Sunken slabs are very common in building structures. Mostly toilet slabs are sunken to accommodate services pipes inside it.
Manually designing a sunken slab has been a confusion for young civil & structural engineers.
In this video a students question on design of sunken slab is discussed.
A sunken slab can be conservatively designed as a 4-side discontinuous slab.
This may lead to slight over design of the span bottom steel of the slab.
There will be slight fixity at the support due to the monolithic slab beam joint.
However generally the stiffness of this joint is not so huge and hence a minimum steel provided there at the top will ensure design safety.
Even if the top steel provided is slightly lesser, redistribution can take place within limits and ensure the sunken slab is safe.
If you want to learn more about redistribution here is a blog
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