Masonry Magazine September 1968 Page. 22
I'm as determined by prism tests. In the prism test, 16-inch-high piers, as shown in Fig. 4, are built up using the same units, mortar, and workmanship proposed for the job. The prisms, after sufficient aging, should be tested in accordance with the applicable provisions of Standard Method of Test for Compressive Strength of Molded Concrete Cylinders, (ASTM C 39.)
Allowable stresses determined from the prism test are generally higher than those based on unit strength (Table I). In addition, since the prism incorporates the units, the mortar, the grout, and the workmanship of the mason, a designer is inclined to place more confidence in this method of determining f'm.
To those familiar with reinforced concrete design, it will become apparent from the figures above that the I'm parameter applying to concrete masonry is comparable to f'c which is a well-recognized symbol for the ultimate compressive strength of concrete as determined from compressive strength tests on cylinders.
Design Stresses
Criteria for converting f'm into design stresses, fm, is found in all major building codes today, such as, the Uniform Building Code of the International Conference of Building Officials, and Building Code Requirements for Reinforced Masonry, promulgated by the National Bureau of Standards, see Table III. The latter document forms the basis of requirements on reinforced masonry in other major codes, such as the (SBCC) Southern Standard Building Code, (BOCA) Basic Building Code, and the National Building Code sponsored by the American Insurance Association.
Effective Wall Section
(Hollow Masonry Units)
It was pointed out in Table I that compressive strength of hollow-unit masonry (f'm) is based on net area. Since hollow concrete masonry units are of uniform consistent configuration, the net area used in design calculations is predictable and will vary only with the extent of grouting of vertical cells. The Table II shows these equivalents for walls of nominal 6-inch, 8-inch, and 12-inch block, and for various combinations of grouted cells. For example, the wall of 8-inch three-core block shown in Fig. 5 (a) with vertical reinforcement 32 inches o.c., has an equivalent solid (net area) thickness of 4.9 inches.
In flexural compression, the effective design section of a reinforced hollow-unit concrete masonry wall is similar to a T-beam, Fig. 5 (b). Width of the compression flange (b) is assumed equal to six times the nominal wall thickness (t) when the wall is laid in running bond, and three times wall thickness when laid stack bond. The width of the T-beam stem (b') is equal to the width of the filled core plus the adjacent cross webs (normally 6 inches for a three-core block). Location of T-beam neutral axis and computations then follow procedures standard for reinforced concrete.
Walls subject to both axial and flexural compressive stresses are proportioned so that the summation of the ratios of calculated to allowable stresses does not exceed unity.
Combined Stresses:
+
Fm Fa not to exceed 1.0
Where:
f
Im
Fa
Fm
Computed axial unit stress,
Computed flexural unit stress,
Allowable axial stress,
Allowable flexural stress.
In the case of concentrated loads, the wall length considered effective is: (1) the distance between loads, or (2) the width of the bearing plus four times the wall thickness.
Shear calculations in reinforced concrete masonry walls follow reinforced concrete design procedures also, except that the factor (j) is still employed, and web reinforcement is provided to carry the entire shear when the computed shear stress exceeds that allowed on the plain masonry.
v=V
bjd
v Shear stress
V- Total shear
Where
b Width of compression face of flexural member
Distance from extreme compression fiber to centroid of tension reinforcement
d
j Ratio of distance between centroid of tension and centroid of compression to the depth, d.
Of special interest in the design of multi-story load-bearing structures, Figure 5 (c) shows the wall section assumed effective in longitudinal shear walls.
Advantages
Reinforced concrete masonry consists of the same materials as reinforced concrete and has similar physical and structural properties; however, reinforced concrete masonry holds some distinct advantages. The need for watertight forms, and for carpenters to erect them, is eliminated. Also eliminated is the need for special finishing of the walls. In many applications, the exposed masonry provides a finished wall surface; in other cases only painting is required. The problems of form marks, and changes in color and texture of exposed concrete are eliminated. Required materials are available from local suppliers; construction is performed by local contractors and labor.
Of course, the usual concrete masonry qualities are preserved; fire resistance of lightweight block exceeds that of the same thickness of heavyweight poured concrete; durability is unchanged; and acoustical characteristics are enhanced. The latter is true because the surface texture provides excellent sound sound absorption, while the grouted interior of the wall increases its resistance to sound transmission.