Experimental Study on Bearing Capacity of Rectangular Steel Tube Concrete Axially Compressed Members

Literature Overview

This paper, authored by Han Linhai of Fuzhou University and Yang Youfu of Harbin Institute of Technology, was published in the China Civil Engineering Journal in 2001 (Volume 34, Issue 4, pages 22-31). Funded by the Fok Ying Tung Education Foundation (Grant No. 0501064), the study presents a systematic experimental investigation on twenty-four rectangular cross-section steel tube concrete (RFTC) axially compressed short columns. The research addresses a critical gap in structural engineering practice, where rectangular steel tube concrete members are increasingly used in buildings, bridges, and industrial structures due to their superior strength-to-weight ratio, ductility, and construction efficiency compared to conventional reinforced concrete columns.

Core Experimental Parameters and Design Variables

The experimental program was designed around two primary variables that govern the structural behavior of RFTC columns. The confinement effect coefficient (denoted as α) was varied from 0.5 to 1.3, representing different levels of steel tube-to-concrete interaction intensity. The cross-sectional aspect ratio (b/h) ranged from 1.0 to 1.75, covering square sections through moderately elongated rectangular profiles. These parameter ranges were selected to encompass the typical design envelope encountered in practical engineering applications.

The specimen dimensions, steel grades, and concrete strengths were selected to reflect common construction practices in China during the early 2000s. The short-column configuration ensures that failure is governed by material strength and confinement effectiveness rather than buckling instability, which allows for a clearer understanding of the fundamental mechanical interaction between the steel tube and the confined concrete core.

Key Experimental Findings

Influence of Confinement Effect Coefficient

The study demonstrates that the confinement effect coefficient has a pronounced and non-linear influence on the ultimate bearing capacity of RFTC columns. As α increases from 0.5 to 1.3, the strength capacity rises significantly, confirming the well-established principle that steel tube confinement enhances the triaxial stress state of the core concrete. At lower α values (around 0.5), the steel tube provides minimal lateral restraint, and the member behavior approaches that of an unstrengthened concrete column. As α increases beyond 1.0, the confinement benefit continues to grow, but the rate of improvement diminishes slightly, suggesting an approaching plateau in confinement effectiveness.

Influence of Cross-Sectional Aspect Ratio

The aspect ratio effect is more nuanced. Columns with square sections (b/h = 1.0) exhibit the highest specific bearing capacity, while those with higher aspect ratios (b/h = 1.75) show reduced strength per unit cross-sectional area. This reduction is attributed to the increased susceptibility of the shorter-side steel plates to local buckling under the lateral pressure from the expanding concrete core. The longer sides, being more distant from the neutral axis in bending modes induced by eccentric loading imperfections, contribute less effectively to confinement. This finding carries significant implications for the design of rectangular columns in tall building cores and industrial frame structures.

Comparison of Design Codes and Standards

A major contribution of this paper is the comparative evaluation of four design approaches for calculating the bearing capacity of rectangular steel tube concrete columns:

The GJB (Chinese standard) results align most closely with the experimental data, which is unsurprising given that the experimental parameters and material properties were calibrated to Chinese construction norms. However, the study also notes that all four codes produce results that are on the safe side relative to actual test values, which is a conservative bias generally acceptable in structural design but potentially disadvantageous for cost optimization in large-scale projects.

Integration with Engineering Practice

In practical steel pipe and pipe fitting applications, the principles established in this study have direct relevance to the design of structural columns in petrochemical plants, offshore platforms, and industrial warehouses where rectangular hollow sections filled with concrete are employed. The confinement effect coefficient concept can be extended to evaluate the performance of concrete-filled pipe fittings, such as large-diameter elbows and tees, where internal concrete filling is sometimes used to enhance buckling resistance and impact toughness.

For pipe fitting manufacturers, the aspect ratio findings suggest that rectangular hollow sections with b/h ratios exceeding 1.5 should be designed with additional consideration for local buckling of the shorter-side walls. In the context of pipe fitting forming processes—particularly roll forming and hydraulic expansion—the geometric proportions must be carefully controlled to ensure uniform wall thickness distribution, as non-uniformity can create weak zones susceptible to premature buckling under compressive loading.

Key Questions and Reflections

The experimental study raises several important questions for further investigation. First, the study is limited to short columns under pure axial compression, but real-world structures inevitably experience eccentric loading due to construction tolerances, seismic actions, and wind loads. The interaction between eccentricity and the confinement effect in rectangular sections remains an area requiring further experimental and analytical study. Second, the study does not address cyclic loading behavior, which is critical for seismic design of concrete-filled steel tube columns in earthquake-prone regions. Third, the long-term behavior under sustained loads, including creep and shrinkage effects on the concrete core and the resulting redistribution of stresses to the steel tube, warrants further attention.

From a quality control perspective, the study reinforces the importance of precise dimensional control during pipe manufacturing. Variations in wall thickness, cross-sectional flatness, and corner radius all affect the confinement coefficient and, consequently, the structural performance. Non-destructive testing methods such as ultrasonic thickness measurement and eddy current inspection should be applied rigorously to ensure that the as-built geometry matches the design assumptions.

Study Insights and Implications

This paper represents a foundational contribution to the understanding of rectangular steel tube concrete structural behavior. The systematic parametric study, combined with multi-code comparison, provides engineers with both theoretical understanding and practical design guidance. The finding that GJB offers the most accurate predictions for Chinese practice is particularly valuable for domestic engineering projects, while the comparative analysis serves as a benchmark for evaluating the applicability of international codes to local conditions. The research underscores the importance of experimental validation in structural design, reminding practitioners that theoretical models and code provisions, while essential, must be verified against physical test data to ensure reliable and economical designs.