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Design guidelines
Prior to the design of reinforced concrete beam begin, there are certain assumption that need to be made. these guidelines are provided by certain codes and researchers. It should be known that, experience of designer plays significant role in making these assumptions.Beam depth (h)
There is no unique procedure for computing overall beam depth(h) for design. However, certain guidelines can be followed to compute beam depth such that the deflection requirements can be satisfied.- ACI 318-11 provides suggested minimum thickness for non-prestressed beams unless deflections are calculated. t
- Canadian Standard Association (CSA) provides similar table except for one end continuous which is l/18.
Minimum thickness, h |
|||
Simply supported | One end continuous | Both end continuous | Cantilever |
Members not supporting or attached to partitions or other construction likely to be damaged by large deflections | |||
l/16 | l/18.5 | l/21 | l/8 |
Notes: Values given shall be used directly for members with normal weight concrete and Grade 420 reinforcement. For other conditions, the values modified as follows: a) For lightweight concrete having equilibrium density (wc) in the range of 1440 to 1840 kg/m3, the values shall be multiplied by (1.65 – 0.0003wc) but not less than 1.09. b) For fy other than 420 MPa, the values shall be multiplied by (0.4 + fy/700). |
- The depth of beam can also be estimated based on span/depth ratio. IS 456 2000 provides span to depth ratio to control deflection of beam as provided in Table 2.
Beam span | Beam type | Span/depth ratio |
Up to 10m | Simply supported | 20 |
Cantilever | 7 | |
Continuous | 26 | |
Greater than 10m | Simply supported | 20*10/span |
Cantilever | - | |
Continuous | 26*10/span |
Beam width (b)
The ratio of beam depth to its width is recommended to be between 1.5 to 2 with upper bound 2 being the most common used. The reinforcement arrangement is one of major factors that specify beam width. So, minimum bar spacing shall be considered while beam width is estimated. The width of the beam shall be equal or less than the dimension of the column supporting the beam.
Steel reinforcement
ACI 318-11 provided minimum and maximum reinforcement ratio. The reinforcement ratio is an indicator of the amount of steel in a cross section. So, any values between this range can be used for beam design. Nonetheless, the choice is influenced by ductility requirement, construction and economic consideration. lastly, it is recommended to use 0.6*maximum reinforcement ratio.


Spacing between bars
ACI 318-11 specify minimum spacing between bars equal to bar diameter or 25mm. This minimum spacing shall be maintained to guarantee proper placement of concrete around steel bars. Additionally, to prevent air pockets below reinforcements, and ensure good contact between concrete and bars so as to achieve satisfactory bond. If two layer of steel bars are placed in a beam, then the distance between them shall not be less than 25 mm.
Concrete protection for reinforcement
the designer must maintain minimum thickness or concrete cover outside of the outermost steel to provide the steel with adequate concrete protection against fire and corrosion. According to ACI Code 7.7, concrete cover of 40 mm for cast in place beams, not exposed directly to the ground or weather. At least 50mm cover, if the concrete surface is to be exposed to the weather or in contact. To simplify construction and thereby to reduce costs, the overall dimensions of beams, b and h are almost rounded up to nearest 25 mm.
Design of rectangular reinforced concrete beam procedure
The design of concrete beam includes the estimation of cross section dimension and reinforcement area to resist applied loads. There are two approaches for the design of beams. Firstly, begin the design by selecting depth and width of the beam then compute reinforcement area. Secondly, assume reinforcement area, then calculate cross section sizes.The First approach will be presented below
The following procedure used for the design of rectangular reinforced concrete beam:- Initially, select beam effective depth (d) and width (b). Effective depth can be computed using beam depth (h).

- Then, calculate the required flexural resistance factor assume ?=0.9

- After that, find reinforcement ratio corresponding to the computed flexural resistance compute above,

- The reinforcement ratio shall be less than maximum reinforcement ratio and greater than minimum reinforcement ratio.
- Minimum reinforcement ratio,

- Maximum reinforcement ratio

- Either reinforcement ratio can be employed but the latter will ensure that the strain in steel is at least 0.005.
- Thereafter, compute reinforcement area,

- Then, find number of bar by dividing reinforcement area over the area of a single bar.
- Lastly, check whether the bar can be placed within selected width of the cross section,

- The value of S shall not be less than 25 mm which the minimum required spacing between adjacent bars.
