Wind Speed vs Wind Load

The Why

The formula that converts wind velocity to wind pressure has many variables. Each variable affects the resulting pressure for a given wind velocity.  A 100mph wind velocity for example can produce pressures as low as in the mid-teens (psf) to upwards of 50psf, even 60psf, and more*.  These variables include such features as building height, location of the area on the building, the terrain around the building, whether the building is on or near a hill, slope of the roof, size of the area in consideration, and more.

There are even two types of wind speeds and pressures that complicate things further, Allowable stress design and Load Factored Design as well as different wind codes that govern (Some use ASCE 7-05, most use ASCE 7-10, and there’s now ASCE 7-16 with yet different conversion factors for some building surfaces for the same wind velocity).  Engineers deal with a common denominator when designing structures (wind pressure) which sets a uniform standard anywhere (yes even on the moon) for a force that a product can resist.

What a product rating really looks like and what they are rated to

So you might be asking, ok if you can’t directly tell me what your product is rated to in MPH then what wind load will they handle.  Each product approval will have a very detailed engineering document that list all the approved ways a product can be installed and what the rating is for each of those installation methods.  The table to the left shows the design pressure for our accordion shutters based on the span (height) of the shutter.  As the shutter gets taller the design pressure decreases. Larger shutters will always have a lower design pressure than a smaller opening span.  To further complicate things, the type of screws holding the shutter to the wall, the amount of embedment (depth the screw goes into the wall) and the type of structure the shutter is attaching to – wood, concrete or block.  All these things will factor into determining the engineered design pressure.  If you would like an example of these engineering documents, here is our construction drawings for our accordion shutters – Click here 

Bottom Line Thought

Anyone that states your hurricane shutters or windows are rated to 140mph,… or 160mph simply isn’t accurate.  Products are rated by a design pressure (negative and positive pressure) not by wind speed.  Every product we install will meet or exceed your local building code for hurricane protection.

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