Calculating Wind Load For Banners
With fundamental math skills and a basic calculator, you will be able to calculate the wind load on a banner. Unlike buildings, banners are incredibly flexible and easily subjected to changes in wind patterns and intensity. This creates tension on the anchor points and, for safety purposes, makes it essential always overestimate the average wind speed that a banner is exposed.
To calculate the wind load for your banner, you will need to know the maximum wind speed that that banner will be routinely exposed to in your area. The size of your banner is also essential when doing this calculation. For safety purposes, many professional banner manufacturers recommend using a minimum estimated wind speed of 75 mph. Certain areas may experience wind speeds that exceed 75 mph during natural events such as hurricanes, but it is highly encouraged to remove banners during those occurrences. Please note that these values are estimates and should be adjusted to your project specifications.
In Edmonton, we mainly see strong winds in the springtime coming from the north or the west with over 100km/hr winds because of the temperature change.
As we see in the picture, the banner was not fastened correctly, using improper grommets and bolted to the side of the building. The banner in the picture is cut at the top from excessive rubbing on the edge. The banner also has an opening underneath for the wind to go behind and create an excess force on the grommets.
Calculating wind load on a banner is a crucial step when you are looking for longevity.
How to Calculate the Wind Load on Your Banner:
1. Calculate the square footage area of the banner by multiplying the length by the width. For example; a 10-foot by 10-foot banner would equal 100 square feet (sf).
2. Calculate the pressure per square foot (psf) by taking the estimated wind speed squared and then multiplied by .00256. When calculating wind load for a banner, assume a minimum sustained wind speak of 75 mph. This equates to approximately 15 psf (75 x 75 x .00256).
3. Then you will multiply the banner area by the pressure per square foot (100 sf x 15 psf). This shows that the total wind load of the banner at ground level is 1,500.
4. Next, you will multiply the banner pressure of 1, 500 by the appropriate drag coefficient. According to “Fabric Architecture”, the minimum drag coefficient should be 1.45, which reflects the wind speeds at 15 feet above the ground. For our example, we would take the banner pressure of 1,500 x 1.45 and get a total wind load of a 10×10-foot banner at 15 feet above ground to be 2,175 pounds.
5. Lastly, you will divide the final wind load by the number of fixtures to calculate the load per fixture. Assume in our example that there are 20 grommets spaced 2 feet apart from one another around the 10×10-foot banner. With the final wind load being 2,175 lbs, divide that by the 20 grommets, and the load per fixture equates to approximately 109 lbs. (2,175 / 20).
Keep in mind that by cutting evenly spaced slits across a banner to allow the wind to pass through, the wind load may be reduced. However, according to “World Wide Graphics”, this only reduces the wind load by approximately 10-15 percent.