Axial Compression Behavior of Rectangular Hollow Sandwich Steel Tube Concrete Members

Literature Overview

The paper by Xie Li, Chen Mengcheng, and Huang Hong from East China Jiaotong University and Nanchang University (Industrial Construction, Vol. 43, No. 5, 2013, pp. 128-131) presents an experimental study on six rectangular hollow sandwich steel tube concrete (RH-SCFSTC) short column specimens and one solid rectangular steel tube concrete (SCFSTC) short column for comparison. The study examines the influence of the length-to-width ratio of inner and outer steel tubes on the mechanical performance of short columns under axial compression. ABAQUS finite element analysis was used to simulate the full loading process, with results showing good agreement with experimental data.

Core Technical Findings

The experimental results reveal interesting trends in the load-bearing capacity of sandwich SCFSTC members:

Comparison Condition	Load Capacity Trend	Primary Mechanism
Same inner tube, larger outer tube	Slightly higher capacity	Increased concrete volume and confinement
Same outer tube, larger inner tube	Slightly lower capacity	Reduced concrete volume and inner tube local buckling
Solid SCFSTC vs. hollow sandwich SCFSTC	Solid slightly higher	Full concrete section provides uniform confinement

The key finding is that the hollow sandwich configuration, while offering reduced material usage and potential for core utilization (such as for piping or cable routing), does not necessarily outperform solid SCFSTC members in terms of pure axial load capacity. The reduction in concrete volume and the local buckling of the inner tube are the primary factors responsible for the capacity reduction.

Local Buckling of the Inner Tube

The local buckling of the inner tube is a critical failure mechanism that warrants detailed discussion from a welding and fabrication perspective. In sandwich SCFSTC members, the inner tube is confined by concrete on all sides, but the confinement pressure is not uniform. At the corners of the rectangular section, the stress concentration is higher, and the inner tube is more susceptible to local buckling. The weld seams of the inner tube, if present (in the case of welded rectangular tubes), can act as initiation points for buckling.

Buckling Type	Location	Trigger Condition	Preventive Measure
Local plate buckling	Flat faces of inner tube	High compressive stress	Increase wall thickness
Corner buckling	Corners of inner tube	Stress concentration	Fillet weld reinforcement
Global inner tube buckling	Along length	High slenderness ratio	Add internal bracing
Weld-initiated buckling	At weld seams	Residual stress + imperfection	Post-weld stress relief

Fabrication and Welding Challenges

The fabrication of rectangular hollow sandwich SCFSTC members involves several complex welding operations:

Inner tube fabrication: Rectangular tubes are typically formed from steel plates by bending and welding. The corner welds are critical and should be full-penetration fillet welds to ensure structural continuity.
Outer tube fabrication: Similar to the inner tube, but with larger dimensions requiring more robust welding procedures.
Inner-outer tube connection: In some configurations, the inner and outer tubes are connected by transverse stiffeners or through the concrete itself. If welded connections are used, the fit-up tolerance and welding procedure must be carefully controlled.
Concrete placement: The concrete must be placed between the inner and outer tubes, and the welding of any access openings or pour holes must be carefully executed to avoid disturbing the concrete.

Finite Element Modeling Considerations

The ABAQUS finite element model used in this study should incorporate the following elements to accurately capture the behavior:

Model Element	Purpose	Key Parameters
Concrete material model	Captures nonlinear behavior	CDP model with damage parameters
Steel material model	Captures yielding and hardening	Bilinear or multilinear kinematic hardening
Contact elements	Inner-outer tube interaction	Friction coefficient, penalty method
Mesh refinement	Near weld regions	Element size of 2-5 mm
Boundary conditions	Realistic restraint	Fixity at ends, load application

The quality of the finite element model depends heavily on the accurate representation of weld geometry and residual stresses. In practice, weld-induced residual stresses can reduce the buckling strength of the inner tube by 10-20%, which should be accounted for in the analysis.

Study Insights and Reflections

This study provides valuable experimental data on the axial compression behavior of rectangular hollow sandwich SCFSTC members, but the sample size is relatively small (six specimens plus one comparison). The findings suggest that the sandwich configuration is not inherently superior to solid SCFSTC in terms of pure load capacity, which is an important consideration for design engineers evaluating the economic viability of sandwich sections. The local buckling of the inner tube is a critical failure mode that must be addressed through appropriate design and fabrication practices. From a welding perspective, the quality of welds in both the inner and outer tubes, as well as any connecting welds, directly influences the overall structural performance. Future research should expand the specimen database, investigate cyclic loading behavior for seismic applications, and develop design equations that account for the unique failure mechanisms of sandwich SCFSTC members.