Civil Engineering Reference
In-Depth Information
FIGURE 4.9
Damage caused by a soft story at the first-floor level. The damage occurred during the Izmit
earthquake in Turkey on August 17, 1999. (
Photograph by Mehmet Celebi, USGS.
)
the structural design. For example, after the Kobe earthquake in Japan on January 17, 1995,
it was observed that there were a large number of 20-year and older high-rise buildings that
collapsed at the fifth floor. The cause was apparently an older version of the building code
that allowed a weaker superstructure beginning at the fifth floor.
While damage and collapse due to a soft story are most often observed in buildings, they
can also be developed in other types of structures. For example, Figs. 4.12 and 4.13 show
an elevated gas tank that was supported by reinforced concrete columns. The lower level
containing the concrete columns behaved as a soft story in that the columns were unable to
provide adequate shear resistance during the earthquake.
Concerning the retrofitting of a structure that has a soft story, the National Information
Service for Earthquake Engineering (2000) states:
There are many existing buildings in regions of high seismic risk that, because of their
structural systems and/or of the interaction with non-structural components, have soft stories
with either inadequate shear resistance or inadequate ductility (energy absorption capacity) in
the event of being subjected to severe earthquake ground shaking. Hence they need to be retro-
fitted. Usually the most economical way of retrofitting such a building is by adding proper
shear walls or bracing to the soft stories.
4.4.2 Pancaking
Pancaking occurs when the earthquake shaking causes a soft story to collapse, leading to
total failure of the overlying floors. These floors crush and compress together such that the
