TM 5-809-3/NAVFAC DM-2.9/AFM 88-3, Chap. 3
and location of control joints within the total length of a wall may significantly affect element rigidities,
especially flexural deformation.
(2) Openings for doors, windows, etc., reduce the rigidity of shear wall elements. If openings are
significantly large or are significantly large in number, they should be considered in rigidity analyses as given
in paragraph 7-7.
(3) A shear wall element which is structurally intern-al at its end with a shear wall that is normal to the
element, forming an "L" or "T" in-plan shape, is called a corner element. The rigidity of a corner element is
greater than that of a straight element. The amount of increase in rigidity is difficult to quantify but may be
taken into account empirically when rigidity analyses are done using the method given in this chapter.
(4) Since shear walls are by nature, very rigid, rotation of the foundation can greatly influence the
overall rigidity of a wall. However, the rotational influence on relative rigidities of walls for purposes of
horizontal force distribution may not be as significant. Considering the complexities of soil behavior, a
quantitative evaluation of the foundation rotation is generally not practical, but a qualitative evaluation,
recognizing the limitations and using good judgment, should be a design consideration. It is usually assumed
either that the foundation soil is unyielding or that the soil pressure varies linearly under the wall when the
wall is subjected to overturning. These may not always be realistic assumptions, but are generally adequate
for obtaining the relative rigidities required for design purposes.
7-6. Distribution of Forces to Shear Walls.
a. General. The distribution of lateral forces by different types of diaphragms is discussed in TM 5-809-
10/NAVFAC P-355/AFM 88-3, Chap. 13, Seismic Design For Buildings. A brief description is provided
herein.
b. Translational shears. The distribution of lateral story level shears from a diaphragm to the vertical
resisting elements (in this case, masonry walls acting as shear walls) is dependent upon the relative stiffness
of the diaphragm and the shear walls. A rigid diaphragm is assumed to distribute horizontal forces to the
masonry shear walls in direct proportion to the relative rigidities of the shear walls. Under symmetrical
loading, a rigid diaphragm will cause all vertical shear wall elements to deflect equally with the result being
that each element will resist the same proportion of lateral force as the proportion of rigidity that element
provides to the total rigidity of all the elements in the same level and direction. Flexible diaphragms, on the
other hand, are considered to be less rigid than shear walls and will distribute the lateral forces to the wall
elements in a manner analogous to a continuous beam without regard to the rigidity of the walls. A flexible
diaphragm is considered incapable of resisting torsional rotational moments (see below).
c. Rotational shears. In a rigid diaphragm, when the center of gravity of the lateral forces fails to coincide
with the center of rigidity of the supporting shear wall elements, a torsional moment will be generated within
the rigid diaphragm. Provisions will be made to account for this torsional moment in accordance with TM
5-809-10/NAVFAC P-355/AFM 88-3, Chap. 13, Seismic Design For Buildings.
d. Maximum shear wall deflection. Roof and floor diaphragms, must be capable of transmitting horizontal
shear forces to the shear walls without exceeding a deflection that which would damage the vertical elements.
The maximum allowable deflection for horizontal diaphragms in buildings utilizing masonry shear walls will
be as follows:
Where:
Fb = The allowable flexural compressive stress in masonry, psi.
= (0.33)f'm
= (1000)f*m for CMU
t = The effective thickness of the wall, inches.
This equation is neither exact nor technically correct. However, its primary function is to force the designer
to think about limiting the deflection of the diaphragm to a value that will not adversely affect, architecturally,
the completed wall.
7-7. Effects of Openings in Shear Walls. The effects of openings on the ability of shear walls to resist
lateral forces must be considered. If openings are very small, their effect on the overall state of stress in a
shear wall will be minor. Large openings will have a more pronounced effect. When the openings in a shear
wall become so large that the resulting wall approaches an assembly similar to a rigid frame or a series of
7-4
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