Axial Compression Behavior of Welded Rectangular Steel Tube Concrete Composite T-Shaped Short Columns

Literature Overview

The research by Du Guofeng, Xu Lihua, Xu Haoran, and Wen Fang (2008), published in the Journal of Xi'an University of Architecture and Technology (Vol. 40, No. 4, pp. 549–555), investigates the axial compression behavior of a novel welded rectangular steel tube concrete composite T-shaped column (WRC-T column). Supported by the Hubei Provincial Natural Science Foundation (Grant 2003ABA059), this study addresses the need for efficient structural solutions in multi-story buildings where space constraints and load requirements demand innovative column geometries. The T-shaped configuration combines two rectangular steel tube concrete sections into a single composite column, offering enhanced load capacity and stiffness within a compact footprint.

Structural Configuration and Design Parameters

The WRC-T column consists of two rectangular steel tube concrete sub-sections welded together to form a T-shaped cross-section. The key design parameters investigated include:

Twenty WRC-T column specimens were fabricated and tested under axial compression. Each specimen was loaded to failure, with load-deformation curves and ultimate capacities recorded. The specimens exhibited ductile failure modes characterized by progressive local buckling of the steel tube walls, followed by concrete crushing under triaxial confinement.

Experimental Results and Parametric Analysis

The experimental results demonstrated that the two components of the WRC-T column work together effectively in composite action:

1. Load-deformation behavior: The load-deformation curves exhibited a clear elastic stage, a yielding plateau, and a post-peak descending branch, indicating good ductility. The steel tubes confined the concrete, delaying concrete crushing and allowing the column to sustain loads beyond the sum of the individual component capacities.
2. Failure mode: Failure initiated at the web region where the two tube sections intersect, with progressive outward bulging of the steel tube walls. The concrete in the confined region achieved higher compressive strength than unconfined concrete, confirming the effectiveness of the composite action.
3. Parametric effects: The confinement effect coefficient had the most significant influence on axial capacity, followed by the limb-to-web ratio. The aspect ratio had a moderate effect, primarily influencing the post-peak ductility rather than the ultimate capacity.

Numerical Simulation and Validation

The study employed ABAQUS 6.5 finite element software to model the WRC-T column specimens using C3D8R (8-node linear reduced-integration brick element with hourglass control) solid elements. The numerical model incorporated:

• Elastic-plastic material model for the steel tube with isotropic hardening
• Concrete damage plasticity model for the concrete fill
• Interface behavior between steel and concrete representing bond and slip
• Geometric nonlinearity to capture large deformation and buckling

The comparison between numerical and experimental results showed good agreement, validating the finite element model for parametric studies and design optimization. The use of C3D8R elements with hourglass control proved effective in simulating the large deformation and local buckling behavior of the steel tube walls.

Welding Considerations for WRC-T Column Fabrication

The fabrication of WRC-T columns involves critical welding operations at the junction between the two rectangular steel tube sections:

1. Weld joint design: The T-junction between the web and the two limb sections creates a complex stress concentration zone. The weld configuration should be designed to minimize stress concentration, potentially using fillet welds with generous leg dimensions or full-penetration groove welds at critical locations.
2. Welding procedure qualification: The welding procedure should be qualified according to applicable standards (e.g., AWS D1.1, ISO 15614, or GB/T 985) for the specific steel grade and wall thickness. Preheat temperature, interpass temperature, and heat input should be controlled to prevent cold cracking and minimize distortion.
3. Residual stress management: The welding residual stresses at the T-junction superimpose on the axial compressive stress during service, potentially accelerating buckling of the steel tube walls. Post-weld stress relief heat treatment or mechanical peening of the weld toe may be beneficial.
4. Quality assurance: Non-destructive testing (NDT) of the weld joints is essential. Ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) should be used to detect internal defects such as lack of fusion, porosity, and slag inclusions. Visual testing (VT) and magnetic particle testing (MT) should be applied to detect surface-breaking defects.

Engineering Application and Design Recommendations

The WRC-T column offers several advantages for practical engineering applications:

• Space efficiency: The T-shaped configuration provides high load capacity within a compact footprint, suitable for dense building layouts.
• Constructability: The modular design using standard rectangular steel tubes simplifies fabrication and field assembly.
• Structural efficiency: The composite action between steel and concrete is fully exploited, achieving higher capacity-to-weight ratio than conventional columns.

For engineers specifying WRC-T columns, the following recommendations emerge from this study:

• The confinement effect coefficient should be optimized to balance steel and concrete contributions.
• The limb-to-web ratio should be selected to minimize stress concentration at the T-junction.
• Welding quality must be rigorously controlled, with NDT coverage proportional to the structural importance of the column.
• The finite element model validated in this study can be adapted for detailed design analysis of specific WRC-T column configurations.

Study Insights and Outlook

This study contributes to the growing body of knowledge on composite steel-concrete columns with non-circular cross-sections. The demonstration that the WRC-T configuration achieves effective composite action is encouraging for practical adoption. However, several aspects warrant further investigation, including the behavior under eccentric loading, the effect of cyclic loading (relevant for seismic design), and the long-term durability of the composite section under environmental exposure.

For steel pipe manufacturers and welding engineers, the study highlights the importance of producing steel tubes with consistent wall thickness and surface quality, as these factors directly influence the composite action and welding quality at the T-junction. The welding of tubular sections to form composite geometries is a specialized skill that requires careful procedure development and quality control to ensure structural integrity.