Flat slab buildings have very low lateral stiffness and hence they are prone to large deflections even for very small levels of shaking. This excessive drift can further amplify the secondary moments on the columns due to P-Delta Effect. This is the reason ACI does not allow the use of this type of construction in high seismic regions and even forbids it use for some SDC.
Regarding the effective width calculation and other aspects of the design of flat slabs you may refer the ACI-ASCE joint publication "GUIDE TO DESIGN OF REINFORCED TWO-WAY SLAB SYSTEMS (ACI 421.3R-15)". Section 7.2 provides a method of calculating the effective width of the slab for lateral load purpose and also provides a method for designing these for seismic/ Lateral load.
However do read the below extract from the same specification. I would suggest you add a secondary lateral load system by terms of shear wall etc, and use the slab column system just for gravity loads.