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This post was last edited by Freestyle-sky on 2018-5-8 at 12:19. It originates from the “ANSYS Analysis and Design” WeChat account. Original works are respected; those interested in analysis and design may follow this account, as it updates with relevant content on a weekly basis. I heard this story just today, and I decided to record it right away – partly because I was afraid of forgetting it, and partly because I wanted to share it with everyone. In a project carried out by an engineering company, due to the extremely tight schedule, it was necessary to erect a long, slender tower on site first as a reference point for the entire construction process. Only the compressive elements of the tower were calculated; ladder platforms, pipes attached to the tower, and internal components were not installed simultaneously. What was erected on site was essentially just a bare tower with a shell and a skirt. Later, construction was suspended for other reasons, but this bare tower remained standing at the construction site for a long time, enduring a harsh winter. During periods of strong winds in winter, workers noticed that the tower swayed violently, just like branches in the wind. After passing through that long winter, inspections of the tower revealed significant cracks in the welds between the skirt and the lower head. In fact, the cause of this crack is a \"fatigue crack\" resulting from periodic vibrations of the tower caused by resonance under wind load. Although resonance was taken into account during the calculations, no practical anti-resonance measures were considered, which led to fatigue failure at the weld between the skirt and the lower head. (1) This is also why a technical requirement was later added for towers that are prone to resonance: \"Attachments such as pipeline systems attached to the tower, ladder platforms, and insulation layers should be installed simultaneously with the tower, and it is not allowed for the tower to stand bare at the site for an extended period of time.\" (2) This is also why Appendix F, \"Precautions against failure of tower-type vessels due to lateral wind resonance,\" was added when SH/T3098 was revised later. 2. When is it necessary to consider the lateral wind load calculation for towers? Since towers placed outdoors experience wind loads, they vibrate in two directions: one is vibration in the direction of the wind, with the vibration direction being consistent with the wind’s flow ； The other type is lateral vibration, where the direction of vibration is perpendicular to the wind direction. The former vibration is part of the conventional design, while the latter type of vibration is also known as wind-induced vibration. However, it is not necessary to consider lateral wind loads for all towers. According to MB/T47041, wind-induced vibrations caused by lateral wind loads need to be calculated only for slender towers with a H/D ratio greater than 15 and a height H greater than 30 meters (where H is the distance from the bottom surface of the tower’s foundation ring to the tangent line at the upper crown, and D is the average diameter of the tower), as longitudinal wind loads do not cause resonance, while lateral wind loads are the actual cause of resonance. 3. What causes resonance induced by lateral wind loads? For slender towers, the wind pressure at the top of the tower is high, resulting in large wind loads. Under the action of lateral wind forces, the wind flows around the cylindrical tower; vortices are formed on the back side of the tower and then detach. Depending on the Reynolds number Re, the formation of vortices under lateral wind forces varies as well. (1) In the range of 150 < Re < 300, it is a transitional phase, and the detachment of the vortices that form is highly irregular ； (2) In the range of 300＜Re＜3e105 (subcritical region), vortices are formed periodically on both sides behind the tower and detach from the tower surface at a certain frequency, resulting in periodic vibrations; this is what is commonly referred to as the Karman vortex street ； (3) In the range of 3e105＜Re＜3.5e106 (the transition zone), the Karman vortex street phenomenon disappears; the vortex shedding becomes irregular, resulting in random vibrations ； (4) However, in the range where Re > 3.5e106 (the supercritical region), the Karman vortex street phenomenon reappears, resulting in periodic vibrations. Fluent simulation of the flow field around Karman vortex streets. In summary, in the transition zone, due to the irregular shedding of vortices, there is no specific frequency that can cause resonance in the tower. In the subcritical and supercritical zones, however, the vortices shed in a periodic manner, resulting in a certain shedding frequency. When this shedding frequency matches the natural vibration frequency of the tower, resonance occurs. If the frequency corresponds to the natural vibration frequency of the tower’s first mode, it leads to first-order resonance; whereas if it matches the natural vibration frequency of higher-order modes, it results in higher-order resonance, thereby causing the tower to be under fatigue stress due to alternating loads over time. 4. What is the method for determining resonance? (NB/T47041-2014) 5. Preventive measures against resonance and their interpretation? The story cited by the author earlier explains the origin of Appendix F of SH/T3098-2011, \"Precautions against failure of tower vessels due to lateral wind resonance\". Below, the author will provide some explanations for the preventive measures listed in Appendix F: Preventive measure (1): The outer diameter of the skirt should be equal to the outer diameter of the lower head (i.e., the outer diameters should align) ； Reason analysis: When the outer diameters are aligned, the moment of inertia and flexural section modulus at the weld cross-section are greater than those when the inner diameters are aligned, which helps to reduce the secondary bending stress in the weld area. Precautionary measure (2): The weld joint between the skirt cylinder and the lower head should be fully penetrated as much as possible; the weld surface should be smooth, with a seamless transition to the outer surface of the lower head ； Reason analysis: Full penetration ensures a certain bending cross-sectional area, reducing internal welding defects. A smooth surface along with seamless transitions help to minimize stress concentration at the welds, thereby reducing the risk of fatigue cracks. Precautionary measure (3): The weld joints between the skirt cylinder and the lower head shall be subjected to 100% MT or PT testing, in addition to 20% UT testing, with a grade of II as the acceptance standard. Reason explanation: There is no need to elaborate on the reasons for non-destructive testing; it is done to ensure welding quality and reduce the risk of internal welding defects. Precaution (4): Add spoilers. Reason explanation: Installing perturbators with axial or spiral fins within the upper 1/3 of the tower height disrupts the wind flow, reduces the likelihood of the formation of Karman vortices, and thus effectively prevents resonance in the tower. Precautionary measure (5): Reduce the natural vibration period of the tower. Reason explanation: Reducing the natural vibration period of the tower means increasing its natural vibration frequency, in order to avoid the vortex shedding frequency and thus reduce resonance. Precautionary measure (6): During calculations, if under normal operating conditions the design wind speed exceeds the first or second critical wind speed, it is necessary to take into account lateral wind resonance. It is recommended to perform calculations for lateral wind resonance of the bare tower (i.e., without insulation, without platforms or ladders, without pipe connections, and without any internal components) after conducting the checks related to wind load moments ； It is recommended that accessories such as ladder platforms and connecting pipelines for the tower be installed simultaneously with the tower itself, and it is not allowed for the tower to stand bare at the site for an extended period. Reason analysis: On the one hand, when the tower is bare, its weight decreases, which results in greater bending moments under wind loads and more pronounced vibration ； On the other hand, since bare towers lack features such as ladder platforms and additional pipes, they are more prone to resonance. Therefore, if the stresses at all critical sections are within acceptable limits when calculating for bare towers, greater reliability is ensured under normal operating conditions. Under normal operation, the additional ladder platforms and pipes, due to their varying positions and orientations, act as flow disruptors (similar to spoilers), reducing the likelihood of the formation of Karman vortices and thus decreasing the risk of resonance.