The nature of wind storms and windinduced damage - Wind Loading of Structures - page 19

In 2005, there was an estimated US$80 billion of insured losses from natural disasters,
of which the majority originated from hurricanes and typhoons.
Some notable wind storms and the losses resulting from them are listed in Table 1.3.
Cyclone 'Tracy' and Hurricane 'Andrew' have already been mentioned, but in fact all the
events listed in Table 1.3 have had a major influence on the insurance industry and
structural engineering profession.
Table 1.3 does not include tornadoes. However, the aggregate damage from multiple
events can be substantial. For example, in the 'super-outbreak' of 3-4 April 1974, the
total damage in the state of Ohio alone was estimated to be US$100 million.
1.5 Wind-generated debris
As well as damage to buildings produced by direct wind forces—either overloads caused
by overstressing under peak loads or fatigue damage under fluctuating loads of a lower
level—a major cause of damage in severe wind storms is flying debris. Penetration of the
building envelope by flying missiles has a number of undesirable results: high internal
pressures threatening the building structure, wind and rain penetration of the inside of the
building, the generation of additional flying debris, and the possibility of flying missiles
inside the building endangering the occupants.
The area of a building most vulnerable to impact by missiles is the windward wall
region, although impacts could also occur on the roof and side walls. As the air
approaches the windward wall its horizontal velocity reduces rapidly. Heavier objects in
the flow with higher inertia will probably continue with their velocity little changed until
they impact on the wall. Lighter and smaller objects may lose velocity in this region or
even be swept around the building with the flow if they are not directed at the stagnation
point (see Chapter 4).
1.5.1 Threshold of flight
Wills et al. (1998) carried out an analysis of debris flight conditions and the resulting
building damage in severe winds. They considered 'compact' objects, sheet objects and
rods and poles (Figure 1.13) and established relationships between the body dimensions
and the wind speed, U f , at which flight occurs and the objects become missiles. For each
of the three categories, these relationships are
(1.13)
(1.14)
(1.15)