Windborne Missiles

DOE-STD-1020-2002

ASD is typically used for the design of steel, timber and masonry construction.

Allowable stresses for the material and the type of loading (axial, shear, bending, etc.) are

determined from applicable codes and specifications. The specified load combinations for ASD

for Performance Categories 1 and 2 should be taken from the applicable material design standard

(e.g. ACI or AISC) for straight winds. Load combinations for Performance Categories 3 and 4

can be less conservative than for Performance Categories 1 and 2. Because the ratio of hazard to

performance probability is smaller by a factor of two, it is judged that the load combinations can

be reduced by 10 percent. The load combinations for Performance Categories 3 and 4 for

straight winds should reflect this reduction. The hazard to performance probabilities for

tornadoes is more than satisfied by the hazard probability, as indicated by the ratio 1/5. The

tornado load combinations for ASD Performance Categories 3 and 4 were somewhat arbitrarily

chosen, based on engineering judgment.

Strength Design (SD) has been used for the design of reinforced concrete structures since

about 1977 (Ref. D-4). Recently a strength design approach was introduced for steel

construction which is called Load and Resistance Factor Design (LRFD) (Ref. D-5). Strength

design concepts are currently being developed for use with timber and masonry construction.

With SD, the nominal strength of the material is reduced to account for uncertainties in material

and workmanship. The reduced material strengths must be greater than or equal to the factored

loads in order to satisfy a postulated limit state. The required conservatism is reflected in the

load factors for loads involving straight winds. In this case, the load factors for Performance

Categories 3 and 4 are increased by ten percent. Load factors for Performance Categories 1 and

2 are recommended in References D-3, D-4 and D-5. Since the performance goals are satisfied

by the tornado hazard probabilities, unit value of load factors can be used for SD. Unit values

are justified in this case, because the material reduction factors account for uncertainties

associated with materials. The load factors for tornadoes are consistent with recommendations

for commercial nuclear power plants as given in ACI 349 (Ref. D-6) for concrete and

ANSI/AISC N690-1984 (Ref. D-7) for steel.

D.4

Windborne Missiles

Windborne missile criteria specified herein are based on windstorm damage

documentation and computer simulation of missiles observed in the field. Reference D-8

documents the occurrence of classes of missiles that are picked up and transported by straight

winds and tornadoes. Computer simulation of tornado missiles is accomplished using a

methodology developed at Texas Tech University. The method is similar to one published in

Reference D-9.

The timber plank missile is typical of a class of missiles that are frequently found in the

windstorm debris. The 2x4 timber plank weighing 15 lbs is typical of the debris from damaged

or destroyed residences, office trailers and storage shacks. It can be carried to heights up to 200

ft in strong tornadoes. The 3-in. diameter standard steel pipe is typical of a class of missiles,

which includes small diameter pipes, posts, light-weight rolled steel sections and bar joists.

These objects are not likely to be carried to heights above 100 ft because of their larger weight to

surface area ratio. Automobiles, storage tanks, trash dumpsters are rolled and tumbled by high

D-6