features, such as asymmetry in plan or vertical
results in increased seismic load effects, as indicated
discontinuity, the assumptions used in developing
in Equation 4-4 and 4-5.
seismic criteria for buildings with regular features
e.
Ductile vs. Brittle Response.
may not apply. For example, planners often omit
Although
partitions and exterior walls in the first story of a
ductile response is highly desirable from an
building to permit an open ground floor; in this case,
earthquake energy dissipation standpoint, ductile
the columns at the ground level are the only
structures will be more flexible, and the designer
elements available to resist lateral forces, and there
must give proper consideration to the resulting drift
is an abrupt change in the rigidity of the vertical
to preclude structural instability and undue damage
elements of the lateral force resisting system at that
to nonstructural elements.
Similar consideration
level. This condition, generally referred to as soft
must be given to structural elements with anticipated
story, is undesirable. It is advisable to carry all shear
brittle response (e.g., shear in concrete columns).
walls down to the foundation. It is best to avoid
These elements must be designed so as to preclude
creating buildings with irregular features; however,
brittle response (e.g., adequate shear strength in
when irregular features are unavoidable, special
concrete columns to permit flexural yielding of
design considerations are required to account for the
column or connecting beams) or designed with
unusual dynamic characteristics and the load transfer
adequate capacity to resist the unreduced demand
and stress concentrations that occur at abrupt
forces. When a building is subjected to earthquake
changes in structural resistance.
ground motion, a pattern of lateral deformations that
varies with time is induced into the structure. At any
d.
Redundancy.
Redundancy is a highly
given point in time, a particular state of lateral
desirable
characteristic
for
earthquake-resistant
deformation will exist in the structure, and at some
design. Redundancy can be achieved with multiple
time within the period in which the structure is
load paths. For example, a multistory steel moment
responding to the ground motion, a maximum
frame building, with all the joints designed to be
pattern of deformation will occur. At relatively low
moment-resisting, has greater redundancy than a
levels of ground motion, the deformations induced
similar building with only selective moment-
within the building will be limited, and the resulting
resisting joints in that a flaw or unexpected failure of
stresses
that
develop
within
the
structural
one joint can be offset by redistribution of loads to
components will be within the elastic range of
the other joints. Redundancy can also be achieved
behavior.
Within this elastic range, the structure
with parallel or "back-up" systems, such as the
will
experience
no
damage.
All
structural
moment-resisting frames in a dual framing system in
components will retain their original strength,
which the frames are designed for a nominal lateral
stiffness, and appearance, and when the ground
force, but are expected to preclude collapse after the
motion stops, the structure will return to its pre-
shear
walls
or
braced
frames
have
failed.
earthquake condition.
At more severe levels of
Redundancy is defined by the reliability factor ∆
ground motion, the lateral deformations induced into
the structure will be larger. As these deformations
described in paragraph 4-1, and lack of redundancy
increase, so will demands on the individual
2-9
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