Seismic Performance of Rectangular CFST Frame Under Low-Cycle Reversed Loading

Literature Overview

This paper, published in the Journal of Xi'an University of Architecture and Technology in 2015 by Li Bin, Yang Xiaoyun, and Gao Chunyan, presents experimental results from a low-cycle reversed loading test on a single-bay, three-story rectangular concrete-filled steel tube (CFST) frame. The frame consists of square CFST columns and rectangular CFST beams. The research investigates failure characteristics, load-bearing capacity, hysteresis curves, skeleton curves, ductility, strength and stiffness degradation, and energy dissipation capacity under seismic loading. The study is particularly relevant to engineers involved in steel pipe fabrication and structural welding, as the seismic performance of CFST frames is fundamentally dependent on the quality of steel tube manufacturing and the integrity of welded joints.

Core Technical Findings

The experimental results demonstrate several critical findings that have direct implications for engineering practice:

• The frame exhibited a "strong column, weak beam" failure mechanism, which is the desired seismic design philosophy for ensuring progressive collapse resistance.
• At failure, plastic hinges formed at all three beam ends and at the first-story column bases, confirming that the plasticity demand was properly distributed.
• Hysteresis loops were full and well-shaped with minimal pinching, indicating excellent energy dissipation and stable cyclic behavior.
• The descending branch of the load-displacement curve was gradual, showing that overall strength and stiffness degradation were not pronounced.
• The displacement ductility coefficient exceeded 3.0 for all specimens, and the inter-story drift angle at failure exceeded 1/50, satisfying the requirements for elastic-plastic deformation under rare earthquakes.
• The equivalent damping coefficient reached 0.441 at failure, which is remarkably high and indicates superior seismic energy dissipation capacity.

Technical Points and Engineering Implications

Material and Fabrication Requirements

The excellent seismic performance of this rectangular CFST frame is not achievable without rigorous attention to steel tube manufacturing and welding quality. From a pipe manufacturing perspective, several factors are critical:

Welding Considerations for CFST Frame Joints

The "strong column, weak beam" mechanism requires that plastic hinges form preferentially at beam ends rather than at column joints. This means the beam-to-column connections must be designed and fabricated with particular care. The welded joints in rectangular CFST frames typically involve:

• End-plate connections: The end plate must be welded to the rectangular tube with full-penetration groove welds. Any lack of fusion, undercut, or porosity in these welds can prematurely weaken the connection and compromise the intended failure mode.
• Column splice welds: First-story column base welds must be designed to develop full plastic capacity. In the test, plastic hinges formed at the first-story column bases, which means these welds must withstand large inelastic deformations without cracking.
• Welding residual stresses: The combination of welding residual stresses and cyclic loading can lead to fatigue cracking in the heat-affected zone (HAZ). Post-weld heat treatment (PWHT) or low-hydrogen welding procedures should be specified for critical joints.

Concrete Fill and Confinement

The confinement provided by the rectangular steel tube is a key contributor to ductility. The rectangular cross-section provides more uniform confinement compared to circular sections, particularly in the corner regions. However, the corners of rectangular tubes are susceptible to local buckling under cyclic loading. The corner radius should be sufficiently large to avoid stress concentrations, and the wall thickness should be adequate to prevent premature local buckling before plastic hinge formation.

Integration with Engineering Practice

In practical CFST frame construction, the following quality control measures should be implemented to ensure the seismic performance demonstrated in this study is achievable:

1. Incoming material inspection: Verify steel tube dimensions, wall thickness uniformity, and chemical composition through ultrasonic thickness gauging and spectrometric analysis.
2. Weld procedure qualification (WPQ): All welding procedures used for beam-to-column joints must be qualified per ISO 15614 or AWS D1.1, with particular attention to the strain capacity of the weld metal and HAZ.
3. Non-destructive testing (NDT): Full-penetration welds at plastic hinge regions should be inspected by phased array ultrasonic testing (PAUT) or time-of-flight diffraction (TOFD) to detect volumetric and planar defects.
4. Concrete placement monitoring: Concrete should be placed in layers with vibration to ensure full compaction within the rectangular tube. Air voids or honeycombing can significantly reduce confinement effectiveness.
5. Dimensional tolerance control: The fit-up tolerance for beam-to-column joints should be maintained within ±2 mm to ensure proper weld preparation and minimize welding distortion.

Key Questions and Reflections

One important observation from this study is the high equivalent damping coefficient of 0.441, which exceeds typical values for conventional steel frames (0.10–0.15). This raises an interesting question: how much of this energy dissipation is attributable to the steel tube yielding, how much to the concrete crushing and friction, and how much to the welding interface behavior? In practice, the welding interface between the steel tube and the end plate can act as a semi-rigid connection, contributing to energy dissipation through micro-slip. However, if the weld is overly rigid, it may concentrate stresses and lead to brittle fracture. This balance is critical in detailed joint design.

Another reflection concerns the applicability of these results to larger-scale structures. The tested frame was a single-bay, three-story model, which simplifies the boundary conditions and load distribution. In real multi-bay, multi-story buildings, the interaction between adjacent bays, the effect of P-Δ (second-order) forces, and the influence of torsional coupling can significantly alter the seismic response. Engineers should not directly extrapolate the ductility and damping values from this model test to full-scale structures without appropriate scale factors and adjustment coefficients.

Study Insights and Reference Value

This study provides valuable experimental data for the seismic design of rectangular CFST frames and confirms that such frames can achieve excellent seismic performance when properly designed and fabricated. The key insight for steel pipe and welding engineers is that the seismic capacity of CFST frames is not solely a function of structural design but is equally dependent on fabrication quality. A single defect in a critical weld or a region of inadequate concrete fill can undermine the entire seismic performance. The study reinforces the importance of comprehensive quality control throughout the fabrication, assembly, and construction phases. For future research, the authors suggest that the seismic performance of CFST frames with different steel grades, concrete strengths, and tube geometries should be further investigated, particularly under combined seismic and fire loading conditions, which is a topic addressed in subsequent literature.