Experimental Research on Seismic Performance of Steel Tube Confined Concrete Frame Structures

Literature Overview

This paper by Li Zhongxian and colleagues from Tianjin University, published in Building Structures in 2004, presents the results of a full-scale shaking table test on a two-bay, three-story CFST frame model. Funded by the Tianjin Municipal Natural Science Foundation Key Project, the research investigated the seismic behavior of a CFST frame designed according to prevailing Chinese codes, evaluating its deformation capacity, load-bearing capacity, ductility, and energy dissipation characteristics under combined vertical and cyclic horizontal loading.

Core Technical Content and Experimental Setup

The test specimen was a one-bay, two-span, three-story frame with round steel tube concrete columns and steel beams, designed in accordance with GB 50011 (Seismic Design Code for Building Structures) and CECS 28 (Technical Specification for Concrete Filled Steel Tubular Structures). The model was subjected to:

1. Constant vertical axial loads representing gravity loads
2. Low-cycle reversed horizontal loading simulating seismic lateral forces

Seismic Performance Evaluation

The experimental results demonstrated several important findings:

• Beam-hinge mechanism formation: The frame successfully formed a beam-hinge failure mechanism under seismic loading, which is the desired failure mode for ductile frames as it ensures energy dissipation in the beams while preserving column integrity for vertical load transfer.
• Ductility performance: The effective ductility coefficient reached 7.54, significantly exceeding the minimum requirement of 4.0 for ductile frames. This indicates excellent deformation capacity beyond yielding.
• Energy dissipation: The hysteresis loops exhibited full, well-shaped characteristics with stable energy dissipation over multiple loading cycles, indicating good seismic energy absorption capacity.
• Load-bearing capacity: The frame maintained its load-carrying capacity throughout the test, with only gradual degradation at large drift ratios.

Comparison with Other Structural Systems

The study concluded that CFST frame structures exhibit superior seismic performance compared to both reinforced concrete (RC) frames and steel frames:

The superior performance of CFST frames is attributed to the complementary behavior of steel tubes and concrete cores: the steel tube provides tensile ductility and confinement, while the concrete core provides compressive strength and mass for inertial load generation. The confined concrete exhibits enhanced compressive strength and strain capacity due to the triaxial stress state imposed by the steel tube.

Welding and Fabrication Considerations for Seismic Performance

From a welding and fabrication quality perspective, the seismic performance of CFST frames depends critically on:

1. Column-beam connection welds: The moment-resisting connections between steel beams and CFST columns are subject to cyclic shear and bending. Weld quality at these connections is paramount—full-penetration groove welds with appropriate heat input control are essential to prevent brittle fracture in the heat-affected zone.
2. Steel tube longitudinal welds: In welded tubes, the longitudinal weld represents a potential initiation site for fatigue cracking under cyclic loading. The weld must be inspected thoroughly using UT or MT methods, and any defects must be repaired to full standard before installation.
3. Joint reinforcement details: Steel stiffeners or diaphragms at beam-column joints must be welded with high-quality connections to ensure proper force transfer and prevent local buckling of the tube wall under cyclic shear.
4. Residual stress management: The welding residual stresses in the steel tube, particularly at longitudinal welds and connection details, interact with seismic stresses. Pre-weld stress relief or controlled welding procedures may be necessary for critical connections.

Standards Compliance and Design Implications

The experimental results validate the seismic design provisions in Chinese codes for CFST structures. The design approach based on capacity design principles—ensuring that plastic hinges form in beams rather than columns—proved effective in practice. The beam-hinge mechanism was achieved through proper detailing of beam-column connections and appropriate relative stiffness between beams and columns.

For engineers designing CFST frames in seismic regions, the study provides confidence that code-compliant designs can achieve excellent seismic performance. However, the study also highlights the importance of:

• Proper detailing of beam-column connections to ensure moment capacity
• Adequate column confinement to prevent buckling
• Quality control of all welded connections, particularly at seismic demand regions

Key Questions and Reflections

While the results are encouraging, several aspects warrant further consideration:

• The test was conducted at model scale with low-cycle reversed loading, which does not fully replicate the dynamic characteristics of real seismic events including high-rate loading and multi-directional components.
• The long-term durability of welded connections under repeated seismic loading and environmental exposure is not addressed.
• The behavior of CFST frames with different tube cross-sectional shapes (square, rectangular) and connection types (moment-resisting vs. rigid bolted) may differ from the round tube configuration tested.

Study Insights and Conclusions

This research provides valuable experimental evidence supporting the use of CFST frames in seismic regions. The demonstrated ductility coefficient of 7.54, well above the code requirement, indicates substantial safety margin in the design approach. The successful formation of beam-hinge mechanism confirms that the capacity design philosophy works effectively for CFST frames. For welding engineers and fabrication specialists, the study reinforces the critical importance of maintaining high weld quality standards at all structural connections, as even minor weld defects can compromise the seismic performance that the structural design intends to achieve.