Seismic Performance of Exterior Steel Ring-Stiffener Joint

We study the seismic performance of the exterior steel ring-stiffener (ESRS) joint, a member that is used to connect a steel beam to a square steel tubular column filled with steel fibre recycled concrete for seismic resistance. To this end, the influence of seismic factors such as axial compression ratio and beam-column linear stiffness ratio on seven specimens is studied. A physical test is conducted on the specimens with a series of cyclic loadings. The testing and analysis of hysteretic loops and skeleton curves of the ESRS joints revealed the seismic performance characteristics including failure mode, rigidity, ductility, bearing-capacity degradation, and energy dissipation capacity. The results reveal (a) damage to the specimens at the beam-end near the joint, (b) no failure at the joints’ core area and column, (c) appearance of a plastic hinge at the beam-end near the joint, and (d) plump spindle-shaped hysteretic loops and normal rigidity degradation curves in the specimens. The specimens are then simulated with the finite element method (FEM) and the results are compared with those of the physical test.


Introduction
Recycled coarse aggregates (RCA) from waste concrete can be extracted by crushing and cleaning concrete debris. Recycled concrete, which is a new type of green building material, is produced by substituting a percentage of its natural coarse aggregates (NCA) with RCA. By this method, waste concrete can be reutilised as a resource, and NCA can be conserved. In regular concrete-filled steel tubes (CFST), the exterior steel tube confines the concrete and the concrete in return supports the steel tube. us, CFST improves the strength, plasticity, and ductility of concrete inside external steel tubes, avoid or delay local buckling of the exterior steel tube, and increase its stability. erefore, the material properties of both concrete and steel tube can be integrated and utilised [1]. e recycled-concrete-filled steel-tube column inherits the advantages of high bearing capacity, good seismic performance, and convenient construction of CFST columns. e literature [2][3][4][5][6][7][8][9][10][11][12] reveals that the performance of CFST can be improved by adding steel fibres to the concrete, and that this technical method could have broad applications in high-rise buildings and large-span structures. Research also shows that the recycled-concretefilled steel tubes can meet the performance of an ordinary steel tube under most conditions. In addition, Li et al. [2] numerically analysed the axial compressive properties of a steel tubular short column filled with steel-fibre highstrength concrete. Yin [3] studied the axial compressive properties of steel-fibre-lightweight-aggregate-concrete-filled steel tubular column by exposing it to flames. Gopal and Manoharan [4] conducted an experimental study on the eccentrically compressed, fibre-reinforced-concrete-filled slender circular steel tubes.
In this study, a new type of ESRS joint, which connects an H-shaped beam linked to a concrete slab to a square steel fibre recycled concrete (SFRC)-filled steel tubular column for improved earthquake resistance, is proposed. e study considers the advantages of the square CFST, SFRC, and ESRS joints, and the influence of slabs on the joints. Seven specimens are fabricated and tested, to study the seismic performance of ESRS. rough a physical test and finite element method (FEM) simulation, the seismic indices of the ESRS joints-failure mechanism, deformability, bearing capacity, and energy dissipation capacity-are analysed. We believe this study will help lay the foundations of a sound theoretical system and promote the development, recycling, and reusing of waste concrete and SFRC in engineering practice.

Experimental Program
2.1. Specimen Design. Seven ESRS joint specimens were designed and fabricated based on the axial compressive ratios, beam-column linear stiffness ratios, numbers of bolts located in the joints' core area, and types of slab (including the recycled concrete slab and composite slab linked to the profiled steel sheet). Among them, specimen SJD-4 had four M10 high-strength bolts of class 10.9, which are located in the joint's core area. Specimens SJD-1∼SJD-6 had a slab that is connected to its beam, while specimen SJD-7 did not. e axial compressive ratio can be validated through the adjusted load of a vertically set hydraulic jack. By adjusting the cross section of steel beams, the beam-column linear stiffness ratio can be controlled. e parameters of the specimens are presented in Table 1. Some specimens are shown in Figure 1.

Material Properties.
e steel tubes are made of Q235 grade steel. Table 2 shows mechanical parameters of the steel used for the specimens, in accordance with the literature [13]. e proportions of SFRC per cubic metric are 584 kg of cement, 499 kg of sand, 855 kg of RCA, 368 kg of NCA, and 233 kg of water. e substitutive rate of the RCA is 70%. e dimensions of the curved steel fibres are 0.7 mm × 0.7 mm × 35 mm. e steel fibre content is 1%, taken from the author's previous research [14,15]. e numerical value of the standard compressive cubic strength for concrete, f cu,k , is 42.6 N/mm 2 . Figure 2, the vertical hydraulic jack with a bearing capacity of 1,000 kN is installed at the top of the column. A ball hinge is installed at the bottom of the column. e left end of the beam is free to move, and it was connected to a 250 kN hydraulic actuator that is linked to the beam of the reaction frame. e hydraulic actuator applies a cyclic load to the left beam-end of a specimen. To avoid instability and offset of the column during the loading, a lateral support is connected to the upper-end of the column. e bearing capacity of each concrete-filled steel tubular column can be calculated per the literature [16][17][18]. e constant vertical load, which depends on the axial compressive ratio, was applied on the top of the column by a hydraulic jack with bearing capacity of 1,000 kN. Figure 3 shows the cyclic loading test adopted for this study. e tests were performed by displacing the specimen. e reversal of cyclic loads was symmetric [19]. One cyclic loading was applied on the specimen at displacement <than15 mm. At displacement ≥15 mm, three cyclic loadings were applied. At displacement ≤than 30 mm, the displacement increment was 3 mm. At displacement >30 mm, the increment was 6 mm. e test was terminated when the applied load was less than 85% of the peak load.

Test Setup. As shown in
Before each testing, the cells and sensors were checked to ensure that they were working well. en, a preload was applied vertically to eliminate the initial imperfections of the test setup and allow the specimen to properly contact the testing device. After the checking and validation of the cells and sensors, a design vertical load was applied in two or three steps, and held for 2 min. Following that, a cyclic force was applied according to the loading procedure.

Test Phenomenon and Failure Mode
e specific failure phenomena are shown in Figure 4. e failure phenomena of specimens are listed in Table 3. e failure characteristics of specimens, obtained via observation and analysis of the failure, are summarised as follows: (1) In the elastic, elastic-plastic, and failure stages, a plastic hinge formed gradually at the beam-end near the joint. With tiny displacements and strains, the column and the joint were elastic. erefore, the designed joint specimens meet the seismic principles of strong column and weak beam and strong joint and weak member.
(2) In the testing phase, ESRS and the joints between them and the column were left undeformed. As the ESRS joint can effectively transmit and distribute the load, the distribution of stresses within the joint's core area was more uniform. is contributes to the seismic performance of joints. (3) Due to the composite effect of the slabs, both the joint's rigidity and the steel beam's stability were enhanced.

Hysteretic
Curves. e hysteretic curves are comprehensive reflections of the responses of structures or members subjected to cyclic loads, and are of great significance to the analysis of their seismic behaviour [20]. e hysteretic force vs. displacement curves of beam-end far from the joints are shown in Figure 5. e following conclusions are drawn from Figure 5: (1) Specimens develop plump, spindle-shaped hysteretic curves, and show improved energy dissipation capacity and displacement ductility. (2) When the displacement is smaller, the specimens are in the elastic stage, develop almost linear hysteretic loops with smaller envelops, and have smaller residual deformations and energy dissipation capacities. As the displacement increases, the specimens move into the elastic-plastic stage. With the increasing displacement, the specimens gain curved hysteretic loops, which move gradually toward the displacement axis, with larger envelop areas, larger energy dissipation capacities, and greater rigidity 2 Advances in Civil Engineering degeneration. With the increasing displacement, cracks begin to appear around the plates and columns. After the specimens have yielded, the loading cycles are applied three times for each displacement. When the three hysteretic loops at a given displacement are compared, the last two loops are smaller than the first loop and the envelop areas of the last two loops decrease. is demonstrates strength degradation, decline of energy dissipation capacity, and damage accumulation in the specimens.
(3) Due to the composite effects of slabs, the specimens have a larger rigidity and bearing capacity in the negative direction than in the positive one.

Skeleton Curves.
A skeleton curve can be obtained by connecting every peak load of the given displacements. e characteristic points can be obtained from a skeleton curve, such as yield displacement and load, peak displacement and load, and ultimate displacement and load. From the skeleton curve, mechanical properties such as ductility, energy dissipation, strength degradation, and rigidity degradation can be obtained. e skeleton curves obtained from the tests are shown in Figure 6. ey reflect the respective influences of axial compressive ratios, beam-column linear stiffness ratio, highstrength bolts, slab types, slab, etc.
(1) e elastic rigidity increases with increasing axial compressive ratio. e skeleton curves and bearing capacities of different specimens are close to each other. (2) Specimens SJD-5 and SJD-2 have the same axial compressive ratio, but the beam-column linear stiffness ratio of specimen SJD-5 is higher. e peak load of SJD-5 is 58% higher than SJD-2 in the positive direction, and 42.7% higher in the negative direction. It follows that the rigidity and bearing capacity of specimens increase with an increasing beam-column linear stiffness ratio.
(3) Specimens SJD-4 and SJD-2 have the same axial compressive ratio, but SJD-4 has high-strength bolts in its joint's core area. e skeleton curves of specimens SJD-4 and SJD-2 are close with each other. It follows that the high-strength bolts in the joint's core area hardly have any influence on the rigidity and bearing capacity. (4) Specimen SJD-2 has a reinforced recycled concrete slab connected to its beam, while SJD-6 has a slab composited with a profiled steel sheet. e peak load of SJD-2 is 35.9% higher than that of SJD-6 in the positive direction, and 9.9% higher than that of SJD-6 in the negative direction. Compared with the specimen with a slab-profiled-steel-sheet composite, the specimen with a recycled reinforced concrete slab has a larger rigidity and bearing capacity. (5) Specimen SJD-2 has a slab, while SJD-7 does not. e peak load of SJD-2 is 30.3% higher than that of SJD-7 in the positive direction, and 57.4% higher than that of SJD-7 in the negative direction. Compared with the specimen without a slab, the specimen with a slab has a larger rigidity and bearing capacity.

Rigidity Degeneration.
Effective rigidity is one of the most important indicators of seismic performance of structures and members. e effective rigidity is presented in terms of the loop rigidity [20,21]: where P ij represents the peak load of the i-th loading cycle at the j-th displacement of Δ j , u ij refers to the peak displacement of the i-th loading cycle at the j-th displacement of Δ j , and n is the number of loading cycles. At the initial loading stage, the rigidity of specimens decreases drastically. At the final loading stage, as the concrete cracks have fully propagated, and the specimens move into the plastic stage, the rigidity drops slowly. us, as shown in Figure 7, following the cracking of concrete and   damage accumulation of steel, the rigidity decreases with the increasing absolute displacement. Furthermore, the initial rigidity increases with the increasing beam-column linear stiffness ratio. For the specimen with a slab connected to its steel beam, the rigidities in both the directions are different at various loading stages. In the initial loading stage, the slab and steel beam are tightly combined, and the initial rigidity in the negative direction is larger than that in the positive direction. In the final loading stage, with the cracks fully extended in the concrete slab, the concrete slab is damaged, the composite effects between the slab and the steel beam disappear, and the final rigidities in both the directions are close to each other.

Ductility.
e ductility of a structure or a member defines its capability to endure plastic deformation, with a relative higher load applied. Ductility is an important parameter representing the deformation capability of a structure or a member, and it is of great significance for the evaluation of seismic performance of structures [22,23]. Ductility is usually expressed by the ductility coefficient. According to the different definitions of deformation, there are three ductility coefficients of structures and members: curvature ductility coefficient, corner ductility coefficient, and displacement ductility coefficient. Among them, the curvature ductility coefficient only indicates the characteristics of a certain section, while the corner ductility coefficient and the displacement ductility coefficient can reflect the ductility of the structure and the member more macroscopically. Ductility is usually expressed by a ductility index (μ). In general, the larger the value of μ, the better the ductility of the joint. Equation (2) is given for calculating the ductility index. In the formula, displacements Δ u and Δ y of the beam-end far from the joint are used to calculate the ductility index, and can be determined according to the method described in the literature [20].
where Δ u represents the displacement corresponding to the peak load P u , and Δ y is the displacement corresponding to the yield load P y . Table 4 shows the yield load, the yield displacement, the peak load, the peak displacement, and the ductility indices of the specimens. e following is a summary of the results from Table 4: (1) e average value of the ductility indices of the specimens is 2.10.  Advances in Civil Engineering 5 (2) e compressive capacity of concrete is significantly higher than its tensile capacity; so, it is reasonable to assume that the concrete can carry only the compressive stress and neglect its tensile capacity in engineering practice. When loading in the positive direction, the beam-end far from the joint moves downward, the concrete slab carries a negligible tensile stress, and the top flange of the steel beam carries nearly the whole tensile force. When loading in the negative direction, the beam-end far from the joint moves upward, a compressive stress is exerted on the concrete slab, and the concrete slab can work in tandem with the top flange of the steel beam to carry the compressive force. e differences in the mechanical properties of the composite beam-slab system can induce significant differences in the bearing capacity and ductility index in both the directions. For example, specimen SJD-6 has a slab connected to its steel beam, and its yield load and peak load in negative direction are 2.3 and 1.7 times those in the positive direction, respectively. Furthermore, its ductility index in the positive direction is 1.8 times that in the negative direction. is is because, in the negative loading, the recycled reinforced concrete slab is compressed and the plate joins with the top flange of the steel beam in resisting the pressure and deformation. In the positive loading, the recycled reinforced concrete slab has poor tensile strength and cracks easily, so that the section size of the plate is reduced, and the tension is basically assumed by the top flange of the steel beam.
(3) A comparison of the ductility indices of specimens with the same axial compressive ratio revealed that the indices of SJD-4, SJD-5, and SJD-6 were relatively lower than those of SJD-2. is indicates that such structural fabrications could have negative effects on the ductility of specimens, as highstrength bolts settled in the joint's core area increased the beam-column linear stiffness ratio and Step 0   Advances in Civil Engineering the recycled concrete slab composited with a profiled steel sheet. (4) e axial compression ratio has little effect on the bearing capacity of the specimen. is is because the specimen is broken in the plastic hinge area of the beam and the column is in the elastic stage.

Energy Dissipation.
e energy dissipation capacity is another important index for studying the seismic performance of structures and members. In a loading-unloading hysteretic loop, the energy absorbed by the loading process is generally larger than the energy discharged by the unloading process. e difference between the energy absorbed and energy discharged is energy dissipation, which is represented by the area enclosed by a hysteretic loop. e plumper the loop and the larger its enclosed area, the larger the energy dissipation capacity. In engineering practice, the energy dissipation coefficient, E, and the effective viscous damping factor, h e , are generally used to evaluate the energy dissipation capacity of structures and members [24,25]. e equivalent damping coefficient (h e ) was defined by formulas (3) and (4). As shown in Figure 8, S BEF and S DEF are areas under the BEF and DEF curves, respectively, and S AOB and S COD are the triangular areas of AOB and COD, respectively.
(4) Table 5 shows the E and h e obtained. e relationship between h e and the loading cycle is shown in Figure 9. As shown in the table, the numerical values of h e are in the range of 0.161-0.198 at a displacement of 36 mm, and their average value is 0.180. e effective viscous damping factors obtained from the tests are almost twice those of the reinforced concrete joints. It follows that the joints studied have better energy dissipation capacity, and hence can meet the requirements of structural seismic design.
With larger effective viscous damping factors, the seismic dissipation capacity of specimens with a slab is better than those without a slab. e axial compressive ratio has negative effects on seismic dissipation capacity, and the effective viscous factor decreases with increasing axial compressive ratio.
A comparison of SJD-4 and SJD-2 reveals that their effective viscous factors are close to each other. From this, the high-strength bolts in the joint's core area have no influence on the seismic dissipation capacity. As shown in Figure 9, the effective viscous factors increase with displacement-the larger the displacement, the greater the energy dissipation. At a given displacement, the energy dissipation capacity decreases with increasing loading cycle. e plastic hinge developing at the beam-end near the joint could improve the energy dissipation capacity of specimens.

Establishment of Finite Element Model.
e C3D8R element and structural meshing method were used to mesh the steel and recycled concrete. A meshed specimen is shown in Figure 10. e loading and boundary conditions of the specimen are shown in Figure 11. e displacement constraints in the x and z directions, and the rotational constraints in the y and z directions are enforced on the top of the column. e displacement constraints in the x, y, and z directions, and the rotational constraints in the y and z directions are applied at the bottom of the column. e column can only move in the vertical direction and rotate in the y-o-z plane. e displacement constraint in the x direction is enforced at the reference point of the beam, so that the beam can move only in the y-o-z plane. Two steps were adopted for the loading: (a) Exertion of a vertical force on the top of the column, and (b) application of cyclic load on the left beam-end.

Material Constitutive Relationship.
e double-straightlines-strengthened model is used as a constitutive relationship of steel. In the case of the constitutive relationship of recycled concrete used in the slab, the model developed by Xiao [24] is used as the compressive constitutive relationship, and the tensile model of ordinary concrete is used as the tensile constitutive relationship [25]. In the case of the constitutive relationship of SFRC used in the column, the model presented by Yang [26] and Meng [27] is used as the compressive constitutive relationship, and the tensile constitutive relationship is the same with the concrete used in the slab. ere are few references for the constitutive relationship of steel fibre recycled concrete in the literature. Upon the addition of steel fibre, the strength gain of the specimen is not obvious and the column is basically in the elastic state during the whole stress cycle. In addition, the method for determining the damage coefficient of concrete proposed by Birtel and Mark [28] is adopted.

Contact Pairs Settings.
For the contact between the steel tube and the concrete, the tangential force is considered along with the Coulomb friction criterion, and the normal force along with the "hard contact." A nonlinear spring unit is generally used to simulate the contact between the top flange of the steel beam and the concrete, as there are studs connecting the top flange of the steel beam to the concrete slab. e embedded connection is used to simulate the contact between the reinforcing bars and the concrete slab. In this case, the reinforcing bars and the concrete slab need to be modelled separately (see Figure 12). Figure 13 illustrates the experimental and simulated hysteretic curves for the joints studied. e hysteretic curves obtained from the tests and the FEM simulation are in good agreement with each other. e error margin between the simulated and experimental curves is less than 10%. We also found that the specimens have spindle-shaped hysteretic curves. ese curves imply that the ESRS joint specimens have better ductility and seismic     Advances in Civil Engineering performance. From analysis, these errors exist despite the weld seams at the joints, the bond slip between the steel tube and the concrete, and assumptions of idealised constitutive relationship and boundary conditions. Figure 14, the skeleton curves obtained from the physical tests and FEM simulation are in good agreement with each other. e skeleton curves are S-shaped, which indicates that the specimens have undergone elastic, elastic-plastic, and plastic stages consecutively.

Skeleton Curves. As shown in
In addition, induced by the composite effect of the slab and the steel beam, the skeleton curves are asymmetrically distributed, and the bearing capacity and rigidity in the positive direction are larger than those in the negative direction. e symmetry of the skeleton curves obtained from the FEM analysis is better than that from the physical tests.
is symmetry is induced by no consideration of the residual deformation of the members, avoiding the torsion of members by imposing a constraint in the FEM analysis. In the test, the residual deformation and torsion of members are inevitable.

Comparison of Typical Specimen Failure
Modes. Figures 15-18 show that the failure modes observed in the tests and the simulation are in agreement with each other. Because the weld seams were not considered, the failure of specimens in FEM occurs at locations with the largest deformation. From observation, the local buckling and rupture failures generally occur at the connections where the H-shaped steel beam is linked to the steel ringstiffener. Table 1, SJD-2 has a steel beam of dimensions 150 mm × 75 mm × 5 mm × 5.5 mm, while SJD-5 has a steel beam of 200 mm × 100 mm × 5.5 mm × 6 mm. e other parameters of the two joints are the same. A comparison of the parameters of SJD-2 and SJD-4 reveals that SJD-4 has high-strength bolts settled at the joint's core area, while SJD-2 does not. e stress distributions of joints SJD-2, SJD-4, and SJD-5 are illustrated in Figures 19-21, respectively.

Stress Analyses of Joints. As shown in
From Figure 19 (SJD-2), as the displacement of the beamend far from the joint reaches 12 mm, the maximum normal stress, 375.4 MPa, occurs at the trapezoidal zone of both the steel beam flanges. e normal stress at the trapezoid zone increases with displacement, and reaches 451 MPa at 18 mm. After 18 mm, the normal stress gradually yields. At 30 mm, the normal stress near the bolt-weld connection partially yields and local buckling occurs at both the flanges. At 36 mm, the lower flange of the steel beam near the bolt-weld connection yields significantly. Although the top flange of the steel beam near the bolt-weld connection has yielded, the deformation is negligible for the supporting and constraint effects of the slab. At 39 mm, the local buckling area of the lower flange near the bolt-weld connection has significantly enlarged, and hence completely failed. Note. "+" denotes positive direction, and "− " denotes the negative direction.      core area not only protect the core area from failure, but also changes the distribution of stresses. At 30 mm, the normal stress at the flanges near the bolt-weld connection increases to a value smaller than that of SJD-2. At the moment, both the flanges have partially yielded. At 36 mm, the yielding areas of the two flanges enlarge, and a larger local buckling is observed. At 42 mm, the local buckling area near the boltweld connection enlarges significantly, and the flanges enter the plastic stage.

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Buckling deformation the weld seam is broken

Conclusions
In this study, the seismic performance of ESRS joints was studied via physical tests and FEM simulation. e following is a summary of the main conclusions of the study: (1) After the elastic, elastic-plastic, and failure stages, a plastic hinge begins to form gradually at the beamend near the joint. For minor displacements and strains, the column and joint are elastic. erefore, the designed specimens of joints meet the seismic principles of strong column and weak beam, and strong joint and weak member. (2) As the ESRS can effectively transmit and distribute the load, the distribution of stresses within the joint's core area is more uniform. is contributes to the seismic performance of joints. Due to the composite effects of slabs, the joints' rigidity, bearing capacity, and energy dissipation capacity, and the local stability of the steel beam's top flange and the integral stability of the steel beam are enhanced. e rigidity and bearing capacity of joints in the negative direction are larger than that in the positive direction. (3) e bearing capacity, strength degradation, ductility, and energy dissipation of the specimens were barely influenced by the axial compressive ratio. Instead, the bearing capacity and rigidity were significantly influenced by the beam-column linear stiffness ratio, and increased with an increase in the ratio. e ductility and energy dissipation capacity of specimens, however, decreased with the ratio. Since the plastic hinge occurred at the beam-end but not in the joint's core area, the bearing capacity, rigidity, and energy dissipation capacity of specimens were not influenced by the high-strength bolts fastened in the joint's core area. Subjected to the same axial compressive ratio, the ductility and energy dissipation capacity of specimens with recycled concrete slab were higher than those of the specimens with the slab-profiled-steel-sheet composite. (4) Compared with the existing reinforced concrete joints, the ESRS joints developed in this paper not only have the advantages of higher ductility, larger effective viscous damping factor, better energy dissipation capacity, and plumper hysteretic curves without pinching, but also simple construction and definite load-transferring mechanism. We believe that the technique will benefit structural and civil engineers in developing better earthquake-resistant buildings, and the recycled concrete will benefit the environment and reduce costs. (5) Large-scale finite element software ABAQUS was used to simulate the seismic performance of the ESRS joints. A joint stress analysis was performed to ensure the correctness of the model. From observation and simulation, the normal stresses in the ring stiffener were generally uniform, increased from the longer side to the opposite shorter side of the trapezoidal zone, and reached the maximum value near the bolt-weld connection. To avoid damage to the ring-stiffener during engineering practice, we can thicken it or add rib-strengthened panels.

Data Availability
All the data used in this study are available from the experimental tests of the composite joints, and data can be obtained from the corresponding author upon reasonable request.

Conflicts of Interest
e author(s) declare(s) that there are no conflicts of interest regarding the publication of this paper.