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GENERAL REINFORCING REQUIREMENTS



The design of steel reinforcing bars are based on the working stress allowables.

a. Minimum bar size. The minimum bar size should 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 should 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, p_e . Examining the elastic theory shows that reinforcing steel added to a masonry element with p > p_e provides less than one half the added strength the same amount of steel added to the member with p < p_e provides. Although using p > p_e 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.


The following Table lists values of p_e , k and j for varying values of f'm.

f'm	pe	k	j
1350	0.0027	0.287	0.904
1500	0.0030	0.287	0.904
2000	0.0040	0.287	0.904
2500	0.0050	0.287	0.904

The following Table 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, p_e the actual steel ratio, p, and the actual depth of the compression stress block, kd; using the respective bar size and spacing.

CMU Thickness	d	pe	Reinforcing	Actual p	Actual k	Actual kd
6	2.81	0.0027	#4 @ 24"	0.0030	0.300	0.84"
6	2.81	0.0027	#5 @ 40"	0.0028	0.292	0.82"
8	3.81	0.0027	#4 @ 24"	0.0022	0.264	1.01"
8	3.81	0.0027	#5 @ 32"	0.0025	0.278	1.06"
8	3.81	0.0027	#6 @ 48"	0.0024	0.274	1.04"
10	4.81	0.0027	#4 @ 16"	0.0026	0.283	1.36"
10	4.81	0.0027	#5 @ 24"	0.0027	0.287	1.38"
10	4.81	0.0027	#6 @ 32"	0.0029	0.296	1.42"
10	4.81	0.0027	#7 @ 48"	0.0026	0.283	1.36"
12	5.81	0.0027	#4 @ 16"	0.0022	0.253	1.47"
12	5.81	0.0027	#5 @ 24"	0.0027	0.287	1.67"
12	5.81	0.0027	#6 @ 32"	0.0024	0.274	1.59"
12	5.81	0.0027	#7 @ 40"	0.0026	0.283	1.64"
12	5.81	0.0027	#8 @ 48"	0.0028	0.292	1.70"

The following Table provides similar information to that given above for CMU with two bars per cell.

CMU Thickness	d	pe	Reinforcing	Actual p	Actual k	Actual kd
8	5.31	0.0027	#4 @ 16"	0.0024	0.274	1.45"
8	5.31	0.0027	#5 @ 24"	0.0024	0.274	1.45"
8	5.31	0.0027	#6 @ 32"	0.0026	0.283	1.50"
8	5.31	0.0027	#7 @ 40"	0.0028	0.292	1.55"
10	7.06	0.0027	#4 @ 16"	0.0018	0.242	1.71"
10	7.06	0.0027	#5 @ 16"	0.0027	0.287	2.03"
10	7.06	0.0027	#6 @ 24"	0.0026	0.283	2.00"
10	7.06	0.0027	#7 @ 32"	0.0027	0.287	2.03"
10	7.06	0.0027	#8 @ 40"	0.0028	0.292	2.06"
12	8.81	0.0027	#5 @ 16"	0.0022	0.264	2.33"
12	8.81	0.0027	#6 @ 24"	0.0021	0.259	2.28"
12	8.81	0.0027	#7 @ 24"	0.0028	0.292	2.57"
12	8.81	0.0027	#8 @ 32"	0.0028	0.292	2.57"
12	5.81	0.0027	#8 @ 48"	0.0028	0.292	1.70"

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