Lightning and Metal Structures

The probability of lightning striking

The probability of a lightning strike is determined by a number of factors:

1.The topography in the area of the subject facility. Probability of strike is higher if the project is located on a mountain top or hill top as opposed to a field.

2. Size and height of the subject structure. A tall building or one covering more ground area is more likely to be struck than a short or small building. A tall, slender structure (such as a steeple or lighthouse) is also a more likely candidate for a strike.

3. Relative location of the subject structure with respect to nearby larger and taller structure. Presence of a very tall structure in proximity to a small, short-building will tend to further reduce the likelihood of a strike to the small building.

4. Frequency and severity of thunderstorm activity in the geographic area of the project.

As can be appreciated by reviewing the above factors, the probabilities of a strike to a metal structure are no more or less than any other kind of structure, as these probabilities have to do with height and size of the structure and its surroundings, rather than its construction materials.

Studying the consequences of a lightning strike

In order to adequately assess risks involved with lightning events, the consequence of a strike must also be studied. In other words, what if lightning does strike a subject building ­ what will happen? Obviously, there is a potential threat to human life associated with a lightning strike in addition to the threat of damage to either the contents of the building, or the building itself, or both. These threats are affected by the following factors:

1.The construction materials used for both framing and roof coverings. If these materials are (electrically) conductive, the threat of fire and explosion are both reduced, also reducing the threat to human life. If these materials are noncombustible the threat of damage to them is reduced, and they will not contribute a fuel source to any fire resulting from a lightning strike.

2. Physical contents of a building. If contents are flammable, or explosive, risks of the perils of fire are obviously increased. If contents are highly sensitive electronic or other equipment highly valuable or irreplaceable items, then the consequence of loss is intensified.

3. Human occupancy. Buildings which are heavily occupied are considered to be at higher risk than unoccupied or sparsely occupied buildings. Also, the type of occupancy has a bearing. If a fire results from a lightning strike, the risk to human life is greater if occupants are handicapped or non-ambulatory and cannot be quickly evacuated.

4. Remoteness of the building. If the building is remote with respect to fire fighting and medical emergency response, the risks of physical loss due to fire as well as human peril are increased.

Because metal products are both an electrical conductor, and a noncombustible material, the risks associated with its use and behavior during a lightning event make it the most desirable construction material available. This fact notwithstanding, and in view of the many variables which contribute to lightning risk, it may, in some cases, be prudent to consider lightning protection. A lightning protection system provides for a continuous conductor from earth to sky (and vice-versa) so that the electrical charge is furnished an obvious path through which to flow, thereby reducing the risk to (electrically) resistive construction materials and human life.

Metal Construction Association/National Fire Protection Association, Quincy, Mass. 1995

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