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.
A slab-column structure acting as the seismic-force-resisting system is likely to be far too flexible for higher seismic design categories (SDCs) and would not deliver performance consistent with other requirements.
For earthquake ground motions, slab-column framing systems designed according to Chapters 1 through 6 can be used in ordinary moment frames, and are appropriate for SDC A or B. If a slab-column frame meets additional requirements described in this chapter, it may be considered as an intermediate moment frame and is allowed for structures assigned to SDC C (ASCE 7-10, Section 12.2). Additional requirements are related to the distribution of slab moments, arrangement of slab reinforcement, and punching shear related issues. For structures assigned to SDC D, E, or F, slab-column frames without beams are generally not permitted as part of seismic-force-resisting systems (ACI 318-14, Section R18.2), with limited exceptions under ASCE 7-10, Section 12.2. For example, one exception would be intermediate moment frames in a dual system with special reinforced concrete shear walls less than 100 ft (30.5 m) in height.
The reasoning behind this is the slab-column frames cannot be detailed for the level of energy dissipation and ductility demanded for special moment frames. In the case of ductility demand, this chapter describes punching-shear related recommendations for slab-column connections, along with the design story drift estimated for the seismic force resisting system
Mandeep Singh Kohli CP, M.ASCE
Sent: 10-27-2020 11:43 AM
From: Christian Parker
Subject: Continuous Drop Panels
I am designing a low-rise concrete flat slab building. The lateral system is an intermediate moment frame incorporating the slab and drops. In order to meet the seismic drift limit, we have made the drop panels continuous in the building's weaker direction. ACI 318-14 Chapter 18 has very different design and detailing requirements for beams than for slabs with drop panels. Is it appropriate to design continuous drop panels (much wider than the columns) as beams, and if so, under what criteria? Are there geometry limits that would preclude considering drop panels as beams?
For everyone's benefit, I would love to hear responses general to all flat slab systems with continuous drops, not specific to my project.
Christian Parker EIT, A.M.ASCE
Structural Project Engineer