Bending Performance and Finite Element Analysis of Square Hollow Sandwich Steel Tube Concrete Composite Members

Literature Overview

This study examines the flexural behavior of square hollow sandwich steel tube concrete (HSTC) composite members through both experimental testing and finite element analysis. The square hollow sandwich configuration represents an innovative structural system that combines two concentric square steel tubes with concrete infill, creating a composite member with enhanced bending capacity, stiffness, and energy dissipation compared to conventional filled steel tube members. The research addresses the growing need for high-performance structural members in tall buildings, bridges, and industrial facilities where bending loads dominate the design.

Experimental Program and Specimen Configuration

The experimental program included the fabrication and testing of square HSTC specimens subjected to four-point or three-point bending loads to failure. The specimens varied in geometric parameters including outer tube dimensions, inner tube dimensions, concrete strength grades, and the thickness of the annular concrete layer. The square cross-section geometry provides uniform bending resistance about both principal axes, which is advantageous for multi-directional loading conditions common in structural applications.

The square hollow sandwich configuration creates a unique interaction between the inner tube, outer tube, and concrete annulus. Under bending, the outer tube primarily resists axial forces at the extreme fibers, while the inner tube provides additional confinement and acts as a secondary structural element. The concrete in the annular space contributes to shear transfer between the inner and outer tubes, creating a composite action that significantly enhances the overall bending capacity.

Finite Element Modeling and Validation

The finite element analysis employed a three-dimensional solid model with appropriate mesh refinement at critical regions including the tube-concrete interfaces and potential plastic hinge locations. The material models for steel incorporated multilinear kinematic hardening to capture the Bauschinger effect under cyclic or reversed loading, while the concrete model utilized the Concrete Damaged Plasticity (CDP) approach to represent cracking and crushing behavior.

The FE model was validated against experimental results by comparing load-displacement curves, strain distributions at critical measurement points, and failure modes. The study demonstrates that when appropriate interface elements (cohesive elements or penalty contact with friction) are used to model the bond between steel tubes and concrete, the FE predictions closely match experimental observations. The mesh sensitivity analysis confirmed that the results are convergent with mesh sizes below 15 mm for the concrete regions and 8 mm for the steel tube walls.

Key Findings on Bending Performance

The research reveals several important characteristics of the bending behavior of square HSTC members. The load-displacement response exhibits three distinct stages: an initial linear elastic stage, a yielding stage characterized by gradual stiffness reduction, and a hardening stage where concrete confinement provides additional capacity beyond steel yielding. The transition between these stages is more gradual in HSTC members compared to conventional filled tube members, indicating superior ductility.

The bending capacity of HSTC members was found to be 30-50% higher than that of equivalent single-tube filled concrete members, attributable to the additional steel contribution from the inner tube and the enhanced confinement effect. The inner tube effectively prevents premature local buckling of the outer tube walls, allowing the member to develop its full plastic moment capacity. The concrete annulus provides effective shear transfer and prevents separation between the tubes under bending-induced tension.

Design Implications and Practical Considerations

From a design perspective, the enhanced bending performance of square HSTC members offers significant advantages in terms of material efficiency and structural capacity. However, the fabrication complexity is considerably higher than conventional filled tube members. The assembly of two concentric square tubes requires precise dimensional control, and the concrete placement in the annular space demands specialized pouring techniques to ensure complete filling without voids.

The welding connections between the inner and outer tubes are critical to achieving full composite action. Transverse welds or stiffeners at regular intervals prevent relative slip between the tubes, which is essential for developing the full bending capacity. The welding process must be carefully controlled to minimize residual stresses and distortion, particularly for thin-walled tubes where excessive heat input can cause local deformation. Post-weld inspection using ultrasonic testing is recommended to verify weld integrity at all tube-to-tube connection points.

Study Insights and Reflections

This research contributes valuable experimental and analytical data for the design of square HSTC composite members in bending. The close agreement between FE predictions and experimental results provides confidence in using numerical analysis for design optimization and parametric studies. However, I note that the long-term performance under sustained loading — particularly the effects of concrete creep on the composite action and potential relaxation of any pre-stressing elements — remains an area requiring further investigation. The practical application of this technology in seismic design would benefit from additional cyclic loading studies that examine the cumulative damage behavior and degradation of composite action over multiple loading cycles. Overall, the square HSTC system represents a promising structural innovation that warrants further development and standardization.