In
the previous section we saw the minimum and maximum spacing of bars in a beam. In this section, we will see the minimum and maximum areas of bars in a beam.
b = width of beam or width of web for a flanged beam in mm
d = effective depth in mm and
fy = Characteristic strength of steel in N/mm2.
• In such beams, large exposed areas will be without reinforcement. In these areas, cracks can occur due to shrinkage of concrete and due to temperature variations.
pt is given by the equation:
If pt is greater than pt,lim, the section is Over reinforced, and it should be redesigned.
There is one more check that we have to do on a beam section. It is called the 'Check for deflection control'. We will see it's details in the next section.
Minimum area of flexural steel for beams
We saw how we determine the actual area of tension steel Ast,p provided in the beam here. Ast,p should be greater than or equal to As, the minimum area
of tension steel specified by the code. It is given in the
cl.26.5.1.1 of the code. According to this clause,
Where As = Minimum area of tension steel required in mm2 
b = width of beam or width of web for a flanged beam in mm
d = effective depth in mm and
fy = Characteristic strength of steel in N/mm2.
Maximum allowable area of flexural reinforcement in beams
If we provide excessive reinforcement in beams, concrete cannot be placed properly and compacted. This happens due to the congestion of reinforcements (Particularly at the column beam junctions) when excessive reinforcement is provided. So the code (cl 26.5.1) has specified an upper limit for the amount of reinforcement that can be given to a beam.
According
to cl 26.5.1(b), area of the tension steel provided should not be more
than 4% of the total cross sectional area of the beam ie., Ast,p ≯ 0.04bD
The
code specifies an allowable limit for the compression steel also. We
will see the details about it when we discuss about the design of
doubly reinforced beams.
Beams with overall depth greater than 750mm
In the case of beams where the depth is more, the area of concrete below the neutral axis will also be more. That means, there is a greater area of concrete under tension. We will be providing the tension reinforcements as nearly as possible to the tension face, satisfying the cover requirements, to get the maximum effective depth. In such beams, the tension reinforcement provided according to the above discussed rules will help to reduce cracks in the region around those tension reinforcements. But cracks will develop above this region where the concrete is in tension and is without reinforcement.• In such beams, large exposed areas will be without reinforcement. In these areas, cracks can occur due to shrinkage of concrete and due to temperature variations.
• We
have to consider the possibility of lateral buckling of the web
when the overall depth is more.
Considering
the above factors, the code (cl.26.5.1.3) requires us to provide side
face reinforcement
along
the two faces of the beams when the overall depth exceeds 750 mm. The
total area of side face reinforcement shall not be less than 0.1
percent of the web area and shall be distributed equally on the two
faces. The spacing of these bars should not exceed 300 mm or the web
thickness whichever is less. This is shown in the fig.4.14 below:
Fig.4.14
Side face reinforcements
Fig.4.14
Side face reinforcements
Check for the Limiting area of steel
We have seen earlier (Eq.3.29) in the analysis of beam section that every beam section have a quantity Ast,lim. So we have to check and confirm that Ast,p is less than Ast,lim. The easiest way to do this is to calculate pt of the beam section and compare with it's pt,lim. pt,lim can be obtained from the table 3.5 which is given below Eq.3.29.pt is given by the equation:
If pt is greater than pt,lim, the section is Over reinforced, and it should be redesigned.
Check whether Ultimate moment of resistance is greater than the ultimate moment acting on the beam
We must check and confirm that MuR of the section is greater than Mu. Calculation of MuR is easy because, in the previous check on pt, we will be confirming whether the beam is Under reinforced or Over reinforced. If it is over reinforced, the section should be redesigned. If it is Under reinforced, the calculation of MuR is easy.There is one more check that we have to do on a beam section. It is called the 'Check for deflection control'. We will see it's details in the next section.
good way of intoduction
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