A Correction to Our Lightning Protection Story in Winter 2006 Pure Power

03/28/2006


Editor’s Note: In the Winter 2006 issue of Pure Power, we ran a story titled Lightning Protection: Optional or Recommended by frequent contributor and CSE editorial consultant Keith Lane, P.E. Several readers questioned the calculations used. The author replies below:

After the publication of “Lightning Protection: Optional or Recommended,” I realized that there was a computational error on the example provided. In the determination of Ae (the equivalent collection area), the equation I provided is correct, but the result does not include the pi x 9 x H(squared) portion. I have rectified the error as follows:

Building #1. Physical Characteristics

Length %%MDASSML%% 100 feet = 0.0305 km

Width %%MDASSML%% 100 feet = 0.0305 km

Height %%MDASSML%% 80 feet = 0.0244 km

Ae is the equivalent collective area of the structure in km squared. It is equivalent to the ground area with the same yearly lightning flash probability as the structure. This value increases with increased height of the building.

Ae = LW + 6H (L+W) + pi9H2= 0.0267

Ae = 0.000930 +0.00893 + 0.01682 = 0.0267

Ng %%MDASSML%% Assume the project is in Seattle, Wash., where the yearly average flash density in the region is 0.1.

(C1) for this example will be 0.5. The structure is surrounded by smaller structures within a distance of 3H. H is equal to the height of the building.

Nd (Lighting Strike Frequency) = (Ng) (Ae) (C1)= 0.1 * 0.0267 * 0.5 = 0.0013.

C2 %%MDASSML%% Assume that it is a metal structure and a nonmetallic roof with a resulting coefficient of 1.00.

C3 %%MDASSML%% Assume that the contents of the building are of standard value and are nonflammable. This will result in a coefficient of 1.00.

C4 %%MDASSML%% Lets assume t hat the building is normally occupied. This will result in a value of 1.00.

C5 %%MDASSML%% Lets assume that the electrical system continuity is not required and that there would be no environmental impact if the electrical system were shut down from a lightning strike. This will result in a value of 1.00.

Nc= (1.5x10-3) / C where

C = (C2) * (C3) * (C4) * (C5)

In our example, C = 1 * 1 * 1 * 1 = 1

Nc = (1.5x10-3) / C = .0015

Nd (Lighting Strike Frequency) is less than Nc (Tolerable Lighting Frequency).

.0013 is less than .0015

In this example, Lightning Protection is Optional based on the Risk Analysis of NFPA 780.

Building #2. Physical Characteristics

Length %%MDASSML%% 100 feet = 0.0305 km

Width %%MDASSML%% 100 feet = 0.0305 km

Height %%MDASSML%% 120 feet = 0.0366 km

Ae is the equivalent collective area of the structure in km (squared). It is equivalent to the ground area with the same yearly lightning flash probability as the structure. This value increases with increased height of the building.

Ae = LW + 6H (L+W) + pi9H2= 0.052

Ae = 0.000930 +0.0134 + 0.0379 = 0.052

Ng %%MDASSML%% Assume the project is in Seattle, Washington %%MDASSML%% The yearly average flash density in the region is 0.1.

(C1) for this example will be 0.5 %%MDASSML%% The structure is surrounded by smaller structures within a distance of 3H. H is equal to the height of the building.

Nd (Lighting Strike Frequency) = (Ng) (Ae) (C1)= 0.1 * 0.052 * 0.5 = 0.0026.

C2 %%MDASSML%% Assume that it is a metal structure and a non-metallic roof with a resulting coefficient of 1.00.

C3 %%MDASSML%% Assume that the contents of the building are of standard value and are non-flammable. This will result in a coefficient of 1.00.

C4 %%MDASSML%% Lets assume t hat the building is normally occupied. This will result in a value of 1.00.

C5 %%MDASSML%% Lets assume that the electrical system continuity is not required and that there would be no environmental impact if the electrical system were shut down from a lightning strike. This will result in a value of 1.00.

N c= (1.5x10-3) / C where

C = (C2) * (C3) * (C4) * (C5)

In our example, C = 1 * 1 * 1 * 1 = 1

N c = (1.5x10-3) / C = .0015

Nd (Lighting Strike Frequency) is greater than Nc (Tolerable Lighting Frequency).

.00260 is greater than .0015

In this example, Lightning Protection is recommended based on the Risk Analysis of NFPA 780.

The Intent of the example was to illustrate how changing variables (height) of the building can effect the recommendation of supplemental lightning protection.

The risk assessment as illustrated above is a tool the engineer can utilize to provide a guideline to the building owner. The final decision to employ a lightning protection system should be made by the building owner.





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