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MILHDBK419A
should be applied to Equation 34. The experimental data to justify the use of Equation 35 for structures
greater than 400 meters (1300 feet) is sketchy. However, since structures even approaching this height are not
expected to be of primary concern, Equations 34 and 35 are expected to be adequate for most design
purposes.
Large flat buildings that do not extend above the median treetop level in the general area will have an
attractive area that is essentially the area of the roof (assuming the roof covers the entire structure). If the
building is several stories high such that it appreciably extends above the prevailing terrain, then its attractive
area is its roof area plus that portion of the attractive area not already encompassed by the roof. Figure 34
illustrates the method for calculating the attractive area of a rectangular structure of length,
and width, w.
The roof area is given by x w. The additional attractive area resulting from the height of the building is
readily determined by recognizing that the areas contributed by the four corners of the building equal a circle
of radius, ra. Both ends of the structure (dimension w) contribute the area of
the sides contribute
The total attractive area is the sum of the roof area
the corners
the ends
and the sides
to produce a total of
Figure 35 indicates that the height to be used in calculating the attractive area of a tall structure should be
the height that the structure extends above the effective (i.e., the level that earth charges would rise to if the
building were not there) levels of the earth. On open, level terrain the height, h, would be the full height of the
roof from grade level.
The number of flashes which can be expected to strike a given structure is equal to the product of the flash
times the attractive area,
density,
of the structure. For example, suppose the relative likelihood of a
lightning strike to a low, flat structure 100 meters on a side, located in Nashville, TN, is desired. From Figure
32, Ty is determined to be approximately 54 thunderstorm days per year. The flash density as given by
Equation 31 is 20.4 flashes/km /year. The proportion of those flashes that are discharges to earth is 24.4
percent (from Equation 32) since the latitude is 36 degrees. Thus approximately 5 flashes/km /year to earth
can be expected. Within the area of the structure (0.01 km2) there will be only 0.05 strikes per year on the
average, or there is a 1 in 20 chance of being struck by lightning in a given year. For the same structure in
Southern California, only a 1 in 330 likelihood of a strike would be expected in a given year.
3.5.2 Cone of Protection.
This ability of tall structures or objects to attract lightning to themselves serves to protect shorter objects and
structures. In effect, a taller object establishes a protected zone around it. With this protected zone, other
shorter structures and objects are protected against direct lightning strikes. As the heights of these shorter
objects increase, the degree of protection decreases. Likewise, as the separation between tall and short
structures increases, the protection afforded by the tall structure decreases. The protected space surrounding a
lightning conductor is called the zone (or cone) of protection.
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