Additional Adverse Effects of Tornadoes
In addition to wind effects, tornadoes produce atmospheric pressure change effects and
missile impacts from windborne debris (tornado-generated missiles).
Atmospheric pressure change (APC) only affects sealed structures. Natural porosity,
openings or breach of the structure envelope permit the inside and outside pressures of an
unsealed structure to equalize. Openings of one sq ft per 1000 cu ft volume are sufficiently large
to permit equalization of inside and outside pressure as a tornado passes (Reference E-1). SSCs
that are purposely sealed will experience the net pressure difference caused by APC. APC, when
present, acts outwardly and combines with external wind pressures. The magnitude of APC is a
function of the tangential wind speed of the tornado. However, the maximum tornado wind
speed and the maximum APC do not occur at the same location within the tornado vortex. The
lowest APC occurs at the center of the tornado vortex, whereas the maximum wind pressure
occurs at the radius of maximum wind, which ranges from 150 to 500 ft from the tornado center.
The APC is approximately one-half its maximum value at the radius of maximum wind speed.
With APC acting on a sealed building, internal pressure need not be considered. The rate of
APC is a function of the tornado's translational speed, which can vary from 5 to 60 mph. A rapid
rate of pressure change can produce adverse effects on HVAC systems. Treatment of these
effects is beyond the scope of this document.
High winds and tornadoes pick up and transport various pieces of debris, including roof
gravel, pieces of sheet metal, timber planks, plastic pipes and other objects that have high surface
area to weight ratios. These objects can be carried to heights up to 200 feet in strong tornadoes.
Steel pipes, posts, light-weight beam sections and open web steel joists having smaller
area-to-weight ratios are transported by tornado winds, but occur less frequently and normally do
not reach heights above 100 ft. Automobiles, storage tanks, and railroad cars may be rolled and
tumbled by severe tornado winds. In extremely rare instances, large-diameter pipes, steel wide
flange sections and utility poles are transported by very intense tornado winds. These latter
missiles are so rare that practicality precludes concern except for SSCs having lower
probabilistic performance goals than Performance Category 4, which are comparable to SSCs
found in commercial nuclear power plants.
Effects on Structures, Systems, and Components
A structure as used herein is an element or collection of elements that provide support or
enclosure of space, e.g. a building. The walls and roof make up the envelope of a structure.
Wind pressures develop on the surfaces of a building envelope and produce loads on the support
structure, which, in turn, transmits the loads to the foundation. The support structure also must
carry dead, live and other environmental loads.
Element failure is quite rare. More frequently the element connections are the source of
failure. A properly conceived wind-force resisting structure should not fail as a result of the
failure of a single element or element connection. A multiple degree of redundancy should be
provided in a ductile structure that allows redistribution of load when one element or connection