Comparative Study on Seismic Performance of Rigid Connections in Composite Steel Tube Concrete Structures

Research Background and Seismic Design Challenges

This study investigates the seismic performance of rigid connection nodes in composite steel tube concrete (CSTC) structures, comparing different connection configurations under cyclic loading conditions. Rigid connections are fundamental to moment-resisting frame systems, and their behavior under seismic loading directly determines the overall structural performance during earthquakes. The composite nature of CSTC columns—combining steel tubes with concrete infill—creates unique challenges for connection design that differ significantly from conventional steel or reinforced concrete connections.

Connection Configurations and Test Setup

The study evaluates multiple connection types, each representing practical alternatives for different structural applications:

The test specimens simulate realistic connection conditions with appropriate boundary conditions and loading protocols that replicate seismic demand. The loading protocol follows the ACI 318 or GB 50011 displacement-controlled cyclic loading procedure, with drift ratios up to 4% to simulate severe seismic events.

Seismic Performance Comparison

The comparative analysis reveals significant differences in seismic performance among the connection types:

Type A connections (full-penetration welds) exhibit the highest stiffness and strength but suffer from brittle failure characteristics and limited ductility. The weld zones experience concentrated stress that leads to premature cracking and rapid strength degradation under cyclic loading. Type D connections (through-bolt with shear studs) demonstrate excellent ductility and energy dissipation but sacrifice initial stiffness, which may be unacceptable in certain structural configurations.

Damage Mechanism Analysis

The study identifies distinct damage mechanisms for each connection type through detailed observation and strain measurement:

• Type A: Weld root cracking initiates at approximately 1.5% drift, propagating through the heat-affected zone. The heat-affected zone exhibits reduced ductility due to microstructural coarsening, creating a weak link in the connection.
• Type B: Fillet weld cracks develop at the plate-to-tube interface, while the embedded plate undergoes yielding before connection failure. The concrete in the tube near the connection experiences splitting due to concentrated bearing stresses.
• Type C: Bolt hole elongation and bearing plate yielding govern failure. The stiffeners play a critical role in preventing tube wall local buckling, and their absence leads to premature connection collapse.
• Type D: Shear stud fracture occurs at high drift levels, providing a well-defined failure mode with significant plastic deformation preceding ultimate failure.
• Type E: The hybrid configuration distributes damage between welded and bolted components, achieving a balance between strength and ductility through controlled yielding sequence.

Welding Quality and Connection Performance

From a welding engineering perspective, the study highlights critical quality requirements for welded connections in CSTC structures:

1. Heat input control: The welding heat input for connections in seismic zones should be limited to 15-25 kJ/cm to minimize heat-affected zone softening while ensuring adequate penetration
2. Preheating requirements: For steel tubes with wall thickness exceeding 20 mm, preheating to 100-150°C is essential to prevent cold cracking in the weld and heat-affected zone
3. Post-weld treatment: Stress relief treatment is recommended for Type A connections to reduce residual stresses that contribute to fatigue crack initiation under cyclic loading
4. Weld inspection: Full volumetric ultrasonic testing (UT) per NB/T 47013 is mandatory for seismic connections, with acceptance criteria at Level B or higher

The interaction between weld residual stresses and the cyclic loading demands creates complex stress states that can accelerate fatigue damage. Engineers should consider residual stress measurement and mitigation strategies during connection fabrication to ensure adequate seismic performance.

Design Recommendations

Based on the comparative study results, the following design guidelines are proposed:

• For seismic intensity zones 8 and 9, prefer Type E (hybrid) or Type D (through-bolt with shear studs) connections for their superior ductility and energy dissipation
• Type A connections should only be used in non-seismic or low-seismic zones where stiffness is prioritized over ductility
• Stiffener design for Type C connections should follow the requirement of minimum 1.5 times tube wall thickness for stiffener thickness, with spacing not exceeding 3 times the tube wall thickness
• The connection zone should be designed to ensure that yielding initiates in the beam rather than the connection, following the strong-column-weak-beam philosophy adapted for composite structures

Study Insights and Practice Implications

This comparative study provides essential guidance for selecting appropriate connection types in CSTC structures subjected to seismic loading. The findings emphasize that seismic design of composite connections requires careful consideration of both strength and ductility, with ductility often being the governing design parameter in seismic regions. For engineers involved in steel pipe fabrication and structural welding, the study reinforces the importance of welding quality in achieving predicted seismic performance. The damage mechanism analysis provides valuable insights for developing inspection criteria and maintenance strategies for existing CSTC structures. The research contributes to the maturation of CSTC technology as a viable alternative to conventional structural systems in seismic-prone regions.