Page 228 - Forensic Structural Engineering Handbook

7.12 CAUSES OF FAILURES

FIGURE 7.8 Airborne missile damage to siding, Hurricane Andrew. (Credit: E. Schroter,
Simpson Gumpertz & Heger Inc.)

negative pressures generally diminish demand on all other surfaces where outside pressures
usually are negative. The condition changes when windows or curtain walls on the wind-
ward face are damaged by airborne debris. Normally such damage will substantially
increase the size and influence of openings on the positively pressurized windward face rel-
ative to openings on building surfaces with negative pressures. Consequently, the interior
of the building will be raised to net positive pressure relative to ambient pressure. Then
interior and exterior pressures become additive on the leeward wall, sidewalls, and the roof,
adding to the potential that components of these surfaces will be damaged.
It is rare that engineered buildings experience severe damage or collapse of the main
wind-resisting systems that support the building against lateral loads. Several factors con-
tribute to the relative success that these systems enjoy compared to component systems.
First, design procedures that are associated with design standards normally contain conser-
vative simplifying assumptions that allow designers to deal with the very complicated and
uncertain wind environment and pressures which normally occur on buildings. This adds a
measure of reserve to the strength of buildings that are designed conventionally. Second,
designers normally ignore the substantial, but uncertain, wind-resisting strengths that are
offered by building frames and components that are not part of the designed lateral load
systems. In particular, frames that are not braced or designed as moment frames, and frames
that are in buildings with shear walls that are assumed to resist lateral loads, have substan-
tial strengths that designers usually ignore. Also, incidental building components, such as
exterior curtain walls and interior partitions, have resistances that are disregarded in con-
ventional design. To the extent that these circumstantial strengths are not considered dur-
ing design, they become strength reserves that help the designed force-resisting systems
respond in extreme wind events. Finally, it is often the building envelope system that fails
first (Fig. 7.9). As this happens and surfaces that receive wind pressures are removed from
a building, the overall forces on the building diminish and the demand on the main force-
resisting system is reduced.