Comparative Axial Compression Performance of Circular and Square Reinforced Steel Tube Concrete Short Columns

Study Overview

Wei Hua, Zhang Zhen, and Wang Haijun published this research in the Journal of Shenyang University of Technology (Volume 39, Issue 3, 2017, pp. 352-356), supported by the Shenyang Science and Technology Plan Project (No. F16-205-1-09). The study investigates and compares the axial compressive performance of circular and square reinforced steel tube concrete (RSTC) short columns, with a focus on load capacity, deformation characteristics, composite action efficiency, and failure modes. A total of 30 specimens were tested, covering hollow steel tubes, plain concrete-filled steel tubes, and reinforced steel tube concrete columns in both circular and square cross-sectional geometries.

Experimental Configuration

The test programme was carefully designed to enable a fair comparison between circular and square cross-sections:

Specimen Category	Circular	Square	Total
Hollow steel tube	Included	Included	2 groups
Plain concrete-filled steel tube	Included	Included	2 groups
Reinforced steel tube concrete (with internal rebar)	Included	Included	2 groups
Total specimens	15	15	30

The concrete cross-sectional area, steel tube cross-sectional area, and rebar cross-sectional area were kept approximately equal between circular and square specimens to ensure a valid comparison. The specimens were subjected to monotonic axial compression loading with full load-displacement recording.

Core Findings: Load Capacity Comparison

The study found that circular steel tube concrete columns exhibit higher load capacity than square steel tube concrete columns under equivalent cross-sectional area conditions. This finding is consistent with the fundamental mechanics of confined concrete:

Uniform confinement in circular sections: In a circular steel tube, the lateral confining pressure is uniformly distributed around the entire concrete core. This uniform confinement leads to a more efficient triaxial stress state in the concrete, maximising the enhancement of concrete compressive strength.
Non-uniform confinement in square sections: In a square steel tube, the confining pressure is not uniform. The corners of the square section experience higher local stresses due to the geometric stress concentration, while the mid-span of each flat face experiences lower confinement. This non-uniformity reduces the overall efficiency of the confinement mechanism.

Quantitative Comparison

Performance Indicator	Circular Section	Square Section	Ratio (Circular/Square)
Ultimate load capacity	Higher	Lower	~1.05-1.15
Peak strain (ductility)	Higher	Lower	~1.10-1.20
Composite action efficiency	Higher	Lower	~1.05-1.10
Post-peak load degradation	More gradual	More abrupt	Qualitative

Deformation and Ductility Characteristics

The circular columns demonstrated superior deformation performance compared to the square columns. The key observations include:

Load-displacement curve shape: Circular columns exhibited a more pronounced post-peak plateau, indicating better energy absorption capacity. Square columns showed a steeper post-peak load drop, suggesting a more brittle failure transition.
Lateral expansion: The lateral expansion of circular columns was uniform and predictable, while square columns showed non-uniform lateral deformation with greater expansion at the corners and less at the mid-span of the flat faces.
Ductility coefficient: The ductility coefficient (defined as the ratio of displacement at ultimate load to displacement at peak load, or alternatively as the ratio of ultimate displacement to yield displacement) was consistently higher for circular columns.

Failure Mode Analysis

The failure modes of the two cross-sectional geometries differed significantly:

Circular Columns

Initial stage: Elastic compression with uniform stress distribution.
Concrete cracking stage: Micro-cracks initiated in the concrete core and propagated radially toward the steel tube.
Steel tube yielding stage: The steel tube yielded locally at the location of maximum lateral expansion, forming a characteristic diamond-shaped local buckling pattern.
Ultimate stage: Progressive local buckling of the steel tube with the concrete core providing continued confinement. The failure was ductile and predictable.

Square Columns

Initial stage: Elastic compression with stress concentrations at the corners.
Concrete cracking stage: Cracks initiated preferentially at the corners and propagated diagonally.
Steel tube yielding stage: The flat faces of the square tube buckled outward, while the corners remained relatively stable. This asymmetric buckling pattern reduced the overall confinement efficiency.
Ultimate stage: The failure was characterised by a combination of local buckling of the flat faces and diagonal cracking of the concrete core. The transition to failure was more abrupt than in circular columns.

Buckling Behaviour

An important finding was that square steel tube concrete columns were more susceptible to buckling deformation than circular columns. This is attributed to the lower flexural rigidity of the flat faces of the square tube compared to the uniform curvature of the circular tube. In a circular tube, the curvature provides inherent geometric stability against local buckling, while the flat faces of a square tube have no such geometric advantage.

Composite Action Efficiency

The composite action efficiency (the ratio of the actual composite column strength to the sum of the individual component strengths) was higher for circular columns than for square columns. This is because:

Better load sharing in circular sections: The uniform confinement in circular columns leads to a more balanced load sharing between the steel tube and the concrete core. Both components reach their yield or ultimate strength at approximately the same strain level.
Reduced load sharing in square sections: The non-uniform confinement in square columns leads to premature failure of the weaker regions (mid-span of flat faces) before the stronger regions (corners) reach their full capacity. This reduces the overall composite action efficiency.

Welding and Fabrication Implications

From a pipe fabrication and welding engineering perspective, the comparison between circular and square steel tubes raises several practical considerations:

Manufacturing Processes

Aspect	Circular Tube	Square Tube
Common manufacturing process	ERW, HFW, LSAW, cold drawing	HFW (hydrostatic forming), LSAW, cold bending
Weld seam configuration	Longitudinal (LSAW, ERW, HFW) or circumferential (spiral)	Longitudinal (4 seams for square)
Wall thickness uniformity	Generally better	May vary between flat faces and corners
Residual stress pattern	Relatively uniform	More complex, especially at corners

Welding Quality for Square Tubes

Square steel tubes require four longitudinal welds (for a square cross-section) or more complex welding sequences (for rectangular or multi-faceted sections). Each additional weld introduces:

Additional residual stresses that interact with the stresses from adjacent welds.
Additional HAZ regions that may have different mechanical properties from the base metal.
Additional potential defect locations that require NDT inspection.

The welding procedure for square tubes must be carefully designed to minimise the interaction between weld residual stresses. A common approach is to weld the seams in a specific sequence (e.g., opposite seams first, then the remaining two) to balance the residual stress field.