Experimental Study on High-Strength Conical Hollow Sandwich Thin-Walled Steel Tube Concrete Axially Compressed Short Columns

Literature Overview

This study, published in the Journal of Xi'an University of Architecture and Technology (Vol. 54, No. 2, 2022, pp. 306-316), investigates the axial compression behavior of high-strength conical hollow sandwich thin-walled steel tube concrete (THSTW-CFDS) short columns. The research team, led by Chen Qingsheng from State Grid Gansu Economic and Technical Research Institute and Pang Yahong from Xi'an University of Architecture and Technology, conducted experimental tests on 5 groups totaling 10 specimens utilizing Q690 high-strength steel and C120 ultra-high-strength concrete. Funded by the National Natural Science Foundation of China (Project No. 51678474), this work addresses a critical need in the field of composite column design for power transmission towers and heavy-load structures.

Core Technical Content and Test Parameters

The specimens are characterized by a conical (tapered) geometry with a hollow sandwich structure, where an inner steel tube is placed concentrically within an outer steel tube, with concrete filling the annular space between them. The hollow rate (ratio of inner tube diameter to outer tube diameter) serves as the primary geometric variable. The test parameters include:

Key Experimental Findings

The experimental results reveal several important trends:

1. Hollow rate effect: As the hollow rate increases, the ultimate bearing capacity decreases progressively. This is consistent with the reduced concrete confinement area available for composite action.
2. D/t ratio effect: Increasing the diameter-to-thickness ratio of both inner and outer steel tubes leads to reduced ultimate capacity, reflecting the increased susceptibility to local buckling of thinner-walled tubes.
3. Stiffener contribution: The introduction of longitudinal stiffeners (ribs) provides measurable improvements in load-bearing capacity:

• At hollow rate 0.72: capacity increase of approximately 7.2%
• At hollow rate 0.82: capacity increase of approximately 7.5%
• At hollow rate 0.85: capacity increase of approximately 10.9%

The observation that stiffeners become more beneficial at higher hollow rates is particularly noteworthy. At higher hollow rates, the concrete annulus becomes thinner, making it more prone to instability. The longitudinal stiffeners effectively prevent premature buckling of the inner tube, thereby maintaining the composite action and enhancing the overall structural efficiency.

Finite Element Analysis and Load-Displacement Behavior

The authors employed ABAQUS finite element software to conduct numerical simulations of the THSTW-CFDS columns throughout the entire loading process. The full stress-strain response was divided into three distinct stages:

Bearing Capacity Calculation Formula

A novel bearing capacity formula was proposed specifically for THSTW-CFDS columns with longitudinal stiffeners. The formula incorporates the nominal confinement effect coefficient, which quantifies the degree to which the steel tubes confine the concrete core. The calculated results showed good agreement with experimental data, validating the proposed approach.

Engineering Practice Implications

From a metallurgical and welding perspective, several observations warrant attention:

• Q690 steel welding considerations: The use of Q690 steel (minimum yield strength 690 MPa) requires careful attention to preheating temperatures (typically 150-250°C), interpass temperature control, and post-weld heat treatment to prevent cold cracking and ensure adequate toughness in the heat-affected zone.
• C120 concrete placement: Ultra-high-strength concrete with such high compressive strength requires meticulous attention to workability, vibration, and curing to avoid voids and honeycombing within the confined annular space.
• Stiffener attachment: The longitudinal stiffeners are typically welded to the inner surface of the outer tube. The weld quality directly influences the composite action and should be inspected by ultrasonic testing or magnetic particle testing.

Study Insights and Reflections

This research contributes significantly to the understanding of hollow sandwich composite columns with conical geometry. The finding that stiffeners provide greater benefit at higher hollow rates suggests a design optimization opportunity: for applications requiring high hollow rates (such as lightweight transmission towers), the incorporation of internal stiffeners becomes not merely beneficial but essential. The proposed bearing capacity formula provides engineers with a practical tool for design verification, though further validation with full-scale specimens and varied loading conditions would strengthen confidence in its applicability.

The conical geometry itself is particularly relevant for transmission tower legs, where tapered members are common. The hollow sandwich configuration offers weight reduction while maintaining structural integrity, and the use of Q690 steel and C120 concrete pushes the envelope of material performance. However, the practical feasibility of manufacturing such complex geometries with consistent quality remains a challenge that requires close coordination between fabrication shops and design engineers.

Conclusion

This study demonstrates that high-strength conical hollow sandwich thin-walled steel tube concrete columns offer a promising solution for heavy-load, weight-sensitive applications. The systematic investigation of hollow rate, D/t ratio, and stiffener configuration provides valuable design guidance. The proposed bearing capacity formula, validated against experimental results, represents a practical advancement in the design methodology for this specialized composite column type. Engineers involved in transmission tower and heavy industrial structure design should consider these findings when evaluating material and geometric optimization strategies.