Static Performance of Rectangular Steel Pipe-Steel Pipe Concrete Composite Trusses Based on Total Strain Crack Constitutive Model

Literature Overview

This paper by Lin Hongyu, Liu Yongjian, and Zhang Shuyun, published in the Journal of Wuhan University of Technology (2011, Vol. 33, No. 9, pp. 91-96), presents a static performance analysis of rectangular steel pipe-steel pipe concrete composite trusses using a total strain crack constitutive model for confined concrete. Supported by the National Western Transportation Construction Science and Technology Project (Project No. 2006319812112) and the Shaanxi Provincial Department of Education Scientific Research Plan Project (Project No. 2010JK669), the study employs the MIDAS/FEA finite element program to analyze three types of trusses: hollow steel pipe trusses, steel pipe-steel pipe concrete composite trusses, and full steel pipe concrete trusses.

Core Technical Content

Composite trusses that incorporate steel pipe concrete members offer a combination of the high compressive strength of concrete-filled steel pipes and the tensile strength of hollow steel pipes. By strategically filling certain truss members with concrete, engineers can optimize the structural performance by leveraging the complementary strengths of steel and concrete in different loading scenarios.

Total Strain Crack Constitutive Model

The total strain crack constitutive model for confined concrete is the theoretical foundation of this study. This model describes the behavior of concrete under multiaxial stress states by incorporating:

Crack formation and propagation: The model defines crack initiation criteria based on strain limits and describes crack propagation through the degradation of the constitutive matrix.
Confinement effect: The lateral confinement provided by the steel pipe increases the compressive strength and ductility of the concrete core, which is captured through the confinement pressure parameter in the constitutive model.
Post-cracking behavior: After crack formation, the model describes the reduced stiffness and the transfer of stresses between the crack faces through aggregate interlock and friction.

Finite Element Modeling

Three types of trusses are modeled and analyzed:

Truss Type	Member Composition	Expected Performance
Hollow steel pipe truss	All members are hollow steel pipes	Baseline reference
Composite truss	Some members hollow, some concrete-filled	Optimized performance
Full concrete-filled truss	All members are concrete-filled steel pipes	Maximum stiffness, higher weight

The finite element model is validated against experimental data to ensure the accuracy of the constitutive model implementation and the boundary condition representation.

Key Findings

The analysis reveals several important structural behaviors:

Node-critical failure: All three truss types fail at the node connections rather than in the member spans. This indicates that the node design is the governing factor in truss capacity, regardless of the member composition.
Failure mode modification: The presence of concrete-filled members in the truss chords changes the node failure mode. In hollow steel pipe trusses, node failure typically involves local buckling or web hole weakening. In composite trusses, the concrete-filled members provide additional resistance to local deformation, shifting the failure mode to different mechanisms.
Capacity improvement: The concrete-filled members in the composite truss effectively increase the node strength and stiffness, leading to higher overall truss capacity and reduced global deformation.
Parametric sensitivity: The analysis identifies the key parameters that influence truss performance, including the number and location of concrete-filled members, the concrete strength, the steel pipe dimensions, and the node geometry.

Welding and Fabrication Considerations

The static performance of composite trusses is intimately connected to the welding quality of the node connections and the member-to-member joints. Several welding-related aspects are critical:

Node Welding Quality

The node connections in steel pipe trusses are typically welded using fillet welds, groove welds, or a combination thereof. The welding quality at these critical locations directly influences the truss performance:

Welding Aspect	Impact on Truss Performance	Quality Requirement
Weld penetration	Load transfer capacity at nodes	Full penetration or specified partial penetration
Weld geometry	Stress concentration factors	Smooth transition, no undercut or excessive reinforcement
Weld defects	Premature crack initiation	Acceptable per applicable code (e.g., GB/T 3323, ASME V)
Weld HAZ properties	Local strength and toughness	Match or exceed base metal properties

Concrete-Filled Member Welding

When welding concrete-filled steel pipe members to hollow steel pipe members at truss nodes, additional considerations arise:

Thermal effects on concrete: The welding heat input can affect the concrete near the weld zone, potentially causing thermal cracking or strength reduction. Welding procedures should include measures to limit the heat input and protect the concrete from excessive temperature.
Residual stress interaction: The welding residual stresses in the concrete-filled member interact with the confining pressure from the concrete. In some cases, the welding may relieve beneficial confining stresses, reducing the composite action in the member.
Access and positioning: The presence of concrete in some members may limit access for welding operations at the nodes, requiring specialized welding procedures and equipment.

Welding Procedure Development

For composite truss fabrication, welding procedures should be developed and qualified with the following considerations:

Heat input control: Limit the heat input to prevent thermal damage to the concrete in adjacent members.
Welding sequence optimization: Develop a welding sequence that minimizes distortion and ensures uniform stress distribution at the nodes.
Post-weld treatment: Consider post-weld heat treatment or stress relief to reduce welding residual stresses, particularly in high-strength steel applications.
Inspection requirements: Specify appropriate non-destructive testing methods and acceptance criteria for all node welds, with increased scrutiny for welds connecting concrete-filled members to hollow members.

Engineering Practice Integration

The findings of this study have direct applications in the design and fabrication of steel pipe concrete composite trusses for bridge systems, industrial structures, and other applications where the combination of high compressive capacity and tensile strength is beneficial.

Design Optimization

The parametric analysis demonstrated in this study provides a framework for optimizing the placement of concrete-filled members in composite trusses. By identifying the members that experience predominantly compressive stresses under service and ultimate loading conditions, engineers can selectively fill these members with concrete to maximize the structural benefit while minimizing the additional weight and cost.

Fabrication Quality Assurance

The node-critical failure mode identified in this study has significant implications for fabrication quality assurance. The welding quality at truss nodes should be subject to the most stringent inspection requirements, including:

100% visual inspection of all node welds
Radiographic or ultrasonic testing of a representative percentage of node welds
Dimensional verification of weld geometry against design specifications
Mechanical testing of weld coupons from qualification tests

Performance Monitoring

For critical composite truss structures, the integration of structural health monitoring systems can provide real-time data on the structural performance. Strain gauges, displacement sensors, and acoustic emission sensors can be strategically placed at the node connections to monitor the development of damage and verify that the actual performance matches the predicted behavior.

Study Insights and Conclusions

This study demonstrates that the strategic use of concrete-filled steel pipe members in truss structures can significantly improve the structural performance by enhancing the strength and stiffness of the critical node connections. The total strain crack constitutive model provides a reliable theoretical framework for predicting the behavior of confined concrete in complex structural systems, and the finite element analysis validated against experimental data offers confidence in the analytical approach.

For the steel pipe fabrication and welding industry, this work reinforces the importance of welding quality at truss nodes as the primary determinant of structural performance. The welding procedures for composite trusses should be specifically developed and qualified for the unique challenges of connecting concrete-filled and hollow steel pipe members, with particular attention to heat input control, welding sequence optimization, and comprehensive quality inspection.

The concept of composite trusses, where the benefits of steel and concrete are combined in a single structural system, represents an important advancement in structural engineering. The success of this concept depends not only on sound design principles but also on rigorous fabrication and welding quality control. By ensuring that the welding quality at all critical connections meets the highest standards, the full structural potential of composite trusses can be realized in practice, delivering structures that are both efficient and durable.