Tornadoes

DOE-STD-1020-2002

the use in this standard fastest quarter-mile and 3 second gusts are deemed to be equivalent

for all practical purposes.

With the wind speed given in fastest quarter-mile wind and an importance factor of 1.0,

the equations in ASCE 7 should be used to obtain design wind pressures on SSCs. Exposure

Category C should always be used with tornado winds regardless of the actual terrain

roughness. Non-conservative results will be obtained with exposure B. Tornadoes traveling

over large bodies of water are waterspouts, which are less intense than land-based tornadoes.

Thus, use of exposure category D also is not necessary. The velocity pressure exposure

coefficient and gust effect factor are obtained from ASCE 7. External pressure coefficients

are used to obtain tornado wind pressures on various surfaces of structures. Net pressure

coefficients are applicable to systems and components. On structures, a distinction is made

between main wind-force resisting systems and components and cladding.

If a structure is not intentionally sealed to maintain an internal negative pressure for

confinement of hazardous materials, or, if openings greater than one square foot per 1000

cubic feet of volume are present, or, if openings of this size can be caused by missile

perforation, then the effects of internal pressure should be considered according to the rules

of ASCE 7. If a structure is sealed, then atmospheric pressure change (APC) associated with

the tornado vortex should be considered instead of internal pressures. (see Table 3-1 and

note below for APC values).

The maximum APC pressure occurs at the center of the tornado vortex where the wind

speed is theoretically zero. A more severe loading condition occurs at the radius of

maximum tornado wind speed, which is some distance from the vortex center. At the radius

of maximum wind speed, the APC may be one-half its maximum value. Thus, a critical

tornado load combination on a sealed building is one-half maximum APC pressure combined

with maximum tornado wind pressure. A loading condition of APC alone can occur on the

roof of a buried tank or sand filter, if the roof is exposed at the ground surface. APC pressure

always acts outward. A rapid rate of pressure change, which can accompany a rapidly

translating tornado, should be analyzed to assure that it does not damage safety-related

ventilation systems. Procedures and computer codes are available for such analyses

(Ref. 3-8).

When using ASD methods, allowable stresses appropriate for the materials shall be used.

Since in this case, the hazard probability satisfies the performance goal, little or no additional

conservatism is needed in the design. Thus, for ASD the tornado wind load combinations are

modified to negate the effect of safety factors. For example, the combinations from ASCE 7

become:

(a) 0.63 (D + Wt)

(b) 0.62 (D + L + Lr + Wt)

3-8