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The following article, written by James E. Waller, P.E., president of RemagenSafeRooms and president of the National Storm Shelter Association, was published in the September/October, 2002 issue of Southern Building. This issue is available on the SBCCI web site www.sbcci.org and is reprinted here by permission of SBCCI.
SURVIVING NATURE'S WRATH AND HUMAN SHORTSIGHTEDNESSJuly 2002 During the past 30 to 40 years, major advances in the science and technology of meteorology and wind engineering have led to public awareness of the risks of injury from severe windstorms, resulted in increased severe weather warning lead times, and led to the development of high quality hurricane and tornado shelters and safe rooms. Unfortunately, there has not been corresponding acceptance by the home-building industry to avail itself of these advances. Except for veterans of devastating windstorm events and for public school officials concerned for protecting students from storm injuries, apathy yet exists regarding preparation for severe wind catastrophes. Predictions for Future Storms Since Hurricane Andrew devastated South Florida in 1992, coastal areas of the United States have been spared catastrophic loss of life and property damage as the result of hurricanes. During the period 1995 through 2000, only 3 of the 23 named tropical storms reaching Category 3 or higher on the Saffir-Simpson scale (minimum wind speed 111 mph) made landfall with the United States. According to Stanley Goldberg, research meteorologist at the National Oceanographic and Atmospheric Administration's Hurricane Research Division, the low incidence of hurricane hits during this period was no accident but, rather, the result of cooler water temperatures in the mid-Atlantic and a phenomenon known as vertical wind shear, which interrupts the formation of hurricanes from tropical depressions. These factors and the presence of a prevailing low pressure trough over the Atlantic, which creates hurricane steering currents such as diverted Hurricane Floyd away from Florida in 1999, have resulted in most major hurricanes being diverted away from the U.S. mainland. Goldberg states that recently higher water temperatures and weaker wind shears have been occurring in the hurricane-developing regions of the mid-Atlantic. He speculates that the low pressure trough will weaken and result in a substantial increase of major hurricanes reaching the coastal areas of the U.S. [E. Kleinberg, "Stronger Storms Forecasted for Decades", Southern Buildings, October, 2001] Well known hurricane forecaster, William Gray, who also participated in the study of hurricane intensities and frequencies, believes that the conditions which were so favorable for hurricanes hitting the U.S. during the middle of the last century will soon again be experienced. Gray estimates that hurricane damage in the U.S. over the next 35 years will probably exceed that experienced during the past 35 years by a factor of ten. [Ibid]. Given the awesome property damage, loss of life, and numbers of grievious injuries which resulted from just two hurricanes, Hugo (1989) and Andrew (1992), during the preceding period, this prophecy should give pause to coastal residents and disaster management officials. Though diverted from hitting Florida's coastline, Hurricane Floyd nevertheless resulted in the largest government-ordered evacuation in U.S. history. Many hundreds of thousands of persons who were directed to evacuate or voluntarily fled coastal regions of the U.S. as Floyd veered dangerously close to the southeastern coastal states were fully exposed to the danger of being struck by the storm while trapped in long lines of traffic created by the evacuation. Had Floyd turned inland farther south than the outer banks of North Carolina, the number of storm casualties could have been in the tens of thousands. Experts further warn that there will be an increase in tornadoes and other severe windstorms in the mainland U. S. and that residential structures, as they are currently built, are highly susceptible to severe damage or destruction. The general frailty of residence buildings increases the likelihood of greater volumes of airborne debris during severe windstorms. More debris impacting frangible elements of enclosed residences (glass doors, windows, garage doors) creates greater numbers of residences which are transformed from enclosed buildings to partially open buildings with attendant increases in internal wind pressures on the roof and walls. Residences which could normally resist design wind pressures when enclosed are frequently "exploded" by higher internal wind pressures which result after debris from destroyed, nearby structures impact and open up the building envelopes. The resulting "chain reaction" of storm debris creation in built-up areas increases the probability of building failures. Tornado Forces Versus Design Wind Loads The National Climatic Data Center presents data indicating that an average of 1,200 tornadoes are reported in the U.S. every year during the preceding decade. The majority of observed tornadoes are rated on the Fujita scale as F0 to F2 tornadoes which result in damage ranging from broken limbs to downed trees, lost shingles to severe roof and window damage, and destroyed mobile homes. Tornadoes occur in the U. S. at any time during the year, largely during the spring in the eastern U. S. and during summer months in the mid-western states, but anywhere in the U. S. without notice. Based on the Federal Emergency Management Agency's table of Wind Zones in the United States and tornado damage assessment research at Texas Tech University, every residence in the U. S. lying east of the Rocky Mountain range is susceptible to F4 - F5 tornado damage (destroyed). Residences in all other areas of the continental U.S. are susceptible to severe (F3) tornado damage. This debunks the myth that only residents of "Tornado Alley" are at risk for dangerous tornadoes. Tornado wind velocities have been typically associated with the corresponding structural damage as indicated by the Fujita Tornado Scale which has been adopted by FEMA and other agencies as the scale for measuring ferocity of tornadoes. Recent post-tornado damage assessment research has indicated flaws in the Fujita scale as it relates to residential buildings. After the May 3, 1999 Oklahoma City area tornadoes, wind engineers discovered that residential damage classified as "extensive" (Fujita F2, 119 - 163 mph), "severe" (F3, 164 - 210 mph), and "destroyed" (F4 - F5, 211 - 320 mph) had resulted from estimated actual wind speeds of 110 - 130 mph, 120 - 140 mph, and 130 - 160 mph respectively. Thus, F2, F3, and F4 - F5 tornado damage had resulted from wind speeds which averaged 86%, 73%, and 55% respectively of the Fujita scale wind velocities associated with comparable damage. [Gardner, et al, The Tornadoes of Oklahoma City of May 3, 1999, Wind Science and Engineering Research Center, Texas Tech University, Lubbock, TX, 2000]. In the Texas Tech study, the residential construction evaluated was typically wood frame on concrete slab with brick veneer. Residential damage was defined as follows: Extensive - more than 20% of roof area damaged, most exterior walls remained intact, more than 80% of residence structure remained intact; building uninhabitable until repaired. Severe - loss of roof and exterior walls; only small interior rooms remained intact. Destroyed - building entirely swept from foundation or totally demolished. Based on the results of Texas Tech University's wind engineers' study, a modified Fujita scale for residential buildings in the U. S is suggested.
The American Society of Civil Engineers Standard, ASCE 7-98, Minimum Design Loads for Buildings and Other Structures, prescribes design wind speeds corresponding to the Modified Fujita Scale speeds for "severe" or "destroyed" in narrow coastal regions of Massachusetts and New York, the coastal states from North Carolina to Texas, and Alaska. Thus, we might conclude that residences constructed in non-coastal regions of the U. S. where the ASCE 7-98 design wind speed is less than 120 mph would be extensively damaged from being hit by a relatively common F2 (modified scale) tornado. Based on structural engineering experience and post-tornado damage assessment, this writer suggests that a substantial percentage of residences built in the U. S. would be unable to sustain forces generated by 85 - 110 mph design wind speeds prescribed by ASCE 7-98 for non-coastal regions without significant damage unless these buildings were to be structurally modified to strengthen connections and create continuous structural load paths from roofs to foundations. In Harm's Way Adding to technical concerns about residential structural stability and strength are population shifts to regions having greater risks of storms. Maps of population density appear to resemble storm intensity maps. National Geographic states, "Unfortunately, people are moving into harm's way at a fast clip." "Half the U. S. population lives in coastal states, with 34 million in Texas and Florida, two favorite hurricane targets." [National Geographic Magazine, July, 1998]. The confluence of these trends points to a condition which became the lead title of a book entitled, Disasters by Design - A Reassessment of Natural Hazards in the United States, by Dennis S. Mileti, Joseph Henry Press, Washington, DC, 1999, wherein the author urges us to adopt a policy of "sustainable hazard mitigation". With respect to violent wind storms, the logical technological conclusion to be reached is that all residences would benefit from having a qualified safe room or storm shelter to provide protection for occupants during severe wind storms. Unfortunately, as Mileti points out, only in an ideal world could we expect everyone to "accurately assess their risk, search for possible adjustments, and adopt techniques that would provide them with adequate protection at a reasonable cost." One can only hope that responsible family members, after considering the potential losses, make the logical sociological determination that the benefits of paying a small, one-time premium for a qualified storm shelter or safe room is warranted. Due to the growing population and the potential for either tornadoes or hurricanes hitting any region, evacuation becomes the least desirable option for survival during severe windstorms except for residents of flood-prone areas. Above Ground or Below Ground Shelter? Caves are the first known storm shelters to be used by man. Subsequently, root cellars, the traditional under-house space used for food storage, became the preferred storm shelter. Residents of areas hit by violent wind storms and tornadoes found that cellars provided undependable protection when houses lifted up from foundations, often dropping them back down on their hapless occupants. Thus, the fully enclosed, underground tornado cellar was for many years accepted as the preferred shelter. Currently, residential structures, except for manufactured housing, are most often built on concrete slabs on grade rather than over cellars. Wind engineers, particularly those engaged in tornado debris impact effects on structures, have determined that relatively inexpensive above ground tornado safe rooms can be constructed and anchored to existing concrete floor slabs, providing as much protection from tornado wind forces and debris impacts as underground shelters, even if the residence is completely destroyed. Based on current engineering knowledge and experience, it is anticipated that there will be little statistical difference in overall safety provided by occupied, high quality underground shelters and above ground, in-residence safe rooms. Therefore, the relevant question is not which type of shelter is preferable but which shelter is more quickly and safely accessible. One study indicates that most tornado victims have less than 60 seconds from the time they are aware of the presence of a tornado to the time that the tornado strikes their residence [Schmidlin & King, "Risk Factors for Death in the 1 March 1997 Arkansas Tornadoes", Quick Response Report # 98, 1997]. Clearly, the shelter should be as close to the living space as possible. Access to the shelter should not be hindered by obstacles or having to find keys or remove stored contents in order to get into the shelter. As an exercise to test family response to a tornado threat, one should try rallying every family member to one place in less than 60 seconds. The response time may very well govern the decision as to what type of tornado shelter to select and where the shelter should be located. Survivability during a tornado is clearly enhanced by promptly heeding severe storm warnings and remaining in protective shelters until announced warnings are discontinued. It is also clear that a storm shelter which may be entered without leaving the residence is preferable to one which must be reached by exposing one's self to lightning, hail, and windborne debris. Like the cave dweller, modern man will most likely discover that storm safety is best sought inside the home. Another factor which influences selection of shelter type is whether the shelter is to be built in new construction or into an existing building. Harvey Ryland, president of the Institute for Business and Home Safety, states that 98% of residence buildings in the U. S. are existing structures. Only 2% of residences are new (constructed within the last year). [P. Zinkewicz, "Stormy Weather", The Rough Notes Magazine, 2001] Above ground safe rooms are more suited to construction in existing residences than underground shelters, based on construction costs and other considerations. For mobile homes and manufactured housing, exterior shelters are safer and easily constructed. Texas Tech University's Wind Engineering Research Center has just released a graphic planning guide for manufactured housing, "Tornado Wind Safety - Shelter Concepts for Aboveground Homes Without Foundations". This well illustrated guide suggests a number of adjunct tornado safe room possibilities for residential buildings which do not lend themselves to interior safe rooms. Future Trends in the Storm Shelter Industry Public acceptance of the protection offered by constructed and manufactured underground storm shelters and above ground, tornado safe rooms will be most influenced by wind research and design professionals, real estate marketers, and state and federal government initiatives, in addition to the constructors and manufacturers of high quality tornado shelters. Advances in wind science and engineering and serious post-tornado structural, economic, sociological, and medical damage assessment studies have provided the basis for much greater understanding of the requirements and cost/benefit relationships for residential wind storm protection. The news media has made the public aware of the devastating effects and risks of serious injuries and deaths to the unprotected public which result from severe wind storms. The Federal Emergency Management Agency has challenged local governments and businesses to promote the construction of tornado shelters and safe rooms. FEMA's Project Impact has been a popular media topic and has been supported and joined by many local communities and counties. The publication of FEMA Publication 320, Taking Shelter from the Storm: Building a Safe Room Inside your House, generated much public interest in storm shelter protection in homes and has provided a stimulus to constructors and storm shelter manufacturers to create products to meet this rising need. Further stimuli for residential shelter installation have been provided by FEMA's Hazard Mitigation Grant Program for storm- impacted regions (FEMA-1360-DR), HUD's initiative enabling home loans to include up to an additional $5,000 for purchase of tornado safe rooms, local governments waiving permit fees for tornado shelter construction, and other state and federal legislation and initiatives. The National Storm Shelter Association On May 3, 2000, in Oklahoma City, a group of storm shelter and tornado safe room designers and manufacturers founded the National Storm Shelter Association (NSSA) for the primary purpose of promoting the highest quality of manufactured and constructed storm shelters. NSSA developed and adopted the Standard for the Design, Construction, and Performance of Storm Shelters, which is a comprehensive standard applicable to all types of tornado safe rooms and underground storm shelters. NSSA's by-laws require producer member applicants to have all storm shelters tested at an NSSA-approved testing facility (presently the Wind Science and Engineering Research Center at Texas Tech University) and to submit shelter engineering calculations and design drawings to an NSSA-approved engineering firm (presently T. R. Arnold & Associates) for verification of compliance with the NSSA Standard. In July 2000, FEMA published Design and Construction Guidance for Community Shelters (FEMA Publication 361). Public shelter design technology continues to benefit from published papers of wind engineers engaged in storm shelter research and development such as the recently published article by Coulbourne, Tezak, and McAllister, "Design Guidelines for Community Shelters for Extreme Wind Events", ASCE Journal of Architectural Engineering, June 2002. A considerable body of technological information has been, and continues to be compiled relating to the effects on structures of wind forces and debris impacts. Further study is needed to quantify the weights and impact velocities of windborne debris impact test missiles associated with various wind velocities. Developers of storm shelter design criteria will certainly encourage the funding of appropriate research and avail themselves of this important technical information as it becomes available. SBCCI of the International Code Council (ICC) and NSSA have signed an agreement allowing SBCCI to proceed with the development of a national consensus standard for community and residential storm shelters using the NSSA Standard as a foundation document. NSSA will continue to develop the industry standard and make it available on its web site until such time as consensus is reached on the ICC/NSSA Standard. The standards to which NSSA members' storm shelters are required to comply may be viewed and downloaded from the NSSA web site (www.nssa.cc). Familiarity with the Standard aids the consumer in judging the qualities of shelters and selecting manufacturers, producers, and shelter design professionals who champion the cause of providing high quality storm shelters. It is hoped that NSSA's commitment to excellence in the storm shelter industry will engender public confidence in the ICC/NSSA Standard and promote the recognition of the NSSA storm shelter seal as a commitment by the producing member that storm shelter products meet or exceed the highest standards for the storm shelter industry.
James E. Waller, P.E., is president of the National Storm Shelter Association and president of RemagenSafeRooms, Monteagle, Tennessee (www.remagensaferooms.com) which manufactures StormCloset® and ClosetVault®. He has been a registered structural engineer for 32 years and has conducted post tornado interviews and building damage assessment as part of a Texas Tech University damage assessment team. |