WIND DAMAGE AND ITS MITIGATION
Wind is one of the manifestations of nature that catches the imagination of humans often inspiring feelings of awe and fear that can be linked with primitive religious experiences. Humanitys exploration of wind began with mythology and continued with the beginning of the sciences, but real understanding of the wind did not begin before the 18th century. The absence of tools for measuring temperature, pressure, and so on impaired real progress for many centuries. About 1600, however, Galileo invented the thermometer and in 1640s his pupil, Torricelli, invented the barometer. In 1625, Francis Bacon published a treatise in which he tried to pursue a scientific understanding of wind apart from Aristotles theories. In 1680, Edmond Halley, an English astronomer, wrote about trade winds an monsoons. He recognized that solar heating of the atmosphere over the equator was the energy source for tropical winds. In 1835, the French engineer, Gaspard Gustav de Coriolis, explained the deflection of the air masses that rotate around the earth and that the global wind circulation gives rise to cyclones and hurricanes.
Cyclones are systems of wind rotating counterclockwise in the northern hemisphere with a diameter varying from 50 to 1,000 miles and a barometric pressure that diminishes towards the centre. Other features of cyclones include the vertical ascent of the wind around the centre, the formation of a cloud above the centre, and condensation with precipitation.
Hurricanes are tropical cyclones of a particularly high intensity. As cyclones, they consist of rotating masses of air that converge spirally toward the centre where the atmospheric pressure is low.
Tornadoes are somewhat similar to hurricanes and even more violent. However, they are not part of the cyclone family. Furthermore, tornadoes have a diameter much smaller than 50 mils which is the minimum of cyclones.
The intensity of hurricanes and tornadoes have been described using recognized scales. The National Weather Services of the United States published a hurricane disaster potential scale which describes five categories of hurricane intensity starting with category no. 1 with wind speed range of 120-150 kmph. Category no. 5, which is the most severe, corresponded to wind speeds of about 250 kmph. While a Category 1 hurricane produces no real damage to structures a Category 5 hurricane is likely to produce complete building failures. Tornadoes are measured on a Fujita scale which describes about 12 classes with increasing intensity ranging from "weak" tornadoes having wind speeds of less than 64 kmph, through to "strong" tornadoes with speeds of 180-250 kmph, "devastating" tornadoes having speed ranges of 331-416 kmph, and through to "incredible" tornadoes with a wind speed range of 251-510 kmph. During an incredible tornado, automobile sized missiles fly through a distance of 100 metres.
Society as a whole, bears the burden of wind loss. Individuals, renters, homeowners, farmers, businesses, and government are all called upon to pay the price of losses attributable to hurricanes, tornadoes, severe local windstorms, and other wind hazards. Insurance represents means by which victims of a wind event may be indemnified if they purchased coverage. Insurance also provides a means whereby the losses of a few are distributed among the many. Losses incurred are distributed through the pricing mechanism of the coverage purchased.
The analysis of any natural hazard including wind hazards, must consider the interrelated phases of mitigation, preparedness, planning, emergency response and recovery. A variety of measures can be undertaken to mitigate the affect of damaging winds. The direct effect of these actions is to lessen property destruction, increase occupant safety, and lower the level of disruption to the community.
One of the most important factors in achieving mitigation of damages is concerned with design codes and code enforcement. The public generally assumes that if a building subject to a building code is issued a permit, is inspected during various phases of construction and is finally issued a certificate of occupancy upon completion of the building must then comply in every respect with the code including the ability to sustain wind loads. This may not necessarily be true, as evidenced by a wide survey carried a few years ago for this purpose. There are several reasons for this.
First is the need for a more rational approach to safety whereby occupant safety rather than the conventional concept of structural safety must be considered. The criteria that ensure structural safety are basically tied to the structure. However, such criteria may not guarantee the safety of the occupants of the structure. In any structure subjected to a hurricane, the potential for injury resulting from non-structural causes may be equal to or greater than that resulting from structural failure. It is therefore fitting to propose a method of structural evaluation that focuses directly on the safety of the occupant and which simultaneously account for the environmental forces, the structural characteristics, and the non-structural failure characteristics. It is felt that the risk of injury or death to an occupant of a structure is a rational measure of occupant safety. The smaller the risk, the greater the level of occupant safety offered by the structure. What is needed is a logical, consistent, and practical method of evaluating these risks.
Another aspect is the empirical nature of some of the code requirements. Over the years, local and model codes have developed empirical provisions to regulate common types of construction (such as wood framed and masonry buildings not exceeding 2 or 3 storeys in height). These empirical provisions often contain minimal requirements for lateral loading and stability against wind. They give no consideration to resistance to high winds and are based on construction practices that have withstood the "test of time" without any type of real review. Technically, therefore, adhering to empirical requirements does not set aside the need to verify compliance with the additional provisions for wind loads, earthquake loads, etc. However, it is still common practice for building permits to be issued for buildings "designed to comply with code empirical requirements that have not been subjected to an engineering analysis to determine if the structure can resist the required wind loads. Clearly periodic reviews of building code provisions in the light of additional wind damage data are essential ingredients of any damage mitigation strategy.
Another aspect of upgrading design and construction practices is to review the present code provisions which exempt small buildings, especially dwellings, from the requirement that a design be done by a licensed person.
Another aspect of the problem is concerned with the quality of code enforcement. The quality of code enforcement varies greatly among jurisdictions. One of the factors determining the quality of a jurisdictions political considerations, the salary level offered to personnel, and the number of qualifications of authorized personnel. There is increased awareness of the importance of the building code enforcement. Furthermore it is becoming standard practice to require that all buildings inspectors be certified, which usually entails attending a specified number of hours of continuing education classes each year. This positive trend must be encouraged in order to foster appreciation of sound construction practices.
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Walters Forensic Engineering | 277 Wellington
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