Experimental Study on Axial Compression Load-Bearing Capacity of Thin-Walled Steel Pipe Concrete Columns

Literature Overview

This paper by Li Yan, Zhan Meisen, and Xiong Jingang, published in 2008 in "Concrete" (No. 11, pp. 47-49), presents an experimental investigation of the axial compression load-bearing capacity of circular thin-walled steel pipe concrete columns. Conducted at the School of Civil Engineering, Nanchang University, the study tested 8 specimens and examined the influence of diameter-to-thickness ratio, slenderness ratio, and hoop reinforcement on the structural performance. The relatively concise nature of the publication reflects the typical format of Chinese technical journals of that era, but the experimental data and observations remain valuable for engineering practice.

Experimental Program and Test Observations

The 8 circular thin-walled steel pipe concrete columns were subjected to axial compression loading until failure. The specimens varied in three primary parameters: the diameter-to-thickness ratio (D/t), the slenderness ratio (L/D), and the presence or absence of hoop reinforcement (stirrup-like confinement rings). The test observations focused on failure modes, load-bearing capacity, and deformation characteristics.

Influence of Diameter-to-Thickness Ratio

The D/t ratio is the most critical geometric parameter governing the local buckling behavior of the steel pipe shell. As the D/t ratio increases (i.e., the wall becomes thinner relative to the diameter), the local buckling resistance decreases, leading to earlier shell instability and reduced composite action between steel and concrete.

The experimental results confirm that thinner-walled pipes exhibit lower ultimate loads and reduced ductility. However, the degree of performance reduction is not linear; there is a threshold beyond which further thinning causes disproportionately large capacity losses.

Influence of Slenderness Ratio

The slenderness ratio (L/D) governs the global buckling behavior of the column. For short columns (low L/D), the failure is dominated by material crushing and local shell buckling. As the slenderness ratio increases, global flexural buckling becomes the governing failure mode, and the ultimate load decreases in accordance with classical column buckling theory. The transition between local and global buckling is influenced by the D/t ratio; thinner-walled pipes tend to buckle locally before global instability occurs, effectively reducing the slenderness ratio at which global buckling becomes critical.

Influence of Hoop Reinforcement

Hoop reinforcement (transverse rings welded or bolted to the exterior of the steel pipe) provides additional lateral restraint to the steel shell, delaying local buckling and maintaining the composite action over a larger deformation range. The experimental results demonstrate that hoop reinforcement can increase the ultimate load by 10-25%, depending on the spacing and thickness of the hoops. This is particularly beneficial for columns with high D/t ratios, where the unconfined shell would otherwise buckle prematurely.

Failure Mode Analysis

The failure modes observed in the tests can be categorized as follows:

• Material crushing failure: Dominant in short, thick-walled columns where the concrete core crushes before significant shell deformation occurs. The failure is characterized by concrete spalling and steel pipe ovalization.
• Local buckling failure: Characteristic of thin-walled columns, where the steel pipe develops diamond-shaped or circumferential wrinkles before concrete crushing. The concrete confinement is lost locally at the buckled regions.
• Combined failure: Most specimens exhibit a combination of local buckling followed by progressive concrete crushing, with the failure propagating from the most severely buckled region.

The load-displacement curves generally show a peak load followed by a gradual or abrupt post-peak descent, depending on the confinement effectiveness. Specimens with better confinement (thicker walls, hoop reinforcement) exhibit more gradual post-peak behavior, indicating greater energy absorption capacity.

Engineering Practice Considerations

For engineers designing thin-walled steel pipe concrete columns, the key design parameters must be carefully balanced. The D/t ratio should be kept below 30 for structural applications to ensure adequate local buckling resistance. When thinner walls are required for economic reasons, hoop reinforcement should be considered as a cost-effective means of restoring confinement capacity. The slenderness ratio must be checked against both local and global buckling criteria, with appropriate interaction effects considered.

From a fabrication perspective, the steel pipe must be manufactured to tight dimensional tolerances, as ovality and wall thickness variations significantly affect the buckling resistance. The concrete must be placed with adequate compaction to ensure full bond between the concrete and the steel pipe interior surface. In practice, this often requires the use of lightweight aggregate concrete or self-consolidating concrete for large-diameter pipes where conventional vibration is impractical.

Study Insights and Implications

This experimental study, while limited in specimen count, provides essential baseline data for understanding the behavior of thin-walled steel pipe concrete columns under axial compression. The systematic variation of D/t ratio, slenderness ratio, and hoop reinforcement establishes clear trends that can guide preliminary design decisions. The findings reinforce the importance of the confinement mechanism in steel pipe concrete columns and highlight the vulnerability of thin-walled configurations to premature local buckling.

For practical engineering applications, the study suggests that thin-walled steel pipe concrete columns can be viable structural members provided that appropriate design measures are taken to control local buckling. The use of hoop reinforcement offers a practical solution that can be implemented without significant changes to the fabrication process. Future research should extend to eccentric loading, cyclic loading for seismic design, and the development of more refined analytical models that capture the interaction between local buckling and concrete crushing in a progressive manner.