Bending-Torsion Bearing Capacity of Hollow Sandwich Steel Tube Concrete Members

Literature Overview

This research investigates the bending-torsion combined bearing capacity of hollow sandwich steel tube concrete (HSSC) members, a composite structural system that integrates an outer steel tube, an inner concrete core, and an intermediate hollow layer. The study addresses a critical gap in structural engineering literature, as most existing research on steel tube concrete focuses on pure bending, pure torsion, or axial compression, while combined loading scenarios—particularly bending-torsion—are encountered frequently in offshore platforms, wind turbine towers, and complex bridge structures. The research employs both experimental testing and numerical simulation to establish load-bearing envelopes, interaction curves, and failure mechanisms under variable bending-torsion ratios.

Core Technical Content and Key Parameters

The hollow sandwich configuration introduces unique mechanical behavior compared to conventional solid-core steel tube concrete (SRC) members. The hollow intermediate layer serves multiple functions: it reduces overall weight by approximately 15–25% relative to solid-core counterparts, provides a pathway for reinforcement or additional materials, and introduces a geometric discontinuity that modifies stress distribution patterns. The study examines outer steel tube specifications typically ranging from Φ219–Φ426 mm in outer diameter with wall thicknesses of 8–16 mm, constructed from Q345B or Q390B grade structural steel conforming to GB/T 1591 and GB/T 699 standards.

The experimental program involves cyclic loading protocols that apply controlled bending moments and torsional moments simultaneously, with the ratio M/T varied to generate a comprehensive interaction envelope. The torsional loading is applied through a dedicated torsion rig that grips the specimen at both ends while a hydraulic actuator applies lateral displacement at the midspan to induce bending. Strain gauges, displacement transducers, and torque sensors capture full-field data throughout the loading history.

Failure Mechanisms and Stress Analysis

The failure behavior of HSSC members under combined bending-torsion loading exhibits distinct characteristics that differ markedly from solid-core SRC members. In the outer steel tube, principal stress trajectories rotate from the pure bending orientation (longitudinal tension/compression) toward a 45-degree helical pattern as torsion increases, indicating a transition from flexural yielding to torsional shear failure. The intermediate hollow layer creates a stress concentration zone at the inner tube junction, where the shear flow from the outer tube must redistribute around the hollow cavity.

The concrete core experiences a complex three-dimensional stress state. Under pure bending, the concrete primarily bears compressive stresses on the compression face and tensile stresses on the tension face. As torsion is introduced, the concrete develops diagonal compressive struts at approximately 45 degrees to the member axis, which interact with the bending-induced compressive zone. This interaction can lead to premature crushing of the concrete in the region where the bending compression and torsional compression coincide, typically at the compression-side corners of the cross-section.

The interface between the steel tube and concrete is particularly critical in the hollow sandwich configuration. The hollow layer reduces the lateral confinement effect that the outer tube would otherwise provide to the concrete, potentially leading to earlier debonding under combined loading. The study quantifies this through interface shear stress measurements and observes that the peak interface shear stress decreases by approximately 20–35% compared to solid-core SRC members of equivalent outer tube dimensions.

Numerical Simulation and Model Validation

Finite element analysis (FEA) is employed using ABAQUS software with a three-dimensional shell-solid coupled model. The outer and inner steel tubes are modeled using S4R shell elements with five integration points through thickness, while the concrete core is discretized using C3D20R 20-node reduced integration solid elements. The concrete is characterized using the Concrete Damaged Plasticity (CDP) model, which captures the nonlinear behavior through independent tensile and compressive damage variables governed by a multi-surface yield criterion.

The numerical model is validated against experimental data by comparing load-displacement curves, strain distributions, and failure modes. The average deviation between predicted and experimental ultimate loads is approximately 8%, with individual specimens showing deviations within ±12%. The model adequately captures the interaction between bending and torsion, including the progressive yielding sequence and the transition from elastic to fully plastic behavior.

Engineering Practice Implications

The research provides direct value for engineers designing composite members subjected to combined bending and torsion, particularly in offshore and marine engineering applications. The established interaction curves can be incorporated into design codes or used as supplementary design tools. For practical engineering applications, the following observations are particularly significant: the hollow sandwich configuration offers a favorable strength-to-weight ratio, but the reduction in concrete confinement must be compensated through enhanced interface treatment, such as corrugated inner tube surfaces or additional shear connectors. The study recommends that for bending-torsion ratios exceeding 0.6, the hollow sandwich configuration should be used with caution, as the combined stress state approaches the concrete crushing limit more rapidly than in solid-core configurations.

The research also highlights the importance of residual stress management in the outer steel tube. Residual stresses from the cold-bending or hot-rolling process can reduce the effective bending-torsion capacity by 5–10%. Engineers should specify stress-relieved material or account for residual stress effects in the design calculations. The interaction between manufacturing-induced residual stresses and service loading deserves further investigation, particularly for large-diameter tubes where the rolling residual stress pattern is more complex.

Study Insights and Reflections

This research represents a meaningful advancement in understanding the behavior of hollow sandwich composite members under combined loading. The systematic variation of the bending-torsion ratio provides a comprehensive picture that can be directly applied to design practice. However, the study primarily focuses on static loading conditions, and the behavior under fatigue loading—particularly relevant for offshore platforms subjected to wave-induced cyclic loads—remains unexplored. Future research should extend to cyclic loading protocols and investigate the cumulative damage effects of repeated bending-torsion loading on the hollow sandwich interface. Additionally, the scale effect of the hollow layer thickness and its optimal proportion relative to the overall cross-section warrant further parametric investigation to establish design guidelines for the hollow sandwich configuration.