wall becomes large.
Overturning may then be a
prescribed for a vertical combination of two
problem, and if the foundation soil is relatively soft,
structural systems.
c.
Configuration and Simplicity. A great deal
the entire shear wall may rotate, causing localized
damage around the wall. Another difficult case is
of a building's resistance to lateral forces is
the shear wall with openings such that it may
determined by its plan layout. The objective in this
respond more like a frame than a wall.
regard is symmetry about both axes, not only of the
building itself, but of its lateral-force-resisting
(c) Braced frames (Figure 2-2) generally
elements and of the arrangement of wall openings,
have the stiffness associated with shear walls, but are
columns, shear walls, and so on. It is most desirable
somewhat less restrictive architecturally. It may be
to consider the effects of lateral forces on the
difficult to find room for doorways within a braced
structural system from the start of the layout, since
frame; however, braces are less obtrusive than solid
this may save considerable time and money without
walls. The concern for overturning mentioned above
detracting significantly from the usefulness or
for shear walls also applies to braced frames. Braced
appearance of the building. Experience has shown
frames may be concentric (c in Figure 2-2) or
that buildings that are asymmetrical in plan have
eccentric (Figure 7-23).
greater susceptibility to earthquake damage than
symmetrical structures with simple and direct load
(d) Horizontal elements in the lateral-force-
paths for lateral forces. The effect of asymmetry is
resisting system include floor and roof diaphragms
to induce torsional oscillations of the structure and
and horizontal bracing systems. Diaphragms may
stress
concentrations
at
re-entrant
corners.
consist of wood sheathing or plywood, steel decking
Asymmetry in plan can be eliminated or improved
with or without concrete fill, or cast-in-place or
by separating L-, T-, and U-shaped buildings into
precast concrete slabs. Diaphragms and horizontal
distinct units by use of seismic joints at the junctions
bracing systems are classified as flexible, stiff, or
of the individual wings. It should be noted, however,
rigid, based on their deformation relative to the
that this causes two new problems: providing floor
vertical-resisting elements. Design and acceptance
joints that are capable of bridging gaps large enough
criteria for these elements are provided in Paragraph
to preclude adjacent structures from pounding each
7-7 of Chapter 7.
other, and providing wall and roof joints that are
capable of keeping out the weather.
Asymmetry
(e)
Structural systems may be used in
caused by the eccentric location of lateral-force-
various combinations.
There may be different
resisting structural elements--such as in the case of
systems in the two directions, or systems may be
a building that has a flexible front because of large
combined in any one direction, or may be combined
openings and an essentially stiff (solid) rear wall--
vertically. FEMA 302 permits the use of R factors
can usually be avoided by better conceptual
applicable
to
the
structural
system
in
each
planning. For example, modify the stiffness of the
orthogonal direction. Specific R values are provided
rear wall or add rigid structural partitions to make
for acceptable dual systems, and the lower R value is
the center of rigidity of the lateral-force-resisting
elements closer to the center of mass.
When a
building has irregular features, such as asymmetry in
2-8
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