ACI guidelines for Reinforced Concrete Analysis and Design

ACI (American Concrete Institute) has developed guidelines to help in design of reinforced concrete beams. Some of them are given below

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. Minimum 2 bars and maximum 6 bars are allowed for apprpriate spacing of rebar in a single layer.

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/f_s ) - 2.5 c_c 

or s = 12(40,000/f_s ) 

where c_c  is the least distance from surface of reinforcement to the tension face, and  f_s 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(0.375 in. or 9.525 mm), #4 (0.5 in. or 12.7 mm), #5 (0.625 in. or 15.875 mm), #6 (0.750 in. or 19.050 mm), #7 (0.875 in. or 22.225 mm), #8 (1.00 in. or 25.400 mm), #9 (1.128 in. or 28.650 mm), #10 (1.270 in. or 32.260 mm) #11 (1.410 in. or 35.810 mm), #14 (1.693 in. or 43.000 mm), #18 (2.257 in. or 57.330 mm)

There should be atleast 2 bars for flexural reinforcement. Minimum amount of reinforcement in a beam should be maximum of the following

         A_s,min  = 3(f '_c )^0.5 bd / f_y

or    A_s,min  = 200 bd / f_y

the values of f '_c  and  f_y 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 f_y 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. cover

  for #11 and smaller bars-    0.75 in. cover

In case of concrete exposed to earth or weather the cover for rebar shall not be less than the following;

  #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.

There should be atleast 1.5 inch clear side cover to the stirrups

 To learn more about calculating the strength of reinforced concrete beam please visit solved Example 9-1, Example 9-2, Example 9-3

You can also use our free online calculator for strength of Reinforced Concrete beam.