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- During the ‘Structural Vibration Measurement Service’ phase carried out at a natural gas transfer station, it was observed that the lightning rods on site were oscillating.
- Discussions with site personnel revealed that the rods swayed considerably at certain wind speeds, and that some rod tips had even been damaged as a result of this swaying.
Physical Infrastructure of the Case:
- The existence of an oscillation state formed at certain wind speeds indicates that the problem arises as a result of the fluid/object relationship.
- Behind a rod under laminar flow, ‘Vortex Eddies’ form at a frequency proportional to the fluid type (Strouhal number – St), flow velocity (U) and rod diameter (L).
If this vortex frequency coincides with the structural natural frequencies of the rod/structure/object, a situation known as ‘Self-Sustaining Vibration’ occurs. The vortex continuously supplies energy to the system, causing the oscillation to continue and leading to fatigue-induced damage to the structure/object after a certain period of time.
- At low wind speeds, the formation of the first mode at a frequency of 0.6 Hz is observed due to vortex shedding occurring in the lower region of the mast, while at high wind speeds, the formation of the second mode at a frequency of 2 Hz is observed due to vortex shedding occurring in the upper/middle region of the mast.
- To prevent structural damage due to fatigue, it is recommended that the mast be securely anchored to the ground using steel cables in a modal configuration.
Result:
- Structural vibration measurements in large static structures (steel construction/reinforced concrete buildings, bridges, towers, poles, etc.) should be compared with ISO 4866, ISO 2631, and DIN 4150 standards and evaluated according to structural damage and/or human comfort conditions.
- Vibrations occurring in these structures can be continuously monitored in real-time using a ‘Structural Health Monitoring System’.
- If amplitude limits are exceeded, detailed structural vibration measurement methods (modal testing, ODS, OMA, Motion Amplification, etc.) should be applied by a specialist team, and corrective actions should be determined.
