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For solid masonry units the value of In is replaced with the moment of
inertia of the gross section. The values of In and Ig for hollow and
solid masonry units used in joint reinforced masonry construction are listed
in Table 29. The values of Ig for solid units may also be used for walls
which utilize combined joint ad cell masonry construction. The values of
Ic for both hollow and solid masonry construction may be obtained using:
EQUATION:
Ic = 0.005 bd3b
(131)
(5) Rebound. Vibratory action of a masonry wall will result in
negative deflections after the maximum positive deflection has been
attained. This negative deflection is associated with negative forces which
will require tension reinforcement to be positioned at the opposite side of
the wall from the primary reinforcement. In addition, wall ties are
required to assure that the wall is supported by the frame (see Figure 57).
The rebound forces are a function of the maximum resistance of the wall as
well as the vibratory properties of the wall and the load duration. The
maximum elastic rebound of a masonry wall may be obtained from Figure 68 of
NAVFAC P397.
c. NonReinforced Masonry Walls. The resistance of nonreinforced
masonry walls to lateral blast loads is a function of the wall deflection,
mortar compression strength, and the rigidity of the supports.
(1) Rigid Supports. If the supports are completely rigid and the
mortar's strength is known, a resistance function can be constructed in the
following manner.
(a) Both supports are assumed to be completely rigid and lateral
motion of the top and bottom of the wall is prevented. An incompletely
filled joint is assumed to exist at the top as shown in Figure 58a. Under
the action of the blast load the wall is assumed to crack at the center.
Each half then rotates as a rigid body until the wall takes the position
shown in Figure 58b. During the rotation the midpoint, m, has undergone a
lateral motion, Xc, in which no resistance to motion will be developed in
the wall, and the upper corner of the wall (point o) will be just touching
the upper support. The magnitude of Xc can be found from the wall
geometry in its deflected position:
EQUATION:
(TXc)2 = (L)2  [h/2 + (h'h)/2]2
(132)
= (L)2  (h'/2)2
where the values of Xc and L are given by:
EQUATION:
Xc = T  [(L)2  (h'/2)2]1/2
(133)
and
EQUATION:
L = [(h/2)2 + T2]
(134)
All other symbols are shown in Figure 58.
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