Mechanical Behaviour of Round-End Steel Tube Concrete Members Under Pure Bending

Literature Overview

The paper by Ren Zhigang and Xiao Meng, published in the Journal of Architecture and Civil Engineering in 2020 (Vol. 37, No. 2, pp. 44–53), investigates the pure bending mechanical performance of round-end steel tube concrete (CFST) members. Funded by the National Natural Science Foundation of China (Project 51178179), this study from the School of Architecture and Civil Engineering at Wuhan University of Technology presents experimental and numerical results on four specimens subjected to pure bending loading. The research is significant for structural engineers working with steel tube concrete systems, as it addresses the behaviour of a relatively novel cross-sectional geometry where the steel tube has rounded ends rather than flat walls.

Core Technical Findings

Experimental Configuration and Test Results

The study employed four CFST specimens with varying aspect ratios and steel ratios under pure bending conditions. Key observations include:

• Significant deflection deformation occurred under pure bending, with specimens still sustaining substantial loads when midspan deflection reached L/25 (where L is the specimen length), demonstrating excellent ductility.
• The deflection curve can be reasonably assumed to follow a sinusoidal half-wave profile.
• The midspan cross-section essentially maintained the plane-section assumption throughout loading.
• At ultimate bending capacity, the steel tube carried the majority of the bending moment.
• The arcuate (rounded) segment of the steel tube provided stronger confinement to the concrete compared to the flat segment.
• Bending capacity increased with steel strength, while changes in concrete strength had minimal influence on bending performance.

Finite Element Modelling Approach

An ABAQUS finite element model was developed and validated against experimental results, showing good agreement. The verified model was then used for parametric studies covering the full loading process. Based on these parametric analyses, a bending capacity formula was proposed that showed good correlation with test data.

Implications for Steel Pipe Manufacturing and Welding Practice

Steel Tube Material Requirements

From a steel pipe manufacturing perspective, this study underscores the critical role of steel grade selection in CFST applications. Since bending capacity is predominantly governed by steel strength, the mechanical properties of the steel tube—particularly yield strength and tensile strength—are the primary design drivers. This has direct implications for:

• Material grade selection: Higher strength grades such as Q355, Q390, or Q420 (per GB/T 1591) should be preferred for CFST applications where bending capacity is the governing design criterion.
• Uniformity of properties: Since the steel tube carries most of the bending moment, property gradients or non-uniformities along the tube length can create weak links. Manufacturers must ensure consistent mechanical properties throughout the pipe length.
• Surface quality: The rounded-end geometry requires high-precision forming. Surface defects, including inclusions, laps, or surface cracks introduced during manufacturing, can initiate failure under bending stresses.

Welding Considerations for Round-End CFST Members

The rounded-end geometry introduces unique welding challenges:

• Joint design at transition zones: The transition from flat to curved wall sections creates regions of geometric discontinuity that concentrate stresses. Welding joints in these areas require careful design to avoid stress concentrations.
• Heat-affected zone (HAZ) sensitivity: In high-strength steel tubes (Q390 and above), the HAZ is susceptible to softening or over-tempering, which can reduce the effective bending capacity. Preheat control and interpass temperature management are essential.
• Residual stress distribution: The combination of bending loads and welding residual stresses can accelerate local yielding. Post-weld stress relief or controlled cooling may be necessary for critical applications.

Confinement Mechanism and Manufacturing Quality

The finding that the arcuate segment provides stronger concrete confinement than the flat segment is particularly relevant to pipe manufacturing. This implies that:

The rounded-end geometry demands tighter dimensional tolerances on wall thickness, as variations directly affect the confinement pressure and hence the composite action. During manufacturing, this translates to stricter control of:

• Hot-rolled coil thickness tolerance for seamless tubes
• Welding distortion control for welded tubes (ERW, HFW, or LSAW)
• Ovality and roundness specifications

Defect Analysis and Quality Control Recommendations

Based on the research findings, the following defects are most critical for round-end CFST members:

1. Wall thickness non-uniformity: Causes uneven confinement pressure and premature local buckling under bending.
2. Surface imperfections at curved sections: Act as stress concentrators under combined bending and confinement stresses.
3. Weld defects at geometric transitions: Lack of fusion or porosity at the flat-to-curved transition can reduce effective load transfer.
4. Residual stress accumulation: From both manufacturing (forming) and welding, can reduce the effective ductility observed in testing.

Recommended non-destructive testing (NDT) protocols should include ultrasonic testing (UT) for wall thickness verification, magnetic particle testing (MT) for surface defects at curved sections, and radiographic testing (RT) for volumetric weld defects at transition zones.

Study Insights and Independent Reflection

This research provides valuable insight into the structural behaviour of an unconventional CFST geometry. From a manufacturing standpoint, the key takeaway is that the rounded-end design shifts the primary structural demand onto the steel tube itself, making steel quality and manufacturing precision paramount. The conclusion that concrete strength has limited influence on bending capacity suggests that material cost optimisation should prioritise steel grade selection over concrete grade escalation. For welding engineers, the geometric complexity of rounded-end tubes demands enhanced weld procedure qualification, particularly for transition zone joints where stress gradients are steepest. The ductility demonstrated at L/25 deflection provides confidence for seismic design applications, but only if manufacturing quality ensures that theoretical ductility is realised in practice.