Page 229 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.13

FIGURE 7.9 Metal roof removed by Hurricane Andrew. (Credit: E. Schroter, Simpson
Gumpertz & Heger Inc.)

When main force-resisting systems of engineered buildings do fail, often it is the result
of structural modifications or deterioration that occurs over time. Obviously, if important
components of the main force-resisting system are compromised, the potential for failure
under extreme loadings is significant. Main force-resisting systems also fail in tornadoes,
usually because conventional design of common buildings that are not shelters usually
acknowledges that the probability that a tornado will strike a particular building is very low.
Consequently, designers usually disregard wind velocities and rapid pressure changes asso-
ciated with tornadoes. At wind speeds typically between 150 mi/h (67 m/s) and 250 mi/h
(112 m/s) and with rapid changes in air pressures, tornadoes can cause pressures on com-
ponents and structures to exceed normal design pressures by severalfold (Fig. 7.10).

FIGURE 7.10 Tornado damage to school, Lancaster, Texas. (Credit: J. Ceruti, Simpson
Gumpertz & Heger Inc.)

Occasionally, high winds cause main force-resisting systems to fail in buildings that are
not specifically designed for lateral loads. For instance, wind pressures can exceed resis-
tances of conventionally framed wood buildings (Fig. 7.11), especially when the buildings
are constructed with minimal connections. Even with inadequate main force-resisting