Axial Compression Behavior of Circular Hollow Sandwich Steel Tube Self-Consolidating Concrete Short Columns

Literature Overview

This study by Xu Peng, Liu Qing, Han Fengxia, Cao Dianyu, and Ning Jun, published in Concrete Magazine in 2019, investigates the mechanical behavior of circular hollow sandwich steel tube columns filled with self-consolidating concrete (SCC). Funded by the National Natural Science Foundation of China, the research examines nine specimens with varying outer steel tube diameter-to-thickness ratios under axial compression loading.

Structural Configuration and Design Rationale

The circular hollow sandwich steel tube column consists of an outer steel tube, an inner steel tube, and self-consolidating concrete filling the annular space between them. This configuration offers several engineering advantages:

Material efficiency: The hollow center reduces concrete volume while maintaining structural performance.
Composite action: Both steel tubes provide confinement to the surrounding SCC, enhancing ductility and post-peak behavior.
Construction simplicity: SCC eliminates the need for vibration during placement, critical for the confined annular geometry.

Experimental Results and Key Findings

The test program comprised nine specimens with systematically varied outer tube diameter-to-thickness ratios (D/t). The following relationships were established:

Parameter Variation	Bearing Capacity Trend	Ultimate Engineering Stress	Ultimate Strain (Inner/Outer Tube)	Ductility
D/t increase (within range)	Linear increase	Decreases	Decreases	Degrades
D/t increase effect on displacement	Large influence on longitudinal and lateral displacement	-	-	-

The linear increase in bearing capacity with D/t ratio is attributed to the increased cross-sectional area of the outer tube contributing to load-carrying capacity. However, the degradation in ductility with increasing D/t ratio reflects the reduced confinement effectiveness as the tube becomes relatively thinner compared to its diameter.

Metallurgical and Materials Considerations

From a materials engineering perspective, several critical aspects merit attention:

Steel tube material properties: The outer and inner steel tubes should be fabricated from structural steel grades with adequate yield strength and elongation to ensure proper composite action. Q235B or Q355B grades (per GB/T 1591) are commonly used, providing a good balance of strength and ductility.

Self-consolidating concrete design: The SCC mix must achieve complete filling of the annular space without vibration. Key mix design parameters include:

Flow diameter: 600-650 mm (per GB/T 50496)
V-funnel time: 4-10 s
L-box ratio: > 0.85
Compressive strength: Typically C40-C60 for structural applications

Welding of steel tubes: The inner and outer tubes must be joined at the column ends to prevent separation under compressive loading. Full-penetration butt welds (typically GTAW followed by FCAW or SAW) are required, with 100% ultrasonic testing per GB/T 11345.

Defect Analysis and Quality Control

Defect Type	Cause	Consequence	Prevention
Incomplete concrete filling	Insufficient SCC workability	Reduced composite action	Verify SCC flow properties before placement
Weld lack of fusion	Inadequate heat input or improper technique	Reduced confinement effectiveness	100% UT inspection of all welds
Tube ovality	Manufacturing tolerance	Non-uniform confinement pressure	Control manufacturing tolerances per GB/T 8163
SCC segregation	Poor mix design	Non-uniform properties	Optimize aggregate gradation and binder content

Engineering Practice Applications

This sandwich steel tube column concept is particularly suited for:

High-rise building cores requiring large column capacity with material efficiency
Seismic regions where ductility is critical
Corrosive environments where the inner tube provides additional protection for the concrete core
Applications where construction speed is paramount due to SCC placement simplicity

Study Insights and Design Recommendations

The research clearly demonstrates that the diameter-to-thickness ratio of the outer tube is the dominant geometric parameter governing both strength and ductility. For optimal design, a D/t ratio should be selected to balance capacity requirements against ductility demands. The use of SCC in this application is particularly advantageous as it ensures complete filling of the annular space without the quality risks associated with conventional concrete placement in confined geometries. Future research should extend to cyclic loading to evaluate seismic performance and to investigate the effects of different steel grades and concrete strengths on the composite behavior.