Torsional Performance of CFST Columns Under Eccentric Compression Loading

Research Motivation and Structural Context

This study by Wang Yuhang and colleagues from Chongqing University addresses a structurally significant but relatively underexplored topic: the torsional behavior of concrete-filled steel tube (CFST) columns subjected to combined eccentric compression and torsion. The research is motivated by real structural applications where such combined loading occurs, particularly in:

1. Curved bridge piers: The rigid connection between curved bridge decks and piers subjects the pier columns to combined axial compression, bending, and torsion due to the eccentricity of the deck loading and seismic lateral forces.
2. Arch bridge main arches: CFST arch ribs in arch bridges may experience torsional effects due to eccentric loading, wind loads, or construction-stage asymmetries.
3. Seismic loading scenarios: Lateral seismic forces on curved or asymmetric structures can induce torsional moments in addition to the primary bending and axial loads.

Experimental Program

Specimen Configuration

Eight CFST column specimens with different cross-sectional shapes were tested under three loading protocols:

The specimens included both square and circular cross-sectional shapes, with varying slenderness ratios (short columns and slender columns) to investigate the influence of geometric parameters on torsional behavior.

Test Parameters

Key Experimental Findings

Cyclic Torsional Hysteresis Behavior

The hysteresis loops under cyclic torsion loading are notably full and plump, with no pinching phenomenon observed. This is a significant finding, as pinching in hysteresis loops typically indicates brittle behavior, energy dissipation capacity degradation, or damage accumulation. The absence of pinching indicates that:

• The CFST column maintains its integrity throughout the cyclic loading
• The steel tube confinement effectively prevents concrete spalling and degradation
• The energy dissipation mechanism is stable and repeatable
• The column has good fatigue resistance under torsional loading

Damage Degradation Characteristics

The degradation of strength and stiffness under cyclic torsion is relatively mild, indicating that CFST columns possess good energy dissipation capacity under torsional loading. This is attributed to the composite action between the steel tube and the concrete core, where:

• The steel tube provides continuous hoop confinement that maintains concrete integrity
• The concrete core provides lateral support to the steel tube, delaying local buckling
• The composite action creates a synergistic energy dissipation mechanism that is more robust than either component alone

Influence of Slenderness Ratio

The comparison between short and slender columns reveals that short columns exhibit more pronounced damage but possess higher ultimate bearing capacity and better energy dissipation capacity. This is consistent with the general behavior of short columns in structural engineering, where the lower slenderness ratio provides greater resistance to buckling and more stable load-bearing behavior.

Effect of Eccentric Compression on Torsional Performance

The combined eccentric compression and cyclic torsion loading reveals several important interaction effects:

1. Stiffness degradation: Square-section specimens under combined loading exhibit more pronounced stiffness degradation compared to pure torsion loading. The eccentric compression introduces additional bending stresses that accelerate the degradation of the composite section's torsional stiffness.
2. Energy dissipation reduction: The eccentric compression reduces the energy dissipation capacity of square CFST columns, likely due to the interaction between bending and torsional stresses that creates unfavorable stress concentrations.
3. Buckling angle modification: At the ultimate state, the eccentric compression alters the buckling (wrinkling) angle of the steel tube. Under pure torsion, the steel tube buckles at approximately 45 degrees to the tube axis (consistent with pure shear buckling). Under combined eccentric compression and torsion, the buckling angle deviates from 45 degrees, reflecting the influence of the additional compressive and bending stresses on the buckling mode.

Engineering Practice Implications

Design Considerations for CFST Columns Under Combined Loading

This research provides critical design guidance for CFST columns subjected to combined eccentric compression and torsion:

• Cross-sectional shape selection: Circular cross-sections may be preferred over square sections for applications where torsional loading is significant, as they exhibit more uniform torsional behavior and less stiffness degradation under combined loading.
• Slenderness ratio control: Shorter columns (lower slenderness ratios) provide better energy dissipation capacity and higher ultimate bearing capacity under torsional loading. Designers should minimize slenderness ratios where torsional effects are expected.
• Steel tube buckling prediction: The modification of buckling angles under combined loading means that standard buckling predictions based on pure torsion or pure compression may be unconservative. Combined loading interaction effects must be explicitly considered in the design.
• Seismic design implications: The good hysteresis behavior and energy dissipation capacity of CFST columns under torsional loading support their use in seismic design of curved bridge piers and asymmetric structures.

Steel Pipe Quality Requirements

For steel tubes used in CFST columns subjected to torsional loading, the following quality requirements are particularly important:

1. Geometric regularity: The tube must have uniform wall thickness and minimal ovality, as geometric imperfections significantly reduce torsional capacity and accelerate buckling.
2. Surface quality: Internal and external surface defects—such as dents, scratches, and weld seams—act as stress concentrators that initiate torsional buckling. Surface quality standards should be more stringent for torsionally loaded applications.
3. Material uniformity: The steel must have consistent mechanical properties throughout the tube length, as localized soft spots can initiate premature buckling under torsional loading.
4. Weld inspection: For welded tubes, the longitudinal weld seam is a potential initiation point for torsional buckling. Ultrasonic testing (UT) of weld seams is essential, with acceptance criteria aligned to the relevant standard (e.g., GB/T 19446 for welded tube inspection).

Study Insights and Reflections

This research fills an important gap in the understanding of CFST column behavior under combined eccentric compression and torsion, a loading condition that is common in practical structures but has received limited experimental investigation. The finding that CFST columns exhibit excellent hysteresis behavior under cyclic torsion—without pinching—is particularly encouraging for seismic design applications. The observation that eccentric compression reduces energy dissipation capacity and modifies buckling angles underscores the importance of considering combined loading effects in design, rather than treating torsion and compression as independent load cases. For steel pipe engineers and quality control professionals, this research reinforces the principle that geometric regularity and material uniformity are not merely manufacturing preferences but critical structural requirements, particularly for applications involving torsional loading where imperfection sensitivity is high. The experimental methodology—combining monotonic, cyclic, and combined loading protocols—is comprehensive and provides a robust basis for design recommendations.