Experimental and Theoretical Analysis of Rectangular Concrete-Filled Steel Tubular Medium-Long Column Axial Compression Members with High-Strength Concrete

Literature Overview

This paper by Guo Lanhui, Zhang Sumei, Wang Yuyin, and Liu Jiepeng (Industrial Construction, Vol. 35, Issue 3, 2005, pp. 75-79) presents an experimental and theoretical investigation of rectangular-section CFST medium-long column members filled with high-strength concrete. Supported by the National Natural Science Foundation of China (Grant No. 59808004) and the Heilongjiang Provincial Outstanding Youth Fund, the research addresses the stability behaviour of CFST columns with intermediate slenderness ratios, a critical design consideration for tall building applications. The study is conducted at Harbin Institute of Technology, a leading institution in structural engineering research.

Core Technical Content

The experimental programme consisted of 10 rectangular-section CFST medium-long column specimens subjected to axial compression loading. The study systematically investigates the effects of three primary parameters: slenderness ratio (λ), steel ratio (ρ_s), and section aspect ratio (b/h, where b and h are the cross-section dimensions). The high-strength concrete used in the specimens has compressive strengths significantly exceeding conventional concrete grades, reflecting the trend toward higher-strength materials in modern structural design.

Experimental Results

The test results reveal the following key observations:

• The stability bearing capacity of the CFST column is primarily governed by the slenderness ratio, with a rapid decrease in capacity as the slenderness ratio increases.
• The section aspect ratio in the range of 1.0 to 1.6 has minimal influence on the stability coefficient and relative mid-span deflection.
• The steel ratio and concrete strength have secondary effects on the stability behaviour, with their influence becoming more pronounced at lower slenderness ratios.

Theoretical Analysis

The authors developed a nonlinear finite element analysis programme specifically tailored for rectangular CFST axial compression members. The programme accounts for:

• Geometric nonlinearity due to large deflections and second-order effects.
• Material nonlinearity of both steel and high-strength concrete.
• Interface interaction between the steel tube and concrete core.
• Initial imperfections and residual stresses in the steel tube.

The comparison between theoretical predictions and experimental results demonstrated good agreement, validating the analytical approach and providing confidence in its use for parametric studies.

Stability Coefficient Analysis

Using the validated nonlinear analysis programme, the authors conducted extensive parametric studies to determine the influence of various factors on the slenderness ratio-stability coefficient relationship curves. The key findings include:

• The stability coefficient decreases rapidly with increasing slenderness ratio, following a pattern similar to the Perry-Robertson formula used in conventional steel column design.
• Higher steel ratios provide greater confinement to the concrete core, delaying concrete crushing and maintaining post-buckling strength.
• Higher steel yield strengths contribute to improved stability capacity, particularly for stocky columns.
• The section aspect ratio has negligible influence on stability within the investigated range, suggesting that rectangular sections with aspect ratios up to 1.6 can be designed similarly to square sections.

Engineering Practice Implications

This research has significant relevance to the steel pipe and fitting industry, particularly for the following applications:

1. Rectangular tube fabrication: The study validates the use of rectangular steel tubes with aspect ratios up to 1.6 in CFST column applications. For pipe fabrication, this means that rectangular tubes can be produced through hot-rolled or cold-formed processes with appropriate dimensional tolerances per GB/T 6728 (Cold-rolled rectangular and square hollow sections).
2. Welded rectangular tube quality: When rectangular tubes are fabricated through welding (e.g., HFW or LSAW processes for larger sections), the weld quality is critical. The longitudinal and transverse welds must achieve full fusion with uniform mechanical properties. The residual stress distribution in the weld zone can affect the local buckling behaviour of the tube walls under axial compression.
3. High-strength concrete placement: The use of high-strength concrete in CFST columns requires careful attention to placement and compaction. The concrete mix design should ensure adequate workability for placement inside the steel tube, with appropriate use of vibrators or pumping equipment. The concrete strength verification should include core sampling and compressive strength testing.
4. Design code compliance: The design of CFST columns must comply with relevant standards including GB 50936 (Technical Code for Concrete-Filled Steel Tubular Structures), CECS 38 (Code for Design and Construction of Concrete-Filled Steel Tubular Structures), and EN 1993-1-1 (Eurocode 3). The stability coefficient derived in this study should be compared with code provisions to ensure conservative design.
5. Quality control programme: The quality assurance programme for CFST column fabrication should include:

• Steel tube dimensional inspection (OD, wall thickness, straightness)
• Steel tube NDT (UT for wall thickness, MT/PT for surface defects)
• Weld joint NDT (RT or UT for volumetric defects, MT/PT for surface defects)
• Concrete compressive strength testing (cubes and cores)
• Hydrostatic pressure testing after concrete filling
• Final dimensional inspection of completed CFST members

Key Questions and Reflections

The study provides valuable insights into the stability behaviour of rectangular CFST medium-long columns but several aspects merit further consideration. First, the influence of initial geometric imperfections (out-of-straightness, ovality) on the stability capacity has not been explicitly addressed, although these imperfections are inherent in manufactured steel tubes. Second, the effect of eccentric loading on the stability behaviour is not examined, which is important for practical applications where perfectly axial loading is rarely achieved. Third, the long-term creep and shrinkage effects of high-strength concrete on the stability behaviour over extended service periods should be investigated.

From a welding engineering perspective, I note that the residual stress distribution in the steel tube significantly influences the local buckling behaviour under axial compression. The longitudinal residual stresses in the weld zone and the transverse residual stresses at the tube corners can reduce the effective buckling load. The welding process parameters (heat input, welding sequence, and cooling rate) should be optimized to minimize adverse residual stress effects. Post-weld stress relief treatment may be beneficial for critical applications where high stability capacity is required.

Study Insights and Implications

This research provides a comprehensive understanding of the stability behaviour of rectangular CFST medium-long columns with high-strength concrete. The validated nonlinear analysis programme offers a powerful tool for parametric studies and design optimization. The finding that section aspect ratios up to 1.6 have negligible influence on stability provides design flexibility for architects and engineers. For the steel pipe fabrication industry, the study reinforces the importance of dimensional accuracy, material quality, and weld integrity in CFST column applications. The research contributes to the ongoing development of design codes and standards for CFST structural systems.