in which the deflections of portions of walls are
performs the function of a plate girder web, the
combined usually are adequate.
pilasters or floor diaphragms function as web
stiffeners, and the integral reinforcement of the
(a) Wall deflections. The rigidity of a wall
vertical boundaries functions as flanges.
Axial,
is usually defined as the force required to cause a unit
flexural, and shear forces must be considered in the
deflection. Rigidity is expressed in kips per inch.
design of shear walls. The tensile forces on shear
The deflection of a concrete shear wall is the sum of
wall elements resulting from the combination of
the shear and flexural deflections (see Figure 7-1). In
seismic uplift forces and seismic overturning
the case of a solid wall with no openings, the
moments must be resisted by anchorage into the
computations of deflection are quite simple; however,
foundation medium unless the uplift can be
where the shear wall has openings, as for doors and
counteracted by gravity loads (e.g., 0.90 of dead
windows, the computations for deflection and rigidity
load) mobilized from neighboring elements. A shear
are much more complex.
An exact analysis,
wall may be constructed of materials such as
considering angular rotation of elements, rib
concrete, wood, unit masonry, or metal in various
shortening, etc., is very time-consuming. For this
forms. Design procedures for such materials as cast-
reason, several short-cut approximate methods have
in-place reinforced concrete and reinforced unit
been developed. These do not always give consistent
masonry are well known, and present no problem to
or satisfactory results. A conservative approach and
the designer once the loading and reaction system is
judgment must be used.
determined.
Other materials frequently used to
support vertical loads from floors and roofs have
(b) Deflection charts. The calculation of
well-
established
vertical-
load-
carrying
deflections is facilitated by the use of the deflection
characteristics, but have required tests to demonstrate
charts. See Figure 7-4 for fixed-ended corner and
their ability to resist lateral forces. Various types of
rectangular piers. Curves 5 and 6 are for cantilever
wood sheathing and metal siding fall into this
corner and rectangular piers. The corner pier curves
category. Where a shear wall is made up of units
are for the special case where the moment of inertia,
such as plywood, gypsum, wallboard, tilt-up concrete
units, or metal panel units, its characteristics are, to a
large degree, dependent upon the attachments of one
unit to another, and to the supporting members.
(1)
Rigidity analysis.
For a building with
rigid diaphragms, there is a torsional moment, and a
rigidity analysis is required. It is necessary to make a
logical and consistent distribution of story shears to
each wall. An exact determination of wall rigidities
is very difficult, but is not necessary, because only
relative rigidities are needed. Approximate methods
7-9
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