stiff or rigid diaphragms is acceptable if torsional

effects are either sufficiently small to be ignored or

indirectly captured.

Vertical lines of seismic

framing in buildings with flexible diaphragms may

be individually modeled, analyzed, and evaluated as

two-dimensional assemblies of components and

elements, or a three-dimensional model may be used

basic analytical procedures; however, only the first

with the diaphragms modeled as flexible elements.

three procedures are prescribed by this document.

Connection modeling is not required for linear

The first two procedures are linear elastic, and the

analysis.

Explicit modeling of a connection is

latter two procedures are nonlinear. Limitations on

required for nonlinear analysis if the connection is

the use of linear elastic static procedures are

weaker than the connected components, and/or the

indicated in Paragraph 5-2b, and conditions when

flexibility of the connection results in a significant

nonlinear procedures are required are provided in

increase in the relative deformation between the

Paragraph 5-4b. The procedures are discussed in the

connected components.

following paragraphs in order of increasing rigor

and complexity.

Advantages, disadvantages, and

(2)

Horizontal torsion.

The effects of

limitations are indicated for each procedure.

horizontal torsion must be considered for buildings

Paragraph 4-11 and Table 4-4 prescribe the

with diaphragms capable of resisting such torsion.

minimum analytical procedure for each performance

The total torsional moment at a given floor level

objective for the various seismic use groups. The

shall be set equal to the sum of the following two

prescribed minimum procedure is intended to apply

torsional moments:

to structures of average complexity for each

performance objective. Unusual, or more complex,

The actual torsion; that is, the moment,

structures may require more complex or rigorous

analytical procedures than the prescribed minimum.

centers of mass at all floors above and including the

given floor, and the center of rigidity of the vertical

seismic elements in the story below the given floor,

and

(1) Basic assumptions. In general, a building

should be modeled, analyzed, and evaluated as a

The accidental torsion; that is, an

three-dimensional

assembly

of

elements

and

accidental torsional moment, *M*ta, produced by

components.

Three-dimensional

mathematical

horizontal offset in the center of mass, at all floors

models shall be used for analysis and evaluation of

above and including the given floor, equal to a

buildings with plan irregularity. Two-dimensional

minimum of 5 percent of the horizontal dimension at

modeling, analysis, and evaluation of buildings with

5-1

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