Axial Compression Behavior of Circular High-Strength Hollow Sandwich Concrete-Filled Steel Tube Members

Literature Overview

The paper by Wang Cancan, Liang Xudong, Zhu Peihong, Wang Xiantie, Pang Yahong, Meng Fandong, and Wu Jianling, published in the Journal of Xi'an University of Architecture and Technology (2021, Vol. 53, No. 3, pp. 366–378), investigates the axial compression behavior of circular high-strength hollow sandwich concrete-filled steel tube (HSCFST) members. The members consist of Q690 high-strength steel tubes with C100 and C120 ultra-high-performance concrete fills, featuring an internal hollow core. Six test specimens were fabricated with varying concrete strength, hollow ratio, and steel ratio. The research was supported by the National Natural Science Foundation of China (51678474) and a technology project from Lishui Zhengyang Electric Power Construction Co., Ltd. (SGTYHT/16-JS-200).

Core Technical Findings

Parametric Test Results

The experimental program systematically varied three parameters: concrete strength (C100 and C120), cross-sectional hollow ratio, and cross-sectional steel ratio. The results revealed clear trends in the axial compression behavior of these high-strength members.

Failure Mode Classification

The study identified two distinct failure modes: "bulging-type" (鼓曲型) and "shear-type" (剪切型). Bulging-type failure specimens exhibited higher post-peak load capacity and better ductility, while shear-type failure specimens showed relatively lower ductility. This distinction is critical for seismic design, as the failure mode directly influences the deformation capacity and energy dissipation of the member.

The bulging-type failure occurs when the steel tube undergoes outward local buckling, which is a ductile failure mode that allows significant deformation before collapse. The shear-type failure occurs when the concrete core fails in shear, which is a more brittle failure mode with limited deformation capacity. The transition between these two failure modes is influenced by the concrete strength, hollow ratio, and steel ratio.

Finite Element Analysis and Design Formulas

ABAQUS finite element analysis was used to investigate the influence of hollow ratio, concrete strength, and inner/outer steel tube diameter-to-thickness ratios on the axial compression behavior. The analysis confirmed the experimental trends and provided additional insights into the stress distribution and deformation patterns within the members. Design formulas for axial compression load capacity and stiffness were proposed, and the calculated results showed good agreement with experimental data.

Interpretation of Technical Points

High-Strength Material Interaction

The use of Q690 steel and C100/C120 concrete represents the frontier of high-strength structural materials. Q690 steel has a yield strength of 690 MPa, which is significantly higher than conventional structural steels such as Q345 (345 MPa) and Q460 (460 MPa). C100 and C120 concrete have compressive strengths of 100 MPa and 120 MPa, respectively, which are several times higher than conventional C40 or C50 concrete.

The interaction between these high-strength materials is complex. The high strength of the concrete reduces the confining effect of the steel tube, as the concrete is less prone to lateral expansion. Conversely, the high strength of the steel tube provides significant confinement to the concrete. The hollow core in the sandwich configuration further modifies this interaction by reducing the concrete volume and changing the stress distribution within the cross-section.

Hollow Sandwich Configuration Advantages

The hollow sandwich configuration offers several advantages over solid CFST members. The hollow core reduces the weight of the member while maintaining structural integrity, which is beneficial for seismic design and transportation. The configuration also provides a channel for utility services or inspection access. However, the hollow ratio must be carefully controlled to maintain adequate load capacity and ductility.

Connection with Engineering Practice

Material Standards and Specifications

The use of Q690 steel and C100/C120 concrete requires adherence to specific material standards. Q690 steel is governed by GB/T 1591 and EN 10025, which specify chemical composition, mechanical properties, and welding requirements. C100 and C120 concrete are governed by GB/T 50082 and specialized specifications for ultra-high-performance concrete (UHPC), which require careful mix design, curing, and quality control.

The welding of Q690 steel tubes requires special attention due to the high strength and potential for delayed cracking. Welding procedure specifications (WPS) must include appropriate preheating temperatures (typically 100–150°C for Q690 steel), interpass temperature control, and post-weld heat treatment (PWHT). NDT protocols should include UT and MT for all critical welds, with acceptance criteria based on GB/T 19804 or ASME B31.3.

Quality Control for High-Strength Concrete

The quality control of C100 and C120 concrete is more demanding than conventional concrete. The concrete must be placed under controlled conditions to ensure full compaction without segregation or voids. The hollow core in the sandwich configuration creates additional challenges for concrete placement, as the concrete must be pumped through a central opening and spread evenly between the inner and outer tubes. Post-placement inspection using ultrasonic pulse velocity (UPV) testing is recommended to verify concrete quality.

Key Questions and Reflections

The transition between bulging-type and shear-type failure modes is a critical design consideration. Engineers must determine the conditions under which each failure mode occurs and design accordingly to ensure ductile behavior. The study provides experimental data for this purpose, but the transition boundaries may vary with member geometry, loading conditions, and material properties.

The proposed design formulas for load capacity and stiffness should be validated against additional experimental data before widespread adoption. The current dataset of six specimens is limited, and the parametric range may not cover all practical design scenarios. Further research with a larger specimen database and broader parameter variations is recommended.

The long-term durability of high-strength concrete in the sandwich configuration is another concern. C100 and C120 concrete may be susceptible to carbonation, chloride ingress, and sulfate attack, which can degrade the concrete strength over time. Protective measures such as corrosion inhibitors, surface coatings, and cathodic protection should be considered for the steel tubes in aggressive environments.

Study Insights and Implications

This research advances the state of the art in high-strength CFST member design by investigating the behavior of Q690 steel and C100/C120 concrete in a hollow sandwich configuration. The identification of two distinct failure modes and the development of design formulas provide practical tools for engineers. The finite element analysis confirms the experimental findings and enables parametric studies beyond the experimental range. For practitioners, the key implications are that high-strength materials can be effectively used in CFST members, but careful attention must be paid to material compatibility, welding quality, concrete placement, and failure mode control. The hollow sandwich configuration offers a promising approach for lightweight, high-strength structural members, but the design must account for the unique behavior of this configuration under axial compression.