Bending Performance of Square Hollow Sandwich Concrete Filled Steel Tube Composite Members

Literature Overview

The paper by Li Minglun et al. (2022), published in the Journal of Shenyang Jianzhu University, investigates the flexural behavior of square hollow sandwich concrete-filled steel tube (CFST) composite members through finite element analysis. The research is supported by multiple national and provincial funding programs, including the National Natural Science Foundation of China (Grant 51808351) and the Liaoning Province Key R&D Program. The authors establish a validated FE model, define five mechanical characteristic points along the load-deflection curve, and propose a superposition-based calculation formula for flexural bearing capacity.

From a steel tube manufacturing and welding perspective, this topic is particularly relevant because square hollow sandwich CFST members involve inner and outer square steel tubes connected through composite action, which introduces significant challenges in tube fabrication, welding of composite joints, and material compatibility.

Core Technical Findings

The study identifies five key characteristic points on the load-midspan deflection curve: elastic stage, yielding point, peak load point, descending branch point, and residual strength point. The authors systematically analyze cross-sectional stress distributions at each of these points, revealing how the hollow ratio, longitudinal reinforcement ratio, material strength, and section aspect ratio influence flexural capacity and stiffness.

The proposed superposition formula yields a mean ratio of 1.030 and standard deviation of 0.080 when compared with FE results, indicating acceptable accuracy for engineering estimation.

Welding and Fabrication Implications

For a steel tube fabrication engineer, the hollow sandwich configuration demands careful attention to several manufacturing aspects. The inner and outer square tubes must be manufactured with tight dimensional tolerances to ensure uniform concrete annulus thickness, which directly affects composite action and stress distribution. The welding of any internal connection between the inner and outer tubes must be performed using processes such as GTAW or GMAW with appropriate heat input control to avoid excessive distortion of the thin-walled inner tube.

The study's finding that longitudinal reinforcement ratio has a significant effect on flexural capacity implies that the welding of reinforcing bars to the inner tube wall must achieve full penetration and adequate mechanical properties. From a metallurgical standpoint, the heat-affected zone (HAZ) of carbon steel tubes exposed to welding processes must be monitored to prevent softening that could compromise the composite member's load-bearing capacity, especially in the yield and post-yield stages identified by the authors.

Study Insights and Reflections

The superposition principle applied in this study is a simplification that assumes independent contributions from the outer tube, inner tube, and concrete layers. In practice, the bond stress at the steel-concrete interface plays a critical role, and this is influenced by surface roughness of the steel tube, which is affected by the manufacturing process. Seamless tubes typically provide better bond performance than welded tubes due to the absence of weld seams that can create local stress concentrations. Engineers working on similar composite members should consider specifying seamless square tubes or at minimum ensuring that weld seams in ERW or HFW tubes are oriented to minimize their effect on flexural stress distribution.

The validation approach using FE analysis with five characteristic points is methodologically sound, but the study would benefit from experimental verification to confirm that the composite action assumptions hold under cyclic loading conditions, which are common in seismic applications. Overall, this work provides a useful analytical framework for the preliminary design of hollow sandwich CFST composite members, but fabrication and welding quality remain critical factors that can significantly influence the actual structural performance.