Masonry Magazine February 1989 Page. 28
Design Requirements
# Effective Area
The effective area for axial compressive stress calculations in reinforced hollow unit concrete masonry walls is defined as the net cross-sectional area (i.e., average net cross-sectional area of the masonry unit plus the grouted area). The net cross-sectional areas of 6, 8, and 12-inch walls are listed in Table 4. The effective area for shear is based on the net section (i.e., mortar bedded area plus the grouted area). Net section and net cross-sectional areas are illustrated in Figure 5.
# Effective Width
In computing flexural stresses where a reinforcing bar resists tensile stresses produced by forces perpendicular to the wall, the effective width shall not exceed six times the wall thickness where masonry is laid in running bond, nor three times the wall thickness where masonry is laid in stacked bond.
# Minimum Thickness
The Uniform Building Code requires that the minimum thickness of concrete masonry walls should not be less than the distance between laterally supporting members, either horizontally or vertically. ACI Standard 531-79 (Revised 1981) limits this minimum thickness to the distance in either direction.
# Reinforcement
In seismic zones 2, 3 and 4 the minimum area of reinforcement (in either direction) should not be less than 0.07 percent of the gross cross-sectional area of the wall. The sum of percentages of horizontal and vertical reinforcement should be at least 0.2 percent. In other areas where reinforcement is used in selected portions of the wall as needed to resist tensile stresses, there is no requirement as to the minimum percentage of reinforcement. The size of bars used in reinforced masonry walls range from No. 4 through No. 11 bars.
The minimum and maximum vertical reinforcement in masonry columns is 0.25 percent and 4 percent, respectively. Minimum size of bar is No. 4 and the minimum number of bars is four.
FIGURE 5-EFFECTIVE AREAS
Net cross sectional area of partially grouted masonry wall-effective area in resisting compression
b) Niet section of partially grouted concrete masonry wall with face shell bedding-effective area in resisting hear
TABLE 3-SUMMARY OF ALLOWABLE STRESS, REINFORCED CONCRETE MASONRY (1)
DESCRIPTION
Compression
Axial, walls
Flexural
Bearing
On Ful Area
On One Third Area or Less
Shear Stress in Masonry
No Shear Reinforcemant
Flexural Members
Shear Walls
M/VO 1
M/Vd <1
MIVO -0
Where Reinforcement Takes
Entire Shear Flexural
Members
Shear Walls
Mive 21
Mive <1
MVd0
Modulus of Elasticity
Modulus of Rigidity
Wall Construction
ACI 531-79
225/4
20
20
201
25
13
Лог
30
30√
15
15
20
20
1000
1000
400
400
TABLE 4-NET CROSS-SECTIONAL AREAS (Sq. in./ft.) IN REINFORCED HOLLOW UNIT CONCRETE MASONRY WALLS*
Wall Thickness
6-inch
8-inch
12-inch
Solid Grouted Wall
68
92
140
Vertical
Cores
Grouted
at
16°0.c.
50
67
99
24°0.c.
43
57
83
32 0.0.
41
54
78
40°o.c.
39
51
73
48°0.c.
38
50
72
Areas based on units having two cores and face shell and web thicknesses equal to the minimum required in ASTM C90.
NATIONAL CONCRETE MASONRY ASSOCIATION
P.O. Box 781, Herndon, Virginia 22070