Tornado Protection: Selecting Refuge Areas in Buildings 

Chapter 2: Effects of High Winds



In buildings hit by tornadoes, the threat to life is due to a combination of effects that 
occur at almost the same time. To understand the tornado damage that can occur in a 
building, the following must be considered: 

¥ wind-induced forces 

¥ changes in atmospheric pressure
 
¥ debris impact 



Wind Effects on Buildings

The wind speeds generated by some tornadoes are so great that designing for these 
extreme winds is beyond the scope of building codes and engineering standards. Most 
buildings that have received some engineering attention, such as schools, and that are 
built in accordance with sound construction practices can usually withstand wind 
speeds specified by building codes. Meeting these code-specified wind speeds can 
provide sufficient resistance to tornadic winds if the building is located on the outer 
edge of the tornado vortex. In addition, if a portion of the building is built to a higher 
tornado design standard, then both building and occupant survival are improved. 
Wind creates inward- and outward-acting pressures on building surfaces, depending on 
the orientation of the surface (e.g., flat, vertical, low-slope). As the wind moves over 
and around the building, the outward-acting pressure increases as the building geometry 
forces the wind to change direction. These pressure increases create uplift on parts of 
the building, forcing the building apart if it is too weak to resist the wind loads. When 
wind forces its way inside or creates an opening by breaking a window or penetrating 
the roof or walls, the pressures on the building increase even more. Figure 2-1 shows 
how wind affects both an enclosed building and a building with openings. 
Heavy building materials (e.g., reinforced masonry or concrete) that are well tied to all 
other building components often survive extreme winds. The weight of these materials 
helps resist uplift and lateral loads, and heavy materials often stop windborne debris 
that can increase damage to the building. However, heavy concrete roof panels and 
heavy masonry walls that are not adequately connected or reinforced have failed 
during severe winds. Lightweight roofing and siding materials such as gravel, 
insulation, shingles, roofing membranes, and brick veneer can also be a problem. 
Building shapes that "catch" the wind, such as overhangs, canopies, and eaves, tend to 
fail and become "sails" in extreme winds. Flat roofs can be lifted off when the wind 
flows over them and increases the uplift pressure at the corners and edges of the roofs.



Atmospheric Pressure Changes

Initially, the pressure outside a building during a tornado is very low compared to the 
pressure inside. In most buildings, however, there is enough air leakage through 
building component connections to equalize these pressures. Also, windborne debris is 
likely to break windows and allow wind to enter. 
The explosion of buildings during a tornado due to atmospheric pressure differences is 
a myth. In reality, the combination of internal pressure and outward pull on the building 
from suction pressure has caused building failures that have forced the walls outward 
and given the building the appearance of having exploded. During an event, doors and 
windows should remain closed on all sides of the building in order to minimize the entry 
of wind into the building. 
building with openings. 




Debris Impact

The extreme winds in tornadoes pick up and carry debris from damaged buildings and 
objects located in the path of the winds (see Figures 2-2 and 2-3). Even heavy, massive 
objects such as cars, tractor trailers, and buses can be moved by extreme winds and 
cause collateral damage to buildings. Light objects become flying debris, or missiles, 
that can penetrate doors, walls, and roofs; heavier objects can roll and cause crushing-
type damage. 
Missiles can travel vertically as well as horizontally (see Figure 2-4). Therefore, 
shelters and refuge areas should provide protection overhead as well as 
Figure 2-3 
on the side. Building walls and roofs can be designed to withstand the im-
Example of severe damage from a windborne 
pacts of these missiles. Protection can be provided at the exterior building 
missile.This metal door was pushed inward by 
wall, or interior barriers can be constructed to provide protection for a 
smaller 
the impact of a heavy object. 
area within the building. 



Figure 2-4 Example of damage from windborne missiles. Medium and small missiles 
penetrating through the roof of a high school.The missile protruding from the roof in the 
foreground is a double 2-inch by 6inch wood board.The portion sticking out of the roof 
is 13 feet long.This missile penetrated a ballasted ethylene propylene diene monomer 
(EPDM) membrane, approximately 3inches of polyisocyanurate roof insulation, and 
the steel roof deck.The missile lying on the roof just beyond it is a 2-inch by 10-inch, 
16-foot-long wood board.The missile protruding from the roof in the background is a 2-
inch by 6-inch, 16-foot-long wood board.



Selecting Refuge Areas

Wind effects on buildings have been studied sufficiently to predict which building 
elements are most likely to successfully resist the extreme wind pressures caused by 
tornadoes and which are most likely to fail. Sufficient material testing and design work 
has been performed for large shelters to develop a refuge area selection guide for any 
building in which such areas are needed. Many buildings contain a small interior area 
or areas that could serve as the best available refuge area or possibly be converted or 
reinforced for refuge area use. 
The selection of refuge areas in existing buildings is discussed in Chapter 4. For more 
information about refuge areas and shelter design, refer to FEMA publication 361, 
Design and Construction Guidance for Community Shelters (see sidebar on page 11). 
10 Tornado Protection: Selecting Refuge Areas in Buildings