by Ralph Buoniconti and Cheryl Smith
August 24, 1992 is a date that many residents of Florida are likely to remember the rest of their lives. That, of course, was the day Hurricane Andrew devastated southern Florida, delivering an unprecedented blow to the states economy and the confidence of residents in the ability of their homes and businesses to withstand another strong hurricane.
As the scope of the changes required became evident to construction professionals, the county created the Metro Dade County Building Code Evaluation Task Force. Studies were conducted to determine how so many buildings failed, and to provide more rigorous testing and evaluation procedures to help improve the performance of hurricane-resistant building materials.
Research identified the loss of windows, doors, and roofing materials as a major cause of catastrophic building loss in a hurricane. The reason is simple to understand: when windows and doors fail, the integrity of the building envelope is breached, producing intense internal pressurization of the structure. The main culprit by far in breaching a buildings "envelope" was found to be windborne debris.
In recognition of this, Dade County adopted tough new building standards in 1993 that govern the performance of glazing. In essence, the South Florida Building Code requires that every exterior opening in a house or business be protected against flying debris propelled by hurricane-force winds. Such protection can consist of shutters or impact-resistant windows, doors and skylights.
Similar codes and testing procedures have been adopted by the counties of West Palm Beach, Monroe and Broward in Florida. In addition, as of July 1998, the Texas Department of Insurance will require hurricane glazing protection in 14 counties at high risk of wind storm damage.
The tests devised in Florida to determine whether adequate protection exists consist of two major components: impact testing and pressure testing. To measure impact performance, two different kinds of "missiles" are shot at the glazing. A large missile, defined as a nine-pound two by four, is launched from an air cannon at 34 mph (50 ft/second). Manufacturers seeking hurricane resistance qualification supply three test specimens, each of which must survive two impactsone in the center and one in the cornerwithout penetration. According to the new codes, windows, doors and skylights installed 30 feet or less from ground level must be made of materials that pass the large missile tests.
A second impact test uses smaller missiles because it is assumed that even in hurricane winds, large objects are unlikely to be traveling at high speeds (if at all) above 30 feet. The test missile is intended to represent roof gravel weighing two grams moving at 55 mph (80 ft/second). Again, manufacturers supply three specimens, but in this test 30 impacts are required on each specimenten in the center, ten along the edge, and ten near the corner. All three windows must survive the impacts without penetration.
However, the tests that evaluate the performance of these materials have themselves had to weather some controversy. The "roof gravel" is a potential source of inconsistency. Some products, may pass with some types of rock and fail with others because some gravel is made of softer minerals or has a more forgiving shape. Current Dade County tests, still widely followed, are due to be updated. Newer ASTM and TDI standards use steel balls weighing exactly two grams in an effort to eliminate the sources of variability.
For the final phase of testing, glazing samples are subjected to cyclic pressure testing. Hurricane winds dont blow at a constant rate, so this test seeks to simulate hurricane force loading at 9,000 wind cycles, where each cycle is a function of the maximum wind speed (converted to pressure) to which the product will be "rated."
Each window, door and skylight manufacturer determines the pressure at which its product will be tested. Dade County requires a minimum of 60 pounds per square foot for the first three floors of a building in a non-coastal environment. Its important to remember, however, that wind pressure requirements vary depending on where the glazing will be installed. Higher pressure performance is necessary near the corners of a building or where surrounding structures cause a special wind effect. In addition, certain coastal conditions can produce pressures in excess of 200 pounds per square foot and therefore require correspondingly more rigorous certification levels.
To pass, a manufacturers specimens must pass the impact tests without penetration and during the cyclic pressure test, the specimens must remain in their frames and have no tear or crack longer than five inches and wider than 1/16 inch through which air can pass. In addition, weathering tests are conducted on all of the glazing materials used in the system to assess how the materials can be expected to perform in a storm even after long-term exposure to the elements.
Ralph Buoniconti is an engineer at GE Structured Products, Pittsfield, MA, with special expertise in testing and regulatory matters. Cheryl Smith is a Structured Products marketing specialist for the hurricane protection industry who works with glazing system manufacturers, architects and developers nationwide.
LEXAN® is a registered trademark of General Electric Company.
One material to which glaziers are turning for glazing that maintains its integrity storm after storm is LEXAN glazing from GE Structured Products of Pittsfield, MA. In repeated tests performed on standard glazing made from LEXAN sheet, the large missile literally bounces off the material. Both vertical glazing for windows and doors made from MR10 sheet and overhead glazing for skylights made from XL10 sheet perform equally well. A high-performance coating developed by the manufacturer helps the materials further resist abrasion and UV damage.
Besides passing Dade County and other south Florida hurricane tests, glazing made from LEXAN sheet complies with all major model building codes. RB and CS
To make buildings less vulnerable to harsh weather conditions that can cause glazing failure, Glasslam NGI of Pompano Beach, FL, has developed Safety-Plus®. This laminated glass product is meant to address the problem of PET films and laminates that hold a glazing lite together, but do nothing to hold the lite in its frame: a potentially lethal threat.
Safety-Plus consists of a three-ply interlayer, the middle layer a heavy-duty PET film. The PET film is cut longer than the glass size and mechanically held into the glazing frame. Silicone used in the installation adheres to the PET film, causing the lite to be held securely in place despite any possible glass breakage, according to the company.
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