Page 505 - Design of Reinforced Masonry Structures

SHEAR WALLS 7.67

7.8.8 Ramifications of Horizontal and Vertical Structural irregularities
Structural irregularities, both horizontal and vertical, can result in forces and deformations
significantly different from those assumed when using equivalent static procedures. As
such they create great uncertainties in the ability of a structure to meet the design objective.
Experience from past earthquakes has shown that buildings having irregularities suffer
greater damage than buildings having regular configurations. This situation prevails even
with good design and construction.
Poor seismic performance of irregular buildings can be attributed to the manner in
which such buildings respond to ground shaking. In a regular structure, inelastic demands
produced by strong ground shaking tend to be well distributed throughout the structure,
which results in dispersion of energy dissipation and damage. By contrast, in irregular
structures, inelastic response can concentrate in the zones of irregularity, which can cause
rapid failure of structural elements in those areas. Other factors can contribute to the poor
performance of irregular structures as well. For example, irregularities can also introduce
unanticipated stresses into the structure, which the designers may overlook when detail-
ing the structural system. The elastic analysis methods typically employed in design of
structures, such as equivalent static procedure, often cannot predict the distribution of
earthquake demands in an irregular structure very well, leading to poor design in the zones
of irregularity.
The configuration of shear walls in a building affects building configuration which, in
turn, can significantly influence its performance in an earthquake. Figure 7.29 shows some
typical shear wall layouts with potential problems leading to poor performance. A building
having a regular configuration can be square, rectangular, or circular. Buildings with minor
reentrant corners are generally acceptable as regular buildings. However, buildings having
large reentrant corners that may create a crucifix form would be classified as irregular even
though they have geometrical symmetry about both axes. This is because the response of
the wings of this type of building is generally different from the response of the building
as a whole. Other plan configurations such as H-shapes that have geometrical symmetry
would be considered as irregular also because of the response of the wings [7.13, 7.14].
In general, irregularities in structural configurations and in load paths are known to be
major contributors to structural damage due to strong earthquake ground motion [7.12].
Therefore, it is extremely important for designers to recognize the presence of irregularities
in a structure. In some cases, even seemingly regular buildings may not perform well during
strong ground shaking. An example of such a building is a core-type building with the vertical
components of the seismic force–resisting system, symmetrically placed, but concentrated
near the center of the building (Fig. 7.29c). Better performance has been observed when the
vertical components are distributed near the perimeter of the buildings Fig. 7.29f).
Structural design involves serious ramifications whenever seismic forces cause a struc-
ture to have horizontal or vertical structural irregularity:

1. When accidental torsional moments cause horizontal structural irregularity Type 1a or
Type 1b in a structure, it is required to be analyzed using a three-dimensional model.
When using such a model, it is required that a minimum of three dynamic degrees of
freedom consisting of translation in two orthogonal plan directions and torsional rota-
tion about the vertical axis of rotation at each level of the structure be used for analysis
(ASCE 7-05 Section 12.7.3).
2. For structures assigned to Seismic Design Category D, E, or F and having a horizontal
structural irregularity Type 1a, 1b, 2, 3 or 4 (see Table 7.6), or a vertical structural
irregularity Type 4 (Table 7.6), design seismic forces are required to be increased
25 percent for connections of diaphragms to vertical elements and to collectors, and for
connections of collectors to vertical elements (ASCE 7-05 Section 12.3.3.4).