As far as possible, opening in slabs should be located in zones where shear stresses are small and bending moment are below maximum. However, small opening for pipe sleeves etc. can be made anywhere in slab. In case of slabs supported directly by columns, from structural view point, they are best located well away from the columns, preferably in the area common to the slab middle strips. Opening of any size may be provided in slab systems if shown by analysis that the design strength is at least equal to the required strength and that specified limits on deflection meets. For strength the strip method is appropriate.
Code Requirements (ACI 318)
13.5.1 Opening of any size are permitted in slabs if analysis shows that both strength and deflection are acceptable.
13.5.2 In lieu of special analysis as required by 13.5.1 opening may be provided in slab systems without beams only in accordance with the following.
220.127.116.11 Opening of any size may be located in the area common to intersecting middle strips.
18.104.22.168 In area common to intersecting column strips, not more than 1/8 the width of column strip in either span shall be interrupted by openings.
22.214.171.124 In the area common to one column strip and one middle strip, opening size is limited to 1/4 the width of column strip in either span.