In
the previous section we completed the analysis of flanged sections. In this section we will consider the design. At the beginning of chapter 9 we saw that two conditions should be satisfied to ensure the integral action between the slab and the beam. There we saw the first condition: The slab and the beam should be cast together. In most cases, the slab and beam are indeed cast together. The form works consists of horizontal board for the bottom side (soffit) of the beam, vertical boards for the sides of the beam, and horizontal boards for the under side of the slab. After tying the reinforcement bars and keeping them in position, fresh concrete is poured from the bottom of the beam. Concrete is poured continuously in such a way that, first the beam is filled up, and then the slab. Necessary compaction is ensured by using a needle vibrator or other such devices.
It may be noted that the stirrups of the beam also play a major role in ensuring integral action between the slab and the beam. These stirrups confine the main bars of the beam. They extend from the beam into the slab, upto the top surface of the slab. They are able to extend upto the top surface because the 'stirrup suspender bars' of the beam are near the top surface of the slab. So sufficient stirrups should be provided for the beam to ensure the integral action. We will learn about the design of stirrups when we discuss 'shear design'.
We will now see the second condition. Consider a slab and beam system used as the roof of a room, as shown in the plan view below:
Fig.10.1
Main bars of a slab given parallel to beam does not contribute towards flange action
We can see that the main bars extend from wall AB to wall CD. This is because, in the design of this particular slab, the slab loads are assumed to be transferred to the walls AB and CD only. No loads are transferred to the other two walls BC and AD. So no loads are transferred to the beam either. Thus all the main bars of the slab are parallel to the beam. In such a case, even if the slab and beam are cast together, the slab will not make a 'contribution' as the flange of the beam.
For the slab to make the 'contribution', a specific quantity of main bars of the slab should be available in a direction perpendicular to the beam. So in the above case, we must provide some 'special bars' perpendicular to the beam. The details of this are given in the cl.23.1.1.b of the code. The requirements of this clause can be illustrated if we draw the section XX shown in the plan view above. This is shown in the fig.10.2 below:
Fig.10.2
Section XX
We can see that the length of each of the 'special bars' on either side of the beam, should not be less than l/4, where l is the span of the beam. The area of these bars should not be less than 60% of the area provided at the midspan of the slab. Also, these bars should be provided near the top surface of the slab. By providing the specified quantity and length of these 'special bars', at the correct position, we can ensure that the slab will act as the flange.
In the fig.10.1 above, distributor bars will be present in a direction perpendicular to the beam. But they are not available near the top surface of the slab through out the length of the beam. Also their quantity may not satisfy the 60% requirement. So we must provide the 'special bars' even though the distributor bars are present in the perpendicular direction to the beam.
Now consider the slab shown in the fig.10.3 below:
Fig.10.3
Main bars of a slab given perpendicular to beam will contribute towards flange action
In this case, the main bars of the slab are provided in a direction perpendicular to the beam. It is a continuous slab. We have already discussed the analysis and design of such slabs. We know that the slab will experience a hogging moment at the beam. So there will be top bars to resist the hogging moment. And these bars will be perpendicular to the slab. So we do not need any 'special bars'. The top bars that we designed at an intermediate support for a slab can be seen here. It may be noted that even if we provide bent-up bars instead of straight bars, the required effect will be obtained because, the bent-up bars will become top bars at supports. In any case, the required length of l/4, and the quantity of 60% mentioned above should be available.
So now we know how to ensure integral action between slab and beam. We must extend this discussion to isolated T-beams and L-beams also. In those cases, the stirrups of the beam should extend to opposite sides of the beam, and travel upto the edges of the slab. This is shown in the fig.10.4 below.
Fig.10.4
Ensuring integral action between slab and beam for isolated flanged beams
A 3D view can be seen below:
Fig.10.5
3D view of bars in isolated T-beams
Fig.10.6
3D view of bars in isolated L-beams
From the above figs., we can see that the number of stirrups will be equal to the number of slab bars. The number of stirrups is calculated using shear force considerations. But the number of slab bars (obtained as spacing in 1m) is calculated from bending moment considerations. We must correlate both these and the larger number should be provided. The diameter of these bars should also be checked.
The presentation given below shows some examples of flanged beams, and the method of their performances.
From the above presentation, it is clear that each structure should be examined carefully to determine the locations where we can design a beam as a 'flanged beam'.
We can now take up the discussion on design procedure. In the next section, we will see the basic requirements like concrete cover, minimum steel etc.,
It may be noted that the stirrups of the beam also play a major role in ensuring integral action between the slab and the beam. These stirrups confine the main bars of the beam. They extend from the beam into the slab, upto the top surface of the slab. They are able to extend upto the top surface because the 'stirrup suspender bars' of the beam are near the top surface of the slab. So sufficient stirrups should be provided for the beam to ensure the integral action. We will learn about the design of stirrups when we discuss 'shear design'.
We will now see the second condition. Consider a slab and beam system used as the roof of a room, as shown in the plan view below:
Fig.10.1
Main bars of a slab given parallel to beam does not contribute towards flange action
We can see that the main bars extend from wall AB to wall CD. This is because, in the design of this particular slab, the slab loads are assumed to be transferred to the walls AB and CD only. No loads are transferred to the other two walls BC and AD. So no loads are transferred to the beam either. Thus all the main bars of the slab are parallel to the beam. In such a case, even if the slab and beam are cast together, the slab will not make a 'contribution' as the flange of the beam.
For the slab to make the 'contribution', a specific quantity of main bars of the slab should be available in a direction perpendicular to the beam. So in the above case, we must provide some 'special bars' perpendicular to the beam. The details of this are given in the cl.23.1.1.b of the code. The requirements of this clause can be illustrated if we draw the section XX shown in the plan view above. This is shown in the fig.10.2 below:
Fig.10.2
Section XX
We can see that the length of each of the 'special bars' on either side of the beam, should not be less than l/4, where l is the span of the beam. The area of these bars should not be less than 60% of the area provided at the midspan of the slab. Also, these bars should be provided near the top surface of the slab. By providing the specified quantity and length of these 'special bars', at the correct position, we can ensure that the slab will act as the flange.
In the fig.10.1 above, distributor bars will be present in a direction perpendicular to the beam. But they are not available near the top surface of the slab through out the length of the beam. Also their quantity may not satisfy the 60% requirement. So we must provide the 'special bars' even though the distributor bars are present in the perpendicular direction to the beam.
Now consider the slab shown in the fig.10.3 below:
Fig.10.3
Main bars of a slab given perpendicular to beam will contribute towards flange action
In this case, the main bars of the slab are provided in a direction perpendicular to the beam. It is a continuous slab. We have already discussed the analysis and design of such slabs. We know that the slab will experience a hogging moment at the beam. So there will be top bars to resist the hogging moment. And these bars will be perpendicular to the slab. So we do not need any 'special bars'. The top bars that we designed at an intermediate support for a slab can be seen here. It may be noted that even if we provide bent-up bars instead of straight bars, the required effect will be obtained because, the bent-up bars will become top bars at supports. In any case, the required length of l/4, and the quantity of 60% mentioned above should be available.
So now we know how to ensure integral action between slab and beam. We must extend this discussion to isolated T-beams and L-beams also. In those cases, the stirrups of the beam should extend to opposite sides of the beam, and travel upto the edges of the slab. This is shown in the fig.10.4 below.
Fig.10.4
Ensuring integral action between slab and beam for isolated flanged beams
A 3D view can be seen below:
Fig.10.5
3D view of bars in isolated T-beams
Fig.10.6
3D view of bars in isolated L-beams
From the above figs., we can see that the number of stirrups will be equal to the number of slab bars. The number of stirrups is calculated using shear force considerations. But the number of slab bars (obtained as spacing in 1m) is calculated from bending moment considerations. We must correlate both these and the larger number should be provided. The diameter of these bars should also be checked.
The presentation given below shows some examples of flanged beams, and the method of their performances.
From the above presentation, it is clear that each structure should be examined carefully to determine the locations where we can design a beam as a 'flanged beam'.
We can now take up the discussion on design procedure. In the next section, we will see the basic requirements like concrete cover, minimum steel etc.,
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can i get this reinforced concrete design as complete pdf
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