Zhuojin Pipe Fitting Co., LtdAxial Compressive Capacity of Inner Circular Outer Square Composite Steel Pipe Concrete Short Columns
Literature Overview
This study by Zhang Yang, Qian Jiaru, Ji Xiaodong, and Cao Wanlin from Tsinghua University and Beijing University of Technology (2011) presents experimental results on the axial compressive behavior of a novel composite cross-section consisting of an inner circular steel pipe and an outer square steel tube, both filled with concrete. Ten specimens were tested under concentric axial compression. The research was funded by the Beijing Science and Technology Plan Major Project (D09050600370000), the Beijing Education Commission Key Project (KZ200910005008), and the Tsinghua University Independent Research Program (20101081766). The paper was published in World Information on Earthquake Engineering (Vol. 27, No. 3, pp. 40-46).
Core Technical Content
The experimental program involved ten short column specimens with the following key characteristics:
| Parameter | Description |
|---|---|
| Cross-section type | Inner circular steel pipe + outer square steel tube, both concrete-filled |
| Number of specimens | 10 |
| Loading condition | Concentric axial compression |
| Key observation at peak load | Square tube longitudinally yielded, transversely not yet yielded |
| Post-peak ductility | At least 70% of peak capacity maintained even at 0.11 average compressive strain |
| Stiffness prediction accuracy | Calculated compression stiffness averaged 83.6% of measured values |
Failure Mode Analysis
The failure mechanism observed was characterized by outward bulging of the square tube walls, followed by local longitudinal tearing. The concrete between the square and circular tubes became friable and locally crushed. This failure mode indicates that the square tube provided significant confinement to the inner circular pipe-concrete assembly, but the confinement action was primarily through the axial load path rather than lateral restraint.
Interpretation of Key Technical Points
Confinement Effect Assessment
A critical finding is that the assumption of steel tubes providing only axial compressive capacity without lateral confinement yielded the best agreement with experimental results. This contradicts the conventional assumption in many CFST design codes that steel tubes provide significant lateral confinement to concrete.
| Design Assumption | Agreement with Test Data | Implication |
|---|---|---|
| Steel provides axial capacity only | Best agreement | Conventional confinement models overestimate capacity |
| Steel provides axial + lateral confinement | Overestimates capacity | Confinement models need recalibration for composite sections |
| Empirical confinement model (Mander-type) | Moderate agreement | Requires section-specific calibration |
Implications for Steel Pipe and Steel Tube Specification
From a steel pipe fabrication and welding perspective, several important observations emerge:
- Square tube fabrication: The square steel tube is typically fabricated by cold bending or roll forming of hot-rolled HFW or ERW pipe, or by welding four flat plates. The longitudinal welds in square tubes are critical stress concentrations, particularly at the corners where bending during fabrication induces complex residual stress states.
- Weld quality requirements: Given that the square tube yields longitudinally at peak load, the longitudinal welds must be designed and inspected to withstand this yielding condition. Full-penetration fillet welds or double-sided V-groove welds with complete fusion are essential.
- Dimensional tolerance: The gap between the inner circular pipe and outer square tube must be precisely controlled to ensure uniform concrete filling and avoid voids that would compromise the composite action.
Residual Stress and Welding Considerations
The manufacturing process of the composite section introduces several welding-related concerns:
- Thermal distortion: Welding the square tube to the inner circular pipe (if welded connections are used) introduces thermal distortion that may affect dimensional accuracy. Pre-welding fixtures and sequential welding procedures are essential.
- Residual stress interaction: The residual stresses from square tube fabrication (cold bending) interact with welding residual stresses, potentially creating regions of elevated tensile stress that are susceptible to stress corrosion cracking (SCC) in aggressive environments.
- Heat-affected zone (HAZ) properties: If the inner circular pipe is a seamless pipe and the outer square tube is welded, the HAZ of the square tube welds may have reduced toughness compared to the base metal, affecting the overall ductility of the composite section.
Engineering Practice Integration
For engineers specifying this composite section type in practice:
- Pipe material selection: The inner circular pipe should be a seamless pipe or high-quality ERW/HFW pipe with minimum yield strength of 345 MPa (Q345 equivalent) and elongation ≥ 20%. The outer square tube should be formed from steel plate meeting ASTM A572 Gr. 50 or equivalent.
- Welding procedure specification (WPS): The welding of connections between the inner and outer members requires a qualified WPS with preheating (100–150°C for carbon steel) and post-weld heat treatment (PWHT) where specified to relieve residual stresses.
- NDT requirements: All longitudinal and circumferential welds should undergo 100% ultrasonic testing (UT) per ASME V or GB/T 11345, with acceptance criteria per ASME B31.1 or equivalent.
- Dimensional control: The concentricity tolerance between the inner circular pipe and outer square tube should be within ±3 mm, and the gap for concrete filling should be uniform within ±5 mm.
Study Insights and Reflections
This research challenges the conventional design philosophy that steel confinement is the primary mechanism for enhancing concrete strength in CFST members. The finding that the steel tubes in this composite section primarily contribute axial capacity, rather than lateral confinement, has profound implications for design methodology.
From a metallurgical and welding standpoint, the observed failure mode — longitudinal yielding of the square tube without transverse yielding — suggests that the square tube's corner regions, which are typically the weakest points due to cold bending, may govern the failure. This highlights the importance of corner radius control in square tube fabrication. A minimum corner radius of 2.5 times the wall thickness (R ≥ 2.5t) is recommended to reduce stress concentration.
The ductility performance is encouraging — maintaining 70% of peak capacity at 0.11 compressive strain indicates good post-yield behavior. However, this ductility depends critically on the quality of the steel pipe material and weld integrity. Any weld defect, such as incomplete fusion, lack of penetration, or slag inclusion, could serve as a crack initiation site that would dramatically reduce ductility.
In conclusion, this study provides valuable experimental data for the design of composite steel pipe concrete sections, and its findings should inform both the structural design methodology and the steel pipe fabrication and welding quality control protocols for such applications.