Calculating wind loads on banners requires basic math skills and a calculator. Unlike calculating a wind load on a static structure, like a building, banners are flexible and flap in the wind, which creates even more tension on the anchor points. For safety purposes, it is best to overestimate the average wind speed to which a banner is subjected. Wind load may be reduced by cutting evenly-spaced slits across a banner to allow wind to pass through; however, according to World Wide Graphics, this only reduces the wind load approximately 10 to 15 percent. Please note that the values below are just estimates and should be adjusted to your project specifications.
To calculate the wind load for your banner, begin by gathering information on the maximum wind speed the banner will be routinely subjected to in your area and the planned size of your banner. For safety purposes, professional banner manufacturers recommend using a minimum estimated wind speed of 75 miles per hour. If your area is expected to experience wind speeds that exceed this range, like during a hurricane, it is a good idea to remove the banner until wind speeds return to normal.
Calculate the square footage area of the banner by multiplying length and width. For instance, a 10-foot by 10-foot banner would be 100 square feet (sf).
Calculate the pressure per square foot (psf). Take the estimated wind speed squared, then multiply by .00256. For banners, assume a minimum sustained wind speed of 75 mph, which works out to approximately 15 psf (75 x 75 x .00256).
Then multiply the banner area by the pressure per square foot to get the total wind load at ground level. In our example this is 1,500 (100 sf x 15 psf).
Next, multiply the banner pressure by the drag coefficient. According to "Fabric Architecture," the minimum drag coefficient should be 1.45, which reflects wind speeds at 15 feet above the ground. In our example this would be 2,175 pounds (1,500 x 1.45). This is the total wind load of a 10-x-10-foot banner at 15 feet above ground.
Finally, divide the final wind load by the number of fixtures to calculate the load per fixture. Assume for our example that there are 20 grommets spaced 2 feet apart around the 10-x-10-foot banner. The load per fixture would then be approximately 109 pounds (2,175 / 20).
Always consult a qualified, licensed designer and engineer before designing and mounting a banner.
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