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With forecasts for the remaining 2008 hurricane season predicting increasing probabilities that at least one hurricane will make landfall somewhere along the U.S. coast, it is timely to look at options for protecting and preserving homes and businesses.
For windows and doors, there are three key considerations:
1. structural resistance to high wind pressure
2. resistance to impact from wind-borne debris
3. water penetration from wind-driven torrential rains.
WIND PRESSURE
The primary consideration is structural integrity. The key to this is keeping the window or door intact to prevent the pressure of high-velocity wind from entering the building and causing catastrophic structural damage.
The jumping-off point for improved structural resistance to the force of high winds is the current basic water performance requirements for windows and doors set forth in AAMA/WDMA/CSA 101/I.S.2/A440-08 NAFS, North American Fenestration Standard/Specification for windows, doors and skylights. The 2009 edition of the I-codes is on track to reference the 2008 edition of this standard.
Basically, AAMA/WDMA/CSA 101/I.S.2/A440-08 and earlier versions delineate different Performance Classes of windows and doors based on increasingly stringent structural requirements. For the most recent edition of the standard, these are R, LC, CW and AW.
For a product to gain entry to one of these performance classes, it must be tested to withstand progressively higher minimum design pressures derived from the maximum wind velocity likely to be experienced at a given geographical location. The majority of the southeast Atlantic and Gulf coasts are rated for maximum wind speeds based on a three-second gust at 33 feet above grade, which results in wind speeds of 120 mph to a maximum of 150 mph at the southern tips of Florida and Louisiana.
The gateway minimum design pressures for the four performance classes are:
• R class: 20 psf (960 Pa), equivalent to approximately an 88-mph wind (commonly used in one- and two-family dwellings)
• LC class: 30 psf (1,440 Pa), corresponding to a 108-mph wind (typically for low- and mid-rise multi-family dwellings and other buildings where larger sizes and higher loading requirements are expected)
• CW class: 35 psf (1,680 Pa), corresponding to a 117-mph wind (low- and mid-rise buildings, which add limits on deflection and heavier use to the LC requirements)
• AW class: 45 psf (2,160 Pa), equivalent to a 133-mph wind (mid- and high-rise buildings where increased loading requirements must meet the most extreme environmental and usage conditions)
Note that these are minimums. For high-risk hurricane zones, optional performance grades should be specified in each class above the minimum requirement. This may be done in increments of 5 psf, up to a maximum cap of 100 psf—the force of 198-mph wind speed—for all classes except AW, for which there is no cap.
IMPACT RESISTANCE
Standing up to high winds is just the beginning. A significant majority of window breakage in major storms is caused by impacts from wind-borne debris. Furthermore, if an opening is penetrated by flying debris, the highly pressurized air can blow off the roof and cause the collapse of the building.
Stronger code requirements for impact-resistant windows have been adopted in many coastal jurisdictions, ranging from those that reference current I-code requirements, up to those in defined High Velocity Hurricane Zones (HVHZ), such as Miami-Dade standards TAS-201, TAS-202 and TAS-203.
The tests for determining compliance with these impact requirements are not trivial.
For example, for windows to be located less than 30 feet above ground level, the impact of large missiles is simulated by impelling a 2 x 4 stud into the product at 50 feet per second (fps), or 34 mph. For windows located more than 30 feet above ground, the impact of roof gravel and other small objects is simulated by firing a shotgun-like pattern of two-gram ball bearings into the window at a speed of 130 fps (88 mph).
To pass these tests, there can be no penetration upon impact, no opening formed larger than 3 inches in diameter or no tear longer than 5 inches.
Products certified to AAMA/WDMA/CSA 101/I.S.2/A440-08 under the American Architectural Manufacturers Association (AAMA) Certification Program have an option to certify for impact resistance to any of these standards or to AAMA 506-08, Voluntary Specifications for Impact and Cycle Testing of Fenestration Products, as well as for optional high performance grades for structural resistance to wind pressure.
WATER PENETRATION
Water penetration through or around otherwise intact openings from wind-driven rain can do a significant amount of physical damage, cause occupant displacement and business interruptions, and lead to extensive restoration expenses.
The capability of a fenestration product to resist water penetration is linked to structural integrity through the design pressure. In order to simulate wind-driven rain, testing to determine the water penetration resistance is conducted at a pressure equal to 15 percent of the design pressure, subject to a minimum of 2.9 psf and a maximum of 12 psf (15 psf in Canada). The exception is the AW class, which is tested for water penetration at a pressure of 20 percent of design pressures.
The test methods subject the exterior surfaces to a water application rate of 5 gal/ft2/hr—roughly equivalent to 8 inches of rain per hour—at these test pressures. To pass these tests, no water is permitted to pass the interior plane of the framing or to penetrate the corner seals of the tested assembly and enter the wall cavity.
The University of Florida has introduced the world’s largest portable hurricane wind and rain simulator—calibrated to recreate real-world effects of wind-driven rain scenarios. The simulator provides a realistic evaluation of building products and test methods intended for hurricane-prone regions. To view a video of the simulator in action, visit:
http://www.aamanet.org/news.asp?sect=1&id=38&newsid=168&showarchive=yes on AAMA’s Web site.
Improved standards and product designs are raising the performance bar for all attributes of construction components and methods, especially windows and doors, to the benefit of all building owners not just coastal dwellers. The products of the future will provide many benefits that reach beyond hurricane protection.
Rich Walker is president and CEO of the American Architectural Manufacturers Association (AAMA), Schaumburg, IL, a leading trade association dedicated to the promotion of quality window, door, curtain wall, storefront and skylight products. Founded in 1936, the organization has been developing product performance standards for 70 years and has been certifying products for over 40 years. The association also engages in product testing, market research and continuing education programs for industry professionals.
NEW DOCUMENT WILL VALIDATE STRINGENT TESTING PROTOCOLS FOR WIND-DRIVEN RAIN RESISTANCE
Following the 2004 hurricane season’s destructive power and the property damage of wind-driven rains, the Florida Building Commission (FBC) sought out the AAMA Southeast Region organization. At the FBC’s urging, AAMA assessed current test methods and developed a standard of performance for testing windows to enhance their ability to resist water penetration under hurricane conditions. The resulting document will serve as an elevated performance characterization but is not intended for building code adoption and enforcement.
AAMA has begun a 12-month review to thoroughly evaluate AAMA 520, Voluntary Specification for Rating the Severe Wind-Driven Rain Resistance of Windows, Doors and Unit Skylights (see Industry News, page 8). The final document is expected to be published next summer, well in advance of the 2009 hurricane season.
A preliminary copy of the document will be shared with AAMA-accredited testing laboratories and other members to ensure the specifications are clear and the test protocols yield repeatable results. The participants’ input will help validate testing equipment and procedures and evaluate a selection of current, hurricane impact-resistant windows. Once released, these specifications will represent the most stringent test standards of their kind in the industry today.
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