Zhuojin Pipe Fitting Co., LtdTheoretical Research on Compression-Bending-Torsion Bearing Capacity of Conical Hollow Sandwich Steel Tube Reinforced Concrete Members
Literature Overview
This study investigates the theoretical bearing capacity of a novel structural member: a conical hollow sandwich steel tube reinforced concrete (CFST) component subjected to combined compression, bending, and torsion. The conical geometry introduces a taper that varies the cross-sectional properties along the member length, while the hollow sandwich configuration—typically consisting of an outer steel tube, a concrete-steel-concrete sandwich layer, and an inner steel tube—creates a multi-layered composite action. The research develops analytical models that account for the geometric taper, the interaction between layers, and the multi-axial load combination.
Core Technical Points
The conical geometry means that the cross-sectional area, moment of inertia, and section modulus all vary linearly along the member axis. This variation introduces additional complexity because the stress distribution at any cross-section depends on the local geometry, not a constant cross-section assumption. The hollow sandwich configuration adds another layer of complexity: the outer and inner steel tubes are separated by the concrete layer, and their interaction under torsion involves shear transfer through the concrete core.
Under combined compression, bending, and torsion, the member experiences:
- Axial compressive stress from the direct load
- Bending stress that varies linearly through the depth
- Torsional shear stress that interacts with the bending stress through the von Mises or Tresca yield criterion
- Confinement effects from the steel tubes on the concrete core
- Geometric taper effects that redistribute internal forces along the member length
Analytical Model Development
| Component | Material Property | Typical Value | Role in Bearing Capacity |
|---|---|---|---|
| Outer steel tube | Yield strength fy | 345–460 MPa | Primary compression and bending resistance |
| Inner steel tube | Yield strength fy | 235–345 MPa | Secondary confinement and torsion resistance |
| Concrete core | Compressive strength fc | 30–80 MPa | Axial load sharing and shear transfer |
| Sandwich layer | Compressive strength fc' | 50–100 MPa | Enhanced confinement zone |
| Taper ratio (D1/D2) | — | 1.0–2.0 | Controls stress gradient along length |
The theoretical model typically employs an equivalent stress approach where the combined loading is reduced to an equivalent uniaxial compression problem using an interaction formula. The interaction formula must account for the non-linear stress-strain behavior of both steel and concrete, the confinement enhancement of the concrete, and the geometric taper that creates a variable section problem.
A key innovation in this research is the development of a modified interaction surface in the compression-bending-torsion space that accounts for the conical geometry. For a prismatic member, the interaction surface is well-established through standards such as AISC 360 and GB 50936. For a conical member, the interaction surface varies along the length, and the critical section may not be at the point of maximum applied moment but rather at a location where the combined stress state exceeds the local capacity.
Process and Standards Analysis
The manufacturing of conical hollow sandwich CFST members presents significant fabrication challenges. The tapered steel tubes must be formed from plate or coil using specialized rolling or spinning equipment. The sandwich configuration requires precise control of the concrete layer thickness, which is typically achieved through a formwork system that maintains the gap between the inner and outer tubes.
Welding considerations are paramount:
- The longitudinal weld of the outer tube must be qualified for the full range of wall thicknesses present along the taper
- The connection between the inner and outer tubes (if welded) must accommodate differential thermal expansion
- Circumferential welds at the sandwich layer boundaries require full-penetration butt welds with post-weld heat treatment
- The weld procedure must be qualified per ASME Section IX or ISO 15614, with specific attention to the conical geometry
Quality control during fabrication includes dimensional inspection of the taper (tolerance typically ±1 mm per meter), ultrasonic testing of all welds, and hydrostatic testing of the sandwich layer to verify concrete integrity.
Integration with Engineering Practice
Conical hollow sandwich CFST members find application in:
- Large-span bridge piers where the taper accommodates varying load demands from top to bottom
- Wind turbine towers where the conical geometry optimizes material usage against wind-induced torsion
- Industrial chimneys where the hollow sandwich configuration provides thermal insulation while maintaining structural capacity
- Seismic-resistant structures where the multi-layer configuration provides ductility through controlled yielding of the steel tubes
The theoretical model developed in this research should be validated against full-scale testing. Engineers should note that the analytical predictions may be conservative for short members (slenderness ratio L/D < 5) where local buckling of the steel tubes governs, and less conservative for slender members (L/D > 10) where global buckling under combined loading is the critical failure mode.
Key Questions and Reflections
The most challenging aspect of this research is the torsion analysis of a hollow sandwich section with a conical taper. The torsional rigidity varies along the length, and the warping constant for the non-prismatic section is difficult to determine analytically. The research should clearly state the assumptions made regarding warping restraint and the boundary conditions at the ends.
From a practical standpoint, the fabrication cost and complexity of conical hollow sandwich CFST members is significantly higher than standard prismatic CFST columns. Engineers must weigh the structural benefits—increased capacity, optimized material usage, improved seismic performance—against the additional fabrication, welding, and inspection costs. The research provides the theoretical foundation, but economic feasibility studies are needed before widespread adoption.
Study Insights and Implications
This research advances the theoretical understanding of multi-axial load-bearing capacity for complex CFST geometries. The developed interaction models can be implemented in structural analysis software to enable more accurate design of tapered CFST members. However, the practical implementation requires close collaboration between designers, fabricators, and welders to ensure that the theoretical assumptions are achievable in practice. The research highlights the need for updated design codes that address non-prismatic CFST members, as current standards such as GB 50936, AISC 360, and EN 1993-1-1 are primarily developed for prismatic sections.