Tornadoes - Continued

DOE-STD-1020-2002

(3-1)

Along with nominal material strength and strength reduction factors, the following

SD load combinations for Performance Category 3 shall be considered:

(a) D + Wt

(b) D + L + Lr + Wt

(3-2)

where:

Wt = tornado loading, including APC and missile, as appropriate.

The notation and rationale for these load combinations are explained in Appendix D.

Careful attention should be paid to the details of construction. Continuous load paths

shall be maintained; redundancy shall be built into load-carrying structural systems; ductility

shall be provided in elements and connections to prevent sudden and catastrophic failures.

Two tornado missiles are specified as minimum criteria for this Performance Category.

The 2x4-in. timber plank weighing 15 lbs is assumed to travel in a horizontal direction at

speeds up to 100 mph. The horizontal speed is effective up to a height of 150 ft above

ground level. If carried to great heights by the tornado winds, the timber plank can achieve a

terminal vertical speed of 70 mph in falling to the ground. The horizontal and vertical speeds

are assumed to be uncoupled and should not be combined. Table 3-4 describes wall and roof

structures that will resist the postulated timber missile. A second missile to be considered is

a 3-in. diameter standard steel pipe, which weighs 75 lbs. Design horizontal impact speed is

50 mph; terminal vertical speed is 35 mph. The horizontal speed of the steel pipe is effective

up to a height of 75 ft above ground level. Table 3-4 summarizes certain barrier

configurations that have been successfully tested to resist the pipe missile. Although wind

pressure, APC and missile impact loads can occur simultaneously, the missile impact loads

can be treated independently for design and evaluation purposes. These are the minimum

missile design criteria and should be reviewed for upgrading with the concurrent use of new

tornado hazard assessment methodology (Reference 3-14).

3-9