structural components.
At different levels of
the
Immediate
Occupancy
Level,
damage
is
deformation, corresponding to different levels of
relatively limited. The structure retains a significant
ground motion severity, individual components of
portion of its original stiffness, and most, if not all,
the structure will be strained beyond their elastic
of its strength. At the Collapse Prevention level, the
range.
As this occurs, the structure starts to
building has experienced extreme damage.
If
experience damage in the form of cracking, spalling,
laterally deformed beyond this point, the structure
buckling, and yielding of the various components.
can experience instability and collapse. At the Life
As components become damaged, they degrade in
Safety Level, substantial damage has occurred to the
stiffness. In general, when a structure has responded
structure, and it may have lost a significant amount
to ground motion within this range of behavior, it
of its original stiffness; however, a substantial
will not return to its pre-earthquake condition when
margin remains for additional lateral deformation
the ground motion stops.
Some permanent
before collapse would occur.
At the Safe Egress
deformation may remain within the structure, and
level, the damage is intermediate between the
damage will be evident throughout. Depending on
Immediate Occupancy and the Life Safety levels. It
how far the structure has been deformed, and in what
should be noted that for given buildings, the relative
pattern, the structure may have lost a significant
horizontal and vertical scales shown on this plot may
amount of its original stiffness, and possibly,
vary significantly, and the margin of deformation
strength.
Brittle elements are not able to sustain
between individual performance levels may not be as
inelastic deformations and will fail suddenly; the
large as indicated in this figure. Figure 1-2 is a
consequences may range from local and repairable
similar curve, representative of the behavior of a
damage to collapse of the structural system.
At
nonductile, or brittle, structure. Note that for such a
higher
levels
of
ground
motion,
the
lateral
structure, there may be relatively little margin in the
deformations induced into the structure will strain a
responses
that
respectively
define
the
three
number of elements to a point at which elements
performance levels. For a given structure and design
behave in a brittle manner, or as a result of the
earthquake, it is possible to estimate the overall
decreased overall stiffness, the structure loses
deformation and force demand on the structure, and
stability. Eventually, partial or total collapse of the
therefore, the point on the corresponding curves
structure can occur.
The structural performance
shown in Figures 1-1 or 1-2 to which the earthquake
levels used in this document relate the extent of a
will push the building. This either will or will not
building's response to earthquake hazards to these
correspond to the desired level of performance for
various possible damage states.
Figure 1-1
the structure. The building should also be checked
illustrates the behavior of a ductile structure as it
for compliance with the allowable story drift levels
responds with increasing lateral deformation. The
prescribed in Table 6-1 to preclude unacceptable
figure is a schematic plot of the lateral force induced
damage to nonstructural systems and components.
in the structure as a function of lateral deformation.
When
structural/seismic
design
is
performed,
Four discrete points are indicated, representing the
modifications to the structural model are made to
discrete performance levels: Immediate Occupancy,
alter its strength, stiffness, or ability to dampen or
Safe Egress, Life Safety, and Collapse Prevention. At
resist induced deformations. These actions will alter
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