The nature of wind storms and windinduced damage - Wind Loading of Structures - page 20

where ℓ is a characteristic dimension for 'compact' objects, t the thickness of sheet
objects, d the effective diameter of rod-type objects, ρ a the density of air, ρ m the density
of the object material, C F an aerodynamic force coefficient (see Section 4.2.2), U f the
wind speed at which flight occurs, I a fixing strength integrity parameter, i.e. the value of
force required to dislodge the objects expressed as a multiple of their weight (for objects
resting on the ground ), and g the gravitational constant.
Equations (1.13), (1.14) and (1.15) illustrate the important point that the larger the
value of the characteristic dimension, ℓ, t or d, the higher the wind speed at which flight
occurs. These equations also show that the higher the value of the density, ρ m , the higher
the wind speed for lift off. Thus as the wind speed in a cyclone builds up, the smaller,
Figure 1.13 Three types of flying debris (after
Wills et al., 1998).
lighter objects—e.g. gravel, small loose objects in gardens and backyards—'fly' first. At
higher wind speeds appurtenances on buildings are dislodged as the wind forces exceed
their fixing resistance, and they also commence flight. At even higher wind speeds,
substantial pieces of building structure such as roof sheeting and purlins may be removed
and become airborne.
As examples of the application of Equation (1.13), Wills et al. (1998) considered
wooden compact objects ( ρ m =500 kg/m 3 ) and stone objects ( ρ m =2700 kg/m 3 ). Assuming
C F =1 and I =1, Equation (1.12) gives ℓ equal to 110 mm for the wooden missile, but only
20 mm for the stone missile, for a lift-off speed of 30 m/s.