Zhuojin Pipe Fitting Co., LtdMechanical Behavior of Circular Hollow Sandwich Steel Tube Concrete Members under Compression-Torsion
Literature Overview
This research examines the structural performance of circular hollow sandwich steel tube concrete (HSSC) members subjected to combined compressive and torsional loading. The sandwich configuration consists of an inner steel tube, a concrete core, and an outer steel tube, with the two steel tubes connected by transverse stiffeners or spiral reinforcement. This hybrid cross-section offers enhanced load-carrying capacity and ductility compared to conventional steel tube concrete (STC) columns, but its behavior under combined compression-torsion is complex and not well understood in existing design codes.
Core Technical Analysis
Stress Distribution and Confinement Mechanism
Under pure axial compression, the sandwich HSSC member benefits from a dual-confinement mechanism: the inner tube confines the concrete core, while the outer tube provides additional lateral restraint through the intermediate stiffeners. The study demonstrates that this dual-confinement effect results in a 20 to 35 percent increase in ultimate compressive strength compared to a single steel tube concrete member of equivalent total steel area.
Under combined compression-torsion, the behavior becomes significantly more complex. The torsional shear stress interacts with the compressive stress to create a multiaxial stress state in both the steel tubes and the concrete core. The study identifies a critical interaction effect where the torsional component reduces the effective confinement pressure on the concrete, leading to earlier concrete crushing and a more brittle failure mode.
| Loading Condition | Axial Load (kN) | Torsion (kN·m) | Failure Mode | Ultimate Capacity (kN) |
|---|---|---|---|---|
| Pure compression | 2400 | 0 | Concrete crushing with steel tube yielding | 2400 |
| Compression + low torsion | 2000 | 80 | Shear-concrete crushing interaction | 2150 |
| Compression + medium torsion | 1600 | 160 | Spiral crack propagation, stiffener failure | 1750 |
| Compression + high torsion | 1200 | 240 | Stiffener shear fracture, premature collapse | 1320 |
Interaction Diagram and Design Implications
The study develops a normalized interaction diagram for the compression-torsion capacity of sandwich HSSC members. The interaction curve is approximately elliptical for low torsion ratios but deviates significantly toward a more conservative shape at higher torsion levels. The deviation is attributed to the progressive loss of confinement effectiveness as torsional shear stresses exceed the yield strength of the intermediate stiffeners.
A key finding is that the torsional capacity is not additive to the compressive capacity. Instead, the interaction follows a nonlinear relationship where the effective compressive capacity at a given torsional level is reduced by a factor that depends on the ratio of torsional shear stress to the yield strength of the outer tube wall.
Fabrication and Welding Considerations
The sandwich construction introduces unique fabrication challenges that directly impact structural performance:
- Stiffener weld quality: The transverse stiffeners connecting the inner and outer tubes are the critical load transfer elements. Weld defects such as incomplete fusion, undercut, and porosity at stiffener welds significantly reduce the torsional capacity. The study recommends full-penetration welds with 100 percent ultrasonic inspection for all stiffener connections.
- Concentricity control: The concentricity tolerance between inner and outer tubes should not exceed 2 percent of the outer tube diameter. Eccentricity introduces unintended bending moments that interact adversely with the combined compression-torsion loading.
- Concrete placement: In the sandwich configuration, the concrete core is placed between the two tubes. The annular space between the tubes must be filled with a bonding agent or thin-layer concrete to ensure load transfer between the inner and outer tubes through the intermediate layer.
Engineering Practice Integration
The research findings have direct implications for the design of industrial structures, offshore platforms, and special mechanical equipment where combined compression-torsion loading is expected. The study recommends incorporating a torsion interaction reduction factor into the design calculations, with values ranging from 0.90 for low torsion ratios to 0.70 for high torsion ratios.
From a quality control perspective, the study emphasizes the importance of verifying stiffener weld integrity through non-destructive testing (NDT). Pulse echo ultrasonic testing (UT) is recommended for weld root inspection, while magnetic particle testing (MT) should be applied to the weld surface for crack detection. The acceptance criteria should be based on ASME Section V or equivalent standards, with no acceptability for planar defects at stiffener weld roots.
Study Insights and Reflections
The most valuable contribution of this research is the demonstration that the sandwich HSSC configuration, while superior under pure compression, requires careful design consideration under combined loading conditions. The interaction between torsional shear and compressive confinement is not trivial and cannot be addressed through simple superposition of uniaxial capacities. Engineers designing sandwich HSSC members for applications involving significant torsional loading should adopt the interaction diagrams developed in this study and ensure that stiffener weld quality is rigorously controlled. The research also highlights the need for updated design codes to address the specific behavior of sandwich HSSC members under combined loading, as current codes primarily address pure axial or pure torsional conditions.