Performance Category 4 - STD-102020020059

DOE-STD-1020-2002

3.2.4

Performance Category 4

The performance goal for Performance Category 4 requires more conservative criteria

than Performance Category 3. In some geographic regions, tornadoes must be considered.

Straight Winds and Hurricanes

For those sites where tornadoes are not a viable threat, the recommended basic wind

speed is based on an annual exceedance probability of 1x10-4. The importance factor is 1.0.

Once the design wind speeds are established and the importance factors applied,

determination of Performance Category 4 wind loads is identical to Performance Category 3,

except as noted below. SSCs in category Performance Category 4 may be designed or evaluated

by ASD or SD methods, as appropriate for the material being used in construction. As with

Performance Category 3, the wind hazard exceedance probability contributes a larger percentage

of the total probabilistic performance goal than Performance Categories 1 or 2. Less

conservatism is needed in the design and evaluation procedure. The degree of conservatism for

Performance Category 4 is the same as Performance Category 3. Thus, the load combinations

for both the ASD and SD are the same for Performance Categories 3 and 4.

Although the design wind speeds in Performance Category 4 are larger than Performance

Category 3, the same missiles are specified (Table 3-3), except the maximum height above

ground is 50 ft instead of 30 ft for Performance Category 4.

Tornadoes

For those sites requiring design for tornadoes, the criteria are based on LLNL tornado

hazard assessment methodology presented in Reference 3-14. Again, as with Performance

Category 3, an annual exceedance probability of 1x10-4 could be justified. However, for the

same reasons given for Performance Category 3, a lower value is recommended. The basic

tornado wind speed associated with an annual exceedance probability of 2x10-6 and an

importance factor of 1.0 is recommended. Once the basic tornado wind speed is determined for

the specified annual exceedance probability and given in fastest-quarter mile, the procedure is as

described for Performance Category 3, except as noted below.

Three tornado missiles are specified for Performance Category 4: a timber plank, a steel

pipe and an automobile. The 2x4 timber plank weighs 15 lbs and is assumed to travel in a

horizontal direction at speeds up to 150 mph. The horizontal component of the timber missile is

effective to a maximum height of 200 ft above ground level. If carried to a great height by the

tornado winds, it could achieve a terminal vertical speed of 100 mph as it falls to the ground.

The second missile is a 3-in. diameter standard steel pipe, which weighs 75 lbs. It can achieve a

horizontal impact speed of 75 mph and a vertical speed of 50 mph. The horizontal speed could

be effective up to a height of 100 ft above ground level. The horizontal and vertical speeds of

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