Page 496 - Design of Reinforced Masonry Structures

7.58 CHAPTER SEVEN

elements so placed that they provide a continuous load path for both gravity and lateral
loads. The assumption is that regular structures provide a reasonably uniform distribution
of inelastic behavior in elements throughout the seismic force–resisting system.
Building configurations can be considered with respect to plan or elevation. A build-
ing that lacks symmetry in terms of location of its force-resisting elements is said to be
irregular. A building can have horizontal structural irregularities or vertical structural
irregularities. Horizontal structural irregularities result from diaphragms with offsets,
with reentrant corners, or with any kind of discontinuity (Figs. 7.23 and 7.24). Vertical
irregularities result from factors such as distribution of mass, stiffness, or strength,
which can result in seismic forces and/or deformations over the height of the structure
that are significantly different from the linearly varying distribution assumed in design.
These irregularities are referred to as dynamic force irregularities and include stiffness
(soft story), weight (mass), and vertical geometric irregularities (Figs. 7.25 and 7.26).
Horizontal or diaphragm characteristics that result in significant amounts of torsional
response, diaphragm deformations, or diaphragm stress concentrations are called hori-
zontal structural irregularities.

(a) (b) (c)
Stiff resisting
elements

Stiff resisting
elements

(d) (e) (f)

Nonsymmetrical
mass

(g) (h)
FIGURE 7.23 Horizontal irregularities: (a) – (c) nonsymmetric and symmetric plans, (d) non-
symmetric plan with reentrant corners, (e) – (g) nonsymmetric distribution of lateral rigidities,
(h) nonsymmetric mass ditribution [7.12].