Page 523 - Design of Reinforced Masonry Structures

SHEAR WALLS 7.85

FIGURE 7.32 Minimum reinforcement requirements for intermediate reinforced masonry shear walls [7.16].
(Courtesy: CMACN.)

that spacing of reinforcement must not exceed 4 ft (MSJC-08 Section 1.17.3.2.5). They are
permitted for structures in Seismic Design Category A, B, and C (not permitted in SDC D,
E, and F) without any height limitations. These walls are assigned an R value of 3½ and
overstrength factor W = 2.5.
o
Figure 7.32 shows minimum reinforcement requirements for intermediate reinforced
masonry shear walls.
Special reinforced masonry shear walls are so called because they are specially rein-
forced to provide large ductility which enables them to deform sufficiently without failure
during large earthquakes. To ensure adequate ductility in these walls, it is required that
design shear strength, fV , shall exceed the shear corresponding to the development of
n
1.25 times the nominal flexural strength, M , of the member, except that the nominal shear
n
strength of these walls, V , need not exceed 2.5 times the required shear strength, V (MSJC-
u
n
08 Section 1.17.3.2.6.1.1). As a result, they are assigned an R value of 5 (highest of all types
of masonry shear walls) and overstrength factor W = 2.5. They are designed according to
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requirements of MSJC-08 Sections 2.3 and 3.3 for ASD and SD, respectively.
Special reinforced masonry shear walls are permitted as lateral force–resisting elements
in structures of all seismic design categories (A through F). Note that only special rein-
forced masonry shear walls are permitted for Seismic Design Category C, D, E, and F

structures (i.e., structures in high seismic regions) because of their high ductile capacity.
Shear walls in Seismic Design Category C, D, and E structures are restricted to a height of
160 ft, but only 100 ft is permitted for Seismic Design Category F structures.
Design of special reinforced masonry wall should comply with requirements specified in
MSJC-08 Secs. 2.3 and 3.3, and detailing requirements for minimum reinforcement specified
in Section 1.17.3.2.6 as follows. These minimum reinforcement requirements are intended to
improve ductile behavior of these shear walls under earthquake loading, and to assist in crack
control. Because the minimum required reinforcement may be provided to satisfy design
requirements, at least one-third of the required reinforcement amount is reserved for the lesser
stressed direction to ensure appropriate distribution of reinforcement in both directions.
1. Walls should be reinforced in both horizontal and vertical directions.
2. The reinforcement should be uniformly distributed.