huangyhg

maximum possible two way slab spans and thickness
the thickness of two way slab is dependent on its unsupported
perimeter, so larger the perimeter, greater will be its thickness.
for a 20 x 24 panel , it is about 6.5", but goes upto 9" for 25 x 35 panel.
when should we consider subdividing slab panel (hence reducing slab thickness), by using simply supporting beams but at the cost of loading main grid beams with concentrated loads which may come right at their midspans.
by flat slab, is the perimeter supported on beams? the moment increase by adding a beam at mid span will not be increased by much... very similar to udl loading.
dik
i don't think there's a set answer, but a conventionally reinf flat slab works very well for 30'x30' with normal live loads, 10.5" thick iirc. i'd start considering a slab with beams for bays much bigger than that. probably go flat plate if it was much smaller than this, like 24'x24'.
optieeng:
if you are in the us, chapter 9.5 of aci 318 gives some minimum thickness of slabs for deflection control.
i would calculate the required thickness for you particular case based on these guidelines. if the required thickness seems excessive, then i would consider adding intermediate beams.

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