Quantcast Strike Likelihood - hdbk419a_vol10129

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MIL-HDBK-419A
3.8 DETERMINING THE NEED FOR PROTECTION.
The degree to which lightning protection is required, is a subjective decision requiring an examination of the
relative criticalness of the structure location and its contents to the overall mission of the facility. Those
structures containing elements vital to the operational mission such as air traffic control towers, radar
installations, navigational aids, and communications centers are examples of facilities which obviously must be
protected.  However, every building or structure does not require that a lightning protection system be
installed.  For example, buildings primarily used for the storage of nonflammable materials do not have a
critical need for protection.
Three of the factors to consider in ascertaining whether a given structure should have a lightning protection
system installed or in determining the relative comprehensiveness of the system are the relative threat of being
struck by lightning, the type of construction, and the nature of the facility.
3.8.1 Strike Likelihood.
The relative likelihood of a particular structure being struck by lightning is a function of the keraunic level,
i.e., the thunderstorm activity of the locality, the effective height of the structure and its attractive area.
Average thunderstorm activity can be determined from the isokeraunic maps shown in Figures 3-2 and 3-3.
Then using the techniques described in Section 3.4, estimate the frequency with which strikes to the structure
may occur. Use this estimation as one of the inputs to the decision process.
3.8.2 Type of Construction.
Steel frame buildings with metal outer coverings offer the greatest inherent protection against lightning
damage. Steel towers also exhibit a high immunity to structural damage. Additional protection for these type
buildings will probably be required only for very critical facilities in highly exposed locations. Steel frame
buildings with nonconductive, but nonflammable, outer coverings (like brick or other masonry) also offer a high
degree of protection against lightning damage.  The greatest hazard is posed by pieces of masonry being
dislodged by stroke currents passing through the outer coverings to reach the structural steel underneath.
Minimal protection consisting of interconnected air terminals to down conductors and steel support columns will
be sufficient to prevent this type of structural damage.
Buildings constructed of nonconductive materials such as wood, concrete blocks, or synthetic materials are the
most susceptible to destructive damage. A complete auxiliary protection system will be required to prevent
lightning damage to buildings utilizing this type of construction.
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