SANJIB DAS
Bentley Technical Support Group
We accept that concrete capacity shall be ignored as per clause 6.3.4 of IS 13920-2016 but this can be logically ok if only Ductile shear governs the design. In cases where DL+LL shear governs the design (i.e. no plastic hinge evidently formed) it will be very conservative to ignore shear capacity of concrete in beam.
The requirement of ignoring concrete capacity is due to unpredicted forces during earthquake. The earthquake forces are instantaneous, the concrete will crack, and plastic hinge will be formed. Even after the earthquake loads, the beams will be loaded with the Dead and live loads. The cracking of concrete, the shear capacity of the bean will be depending on the shear reinforcements only. Thus, ignoring concrete capacity of the beam for shear check is applicable to all load combinations. So even the critical combination is DL+LL, the concrete capacity should be ignored