Zhuojin Pipe Fitting Co., LtdExperimental Study on Square Hollow Sandwich CFST Members Under Combined Compression-Bending-Torsion
Literature Overview
Huang Hong and colleagues (2016, China Civil Engineering Journal, Vol. 49, No. 3, pp. 91-97) investigated the behavior of square hollow sandwich concrete-filled steel tube (SHS-CFST) members subjected to combined compression, bending, and torsion. This research extends the understanding of CFST members from simple loading conditions to the complex multi-axial stress states that occur in real structural applications such as bridge columns, tower structures, and offshore platforms.
Experimental Program and Test Configuration
The test program included five square hollow sandwich CFST specimens and two square solid CFST specimens (as comparison references) subjected to combined compression-bending-torsion loading. The primary variation parameters were slenderness ratio, hollow ratio, and axial compression ratio. The square hollow sandwich configuration features an internal square steel tube that partitions the concrete core, creating two annular concrete zones similar in concept to the circular sandwich CFST studied in other research.
Test Parameters Summary
| Specimen Type | Count | Key Variation Parameters |
|---|---|---|
| Square hollow sandwich CFST | 5 | Slenderness ratio, hollow ratio, axial compression ratio |
| Square solid CFST (reference) | 2 | Comparable parameters |
| Loading condition | All | Combined compression + bending + torsion |
The loading apparatus applied simultaneous axial compression, bending moment, and torsional moment to simulate the realistic stress state in structural members subjected to eccentric loading with torsional components. The torsion-angle relationship was recorded throughout the loading process to capture the complete nonlinear behavior.
Finite Element Analysis and Validation
ABAQUS was employed to develop finite element models that simulated the complete torque-angle relationship for the test specimens. The FE results showed good agreement with experimental curves up to the load capacity point, confirming the model's validity for predicting the elastic and pre-peak behavior. Beyond the peak load, discrepancies between FE and experimental results are common due to numerical instabilities associated with post-peak softening behavior.
Key Technical Findings and Working Mechanism
The parametric analysis revealed several important relationships governing the torsional behavior of square hollow sandwich CFST members:
| Condition | Finding | Mechanism Explanation |
|---|---|---|
| Axial compression ratio n >= 0.2 | Torsional capacity decreases with increasing n | Axial compression consumes steel tube capacity, reducing available torsional resistance |
| Small eccentricity ratio | Higher hollow ratio increases torsional capacity | Larger hollow ratio increases section modulus for torsion |
| Small eccentricity ratio | Higher hollow ratio improves ductility | Distributed concrete zones provide more uniform deformation |
Working Mechanism Analysis
The working mechanism analysis decomposed the total load into contributions from the steel tubes and concrete core. Under combined compression-bending-torsion, the outer square steel tube primarily resists bending and provides overall stability, while the inner tube contributes to torsional resistance by increasing the torsional constant of the section. The concrete core in both annular zones provides compressive resistance and contributes to shear flow resistance in torsion through the membrane action between the two steel shells.
The finding that higher hollow ratios improve torsional capacity at small eccentricities is particularly significant for design optimization. It suggests that material can be redistributed from the concrete core to the steel tubes without compromising torsional performance, potentially reducing overall material consumption.
Engineering Practice Considerations
From a fabrication perspective, the square hollow sandwich CFST configuration requires precise alignment of the inner square tube within the outer square tube. The concentricity tolerance is critical because eccentric placement of the inner tube would create uneven concrete thickness that leads to asymmetric confinement and potential premature failure. Welding of connection plates or stiffeners between the inner and outer tubes must maintain structural continuity without creating stress concentrations that could initiate cracks under torsional loading.
The combined loading condition studied here is representative of real-world scenarios such as bridge piers subjected to seismic lateral forces (creating bending and torsion) under gravity loads (creating axial compression). The research findings directly inform the design of such members where multi-axial interaction effects must be considered.
Study Insights and Reflections
This research fills an important gap in the understanding of CFST member behavior under realistic multi-axial loading conditions. The square hollow sandwich configuration demonstrates that internal steel tubes can enhance torsional performance while potentially reducing concrete volume, offering a material-efficient alternative to solid CFST sections. The interaction between axial compression and torsional capacity highlights the importance of considering load combination effects in design, as the beneficial confinement effect of axial compression does not translate uniformly across all load components. Further research on the fatigue behavior of these members under cyclic combined loading would strengthen the design basis for seismic applications.