Overall Stability of Square Steel Tube Welded T-Steel Composite Section Columns Under Axial Compression

Literature Overview

Published in the Journal of Shenyang University (Natural Science Edition), Vol. 37, Issue 2, 2025 (pages 162-169), this paper by Zhou Le, Yang Xiangyu, Zhao Tongfeng, Li Yixuan, and Zhang Lihui investigates the overall buckling stability of composite columns formed by welding T-shaped steel sections to square steel tubes. The research is supported by the National Natural Science Foundation of China (52478191) and the Shenyang Key Laboratory of Engineering Structure Safety Assessment and Prevention and Control (S230184). The study employs ABAQUS finite element analysis to parametrically evaluate the influence of steel grade, initial geometric imperfections, residual stresses, and regularized slenderness ratio on column stability.

Core Technical Framework

The composite section combines the high confinement efficiency of square steel tubes with the material efficiency of T-shaped steel flanges. This hybrid approach aims to achieve higher load-bearing capacity per unit weight compared to conventional steel tube concrete (CFST) or built-up section columns. However, the welding interface between the T-steel and the square tube introduces complex stress distributions, residual stresses, and potential buckling modes that must be thoroughly characterized.

Parametric Study Variables

Key Findings and Technical Interpretation

Residual stress dominance: The study establishes that residual stress exerts the greatest influence on overall stability performance among all studied parameters. This finding is consistent with established knowledge in welded structural steel — welding-induced residual stresses in the heat-affected zone (HAZ) and weld metal create compressive pre-stresses that reduce the effective buckling load. In the context of composite sections, the welding of T-steel to the square tube creates a complex residual stress field at the interface, including:

• Transverse residual stresses perpendicular to the weld line
• Longitudinal residual stresses along the weld length
• Through-thickness residual stresses that can initiate through-thickness cracking in thick sections

Steel grade effect: Higher steel grades improve column stability, but the improvement diminishes at higher slenderness ratios. This is because at high slenderness, buckling becomes elastic and is governed by the modulus of elasticity (E), which is essentially constant across steel grades (~206 GPa for all structural steels). The benefit of higher yield strength is most pronounced in the inelastic buckling regime (λ̄ < 1.0).

Slenderness ratio interaction: As the regularized slenderness ratio increases, the sensitivity to both initial geometric imperfections and residual stresses decreases. This occurs because at high slenderness, the column behaves more like an ideal elastic strut where the initial conditions have less influence on the ultimate capacity.

Welding Process Analysis and Residual Stress Patterns

From a welding engineering perspective, the residual stress patterns in this composite column are governed by the following factors:

Welding Sequence and Residual Stress Generation

The welding of T-steel sections to square steel tubes involves multi-pass welding with significant heat input. The residual stress development follows the well-established sequence:

1. Thermal expansion phase: The weld zone and adjacent HAZ expand, constrained by surrounding cold material, generating compressive stresses
2. Cooling contraction phase: As the weld cools, it contracts but is restrained by the already-solidified base metal, generating tensile residual stresses in the weld metal and HAZ
3. Plastic deformation accommodation: In thick sections, through-thickness plastic deformation accommodates the differential shrinkage, creating a self-equilibrated residual stress field

Welding Process Parameters Affecting Residual Stress

Connection with Standards

The residual stress levels and their effects on stability must be evaluated against relevant standards:

• GB 50017-2017 (Code for design of steel structures): Specifies design buckling curves and reduction factors for columns with residual stresses
• EN 1993-1-1 (Eurocode 3): Provides buckling curves (a, b, c, d) that implicitly account for typical residual stress patterns
• ASTM A6/A6M: Governs material properties and residual stress expectations for structural steel
• AWS D1.1: Specifies welding procedures and quality requirements for structural steel

Finite Element Modeling Approach

The ABAQUS-based numerical model should incorporate the following elements for realistic simulation:

• Material model: Elastic-plastic constitutive law with appropriate strain hardening for each steel grade
• Geometric imperfections: Half-sine wave initial bow with amplitude L/1000 for global, and appropriate local panel buckling modes
• Residual stress: Superimposed on the initial state, with realistic distributions from welding simulation or experimental measurement
• Contact interface: Between T-steel and square tube, accounting for possible slip at high loads
• Boundary conditions: Pinned-pinned for standard stability analysis, with consideration of actual restraint conditions

Engineering Practice Integration

The findings have direct implications for the design and fabrication of composite steel columns in practical applications:

Design recommendations:

• For columns with λ̄ < 1.0 (stocky columns), residual stress mitigation through welding sequence optimization and post-weld stress relief is most beneficial
• For slender columns (λ̄ > 1.5), initial geometric imperfection control during fabrication and erection becomes the dominant concern
• Higher steel grades (Q460, Q690) provide stability benefits primarily in stocky column applications

Fabrication quality control:

• Ultrasonic testing (UT) of welds to detect lack of fusion, porosity, and cracks that would create stress concentrations
• Measurement of residual stresses using X-ray diffraction or neutron diffraction on critical welds
• Straightness verification of the assembled column to ensure geometric imperfections are within tolerance

Key Questions and Reflections

The study raises several important questions for future research:

1. Welding-induced distortion: The paper focuses on stability but does not address the welding distortion that occurs during fabrication of the composite section. In practice, significant angular and longitudinal distortion can occur, affecting the actual geometric imperfection beyond design assumptions.
2. Cyclic loading behavior: The stability analysis is for monotonic axial compression. In seismic applications, the composite column must withstand repeated cyclic loading, and the interaction between residual stresses, geometric imperfections, and cyclic degradation requires investigation.
3. Fire resistance: The composite section's fire performance depends on the welding quality at the T-steel to tube interface. Under fire conditions, differential thermal expansion between the T-steel and square tube could create additional stresses at the weld.
4. Scale effect: The numerical study likely uses representative dimensions. Full-scale testing would validate the stability predictions and capture size-dependent effects of residual stress distribution.

Study Insights and Outlook

This research provides valuable quantitative data on the relative importance of various factors affecting composite column stability. The clear finding that residual stress is the dominant factor should influence both design codes and fabrication practices. For welding engineers, this reinforces the importance of welding procedure qualification, stress relief procedures, and weld quality assurance in composite structural applications.

The practical implication is straightforward: investing in welding quality control and residual stress management yields the highest return in terms of stability performance improvement. This should be reflected in cost-benefit analyses for fabrication specifications, where the marginal cost of improved welding practices is justified by the enhanced structural performance and safety margin.