Technology Sectors
Disaster Preparedness 2011: New wind-damage-resistant windows
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Gantt Miller |
The word tornado came from the Spanish word "tronada" which means a thunderstorm. Also known as a twister, a tornado is a violent spiral-shaped storm with a rapidly rotating column of air rising upward, forming a vortex. The vortex has relatively low pressure at the center and is shaped like a funnel.
Tornadoes that occur over oceans are called waterspouts, which are usually weaker than tornadoes. Tornadoes are nature’s most violent storms, sometimes reaching ground wind velocities of more than 200 mph, compared with typical hurricane velocities of under 150 mph.
This April, more than 300 people died in more than 600 tornado-velocity storms -- the worst month for U.S. tornadoes, smashing the previous record of 267 set in 1974, according to the U.S. Weather Service. Alabama alone suffered more than 200 fatalities from storms that centered around Tuscaloosa. In late May, Joplin, MO, was devastated by a twister that took another 125 lives in that city of 50,000.
Evolving window technology, testing and certification
Window technology has evolved over the years to the point where windows can be selected not only for their aesthetic qualities, but also for their protective performance capabilities. For example, windows can be made using laminated glass that resists impact from flying debris in hurricanes and, most recently, to meet the more stringent missile standards for tornadoes.
When focusing on the use of laminated glass in wind-hazard areas, it is crucial to understand the importance of the overall system design, including:
- Glass size of the window or door opening;
- Glazing materials thickness and composition;
- Framing component design;
- Glazing installation within the frame;
- The anticipated transfer of event loads to the structure;
- Transfer of glass fragments after the event;
- Water penetration after the event.
Certification testing of hurricane-rated windows conforms to a standardized and approved method, such as the Miami-Dade County hurricane impact test or the ASTME 1996 Standard Specification.
Large missile tests are performed both in-house and at certified test laboratories. A 2 x 4 section of lumber is fired from a special test cannon at specified speeds. Small missile tests use a 2 g steel ball fired from the same test cannon.
The cyclic pressure test subjects a specimen to impact and, after impact, to about four hours or more of positive and negative pressure cycles (9,000 cycles). This works the system back and forth, testing its ability to resist fatigue and the fastener systems to keep it attached. This simulates the forces a major storm will generate on the system and its connections to the structure of the building.
Note that some manufacturers may talk about meeting or passing the large missile test, but unless their products also withstand the cyclic pressure tests, they have not qualified the product as a large missile resistant system.
First AAMA standard for tornado windows
In early 2011, the American Architectural Manufacturers Association (AAMA) released a consensus standard for tornado windows that calls for impact testing from a 15-pound, 12-foot-long wood 2 x 4, fired from a test cannon at a speed of 100 mph, simulating a 250 mph wind. The kinetic energy generated in the tornado impact test has more than five times the impact of a large missile Class E hurricane test.
AAMA 512-11, Voluntary Specifications for Tornado Hazard Mitigating Fenestration Products, uses existing test methods and other procedures to qualify windows and other glazed fenestration products for hazard mitigation. The newly-released document provides a system for rating the ability of windows to withstand impact, pressure cycling and water penetration, which are generally associated with tornado conditions.
Tornado window applications
The new AAMA tornado standard can be applied to curtain wall, storefront and, perhaps, sloped glazing systems, as well as punched windows, from fixed to operating configurations. Last year, Winco Window Company introduced the first operating tornado-rated window.
As with hurricane windows, tornado protection can be incorporated in blast, ballistic, acoustic, thermal, custom and historic window systems.
Impact-resistant glazing or other window impact protection methods are required in the building codes of some areas prone to hurricanes and tornadoes, including coastal Florida and Texas. Laminated glass should comply with local building codes, applicable ASTM standards, Consumer Product Safety Standards and SGCC/ANSI Z97.1 - 1984 (Rev. 1994).
Gantt Miller is chairman of Winco Window Company. He can be reached at:
