(6)
Analysis of
two-dimensional models.
deformation
at
which
its
stiffness
changes
Mathematical models describing the framing along
significantly (usually the yield load or member
each axis (axis 1 and axis 2) of the structure shall be
strength). The stiffness properties of this "yielded"
developed for two-dimensional analysis. The effects
component in the structural model are modified to
of horizontal torsion shall be considered.
If
reflect post-yield behavior, and the modified
multidirectional
excitation
effects
are
to
be
structure is subjected to an increase in lateral-loads
considered, component deformation demands and
(load
control)
or
displacements
(displacement
actions shall be computed for the following cases:
control), using the same shape of the lateral-load
100 percent of the target displacement along axis 1,
distribution, or an updated shape to reflect the
and 30 percent of the target displacement along axis
revised fundamental mode shape. Modification of
2; and 30 percent of the target displacement along
component behavior may be in one of the following
axis 1, and 100 percent of the target displacement
forms:
along axis 2.
Placing a hinge where a flexural
f.
Determination of Actions and Deformations.
element has reached its bending strength; this may
be at the end of a beam, column, or base of a shear
(1) Pushover curve. The general procedure
wall.
for the development of the load/displacement or
Eliminating the shear stiffness of a
pushover curve is as follows.
shear wall that has reached its shear strength in a
(a) An elastic structural model is developed
particular story.
that includes all components having significant
contributions to the weight, strength, stiffness,
Eliminating a bracing element that has
and/or stability of the structure, and whose behavior
buckled and whose post-buckling strength decreases
is important in satisfying the desired level of seismic
at a rapid rate.
performance. The structure is loaded with gravity
loads in the same load combination(s) as used in the
Modifying stiffness properties if an
linear procedures before proceeding with the
element is capable of carrying more loads with a
application of lateral loads.
reduced stiffness.
(b) The structure is subjected to a set of
(d) Step (c) is repeated as more and more
lateral loads, using one of the load patterns
components reach their strength. Note that although
(distributions) described in Paragraph 5-4e(3).
the intensity of loading is gradually increasing, the
load pattern usually remains the same for all stages
(c)
The intensity of the lateral load is
of the "yielded" structure, unless the user decides on
increased until the weakest component reaches a
the application of an adaptive load pattern. At each
5-12
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