This is a general question and the answer is subject to many factors as such.
The answer is tentatively 2.0 metres x 2.0 metres isolated footing.
The foremost criterion is the Safe Bearing Capacity(SBC) of soil(bearing stratum).
Secondly the Final reaction coming to the column. That depends on factors like type of construction like walls, type of occupancy for live loads, dead load intensity and most important the span of beams(spacing of columns).
Assuming a load of 300KN/floor and SBC of 250KN/SqM , 2 metrex 2 metre footing is satisfactory. Generally 1.5m x1.5 m to 2.0mx2.0m is applicable.
area of footing = Load on footing/Safe Bearing Capacity of soil.
it gives the area. You can choose the geometry according to you.
It's based on some failure criterion.
Most important of them are:
Shear failure- one way & punching failure
Flexure failure
Bearing failure
Failure due to excessive bond stress.
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