Page 534 - Design of Reinforced Masonry Structures

7.96 CHAPTER SEVEN

In design, strength of shear walls under the two kinds of forces—axial compression and
flexure—is determined separately. A wall subjected to flexure can be treated based on pro-
visions for flexural design (discussed in Chap. 4). The fundamental assumptions underlying
flexural analysis of reinforced concrete masonry elements are as follows:
1. Maximum compressive strain in masonry = 0.0025.
2. Maximum compressive stress in masonry = 0.80 ′ f .
m
3. Depth of equivalent compression block a = 0.80c (c = distance of neutral axis from the
compression force).
There are additional requirements that reinforced masonry shear walls must satisfy.
1. MSJC-08 Section 3.1.2 requires that members subject to axial load be designed for the
maximum moment that can accompany the axial load. The required strength is to be
determined from the factored load combination of ASCE 7-05 Section 2.3.3 (basic load
combinations for strength design).
2. The flexural capacity of the wall must exceed the cracking moment of the section to
preclude the possibility of a sudden brittle failure (masonry is a brittle material). To this
effect, MSJC-08 Section 3.3.4.2.2.2 requires that the nominal flexural strength of the
wall, M , be not less than 1.3 times of its cracking strength, M (i.e., M ≥ 1.3M ; see
n
cr
n
cr
discussion in Chap. 4). A shear wall is treated as a vertical beam for this purpose. The
cracking moment of the wall section can be determined from Eq. (7.84):
M = S f (7.84)
n r
cr
2
2
where S = section modulus of the wall section, in. (mm )
n
f = modulus of rupture as specified in MSJC-08 Table 3.1.8.2 (for flexural tensile
r
stress normal to bed joints)
For a shear wall, the section modulus can be expressed as
S = tL 2 w (7.85)
n
6
3. The amount of tensile reinforcement in a reinforced masonry shear wall is limited
as specified in MSJC-08 Section 3.3.3.5.1. For masonry members where M /V d
u v
u
≥ 1 (typical for shear walls), the cross-sectional area of flexural tensile reinforce-
ment shall not exceed the area required to maintain equilibrium under the following
conditions:
(a) Strain in extreme tensile reinforcement is 1.5 times the strain associated with the

reinforcement yield stress f for the wall subjected to out-of-plane forces.
y
(b) In intermediate reinforced masonry shear walls, the strain in extreme tensile rein-
forcement is 3 times the strain associated with the reinforcement yield stress f for
y
the wall subjected to in-plane forces.
(c) In special reinforced masonry shear walls, the strain in extreme tensile reinforce-
ment is 4 times the strain associated with the reinforcement yield stress f for the
y
wall subjected to in-plane forces.
(d) Axial forces shall be taken from the loading combination given by Eq. (7.86)
(MSJC-08 Section 3.3.3.5.1d):
P = D + 0.75L + 0.525Q (7.86)
E
u