Page 230 - Forensic Structural Engineering Handbook

7.14 CAUSES OF FAILURES

FIGURE 7.11 Collapse of wood-framed building, Hurricane Andrew. (Credit: L. Morse-
Fortier, Simpson Gumpertz & Heger Inc.)
systems, such buildings usually fail first by components, because these individual elements
also have insufficient strength to resist applied pressures.
Building systems can respond dynamically to wind loads. Certainly, if frequencies in
the wind speed spectrum excite structural frequencies, dynamic amplification can occur.
Additional dynamic responses, called aeroelastic phenomena, also are possible for flexible
structures. These phenomena are divided into three major categories: galloping, vortex-
shedding excitation, and flutter.
Galloping instability occurs when the aerodynamic damping of an asymmetric body is
negative and exceeds, in absolute magnitude, the mechanical damping of a structural sys-
tem. Power transmission lines sometimes oscillate transverse to the wind due to this phe-
nomenon if they have an asymmetric accumulation of ice (cylindrical bodies cannot gallop).
Vortex-shedding excitation is transverse oscillation that occurs when slender bodies, such as
chimneys, alternately shed vortices of wind from opposite surfaces at a frequency that is near
to a natural frequency of the structure. In these cases, the frequency of the pressure changes
that are associated with the turbulence of the vortices “locks in” with the frequency of the
structure, and the structure is forced in a transverse direction at its natural frequency. Flutter
normally occurs when two degrees of body motion, such as rotation and transverse move-
ment, couple in the wind stream. Flutter is the phenomenon that causes street signs on single,
torsionally flexible posts to twist violently in strong winds. Flutter also causes flags (and
sometimes fabric structures) to wave in the wind.

Determination of Wind Forces
Wind forces on structures can be determined by analytical and experimental procedures.
However, before reliable calculations can be made, it is necessary to establish the wind
environment at the site of an investigation. Relevant information includes wind speed
and direction, “gustiness,” and storm duration. Most major airports maintain reliable
anemometers. Also research facilities, such as colleges and universities, and public safety