Page 401 - Design of Reinforced Masonry Structures

6.52 CHAPTER SIX

on the amount of reinforcing material present to resist tension, the compression block may
lie entirely within the flange, or it may extend in the web. In the first case, the section can
be analyzed as a rectangular section, whereas in the second case, the section is required to
be analyzed as a T-section.
The analysis of a T-beam requires determining the effective width of flange based on
consideration of shear lag phenomenon. What this means is that uniform compressive
stress is assumed distributed only over a limited width of the compression flange of a
T-beam instead of over the entire flange width that may be physically present in the wall
cross section (because of the shear lag phenomenon). This limited width is called the
effective width, b . The advantage of such an assumption is that it permits a designer to use
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the conventional flexural analysis methods which are applicable to rectangular beams, as
presented in Chap. 4. A discussion on this topic can be found in references [6.26–6.27].
The effective width for reinforced concrete T-beams is specified in MSJC-08 Section
1.9.6: Effective compressive width per bar. The effective width b of compression area for
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flexural analysis is determined as follows (Fig. 6.36):
1. In running bond masonry and for masonry laid in other than running bond with bond
beams spaced not more than 48 in. center-to-center, b is limited to the least of (a)
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center-to-center bar spacing, (b) six times the nominal thickness of the wall, and (c)
72 in.
2. For masonry laid in other than the running bond, with bond beams spaced more than
48 in. (1219 mm) center-to-center, b is limited to the length of the masonry unit (e.g.,
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16 in. for a nominal 8 × 8 × 16 in. concrete masonry unit).
In Item 1 above, the center-to-center maximum spacing is a limit to keep areas of
compressive stresses from overlapping each other. However, the 72-in. restriction is both
empirical and arbitrary. In Item 2, the limited ability of head joints to transfer stresses when
masonry is in stack bond is recognized by the requirements for bond beams.
Engineers familiar with reinforced concrete design would find it instructive to compare
the provisions for the effective width of T-beams in the ACI Code [6.28]. For reinforced
concrete T-beams, the effective width is limited to the least of the following:
1. One-quarter of the span length of the beam (b ≤ L/4)
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2. The effective overhanging flange width on each side of the web not to exceed eight
times the slab thickness
3. The effective overhanging flange width on each side of the web not to exceed one-half
the clear distance to the next web

Obviously, the effective width of a partially grouted wall section (a vertical T-beam)
would depend on the spacing of reinforcing bars. For example, depending on the design

requirements, the vertical reinforcement in a nominal 16-in. CMU wall may be spaced at
8 in. on centers or at multiples of 8 in. (i.e., at 16, 24, 32, 40, and 48 in. on centers, Fig. 6.36);
the maximum permitted spacing being 48 in.
The aforestated limitations on the effective width of a partially grouted wall have impor-
tant bearing on the flexural resistance of partially grouted walls designed to resist out-of-
plane loads. For example, consider a hollow 8-in. (nominal, t actual = 7.625 in.) CMU wall in
running bond, with reinforcement at 32 in. center-to-center, subjected to out-of-plane loads.
The effective width b of the wall will be the least of the following:
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1. Center-to-center distance between the reinforcement = 32 in.
2. Six times the (nominal) wall thickness = (6)(8) = 48 in.
3. 72 in.