Cumulative Energy Dissipation of Double CFST Column-Steel Beam Joints

Literature Overview

Zhang Dongfang, Zhao Junhai, Zhang Changguang, and Zhang Yufen published this study in World Information on Earthquake Engineering in 2017 (Volume 33, Issue 1, pages 59–68). The research investigates the cumulative energy dissipation behavior of double concrete-filled steel tube (CFST) column-steel beam joints under low-cycle reversed loading. Seven specimens were tested, including six double CFST joints and one conventional square CFST joint for comparison.

Specimen Configuration

The double CFST column consists of an outer square steel tube enclosing an inner circular steel tube, both filled with concrete. This hybrid configuration combines the high confinement capacity of the circular tube with the geometric regularity of the square tube, offering enhanced seismic performance.

Cumulative Energy Dissipation Analysis

Hysteresis Loop Characteristics

The double CFST joints exhibited full and stable hysteresis loops, indicating good energy dissipation capacity. The loops maintained their shape across multiple loading cycles, suggesting that the joint mechanism did not experience progressive degradation. This is a critical requirement for seismic-resistant structures, where joints must survive multiple earthquake cycles without catastrophic failure.

Effect of Anchor Web Stiffener and Vertical Rib Extension

The specimen with an anchor web stiffener and a 120 mm vertical rib extension demonstrated the best cumulative energy dissipation performance. The vertical ribs provide additional shear resistance at the beam-column interface, delaying the onset of web buckling and maintaining the joint's load-carrying capacity under large deformations.

Cumulative Damage Assessment

A key finding is that cumulative damage has a limited effect on the energy dissipation capacity of the double CFST joints. For most specimens, the hysteresis energy dissipation decreased by less than 10% even when the beam-end drift ratio reached 1/23. This indicates that the joint mechanism is robust and not highly sensitive to progressive damage accumulation.

Comparison with Single CFST Joints

All energy dissipation indicators of the double CFST joints were superior to those of the conventional single square CFST joint. The improvement is attributed to:

• Enhanced confinement — The dual tube configuration provides greater lateral confinement to the concrete core, delaying concrete crushing and maintaining joint strength at large deformations.
• Redundant load paths — The outer and inner tubes create multiple load transfer paths, providing redundancy that prevents sudden collapse when one tube yields.
• Improved shear resistance — The additional steel tube contributes directly to shear resistance at the joint panel zone.

Parametric Analysis Results

Welding Quality and Joint Fabrication

Critical Weld Locations

The joint performance is heavily dependent on the quality of specific welds:

1. Beam-to-column flange welds — These welds transfer the majority of the flexural moment. Full-penetration groove welds with E7018 electrodes are required. The weld toe geometry must be ground smooth to reduce stress concentration.
2. Web stiffener welds — The anchor web stiffeners and vertical ribs are welded to the inner tube wall. These welds must be inspected by magnetic particle testing (MT) to detect surface cracks.
3. Tube-to-tube interface welds — In the double CFST column, the connection between the outer and inner tubes (if welded) must be designed to accommodate differential thermal expansion during welding.

Welding Sequence and Distortion Control

The fabrication sequence for double CFST joints is critical:

1. Inner tube preparation — The inner circular tube is fabricated and welded first, with all internal welds completed before the outer tube is assembled.
2. Outer tube assembly — The outer square tube is assembled around the inner tube, with careful attention to concentricity tolerances (typically ±3 mm).
3. Stiffener welding — The web stiffeners and vertical ribs are welded to the inner tube after the outer tube is in place.
4. Beam connection welding — The beam is connected to the column assembly last, allowing for precise alignment.

Residual Stress Management

The double tube configuration introduces complex residual stress patterns:

• The inner tube welding generates compressive residual stresses in the outer tube wall due to thermal expansion of the inner tube.
• The outer tube welding generates tensile residual stresses in the inner tube wall.
• These opposing residual stresses partially cancel each other, which is beneficial for fatigue performance but complicates the prediction of buckling behavior.

Post-weld stress relief by furnace heating at 600–650°C for 2 hours per 25 mm of wall thickness is recommended for critical joints.

FMEA Analysis of Joint Failure Modes

Design Recommendations

Based on the experimental and parametric analysis:

• The double CFST joint configuration is recommended for seismic zones where high energy dissipation capacity is required.
• The vertical rib extension length should be set at 120 mm for optimal energy dissipation performance.
• The beam-column moment ratio should be designed to be at least 1.2 to ensure that plastic hinges form in the beams rather than at the joints.
• Cumulative damage is not a critical concern for these joints under typical seismic loading, as the energy dissipation capacity remains robust even after significant drift accumulation.
• The joint design should follow the capacity design principle, ensuring that the joint strength exceeds the sum of the beam and column plastic moment capacities by a factor of at least 1.2.

Study Insights and Conclusions

This research provides valuable experimental evidence for the seismic performance of double CFST column-steel beam joints. The cumulative energy dissipation analysis reveals that these joints are not only strong but also robust against progressive damage, making them suitable for critical structural applications in high-seismicity regions. From a welding engineering perspective, the joint fabrication requires careful attention to weld quality, residual stress management, and geometric tolerances. The combination of finite element analysis, experimental validation, and parametric study presented in this paper offers a comprehensive framework for the design and quality control of double CFST joints in practice.