strength ratio slowly reduced as the loading displacement increased. For the DCEDC specimen, the strength deterioration values were 0.87 and 0.82 under the positive and negative loadings, the strength ratio fluctuated in a narrow range during the loading process, as a result of the strength loss caused by yielding of the steel connectors and cracking of concrete at loading ends. At a displacement of 15 mm, there was a significant strength deterioration, the ratio reduced to 0.81, due to the effect of slight dislocation between the steel connectors and the embedded steel in the beam. After that, the DCEDC specimen showed no substantial deterioration of strength at a higher level of displacement, and this prevented the failure of the specimen.

Strength ratio

1.0 0.8 0.6 0.4 DCEDC MRC

0.2 0.0

-30

-20

-10

0

10

Displacement/mm

20

30

Fig. 19. Strength deterioration of two specimens.

strength of the surrounding concrete and made it resist larger deformation. Compared with reinforced concrete, the steel components had more stable performance at the plastic state. Meanwhile, the crushing of concrete around the loading ends had adverse effects on stiffness deterioration. It was suggested to consider the details of design for the concrete in that region. The strength ratio as a vital parameter is used to evaluate the

4.5. Energy dissipation capacity The plastic deformation of the connections could dissipate some of the energy by hysteretic behavior, and it reduces the transmission of the energy to other structural members. In other words, good energy dissipation capacity of connections can improve the seismic performance of the entire structural system under strong earthquakes. Fig. 20(a) and

Fig. 20. Energy dissipation for MRC and DCEDC specimens: (a) energy dissipation at each loading displacement, (b) cumulative energy dissipation, (c) calculation criteria of equivalent viscous damping coefficient and (d) equivalent viscous damping coefficient. 10

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Acknowledgments

(b) illustrate the energy dissipation at each level of loading displacement and the cumulative energy dissipation for each specimen. The energy dissipation increased obviously when the displacement exceeded 10 mm, and the two specimens entered the yielding state. Due to the better deformability of the steel connectors with low yield point in DCEDC specimen, the gap in the energy dissipation of two specimens gradually increased. Finally, the ultimate cumulative energy dissipation was about 1.84 times that of MRC specimen. For an imperfectly symmetric hysteretic response and its corresponding closed loop, the equivalent viscous damping coefficient, ξe, can be accurately determined as follows:

ξe =

Aloop ED Aafbea = = + + − − 4πES0 π (Fmax Dmax + Fmax Dmax ) 2πAacp + bdp

The writers gratefully acknowledge the partial support of this research by the Natural Science Foundation of China under Grant No. 51578058, and Beijing Natural Science Foundation of China under Grant No. 8172038. References [1] Restrepo JI, Robert P, Buchanan AH. Tests on connections of earthquake resisting precast reinforced concrete perimeter frames of buildings. PCI J 1995;40(4):44–61. [2] Park R. A perspective on the seismic design of precast concrete structures in New Zealand. PCI J 1995;40(3):40–60. [3] Alcocer SM, Carranza R, Perez-Navaratte D, Martinez R. Seismic tests of beam to column connections in a precast concrete frame. PCI J 2002;47(3):70–89. [4] Blandon JJ, Rodriguez ME. Behavior of connections and floor diaphragms in seismic-resisting precast concrete buildings. PCI J 2005;50(2):56–75. [5] Kim TH, Lee HM, Kim YJ, Shin HM. Performance assessment of precast concrete segmental bridge columns with a shear resistant connecting structure. Eng Struct 2010;32(5):1292–303. [6] Im HJ, Park HG, Eom TS. Cyclic loading test for reinforced-concrete-emulated beam-column connection of precast concrete moment frame. ACI Struct J 2013;110(1):115–26. [7] Parastesh H, Hajirasouliha I, Ramezani R. A new ductile moment-resisting connection for precast concrete frames in seismic regions: an experimental investigation. Eng Struct 2014;70(9):144–57. [8] Bull DK, Park R. Seismic resistance of frames incorporating precast prestressed concrete beam shells. PCI J 1986;31(4):54–93. [9] Yee AA, Hon D. Structural and economic benefits of precast/prestressed concrete construction. PCI J 2001;46(4):34–42. [10] Xue W, Zhang B. Seismic behavior of hybrid concrete beam-column connections with composite beams and cast-in-place columns. ACI Struct J 2014;111(3):617–27. [11] Eom TS, Park HG, Hwang HJ, Kang SM. Plastic hinge relocation methods for emulative PC beam-column connections. J Struct Eng 2016;142(2):4015111. [12] Vasconez RM, Naaman AE, Wright JK. Behavior of HPFRC connections for precast concrete frames under reversed cyclic loading. PCI J 1998;43(6):58–71. [13] Maya LF, Zanuy C, Albajar L, Lopez C, Portabella J. Experimental assessment of connections for precast concrete frames using ultra high performance fiber reinforced concrete. Constr Build Mater 2013;48(19):173–86. [14] Maya LF, Albajar L. Beam-column connections for precast concrete frames using high performance fiber reinforced cement composites. Proceedings of the 6th international rilem workshop on high performance fiber reinforced cement composites. Ann Arbor, USA. 2012. p. 347–54. [15] Yang KH, Oh MH, Kim MH, Lee HC. Flexural behavior of hybrid precast concrete beams with H-steel beams at both ends. Eng Struct 2010;32(9):2940–9. [16] Yang KH, Seo EA, Hong SH. Cyclic flexural tests of hybrid steel-precast concrete beams with simple connection elements. Eng Struct 2016;118:344–56. [17] Ersoy U, Tankut T. Precast concrete members with welded plate connections under reversed cyclic loading. PCI J 1993;38(4):94–100. [18] Kim JH, Cho YS, Lee KH. Structural performance evaluation of circular steel bands for PC column-beam connection. Mag Concr Res 2013;65(23):1377–84. [19] Li SF, Li QN, Zhang H, Jiang HT, Yan L, Jiang WS. Experimental study of a fabricated confined concrete beam-to-column connection with end-plates. Constr Build Mater 2018;158:208–16. [20] Vidjeapriya R, Jaya KP. Experimental study on two simple mechanical precast beam-column connections under reverse cyclic loading. J Perform Constr Facil 2012;27(4):402–14. [21] Vidjeapriya R, Hasan NMU, Jaya KP. Behavior of precast beam-volumn stiffened short dowel connections under cyclic loading. Adv Struct Eng 2014:2343–53. [22] Vidjeapriya R, Vasanthalakshmi V, Jaya KP. Performance of exterior precast concrete beam- column dowel connections under cyclic loading. Int J Civ Eng 2014;12(1A):82–94. [23] Ghayeb HH, Razak HA, Sulong NHR. Development and testing of hybrid precast concrete beam-to-column connections under cyclic loading. Constr Build Mater 2017;151:258–78. [24] Bahrami S, Madhkhan M, Shirmohammadi F, Nazemi N. Behavior of two new moment resisting precast beam to column connections subjected to lateral loading. Eng Struct 2017;132:808–21. [25] Duran B, Tunaboyu O, Atli KC, Avsar O. Seismic performance upgrading of substandard RC frames using shape memory alloy bars. Smart Mater Struct 2019;28(8):085007. [26] Yurdakul O, Tunaboyu O, Avsar O. Retrofit of non-seismically designed beamcolumn joints by post-tensioned superelastic shape memory alloy bars. B Earthq Eng 2018;16(11):5279–307.

(9)

where ED is the dissipated energy within a given cycle, Aloop is the area of the corresponding closed loop in the total restoring force–displacement diagram. ES0 is the elastic strain energy associated with the positive and negative maximum force, F+ max and Fmax, and positive and negative displacement, D+ max and Dmax, reached in the loop, Aafbea and Aacp+bdp are the area of the hysteretic loop and the summation of the triangle areas, respectively, as shown in Fig. 20(c). The equivalent viscous damping coefficient is used to compare the energy dissipation capacity of the two test specimens, as shown in Fig. 20(d). It was apparent that the steel connectors improved the deformation ability of the connection, resulting in higher dissipated energy relative to the MRC specimen. 5. Conclusions In this paper, a new type of damage controllable energy dissipation (DCED) beam to column connection is developed, which controls the position of plastic hinges and dissipates most of energy through replaceable steel connectors. The cyclic reversed loading tests were conducted on the 1/2 scale specimens to identify structural characteristics and verify the seismic performances of the DCED connection. The test results indicate that the DCED connection shows the typical flexural failure mode, plastic hinges are formed in the steel connector and little damage is in the connection core and the column. The steel connectors with low yield point first yield and present good deformability. Strain continuity between components associated with the steel connectors indicates that the load is effectively and reliably transferred to the beam and column by bolts and end plates. Compared with the cast-in-place connection, the DCED connection has the similar bearing capacity, exhibits the fuller hysteretic responses with more excellent energy dissipation throughout the loading protocol, and the cumulative energy dissipation is about 1.84 times that of the cast-in-place connection. The steel components embedded in the beam and column improve the tensile strength of surrounding concrete and make it resist larger deformation, as a result, the deterioration in stiffness of DCED connection is mitigated and the strength degradation ratio is within a reasonable range. The proposed DCED connection can achieve great seismic performance under the premise of simple installation and fast assembly. Additionally, the crushing of concrete around the loading ends results in the failure of the DCED connection. It is suggested that the enhancement of concrete at this position should also be considered during the design. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work.

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