Axial Compression Capacity of High-Strength Steel Tube High-Strength Concrete Short Columns

Literature Overview

This paper by Zeng Zhiwei and colleagues from Guangzhou University, published in 2022 in the journal "Building Structure" (Vol. 52, No. 18, pp. 72-77), presents experimental research on the axial compression bearing capacity of short columns composed of high-strength steel tubes filled with high-strength concrete. Supported by the National Natural Science Foundation of China (Grants No. 51878188 and 51678169), this study addresses an important trend in structural engineering: the combined use of high-strength steel and high-strength concrete to achieve higher bearing capacities in more compact structural members.

Experimental Program and Test Setup

The study tested 27 specimens arranged in 9 groups, comparing high-strength steel tube high-strength concrete (HSSC-HSC) columns with high-strength steel tube ordinary concrete (HSSC-OC) columns. The test program is summarized below:

The confinement coefficient (套箍系数) was varied to investigate its influence on the bearing capacity and ductility of the columns.

Core Technical Findings

The experimental results reveal several important findings regarding the behavior of high-strength steel tube high-strength concrete columns:

1. Load-displacement curve differentiation: The load-displacement curves of HSSC-OC specimens and HSSC-HSC specimens exhibit clear differences. The HSSC-OC specimens demonstrate superior ductility compared to HSSC-HSC specimens. This finding is significant because it suggests that the combination of high-strength steel and high-strength concrete, while providing higher bearing capacity, may result in reduced ductility.
2. Confinement effect on bearing capacity: The steel tube confinement effectively improves the ultimate bearing capacity of concrete, with the improvement being more pronounced for high-strength concrete. The confinement effect transforms the inherently brittle high-strength concrete into a material with certain ductility.
3. Ductility enhancement through confinement: The ductility of HSSC-HSC columns increases with the confinement coefficient. This finding is important for seismic design, as it demonstrates that the confinement effect can be used to improve the ductility of high-strength concrete columns.
4. Code formula applicability: The calculated values based on the formula in GB 50936-2014 agree well with the measured values for all specimens, indicating that the code formula is applicable to both HSSC-OC and HSSC-HSC columns.

Material Interaction Analysis

The interaction between the high-strength steel tube and the high-strength concrete infill is the key mechanism governing the behavior of HSSC-HSC columns. The steel tube provides lateral confinement to the concrete, preventing the tensile cracking and spalling that would otherwise occur under axial compression. The confinement effect is more effective for high-strength concrete because:

• High-strength concrete has a higher compressive strength, which means that the lateral expansion pressure exerted on the steel tube is greater.
• The confinement pressure from the steel tube is more effective at preventing the brittle failure of high-strength concrete, as the concrete is more sensitive to lateral expansion.
• The combination of high-strength steel and high-strength concrete creates a more efficient load-carrying system, where both materials contribute significantly to the overall bearing capacity.

The reduction in ductility observed in HSSC-HSC columns compared to HSSC-OC columns is attributed to the brittle nature of high-strength concrete. Even with steel tube confinement, the high-strength concrete retains more of its brittle characteristics compared to ordinary concrete. The confinement coefficient provides a means to improve the ductility, but the fundamental material properties of high-strength concrete limit the achievable ductility.

Engineering Practice Implications

For structural engineers designing columns using high-strength steel tubes and high-strength concrete, this study provides several practical recommendations:

1. The GB 50936-2014 formula is validated for HSSC-HSC columns, providing confidence in the design methodology for these advanced structural members.
2. The confinement coefficient should be carefully selected to balance bearing capacity and ductility requirements. Higher confinement coefficients provide greater bearing capacity and ductility but may result in less economical designs.
3. For seismic applications, the ductility of HSSC-HSC columns should be carefully evaluated, and additional ductility enhancement measures may be required for columns in high-seismicity regions.
4. The load-displacement curves should be used to establish performance-based design criteria, with appropriate limits on the displacement ductility ratio.
5. The failure modes should be carefully monitored during testing and construction, as the brittle failure of high-strength concrete can be sudden and catastrophic.

Key Questions and Reflections

An important question arising from this study is the long-term behavior of HSSC-HSC columns under sustained loading. The confinement effect is effective under short-term loading, but the long-term creep and shrinkage of high-strength concrete may affect the confinement pressure and the overall behavior of the column. The interaction between the steel tube and the concrete may evolve over time, and this evolution should be considered in the design of long-term structures.

Another reflection concerns the weld quality of the steel tube connections. In practical applications, HSSC-HSC columns are often spliced using welded connections, and the weld quality can significantly affect the overall behavior of the column. The study does not address the weld quality aspect, but it is a critical consideration for the design and construction of HSSC-HSC columns in practice.

Study Insights and Implications

This research provides valuable experimental data on the axial compression behavior of high-strength steel tube high-strength concrete short columns. The validation of the GB 50936-2014 formula for HSSC-HSC columns provides confidence in the design methodology, while the identification of the ductility reduction in HSSC-HSC columns highlights an important design consideration. For steel pipe engineers, the key takeaway is that the combination of high-strength steel and high-strength concrete offers significant bearing capacity advantages but requires careful attention to ductility requirements, particularly in seismic applications. The confinement coefficient provides a practical design parameter for balancing bearing capacity and ductility, and the experimental data provides a reliable basis for the design of HSSC-HSC columns in structural engineering practice.