Experimental Study On Hysteretic Behavior of Concrete Filled Square CFRP Steel Tubular Beam- Column


 In order to study the hysteretic behavior of concrete filled square CFRP steel tubular Beam-Column under different influence factors, 12 specimens were designed, and the failure mode, middle section lateral force-deflection(P-Δ) curve, middle section bending moment-curvature(M-φ) curve and middle section deflection-deformation(Δ−Δ') curve were studied. Axial compression ratio and longitudinal CFRP reinforcement coefficient as influencing factors, the effects of axial compression ratio and longitudinal CFRP reinforcement coefficient on P-Δ skeleton curve, M-φ skeleton curve, strength and stiffness degradation, ductility, cumulative energy consumption and other indexes were studied; the P-Δ curve and deformation mode of the specimens were simulated by ABAQUS, and the effects of axial compression ratio, slenderness ratio and other main parameters on the hysteretic performance of the members were studied. The test results show that CFRP has good lateral restraint and longitudinal reinforcement effect on CFST, and the local buckling of CFST is delayed. The P-Δ curve and M-φ curve of all specimens are full. In addition, the steel tube and CFRP have good synergy in both longitudinal and transverse directions. The change of axial compression ratio and longitudinal CFRP reinforcement coefficient has no significant effect on the strength degradation. The increase of axial compression ratio and longitudinal CFRP reinforcement coefficient can improve the flexural capacity and stiffness of the specimens, and slow down the stiffness degradation, but reduce the ductility and cumulative energy consumption of the specimens. The finite element software ABAQUS is used to simulate the P-Δ curve and deformation mode of specimens. It is found that the simulation results are in good agreement with the experimental results. Based on the model analysis of the main parameters, it is found that the increase of steel yield strength and CFRP layers can improve the bearing capacity of the specimens, and the axial compression ratio has the most significant effect on the specimens.


Introduction and research significance" 36
In recent years, earthquakes are more and more widespread in the world. The distribution of 37 seismic zones is not uniform, but they are widely distributed. Some scholars have carried out 38 extensive and in-depth research on the seismic design of building structures, and the 39 earthquakes have caused huge economic losses and casualties. To deal with the threat of 40 earthquake disaster to buildings, the research on hysteretic behavior of building structures is 41 more and more extensive [1][2]. Nowadays, the most commonly used composite structure is 42 steel-concrete composite structure. It is a composite structure composed of steel and concrete, 43 which mainly uses the advantages of compressive performance of concrete and tensile 44 performance of steel [3][4]. This composite structure is not only convenient for construction, 45 but also saves a lot of materials, so as to achieve the goals of reducing the cost, reducing the 46 weight of components and shortening the construction period. Therefore, the steel -concrete composite structure is widely used in practical engineering [5]. bearing capacity was reduced significantly after fire damage, while concrete -filled CFRP-steel 58 tube specimens' fire resistance was better than that of ordinary concrete-filled steel tube 59 specimens [9][10]. In practical application, members often also bear hysteretic loads, such as 60 wind and earthquake load [10][11]. 61 In view of this, 12 groups of square CFRP concrete-filled steel tubular specimens are 62 designed in this paper. Referring to the hysteretic test of concrete -filled steel tubular, the failure 63 mode, P- curve, M- curve and ' curve of each group of specimens are studied. The axial 64 compression ratio and longitudinal CFRP reinforcement coefficient are taken as the influencing 65 factors to study their influence on P- skeleton curve and M- skeleton curve. ABAQUS is used 66 to simulate the P- curve and deformation mode of the specimens. On this basis, the influence of 67 axial compression ratio, slenderness ratio and other main parameters on the hysteretic 68 performance of the member is studied, so as to provide some theoretical reference for engineering practice. Cold-formed steel tubes were used for the S-CF-CFRP-ST specimens, in which the inner 74 chamfer radius at the bending angle was 5mm. The steel tubes' material properties are shown 75 in Table 1. 76 Table 1 The material properties of steel tube used in experiment 77 properties are shown in Table 3. 89 The adhesive and base adhesive are building structural adhesives produced by China Institute 90 of construction science and technology in the test.  In which: N 0 is the axial compression applied to the specimens. 99 The specimens' calculated length (L) was 2000mm. The steel tube's outer length (B s ) was 100 140mm. The tube's wall thickness (t s ) was 4mm, and the number of transverse CFRP layers 101 (m t ) was 1, where m l was the number of longitudinal CFRP layers, and y was the specimens' 102 displacement in the yield state. All specimens' specific parameters are shown in Table 4. 103 In the process of the test, P and  were collected directly by the INV-306D intelligent signal 132 acquisition system, which was connected to the vertical actuator of Electro-hydraulic 133 Servo-system, and the P- curves were drawn at the same time. When the specimens with a small axial compression ratio (n  0.2) were loaded to 3 y , a slight 148 deformation occurred in the compression area near the midsection. With unloading and 149 reverse loading, the convex deformation flattened again, and the increases in the convex 150 deformation were proportional to the increase in displacement. At this time, the transverse 151 CFRP at the bending angle began to fracture sporadically. When they were loaded to 5  y , the convex deformation developed significantly and the sound of the CFRP splitting could be 153 heard. At this time, a large number of transverse CFRPs fractured at the bending angle, and 154 then the longitudinal CFRPs also fractured, as shown in Fig. 2(a). When loaded to 7 y~8  y , a 155 large number of them fractured, and finally, the steel tube was destroyed. In the specimens 156 without longitudinal CFRP, when the deflection was large during the later stage of loading, a 157 large number of transverse CFRPs fractured at the bending angle, and finally, the steel tube 158 was destroyed swiftly, as shown in Fig. 2  In general, as the  and n increased, the extent of the specimen's damage decreased.
where:  is the curvature of the middle section, u m is the deflection of the middle section, L is 213 the calculated length of the specimen, M is the bending moment of the middle section, P is the 214 lateral bearing capacity, and N 0 is the axial force. It can be seen from Fig.9 that the M- hysteretic curves of specimens are shuttle shaped, and 220 there is no obvious pinch phenomenon. When the force control is adopted at the initial stage of 221 loading, the deformation of the specimen is elastic deformation, When the displacement control is 222 adopted, the component produces a less obvious "Bauhinia" effect.

238
In order to ensure the accuracy of the test results, the transverse strain curves (P- t curves) of 239 steel tube and CFRP at two test points of A1 group specimens are taken, as shown in F ig. 12 (a) 240 and Fig. 12 (b). Similarly, the longitudinal strain curve (P- 1 curve) at the same two test points of group A2 are taken, as shown in Fig. 12 (c) and Fig. 12 Figure 14 shows the strength degradation of the specimen. It is obvious from Figure  258 15 that the strength degradation of the specimen is not obvious.

Stiffness degradation
The stiffness EI of each cycle was determined according to the method of reference [ 11]. Figs. 262

(a) and (b) show the effects of axial compression ratio and longitudinal CFRP reinforcement 263
coefficient on the stiffness degradation of specimens, respectivel y, where EI =0 is the initial 264 stiffness of the specimens. It can be seen from Figure 15 that the increase of axial compression 265 ratio can delay the stiffness degradation of the specimen. In addition, the increase of longitudinal 266 CFRP reinforcement coefficient can delay the stiffness degradation of the specimen.

Displacement ductility factor 270
The ductility of the specimen is calculated by the following displacement ductility coefficient μ: 271 The comparison of the ductility coefficient of each group of specimens is shown in Fig. 16. 274 Since the load of n = 0 specimen does not drop to 85% of its peak bearing capacity at the end of 275 the test, it is impossible to determine its ductility coefficient, which is taken as a larger value in 276 comparison. It can be seen from Figure 16 Fig. 17 (a) and Fig. 17 (b) respectively show the influence of axial compression ratio and 285 longitudinal CFRP reinforcement coefficient on the cumulative energy dissipation E of 286 specimens [14-15]. It can be seen that the increase of axial compression ratio will reduce the energy dissipation capacity of the specimens, which is due to the poor ductility of the specimens 288 with large axial compression ratio. The residual bearing capacity of the specimens with large axial 289 compression ratio is lower than that of the specimens with small axial compression ratio. In 290 addition, the increase of longitudinal CFRP reinforcement coefficient can improve the energy 291 dissipation capacity of the specimens. According to the method of reference [14-15], the energy dissipation coefficient he is determined. 293 Figure 18 shows the relationship between energy dissipation coefficient and displacement of 294 =0.34 specimen in the last cycle of each load level. It can be seen from the figure that when the 295 specimen yields, the energy dissipation coefficient of the specimen with a xial compression ratio is greater than that of the specimen without axial compression ratio, which indicates that the axial 297 compression ratio is beneficial to the seismic performance of the specimen within a certain range.

Finite element calculation model
The element selection, mesh generation and interface model treatment method of specimens 311 are consistent with those of CFST members. Figure 19 shows the boundary conditions for the 312 finite element simulation of specimens. that the surface load is applied on the end plate and the lateral hysteretic force is applied on the 319 middle section. In order to ensure that the loading mode is consistent with that in the test process, 320 the loading-displacement control mode is adopted. 321  It can be seen that with the increase of n, the bearing capacity and the stiffness of the elastic stage 356 of the member decrease significantly. The shape of the curve also has obvious changes: when n=0, 357

Comparison of finite element simulation and test results
there is no descending segment in the P- skeleton curve. With the increase of n, the second-order 358 effect of axial force is more obvious, and the descending segment appears in the curve, and the 359 Crushed Crushed amplitude of the descending segment increases. Effect of  on P- skeleton curve of specimens is shown in Figure 25. It can be seen that the bearing capacity and the stiffness of the elastic stage of 361 the member decrease significantly with the increase of  and the shape of the curve also has 362 obvious changes: the stability coefficient decreases with the increase of  and the second-order 363 effect caused by the constant axial force is more obvious. curve of members. It can be seen that with the increase of m l , the shape of the skeleton curve and 367 the stiffness of the elastic stage are basically unchanged, and the bearing c apacity of the member 368 is slightly improved. Figure 27 shows the effect of the number of transverse CFRP layers on the can be seen that with the increase of f y , the shape of the skeleton curve and the stiffness of the 375 elastic stage are basically unchanged, and the bearing capacity of the component is improved. 376 Figure 29 shows the effect of concrete strength on the P- skeleton curve of members. It can be 377 seen that with the increase of f cu , the shape of skeleton curve and the stiffness of elastic stage are 378 basically unchanged, and the bearing capacity of members is slightly improved. 379