the design of the structure to resist ground shaking is
seismically active zone, it is generally expected that
appropriate. Where a site geological hazard exists, it
the structure will be subjected to many small
may be feasible to mitigate it, either by itself or in
earthquakes, some moderate earthquakes, one or
connection with the design of the structure. It is also
more large earthquakes, and possibly a very severe
possible that the risk from a site hazard is so extreme
earthquake.
In general, it is uneconomical or
and difficult to control that construction on the site
impractical to design buildings to resist the forces
will not be cost-effective. Chapter 3 and Appendices
resulting from the very severe or maximum credible
F and G provide guidance for the evaluation and
earthquake within the elastic range of stress; instead,
mitigation of site geological hazards.
the building is designed to resist lower levels of
force, using ductile systems. When the earthquake
motion is large to severe, the structure is expected to
2-4.
Behavior of Structures.
yield in some of its elements. The energy-absorbing
capacity (ductility) of the yielding structure will limit
Buildings and other structures are composed of
the degree of life-threatening damage; buildings that
horizontal and vertical structural elements that resist
are properly designed and detailed can survive
lateral forces. The horizontal elements, diaphragms
earthquake forces substantially greater than the
and horizontal bracing, are used to distribute the
forces associated with allowable stresses in the
lateral forces to vertical elements.
The vertical
elastic range.
Seismic design concepts must
elements that are used to transfer lateral forces to the
consider building proportions and details for their
ground are shear walls, braced frames, and moment
ductility and for their reserve energy-absorbing
resisting frames.
The structure must include
capacity for surviving the inelastic deformations that
complete lateral and vertical-force-resisting systems
would
result
from
the
maximum
expected
capable of providing adequate energy dissipation
earthquake. Special attention must be given to the
capacity to withstand the design ground motions
connections that hold together the elements of the
within the prescribed limits of deformation and
lateral-force-resisting system.
strength demand.
b.
Analysis of Structural Response.
As
a.
Demands of Earthquake Motion. The loads
indicated above, the response of structures to severe
or forces that a structure sustains during an
ground motion is a complex combination of elastic
earthquake result directly from the distortions
and inelastic actions. Additionally, as yielding is
induced in the structure by the motion of the ground
initiated
in
individual
structural
elements,
on which it rests. Ground motion is characterized by
subsequent loads are redistributed among the
displacements, velocities, and accelerations that are
remaining elastic elements. Linear analyses assume
erratic in direction, magnitude, duration, and
that the response can be adequately represented by
sequence. Earthquake loads are inertia forces related
an elastic mode of the structure with various
to the mass, stiffness, and energy-absorbing (e.g.,
response modification factors to represent ductility or
damping and ductility) characteristics of the
the energy absorption capabilities of the structure.
structure. During the life of a structure located in a
Linear elastic and dynamic analyses with a global
2-4
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