Windows that withstand hurricanes

Hurricanes are nothing new for folks in coastal areas, but what is new and evolving is our understanding of how hurricanes affect our buildings, and especially the costs that are incurred when a nasty hurricane engulfs an area.

The first thing we found is that, from a cost and destruction standpoint, the worst line in the sand is when the windows are breached and the hurricane comes inside the building, playing havoc with furniture, interior finishes, hvac equipment, electrical systems, elevators, etc. etc. Further, by allowing intense wind pressures inside building, we could be weakening them structurally, including blowing off the roof. If we could just keep the storm outside of the interior spaces, we could save billions in damage costs, and residents could continue to use their homes soon after the dust settles.

The second thing we learned is that, during hurricanes the windows were being penetrated by wind-borne projectiles. Closer to the ground, these projectiles included "large missiles" such as trees, branches and construction materials. Higher up, the damage was being caused by "small missiles" such as rocks and gravel.

The third thing we learned is that even if a window is broken, if it somehow holds together throughout the storm, the cost to replace the window will be enormously less than restoring the entire building.

These ideas gelled after Hurricane Andrew in August 1992. That monster was a Category 5, and to the date of this writing (just before Irma pounces on Florida), was the most destructive hurricane to ever reach Florida. Andrew killed 65 people and caused $26.5 billion in damages. In today's dollars, that would be $774 billion! I personally know a family who locked themselves in a closet during the peak of the storm, to find that when they emerged, the closet was the only thing left of the entire house.

The smart but battered folks in Miami and Dade Counties set to work developing building codes designed to keep the weather from penetrating the buildings through the windows. Looking around for standards they could build upon, they discovered that the aviation industry already had a standard for impact resistant airplane windshield glass. Airplanes can suffer greatly from crashing into birds, so a test method was invented that propelled chickens against airplane windshields. To learn more about this interesting crossroads of safety testing and cuisine, see Chicken Gun. The chicken gun was also known as the chicken cannon, turkey gun, or my favorite, the rooster booster.

So the Miami Dade folks blended all this together and, along with the window and testing industries, developed test methods for hurricane windows. It is called ASTM E1886, Standard Test Method for Performance of Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems Impacted by Missile(s) and Exposed to Cyclic Pressure Differentials. It was then supplemented by ASTM E1996 which specifies missiles and speeds for each type of missile. 

For windows up to 30 feet above the ground, the Large Missile test is used. Depending upon the protection level, either a 1'-9", 4' or 8' long piece of 2x4 lumber is shot at the window at a speed of 50, 40 or 80 feet per second (30.1, 27.3 or 54.5 mph).

For higher windows, the Small Missile test is used. Here, (10) steel balls of 2 grams each are fired at the window three times, all at 130 fps (88.64 mph).

In both cases, the test chamber pressure upon which the glass is mounted is cycled so that the glass bulges inward and also outward thousands of times.

Windows that pass these tests are then awarded a Notification of Acceptance (NOA) and can then be sold in Miami-Dade and other Florida counties for new construction.

After Hurricane Irma hits Florida, there will be many people interested in seeing if the hard work of Miami-Dade Counties will bear fruit and provide safer, better protected buildings. I know I will be watching.