Experimental Study on Mechanical Properties of Steel Tube-Confinement Reinforced Concrete Compression Members

Literature Overview

This paper by Gao Chunyan, Liu Mingyang, Duan Yaxin, and Guo Dawei, published in the Journal of Xi'an University of Architecture and Technology (Natural Science Edition) in 2019, presents an experimental investigation on the mechanical behavior of steel tube-confined reinforced concrete (RC) compression members. The research was funded by the National Natural Science Foundation of China (51368042) and the Inner Mongolia Natural Science Foundation (2018MS05041). The study was conducted at the Inner Mongolia University of Science and Technology, with engineering collaboration from Baogang Northwest Construction Co., Ltd.

Experimental Program and Core Findings

The experimental program comprised twelve specimens with systematically varied parameters: steel tube diameter-to-thickness ratio (D/t = 55 and 110), specimen slenderness ratio (λ = 12 and 24), and loading eccentricity ratio (e/h = 0, 0.23, and 0.46). This parametric design allows for the isolation of individual parameter effects on the mechanical behavior of the specimens.

Experimental Parameters Matrix

Failure Modes

The failure behavior varied significantly with the loading condition:

• Axially loaded specimens (e/h = 0): The typical failure mode involved outward bulging or tearing of the steel tube at the mid-height region, accompanied by the formation of a distinct diagonal shear crushing band in the core concrete. This failure pattern indicates that the concrete core experienced a combination of compressive and shear stresses, with the diagonal band representing the zone of maximum compressive strain.
• Eccentrically loaded specimens (e/h > 0): The failure location concentrated at the cutout (notch) region of the specimen. On the tension side, the concrete was cracked by tensile stress, while on the compression side, localized crushing occurred. This asymmetric failure pattern is characteristic of flexural-compressive failure in eccentrically loaded members.

Parameter Effect Analysis

The experimental results revealed the following key trends:

1. D/t ratio effect: The steel tube diameter-to-thickness ratio had a relatively minor influence on both the failure mode and the load-bearing capacity of the specimens. This finding is somewhat surprising, as a higher D/t ratio generally implies a thinner steel tube wall with lower confinement effectiveness. However, the range of D/t values tested (55 to 110) may not have been wide enough to capture a significant difference.
2. Slenderness ratio effect: The specimen slenderness ratio had a significant influence on both the failure mode and the load-bearing capacity. Higher slenderness ratios led to reduced load-bearing capacity due to increased second-order effects (P-Δ effects) and earlier buckling of the steel tube.
3. Eccentricity ratio effect: The loading eccentricity ratio also had a significant influence on the failure mode and load-bearing capacity. Higher eccentricity ratios resulted in more pronounced flexural behavior, reduced load-bearing capacity, and a shift in the failure location from the mid-height to the cutout region.

Engineering Practice Implications

The research addresses a practical structural component that combines the advantages of reinforced concrete (high compressive strength and fire resistance) with the confinement benefits of a steel tube (enhanced ductility and concrete confinement). This hybrid system is particularly suitable for applications requiring high load-bearing capacity with limited cross-sectional dimensions, such as bridge piers, building columns in high-rise structures, and offshore platform legs.

From a fabrication and quality control perspective, several considerations arise:

• Steel tube quality: The steel tube must have uniform wall thickness and mechanical properties. Variations in wall thickness can lead to uneven confinement and premature local buckling. Ultrasonic thickness measurement should be performed during inspection.
• Concrete placement: The concrete must be properly compacted within the steel tube to avoid voids or honeycombing, which would compromise the composite action between the steel tube and the concrete core. Pumping methods suitable for confined spaces must be used, and vibration must be applied carefully to avoid damaging the steel tube.
• Welding of internal reinforcement: If the steel tube is a welded tube, the quality of the longitudinal and circumferential welds is critical. The welds must be free of defects that could initiate cracks under cyclic or seismic loading.

Study Insights and Reflections

The experimental study provides valuable data for the development of design formulas for steel tube-confined RC compression members. The finding that the D/t ratio has a relatively minor effect on capacity within the tested range is practically useful, as it suggests that the design can be less sensitive to variations in tube geometry. However, this conclusion should be validated with a wider range of D/t values before being adopted in design codes.

The significant effect of slenderness ratio on capacity is well-established in the literature for conventional RC columns and is expected to apply to steel tube-confined RC columns as well. The eccentricity ratio effect is also consistent with general structural mechanics principles. The more interesting finding is the localization of failure at the cutout region for eccentrically loaded specimens, which suggests that the cutout creates a stress concentration that governs the failure mechanism.

The study could be further enhanced by incorporating finite element analysis to complement the experimental data, particularly for exploring parameter combinations that were not tested experimentally. Additionally, the long-term behavior of steel tube-confined RC members, including creep and shrinkage effects on the composite action, remains an area for future research.

The research contributes to the growing body of knowledge on hybrid steel-concrete composite structures, which are increasingly adopted in modern construction for their superior structural performance and material efficiency.