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W salam, In monolithic construction , T or rec beam is not a matter of choice of designer but its indicated by analysis that if depth of compression block lies below flange (flange = slab thk in beam-slab system) then the compressive force in concrete is balanced by a certain width of flange + total width of web, otherwise only web balance compressive force i.e example of rec section carrying slab load in monolithic construction. Example of rec beam sections also exists in some specific cases of construction for eg RC beams supporting metal deck system,hollow core slab panels and precast construction system. ETABS checks the eqn "a < or > ds" for maximum analyzed Mu-bott and design positive R/F accordingly.

The beam with larger depth should be the main beam, and the beam smaller depth should be the secondary beam. By main beam, I mean that the beam has ability to attract forces from adjoining members because of its larger stiffness. Keeping that thing in mind, secondary beams should have been transferring the reaction directly to main beams, and main beams should have been transferring the loads of secondary beams, which are carrying the load of metal deck and the live load, to concrete beams. In the figure, they have placed larger and smaller size beams in the same directions. If smaller size are able to carry load, as is evident from the standing structure, why use larger beams. They have wasted material.