235 / 2022-06-14 19:53:43

Local compressive behavior and seismic performance of ECC ring beam connection

ECC; ECC-encased CFST column; Beam-column connection; Local compression; Seismic resistance.

Concrete filled steel tube (CFST) composite columns have been extensively investigated and employed because of their high ultimate bearing capacity and ductility. The design of the beam-to-column connections have significant effect on the mechanical performance and safety of a CFST composite frame structure. However, limited research can be found on the connection between a CFST composite column and a reinforced concrete (RC) beam. Recently, a new type of RC ring beam connection has been recently proposed linking the CFST composite column and RC beam. The RC ring beam connection has a definite force transfer mechanism and greatly simplifies the construction process. However, due to the disconnection of the steel tube, the compressive behaviour of the ring beam connection is one of the most important factors that affect the mechanical performance, and the load carrying capacity inevitably decreases for this kind of connection.

To reduce the damage degree and ensure the connection zone have a sufficient strength for safely transferring the load from the CFST composite column, a novel through-beam ECC ring beam connection, which is designed for linking an CFST composite column (ECC-encased CFST column) and RC beam, is proposed (Fig. 1). The steel tube in the ECC-encased CFST column is completely interrupted and the RC ring beam connection is replaced by an ECC ring beam connection. The ECC ring beam connection consisted of a core region and a ring beam encircling it. Several stirrups and circumferential steel reinforcements were used inside the encircling ring beams. Due to the superior high ductility of ECC ring beam which can provide persistent lateral restraint to the joint core region, introducing ECC in the ring beam connection region is effective to improve the load carry capacity under local compressive loading, as well as control the crack width of the ring beam connection. Meanwhile, additional longitudinal bars in the connection anchored into the outer ECC layer of the column are designed to transfer the axial force and bending moment from the column. Both the enhanced ring beam connection and the additional longitudinal bars make the connection zone have sufficient strength for safely transferring the load from the CFST composite column. More importantly, due to the interruption of the steel tube, ECC is cast continuously in the frame beam end and the ring beam connection zones, and the longitudinal steel bars in the frame beam can pass through the connection zone directly. The force transfer mechanism of the bending moment and shear force from the frame beam is definite, similar to that of the normal RC structures. It is well known that the connection region is subjected to high shear stress. Stirrups arranged in the core region are useful to restrain the ECC and participate in joint shear resistance. Also, the shear capacity of the proposed connection is expected to be improved by the excellent shear resistance of the ECC ring beam under reversed cyclic loading.

To investigate the compressive behaviours of ECC ring beam connections, local compressive tests on 15 ring beam connections with various matrix type, width ratio, height-to-width ratio and reinforcement ratio are reported. According to the test results, ECC connections showed a higher load carrying capacity and more ductile behaviour than regular concrete connections. Compared with concrete ring beam connections, the displacement ductility of the ECC ring beam connections was improved by 25.7% to 62.2%. The energy dissipation of the ECC connection specimens was 1.5 and 1.7 times larger than that of the concrete connections, respectively. The test results also indicated that the width ratio had the greatest impact on delaying cracking in the ECC connection. Increasing the reinforcement ratio can efficiently increase the peak load. Increasing the width ratio resulted in an obvious increase in the energy dissipation of the ECC ring beam connections.

To investigate the seismic behaviours of ECC ring beam connections, seven ring beam connection specimens, including two concrete specimens and five ECC specimens, with different reinforcement ratios, ring beam sizes and axial compression ratios, were tested under reversed cyclic loading. The seismic performance of the proposed connection was evaluated in terms of failure mode, hysteresis characteristics, stiffness degradation, ductility and energy dissipation. Based on the test results, introducing ECC in ring beam region resulted in a higher load bearing capacity, ductility and energy dissipation than those observed for concrete specimens. The relative reinforcement ratio had a significant effect on the failure modes of the ECC connections. Additionally, the increase in the circumferential reinforcement ratio and joint size have favourable influences on the seismic behaviours of ECC connections. These test results verified the potential applications of ECC material in ring beam connections for ECC-encased CFST columns.