Seismic Performance and Plastic Hinge Calculation Method of Recycled Concrete-Filled Steel Tube Composite Columns with Built-in Steel Sections

Overview of the Study

This research examines the seismic performance of composite columns that combine recycled concrete-filled steel tubes with built-in steel sections (such as H-beams or I-beams). The plastic hinge calculation method proposed in the study provides engineers with a practical tool for designing these columns to withstand seismic loading while utilizing recycled construction materials. The study addresses both the structural performance and the sustainability aspects of using recycled concrete in critical structural elements.

Steel Tube Material and Welding Considerations

The steel tube in this composite column system serves as both a formwork during construction and a structural component in service. The material selection typically involves carbon structural steel tubes conforming to GB/T 8163 or ASTM A53 Grade B, with wall thicknesses ranging from 6 to 20 mm depending on the column diameter and design loads. When built-in steel sections are embedded within the tube, the welding connections between the steel section and the tube wall become critical structural elements.

The welding of the built-in steel section to the steel tube wall requires careful thermal management. The presence of recycled concrete, which may have different thermal conductivity than virgin concrete, affects the cooling rate of the weld and consequently the HAZ microstructure. Preheating temperatures of 100-150°C are recommended for Q345-grade connections to prevent cold cracking, particularly when the recycled aggregate content exceeds 30%.

Plastic Hinge Behavior and Seismic Performance

The plastic hinge mechanism in these composite columns develops at predetermined locations where the section is intentionally weakened or where the steel tube yields in tension while the concrete core provides compressive resistance. The built-in steel section contributes significantly to the flexural capacity and ductility of the plastic hinge zone. The interaction between the steel tube, built-in section, and recycled concrete creates a complex stress distribution that must be accurately modeled for reliable seismic design.

Key Performance Indicators

• Ductility ratio (μ): typically 3.5-6.0 for properly designed composite columns
• Energy dissipation capacity: measured through cyclic loading tests
• Drift ratio at collapse: 3%-5% for acceptable seismic performance
• Strength degradation rate: less than 10% over 10 cycles at 2% drift
• Recycled aggregate content effect: 30% replacement shows minimal performance reduction; 50% requires design adjustments

The plastic hinge calculation method accounts for the composite action, the contribution of the built-in steel section, and the confinement effect of the steel tube on the recycled concrete core. The method provides a systematic approach to determining the moment-curvature relationship and the plastic hinge length, which are essential inputs for nonlinear pushover analysis and time-history analysis.

Welding Quality and Seismic Reliability

The reliability of the plastic hinge mechanism depends critically on the weld quality at the connection between the built-in steel section and the tube wall. Incomplete weld fusion, porosity, or lack of penetration can create weak points that initiate failure prematurely, compromising the intended ductile behavior. The welding procedure specification (WPS) must be qualified per NB/T 47014 for the specific material combination and thickness range, with full radiographic testing (RT) or ultrasonic testing (UT) required for all fusion welds in the plastic hinge zone.

Engineering Practice Recommendations

For engineers designing seismic-resistant composite columns with recycled concrete-filled steel tubes, the following recommendations emerge from this study:

1. Use Q345B or higher grade steel for both the tube and built-in section to ensure adequate ductility in the plastic hinge zone
2. Qualify welding procedures specifically for the steel tube to steel section connection, accounting for the thermal effects of the surrounding concrete
3. Implement 100% NDT coverage (RT or PAUT) at all welds within the plastic hinge region
4. Control recycled aggregate content to not exceed 40% for columns in high seismic zones, as higher replacement ratios may reduce the confinement effectiveness
5. Design the plastic hinge location to coincide with the weld-free zone, avoiding the weld HAZ as a potential crack initiation site

Study Insights and Conclusions

This research makes a meaningful contribution to sustainable structural engineering by demonstrating that recycled concrete can be effectively used in seismic-resistant composite columns without significant performance degradation. The plastic hinge calculation method provides a practical design tool that accounts for the complex composite behavior. From a welding and steel pipe engineering perspective, the critical insight is that weld quality directly governs the seismic reliability of these composite systems, and the recycling of concrete introduces additional thermal and mechanical variability that must be managed through rigorous welding procedure qualification and inspection protocols.