Static Performance Analysis of Rectangular Steel Tube-Steel Tube Concrete Composite Truss Using Total Strain Crack Constitutive Model

Literature Overview

This paper by Lin Hongyu, Liu Yongjian, and Zhang Shuyun from Xi'an University of Science and Technology and Chang'an University (Journal of Wuhan University of Technology, Vol. 33, Issue 9, 2011, pp. 91-96) investigates the static performance of rectangular steel tube-steel tube concrete (CFT) composite trusses using a total strain crack constitutive model for confined concrete. The study, supported by the National Western Transportation Construction Science and Technology Project (2006319812112) and the Shaanxi Provincial Department of Education Science Research Plan Project (2010JK669), employs MIDAS/FEA finite element analysis to compare the performance of empty steel tube trusses, steel tube-CFT composite trusses, and CFT trusses.

Core Technical Findings

The study demonstrates that the total strain crack constitutive model effectively captures the behavior of confined concrete in CFT members and provides accurate predictions of the static performance of composite trusses. The key findings include:

• The finite element model based on the total strain crack constitutive model can accurately reflect the static performance of steel tube-CFT composite trusses
• The failure of all trusses occurs at the joints, but the failure mode differs depending on whether the main tube is filled with concrete
• The ultimate bearing capacity of all trusses is controlled by the joint behavior
• Filling the main tube with concrete effectively improves the strength and stiffness of the joints, thereby increasing the overall truss bearing capacity and reducing overall deformation

The study also validates the finite element model by comparing the predicted results with experimental data, demonstrating good agreement between the two.

Technical Parameters and Constitutive Model

The finite element analysis and constitutive model parameters are summarized below:

The total strain crack constitutive model accounts for the cracking behavior of concrete under multiaxial stress states, which is particularly important for the analysis of CFT members where the concrete is confined by the steel tube. The model incorporates the effect of confinement on the compressive strength and ductility of the concrete, as well as the cracking behavior of the concrete under tensile and shear stresses.

Joint Behavior and Failure Mode Analysis

The study's finding that the failure of all trusses occurs at the joints is particularly important for the design and fabrication of steel tube trusses. The joint behavior is influenced by several factors, including:

• The connection type and welding quality between the main tube and the diagonal members
• The presence or absence of concrete in the main tube
• The geometric configuration of the joint, including the angle between the main tube and the diagonal members
• The material properties of the steel tube and concrete

The study demonstrates that filling the main tube with concrete changes the joint failure mode from a brittle failure to a more ductile failure. This is attributed to the confinement effect of the steel tube on the concrete, which increases the ductility of the joint and allows for greater deformation before failure.

Welding and Manufacturing Considerations

From a steel pipe manufacturing perspective, the quality of the joint connections is critical for the performance of steel tube trusses. The welding process must be carefully controlled to ensure that the welds are free of defects and that the weld metal has adequate strength and ductility. The following welding considerations are important:

• The welding process should be selected based on the material thickness and the required weld quality
• The welder qualification and welding procedure specification must meet the relevant standards
• Non-destructive testing (NDT) should be performed on all critical welds to detect any defects
• The weld metal composition should be compatible with the base metal to ensure adequate strength and ductility

The study's findings also highlight the importance of the geometric configuration of the joint. The angle between the main tube and the diagonal members, the thickness of the main tube wall, and the presence of reinforcement plates all influence the joint behavior and failure mode. Engineers should carefully design the joint geometry to optimize the load transfer and minimize stress concentrations.

Engineering Practice and Reflections

In practice, steel tube-CFT composite trusses are widely used in bridge structures, including truss bridges, bridge piers, and bridge decks. The study's findings provide valuable insights into the behavior of these trusses under static loading and the role of concrete in improving the joint behavior and overall truss performance.

The study also highlights the importance of using an appropriate constitutive model for the confined concrete in the finite element analysis of CFT members. The total strain crack constitutive model is particularly suitable for capturing the cracking behavior of concrete under multiaxial stress states, which is important for the analysis of CFT members where the concrete is confined by the steel tube.

From a quality control perspective, the study underscores the importance of ensuring high-quality welding and fabrication of the steel tube trusses. Any defects in the welds or geometric imperfections in the steel tubes can significantly reduce the joint behavior and overall truss performance. Engineers should implement rigorous quality control measures during fabrication and assembly, including NDT of all critical welds and inspection of the geometric configuration of the joints.

Summary and Reference Value

This paper provides valuable insights into the static performance of rectangular steel tube-CFT composite trusses and the role of concrete in improving the joint behavior and overall truss performance. The use of a total strain crack constitutive model for confined concrete provides an accurate and practical framework for the finite element analysis of CFT members. Engineers working on steel tube trusses should consider incorporating these findings into their design and analysis procedures, particularly for applications where the joint behavior is critical. The study also highlights the importance of high-quality welding and fabrication in ensuring the performance and safety of steel tube trusses.