Finite Element Analysis and Experimental Study of Internal Stiffened Chord Nodes in Planar Steel Pipe Trusses

Literature Overview

This paper by Chen Yiyi, Chen Jianxing, Wang Wei, Jiang Xiaofeng, Chen Yangji, Gu Sichun, Zhou Chun, and Cao Guofeng, published in Building Structures (Vol. 34, No. 11, 2004, pp. 30-33), investigates the behavior of internal stiffened chord nodes in welded steel pipe trusses. The authors conducted both finite element analysis and scaled model testing to understand the force transmission path through internal stiffeners and to propose effective construction measures for mitigating stress concentration at the node region. The study provides theoretical basis for the design of these critical structural connections.

Core Technical Content and Force Transmission Analysis

Node Configuration and Stiffener Function

In welded steel pipe trusses, chord nodes often require internal stiffeners to enhance load transfer capacity between the chord tube and the brace or web member. The internal stiffener acts as a critical load-bearing component that bridges the gap between the brace end and the chord tube wall, providing a direct force path that reduces stress concentration at the chord-brace intersection. Without adequate stiffening, the chord tube wall may undergo local yielding or buckling under concentrated brace forces.

The finite element analysis reveals that the force transmission path through an internal stiffened node follows a complex three-dimensional stress distribution. The brace load is initially transferred to the stiffener plate through the weld connection, then distributed through the stiffener to the chord tube wall, and finally dispersed into the chord tube cross-section through bending and membrane actions. The stiffener effectively converts a localized point load into a distributed load on the chord tube wall.

Finite Element Analysis Results

The finite element model employed in this study captures the nonlinear material behavior of the steel, including yielding and strain hardening, as well as the geometric nonlinearity associated with large deformations at the node region. The mesh density near the weld line and stiffener-chord intersection is critical for accurately capturing stress concentrations.

The analysis demonstrates that the stiffener plate experiences significant bending moments and shear forces, confirming its role as an active structural member rather than a passive reinforcement. The stress distribution on the stiffener shows maximum values at the weld line connecting the stiffener to the chord tube, with a rapid decay toward the stiffener edge.

Experimental Validation

The scaled model tests were designed to validate the finite element analysis results. The test specimens replicated the geometric proportions and material properties of full-scale nodes, with appropriate scaling factors applied to loads. The experimental results showed good agreement with the numerical predictions, confirming the accuracy of the finite element model and the validity of the proposed design methodology.

Construction Measures for Stress Concentration Mitigation

Identification of Stress Concentration Zones

The study identifies several critical stress concentration zones within the internal stiffened chord node:

1. The weld toe where the brace meets the chord tube wall
2. The intersection of the stiffener plate with the chord tube inner surface
3. The stiffener edge where it terminates within the chord tube
4. The chord tube wall at the brace penetration opening

These zones are prone to fatigue cracking under cyclic loading, which is particularly relevant for bridge and crane truss applications where dynamic loads are present.

Proposed Construction Measures

The authors propose several effective construction measures to mitigate stress concentration:

The fillet weld at the stiffener-chord intersection is particularly important, as it ensures full load transfer between the stiffener and the chord tube. The weld should be a full-penetration groove weld or an equal-leg fillet weld with adequate throat thickness, typically not less than 0.7 times the stiffener thickness.

Engineering Practice and Welding Considerations

Welding Quality Requirements

The welding quality of internal stiffened chord nodes is critical for structural integrity. The following welding requirements should be enforced:

• Preheat temperature of 80-120 degrees Celsius for Q345 steel to prevent cold cracking
• Interpass temperature control between 100 and 250 degrees Celsius
• Post-weld heat treatment for thick sections to relieve residual stresses
• 100% ultrasonic testing of all load-bearing welds
• Visual inspection of all welds for surface defects

The internal stiffener welds are particularly challenging due to limited access within the chord tube. Special welding positions and techniques may be required, including vertical and overhead welding. The use of consumables with appropriate hydrogen diffusibility, such as low-hydrogen electrodes or flux-cored wire, is essential to prevent hydrogen-induced cracking.

Residual Stress Management

Welding of internal stiffeners introduces significant residual stresses in the chord tube wall, which can affect the fatigue performance and buckling resistance of the truss member. The residual stress pattern typically shows tensile stresses along the weld line and compressive stresses in the surrounding material. For critical applications, stress relief heat treatment or mechanical vibration stress relief should be applied after welding.

Key Questions and Reflections

The paper provides valuable insights into the behavior of internal stiffened chord nodes, but several questions remain open for further investigation. The fatigue performance of these nodes under realistic loading spectra, including multi-axial stress states, is not fully addressed. The long-term behavior under creep conditions, relevant for high-temperature applications, also warrants further study. Additionally, the interaction between internal stiffener welds and the chord tube buckling behavior under combined axial and bending loads could be more thoroughly examined.

From a practical standpoint, the accessibility of internal stiffener welds poses a significant challenge for quality assurance. The use of robot-assisted welding or specialized inspection techniques, such as phased array ultrasonic testing, may be necessary to ensure weld quality in confined spaces. The paper's emphasis on the stiffener as an important load-bearing component underscores the need for rigorous design and fabrication standards for these connections.

Study Insights and Implications

This study makes a significant contribution to the understanding of internal stiffened chord nodes in steel pipe trusses. The combination of finite element analysis and experimental validation provides a reliable basis for design practice. The proposed construction measures for stress concentration mitigation are practical and implementable in current fabrication facilities. For welding engineers and fabricators, the paper highlights the critical importance of weld quality in internal stiffener connections and the need for specialized welding techniques in confined spaces. The findings have direct implications for the design of bridge trusses, crane structures, and other steel pipe truss applications where internal stiffening is required to ensure adequate load transfer capacity.