Hello to everyone; In addition to the above, you have taken the pavement load of 0.17 tf/m^2 for all floors, correct this load to 0.350 - 0.400 tf/m^2 for hollow fill. Then things will change a little. Also, you have pushed the floor design of the top floor (I think for the duplex apartment staircase), it is possible to come up with a better solution by approaching it more simply. I am also in favor of statically necessary for large-sized heavy stairs, but mezzanine beams should not be placed for normal residential stairs. Because the mezzanine beam creates a short column situation, if necessary arrangements are not made, it will bring more harm than good. Good work...