Interface Load Transfer Characteristics of Steel Tube Concrete Truss Chords

Literature Overview

Zhao Baojun, Zhang Zhiheng, Cheng Gao, and Wang Pengqi conducted a finite element study on the interface load transfer characteristics of concrete-filled steel tube (CFST) truss chords, published in the Journal of Hefei University of Technology (Natural Science Edition), Volume 46, Issue 7, 2023, pages 957-962. Supported by the National Natural Science Foundation of China (Grants 51778058 and 51978061), the China Postdoctoral Science Foundation (2020M673601XB), and the Shaanxi Provincial Transportation Science and Technology Fund (17-30T, 19-14K), this work focuses on revealing the working mechanism of CFST truss chords through analysis of the steel-concrete interface behavior.

Core Technical Methodology

The researchers modeled three distinct interface conditions between the steel tube and the concrete core to investigate their influence on structural behavior:

The viscous contact model, which represents a finite interface shear stiffness, was identified as the most realistic representation of the actual steel-concrete interface behavior. This approach allows for the calculation of interfacial shear stress, axial stress distribution in both the steel tube and concrete core, and the shear transfer length along the truss panel.

Key Findings on Stress Distribution

The study reveals significant differences in axial stress and interfacial shear stress among the three interface conditions. Under the bonded condition, the steel tube and concrete core share the axial load proportionally to their respective stiffnesses throughout the entire panel length. Under the frictionless condition, each component carries only its own self-weight and any directly applied loads, with no interaction between them. The viscous contact condition produces intermediate results that closely approximate the actual behavior observed in practice.

A particularly important finding is that in the elastic working range, the shear transfer length remains constant and independent of the axial force magnitude. Within the shear transfer length, both the axial stresses in the steel tube and concrete core, as well as the interfacial shear stress, follow an exponential distribution along the truss panel length. Beyond the shear transfer length, the axial stresses in both components remain constant, indicating full composite action has been achieved.

Engineering Design Implications

For CFST truss bridge design, the shear transfer length is a critical parameter that governs the effective panel length required to achieve full composite action. The exponential stress distribution within this length means that the maximum interfacial shear stress occurs at the panel end where the load is introduced, and it decays exponentially toward the panel center. This has direct implications for connection design at truss nodes, where the load introduction occurs and the interface shear demand is highest.

Welding and Fabrication Quality Considerations

The quality of the steel-concrete interface in CFST truss chords is directly related to the concreting process and the surface condition of the steel tube. In practice, the interface bond strength is influenced by the steel tube surface roughness, the concrete mix design, the concreting method (pump placement versus drop placement), and the degree of compaction. For truss chords that may experience tension, the interface must resist tensile separation forces, which is a more demanding requirement than for compression chords.

From a welding perspective, the truss node connections introduce localized stress concentrations that affect the interface behavior. The weld geometry at the chord-to-diagonal or chord-to-end-post connection should be designed to minimize stress concentrations at the steel tube surface, as these concentrations can propagate into the concrete core and reduce the effective shear transfer length. Full-penetration groove welds at truss nodes are preferred over fillet welds to ensure continuous load transfer and reduce the risk of weld toe cracking under fatigue loading.

Study Insights and Outlook

This research provides a rigorous theoretical framework for understanding the composite action in CFST truss chords. The identification of the viscous contact model as the most realistic interface representation offers a practical modeling approach for future design studies. The finding that the shear transfer length is independent of axial force in the elastic range simplifies design calculations, as engineers do not need to recalculate this parameter for different load levels. However, this independence may not hold in the inelastic range, which warrants further investigation for seismic design applications where large deformations occur.