Masts play an essential role in modern lighting systems. While some lighting solutions might be able to make use of existing infrastructure—think of floodlights affixed to the roof of a football stadium, for instance—most require dedicated fixtures to be used. That’s where high masts come in, providing the requisite height needed to light an area effectively.
As is the case with any fixture above a certain height, masts are susceptible to wind and other forces. A strong enough breeze can cause a mast to shake and vibrate, which has an onward impact on structural stability. Long-term exposure to high winds can lead to everything from the appearance of small cracks through to all-out failure . And wind isn’t the only thing that can cause a mast to vibrate, either—as we’ll come to see.
Naturally, for safety reasons alone it’s important to minimise the risk of a mast failing as much as possible. With the addition of a heavy (and expensive) object like a luminaire, reducing that risk becomes even more important than it already is.
In this short guide, we’re looking at the impact of wind and vibration on masts and light poles—and how you can protect your own fixtures.
* A quick note before we begin: we’re using the term “masts” here, but the same guidance also applies to light poles and other support structures, too. *
Why does vibration cause damage to a mast?
Everyone knows that high winds can cause damage. Strong winds exert significant pressure, which can cause objects to bend, break, or collapse. Larger structures like buildings, signs, and trees are particularly susceptible to these dangers.
While the sheer force of a high wind can of course cause damage to a mast, however, the main issue here is actually that of vibration. When the wind blows against a mast, it causes the structure to vibrate—and those vibrations can be categorised in two different ways.
- First mode vibration
First mode—or “harmonic” vibration—is the term we use to describe a swaying mast. Imagine you’re holding a bamboo cane towards its base and shaking it gently from side-to-side. Most of the movement will occur at the top of the cane, and that’s exactly what happens to a mast under the effects of first mode vibration. - Second mode vibration
Take the same scenario as above, but now imagine you’re applying some real force as you shake the base of the cane. As the movement picks up, it won’t just sway at the top—it’ll start to oscillate at the midpoint as well, eventually bending from side-to-side. This is what’s known as second mode, or “resonance” vibration.
First and second mode vibration alike can be damaging to masts. Of the two, though, it’s second mode that tends to have the greatest impact. That’s all because of something called vibration frequency. The frequency of second mode vibration is typically much higher than first mode, enough to cause the kind of oscillation mentioned above. As a result, second mode vibration can eventually cause a mast to crack through what’s known as “material fatigue”.
That’s not to say that first mode vibration is harmless, however. While the vibration frequency may be lower, it will eventually result in material fatigue if left unchecked.
What else can cause masts to vibrate in this way?
While windage is the primary cause of mast vibration, it isn’t the only one. At airports, for instance, the turbulence generated by aircraft can also have an impact. At sea ports and terminals, meanwhile, the movement of vehicles like gantry cranes and terminal tractors can lead to what’s known as “ground transmission”. This is when vibrations pass through the floor into a mast.
Can the design of a mast help to protect against vibrations?
Absolutely, and this is where the specialist expertise offered by a provider like Midstream Lighting comes into play. A good lighting design will always take account of the local environment, and include masts that can withstand external factors like windage. Some of the factors that can have an influence here include:
- Height. The higher the mast, the more susceptible it is to wind-induced vibration.
- Materials. Strong materials like steel and aluminimum can mitigate vibration.
- Thickness. Thick mast walls can help to prevent excessive vibration.
- Shape. Round masts are less susceptible than square ones due to aerodynamics.
- Luminaire weight. Heavier luminaires can increase a mast’s susceptibility to vibration.
How do I minimise vibration and its impact on my own masts?
As mentioned above, a good lighting partner will factor windage and vibration into their designs. To ensure that you’re getting the right solution for your needs, though, you may want to ask questions such as:
- “Have you considered the environmental conditions at my site?”
Everything from geography to the built environment can have an impact on windage and vibration. The flat terrain at an airport means there are usually few objects to break up the flow of the wind, for instance. And the coastal winds at ports can also be stronger than they are inland. - “Can the masts you’ve chosen support the luminaires you’re planning to use?”
Heavy luminaires and overly tall masts are a bad combination, one that’s likely to result in structural problems sooner rather than later. A smart lighting design will always strike the right balance between weight and stability. - “Do you use damping technology in your masts?”
Dampers are designed to reduce vibration in a mast, with different types available to help tackle first and second mode vibrations. First mode dampers are designed to prevent sway at the top of a mast, while second mode dampers actually change the vibration frequency of the mast itself.
Once a mast has been installed, you can also help to ensure its longevity by performing regular visual checks.
These should be carried out:
- One week after installation.
- One month after installation
- Six months after installation
- Annually thereafter.
Key issues to check for include:
- Loose or damaged bolts.
- Missing parts higher up the mast, such as pole caps or parts of the luminaire.
- Rust, corrosion, cracks, or other signs of wear.
- Extreme movement of the mast in windy conditions.
If you notice any of the above, then you should speak to your lighting provider as soon as possible. Reputable companies will always offer an extensive warranty for fixtures and luminaires, ensuring that you’re covered in the event of any issues.
Further questions on masts or lighting design? Please get in touch—we’re always happy to help. And, for more helpful advice, why not check out:
- Upgrading to LED floodlights using existing masts: considerations
- Do I need planning permission for floodlights?
- Why are LED floodlights more efficient?