(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

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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