TM 5-809-3/NAVFAC DM-2.9/AFM 88-3, Chap. 3
Note that when the reinforcing steel is being checked, the minimum axial stress, ~a, must be used. Note also
that it is conservative to not consider axial loading (fa = 0) when checking the reinforcing steel stress.
d. Shear equations. The shear stress at the bottom of the wall is determined by the following equation:
Ra
fv '
(psi)
(eq 6-17)
bwd
Where:
bw = The width of the masonry element effective in resisting out-of-plane shear as given in
chapter 5, inches.
d = The depth of the masonry element effective in resisting shear, given "d1" for one reinforcing bar
per cell and "d2" for two bars per cell in chapter 5.
6-5. P-delta effect. The "P-delta effect" is the increase in moment and deflection resulting from multiplying
the mid-height defection of a wall (due to lateral and eccentric loadings as discussed above) by the summation
of the axial load, P, at the top of the wall and the weight of the top half of the wall. When the height to
nominal thickness ratio of the wall is less than 24, the "P-delta effect" is minor and may be neglected. For
walls where the height to nominal thickness ratios is greater than 24, the mid-height deflection, )s, will be
computed as follows:
When Mmid < Mcr;
(5)(Mmid)(h 2)(144)
)s '
(in)
(eq 6-18)
(48)(Em)(Ig)
When Mcr < Mmid < Mr;
(5)(Mcr)(h 2)(144)
(5)(Mmid & Mcr)(h 2)(144)
)s '
%
(in)
(eq 6-19)
(48)(Em)(Ig)
(48)(Em)(Icr)
Where:
h = The wall height, feet
Mmid = The moment at the mid-height of the panel, including the "P-Delta effect", inch-pounds.
Ig = The gross moment of inertia of the wall cross section, in4.
Icr = The cracked moment of inertia of the wall cross section, in4
Mrm = The allowable resisting moment of the masonry wall, inch-pounds.
6-6. Walls with openings. Walls at the edge of openings or between openings are required to resist
additional tributary axial and lateral loads. The additional tributary axial loads are due to the weight of
masonry above the opening and vertical loads applied to the tributary masonry above the opening. The
additional tributary lateral loads are the lateral loads on non-masonry wall components (doors, windows, etc.)
that are laterally supported by the adjacent masonry wall elements. The tributary load area width will be
measured from the centerline of the openings. Masonry wall elements between and alongside openings that
are subjected to combined loading will be designed in accordance with equations 6-13 through 6-16. Due
allowance will be made for eccentricity.
6-7. Design aids. Appendix B contains design aids that may be used in the design of reinforced masonry
walls. Tables B-1 through B-14 provide the properties of wall stiffeners with varying reinforcement (size,
spacing and number of bars per cell), varying wall thickness (6, 8, 10, and 12 inch nominal thickness) and two
mortar types (S and N). Tables B-15 through B-50 provide reinforcing steel sizes and spacings for varying
wall heights, lateral loads, wall thicknesses, axial loads (with and without eccentricity), using type S mortar.
6-8. Design examples. The following design examples illustrate the development and use of the design aids
in Appendix B.
a. Design example 1. This illustrative example considers only one combination of wind and eccentric axial
loading. When performing a complete wall design, all appropriate load combinations must be considered.
(1) Given--
(a) 12-inch CMU loadbearing wall
(b) Wall height (h) = 24 ft
(c) Lateral wind load (w) = 25 lb/ft2
(d) Axial load (P) = 1500 lb/ft
6-4
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