My input file contains 2 load cases - case 1 and 2. For member 43, case 2 produces a larger value of shear force along local Y axis than case 1. However, the concrete design report indicates case 1 as being critical for shear design, and not case 2. How do you explain this?
The definition of the word critical in the shear design output in not on the basis of which among the various load cases has a larger amount of shear force, but which one requires the largest amount of stirrup reinforcement.
To answer your question, in all likelihood, you will see this happen when both load cases require the same amount of stirrup steel.
Design is carried out for all the load cases. The steel area values for all the cases are then sorted in the ascending order from low to high. If more than one case ends up requiring that highest steel area value (same area required for multiple load cases), the first among those load cases is reported as critical.
Another possibility is that torsion in the load case reported as critical may be higher than the one which has the highest shear force. Stirrups are designed for shear and torsion, not just shear.