Experimental Research and Finite Element Analysis of Local Load Transfer in Rectangular Steel Tube Truss Joints

Literature Overview

This paper by Chen Yu, Chen Yiyi, and Huang Yongqiang (2005), published in Industrial Construction (Vol. 35, No. 11, pp. 10-13), presents an experimental study on the local load transfer behavior at rectangular steel tube truss joints, complemented by finite element analysis. The research, conducted at Tongji University, investigates the through-thickness (Z-direction) stress distribution in the web of the main truss upper chord member when subjected to tension from secondary truss members, and evaluates the effectiveness of internal stiffeners.

Core Experimental Findings

The experimental results demonstrate that when the secondary truss members reach nominal yielding, the Z-direction stress in the web of the main truss upper chord member does not exceed the yield point. This finding is significant because it indicates that the local load transfer mechanism at rectangular steel tube truss joints is governed by membrane stress distribution rather than local web crippling, at least within the tested loading range.

The finite element analysis results closely match the experimental observations, validating the numerical approach for predicting local load transfer behavior. The study further compares the Z-direction stress distribution with and without internal stiffeners in the main truss upper chord member, providing quantitative evidence for the effectiveness of stiffener design.

Technical Parameters and Test Configuration

Welding and Fabrication Considerations

From the perspective of steel pipe fabrication and welding, the local load transfer at truss joints has direct implications for weld design and joint detailing. The connection between the secondary truss member and the main truss upper chord member typically involves either a welded T-joint or a welded K-joint configuration. The quality of these welds, including weld penetration, weld toe geometry, and residual stress distribution, directly affects the local load transfer efficiency.

For rectangular steel tube members fabricated from HFW or ERW welded pipe, the existing weld seam in the pipe wall introduces an inherent geometric and metallurgical discontinuity. When this seam is located in the web region near the joint, it can act as a stress concentrator and a potential crack initiation site. The welding procedure for the truss joint connection must therefore account for the interaction between the pipe's longitudinal weld and the joint connection weld.

Finite Element Analysis Insights

The finite element model used in this study captures the complex three-dimensional stress state at the joint, including the membrane stress distribution, bending stress contribution, and the interaction between the flange and web of the rectangular tube. The through-thickness stress is a critical parameter because it governs the initiation of through-thickness cracks, which are a well-recognized failure mode in tubular structures.

The comparison between stiffened and unstiffened configurations provides valuable data for stiffener design optimization. Internal stiffeners increase the local buckling resistance of the web and redistribute the stress distribution, reducing the peak Z-direction stress. The optimal stiffener spacing and thickness depend on the ratio of the member dimensions and the magnitude of the applied loads.

Key Reflections and Engineering Implications

The finding that Z-direction stress remains below yield at the nominal yield of secondary members suggests that rectangular steel tube truss joints have a reasonable reserve against through-thickness cracking under normal service loads. However, this conclusion is specific to the tested geometry and loading conditions. For joints with higher load concentrations, thinner walls, or larger member size ratios, the Z-direction stress may approach or exceed the yield limit, necessitating the use of internal stiffeners.

For engineering practice, this study provides a validated methodology for assessing the local load transfer capacity of rectangular steel tube truss joints. The combination of experimental testing and finite element analysis offers a reliable basis for joint design, and the simplified testing approach of using tension plates to simulate the secondary member loading provides a practical experimental method for future investigations.

Study Insights and Recommendations

This research contributes valuable experimental and analytical data for the design of rectangular steel tube truss joints. Engineers should incorporate through-thickness stress assessment into the joint design process, particularly for joints with high load concentrations or thin-walled members. The finite element methodology validated in this study can be extended to more complex joint configurations and loading scenarios. Internal stiffeners should be considered as a standard design feature for joints where the Z-direction stress exceeds a specified threshold, with the stiffener geometry optimized based on the stress distribution predicted by validated finite element models.