Finite Element Analysis of Pure Bending Mechanical Properties of Rectangular Hollow Sandwich Steel-Concrete Beams

Introduction and Research Context

Rectangular hollow steel tubes filled with concrete (RHSC) represent an important structural member category in modern construction, offering advantages of high strength-to-weight ratio, design flexibility, and efficient material utilization. This study focuses on the pure bending behavior of rectangular hollow sandwich steel-concrete beams through finite element analysis (FEA), providing detailed insights into stress distribution, deformation mechanisms, and failure modes that are difficult to capture through experimental testing alone.

Finite Element Modeling Approach

The FEA model employs a three-dimensional solid formulation with appropriate material constitutive models for both the steel and concrete components. The following technical specifications characterize the numerical model:

The concrete damage plasticity model incorporates key parameters including the dilation angle (30°), flow potential eccentricity (0.1), and the ratio of second to first uniaxial compressive yield stress (0.833). These parameters were calibrated against experimental data to ensure the model accurately reproduces the confined concrete behavior within the steel tube.

Mechanical Behavior Under Pure Bending

The analysis reveals several distinctive features of the bending response:

Elastic phase: The beam behaves as a composite section with an effective moment of inertia significantly higher than either the steel tube or concrete column alone. The neutral axis shifts toward the concrete core due to the difference in elastic moduli (E_steel ≈ 206 GPa vs. E_concrete ≈ 30–40 GPa).

Yield phase: Yielding initiates in the steel flanges at the extreme fibers, followed by progressive plasticization extending inward. The concrete core contributes to moment resistance primarily through compression in the upper zone and tension in the lower zone, with the steel tube providing lateral confinement that enhances concrete compressive strength by 15–30%.

Plastic phase: Full plastic hinge formation occurs when the entire steel cross-section yields and the concrete core experiences crushing in the compression zone. The ultimate moment capacity is governed by the interaction between steel plastic resistance and concrete confined compressive strength.

Failure Mode Analysis

The FEA results identify three primary failure modes:

1. Steel flange local buckling: Occurs in slender sections (high width-to-thickness ratios) where the compression flange buckles before the concrete core reaches its crushing strength. This mode is governed by the plate buckling stress and is prevented by maintaining b/t < 12ε for the compression flange.
2. Concrete core crushing: Dominates in stocky sections where the confined concrete reaches its ultimate strain (typically 0.004–0.006) before significant steel yielding occurs. The failure surface propagates diagonally from the top compression zone toward the neutral axis.
3. Interface debonding: When the bond strength between steel and concrete is insufficient (typically below 0.5 MPa), relative slip develops at the interface, reducing the effective composite action and leading to premature failure at moments 20–40% below the theoretical capacity.

Design Implications and Code Comparison

The study provides quantitative data for comparing theoretical capacity with code-based design values. For Chinese code GB 50017 and Eurocode 4 provisions, the FEA-predicted ultimate moments generally exceed code values by 10–25%, suggesting that current design codes are conservative for rectangular hollow steel-concrete beams. This conservatism may be partially justified by safety margins but could also indicate opportunities for more efficient design.

The study recommends the following practical design considerations:

• Lateral support: Rectangular sections are more susceptible to lateral-torsional buckling than circular sections. Lateral bracing spacing should not exceed 15 times the weak-axis depth.
• Shear connector design: When using sandwich construction with separate steel tubes and concrete, mechanical shear connectors (studs or bolts) should be designed to transfer at least 50% of the horizontal shear force at the interface.
• Detailing at supports: The transition from pure bending to combined bending-shear near supports requires careful design to prevent premature shear failure.

Study Insights and Reflections

The finite element approach offers unique advantages for understanding the internal mechanics of composite members, particularly in regions where experimental instrumentation is impractical. The ability to visualize stress fields, plastic strain distributions, and damage evolution provides engineers with insights that inform not only capacity assessment but also failure mode prediction and progressive collapse resistance evaluation.

However, the accuracy of FEA predictions depends critically on the quality of material models and contact definitions. The concrete damage plasticity model, while widely used, has known limitations in capturing tensile cracking behavior and post-peak softening. Future work should explore more sophisticated constitutive models, such as cohesive zone models for the steel-concrete interface, to improve prediction accuracy for complex loading scenarios.

Conclusion

The finite element analysis provides comprehensive characterization of the pure bending behavior of rectangular hollow steel-concrete beams, revealing the interplay between steel plasticity, concrete confinement, and interface bonding. The results demonstrate that these composite members offer substantial reserve capacity beyond code-based design values, while also identifying critical design parameters that govern failure mode selection. Engineers should leverage these insights for more efficient and reliable design of steel-concrete composite structural systems.