stiff or rigid diaphragms is acceptable if torsional
CHAPTER 5
effects are either sufficiently small to be ignored or
ANALYSIS PROCEDURES
indirectly captured.
Vertical lines of seismic
framing in buildings with flexible diaphragms may
5-1.
General.
be individually modeled, analyzed, and evaluated as
two-dimensional assemblies of components and
a.
Introduction. This chapter defines four
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
Mt, resulting from the eccentricity between the
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
b.
Mathematical Modeling.
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, Mta, 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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