Page 239 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.23

FIGURE 7.17 Collapse of garage level below apartments, Reseda, California. (Credit:
J. Dewey, U.S. Geological Survey.) (Source: Geologic Hazards Photos, a compilation by the
National Geophysical Data Center of the U.S. Department of Commerce, National Oceanic and
Atmospheric Administration.)

Unfortunately, buildings often do not respond as the designers envisioned. While a
designer might plan a system to support lateral loads, and might provide adequate strength
and detailing in that system, in fact, buildings respond according to their mass and stiffness
distributions, and not necessarily according to the design. Building components “attract”
loads roughly in proportion to their relative stiffnesses compared to the stiffnesses of other
elements that act in parallel.
Seismic response involves large, inelastic deformations. Therefore, by design (and
necessity), stiff elements that are not part of the “intended” load path for seismic forces will
be subjected to the large deformations allowed by the anticipated response of the elements
designed to support seismic loads. As such, elements that are not part of the lateral load sys-
tem may sustain serious damage, if they are not properly detailed.
It is very common for brittle architectural components and nonstructural infill elements
to be damaged by earthquakes. These building components often are not designed to
accommodate the interstory deformations that are imparted by the building motion. Such
elements can fail due to frame motions in their plane; or if they are lightly reinforced or
inadequately supported, they can fail out of plane from the forces generated by their own
inertias. In older, unreinforced masonry structures, collapses of walls (Fig. 7.18), parapets
(Fig. 7.19), and other components (Fig. 7.20) are common in seismic events. When the
support of the entire structure is compromised by these local failures, total collapse often
follows.
Buildings in close proximity to adjacent structures can pound against each other.
Large-amplitude, out-of-phase sway of adjacent buildings can close gaps between build-
ings that are directly next to each other, particularly at upper levels. When contact is
severe enough to damage columns or bearing walls, partial or total collapse of one or both
buildings is possible.
Large-amplitude sway also leads to P-delta response that must be evaluated. As a build-
ing deforms laterally, the weights of each floor are displaced relative to the floor below.