Some of the guidelines by ACI (American Concrete Institute) are given below to help in design of
reinforced concrete beams.
Spacing of Reinforcement
1. Minimum clear spacing between parallel bars in a layer should be
equal to diameter of the bar but not less than 1 inch.
2. Where parallel reinforcement is placed in two or more layers,
bars in the upper layers shall be place directly above the bars in
the bottom layers with clear spacing between the layers not leass
than 1 in.
3. Maximum spacing of reinforcement bars closest to the tension face
shall not exceed the following;
s = 15 (40,000/fs ) - 2.5 cc
or s = 12(40,000/fs )
where cc is the least distance from surface of
reinforcement to the tension face, and fs is the
calculated stress in reinforcement closest to the tension face.
The table given below is showing minimum beam width required for a
reinforced concrete rectangular section as per ACI guidelines
#14 and #18 are rarely used for beams. This table is based on the
assumption that maximum aggregate size does not exceed three-fourth
of the clear space between bars, and the #3 bars are used as stirrup.
The diameter of different bar sizes are given in brackets;
#3(0.375in), #4 (0.5 in.), #5 (0.625 in.), #6 (0.750), #7 (0.875),
#8 (1.00 in.), #9 (1.128 in.), #10 (1.270 in.) #11 (1.410 in.), #14
(1.693 in.), #18 (2.257 in.)
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Minimum amount of reinforcement
in a beam should be maximum of the following
As,min = 3(f
'c )0.5 bd / fy
or As,min = 200 bd / fy
the values of f 'c and fy are
taken in psi (pound per square inch), and the concrete used should
not have f 'c less than 2500 psi.
Commonly used concrete has f 'c ranging from 3000
psi to 6000 psi, and steel with fy of 40000 psi, 50000
psi or 60000 psi.
Minimum cover requirement:
In case of concrete not exposed to weather or in conctact with
ground the cover for re-bar shall not be less than the following
for #14 and # 18 bars- 1.5 in.
for #11 and smaller bars- 0.75 in. cover
In case of concrete exposed to earth or weather;
#6 through #18 bars- 2 in. cover
#5 and smaller- 1.5 in. cover
In case of cast against and permanently exposed to earth, a cover of
3 in. should be provided.
To learn more about strength of reinforced concrete beam please
example 9-1 ,
For strength of RC beam visit
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and bending moment
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