While marker balls can and do affect the vibration of overhead tensioned cables, the effects are not well-documented. Although marker balls are not considered perfect vibration reflection points, we know on theoretical grounds that their mass tends to block the flow of vibration energy. The marker balls in essence create “subspans” in which vibration energy entrapment could occur between marker balls, hindering the ability of the vibration to reach the damping devices that are typically located at the ends of the span.
Damaged Marker Ball
While it is possible that marker balls could hinder vibrational energy from reaching damping devices at span ends (or elsewhere in a span), AFL does not have many reports of damage that have been attributed to marker ball effects on vibration. Therefore, it is possible that the hindering effects of marker balls, while theoretically possible, may not in fact commonly occur in field applications to the extent that damping performance is significantly hindered. This may be due to several factors, including:
- Randomness of vibration frequencies, limiting the amount of damaging vibration cycles.
- Damping created by the presence of marker balls.
- Hindering of vibration level increases due to the presence of the marker balls.
A conservative approach for vibration protection is to apply dampers in the span between marker balls – this can be done between every set of marker balls, or dispersed throughout depending on how conservative the damping scheme is required to be. Another option is to leave the damping scheme as if no marker balls are applied – this option introduces an unknown amount of risk, however this option has likely been taken by many if not most utilities in years past.
For special cases, AFL would recommend evaluating the presence of marker balls more in-depth. These would include:
- Heavy marker balls being applied.
- Long or high tension spans
- Spans in very severe conditions such as large water/canyon crossings.
- Spans with dampers placed throughout the span (i.e. mid-span dampers).