Many of our customers who rent temporary fence will also ask for privacy screen. It goes by many different names – privacy mesh, wind screen, and even scrim – but the purpose is usually the same: to block visibility of a particular area.
Sometimes, it’s an event customer who wants to hide trailers or generators from their attendees to preserve the aesthetic of the event. Organizers of paid festivals may also want to block visibility of the stage from those outside the event. When it comes to construction clients, privacy screen is usually used as a security measure as well as a barrier for dust and debris.
Regardless of what the privacy screen is hiding, clients often feel compelled to add holes or slits to their screening in an attempt to mitigate the effects of wind.
Wind vents are holes or slits cut into vinyl or fabric screening to help reduce wind loading. Most often, they are “half-moon” or semi-circular slits – but we have even seen large slashes haphazardly cut into the screen by customers desperately trying to keep their fence upright. If you’ve ever attended a parade or march, you may have noticed circular holes punched into the parade banners. These are also wind holes, and are intended to serve the same purpose as wind slits.
Some large format printers will add wind vents if asked by the client. Other banner suppliers may refuse, citing the risk of premature ripping and questioning the effectiveness of wind vents.
Let’s revisit the parade example. You may have seen a pair of people marching in a parade, each hoisting a post with a banner spanning between them. The banners will often have wind holes to help lessen the wind load and prevent the banner from acting like a big sail.
The same applies for screening on temporary fence. Temporary structures, by their very nature, are not as stable as their permanent counterparts. Adding a screen, banner or sign – anything that can catch the wind – makes it even more likely that the fence will blow over.
Apparently, it’s not clear when wind vents started to be used. It’s possible that the idea was borrowed from parachutes, which are designed with an “apex hole” to make the airflow more stable and prevent the parachute from flapping around randomly. The idea of using wind holes to stabilize air flow around a solid object is supported by the research of aerodynamics expert B. G. de Bray, conducted in 1956.
In his experiments, B. G. de Bray used a wind tunnel to test how a flat plate with holes in it performed in an air stream. Although the experiments showed that the holes made air flow more stable, the effect on drag was negligible. Even when holes made up 20% of the area of the plate, the reduction in drag was only 5% in 120 km/h winds.
A modified version of this test was performed by The Conversation, a digital publication focused on news from the academic and research community. The authors used a wind tunnel to test the performance of small-scale fabric banners. Some banners had rectangular holes, while others had rectangular flaps.
Although this frequently-referenced experiment demonstrated that wind vents do lessen wind loading, the reduction was marginal. Interestingly, the test found that wind vents can actually increase wind load on a banner at low speeds.
Other wind tunnel tests have also found that wind vents can increase, rather than decreasing, wind loading. In two separate experiments conducted by Kalamazoo Banner Works, it was determined that the wind holes were detrimental due to the vibration and stress they caused to the fabric of the banners.
What’s more is that the vibrations caused by the wind vents could negatively impact the integrity of the supporting structure to which the banner was tied – for instance, a lamp post or metal frame. Wind captured in the area between the wind vents (creating something like an air pocket) placed additional force on the mounting brackets.
What does this mean for those who want to adorn their temporary fence with privacy screen or branded banners? Our experience tells us that adding wind vents to your screening simply isn’t enough to prevent your fence from blowing over. Even mesh screening with a relatively open weave will add weight to your fence and increase the wind loading, and we have seen it cause fence to tip over.
If wind vents don’t do much for wind load – and negatively impact the appearance of your beautiful branded banners – then what’s the alternative?
The authors of the wind tunnel study recommend reducing the overall size of your banner to help lessen the wind load. Even a modest decrease in the overall dimensions can be as effective as adding a few wind vents.
When that’s not possible – for instance, when you are required to block visibility of an entire area with fence privacy screen – your only option is to reinforce the fence. We strongly recommend using concrete blocks, wind braces, concrete jersey barriers, or some kind of anchoring solution like a t-post to properly secure your fence if you plan to hang anything from it. This includes wind screen, debris netting, banners, and even signage.
Although sandbags are commonly used to stabilize temporary fence, they may not be heavy enough when privacy screen is involved.
If you are adding privacy screening or banners to fence that is not anchored, weighted down, or otherwise reinforced with some stability measure, it’s not a question of if it will blow over – but when.
We’re here to answer any questions you may have.