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5 GENERAL COMMENTS It is generally assumed that it is not economical to design a structure that will suffer no damage if it is hit by a tornado except where damage to that structure would seriously endanger the population at large, as in the case of nuclear power plants. It is feasible and desirable, how- ever, to design public buildings that will not collapse in a tornado and will thus provide a safe refuge for the public. This is particularly true in hurricane-prone areas, where such buildings may be used as emergency evacuation centers in weather conditions that often generate tornadoes. The performance of the public, unreinforced masonry buildings examined in this survey was extremely poor. South Carolina has the highest earthquake risk of any state on the East Coast and is subject to hurricanes, so it is surprising that unreinforced masonry is used so extensively in the area when its poor performance in extreme winds and earthquakes has long been known. Since the cost of reinforcing masonry is small compared with the overall construction costs and the returns are so great, there seems little excuse for the continued use of unreinforced masonry in places of public assembly. The collapse of the Northwood Village Shopping Center and the sub- sequent testing and analysis highlighted serious shortcomings in its form of construction. Unfortunately, it is a form of construction used in hundreds of shopping centers in the region. The use of a moment- resisting frame would add relatively little to the total cost yet would increase the safety of such buildings significantly. The better per- formance of buildings constructed in this way clearly shows the advan- tage to be gained from this form of construction. With regard to domestic dwellings, a number of simple improvements could be made to conventional construction that, although they would not prevent minor damage in the event of a tornado, would probably prevent collapse and loss of life. In September l984 the author conducted a similar damage survey in North Carolina following Hurricane Diana (Mitchell et al., in press). North Carolina has a statewide residential building code (North Carolina Building Code Council, l968) that contains specific requirements for improving the wind resistance of domestic dwellings. These requirements were developed following a series of devastating hurricanes in the l950s. At present they apply only to 36
37 coastal areas of the state. For timber-frame construction the code requires that rafters and roof trusses be secured to the framing by metal clips and that the frames be tied to the foundations. This may be achieved by 3/8-in.-diameter rods, not more than 8 ft apart, running from the top of the frames to the foundation. A similar requirement exists for masonry buildings. In Hurricane Diana the wind speeds are thought to have been on the order of l00 mph. Buildings built to the building code appeared to have performed well. Many buildings predating the code lost all of their roofs, although the walls remained standing. Several examples of this type of failure were also found in the paths of the South Carolina tornadoes (Figure 47). These buildings would probably have suffered very little damage had they possessed the connections between roof and foundation required by the North Carolina Uniform Residential Building Code. Indeed, there was a strong similarity between the nature of the damage to poorly connected buildings caused by the South Carolina tor- nadoes and that caused by Hurricane Diana and Hurricane Alicia (which was of similar strength; see Savage et al., l984)). This similarity suggests that the wind speeds in these tornadoes may not have been exceptionally high in many instances and that relatively minor changes in building techniques could significantly reduce the risk of damage and loss of life. Based on the experience in North Carolina, the addition of ties and clips adds relatively little to the overall cost of a building. unanchored mobile homes are clearly dangerous structures in tor- nadoes, but the performance of at least one mobile home in this survey suggests that while conventional tie-down techniques may give good protection in normal storms they do not render a mobile home a safe location in a tornado. The nature of these tornadoes was very unusual with respect to their dimensions and rate of travel. The large diameter of the disturbances probably accounts for the nature of the damage observed. For example, at the Northwood Village Shopping Center all of the debris blew in one direction, which is consistent with the shopping center being to the east of the center of the tornado. Since the tornado was reportedly traveling in a northeasterly direction at about 60 mph, the highest wind speeds would have been to the right of its center, where the forward motion would increase the windspeed caused by the circulation. No seri- ous structural damage was observed in the area where the debris was blown in a direction opposite to the forward motion of the tornado. The path width was significant in other ways. Tornadoes in this area usually have path widths of a few hundred feet and are relatively slow moving. Had this been the case at Bennettsville, it would not have been appropriate to have studied the department store in a conventional wind tunnel. The width of the store, 200 ft, would have been a substan- tial proportion of the width of the tornado, and the wind regime would have differed appreciably from that modeled in the wind tunnel. In- stead, the Bennettsville tornado was over 6,000 ft in diameter and possessed a high forward velocity. Under these circumstances there would be little variation of wind speed and direction across the build- ing, and conditions should have been similar to those created in the wind tunnel.
38 FIGURE 47 Typical roof failure caused by the South Carolina tornadoes. (Photograph courtesy The State.) As a consequence of the unusually wide path widths, direct compari- sons could also be made between the performance of buildings in this storm and buildings subjected to recent hurricanes. Care should be taken, however, in applying these conclusions to large buildings subjected to tornadoes with small diameters. Although South Carolina is not in one of the well-known tornado areas, there have in fact been 33 killer tornadoes in the state since recordkeeping began in l9l2. In one outbreak in l924, 77 people were killed. All three towns in this survey had been hit by tornadoes before. In many respects the state appears to have been well prepared to cope with tornadoes. The National Weather Service had established a system of weather spotters and for several years had held tornado aware- ness weeks prior to each tornado season. One was held just four weeks before this outbreak. Some towns had siren systems to warn of tor- nadoes, and schools conducted regular tornado drills. When the outbreak occurred, the National Weather Service issued timely advisories and warnings. The emergency services appeared to have functioned well, and the utility companies restored services in a reasonable amount of time. Further information on the response of the National Weather Service and the community to the storm can be found in National Oceanic and Atmos- pheric Administraion (l984). The Building Officials Association of South Carolina has made re- peated efforts to get the state legislature to mandate the adoption of
39 building codes and the certification of inspectors. Unfortunately, the state government has maintained a laissez-faire attitude toward building control. This, combined with the relatively low income level and gener- ally gentle climate, has tended to encourage the construction of unsuit- able buildings. Except on the coast, windy days are a rarity. The relatively high design wind speeds are due to the passage of earlier hurricanes and severe thunderstorms. At the time of these tornadoes, there had not been a severe hurricane in the area for more than 20 years, and nobody had been killed in a tornado for over l0 years. In the absence of strong building control, methods of construction have apparently developed that are unable to withstand the extreme wind conditions that occasionally occur in the area. Unlike hurricanes, tornadoes rarely give people enough time to choose where they will take shelter, and places of public assembly must be designed so that they do not collapse when hit by a tornado. The public must be made aware that buildings can resist tornadoes, and the building profession must be made aware of how this can be done. Better trained engineers and building inspectors, and perhaps a change in the attitudes of insurers and building financers, could bring about the necessary improvements in public buildings. Foremost among these should be the discontinuation of the use of unreinforced masonry in a structural system. For domestic dwellings and other nonengineered low-rise structures, local or state governments must adopt a more positive attitude toward the adoption and enforcement of specific and easily understood building code requirements concerning wind resistance. This process should be greatly facilitated by the recent introduction of a standard for walls in hurricane-force winds (Southern Building Code Congress International, l984). Designs meeting the requirements of this standard are deemed to satisfy the wind loading provisions of the Standard Building Code for walls, nominally 8 ft high, constructed of wood stud, brick, or concrete masonry units. The standard is intended to be used in areas where the design wind speed, based on a l00-year recurrence interval, exceeds 80 mph. This includes all of South Carolina. Use of this standard would significantly improve the ability of future low-rise structures to resist this type of event. For added protection of the occupants, the practice of constructing basements should be encouraged where conditions permit. Where under- ground construction is difficult, the provision of a reinforced interior room could be an acceptable alternative.
