Finite Element Analysis of Axial Compression Performance of Square Hollow Sandwich Steel Tube Steel Fiber High-Strength Concrete Short Columns

Literature Overview

This study employs finite element analysis (FEA) to investigate the axial compression behavior of short columns composed of square hollow sandwich steel tubes filled with steel fiber reinforced high-strength concrete (SFHSC). The sandwich steel tube configuration, consisting of an outer and inner steel tube with a gap or infill between them, offers unique confinement characteristics compared to conventional single-wall steel tubes. The research aims to understand how the sandwich configuration, combined with the steel fiber reinforcement in the concrete, influences the load-bearing capacity, ductility, and failure mechanisms of the composite column.

Core Technical Points

Sandwich Steel Tube Configuration

The sandwich steel tube consists of two concentric square steel tubes with a gap between them. The gap may be filled with additional concrete, grout, or left empty depending on the design intent. This configuration provides several advantages: it increases the overall stiffness of the column, provides additional confinement through the inner tube, and allows for independent control of the outer and inner tube properties. The outer tube resists the primary axial load and provides external confinement, while the inner tube provides additional internal confinement and helps to distribute the lateral pressure more uniformly.

Finite Element Modeling Approach

The FEA model employs a three-dimensional solid element formulation with appropriate material models for each component. The SFHSC is modeled using a constitutive model that captures the nonlinear behavior of high-strength concrete, including the influence of steel fibers on the tensile and post-peak behavior. The steel tubes are modeled using an elastic-plastic material model with kinematic hardening to account for cyclic loading effects. Contact elements are defined between the concrete and the inner and outer steel tubes to simulate the bond and friction at the interfaces.

The finite element mesh is refined in regions of expected high stress concentration, such as the corners of the square tubes and the interface between the concrete and the steel tubes. A convergence study is conducted to ensure that the mesh density is sufficient to capture the stress and strain distributions accurately.

Material Constitutive Models

The SFHSC material model incorporates the effect of steel fibers on the tensile strength, post-crack behavior, and energy absorption capacity. The stress-strain relationship for SFHSC is characterized by a linear ascending branch, a peak plateau, and a post-peak descending branch that is less steep than that of plain concrete due to the fiber bridging effect. The tensile strength of SFHSC is typically 1.5-2.5 times that of plain concrete of the same compressive strength, and the post-crack tensile strength is maintained over a significant strain range due to fiber pull-out and bridging.

The steel material model uses a bilinear elastic-plastic relationship with a strain hardening modulus of 0.01-0.02 times the elastic modulus. The yield strength and ultimate strength are taken from the material test data, and the Poisson's ratio is set to 0.3 for elastic behavior and 0.49 for plastic behavior.

Axial Compression Response

The FEA results show that the sandwich steel tube configuration significantly enhances the load-bearing capacity and ductility of the SFHSC column compared to a single-wall steel tube column. The inner tube provides additional confinement that delays the onset of concrete crushing and maintains the post-peak load capacity. The steel fibers in the SFHSC improve the tensile bridging capacity, reducing the rate of strength degradation after cracking and enhancing the energy absorption capacity.

The load-displacement curve exhibits a well-defined peak followed by a gradual descending branch, indicating good ductility. The peak load is 20-35% higher than that of a single-wall steel tube column with the same overall dimensions, and the ductility factor is 1.5-2.0 times greater. The failure mode involves progressive yielding of the outer and inner tubes, with concrete crushing occurring in the confined core region between the tubes.

Process and Standards Analysis

Concrete Placement and Curing

The placement of SFHSC within the sandwich steel tube requires careful attention to ensure complete filling of the gap between the inner and outer tubes. The concrete should be placed in layers of 200-300 mm, with each layer compacted using internal vibration to ensure dense packing and eliminate voids. The steel fibers should be uniformly distributed in the concrete mix, and any segregation or balling of fibers should be avoided. Curing should be maintained for at least 14 days to ensure proper strength development of the SFHSC.

Welding and Connection Details

The inner and outer steel tubes are typically connected at the top and bottom using welded end plates or gusset plates. The welds should be full-penetration butt welds meeting the requirements of GB/T 19804 or equivalent standards. The connection details should be designed to ensure full load transfer between the tubes and prevent local buckling at the connection zones. The welds should be inspected using ultrasonic testing (UT) to verify full fusion and absence of defects.

Standards Compliance

The design and fabrication of the sandwich steel tube SFHSC column should comply with relevant standards including GB 50017 for steel structures, GB 50010 for concrete structures, and JGJ/T 239 for steel tube concrete structures. The material specifications should meet the requirements of GB/T 700 for carbon steel, GB/T 1499 for reinforcing steel, and GB/T 50119 for steel fibers. The testing protocols should follow GB/T 50152 for concrete testing and GB/T 228 for steel testing.

Quality Control and Defect Analysis

Common Defects

Several defects may compromise the performance of the sandwich steel tube SFHSC column. Void formation in the gap between the inner and outer tubes can occur if the concrete is not properly vibrated, leading to reduced load transfer and confinement effectiveness. Cracking of the SFHSC before the steel tubes yield may indicate insufficient confinement or premature concrete failure. Local buckling of the steel tubes at the mid-edges or corners can occur if the wall thickness ratio is too high or the confinement is inadequate.

Countermeasures

To prevent void formation, the concrete should be placed in thin layers with thorough vibration, and the gap should be designed to allow adequate access for vibration equipment. To prevent premature concrete cracking, the steel tube wall thickness and the inner tube confinement should be designed to provide adequate lateral restraint. To prevent local buckling, the wall thickness ratio (D/t) should be kept below 25-30, and additional stiffeners may be added at critical locations.

Study Insights and Engineering Implications

This study demonstrates that the sandwich steel tube configuration, combined with steel fiber reinforced high-strength concrete, offers a promising approach to achieving high load-bearing capacity and ductility in composite columns. The FEA results provide valuable insights into the stress distribution, deformation patterns, and failure mechanisms of the column system, which can guide the design and optimization of practical structures.

The key finding is that the inner tube provides significant additional confinement that enhances the post-peak behavior of the column. This is particularly important for applications where ductility and energy absorption are critical, such as seismic-resistant structures or columns subjected to impact loading. The steel fibers in the SFHSC contribute to the tensile bridging capacity and post-crack toughness, which are essential for maintaining the integrity of the column after cracking has initiated.

From a practical standpoint, engineers should consider the sandwich steel tube configuration as an alternative to conventional single-wall steel tubes when higher ductility and load-bearing capacity are required. The FEA model developed in this study can be used as a design tool to optimize the geometry, material properties, and reinforcement configuration of the column system for specific project requirements.

The study also highlights the importance of fabrication quality in achieving the designed performance. The placement of SFHSC within the sandwich tube, the quality of the weld connections, and the dimensional accuracy of the steel tubes are all critical factors that must be controlled during construction. Engineers should establish comprehensive quality control procedures to ensure that the fabrication and assembly of the column system meet the design requirements.