Masonry Magazine October 1975 Page. 16
The calculation for the section modulus of hollow units becomes somewhat complicated and is as follows:
S = bd³ - (b₁d³+b₂d³+b3d3... bnd³) / 6d
where:
b₁... b₁ = width of cores, in. (cm), see Fig. 12
d₁...d depth of cores, in. (cm), see Fig. 12
The illustration for this calculation method, Fig. 12, is based on a 3-core unit with nominal length of 12 in. (3.048 cm). However, the formula can be adapted to fit other sizes of units and coring patterns. All dimensions shall be actual dimensions.
Once the ultimate transverse strength has been determined by the test of three specimens, the designer should select a safety factor to apply to arrive at an allowable design stress. This safety factor should be based upon the designer's experience, type of workmanship expected, the in situ loadings expected or as recommended by the design standard.
CONCLUSION
The methods of tests described in this Technical Notes provide the design professional with methods to determine allowable design stresses which may be used in the rational design of brick masonry. The values derived from tests will remain valid only so long as the brick properties, mortar properties and workmanship remain relatively close to that specified. The next issue of this Technical Notes series will detail the quality control tests available to insure the designer that he is obtaining masonry properties of sufficient quality to achieve the performance desired.
b1
bo
b
ba
FIG. 12
Typical Hollow Brick Cross Section