Wind Loads on Tents
If you install or own event tents as part of your business, you need to know and understand the effect of wind on the safe operation of your tenting equipment. Every year, we hear of people being killed or injured by the unsafe use of tents in severe weather conditions. Aside from the devastating effects of the personal tragedy and the negative business consequences, these events bring a higher level of scrutiny, oversight and bureaucracy to the entire tenting industry. With a little knowledge and some safe practices, nearly all such incidents can be avoided. Safety for all tent users is the ultimate goal.
If the wind gets nasty, GET OUT!
The first and most important rule of safe tenting is that tents are not safe shelters in high wind conditions. If there is any doubt, move the party indoors to a safer location like a permanent structure.
Know Your Tent’s Engineering
Any reputable tent manufacturer should be able to provide you engineering for your tent. (Or tell you that your tent is not engineered!) You should study this document and ask questions so you fully understand what wind speed your tent can safely withstand and how to correctly anchor your tent. Both of these can vary significantly depending on the structural components used by your manufacturer and the size, shape and style of your particular tent.
Get Good at Driving and Pulling Stakes
Proper anchoring of engineered tents is the best possible defense against violent weather and the cheapest insurance you can purchase as a tent operator. Rather than skimping on the labor-intensive activity of driving and pulling stakes, embrace it as a necessary part of doing business and find ways to get good at it. Train and encourage your install crews to stake and give them the tools to do it safely and efficiently. The capital costs associated with mechanical stake drivers and mechanical stake pullers are easily offset by the daily labor savings for tent crews, and the reduced risk that results from consistently installing strong, safe tents.
The Tent Rental Division (TRD) of the Industrial Fabrics Association International (IFAI) has published a very useful tent staking guide. This pamphlet describes the holding power of stakes under various conditions, and it should be referred to for every tent installation. This same industry association has also recently published an online tool for safely calculating tent ballasting requirements, for when staking is not an option.
Wind Speed vs. Force, an Exponentially Dangerous Relationship
Determining the exact force the wind is exerting on a tent involves several factors, but by far the most important is the wind speed itself. The key thing to know is that the wind pressure increases exponentially with speed. A doubling of the wind speed increases the pressure by a factor of four. A tripling of the wind speed increases the pressure by a factor nine!
Don’t be Fooled by the ‘Zone of Complacency’
A common misconception occurs when installed tents appear to be safe and stable because the fabric is held in shape under light wind conditions. But when the wind suddenly gusts from 10 mph to 40 mph, the anchoring of the tent is suddenly required to resist 1600% more force. (Four times the wind speed, squared = 16 times more force.) The transition from the ‘zone of complacency’ to the ‘zone of terror’ can occur alarmingly fast in situations like this.
Watch the Weather, and Have a Plan
If you have tents in the air, you must watch the weather. Anchor tents appropriate to the weather conditions (with a safety factor!) and follow your manufacturer’s instructions. And if the weather turns, have a pre-arranged plan with your customer about moving the party elsewhere. Many dealers are taking the step of writing into the rental contract an extreme weather evacuation plan, identifying customer contacts in case of severe weather, and designating the person who monitors the weather and ultimately makes the call to evacuate the tent. In many cases, storms of this violent nature are not long lasting, and a short term move to a safe facility could save many lives but does not need to ruin an evening.
Tents Usually Blow UP, not Down
In high wind conditions, tents can often act like a wing, creating lift on the back/leeward side of the tent that is greater than the forces on the side facing into the wind. If not anchored correctly, tent stakes can be pulled from the ground, or on pole tents, side poles can begin to move causing the outguys to fail. Extra care and anchoring should be applied on this leeward side if you find yourself applying extra anchoring in windy conditions.
Tent Site Exposure
Tent ‘exposure’ is a term used by engineers and code officials to indicate, as you would expect, how exposed the tent site is to windy conditions. These various exposure conditions range from ‘A’ (the most protected, but now virtually unused) to ‘B’ and ‘C’ (the most common) and finally ‘D’ (the most exposed). These exposures are precisely defined in the building code based on the distance of the tent location to various natural or man-made ‘structures’ in the vicinity of the tent.
This concept is important for tent engineering (most engineered tents are designed to meet ‘C’ exposures) and site specific tent permitting. From a practical standpoint, it is also important to evaluate your tent site for the same general considerations. If you are installing a tent in a wide-open area, with open water nearby, or at the top of an open slope – in other words a location with more exposure – you should take care to provide the extra anchoring these sites deserve.