Compression-Bending-Shear Behavior of Conical Hollow Sandwich Steel Tube Concrete Members

Literature Overview

This paper by Han Yi and Wang Wenda (Gansu Agricultural University and Lanzhou University of Technology, 2021) investigates the combined compression-bending-shear behavior of conical hollow sandwich steel tube concrete (CHSSC) members using ABAQUS finite element analysis. The CHSSC member is an innovative structural component that combines a tapered (conical) outer steel tube, a hollow inner steel tube, and concrete filling the annular space between the two tubes. This configuration offers advantages in terms of weight reduction, material efficiency, and potential for modular construction.

Core Technical Points

Numerical Modeling Approach

The authors developed a calibrated finite element model in ABAQUS to simulate the complex nonlinear behavior of the CHSSC member under combined loading. The model incorporates:

• Concrete damage plasticity model to capture cracking, crushing, and confinement effects
• Bilinear or isotropic hardening model for the steel tubes
• Contact interaction between the inner and outer tubes through the concrete medium
• Geometric nonlinearity to account for the conical geometry and large deformations

Parametric Study Results

The study systematically varied seven parameters to identify their influence on the lateral shear capacity and overall structural response:

A particularly important finding is that when the inner steel tube diameter-to-thickness ratio (D/t) exceeds 60, the increase in ultimate load capacity becomes marginal. This establishes a practical upper limit for the inner tube slenderness in CHSSC member design.

Failure Modes

The numerical analysis identified several distinct failure modes depending on the loading parameters. Under high shear span ratios, flexural failure dominates with yielding of the steel tubes at the critical section. Under low shear span ratios, shear failure becomes critical, with diagonal cracking in the concrete and potential shear buckling of the outer tube. The conical geometry introduces additional complexity, as the varying cross-section creates non-uniform stress distribution and potential stress concentrations at the taper transition.

Interpretation of Technical Insights

Interaction of Loading Parameters

The study reveals a clear hierarchy in parameter importance. The shear span ratio and axial compression ratio are the dominant factors controlling failure mode transition and capacity. This is consistent with classical beam-column interaction theory, where the shear span ratio determines the relative contribution of shear versus bending, and the axial compression ratio modifies both the flexural and shear capacity through the P-M interaction.

The finding that hollow ratio and cone angle have minor effects on shear capacity is somewhat counterintuitive but can be explained by the mechanism of shear transfer. In sandwich concrete members, shear is primarily transferred through the concrete medium and the bond/friction at the tube-concrete interfaces, rather than through the steel tubes alone. Therefore, changes in the hollow ratio (which affects the steel tube proportions) have less impact on shear capacity than on flexural capacity.

Conical Geometry Effects

The conical shape of the outer tube introduces geometric nonlinearity that affects the structural response in several ways. The taper creates a varying moment of inertia along the member length, which modifies the bending stress distribution and introduces secondary moments. The hollow inner tube, combined with the conical outer tube, creates a variable annular concrete section that affects the confinement pressure distribution. These geometric effects are captured in the nonlinear finite element analysis but would be difficult to account for in simplified analytical models.

Standards and Code Relevance

The CHSSC member does not yet have dedicated provisions in current design codes. The closest applicable standards include:

The study's findings suggest that existing code provisions for circular steel tube concrete members can serve as a starting point, but modifications are needed to account for the conical geometry, the hollow inner tube configuration, and the combined shear-bending interaction specific to this member type.

Engineering Practice Integration

From a steel pipe manufacturing and fabrication perspective, the CHSSC member presents several challenges and opportunities:

• Conical tube fabrication: The tapered outer tube requires specialized manufacturing techniques such as hydroforming, spinning, or laser cutting with subsequent forming. ERW or seamless steel tubes can be used as starting stock, but the tapering process must maintain wall thickness uniformity and avoid excessive thinning at the smaller diameter end.
• Welding of conical joints: Field or shop welding of conical tube segments requires careful procedure qualification. The varying wall thickness and diameter create challenges for maintaining consistent weld geometry and penetration. SAW or FCAW processes are preferred for thicker sections, while GTAW is suitable for thinner tubes.
• Inner tube installation: The hollow inner tube must be precisely positioned within the conical outer tube to maintain a uniform annular gap for concrete filling. This requires careful dimensional control and temporary bracing during construction.
• Quality inspection: The conical geometry complicates non-destructive testing. UT and MT methods must be adapted for the varying curvature, and phased array UT (PAUT) may be necessary to detect internal defects in the tapered sections.

Key Questions and Reflections

Several aspects of this research merit further consideration. First, the numerical model relies on calibrated material models, and the accuracy of the predicted failure modes depends on the validity of these models under the complex stress states present in CHSSC members. Experimental validation with physical specimens would strengthen the findings significantly. Second, the study focuses on quasi-static loading, but the CHSSC member may be applied in seismic or impact loading scenarios where dynamic effects and strain rate sensitivity become important. Third, the long-term performance under sustained combined loading, including creep and shrinkage of the concrete component, has not been addressed.

Study Insights and Implications

This research contributes to the growing body of knowledge on advanced steel tube concrete composite members. The CHSSC configuration offers potential advantages in material efficiency and weight reduction, which are increasingly important in modern structural design. The identification of critical parameters—particularly the shear span ratio, axial compression ratio, and tube slenderness ratio—provides practical guidance for the rational design of CHSSC members. The finding that the inner tube D/t ratio should be limited to approximately 60 establishes a clear design criterion that can be incorporated into future code provisions. For steel pipe manufacturers, the CHSSC application represents an emerging market requiring specialized conical tube production capabilities, precise dimensional control, and comprehensive quality documentation. The study demonstrates that finite element analysis is a powerful tool for investigating novel structural configurations, but it also underscores the need for experimental validation to ensure the reliability of numerical predictions in engineering design.