TM 5-809-3/NAVFAC DM-2.9/AFM 88-3, Chap. 3
5-6. Basic reinforcement requirements. The design of steel reinforcing bars will be based on the working
stress allowables given in table 5-8.
a. Minimum bar size. The minimum bar size will be No. 4.
b. Maximum bar sizes. The most commonly used, and preferred, reinforcing steel bar sizes in CMU walls
are Nos. 4, 5 and 6. When the design requires the use of larger bars, the bar size will not exceed No. 6 bars
in 6-inch CMU walls, No. 7 bars in 8-inch CMU walls and No. 8 bars in 10-inch and 12-inch CMU walls.
This provides reasonable steel ratios, reasonable splice lengths, and better distribution of reinforcement. The
maximum bar size in masonry columns should be No. 9.
c. Maximum flexural reinforcement. There is no maximum flexural reinforcement limit in the working
stress design method, however there is a practical maximum. It is not efficient to use a steel ratio, p, that is
greater than the balanced-stress steel ratio, pe. Examining the elastic theory shows that reinforcing steel added
to a masonry element with p > pe provides less than one half the added strength the same amount of steel
added to the member with p < pe provides. Although using p > pe is not efficient use of the reinforcement,
in some instances it may be more economical from a total wall cost standpoint to increase the reinforcement
in lieu of increasing the wall thickness. Thus, the decision to use more than balanced steel becomes an
economic one and should be decided on a case by case basis.
(1) Table 5-9 lists values of pe, k and j for varying values of f'm.
(2) Table 5-10 may be used by designers to determine the bar size and spacing that will achieve a near
balanced-stress ratio for varying wall thicknesses with one bar per cell. The table also provides the depth to
the reinforcement, d; the balance-stress steel ratio, pe the actual steel ratio, p, and the actual depth of the
compression stress block, kd; using the respective bar size and spacing.
(3) Table 5-11 provides similar information to that given in table 5-10 for CMU with two bars per cell.