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July - August 2003

Blowin' in the Wind
    The Latest in Skylight Codes for Storms
by John Lawton and Roland Temple

   

Following the devastation of Hurricane Andrew in 1992, South Florida Building Code (SFBC) officials adopted the first mandatory glazing impact standards in the United States. These standards were based upon existing Australian standards that were developed after a devastating cyclone in 1974. Research following the cyclone indicated the resulting damage was caused primarily by windborne debris from destroyed structures and fluctuating pressures. A significant conclusion was that buildings designed to withstand a single wind gust were not adequately designed for erratic wind direction and speeds. As with Hurricane Andrew, the cyclonic winds studied in Australia were sustained and changed direction slowly. As structures came apart, they carried tons of debris. We have all seen the now-famous photograph of the 2 x 4 driven through the palm tree. This photograph was shown at many code hearings after Hurricane Andrew. 

The Next Step
Soon after the Miami-Dade code protocols were adopted, the Southern Building Code Congress International (SBCCI) created the Wind Load Committee. This group was charged with developing criteria similar to Miami-Dade for the Standard Building Code (SBC) for windborne debris protection beyond the high wind zones similar to Miami or South Florida. 

Wind engineers and manufacturers were concerned that the Miami-Dade protocols were too restrictive for lower wind speed zones. The mission of the committee was to develop a standard with different levels of protection for varying wind speed zones. At the time, American Society of Civil Engineers Standard 7 (ASCE-7) - Minimum Design Loads for Buildings and Other Structures - and other wind speed maps were developed for fastest mile. Parallel work at ASCE moved to change the wind speed maps to reflect three-second-gusts, a more accurate measurement of the changing winds in high-wind events.

The other work on windborne debris protection at SBCCI, ASCE, American Society for Testing and Materials (ASTM) and Texas Windstorm Insurance Association was following the SFBC standards as a baseline. The test protocols were amended with regard to weights and speeds of the cannon-fired projectile test used in the Miami-Dade protocol. The number of “hits” per specimen was also a difference to be addressed. However, all of the standards were directed toward vertical fenestration (windows and doors)—not skylights—a major concern for the skylight industry. 

This is where the story changes for the sloped glazing and skylight segment of the fenestration and building industries. In its first editions, the ASTM standards were developed with three wind zones and multiple missile weights and speeds, as determined by the applicable wind zone, exposure and height. But a major milestone was reached in the 2001 edition of the ASTM standards. The task group and representatives from Miami-Dade Product Approval developed a standard that could be implemented by all jurisdictions under the Florida Building Code. 

The new standard has four wind zones (with wind zone 4 being greater than 140 mph) and two impacts per specimen. The missile weights and speeds match the Miami-Dade protocols for wind zone 4. The major difference is the pass/fail criteria. Miami-Dade protocols allow no penetration on impact and a crack or tear no greater than 5 inches in length and no wider than 1/16-inch. The ASTM standards allow penetration on impact for wind zones 1 through 3; however, when the cycling is completed, there may not be a tear greater than 5 inches or an opening through which a 3-inch sphere can pass. The ASTM standards incorporate the same pass/fail criteria for wind zone 4 as the Miami-Dade protocol, but the standards do allow smaller missile sizes to be used for skylights in wind zone 4.

Skylights and Windborne Debris
After the first printing of the ASTM standards E-1886 and E-1996, additional research modeling high wind events was conducted. The research indicated the belief of the skylight industry that unit skylights were not being treated equitably by being considered as windows and doors. The ASTM task group responsible for E 1886 and 1996 went “back to the drawing board” to determine how to address unit skylights. 

The industry defines unit skylights as an assembly of a complete factory glazed, glass, or plastic skylights for both residential and commercial construction (from AAMA/WDMA 1600/IS7 Voluntary Specification for Skylights). From the research models, the task group determined that debris from adjacent roofs would be the most likely source of windborne debris specific to skylights. After careful consideration and review of the weight of shingles and roofing tiles, the weight of 4.5 pounds for the 2 x 4 missile was deemed appropriate for unit skylights. This became the only difference in the standards between vertical fenestration and skylights. All test specimens of skylights, whether glass or plastic glazed, would be required to meet all other requirements for every other fenestration products.

Prior to the implementation of the 2000 International Codes, few coastal areas outside of South Florida and seaward areas in Texas were concerned about windborne debris protection. The 2000 International Codes, in particular the International Building Code (IBC) and International Residential Code (IRC), contained provisions for structures constructed in windborne debris areas. As more and more East and Gulf Coast states elect to adopt variations of the I-Codes as their state building codes, the issue of windborne debris protection creates much debate and awareness. Thus, windborne debris requirements are now being enforced as far north as Long Island, New York. Even though South Carolina has established a moratorium on the wind debris requirements of the I-Codes, several coastal towns and cities have chosen to enforce the windborne debris provisions of the code. In Texas, the Texas Department of Insurance is accepting products tested in accordance with the requirements of the IRC.

With the development of new products from the skylight industry, many code officials, builders and homeowners are now dealing with impact products that meet the appropriate energy codes implemented by their individual state. The Southern climate zones requiring impact protection must also deal with solar heat gain factors, while the more Northern climates must deal with u-factor requirements. Skylight products that incorporate insulating glass lites and low-E glass coating have been introduced in order to meet energy requirements in addition to impact.

Education
Education is the biggest hurdle the industry must clear. Consumers ask, “I need a skylight for 120 mph; what do you have?” The difficulty comes in answering the question, or explaining the factors to determine the actual product needed or required. Some of the considerations are: 

• The wind speed zone - 110, 120 or higher;

• Proximity to the shoreline;

• Mean roof height (How tall is the building?); and 

• Placement on the roof. 

All of these variables must be determined before a product is recommended. The building codes (IBC, IRC, FBC and ASCE 7) have a table labeled as Components and Claddings. This table contains the values for wind speed, glazed area, zone of the roof and mean roof height for a particular region. The design pressures are provided in both positive and negative values, as the determining design pressure for a skylight is the negative value (the lower pressure) or the pressure required to lift the skylight from the roof. If an impact product is required, this value is the design pressure under which the unit must be cycled after impact. 

Tomorrow
To date, Miami-Dade Product Approval has not accepted the latest ASTM standards as equivalent to their existing protocols. The industry, ASTM task groups and Miami-Dade representatives are continuing to work to resolve the differences. Whether it is pass/fail, shutters or essential facilities, the code officials, industry representatives, builders and homeowners must have one standard to determine impact protection. This is a life safety issue first and foremost, then reduction of property loss. 

The latest editions of the ASTM standards are being proposed for the Florida Building Code for all regions except for High Velocity Hurricane Zones (HVHZ) until Miami-Dade accepts equivalency. 

John Lawton is the manager of Skylight Product Management and Roland Temple is the skylight code compliance and certification coordinator for Velux America, based in Greenwood, S.C.

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