Axial Compression Performance of CFST-RPC Long Columns

Literature Overview

The paper by Ji Wenyu, Luo Hua, and Yang Guojing (2014), published in China Railway Science, Volume 35, Issue 1, reports on an experimental investigation of the axial compression performance of concrete-filled steel tube reactive powder concrete (CFST-RPC) long columns. The authors conducted axial compression tests on 14 CFST-RPC long column specimens to study the effects of slenderness ratio and confinement ratio at large slenderness ratios on the axial compression behavior. The specimens exhibited predominantly elastic instability failure modes, with smaller slenderness ratio specimens showing both flexural buckling and local compressive bulging failures. The load-strain curves were divided into elastic, elasto-plastic, and unloading stages. This study is relevant to pipe and structural engineers because RPC-filled steel tubes are increasingly used in high-performance structural applications, and the understanding of long column behavior is critical for the design of slender members in bridges, towers, and industrial structures.

Core Technical Points and Experimental Methodology

The experimental program involved 14 CFST-RPC long column specimens with varying slenderness ratios and confinement ratios. The key parameters investigated include the slenderness ratio (lambda), defined as the effective length divided by the radius of gyration, and the confinement ratio, which is the ratio of the steel tube cross-sectional area to the total cross-sectional area. The following table summarizes the key experimental parameters and findings:

The study found that as the slenderness ratio increases, the ultimate bearing capacity and ultimate displacement of CFST-RPC long columns decrease gradually. At large slenderness ratios, the confinement ratio has a diminishing effect on the ultimate bearing capacity, and the confining effect of the steel tube on RPC is less effective than that on ordinary concrete. This is an important finding because it suggests that the design of slender CFST-RPC columns should not rely solely on the confinement effect of the steel tube, and additional measures such as intermediate bracing or increased tube thickness may be necessary.

Failure Mode Analysis and Material Behavior

The failure modes observed in the tests are classified as follows:

1. Elastic instability failure: This is the dominant failure mode for most specimens, characterized by sudden lateral deflection and loss of load-carrying capacity without significant plastic deformation. This mode is typical of slender columns and is governed by the Euler buckling theory.
2. Local compressive bulging failure: Observed in specimens with smaller slenderness ratios, this mode involves local outward bulging of the steel tube wall under compressive stress, accompanied by flexural buckling. This mode indicates a combined failure mechanism where both global and local instability contribute to the collapse.
3. Ductile failure characteristics: Specimens with smaller slenderness ratios exhibited more ductile failure characteristics, with larger deformations before failure. As the slenderness ratio increases, the failure becomes more brittle and instability-dominated.

From a steel pipe manufacturing perspective, the local compressive bulging failure mode is particularly concerning because it involves the deformation of the steel tube wall, which is directly related to the manufacturing quality of the tube. Tubes with wall thickness variations, ovality defects, or residual stresses from the welding or rolling process may be more susceptible to local bulging under compressive loading. Therefore, the quality control of steel tubes used in CFST-RPC columns should include strict inspection of wall thickness uniformity, ovality, and surface defects.

Confinement Ratio and Steel Tube Design

The confinement ratio is a critical design parameter for CFST columns, as it determines the degree of lateral confinement provided by the steel tube to the core concrete. The study found that at large slenderness ratios, the confinement ratio has a diminishing effect on the ultimate bearing capacity, and the steel tube's confining ability on RPC is less effective than on ordinary concrete. This finding has important implications for the design of slender CFST-RPC columns:

The reduced confinement effect of the steel tube on RPC, compared to ordinary concrete, may be attributed to the higher strength and lower strain capacity of RPC. RPC typically has a compressive strength of 120 MPa to 200 MPa or higher, with a relatively low strain at peak stress. This means that the steel tube may not be able to provide sufficient lateral confinement to prevent the RPC from failing in a brittle manner before the tube yields. In practice, this means that the design of CFST-RPC columns with large slenderness ratios should consider the use of higher-strength steel tubes with improved ductility, or the incorporation of additional confinement elements such as spiral reinforcement within the tube.

Engineering Practice Implications

For engineers involved in the design and fabrication of CFST-RPC structures, this study provides several practical recommendations:

• The slenderness ratio should be carefully controlled in the design of CFST-RPC columns, with a preference for smaller slenderness ratios to ensure ductile behavior and adequate confinement.
• For slender CFST-RPC columns, the design should not rely solely on the confinement effect of the steel tube, and additional measures such as intermediate bracing, increased tube thickness, or the use of higher-strength steel grades should be considered.
• The quality of the steel tubes used in CFST-RPC columns should be strictly controlled, with particular attention to wall thickness uniformity, ovality, and surface defects, as these factors can significantly affect the local buckling behavior.
• The welding quality of the steel tubes, particularly for longitudinally welded tubes, should meet high standards to ensure uniform load distribution and prevent premature failure at weld locations.

Study Insights and Reflections

This paper contributes valuable experimental data on the axial compression behavior of CFST-RPC long columns, filling a gap in the existing knowledge base. The findings regarding the diminished confinement effect at large slenderness ratios are particularly important for the design of slender CFST-RPC members, which are increasingly used in high-rise buildings, bridges, and industrial structures. The study also highlights the need for further research on the interaction between the steel tube and RPC under various loading conditions, including eccentric compression, bending, and combined loading.

From a welding and pipe fabrication perspective, the study underscores the importance of high-quality steel tubes for CFST-RPC applications. The local bulging failure mode observed in some specimens suggests that the tube wall thickness and manufacturing quality are critical factors in determining the structural performance. Engineers should ensure that the steel tubes used in CFST-RPC columns meet the requirements of relevant standards such as GB/T 8162, GB/T 8163, or ASTM A530 for seamless tubes, and GB/T 3091 or ISO 21970 for welded tubes.

Summary

This literature provides important experimental insights into the axial compression behavior of CFST-RPC long columns, highlighting the transition from ductile to elastic instability failure as the slenderness ratio increases and the diminished confinement effect at large slenderness ratios. The findings have direct implications for the design and fabrication of CFST-RPC structures, particularly in terms of slenderness ratio control, steel tube quality, and the need for additional structural measures in slender members.