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Table 28. Deflection Criteria for Masonry Walls

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Table 28
Deflection Criteria for Masonry Walls
Wall Type
Support Type
Support Rotation
Reusable
One-way
0.5deg.
Two-way
1.0deg.
Non-Reusable
One-way
1.0deg.
Two-way
2.0deg.
(e) When designing masonry walls for blast loads using response
chart procedures of Section 1, the effective natural period of vibration is
required.  This effective period of vibration when related to the duration
of the blast lading of given intensity and a given resistance of the masonry
wall determines the maximum transient deflection, Xm, of the wall.  The
expression for the natural period of vibration is presented in Equation (1),
Section 1, where the effective unit mass, me, has been described
previously and the equivalent unit stiffness, KE, is obtained from the
resistance-deflection function.  The equivalent stiffness of one-way beams
is presented in Table 7 which may be used for one-way spanning walls except
that a unit width shall be used.  Methods for determining the stiffness and
period of vibrations for two-way walls are presented in Section 6-8 of
NAVFAC P-397.
(f) Determining the stiffness in the elastic and elasto-plastic
range is complicated by the fact that the moment of inertia of the cross
section along the masonry wall changes continually as cracking progresses,
and further by the fact that the modulus of elasticity changes as the stress
increases.  It is recommended that computations for deflections and
therefore, stiffnesses be based on average moments of inertia, Ia, as
follows:
EQUATION:
Ia = In + Ic
(130)
2
where,
In = the moment of inertia of the net section, in4
Ic = the moment of inertia of the cracked section, in4
2.08-209


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