In the previous section we saw the critical sections for shear design in simply supported beams and the beams in framed structures. In this section we will see some more cases.
Case 4
In this case we consider the situation in which there is no compressive force acting on the support portion of the beam. A suspended beam is an example of this situation. It is shown in fig 13.70 below:
Fig.13.70
Suspended beam
• Here the shear strength of the beam at the support region is not enhanced.
• This is because of the absence of any reaction force from a bottom support.
So the critical section should be taken at the face of the support. This is shown in fig.13.71 below:
Fig.13.71
Critical section for suspended beam
Case 5
In this case also, we consider a situation in which there is no compressive force acting on the support portion of the beam. This time we consider an 'inverted T-beam'. It is shown in fig.13.72 below:
Fig.13.72
Inverted T-beam
• The load will be acting at the top surface of the slab
• But this top surface of the slab is at a lower level than the top surface of the beam.
• Because of this, the compressive force cannot be achieved at the support portion of the beam.
So the critical section should be taken at the face of the support as shown in the fig.13.73 below:
Fig.13.73
Critical section for inverted T-beam
Case 6
In this case we consider a beam which is supported on another beam. The supporting beam is called Primary beam and the supported beam is called the secondary beam. This is shown in the fig.13.74 below:
Fig.13.74
Secondary beam supported on a primary beam
In this case, the critical section for the secondary beam is at the face of the support as shown in the next fig.13.75
Fig.13.75
Critical section for the secondary beam
In this case, special care should be taken in the shear design of the primary beam also. It is recommended that extra full depth stirrups, also known as ‘hanger stirrups’ should be provided in the primary beam as shown in the fig.13.76 below:
In this case, special care should be taken in the shear design of the primary beam also. It is recommended that extra full depth stirrups, also known as ‘hanger stirrups’ should be provided in the primary beam as shown in the fig.13.76 below:
Fig.13.76
Details of hanger stirrups
When we analyse the secondary beam, we will get the Shear force at the support. This is force acts as a concentrated load on the primary beam. It is multiplied by the suitable load factors to get the factored load. If we denote this factored load as R, then the area Asv of the extra hanger stirrups can be obtained by the following equation:
Eq.13.65:
The above equation follows from the basic equation: Force (R) = Stress (0.87fy) x Area (Asv)
If the force R is large, bent-up bars should also be used in addition to the above extra stirrups. This is shown in the fig.13.77 below:
Fig.13.77
Additional bent up bars when R is large
This completes our discussion on ‘Critical sections’ for shear design. From this discussion it is clear that we must examine each structural element carefully to determine the position of the critical section. We can now proceed to do the complete shear design of a beam.
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