Seismic Performance Test Study of Steel Tube Concrete Hybrid Columns

Introduction and Research Significance

Steel tube concrete hybrid columns represent an evolution of the conventional concrete-filled steel tube (CFST) concept, incorporating additional reinforcing elements such as internal steel bars, external steel plates, or combined reinforcement systems to enhance seismic performance. This research conducts experimental tests on hybrid columns subjected to low-cycle reversed loading to simulate seismic demand, providing empirical data that validates analytical models and informs design code provisions. The hybrid configuration addresses the limitation of conventional CFST columns in providing adequate shear capacity and ductility in the plastic hinge regions, which is critical for performance-based seismic design of moment-resisting frames.

The study is particularly relevant to current engineering practice in China, where the GB 50011-2010 (2016 edition) seismic design code and the GB 50936-2014 code for concrete-filled steel tube structures provide design guidelines but lack comprehensive provisions for hybrid column configurations. The experimental data generated fills this gap and provides a basis for future code amendments.

Specimen Design and Testing Protocol

The test specimens include several hybrid column configurations designed to isolate the contribution of different reinforcement elements to seismic performance:

The test protocol follows the displacement-controlled cyclic loading procedure specified in GB/T 5123 and JGJ/T 101, with drift ratios ranging from 0.2% to 4.0% in increments of 0.2%. Each drift level is loaded and unloaded twice (two cycles per drift level), and the peak drift level is held for three cycles to evaluate the degradation behavior. The axial load is applied through a hydraulic jack at the top of the column, simulating a constant gravity load of 0.3-0.6 P0 (where P0 is the axial capacity of the column).

Experimental Results and Failure Mechanisms

The test results demonstrate that hybrid columns exhibit substantially improved seismic performance compared to the conventional CFST control specimen:

• H-1 (Control): Exhibits typical CFST behavior with steel tube local buckling initiating at 1.5% drift, followed by concrete core crushing at 2.5% drift. The hysteretic loops show significant pinching due to the loss of confinement after concrete crushing
• H-2 (Internal bars): The longitudinal bars provide additional tensile capacity in the plastic hinge region, increasing the shear capacity by approximately 20%. The bars also help to hold the concrete core together after crushing, improving post-peak ductility
• H-3 (External plate): The external steel plate provides additional lateral confinement and shear reinforcement. The plate acts as a continuous confining element that prevents concrete spalling and maintains load-carrying capacity beyond the concrete crushing point
• H-4 (Combined reinforcement): Shows the most robust seismic performance with peak loads exceeding 15% above the control, ductility ratios above 6.0, and stable hysteretic loops without significant pinching throughout the loading range
• H-5 (Square tube with ribs): The internal ribs improve the concrete-tube bond and distribute the confining pressure more uniformly. The square geometry results in slightly lower ductility compared to circular configurations but provides better torsional resistance

The energy dissipation capacity, quantified by the area enclosed within the hysteretic loops, increases by 40-80% for the hybrid configurations compared to the control. The equivalent damping ratio, calculated from the hysteresis loops, ranges from 0.15 to 0.25 for the hybrid specimens, compared to 0.10-0.15 for the conventional CFST column.

Welding and Fabrication Quality Control

The hybrid column configurations introduce several weld details that require rigorous quality control:

1. Longitudinal bar anchorage welds: The internal reinforcement bars are welded to internal anchor plates or to the tube wall through embedded weld pads. These welds must be designed for full tensile strength of the bar and inspected by MT and UT. The weld leg size should be at least 0.7 times the bar diameter
2. External plate attachment welds: The external steel plates are typically welded to the tube surface using full-length fillet welds or intermittent welds with a spacing not exceeding 200 mm. The weld design must accommodate the cyclic bending of the plate during seismic loading. Fatigue assessment per EN 1993-1-9 or GB/T 19804 should be performed for the weld details
3. Internal rib welds: In square tube configurations, internal ribs are welded to the tube walls to improve concrete confinement. These welds are subject to complex multiaxial stress states and should be designed as full-penetration groove welds with a throat thickness not less than the rib thickness
4. End plate welds: The end plates at the column top and bottom, which connect to the beam or foundation, are critical for force transfer. These welds must be designed for the full moment and shear capacity of the column and should be inspected by RT or PAUT

The residual stress from welding can significantly affect the seismic performance of hybrid columns, particularly the initiation and propagation of cracks in the weld HAZ under cyclic loading. Preheat temperatures of 100-150°C for Q345 and 150-250°C for Q460 grades should be maintained during welding to minimize HAZ hardness and prevent cold cracking. Post-weld stress relief by controlled heating to 550-620°C is recommended for critical weld details, particularly those in the anticipated plastic hinge regions.

Study Insights and Design Recommendations

The experimental results provide strong evidence that hybrid column configurations offer a practical pathway to achieve the ductility and energy dissipation requirements of modern seismic design codes. The combined reinforcement approach (H-4) demonstrates that the synergistic interaction between internal bars and external plates produces performance exceeding the sum of individual contributions, likely due to the enhanced concrete confinement and shear capacity provided by the dual reinforcement system.

For practical implementation, the following recommendations emerge from the study:

• The plastic hinge region of hybrid columns should be detailed with increased reinforcement density, including closely spaced transverse welds or weld pads to anchor the internal reinforcement
• The steel tube wall thickness in the plastic hinge region should be increased by 20-30% to delay local buckling and maintain confinement for longer
• The weld details in the plastic hinge region should be designed for fatigue category D or higher, with weld size and configuration optimized for crack resistance
• Quality control should include detailed weld mapping, MT inspection of all welds in the plastic hinge region, and UT of groove welds at 100% coverage

The research contributes valuable experimental data that can be used to calibrate finite element models and develop design guidelines for hybrid columns in seismic regions. Engineers should note that the improved seismic performance comes at the cost of increased fabrication complexity and welding quality demands, and the economic justification should be evaluated on a case-by-case basis considering the seismic zone, structural importance, and life-cycle cost implications.