Zhuojin Pipe Fitting Co., LtdResidual Stress Distribution in Stiffened Steel Tubes for Large-Span Transmission Towers
Literature Overview
This study by Liu Haifeng, Bian Qi, Fu Dahong, Zhu Binrong, and Yang Junfen, published in Building Structures in 2020, investigates the residual stress distribution characteristics in stiffened steel tubes intended for use as main members in large-span transmission towers. The research was supported by the National Natural Science Foundation of China (grant 51408569). Stiffened steel tubes are formed by welding longitudinal or transverse stiffening ribs to the inner wall of conventional steel tubes, creating a novel structural member that combines the advantages of circular tube cross-sections with enhanced local buckling resistance.
Research Methodology and Specimen Design
The research program involved the fabrication of twelve test specimens, comprising three different stiffening rib configurations and one configuration without stiffening ribs, with three specimens for each configuration. The residual stress measurements were conducted using the blind hole method, which is a well-established technique for measuring residual stresses in metallic components. This method involves drilling a small blind hole at the measurement location and measuring the strain relief using a strain gauge rosette, from which the residual stress can be calculated based on elastic constants and calibration factors.
The three stiffening rib configurations were designed to address the engineering requirements of large-span transmission towers, where the main members must resist significant compressive loads, bending moments, and potential buckling. The stiffening ribs are welded to the inner wall of the steel tube to provide additional local stiffness and improve the overall structural performance of the tube.
Key Findings and Residual Stress Characteristics
The study established that the residual stress field in stiffened steel tubes can be considered as the superposition of the residual stress fields from the steel tube butt weld and the stiffening rib welds. This superposition principle simplifies the analysis of the complex residual stress distribution in these composite members.
| Residual Stress Characteristic | Stiffened Steel Tube | Non-Stiffened Steel Tube |
|---|---|---|
| Residual tensile stress peak value | Essentially the same | Reference baseline |
| Residual tensile stress peak location | Essentially the same | Reference baseline |
| Residual compressive stress peak value | Higher | Lower |
| Residual compressive stress peak location | Significantly different | Reference baseline |
| Stiffening rib weld residual stress | Obvious tensile stress present | Not applicable |
| Rib weld tensile stress magnitude | Lower than butt weld tensile stress | Not applicable |
The residual tensile stress peak values and locations in stiffened steel tubes were found to be essentially the same as those in non-stiffened steel tubes, indicating that the addition of stiffening ribs does not significantly alter the primary tensile residual stress field generated by the tube butt weld. However, the residual compressive stress peak values in stiffened tubes were higher than those in non-stiffened tubes, and the locations of these compressive stress peaks exhibited significant variation. This suggests that the welding of stiffening ribs introduces additional compressive residual stresses that redistribute the existing stress field.
At the stiffening rib weld locations, obvious residual tensile stresses were observed, but these tensile stresses were significantly lower than the residual tensile stresses at the tube butt weld locations. This finding is important for fatigue assessment, as fatigue cracks typically initiate at locations of maximum tensile residual stress. The lower tensile stresses at rib welds suggest that these locations are less susceptible to fatigue crack initiation compared to the butt weld locations.
Welding Process Analysis
The residual stress distribution in stiffened steel tubes is directly determined by the welding sequence and parameters used during fabrication. The butt weld of the steel tube generates a primary residual stress field characterized by high tensile stresses at the fusion line and compressive stresses in the surrounding base metal. The subsequent welding of stiffening ribs to the inner tube wall introduces secondary residual stress fields that superimpose on the primary field.
The welding sequence is critical for controlling the final residual stress distribution. A recommended sequence involves completing the tube butt weld first, followed by the welding of stiffening ribs in a symmetric pattern to minimize additional distortion. The welding parameters, including current, voltage, travel speed, and heat input, must be carefully controlled to limit the thermal cycle severity and reduce the magnitude of residual stresses. Low-heat-input welding processes, such as pulsed GTAW or CMT welding, can be employed to minimize the thermal distortion and residual stress generation during stiffening rib attachment.
Post-weld stress relief treatments, such as thermal stress relief annealing at temperatures of 550 to 650 degrees Celsius for carbon-manganese steels, can be applied to reduce the overall residual stress magnitude. However, for stiffened steel tubes intended for large-span transmission towers, where the tubes may be very large in diameter and length, thermal stress relief may be impractical due to the risks of distortion and the logistical challenges of heating large components uniformly. In such cases, mechanical stress relief methods such as shot peening or local vibration stress relief (LVSR) may be more appropriate.
Engineering Practice Implications
The findings of this study have important implications for the design and fabrication of stiffened steel tubes for large-span transmission towers. The superposition principle for residual stress fields provides a practical framework for predicting residual stress distributions in complex stiffened tube configurations. The observation that residual tensile stress peaks are primarily determined by the tube butt weld rather than the stiffening rib welds suggests that the butt weld quality and residual stress management should be the primary focus of fabrication quality control.
For fatigue-critical applications in transmission towers, where the main members are subjected to cyclic loading from wind and ice loads, the residual stress distribution is a critical design parameter. High residual tensile stresses can significantly reduce the fatigue life of the structure by increasing the effective stress range at critical locations. The study's findings indicate that the tube butt weld locations should be the primary focus of fatigue assessment and residual stress management efforts.
Study Insights and Reflections
This study provides valuable quantitative data on residual stress distributions in a novel structural member type that is being considered for large-span transmission tower applications. The use of the blind hole method for residual stress measurement is well-suited to the practical needs of fabrication quality control, as it is a relatively straightforward technique that can be applied to finished components without significant preparation. The superposition principle established in this study offers a practical tool for predicting residual stress distributions in various stiffening configurations, enabling engineers to optimize the stiffening rib layout for minimum residual stress impact. For welding engineers involved in the fabrication of stiffened steel tubes, this research reinforces the importance of careful welding sequence planning, appropriate welding parameter selection, and consideration of post-weld stress relief measures to ensure the long-term structural integrity of these critical transmission tower components.