Interface Load Transfer Characteristics of CFST Truss Chord Members

Literature Overview

This 2023 paper by Zhao Baojun, Zhang Zhiheng, Cheng Gao, and Wang Pengqi, published in the Journal of Hefei University of Technology (Natural Science Edition), Volume 46, Issue 7, pages 957-962, investigates the interface load transfer characteristics between the steel tube and concrete core in concrete-filled steel tube (CFST) truss chord members. The authors are affiliated with Shaanxi Transportation Holding Group, Chang'an University, and Tibet Tianlu Co., Ltd. The research is supported by the National Natural Science Foundation of China (Grants 51778058, 51978061), the China Postdoctoral Science Foundation (2020M673601XB), and Shaanxi Provincial Transportation Science and Technology projects (17-30T, 19-14K).

Core Technical Content

The study employs finite element modeling to analyze how axial loads are transferred between the steel tube and the concrete core through the interface in CFST truss chord members. Three distinct interface conditions are examined to bracket the range of possible behavior:

Interface Modeling Conditions

The authors analyze both the top chord (compression member) and bottom chord (tension member) of the truss panel, examining axial stress distributions in the steel tube and concrete, interfacial shear stress, and the shear transfer length.

Key Results

1. Significant differences between interface conditions: The axial stress in the steel tube and the interfacial shear stress vary considerably depending on the assumed interface condition, highlighting the importance of accurate interface modeling.
2. Viscous contact as the most realistic model: The finite shear stiffness condition best represents the actual behavior of the steel-concrete interface, as it captures the gradual transfer of load through friction and mechanical interlock.
3. Shear transfer length is constant in elastic range: Under elastic working conditions, the shear transfer length remains constant and independent of the applied axial force. This is a significant finding for structural analysis simplification.
4. Exponential stress distribution within transfer length: Within the shear transfer length zone, both the axial stress in the steel tube and concrete, as well as the interfacial shear stress, follow an exponential function distribution along the truss panel length.
5. Uniform stress beyond transfer length: Outside the shear transfer length zone, the axial stresses in both the steel tube and concrete remain constant, indicating that full composite action has been achieved.

Standards and Engineering Practice Analysis

The shear transfer length concept is directly analogous to the development length concept in reinforced concrete design, where reinforcement bars require a certain embedment length to develop their full tensile capacity through bond stress. In CFST truss members, the shear transfer length represents the distance over which the steel tube and concrete core achieve their respective share of the total axial load through interfacial shear stresses.

For truss bridge design, this knowledge is critical for:

1. Strength calculations: The effective composite action only develops over a finite length from the joint, meaning that sections near joints may not fully benefit from the composite action.
2. Stiffness calculations: The stiffness of the chord member varies along its length due to the progressive development of composite action.
3. Joint detailing design: The connection details at truss joints must account for the fact that the steel tube and concrete may carry different stress levels near the joint.

From a steel pipe manufacturing perspective, the quality of the interface between the steel tube and concrete is paramount. Any voids, honeycombing, or incomplete concrete placement at the interface would reduce the effective shear transfer capacity, effectively increasing the shear transfer length and potentially compromising the structural performance. The welding quality of the steel tube itself — whether it is a seamless pipe, ERW welded pipe, or LSAW longitudinally submerged arc welded pipe — affects the overall structural integrity but does not directly influence the interface shear transfer mechanism.

Study Insights and Reflections

This paper makes a valuable contribution to the understanding of CFST truss behavior by isolating and characterizing the interface load transfer mechanism. The finding that the shear transfer length is constant in the elastic range is particularly useful for design, as it means that engineers can use a single characteristic length value regardless of the load level, simplifying the analysis considerably. The exponential distribution of stresses within the transfer length zone provides a mathematical basis for calculating the stress state at any point along the chord member.

For engineers designing CFST truss bridges, the practical implication is that the composite action is not instantaneous at the joint. There is a transition zone where the steel tube carries a disproportionate share of the load, and the concrete gradually takes up its share through interfacial shear. This has implications for fatigue assessment, as the stress concentrations near the joint may be higher than those predicted by simple composite section analysis. The viscous contact model, being the most realistic representation of the interface, should be the default choice in finite element analyses of CFST truss structures.