Axial Compression Performance of Circular Hollow Sandwich Steel Tube Self-Consolidating Concrete Short Columns with Varying Diameter-to-Thickness Ratios

Literature Overview

This 2019 study published in the journal Concrete by researchers from Xinjiang University investigates the axial compression behavior of circular hollow sandwich steel tube columns filled with self-consolidating concrete (SCC). The sandwich construction—comprising an outer steel tube, a concrete core, and an inner steel tube—creates a confined composite system whose performance is significantly influenced by the outer tube's diameter-to-thickness ratio. Nine specimens were tested under axial compression to characterize the load-bearing capacity, ultimate engineering stress, strain behavior, and ductility.

Core Technical Findings

Influence of Outer Tube Diameter-to-Thickness Ratio

The approximately linear relationship between bearing capacity and outer tube diameter-to-thickness ratio within the studied range is a practical and useful finding. It suggests that, for a given outer diameter, increasing wall thickness (thereby decreasing the D/T ratio) reduces capacity, while reducing wall thickness (increasing D/T ratio) increases capacity—up to the point where local buckling becomes critical. The simultaneous reduction in ultimate engineering stress indicates that the composite column transitions from a more stress-efficient to a more volume-dependent failure mode as the D/T ratio increases.

Mechanical Behavior and Failure Mechanisms

The reduction in ultimate strain of both outer and inner steel tubes with increasing D/T ratio indicates that the confinement effectiveness diminishes as the outer tube becomes relatively thinner. In sandwich columns, the outer tube provides lateral confinement to the concrete core, which in turn restrains the inner tube. When the outer tube wall is thin (high D/T ratio), it is more susceptible to local buckling under the radial pressure from the confined concrete, reducing its ability to maintain confinement integrity throughout the loading history.

The significant influence on transverse displacement is particularly relevant for practical design. In real structures, transverse deformation affects the fit of connections, the stress distribution in adjacent members, and the overall structural integrity under combined loading. Engineers designing sandwich steel tube columns must account for these deformation characteristics when detailing connections and specifying tolerance requirements.

Engineering Practice Integration

For structural engineers specifying sandwich steel tube columns, this research provides a clear guideline: the outer tube diameter-to-thickness ratio should be selected to balance capacity requirements against ductility demands. In seismic applications where energy dissipation through ductile deformation is essential, a lower D/T ratio (thicker wall) is preferred despite the lower nominal capacity. In gravity-dominated structures where capacity is the primary concern and brittle failure is acceptable within code limits, a higher D/T ratio may be economical.

The use of self-consolidating concrete in these columns eliminates the need for vibration during placement, which is particularly advantageous for the confined interior space between the inner and outer tubes. The flowability and pumpability of SCC ensure complete filling of the annular space without voids, maintaining the integrity of the composite action.

Study Insights and Reflections

This work contributes to the growing body of knowledge on sandwich composite columns, a structural system that offers excellent strength-to-weight ratios and enhanced fire resistance compared to conventional steel tubes. The systematic variation of D/T ratio provides a parametric basis for rational design. Future investigations should extend to cyclic loading to characterize the hysteretic behavior and cumulative damage mechanisms, which are essential for seismic design of sandwich steel tube columns. Additionally, the interaction between the outer tube local buckling and the concrete core crushing deserves deeper finite element modeling to establish predictive failure criteria for practical design.