Experimental Study on Axial Compressive Capacity of Composite T-Shaped CFT Columns

Literature Overview

The paper by Xu Lihua, Du Guofeng, Wen Fang, and Xu Haoran from Wuhan University, published in the China Civil Engineering Journal in 2009, presents a systematic experimental investigation on Welded Rectangular Composite T-shaped Concrete-Filled Steel Tubular columns (WRC-T). This work was supported by the Hubei Provincial Construction Science and Technology Program (K200513) and represents one of the earlier domestic studies on unconventional CFT cross-sections. The authors designed and tested 34 specimens under axial compression, varying the slenderness ratio, the arm-to-web ratio, and the confinement effectiveness coefficient. The study ultimately proposes practical calculation formulas for both strength capacity and stability capacity of WRC-T columns.

Core Technical Content and Key Findings

The fundamental innovation of this study lies in the proposal of a welded T-shaped CFT column configuration that combines two rectangular CFT tubes into a T-section. This configuration leverages the confinement effect of concrete on the steel tube walls while simultaneously utilizing the steel tubes as composite structural elements. The authors conducted 34 axial compression tests, measuring load-longitudinal strain curves, ultimate strength capacity, and stability capacity for each specimen.

A critical finding is the classification of WRC-T columns based on their slenderness ratio L/i. Specimens with L/i ≤ 14.4 behave as short columns, exhibiting failure modes dominated by material strength rather than buckling. Specimens with 14.4 < L/i ≤ 33.6 behave as intermediate-length columns, where both material strength and elastic instability contribute to the failure. This classification provides a practical boundary for engineers to determine whether strength-based or stability-based design formulas should govern.

The experimental results demonstrated that the two constituent rectangular tubes in the T-section work cooperatively under axial loading. The concrete core in each tube provides lateral confinement to the steel walls, while the steel tubes restrain the concrete against lateral expansion. This synergistic behavior results in improved ductility and energy absorption compared to conventional rectangular CFT columns of equivalent cross-sectional area.

Interpretation of Technical Parameters

The study examines three primary parameters influencing the axial compressive behavior of WRC-T columns. The confinement effectiveness coefficient reflects the degree to which the steel tube constrains the concrete core, which is directly related to the width-to-thickness ratio of the rectangular tube walls. A higher confinement effectiveness coefficient indicates better composite action and higher ultimate strength.

The arm-to-web ratio, defined as the ratio of the flange tube width to the web tube width, affects the distribution of axial force between the two components of the T-section. A higher arm-to-web ratio means the flange tubes carry a proportionally larger share of the load, which can influence the local buckling behavior of the web tube.

The slenderness ratio L/i governs the transition from strength-controlled failure to stability-controlled failure. The critical slenderness ratios identified in this study (14.4 and 33.6) are somewhat lower than those typically observed for conventional circular CFT columns, which may be attributed to the reduced torsional rigidity of the T-shaped cross-section compared to symmetric circular sections.

Standards and Code Comparison

The proposed calculation formulas in this paper were benchmarked against existing provisions in domestic and international codes, including GB 50017 for steel structures, the Chinese code for concrete-filled steel tubular structures, and relevant provisions in Eurocode 4. The regression analysis of experimental data yielded formulas that showed good agreement with test results, with calculation-to-test ratios typically falling within the range of 0.9 to 1.1.

Engineering Practice Integration

From a practical standpoint, the WRC-T column concept offers several advantages for building construction. The T-shaped cross-section is particularly suitable for moment-resisting frames where the flange tubes can serve as connection interfaces for beam-column joints, while the web tube provides lateral stability. The welded fabrication approach allows for on-site assembly of modular components, reducing construction time.

However, several engineering concerns warrant attention. The weld quality at the T-junction between the flange and web tubes is critical, as this region is susceptible to residual stress concentration and potential fatigue cracking under cyclic loading. Engineers should specify appropriate welding procedures, such as GTAW for root passes followed by GMAW for fill and cap passes, and ensure thorough non-destructive testing including UT and MT at the weld junction.

The concrete pouring process into the T-shaped cavity also presents challenges. Ensuring complete and uniform concrete fill without voids, particularly at the re-entrant corner of the T-section, requires careful formwork design and vibration techniques. The use of self-compacting concrete may help mitigate these concerns.

Key Questions and Reflections

One question that arises from this study is whether the proposed formulas adequately account for the interaction effects between the two tube components under eccentric loading conditions. The study focuses exclusively on axial compression, but practical columns in buildings are almost always subjected to combined axial force and bending moment. The eccentricity of loading can significantly alter the stress distribution within the T-section, potentially leading to premature local buckling of one tube component.

Another consideration is the long-term durability of the welded T-junction. In aggressive environments, such as coastal regions or industrial areas with high sulfur dioxide concentrations, the weld region may be more susceptible to corrosion than the parent tube material. Proper corrosion protection, including coating and cathodic protection, should be specified for the T-junction area.

The study also raises the question of seismic performance. The T-shaped CFT column may exhibit different hysteretic behavior compared to conventional CFT columns, particularly in terms of energy dissipation capacity and ductility under cyclic loading. Further research on the seismic performance of WRC-T columns would be valuable before widespread adoption in seismic regions.

Study Insights and Implications

This paper represents a meaningful contribution to the field of composite steel-concrete structures. The systematic testing of 34 specimens provides a solid experimental foundation for the proposed design formulas. The identification of critical slenderness ratios for WRC-T columns offers practical guidance for engineers selecting appropriate column dimensions for specific applications.

The study underscores the importance of considering confinement effectiveness when designing CFT columns with unconventional cross-sections. The composite action between steel and concrete is not automatic but depends on geometric parameters such as the width-to-thickness ratio of the steel tube walls. Engineers should carefully verify that the steel tube walls are sufficiently thick to provide effective confinement without undergoing premature local buckling.

Overall, the WRC-T column concept shows promise for applications where a T-shaped cross-section is structurally advantageous, such as in moment-resisting frames with integral beam-column connections or in braced frames where the web tube can serve as a brace attachment point. The proposed design formulas provide a starting point for practical engineering applications, though further research on eccentric loading, seismic behavior, and long-term durability is recommended before extensive use in critical structures.