Mechanical Performance of Rectangular CFST Column-Concrete Beam Through-Rebar Joint Under Low-Cycle Loading

Literature Overview

The paper by Zhang Yanxia et al. (2022), published in Engineering Mechanics (Vol. 39, No. 4, pp. 138-150), addresses a critical structural engineering challenge in high-rise construction: the connection between rectangular concrete-filled steel tube (CFST) columns and reinforced concrete beams. Traditional connections such as welded gusset plate joints and sleeve gusset plate joints suffer from indirect force transfer, complex fabrication, and difficult quality assurance during on-site construction. The authors propose a through-rebar joint system developed for the Beijing CBD Z13 super-high-rise project, supported by multiple national and municipal research grants including NSFC Grant 51778036.

Core Technical Content and Experimental Methodology

The study investigates the seismic performance of the proposed through-rebar joint through low-cycle reversed loading tests on multiple specimens with varying reinforcement ratios. The experimental programme systematically examines four key performance indicators: hysteretic behaviour, failure mode, energy dissipation capacity, and ductility. The specimens are compared against two conventional joint types: the welded gusset plate joint and the sleeve gusset plate joint.

From a welding and fabrication perspective, the through-rebar joint eliminates the need for extensive field welding of gusset plates, which is particularly advantageous given that field-welded joints in CFST structures are prone to defects such as incomplete fusion, porosity, and hot cracking due to the thick-walled nature of rectangular steel tubes and the thermal constraints imposed by the surrounding concrete. The joint relies on rebars that pass directly through the hollow steel tube section, creating a more direct force path from beam to column.

Key Experimental Findings

The experimental results demonstrate that the through-rebar joint exhibits significantly stronger load-bearing capacity compared to the other two joint types. The hysteretic loops are well-defined and full, indicating excellent energy dissipation under cyclic loading. The failure mode observed is characterised by plastic hinge formation at the beam end rather than at the joint itself, which satisfies the fundamental seismic design principle of "strong joint, weak member."

Theoretical Analysis and Design Recommendations

The authors develop a theoretical model for the bending capacity of the through-rebar joint and derive a design formula applicable to practical engineering. A key recommendation is that the steel gusset section should be designed to carry 80% of the beam-end shear force. This design criterion provides a rational balance between structural safety and material economy.

From a metallurgical and materials science standpoint, the joint design implicitly addresses several important considerations. The rectangular CFST column, typically fabricated from seamless steel or HFW welded steel conforming to GB/T 8162 or GB/T 3091, must accommodate through-rebar penetrations without compromising the structural integrity of the tube wall. The steel tube wall thickness and the reinforcement arrangement must be carefully coordinated to prevent local buckling of the tube wall at the penetration locations under combined axial compression and bending.

The theoretical formula for bending capacity likely incorporates the contribution of the steel tube, the concrete core, and the through-rebars in a composite manner. The 80% shear design criterion for the gusset plate reflects an understanding that the remaining 20% of shear is transferred through the bond between the through-rebars and the concrete core, as well as through the bearing capacity of the tube wall itself.

Engineering Practice Integration and Welding Considerations

In practical engineering applications, the through-rebar joint offers several advantages that are particularly relevant to welding engineers and fabrication specialists. First, the reduction in field welding significantly decreases the risk of weld defects that are difficult to repair in elevated construction environments. Second, the joint design allows for prefabrication of the steel tube sections with pre-cut openings for rebar penetration, which can be controlled in a shop environment with proper welding procedures, non-destructive testing, and quality assurance protocols.

The fabrication of rectangular CFST columns requires attention to the welding quality of the tube seams themselves. For seamless tubes, the microstructural uniformity is inherently superior, but for HFW or ERW welded tubes, the weld seam must be inspected for defects such as lack of fusion, slag inclusion, and undercut. The steel grade selection—typically Q235B or Q355B per GB/T 1591—directly influences the welding parameters and the heat-affected zone (HAZ) properties.

For the steel gusset plate component of the joint, if welding is required at the connection between the gusset and the tube wall, the welding procedure should follow applicable standards such as GB 50661 or AWS D1.1. The preheat temperature, interpass temperature, and post-weld heat treatment must be carefully controlled based on the carbon equivalent of the base metal. Typical defects to monitor include cracking in the HAZ, distortion of the tube wall, and residual stress concentrations at the weld toe.

Key Questions and Reflections

The study raises several important questions for further investigation. First, while the through-rebar joint demonstrates superior seismic performance, what are the long-term durability implications of the rebar-concrete-steel tube interface under repeated loading and potential corrosion? The bond strength between rebars and concrete, and the compatibility between steel tube and concrete under cyclic strain, warrant further investigation, particularly in chloride-laden environments typical of coastal high-rise construction.

Second, the paper does not extensively discuss the manufacturing tolerances and assembly accuracy required for the through-rebar joint. In practice, the alignment of rebars through the tube openings, the fit-up of the gusset plate, and the dimensional accuracy of the tube penetrations all influence the final joint performance. Tighter manufacturing tolerances may be required compared to conventional joints, which could increase fabrication costs.

Third, the comparison with welded and sleeve gusset joints is valuable, but the study would benefit from additional comparison with bolted connection alternatives and with internal diaphragm joints, which are another common approach for CFST-to-beam connections.

Study Insights and Implications

This research contributes significantly to the structural engineering knowledge base for CFST construction in high-rise buildings. The through-rebar joint represents a practical solution that addresses the well-recognised challenges of force transfer and construction quality in CFST-to-beam connections. The combination of experimental validation and theoretical analysis provides engineers with a reliable basis for design.

From the perspective of welding and fabrication engineering, the joint design philosophy of minimising field welding in favour of bolted or mechanical connections is consistent with modern trends in prefabricated construction. The emphasis on shop fabrication quality and the reduction of on-site welding activities aligns with best practices for ensuring structural integrity in complex high-rise projects.

The design recommendation of 80% shear capacity for the gusset plate is a practical engineering criterion that balances safety with economy. Engineers should verify this criterion against project-specific loading conditions and applicable design codes such as GB 50011 and GB 50010, ensuring that the joint design meets all relevant seismic and serviceability requirements.

This study exemplifies the value of integrating experimental mechanics, theoretical modelling, and practical engineering considerations in the development of new structural connection systems for composite construction.