Source: IHPA Director Frank L. Bennardo, P.E. of Engineering Express, Deerfield Beach, FL
Take a look at those overhead webcam views of traffic on the highway. Did you ever notice how when cars speed up, they spread out, and when they slow down, they come closer together? That phenomenon is very similar to air molecules during a high wind event.
When a high wind storm such as a hurricane or tropical storm spreads its winds across the land, it doesn’t just speed up the wind. It comes in gusts, constantly speeding and slowing down, across narrow pathways. In one area a big burst while just feet away a whole different scenario could be taking place. These wind events speed up and slow down wind molecules just like that of the traffic on the highway.
This effect is what engineers are really designing for in a structure. An “enclosed” building, typical of most structures, becomes pressurized inward and outward hundreds if not thousands of times from these gusty winds and molecules rapidly expanding and lowering pressure. They whip around corners and create vacuums where pressure is multiplied by both the internal and the external forces acting on a structure. The calculation for pressure is velocity squared, so pressure forces increase exponentially as wind velocities increase.
When you put up storm shutters, you are not only protecting an opening (such as a door or window) from being hit by flying debris, but you are also providing another form of protection that is often not apparent to the public. This “other” form of protection is the most important one for us to pay attention to. The build up of pressure behind the shutter can break glass or other cladding that isn’t designed to handle the same forces that the shutter system is. The shutter’s function then becomes to maintain the closure of the building so the internal pressurization isn’t multiplied and cascaded to a critical failure.
Even for more modern windows, compliant with today’s codes, the systems are not designed to handle the potentially thousands of cycles of pressure and small debris that can occur in a hurricane. If a window, for example, is rated for large missile impact (using ‘laminated glass’ – two layers of glass surrounding a synthetic material), you are protected. If not, then a shutter system should be used to keep the opening protected.
So consider all of these factors when looking for storm protection solutions and protecting yourself during a storm. Consult a licensed contractor that specializes in the installation of hurricane protection products before making any decisions. Most importantly, make sure the window, door, or protection system you are using has a Product Approval Number. Visit www.inthpa.com for more information.
And don’t be fooled by that cliché excuse ‘It stayed up during the last storm’, because the gusts of Mother Nature may not blow in your favor next time…
