Page 499 - Design of Reinforced Masonry Structures

FIGURE 7.26 Irregularities in force transfer

TABLE 7.6 Types of Horizontal Structural Irregularities [7.13]

Irregularity type Description
1a Torsional Torsional irregularity is defined to exist where the maximum story drift, computed
irregularity including accidental torsion, at one end of the structure transverse to an axis
is more than 1.2 times the average of the story drifts at the two ends of the
structure. Torsional irregularity requirements in the reference sections
(of ASCE 7-5 Table 12.3-1) apply only in structures in which the diaphragms
are rigid or semirigid.
1b Extreme Extreme torsional irregularity is defined to exist where the maximum story drift,
torsional computed including accidental torsion, at one end of the structure transverse
irregularity to an axis is more than 1.4 times the average of the story drifts at the two ends
of the structure. Extreme torsional irregularity requirements in the reference
sections (of ASCE 7-5 Table 12.3-1) apply only to structures in which the
diaphragms are rigid or semirigid.

2 Reentrant Reentrant corner irregularity is defined to exist where both plan projections of the
corner structure beyond a reentrant corner are greater than 15% of the plan dimension
irregularity of the structure in the given direction.
3 Diaphragm Diaphragm discontinuity irregularity is defined to exist where there are diaphragms
discontinuity with abrupt discontinuities or variations in stiffness, including those having cutout
irregularity or open areas greater than 50% of the gross enclosed diaphragm area, or changes
in effective diaphragm stiffness of more than 50% from one story to the next.
4 Out-of-offsets Out-of-offsets irregularity is defined to exist where there are discontinuities in a
irregularity lateral force resistance path, such as out-of-plane offsets of the vertical elements.
5 Nonparallel Nonparallel systems irregularity is defined to exist where the vertical lateral
systems force–resisting elements are not parallel to or symmetric about the major
irregularity orthogonal axes of the seismic force–resisting system.

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