Pseudo-Static Testing of Rectangular Steel Tube-Concrete Frame Seismic Performance

Literature Overview and Research Background

The paper by Li Bin, Yang Xiaoyun, and Gao Chunyan, published in the Journal of Xi'an University of Architecture and Technology (Natural Science Edition) in 2015, presents a comprehensive pseudo-static experimental investigation on a single-span, three-story rectangular steel tube-concrete (CFST) frame. The test specimen comprised square steel tube-concrete columns and rectangular steel tube-concrete beams, subjected to low-cycle reversed lateral loading to simulate seismic action. The study focuses on the failure characteristics, load-carrying capacity, hysteretic behavior, skeleton curves, ductility, strength and stiffness degradation, and energy dissipation of the frame system.

This research is particularly significant in the context of steel tube-concrete composite structures, which have gained increasing attention in seismic-prone regions due to their superior strength-to-weight ratio, ductility, and fire resistance compared to conventional reinforced concrete or steel frames. From a welding and pipe fitting engineering perspective, the connection design and fabrication quality of steel tube components play a critical role in determining the seismic performance of such frames.

Core Experimental Findings and Interpretation

The experimental results demonstrate several important findings that are directly relevant to the design and fabrication of steel tube-concrete structures:

1. The frame exhibited a "strong column-weak beam" failure mechanism, which is the ideal failure mode for seismic design as it ensures that plastic hinges form at beam ends rather than at column locations. During failure, plastic hinges appeared at all third-floor beam ends and at the first-floor column bases.
2. The hysteretic loops were full and well-defined with no significant pinching phenomenon, indicating excellent energy dissipation capacity and stable cyclic behavior.
3. The descending branch of the load-displacement curve was relatively gentle, suggesting that the overall strength and stiffness degradation of the specimen was not pronounced even after significant inelastic deformation.
4. The displacement ductility coefficients of all specimens exceeded 3.0, and the inter-story drift angles at failure all exceeded 1/50, satisfying the requirements for inter-story elastic-plastic deformation under rare earthquake conditions.
5. The equivalent damping coefficient he reached 0.441 at failure, demonstrating substantial energy dissipation capability.

Welding and Connection Implications for Engineering Practice

From the perspective of steel tube fabrication and welding engineering, several critical aspects deserve careful attention:

Welding Connection Design Considerations

The seismic performance of the CFST frame is heavily dependent on the quality of welded connections between steel tube members. The beam-column joints, which are the critical regions for plastic hinge formation, require special attention to welding quality. The following factors are of paramount importance:

• Welding process selection: For seismic connections, GTAW (Gas Tungsten Arc Welding) or FCAW (Flux-Cored Arc Welding) processes are typically preferred due to their ability to produce high-quality welds with minimal distortion. SAW (Submerged Arc Welding) may be used for thicker steel tube walls but requires careful preheating and post-weld heat treatment.
• Heat-Affected Zone (HAZ) properties: The HAZ of welded joints in seismic connections must maintain adequate toughness and ductility. For low-temperature applications or in seismic zones, the HAZ hardness should be controlled to prevent brittle fracture. Preheating temperatures of 100-150°C are typically recommended for carbon steel tubes with wall thicknesses exceeding 25 mm.
• Residual stress management: Welding residual stresses can adversely affect the fatigue and seismic performance of connections. Stress-relief annealing or controlled welding sequences should be employed to minimize residual stresses in critical connection regions.

Steel Tube Specifications and Quality Control

The steel tubes used in CFST frames must meet specific mechanical property requirements to ensure the overall seismic performance of the structure:

Common Defects and Countermeasures

In the fabrication of steel tube-concrete frame components, the following welding defects are commonly encountered and must be controlled:

• Undercut: Can act as stress concentrators in cyclic loading conditions. Countermeasure: Optimize welding parameters, reduce welding current, and use appropriate electrode angle.
• Porosity: Reduces effective weld cross-section and can initiate fatigue cracks. Countermeasure: Ensure proper gas shielding, clean base metal surfaces, and use appropriate welding wire.
• Lack of fusion: Particularly critical in full-penetration butt welds. Countermeasure: Adequate preheating, proper root preparation, and skilled welder technique.
• Cracking: Can be catastrophic in seismic connections. Countermeasure: Control carbon equivalent (CE ≤ 0.45), preheat, and apply post-weld heat treatment.

Key Questions and Reflections

The study raises several important questions for engineering practice:

1. How does the welding quality of beam-column connections specifically influence the formation and propagation of plastic hinges observed in the test? The paper does not provide detailed information on the welding procedures used for the connections, which is a notable gap.
2. What is the long-term durability of the steel tube-concrete interface under cyclic loading, particularly considering potential corrosion at welded joints?
3. How would the seismic performance change if higher-strength steel tubes (e.g., Q420 or Q460) were used, and what additional welding considerations would be required?

Study Insights and Implications

The research confirms that rectangular steel tube-concrete frames can achieve excellent seismic performance when properly designed and fabricated. The "strong column-weak beam" failure mechanism observed in the test is consistent with modern seismic design philosophy and validates the structural system for practical engineering application.

From a welding and fabrication standpoint, the key implication is that maintaining the ductility and toughness of welded connections is essential for achieving the full seismic potential of CFST frames. Welding procedure qualifications (WPQR) should specifically address cyclic loading conditions, and weld inspection should include both volumetric (RT or UT) and surface (MT or PT) methods to ensure complete weld quality.

The displacement ductility coefficient exceeding 3.0 and the equivalent damping coefficient of 0.441 indicate that the energy dissipation mechanism is primarily through plastic deformation of the steel tubes and their welded connections. This underscores the importance of ensuring that the steel tube material and weld metal have adequate elongation and impact toughness properties, particularly at the expected minimum service temperature.

For future engineering projects, it is recommended to conduct supplementary welding qualification tests that simulate the cyclic loading conditions experienced during earthquakes. Additionally, the use of consumable materials with controlled carbon equivalent and adequate impact toughness should be mandated in the welding specifications for seismic CFST frame connections. The integration of finite element analysis with experimental validation, as demonstrated in this study, provides a reliable basis for design optimization and code development.