Page 399 - Design of Reinforced Masonry Structures

6.50 CHAPTER SIX

FIGURE 6.35 Position of reinforcement inside the cells of concrete masonry units: (a) reinforcing bars at
the center of cells, (b) reinforcing bars near each face of cells.

reversible in direction, and any wall face can be subjected to tension. Therefore, only one
reinforcing bar (near the tension face of the wall) is assumed effective at any given time; the
contribution of reinforcing bar near the compression face in resisting flexure is ignored.
With the reinforcing bars at the center of the wall, as is generally the case, the effective
depth of the wall section equals half the actual thickness of the masonry unit. For example,
the nominal thickness of concrete masonry units is typically 6, 8, 10, 12, or 16 in., with
3
actual thickness being / 8 in. smaller in each case. Thus, for a 12-in. nominal wall, the
3
5
actual thickness would be 11 / 8 in. (= 12 − / 8 = 11.625 in.), and the effective depth d for
1
such a centrally reinforced wall, would be 5.81 in. (= / 2 × 11.625 in.). On the other hand, if
the reinforcement were placed near the tension face of the wall, the effective depth would
be about 9 in. for the same 12-in. nominal wall, much larger than in the case of centrally
reinforced-wall. This increased effective depth results in increased flexural resistance which
can be used as an effective alternative to meet the higher moment strength requirements
without increasing the wall thickness. However, this advantage may be offset by the cost of
providing larger amount of reinforcing (on both faces of the cells). In general practice, for
reasons of economy, it may be preferred to place the reinforcement at the center of the cell
rather than near both faces of cells (one near each face of the same cell). Table 6.8 shows
effective depths for walls of different nominal thicknesses for both alternatives.

TABLE 6.8 Dimensions to Calculate Effective Areas of Concrete
Block (ASTM C 90, Ref. 6.25)

Width (in.) Assumed Dimension* for Design (in.)
†
†
Nominal Actual d d 2 X ‡ t s t w t e
1
6 5 / 8 2.81 — 6 1 1 1
5
5
1
1
8 7 / 8 3.81 5.31 6 1 / 4 1 1 / 4
1
3
5
3
10 9 / 8 4.81 7.06 6 1 / 8 1 / 8 1 / 8
1
1
1
5
12 11 / 8 5.81 8.81 6 1 / 2 1 / 8 1 / 8
*
See Fig. 6.21 for definitions of notations.
†
d = d = effective depth for centrally reinforced wall, Fig. 6.35a.d =
2
1
d = effective depth for reinforcement placed near the far inside face of the
cell, Fig. 6.35b.
‡ 2
x = length of void measured parallel to unit’s length .