Coping with Nature s Most Powerful Storms

Transcription

1 Coping with Nature s Most Powerful Storms Intertek, the NSSA and Texas Tech Team Up to Create Certification Program for Tornado Resistant Products Rick Curkeet, Chief Engineer, Intertek Larry Tanner, Research Associate, Texas Tech University In an average year more than 1,200 tornados touch down in the United States. Many cause extensive property damage, injuries, and deaths. In fact, since 1950 the average annual death toll from tornados is 89. While there s nothing to suggest that tornados themselves are growing more powerful, it is clear that there is continuing growth in populations and in the number of inhabited buildings in areas that are prone to tornado activity. With an increase in inhabitants of tornado prone areas comes increased risk of damage to personal property and potential loss of life. Aware of this increasing risk, as well as the need for the proper means of evaluating shelter system and component designs, Intertek, the National Storm Shelter Association (NSSA), and Texas Tech University have developed a tornado resistant product certification program. As part of the program, testing will be conducted to applicable requirements of the Federal Emergency Management Agency (FEMA) and the NSSA. By determining products wind pressure load resistance, debris impact resistance, structural integrity, and door and latching device capacity, the program will assist manufacturers in determining if products are fit for use in tornado prone areas of the United States. 14

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3 Evaluation of Shelter Design: FEMA Guides 320 and 361 In 1998, FEMA published recommendations and minimum construction guidelines for in-home tornado shelters Taking Shelter from the Storm Building a Safe Room in Your House FEMA 320 and in 1999 FEMA published Design and Construction Guidance for Community Shelter FEMA 361. FEMA s tornado shelter guides provide specific recommendations for basic construction. Typically reinforced concrete walls and ceilings are considered adequate along with several proprietary construction designs. Engineers at the Texas Tech University Wind Science & Engineering Research Center worked closely with the FEMA team in analyzing tornado wind forces and the range of debris impacts that can be anticipated in tornados. This effort lead to the development of a test procedure that has been used as a basis for evaluation of shelter design elements. As indicated in FEMA s National Performance Criteria for Tornado Shelters, shelter doors had been dealt with in a generic manner. At the time, few door manufacturers had attempted to design doors, frames and hardware systems specifically for shelter applications. In recent years, as the interest in building in-home and community tornado shelters has grown so has the need for well engineered and designed entry systems. Evolution of the Above Ground Safe Room Prompted by the tornados that devastated a third of the city of Lubbock, Texas on May 11, 1970, killing 26 residents, the Department of Civil Engineering at Texas Tech University began researching storms. Since that date over 130 storms have been researched, thirty of which were hurricanes and tropical depressions. From the outset of the storm documentations, researchers frequently observed small, un-reinforced structures surviving amid the terrible storm destruction. The genesis of the above ground safe room occurred in 1974 when researchers documenting the Super Outbreak found a surviving closet in Xenia, Ohio. Research subsequently began to develop engineering designs that could withstand the perils of extreme winds, particularly tornados. Researchers knew that storm perils included wind pressures and debris impacts, but did not yet know what type of debris and wind speed shelters should resist. Wind research in the 1960s and early 1970s was still in its infancy and very little was known about extreme winds. However, researchers at Texas Tech, through their numerous investigations and analysis of failed structures, dispelled the myths and determined that damage produced by the strongest of storms seldom exceeded pressures produced by wind speeds in excess of 200 mph. Given the engineers penchant for safety factors, a wind speed of 250 mph was selected for the design of shelters for the most extreme of tornados. Having selected the wind speed, researchers sought to characterize the type and size of the projectiles commonly found in the debris field. Although cars, steel beams and roofing materials were routinely observed, a wooden 2 x 4 board approximately in length and weighing 15 lbs was selected. This wooden board was typical of a standard roof rafter found in stick frame residential structures the most common and most prolifically destroyed structures. Since numerical analysis alone is not dependable in accurately depicting the behavior of a material or assembly impacted by the projectile, full-scale testing was developed at Texas Tech. The method and type of projectile launcher has evolved into what is used today an air pressure cannon that shoots a muzzle loaded projectile. Given the development of design parameters, research on common building assemblies was conducted, and in 1974 the first engineered above ground safe room was introduced. Subsequent research since that time led to numerous designs, which were incorporated and released in FEMA 320. Months later when Oklahoma City and Wichita, Kansas were struck by a number of lethal tornados, the first known above ground safe room to have experienced an F-5 tornado was discovered. Reinforcing the Need for Quality Shelters As a result of the Oklahoma City tornado of 1999, President Clinton suggested that homes, schools, federal and state buildings should be rebuilt to include shelters. A FEMA sponsored home shelter incentive program was authorized and FEMA 361, Design and Construction Guidance for Community Shelters, was written in support of the large shelter initiative. Due to the incentive programs, the states of Oklahoma and Kansas were inundated with contractors and manufacturers. Building officials were unschooled in FEMA 320 and construction and manufacturing problems were rampant. One year after the 1999 tornado, a group of contractors, manufacturers and engineers banded 16

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5 together to form the National Storm Shelter Association (NSSA) which was dedicated to providing a high quality, safe product consistent with the guidelines of FEMA 320 and FEMA 361. NSSA developed a standard for below ground and above ground shelters which includes buoyancy and wind pressure guidance as well as impact testing protocols. This standard is currently under review by the International Building Code (IBC) and will be included in the next printing of the IBC to provide guidance for construction and code enforcement of tornado and hurricane shelters. Shelters Need Doors That Will Resist a Storm s Impact A key effort in each storm investigation and building performance documentation was to study the behavior of doors subjected to the pressures and impacts brought by extreme storms. Given the routine trauma that doors are subjected by a tornado, researchers at Texas Tech developed a test protocol that required the door slab to be perforation resistant and sufficiently stiff to prevent disengagement of the hardware. Multiple lock and hinge points were required to provide redundancy and factors of safety. The test requires both multiple missile impacts with a 15 pound 2 x 4 propelled at 100 miles per hour and a structural load test at 250 pounds per square foot. The impact energy is over 5000 footpounds- the equivalent of dropping a 100 pound weight from 50 feet. The requirements are that the missile not penetrate the assembly or cause a deformation greater than 3 toward the inhabited side and that the door main closed under the structural test load. This protocol further requires that at the conclusion of the test that at least two hinges and two lock points remain engaged; that the door deflection is less than 3-in.; and that no opening or separation of the door from the frame be large enough to allow passage of the 2x4 projectile. Door functionality after the test is not a requirement. Due to the extreme conditions of impact and wind loading simulated in the tornado test procedures, virtually all assemblies expected to meet the requirements are specialty products that include heavier than normal construction and reinforcements. The door, frame and hardware must also be evaluated as a system to assure that they will work as an assembly under the extreme loads involved. Since many of the design elements in these components cannot be readily verified after manufacture or assembly, third-party inspection and certification becomes an essential component of a system. This step ensures consumers and code officials that the shelter doors will perform as expected even in worst-case scenarios. To best meet this need, Intertek has established a certification program that includes documentation of the design and installation details for door assemblies, including qualified hardware and anchorage requirements, for tornado shelter applications. Once these assemblies are qualified by testing to the impact and wind load requirements, manufacturers will be eligible to enter into a followup inspection and WH Mark labeling program. This will provide architects, specifiers, code officials and consumers with the independent assurance that doors meet these critical requirements. Qualified manufacturers will also be eligible to have their products listed on the NSSA s listing of tornado shelter qualified products. Certification and labeling does more than just verify that a production door is the same construction as a door that has been tested. It carries with it requirements that the manufacturer provide installation instructions and assembly details that are critical to the door s performance. Even the strongest door can fail if a poor quality latch is used or the frame isn t properly anchored. Tornado shelters clearly represent an issue of life safety. Should the shelter fail when needed, the result is likely to be severe injury or death of occupants. Thus a high level of reliability and integrity is a very important part of the safety picture. Enforcing Code Requirements The International Code Council and NSSA are working to develop comprehensive and enforceable code requirements that can be adopted by local and state building code bodies to make sure that as new shelters are built they will provide the level of protection needed. Intertek, NSSA, and Texas Tech are working together to provide consumers, builders, manufacturers and code authorities the tools that will allow for shelters to be protected to the necessary level of performance. There is no doubt that some day when the storm warning sirens sound, many more people will be able to quickly secure themselves and loved ones in a properly designed and constructed shelter. They will also have the comfort in knowing that the entire system including the door and hardware may well save their lives. For f urther information, please contact icenter@intertek.com or for more information on the program itself visit NSSA/News/news.htm. 18