Axial Compression Ultimate Bearing Capacity Calculation Method for Steel Tube Fly Ash Concrete Short Columns

Literature Overview

The paper by Zhang Yuzhuo, Liu Jinlong, Xu Qian, and Wang Qinghe, published in the Journal of Architecture and Civil Engineering in 2023, investigates the axial compression ultimate bearing capacity of steel tube fly ash concrete (FAC) short columns. This research is motivated by the need to promote the utilization of coal gangue (fly ash) as a coarse aggregate in steel tube concrete structures, addressing both environmental sustainability and material resource efficiency. The study was conducted by researchers from Shenyang Jianzhu University, Liaoning Technical University, Southeast University, and Harbin Institute of Technology, with support from the National Natural Science Foundation of China.

Experimental Program and Test Configuration

The experimental program comprised 18 specimens: 6 steel tube normal concrete (NTC) short columns and 12 steel tube fly ash concrete (FAC) short columns. The test matrix was designed to vary key parameters including concrete strength grade, confinement ratio, steel ratio, and fly ash replacement ratio. All specimens were subjected to axial compression loading until failure.

Test Matrix Summary

Key Findings and Analytical Results

The study presents several important findings that have direct relevance to the design and application of steel tube concrete structures using recycled or alternative aggregates:

1. Failure Modes: All specimens exhibited local bulging and shear failure patterns under axial compression, which is consistent with the typical failure behavior of confined concrete columns. The steel tube effectively confined the core concrete, delaying failure and enhancing ductility.
2. Confinement Effect Enhancement: Compared to steel tube normal concrete columns, the steel tube provides a better lateral confinement effect on the core fly ash concrete. This finding suggests that the confinement mechanism is even more effective when the core material has slightly lower inherent strength, as the steel tube compensates for the reduced concrete strength through enhanced confinement.
3. Effect of Steel Ratio and Confinement: Increasing the confinement ratio and steel ratio at the same replacement ratio significantly improves the bearing capacity of the columns. This confirms the well-established principle that steel tube confinement is a primary mechanism for enhancing the performance of concrete-filled tubes.
4. Effect of Fly Ash Replacement Ratio: The axial compression bearing capacity decreases with increasing fly ash replacement ratio, but the maximum reduction does not exceed 11%. This is a critical finding, as it demonstrates that the impact of fly ash substitution on structural capacity is relatively modest and manageable through design adjustments.
5. Statistical Correlation: The correlation coefficient between fly ash coarse aggregate and bearing capacity is -0.33, which is not statistically significant. This indicates that the fly ash replacement ratio alone is not the dominant factor governing bearing capacity, and other parameters such as steel ratio and concrete strength play more significant roles.

Comparison of Existing Calculation Methods

The study evaluated the applicability of two existing standards—GB 50936-2014 (Technical Code for Concrete-Filled Steel Tubular Structures) and T/CECS 625-2019—for calculating the ultimate bearing capacity of steel tube fly ash concrete short columns. Both methods were found to be applicable, and with the introduction of correction factors, the average relative error in bearing capacity calculation was maintained within 3%. This is a significant finding, as it means that existing design codes can be used with minor modifications for steel tube concrete structures incorporating fly ash aggregates.

Technical Interpretation from a Metallurgical and Welding Perspective

The findings of this study have important implications for the selection and fabrication of steel tubes used in concrete-filled tube structures, particularly when alternative aggregates are employed. From a metallurgical standpoint, the steel tube serves as both a structural member and a confining element, and its material properties directly influence the composite behavior of the column.

Steel Tube Material Selection

The choice of steel grade for the tubes is critical, as the steel ratio (ratio of steel cross-sectional area to total cross-sectional area) is identified as a key parameter influencing bearing capacity. For columns where fly ash replacement is employed and additional confinement is needed, higher steel grades such as Q345 or Q390 (per GB/T 1591) may be selected to increase the steel ratio without excessively increasing tube wall thickness. The mechanical properties of the steel tube, including yield strength, tensile strength, elongation, and impact toughness, should be verified through standard mechanical testing in accordance with GB/T 228 and GB/T 229.

Welding Considerations for Steel Tube Concrete Columns

While the study focuses on short columns under axial compression, the fabrication of steel tube concrete structures often involves welded connections between tube segments and between tubes and other structural elements. The following welding considerations are relevant:

• Joint Design: For columns requiring field assembly, butt joints between tube segments should be designed with full-penetration welds. The weld metal should match or slightly exceed the strength of the base metal to ensure that the weld does not become a weak link.
• Preheating Requirements: For thicker tube walls (typically above 20 mm), preheating is recommended to prevent cold cracking in the HAZ. The preheating temperature should be determined based on the carbon equivalent (CE) of the steel, calculated according to ISO 4063 or GB/T 19792.
• Post-Weld Treatment: For columns subjected to high axial loads, post-weld heat treatment may be necessary to relieve residual stresses and ensure adequate HAZ toughness.

Quality Control for Concrete Filling

The quality of concrete filling is equally important for achieving the intended composite action. The concrete should be placed in layers with adequate compaction to avoid voids between the steel tube and the concrete. Ultrasonic testing can be employed to detect voids or honeycombing within the filled tubes, and the results should be documented as part of the quality assurance record.

Design Implications and Correction Factors

The study proposes correction factors to be applied to the existing bearing capacity formulas in GB 50936-2014 and T/CECS 625-2019 for steel tube fly ash concrete short columns. The fact that the average relative error is within 3% after applying these corrections is highly encouraging for engineering practice, as it means that engineers can continue to use familiar design methodologies with only minor adjustments.

The maximum reduction in bearing capacity of 11% due to fly ash replacement provides a clear upper bound for design considerations. Engineers should account for this reduction by either increasing the steel ratio, increasing the concrete strength grade, or applying the proposed correction factors. The non-significant correlation between fly ash replacement ratio and bearing capacity (-0.33) suggests that the replacement ratio should be treated as a secondary design variable, with steel ratio and concrete strength being the primary variables for capacity optimization.

Study Insights and Reflections

This study makes a valuable contribution to the field of sustainable construction by demonstrating that fly ash can be effectively utilized as a coarse aggregate in steel tube concrete structures with minimal impact on structural performance. The finding that existing design codes are applicable with minor corrections is particularly significant, as it reduces the barriers to adoption of fly ash-based concrete in steel tube structures.

From a quality control perspective, the study highlights the importance of ensuring adequate steel confinement through proper steel ratio selection and high-quality steel tube fabrication. The welding and fabrication quality of the steel tubes should be maintained to the same standards as those used for normal concrete-filled tubes, as the confinement mechanism is equally critical regardless of the core concrete composition.

The environmental benefits of utilizing fly ash as a construction material are substantial, and this study provides the technical justification for its application in steel tube concrete structures. Future research could extend to long-term durability studies, including corrosion resistance and carbonation depth measurements, to ensure that fly ash-filled steel tube concrete columns maintain their performance over the design service life.