Seismic behavior of a new through-core connection between concrete-filled steel tubular column and composite beam
Introduction
The structural system with composite beam and concrete-filled steel tubular (CFST) column has been widely used in building structures, especially in high-rise and super high-rise buildings. In a practical building, the performance of the beam-column connections plays a critical role. Generally, the connections between composite beams and CFST columns can be classified into two types [1]: one is the non-through core connection, which is usually constructed by directly welding the steel beam to the column, like interior-diaphragm connections [2] and exterior-diaphragm connections [3];the alternative is through core connection which usually consists of a beam or other elements completely passing through the column, such as bolted end-plate connections [4], through-diaphragm connections [5,6] and through-beam connections [7].
Alostaz YM et al. [8,9] conducted experimental and FE studies on a variety of connection details for connections to CFST columns. Results indicated that the direct welding of connection-stub to the steel tube was the most convenient approach. However, this connecting method would lead to a large deformation demand on the tube wall, and the high local distortion would subsequently lead to fracture failure at the flange to column weld or at the tube surface of the column. Connections that utilize embedded components could distribute the girder loads directly to the concrete core, and effectively alleviate the stress concentration on the tube wall. But the connection performance is sensitive to the embedded elements, and the embedded plates often involve complex welding work inside the tube. Latter Alostaz Y M started further researches on the through-core connection [10]. Three kinds of through-core connections with different penetration extents (i.e. one with only web plate and headed studs through the column, one with only flanges through the column, and one with the whole girder through the column) were tested and analyzed. The comparison indicated that connections with continuous web displayed the lowest strength and stiffness. The continuous flange though-core connection presented satisfactory ductility and strength development but poor energy dissipation ability. Connection with the entire girder extended through the CFT column could sufficiently develop the full plastic bending strength of the connected girder, and exhibited favorable elastic-plastic cyclic performance. Relative works on through beam connection [1,7] also showed that the continuous beam construction could effectively transfer the beam force to the core concrete. Then the through beam connection offered the most effective method in developing the rigid connection, and can be used for connecting the heavy steel beam to CFT columns in high seismic risk regions.
Through-beam connections possess comprehensive connecting performance and have been applied in many engineering projects. While in the practical applications, this connection also has constructional difficulty, which is the inconvenient concrete pouring into the CFST column when the girder flange dimension is close to the column diameter. The wide flange may interference the concrete from passing through the joint region, which may bring defects to the CFST column. To solve this problem, a modified connection configuration is proposed (see Fig. 1) in this paper, which has the flange width get reduced at the continuous girder within the “passing through” (PT) region. The flange reduction would reduce the load transferring path under negative moment conditions and may bring weakening effect to the connection behaviors. Previous studies also indicated that the steel bars can help the internal force transfer from girder flange to the core concrete [9]. And research work in ref. [11, 12] also indicated that embedded deformed bars can significantly improve the connection's bearing capacity. Therefore, to limit the adverse effect of flange width reduction in the proposed connection, the longitudinal reinforcements are added in the RC slab, and those reinforcements are also passing through the CFST column. Therefore, to limit the adverse effect of flange width reduction in the proposed connection, the longitudinal reinforcements are added in the RC slab, and those reinforcements are also passing through the CFST column. As given in Fig. 1, these through core reinforcements (TCR) located above the girder can participate the load bearing under negative moment conditions, then making up the negative influence of girder flange reduction to the connection. FE analysis by us proved that the capacity of specimen CFCJ1 (illuminated in Section 2.1 later on) would decrease by 20% at least suppose the through core reinforcements were removed. Moreover, it is noticed that the composite effect of the I-steel beam and RC slab can effectively improve the seismic performance of the connection [[13], [14], [15], [16], [17]].
To investigate the mechanical behaviors of the modified through-beam connection and the influence of beam-flange reduction measure, a series of experimental researches are conducted. Quasi-static cyclic tests on four specimens with different reduction degrees of flange width are conducted, and corresponding 3D FE models are established. Based on that, the load-bearing capacity, failure mode, hysteretic curves, skeleton curves, ductility, stiffness degradation and energy dissipation capacity of the joints are investigated. The influence of flange width reduction degree on the seismic performance of the joints is also analyzed. Then the appropriate reduction degree of flange width is suggested for both upper and lower flange.
Section snippets
Test specimens
Four test specimens (labeled as CFCJ1~CFCJ4) of full sized side-column connections in actual building are tested, among which the beam flanges in “PT” region has different reduction extent except CFCJ1. Essential information of the specimens is shown in Table 1 and Fig. 1.
The nominal dimension of the CFST column and the steel beam is 400 × 400 × 12 mm (Height× Width ×Thickness), H300 × 150 × 200 × 8 × 12 mm (Height × upper flange width × bottom flange width × web thickness × flange thickness)
General
Since the number of test specimens is limited, and the internal working mechanism cannot be obtained directly through the failure modes. Then together with experimental studies, the numerical simulations are also performed. The FE model of tested specimens are established in ABAQUS, as shown in Fig. 6(a). Eight-node reduced integral format 3D solid elements (C3D8R) are used to model the square steel columns, the steel beams, the concrete and the loading plates. The reinforcing bars in the slab
Load-displacement relationship
The hysteretic curves can be observed in Fig. 7 and the load-displacement skeleton curves that obtained from the linking of every cyclic peak point load were showed in Fig. 8. It is obvious that FE results are generally in consistent with experimental results. However, the behavior of the connection, especially the behavior under reverse loading was not imitated to be ideal, the proper reason is due to the bias between the FE model and the act, and one main reason for it may be not considering
Analysis of equivalent plastic strain and stress
In ABAQUS, the equivalent plastic strain (PEEQ) describes the accumulation of plastic strain of materials, and this coefficient is generally used to analyze the plasticity development. When the material enters yielding state, the PEEQ value will be larger than zero, and will get increased as the developing of plasticity. Therefore the plastic zone conditions in the connection can be distinguished based on the PEEQ contours diagram. Fig. 10 shows the PEEQ contours diagram at failure state of the
Conclusion
Traditional through-beam connection would have concrete pouring problems when the flange width of the girder is close to the CFST column. To solve this problem, a modified construction with reduced girder flange width within the “PT” region is proposed. The quasi-static cyclic loading tests on four specimens and reasonable FE analyses are performed. Then to understand the real and exact performance of the proposed connection, four quasi-static cyclic loading tests were performed and
Acknowledgments
This research work is financially supported by the National Key R&D Program of China (2016YFC0701201).
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